Loretta Guidi
email: loretta.guidi@uniurb.it

M Antonio Todaro

Marco Ferraguti

Maria Balsamo

Reproductive system of the genus Crasiella (Gastrotricha, Macrodasyida) Authors

Keywords: Crasiella, Gastrotricha, Macrodasyida, Reproductive System, Spermatozoon

Crasiella diplura from Sweden and Crasiella sp. from Italy were studied alive and with transmission electron microscopy (TEM). The two species are simultaneous hermaphrodites and share the same reproductive system layout: paired ovaries extend along the posterior part of the intestine and join mid-dorsally, while bilateral, clubshaped testes lie at the sides of the anterior gut, extending as deferentia that fuse on the mid-ventral plane and open into a single pore; gametes mature in a caudocephalic and centripetal direction. The bulky, sac-like, frontal organ is lined by a simple epithelium, and lies dorsolaterally to the intestine, on the left side of the body. The spindleshaped caudal organ is musculo-glandular and is located ventrolaterally to the gut on the right side. It is characterized by the presence of a roughly Y-shaped internal channel that opens into two pores close to each other, which function for the intake and outlet of the (auto)sperm, respectively. The spermatozoa, which are peculiar and similar in the two species, are characterized by a long and complex acrosome consisting of four ultrastructurally distinct regions, three of which find equivalence in other gastrotrich species. The flagellum lacks a striated cylinder. Anatomy and ultrastructure enable us to hypothesize a modality of sperm transfer in Crasiella that is similar to that observed in Macrodasys.

Introduction

Gastrotricha is a phylum comprising about 750 species of acoelomate microInvertebrebrates, which are well represented in benthic communities and are grouped into two orders: Macrodasyida, consisting of 300 marine species, which are generally simultaneous or sequential hermaphrodites with cross-fertilization; and Chaetonotida, comprising 450 mostly freshwater, facultative parthenogenic species, with few marine, hermaphroditic genera (Neodasys, Heteroxenotrichula, Xenotrichula, Diuronotus, Musellifer) [START_REF] Balsamo | Taxonomic and nomenclatural notes on freshwater Gastrotricha[END_REF][START_REF] Hummon | Analytic taxonomy and notes on marine, brackish-water and estuarine Gastrotricha[END_REF]).

The reproductive system of Macrodasyida is anatomically quite complex and diverse, probably because of the different modalities of internal cross-fertilization. Usually, there is a pair of testes, two (or one) ovaries and two sexual accessory organs, which are respectively known as frontal and caudal organs from their relative positions in the trunk region (Ruppert 1978a). The elongated testes lie at the sides of the anterior intestine and extend posteriorly in sperm ducts, opening ventrally into either a common or separated pores. The ovaries are lateral to the caudal intestine, and oocytes generally mature in a caudo-frontal direction, gradually shifting dorsally. In some taxa (e.g. Cephalodasys and Dolichodasys) oocytes mature in a fronto-caudal direction. The dorsal fusion of the two ovaries in order to form an unpaired uterus has been reported in Dactylopodola typhle [START_REF] Kieneke | Anatomy and ultrastructure of the reproductive organs in Dactylopodola typhle (Gastrotricha : Macrodasyida) and their possible functions in sperm transfer[END_REF]. A connection made of either a solid cell band, or an epithelial duct between the ovary and an external, dorsal pore (mating pore?), has only been described in Acanthodasys thrinax and Diplodasys ankeli (Thaumastodermatidae), and Dactylopodola typhle, respectively (Ruppert 1978b[START_REF] Kieneke | Anatomy and ultrastructure of the reproductive organs in Dactylopodola typhle (Gastrotricha : Macrodasyida) and their possible functions in sperm transfer[END_REF].

The sac-like frontal organ is lined by an epithelium and lies in proximity to the most mature egg; it displays an external pore through which it probably receives allosperm during copulation, thus functioning as a seminal receptacle. An internal pore allows the stored sperm to leave the organ and fertilize the eggs. The elongated caudal organ is wholly muscularized and lies ventrally; it opens close to or within the anus and almost certainly acts as a copulatory organ [START_REF] Ruppert | Gastrotricha[END_REF]). In the Macrodasys species, the caudal organ forms a thin protrusile tube within which the autosperm are packed (Ruppert 1978a), while in some Urodasys species it has a cuticularized stylet [START_REF] Schoepfer-Sterrer | Five new species of Urodasys and remarks on the terminology of the genital organs in Macrodasyida (Gastrotricha)[END_REF][START_REF] Fregni | Two new species of Urodasys (Gastrotricha, Macrodasyidae) with cuticular stylet[END_REF][START_REF] Balsamo | Living without mitochondria: spermatozoa and spermatogenesis in two species of Urodasys (Gastrotricha, Macrodasyida) from dysoxic sediments[END_REF]. Mating and sperm cross-transfer modalities have been described in detail by Ruppert (1978a, b) on the basis of ultrastructural observations of Macrodasys sp. and different species of Thaumastodermatidae. In addition, hypotheses about these processes have been advanced by [START_REF] Teuchert | Zur Forpflanzung und Entwicklung der Macrodasyoidea (Gastrotricha)[END_REF][START_REF] Balsamo | Reproductive system and spermatozoa of Paraturbanella tessieri (Gastrotricha, Macrodasyida): implications for the sperm transfer modality in Turbanellidae[END_REF], [START_REF] Kieneke | Anatomy and ultrastructure of the reproductive organs in Dactylopodola typhle (Gastrotricha : Macrodasyida) and their possible functions in sperm transfer[END_REF][START_REF] Guidi | Unusual spermatozoa and reproductive modalities of Xenodasys eknomios (Gastrotricha: Xenodasyidae)[END_REF] based upon microscopical and ultrastructural data on species of other families.

The studies carried out in the last 30 years have proved that the morphology of the reproductive organs and the ultrastructure of spermatozoa are very important diagnostic characters for taxonomic use, as well as being a valid tool for phylogenetic purposes (e.g. [START_REF]Interstitial gastrotrichs in some south Florida beaches[END_REF][START_REF] Evans | Morphological variability in warm-temperate and subtropical populations of Macrodasys (Gastrotricha: Macrodasyida: Macrodasyidae) with description of seven new species[END_REF][START_REF] Marotta | Sperm ultrastructure of Macrodasys caudatus (Gastrotricha: Macrodasyida) and a sperm based phylogenetic analysis of Gastrotricha[END_REF][START_REF] Kieneke | Morphology and function of reproductive organs in Neodasys chaetonotoideus (Gastrotricha: Neodasys) with a phylogenetic assessment of the reproductive system in Gastrotricha[END_REF].

However, no morphological data on the reproductive organs and spermatozoa of the Planodasyidae family are yet available. Herein we provide a description of the reproductive system and the spermatozoon of two species of Crasiella, which is one of the two genera of the family. We also suggest a possible modality of mating and fertilization in the two studied species of this genus, and compare this to the information available on other species of Macrodasyida. [START_REF] Clausen | Crasiella diplura gen. et sp. n. (Gastrotricha, Macrodasyoidea)[END_REF] were collected in September 2007 during a two-week workshop held at the Sven Lovén Centre for Marine Sciences on Tjärnö, an island on the Swedish west coast (for details see [START_REF] Willems | Meiofauna of the Koster-area, results from a workshop at the Sven Lovén Centre for Marine Sciences (Tjärnö, Sweden)[END_REF]. Sand samples of Crasiella sp. had also been collected in July 2002 at S. Isidoro, Lecce, Italy.

Methods

Sand samples of Crasiella diplura

Gastrotrichs were extracted from the sediment by the narcotization-decantation technique, using an isosmotic (7%) magnesium chloride solution [START_REF] Giere | Meiobenthology: the microscopic motile fauna of aquatic sediments[END_REF]. The fauna-containing supernatant was then poured directly into a 5-cm diameter Petri dish and scanned for specimens under a Wild M3 dissecting microscope set at 50x magnification.

For optical microscopy, the gastrotrichs were removed with a micropipette from the Petri dish, fresh-mounted on slides and observed using a Nikon Eclipse 80i equipped with Differential Interference Contrasts (Nomarski).

During observation, the animals were photographed with a DS -5M Nikon digital camera and measured with the Nikon NIS software.

For the transmission electron microscopy, 10 specimens of Crasiella diplura and five of Crasiella sp. were fixed overnight in 2% glutaraldehyde in a 0.1 M sodium cacodylate buffer (pH 7.4), and stored in a 0.1 M sucrose/sodium cacodylate buffer. In the lab the specimens were post-fixed in 1% osmium tetroxide buffered with 0.1 M sodium cacodylate, washed in a clean 0.1 M sodium cacodylate buffer, dehydrated in a graded ethanol series and embedded in Araldite. Semi-thin and ultra-thin sections were cut with a LKB Ultrotome 2088V, and were contrasted with toluidine blue and lead citrate respectively. The semi-thin sections were observed in transmission light under a VANOX AHBT3 Olympus optical microscope, whereas the ultra-thin sections were observed under a Philips CM10 transmission electron microscope. The locations of some of the morphological characteristics along the body are given in percentages of units (U) of the total body length, measured from the anterior to the posterior.

Three-dimensional reconstruction of the spermatozoon of C. diplura was performed by CimatronE software (Cimatron Ltm, Tel Aviv, Israel; www.cimatrom.com).

Results

Reproductive apparatus

The overall arrangement of the reproductive system is the same in the two species of Crasiella examined. Mature individuals are simultaneous hermaphrodites. One pair of club-shaped testes lies at the sides of the anterior gut, extending into two sperm ducts which fuse on the mid-ventral plane and open into a single ventral pore (Fig. 1ac,e). Two bilaterial ovaries run along the posterior part of the intestine and join mid-dorsally (Fig. 1a,d). In the posterior trunk region, there are two accessory organs, both of which are very close to each other. The voluminous, sac-like frontal organ is lined by an epithelium and lies dorsolaterally to the intestine on the left side of the body, between the largest oocyte and the caudal organ. The spindle-shaped caudal organ is musculo-glandular and located ventrolaterally to the gut on the right side of the body (Fig. 1a,d).

Testes

The two testes extend from U34 to U50 and narrow into sperm ducts which open into a single ventral pore at U66. The germinal epithelium is arranged along the entire length of each testis, and gametes mature in a caudo-cephalic and centripetal direction so that early germinal cells, spermatogonia and spermatocytes lie in the posterior part of each gonad, while spermatids gather in the internal lumen of the most anterior part (Fig. 2a).

Spermatogenesis

In spermatids, the nuclear chromatin progressively condenses while the nucleus starts elongating. At the same time, a long mitochondrion, arising from the fusion of many smaller mitochondria, sticks to the nuclear envelope and gradually surrounds it, before then sinking into the nucleus. A manchette of microtubules is visible around the nucleusmitochondrion complex (Fig. 2a,b). At that point, the acrosome starts to develop: the cytoplasm of the spermatid is almost entirely filled with large vesicles containing a homogeneous, moderately electron-dense material, and numerous small, strongly electron-dense vesicles. These progressively pile up, forming the basal portion of the acrosome while their content condenses. As the nuclear-acrosomal complex is growing, the axoneme is already apparent and starts lengthening (Fig. 2c,d). Once the nucleus has reached its definitive size, it rotates medially and then posteriorly with respect to the axoneme. Due to this transformation, the spermatid takes on a clear U-shape (Fig. 2e). When the acrosome formation has been completed, spermatids enter the lumen of the anterior portion of each testis and move in a caudal direction. Their flagellum rotates with respect to the nucleus, getting in line with the nuclear-acrosomal complex.

Mature spermatozoa migrate caudally along the testis lumen towards the sperm duct, which is completely surrounded by circular muscles belonging to the trunk musculature (Fig. 2f).

Spermatozoa

The spermatozoon of Crasiella diplura is long, filiform cells containing, in sequence, an acrosome, a nucleusmitochondrial complex, and a tail. The acrosome is very long (about 50 µm) and is sub-divided into four different morphological regions over its length (Figs. 2g,3a,b). The apical region, which is 8 µm long, is a helically twisted tube with an inner structure made of piled disks (Fig. 2h,o); its diameter increases in a basal direction from 0.1 to 0.2 µm. The next region is a straight tube, which is 7 µm in length and surrounded by a helical ridge (Fig. 2i,p), with a diameter increasing from 0.2 to 0.3 µm. The third region, 4 µm long, is a straight tube surrounded by numerous short radial fibres (Fig. 2j,q). The last region is much larger, being 31 µm in length and 0.8 µm in diameter, and has large, piled, globular vesicles (0.7 µm in diameter). Between these large vesicles there are several smaller globular vesicles (0.14 µm in diameter), which are embedded within a very electron-dense material (Fig. 2k,r).

The nucleo-mitochondrial portion is about 20 µm long and has a mitochondrion (0.5 µm in diameter) that is surrounded by the nucleus, forming a characteristic "spring", which is a ribbon of compacted chromatin spirally coiled around the mitochondrion for a total of 20 turns (Figs. 2g,l,s,3a,b). The flagellum has a 9x2+2 axoneme, characterized by a dense and prominent central sheath surrounding the central tubules; a wide space separates the axoneme from the tail membrane. Another peculiarity of the sperm tail in Crasiella is the fact that, apparently, the plasma membrane is curled (Figs. 2m,t,3a,b).

The spermatozoon of Crasiella sp. differs from that of Crasiella diplura in the following ways: 1. the large globular vesicles of the acrosomal base are less electron-dense; 2. the small globular vesicles measure 0.07 µm vs 0.14 µm in diameter and are more electron-dense; 3. the granular material surrounding the vesicles is less electrondense; 4. the mitochondrion inside the nucleus is much thinner, 0.18 µm vs 0.5 µm (Fig. 2n,u,v).

Ovaries

The two lateral ovaries extend along the intestine from U60 to U76. Each ovary is formed by a few oocytes (4-5) that migrate anteriorly and dorsally during maturation. The fully-grown oocyte lies anterior to the frontal organ and dorsal to the intestine (Fig. 4a-d). No epithelial sheath enveloping the germ cells was observed. Circular and longitudinal trunk muscles surround the ovaries. The early oocytes have many free ribosomes and mitochondria; the nuclear chromatin is granulated and moderately electron-dense, and the nucleolus is large and well organized (Fig. 4b). Later oocytes contain yolk granules, which progressively increase both in number and size during oogenesis, until they finally fill the entire cytoplasm. The nuclear chromatin is moderately electron-dense and the nucleolus is absent (Fig. 4c). Mature oocytes are very large cells (100 µm in diameter), which are packed with yolk granules, free ribosomes and fragments of rER. The egg envelope consists of a thin, inner layer of electrondense granular material and an outer layer of densely packed microfilaments (Fig. 4d,e).

Frontal organ

The frontal organ is a large sac (86 µm long, 26 µm wide), extending from U65 to U80. An external, well-defined dorsolateral pore is present, and another small pore is directed anteriorly towards the most mature oocyte. The frontal organ is lined by a monolayered columnar epithelium, delimiting a cavity in which usually numerous active spermatozoa are visible in living specimens (Fig. 5a). The epithelial cells have many microvilli extending into the inner cavity, and a cytoplasm with secretory vesicles of different sizes, varying from 0.2 µm to 1.5 µm. The spermatozoa inside the lumen are surrounded by a uniformly electron-dense material. Circular and longitudinal muscles, belonging to the trunk musculature, surround the frontal organ (Fig. 5c).

Caudal organ

The caudal organ is spindle-shaped (70 µm long, 30 µm large) and extends from U73 to U84. It is formed by an anterior, glandulo-muscular region (50 µm long) and by a posterior muscular region (20 µm in length). This organ is completely surrounded by a thick basal lamina and well-developed circular muscles, and also contains an internal duct (Fig. 5b,f).

This internal duct is Y-shaped, with one branch lying inside the anterior region and forming a blind end (Fig. 5b, e), while the other two branches extend along the posterior region and open into the exterior through two distinct ventral pores (Fig. 5b,e,f,g).

The anterior, blind duct is surrounded by cells with the cytoplasm full of secretory granules. Numerous microvilli extend into the lumen. Inside it, in some cases spermatozoa were observed (Fig. 5d,e).

Of the two ducts lying in the posterior region, the main duct is 10 µm long and 8 µm wide, and ends with a "feeder bib" extremity (12 µm long) which opens into a very small ventral pore (1.5 µm in diameter, Fig 5b). The second duct, the accessory one, is straight and much longer (25 µm long, 3 µm wide). It lies ventrally to the first and opens into a large ventral pore (6 µm in diameter) (Fig. 5f). The lumen of the main duct is covered with a large number of packed secretion granules, which are small, round or ovoid in shape, and contain a homogeneous, moderately electron-dense material. The cytoplasm of the cells surrounding the lumen is filled with secretion granules (80x200 nm), globular mitochondria, numerous stacked membranes (endoplasmic reticulum and dictyosomes), ribosomes that are both free and attached to the reticulum membranes, and many groups of secretion granules. The nuclei often have annulated membranes (Fig. 5e,j). These cells also form many, very long and extremely thin cytoplasmatic protrusions, which overlap each other, thus forming a multi-membranous sheath all over the lumen. The secretion granules are regularly stuck into these cytoplasmic protrusions. The content of the granules is progressively released into the lumen, and appears as a granular material (Fig. 5e,k). The lumen of the feeder bib-shaped extremity of the main duct is covered with typical secretion granules, which are ovoid in section, slightly electron-dense, and are regularly arranged in rows. Only the terminal part of the duct lacks these granules, and is wholly surrounded by large cells that are peculiar for their irregular shape and numerous interdigitations. They contain many mitochondria, a well-developed Golgi apparatus, a lot of ribosomes that are both free and attached to the rER membranes, and secretion granules (Fig. 5h,i).

The accessory duct is connected to the anterior end of the main duct and extends posteriorly along the medial side of the caudal organ up to a ventral pore that is clearly visible in both light and electron microscopy (Fig. 5f,g).

The lumen of the accessory duct is covered with at least three types of secretion granules: the first kind are large and ring-shaped and made up of coarse granular material; the second kind are ovoid or round and full of coarse granular material; the third kind are small, cylindrical granules and consist of homogeneous material. The cells surrounding the lumen have many organelles and groups of all of the types of secretion granules. Numerous elongated microvilli extend from the cell surface into the luminal space (Fig 5g).

Discussion

Members of the Crasiella genus are hermaphroditic, with separate female and male gonads, as is the case in most species of Macrodasyida. The paired condition of both testes and ovaries is very common in macrodasyidan gastrotrichs, as is the caudo-cephalic direction of the maturation of spermatozoa and oocytes [START_REF] Ruppert | Gastrotricha[END_REF][START_REF] Fischer | Ultrastructure of penetrated spermatozoa, ovary, and oogenesis of Dactylopodola baltica (Gastrotricha, Macrodasyida)[END_REF][START_REF] Balsamo | Reproductive system and spermatozoa of Paraturbanella tessieri (Gastrotricha, Macrodasyida): implications for the sperm transfer modality in Turbanellidae[END_REF].

The presence in Crasiella of two club-shaped and hollow testes which extend into sperm ducts confirms the basic plan of the male system of Macrodasyida [START_REF] Ruppert | Gastrotricha[END_REF][START_REF] Fischer | Ultrastructure of spermiogenesis and spermatozoa of Cephalodasys maximus (Gastrotricha, Macrodasyida)[END_REF][START_REF] Guidi | Spermiogenesis and spermatozoa in Acanthodasys aculeatus (Gastrotricha, Macrodasyida): an ultrastructural study[END_REF]). However, it has been stated that Lepidodasys sp. and Urodasys anorektoxys have strip-shaped testes, with no inner lumen or defined sperm ducts [START_REF] Guidi | Spermatology of the genus Lepidodasys Remane, 1926 (Gastrotricha, Macrodasyida): towards a revision of the family Lepidodasyidae Remane[END_REF][START_REF] Balsamo | Living without mitochondria: spermatozoa and spermatogenesis in two species of Urodasys (Gastrotricha, Macrodasyida) from dysoxic sediments[END_REF]). The paired testes of Dactylopodola typhle are also stripshaped, without distinct sperm ducts, but they do have a testicular lumen [START_REF] Kieneke | Anatomy and ultrastructure of the reproductive organs in Dactylopodola typhle (Gastrotricha : Macrodasyida) and their possible functions in sperm transfer[END_REF]. The germinal epithelium of the Crasiella species forms the testes wall, like in many species of Macrodasyida [START_REF] Ruppert | Gastrotricha[END_REF].

The fusion of the two sperm ducts in a single median, ventral pore is a feature that Crasiella shares with Turbanellidae, Cephalodasys and Megadasys [START_REF] Remane | Morphologie und Verwandschaftsbeziehungen der aberranten Gastrotrichen I[END_REF][START_REF] Schmidt | Interstitielle Fauna von Galapagos. IV. Gastrotricha[END_REF], whereas most Macrodasyida species have two separate, ventral male pores [START_REF] Ruppert | The reproductive system of gastrotrichs. I. Introduction with morphological data for two new Dolichodasys species[END_REF]Ruppert 1978a, b;Guidi et al. 2009, Hummon and[START_REF] Hummon | Analytic taxonomy and notes on marine, brackish-water and estuarine Gastrotricha[END_REF]. Only a few species have a direct connection between the sperm duct/s and the caudal organ (Mesodasys, Thaumastodermatidae [START_REF] Ruppert | Gastrotricha[END_REF][START_REF] Ferraguti | The comparative ultrastructure of sperm flagella central sheath in Clitellata reveals a new autapomorphy of the group[END_REF]Balsamo 1994, Fregni et al. 1999).

The lack of a sheath enveloping the oocytes is a feature shared by the Crasiella species with other species of Macrodasyida (Mesodasys laticaudatus, Mesodasys adenotubulatus, Paraturbanella teissieri; [START_REF] Ferraguti | Sperm morphology and anatomy of the genital organs in Mesodasys laticaudatus Remane, 1951 (Gastrotricha, Macrodasyida)[END_REF][START_REF] Fregni | Morphology of the reproductive system and spermatozoa of Mesodasys adenotubulatus (Gastrotricha, Macrodasyida)[END_REF][START_REF] Balsamo | Reproductive system and spermatozoa of Paraturbanella tessieri (Gastrotricha, Macrodasyida): implications for the sperm transfer modality in Turbanellidae[END_REF]). An incomplete ovarian covering has been reported in some species (Dolichodasys carolinensis, Acanthodasys thrinax, Diplodasys ankeli, Tetranchyroderma bunti, Thaumastoderma heideri, Macrodasys sp.; Ruppert andShaw 1977, Ruppert 1978a, b). A thin epithelial lining the oocytes has been hypothesized to be the plesiomorphic condition in Gastrotricha by [START_REF] Kieneke | Morphology and function of reproductive organs in Neodasys chaetonotoideus (Gastrotricha: Neodasys) with a phylogenetic assessment of the reproductive system in Gastrotricha[END_REF].

Spermatogenesis

The occurrence of spermatogenesis in a caudo-cephalic and centripetal direction is typical of most Macrodasyida [START_REF] Hummon | Gastrotricha[END_REF][START_REF] Ruppert | Gastrotricha[END_REF][START_REF] Fischer | Ultrastructure of spermiogenesis and spermatozoa of Cephalodasys maximus (Gastrotricha, Macrodasyida)[END_REF]. The major steps in the process have been documented in five species belonging to five families: Turbanella cornuta, Lepidodasys sp., Cephalodasys maximus, Acanthodasys aculeatus, and Urodasys anorektoxys [START_REF] Teuchert | Elektronenmikroskopische Untersuchung über die Spermatogenese und Spermatohistogenese von Turbanella cornuta Remane (Gastrotricha)[END_REF]Fisher 1994;[START_REF] Guidi | Spermiogenesis and spermatozoa in Acanthodasys aculeatus (Gastrotricha, Macrodasyida): an ultrastructural study[END_REF][START_REF] Guidi | Spermatology of the genus Lepidodasys Remane, 1926 (Gastrotricha, Macrodasyida): towards a revision of the family Lepidodasyidae Remane[END_REF][START_REF] Balsamo | Living without mitochondria: spermatozoa and spermatogenesis in two species of Urodasys (Gastrotricha, Macrodasyida) from dysoxic sediments[END_REF]. Like in Turbanella cornuta (Turbanellidae), Lepidodasys sp. (Lepidodasyidae) and Cephalodasys maximus (Cephalodasyidae) the spermatogenesis in species of Crasiella imply a two-step process in the formation of the nuclear-mitochondrial complex. The fusion of mitochondria into a single element is followed by the penetration in the nucleus by this mitochondrial element. A comparable two-step process has also been described in Acanthodasys aculeatus (Thaumastodermatidae), but here the mitochondria are incorporated into the nucleus one by one, and it is only afterwards that they fuse into a single element [START_REF] Guidi | Spermiogenesis and spermatozoa in Acanthodasys aculeatus (Gastrotricha, Macrodasyida): an ultrastructural study[END_REF]. In Urodasys anorektoxys (Macrodasyidae), spermatogenesis is completely different, because the single mitochondrion gets lost at the end of the process, thus forming a highly specialized sperm that is lacking any mitochondria [START_REF] Balsamo | Living without mitochondria: spermatozoa and spermatogenesis in two species of Urodasys (Gastrotricha, Macrodasyida) from dysoxic sediments[END_REF]). In species of Macrodasys, which is the other genus of the family, the mitochondrion wraps itself externally around both the nucleus and the acrosome of the mature sperm and remains in this position. Hence, in Macrodasys the formation of the nuclear-mitochondrial complex likely imply a single step, i.e. the fusion of the mitochondria in a single element [START_REF] Marotta | Sperm ultrastructure of Macrodasys caudatus (Gastrotricha: Macrodasyida) and a sperm based phylogenetic analysis of Gastrotricha[END_REF]. This shortened process in species of Macrodasyidae seems to be related to a secondary evolutionary process toward the loss of mitochondria, rather than to a primitive modality of spermatogenesis (Guidi et al. unpublished data).

Sperm

The spermatozoa of the two Crasiella species resemble those of other macrodasyidan gastrotrichs in several ways. They have a long and complex acrosome with four different regions: 1) an apical helix with piled discs; 2) a straight tube with helical ridges; 3) a straight tube with radial fibres; and 4) a long and wide region formed by piled vesicles.

The aspects of these regions differ considerably, but three of them have an equivalent in other species. Region one is, in fact, also found in Urodasys anorektoxys [START_REF] Balsamo | Living without mitochondria: spermatozoa and spermatogenesis in two species of Urodasys (Gastrotricha, Macrodasyida) from dysoxic sediments[END_REF]), Acanthodasys aculeatus [START_REF] Guidi | Spermiogenesis and spermatozoa in Acanthodasys aculeatus (Gastrotricha, Macrodasyida): an ultrastructural study[END_REF], and in the Thaumastodermatinae examined so far; region two is present in Acanthodasys aculeatus, while region four is similar to that observed in Xenodasys eknomios [START_REF] Guidi | Unusual spermatozoa and reproductive modalities of Xenodasys eknomios (Gastrotricha: Xenodasyidae)[END_REF] and Thaumastoderma mobjergi (Ferraguti M., Marotta M., Guidi L., Todaro M.A. unpublished data). A nucleo-mitochondrial region, formed by a spring-shaped nucleus surrounding the mitochondrion, is also present in all of the Thaumastodermatidae, Lepidodasyidae and Turbanellidae studied so far, as well as in Mesodasys [START_REF] Ferraguti | Sperm morphology and anatomy of the genital organs in Mesodasys laticaudatus Remane, 1951 (Gastrotricha, Macrodasyida)[END_REF]. It is, however, absent in Xenodasys where, surprisingly, the nuclear spring surrounds part of the acrosome [START_REF] Guidi | Unusual spermatozoa and reproductive modalities of Xenodasys eknomios (Gastrotricha: Xenodasyidae)[END_REF]. In Crasiella, the flagellum is devoid of a striated cylinder, which is a characteristic present in Thaumastodermatidae, Cephalodasyidae [START_REF] Hummon | Analytic taxonomy and notes on marine, brackish-water and estuarine Gastrotricha[END_REF] and Macrodasyidae, with the exception of Urodasys anorektoxys [START_REF] Balsamo | Living without mitochondria: spermatozoa and spermatogenesis in two species of Urodasys (Gastrotricha, Macrodasyida) from dysoxic sediments[END_REF]. The axoneme has a curious modification of the central axonemal apparatus, which is the prominent central sheath that is known only from Clitellata and was regarded as an autapomorphy of that group [START_REF] Ferraguti | The comparative ultrastructure of sperm flagella central sheath in Clitellata reveals a new autapomorphy of the group[END_REF]. Accordingly, the prominent central sheath of the axoneme is the only characteristic which clearly separates the spermatozoon of Crasiella from those of the other gastrotrichs.

Accessory sex organs and possible fertilization modality

The two accessory sexual organs of the Crasiella species are always visible, well-developed, and close to each other, although they are clearly separate: their morphology and arrangement correspond most closely to those of species of Macrodasys (Macrodasyidae). The shape and size of the frontal organ, as well as the relative position of its external and internal pores, confirm the observations on most Macrodasys species (e.g. [START_REF] Evans | Morphological variability in warm-temperate and subtropical populations of Macrodasys (Gastrotricha: Macrodasyida: Macrodasyidae) with description of seven new species[END_REF]).

The structural complexity of the glandulo-muscular caudal organ of Crasiella resembles that of the same organ in the species of Macrodasyidae. In Macrodasys the posterior part of the caudal organ contains a thin tube, which has a copulatory function (Ruppert 1978a;[START_REF] Evans | Morphological variability in warm-temperate and subtropical populations of Macrodasys (Gastrotricha: Macrodasyida: Macrodasyidae) with description of seven new species[END_REF], while in Urodasys the organ is V-shaped and formed by two distinct, elongated muscular regions. These are almost parallel to each other, with the left one being glandular in nature, while the right one has a funnel-like, copulatory stylet. The two regions are connected by a thin anterior duct [START_REF] Schoepfer-Sterrer | Five new species of Urodasys and remarks on the terminology of the genital organs in Macrodasyida (Gastrotricha)[END_REF][START_REF] Fregni | Two new species of Urodasys (Gastrotricha, Macrodasyidae) with cuticular stylet[END_REF], and open to the outside into a common pore.

The caudal organ is apparently very similar in the species of Crasiella and Macrodasys, because of its shape, and a well-defined, anterior glandular region. However, only in Crasiella it shows two distinct ducts opening into two distinct pores. The presence of a short, blind duct in the glandular region, and of the only circular musculature are two additional features that clearly distinguish the caudal organ of Crasiella from that of the other Macrodasyidae.

A possible mechanism for cross-fertilization in Crasiella is hypothesized here on the basis of morphological grounds and by making a comparison with the few observations available on the fertilization modalities in other Macrodasyida.

A mature individual would first transfer spermatozoa from the testes to the caudal organ. It accomplishes this transfer through ventral body flexions, bringing the male pore close to the pore of the accessory duct of the caudal organ.

Spermatozoa enter the accessory duct, and travel to the blind duct of the glandulo-muscular region. They are then stored in the secretion material produced by the surrounding glandular cells (i.e. the abundant electron-dense material which has been observed with TEM) until to be transferred to the mating partner. At mating, the glandulo-muscular region pushes spermatozoa from the blind duct through the main duct up to its feed bib-shaped extremity, which might be protruded into the pore of the partner"s frontal organ. Active sperm can then reach the mature oocyte via the internal pore of the frontal organ. The supposed fertilization mechanism in Crasiella has some correspondence with that described by Ruppert (1978a) for Macrodasys sp. Here, spermatozoa are transferred during copulation from the male pores of an individual to its own caudal organ through its pore. Inside the caudal organ, they are packed into the thin, copulatory tube at the same time that the latter is placed into the partner"s frontal organ. Then, the copulatory tube breaks off and releases the spermatozoa, which now head to the mature oocyte. Excess of spermatozoa are reabsorbed. Some functional similarities and possible homologies between different parts of the reproductive system in the two genera can be suggested on the basis of ultrastructural observations. The lack of a direct connection between the male gonads and the caudal organ in both implies the need to charge the latter with autosperm before fertilization.

In fact, the hypothesized mechanisms of fertilization in Macrodasys and Crasiella appear to be similar in that they comprise two subsequent phases, the first for transferring autospermatozoa from the male pore to the caudal organ, and the second for transferring sperm to the frontal organ of the partner by means of a specialized anatomic structure belonging to the caudal organ.

According to Ruppert"s (1978a) description, the terminal part of the caudal organ of Macrodasys, called "antrum' by [START_REF] Remane | Gastrotricha. In: Bronns HG (ed) Klassen Ordnungen des Tierreichs[END_REF], is a glandular region specialized in receiving autosperm, and thus it may be regarded as being at least functionally correspondent to the accessory lumen of the caudal organ of Crasiella. Similarly, the "copulatory tube" of Macrodasys (Ruppert 1978a) would have a functional correspondence with the "feeder bib-shaped" extremity of the caudal organ of Crasiella, since both structures likely transfer autosperm to the partner"s frontal organ. However, the timing of the two phases appears to be different for species of Macrodasys and Crasiella, respectively. In the former, the packaging of autosperm into the copulatory tube occurs during copulation, when the tube is already inside the partner's frontal organ ; in Crasiella, on the contrary, the autosperm are packaged into the main duct of the caudal organ prior to copulation. All other species of Macrodasyida for which information on reproductive anatomy and biology is available appear to be too different from the Crasiella species to be usefully compared.

Conclusions

This study has allowed us to trace a consistent plan of the reproductive system in two species of the genus Crasiella.

The features of the accessory organs and, in particular, the highly complex structure of the caudal organ, suggest that the Crasiella species are phylogenetically close to the Macrodasys species. If the functional similarities that have been proposed for the two genera can be proved to be real homologies, these would strongly support the proximity of the genus Crasiella to the Macrodasyidae, which has been suggested by the phylogenetic analysis of [START_REF] Hochberg | Macrodasyida (Gastrotricha): A cladistic analysis of morphology[END_REF]. A morphological study of species of Planodasys, the only other genus of Planodasyidae, would be the best way to test this hypothesis.

Fig. 3

 3 Fig. 3 Graphic reconstructions of the mature spermatozoon of Crasiella diplura. a Schematic drawing of the spermatozoon. I-IV: different regions of the acrosome, from the more distal (I) to the more proximal (IV) (see the description in the text). The nucleus is spiralized around the mitochondrion and the flagellum is surrounded by a curled plasma membrane (arrow). b 3D model of the spermatozoon showing the same regions of the spermatozoon represented in the 2D drawing. cs central sheath; f flagellum(* in cross section); m mitochondrion; n nucleus.

Fig. 4

 4 Fig. 4 Ovaries of Crasiella diplura (TEM microscopy). a Cross section of the body of a whole animal at the level of the early oocytes. The sperm ducts are sectioned close to their fusion. b An early oocyte. c Three later oocytes. d Mature dorsal oocyte packed with yolk granules (longitudinal section). e Egg envelope: the inner thin layer made of electron-dense granular material and the outer one made of densely packed microfilaments are visible. ee egg envelope, eo early oocyte, mo mature oocyte, sd sperm duct. Scale bars a-d = 8 µm; e = 1 µm

Acknowledgments

We would like to thank the referees for their helpful comments. M.A. Todaro is thankful to the organizing committee of the meiofauna workshop at the Sven Lovén Centre for Marine Sciences at Tjärnö, during which Crasiella diplura was collected. The research benefitted from a M.I.U.R. grant to MB (PRIN 2007 -Sexuality, reproduction and specific identification of Gastrotricha and Placozoa).

Legends of figures


N Sewraj

N Merbahi

J P Gardou

P Rodriguez

F Marchal

ELECTRIC AND SPECTRO SCO PIC ANALYSIS OF A PURE NITROGE N MONO-FILAMENT ARY DIELECTRIC BARR IER DISC HARGE (MF-DBD) AT 760 TORR

Keywords: Dielectric barrier discharge, Filamentary discharge, Nitrogen, Electrical analysis, Optical emission spectroscopy, Vacuum ultraviolet and ultraviolet radiations, Visible light and near Infrared emissions PACS: 52.80.-s -52.70.Ds -51.50.+v -32.30.Jc -32.20.Lg -52.70.Kz

As far as we know, VUV emissions of nitrogen MF-DBDs have never been reported before.

Mono-filamentary dielectric barrier discharge (MF-DBD), occurring within 1-mm gap of atmospheric pressure pure nitrogen and operating with a sinusoidal electric supply at about 8 kHz, is studied in this paper.

A thorough electrical analysis allows experimental determination of the ignition and extinction voltages, respectively (15750 ± 50) V and (2097 ± 7) V, the injected energy (158 ± 2) J and charge (17.22 ± 0.22) nC in a single filament. The mean axial reduced electric field is equal to (644 ± 2) Td at ignition. An empirical technique is proposed to evaluate these discharge parameters by avoiding bulky calculations. Optical emission spectroscopic measurements of the vacuum ultraviolet (VUV), ultraviolet (UV), visible and near infrared (IR) emissions are presented and discussed. Two atomic nitrogen lines attributed to the decay of the N[2s 2 p 2 3s 2 P] triplet towards N[2s 2 2p 3 2 D°] level are observed at 150 and 175 nm, together with the Lyman-Birge-Hopfield

) dominates the UV and visible-blue spectra. The (0-0) transition of the First Negative system (N 2

) and the Herman infrared transitions (N 2 [C" 5 Π u ]→ N 2 [A' 5 Σ + g ]) also present. Both our VUV and near IR spectra are consistent with recently reported results in hollow cathode and cylindrical DBDs. The electrical and spectroscopic experimental results reported here are useful for ongoing and forthcoming modelling of filamentary nitrogen dielectric barrier discharges.

INTRODUCTION

Cold non-equilibrium atmospheric plasmas achieved with Dielectric Barrier Discharges (DBDs) are of great interest for production of energetic photons (hν > 10 eV), radicals or ion sources for industrial use. Some of these applications concern photo-chemical treatment of water [START_REF] Azrague | Hydrogen peroxide evolution during V-UV photolysis of water during the degradation of organic compounds[END_REF], Gonzalez et al 2004], volatile organic compound (VOC) remediation [START_REF] Biomorgi | Coupling of Adsorption on Zeolite and V-UV Irradiation for the Treatment of VOC Containing Air Streams[END_REF], Koutsospyros A et al 2004], biochemical decontamination and remediation of toxic gases [Herrman H W et al 1999], realisation of non-coherent vacuum ultraviolet (VUV: 100 nm < λ < 200 nm) or ultraviolet (UV: 200 nm < λ < 400 nm) sources [Kurunczi P et al 1999, Masoud N et al 2004], material deposition [START_REF] Babayan | Deposition of silicon dioxide films with an atmospheric pressure plasma jet Plasma Sources[END_REF], H 2 generation for fuel cells and diesel reforming [START_REF] Qiu | Hydrogen Generation in a Microhollow Cathode Discharge in High-Pressure Ammonia-Argon Mixtures[END_REF], Becker K H et al 2004], exhaust treatment [START_REF] Dietz | 31 st IEEE ICOPS[END_REF] … As far as non-coherent radiation DBD sources are concerned, VUV sources are of topical interest for a variety of industrial purposes such as plasma processing [START_REF] Kogelschatz | From ozone generators to flat television screens: history and future potential of dielectric-barrier discharges[END_REF],

surface cleaning [START_REF] Korfatis | 8th Symp. on Atomic, Cluster, and Surface Physics[END_REF] and modification [START_REF] Wagner | The barrier discharge: basic properties and applications to surface treatment[END_REF][START_REF] Borcia | Dielectric barrier discharge for surface treatment: application to selected polymers in film and fibre form Plasma Sources[END_REF], sterilization, decontamination and medical care. Recently some emerging scientific investigations were performed in neon [START_REF] Carman | Enhanced performance of an EUV light source (λ = 84 nm) using short-pulse excitation of a windowless dielectric barrier discharge in neon[END_REF]. Rare gas DBDs can be used for excilamps [Zhang J-Y andBoyd I W 2000, Boichenko A M et al 2004], mercury-free lamps [START_REF] Jinno | [END_REF], for oxidation of silicon at 250 °C [START_REF] Kogelschatz | High-intensity sources of incoherent uv and vuv excimer radiation for low-temperature materials processing E-MRS-IUMRS-ICEM[END_REF]. Operating in xenon or neon, they produce VUV or EUV (10 nm < λ < 100 nm) emissions with an efficiency as high as 50 to 60% [START_REF] Vollkommer | Vollkommer F and Hitzschke L 1996 Pulsed dielectric barrier discharges -a breakthrough in UV radiation generation[END_REF][START_REF] Hitzschke | Hitzschke L and Vollkommer F[END_REF][START_REF] Mildren | Enhanced performance of a dielectric barrier discharge lamp using short-pulsed excitation[END_REF][START_REF] Carman | Computer modelling of a shortpulse excited dielectric barrier discharge xenon excimer lamp (λ ∼ 172 nm)[END_REF], Carman R J et al 2004, Merbahi N et al 2007, Carman R J et al 2010]. In order to understand their behaviour, stable MF-DBDs were must be achieved at frequencies around tens of kilohertz [Adler F and Muller S 2000, Sewraj N et al 2009[START_REF] Merbahi | Luminescence of argon in a spatially stabilized mono-filamentary dielectric barrier micro-discharge: spectroscopic and kinetic analysis[END_REF], Merbahi N et al 2010]. In such circumstances, for each half-period all the observed emissions can be unambiguously attributed to a single filament whose time-origin is well defined by the fast rising supply current.

During the last three decades a growing attention has been paid to atmospheric DBDs containing nitrogen because of their potentiality for a wide variety of applications [Kogelschatz U et al 1999]. Polymer surfaces are often smooth and it is rather difficult to wet them, or to paint or print on them or to deposit adherent metal coatings. Such surfaces can be activated by a DBD treatment in open air. An increasing number of investigations are devoted to the decomposition of nitrogen and sulphur oxides in flue gases, and of VOCs emanating from various large-scale industrial plants, for which dc corona discharges technique is used. However, for applications where high specific powers or low gas temperature are required, DBDs constitute a good alternative to corona discharges.

Atmospheric DBDs are usually diffuse (homogeneous) or multi-filamentary [START_REF] Okazaki | Appearance of stable glow discharge in air, argon, oxygen and nitrogen at atmospheric pressure using a 50 Hz source[END_REF]. Homogenous nitrogen based DBDs are often devoted to surface treatment [START_REF] Wagner | The barrier discharge: basic properties and applications to surface treatment[END_REF]. Nevertheless, random filaments which may occur at ignition or, to a lesser extent, during the steady state are detrimental to the quality of the treatment. For both type of DBDs, a good knowledge of the electrical, spectroscopic and kinetic behaviour of mono-filamentary discharge is required in order to optimize specific applications. In order to study filamentary nitrogen DBDs a few authors have achieved laboratory multi-filamentary [START_REF] Lukas | Dielectric barrier discharges with steep voltage rise: mapping of atomic nitrogen in a single filaments measured by laser-induced fluorescence spectroscopy[END_REF] or mono-filamentary DBDs in mixtures [START_REF] Kozlov | Investigation of the filamentary and diffuse mode of barrier discharges in N2/02 mixtures at atmospheric pressure by cross-correlation spectroscopy[END_REF] containing nitrogen. In contrast to rare gas DBDs, those occurring in nitrogen yield a variety of emissions over an extended spectral range from VUV to infrared.

The intent of this paper is to perform a simultaneous experimental analysis of both the electrical and spectroscopic performance of a pure nitrogen MF-DBD. The original feature is that spectroscopic measurements are performed on spatially stable nitrogen MF-DBDs, where emissions are recorded from VUV to near infrared emissions. It also constitutes a first step in providing essential electrical and spectroscopic data which will allow validation of electro-dynamic and kinetic modelling of these MF-DBDs which can be more easily controlled and modelled than multifilamentary ones.

THE EXPERIMENTAL SETUP

Fig. 1: The experimental setup for electric and emission spectroscopic analysis of a MF-DBD (including photography)

The experimental setup allowing electrical and spectroscopic investigations of pure nitrogen MF-DBDs is shown in figure 1. It consists of a DBD chamber (cell) associated to a pumping/filling system, an electric supply as well as the electrical and spectroscopic measuring devices. The MF-DBD cell is a closed cylindrical glass chamber provided with two diametrical side-on fittings, each one equipped with a high voltage vacuum feed-through isolated (15 kV) from the chamber. Each electrode is a 4-mm diameter cylindrical aluminium rod, covered with a d die = 0.5 mm thick insulating MAKOR on its flat front-end (relative electrical permittivity: ε R = 4.0). Such small plate-to-plate double barrier configuration electrodes ensures establishment of a unique filamentary barrier discharge for every half-cycle of the driving sinusoidal voltage u sup (t) [START_REF] Merbahi | Luminescence of argon in a spatially stabilized mono-filamentary dielectric barrier micro-discharge: spectroscopic and kinetic analysis[END_REF] and was previously used to investigate rare gas DBDs. The gap distance was maintained at d =1 mm, so as to achieve stable MF-DBDs in pure nitrogen at a pressure of P N2 = 760 Torr, with our electric supply at a frequency of f = 7.8 kHz. Daily refreshed, the DBD cell was filled with 99.999% pure nitrogen (Air Liquide) and before filling up, it was pumped down to about 10 -7 Torr. The impurities initially present in our gas were: O 2 , H 2 , CO, CO 2 , The front-end of the reactor was equipped with a transparent MgF 2 window for spectroscopic measurements. To record the visible and near infrared emission spectra we used a 0.5 m imaging visible spectrophotometer (V_MC: Acton Spectra Pro 2556i, in the Czerny-Turner configuration). The detecting device was a 32-bits 2D Intensified Charge Coupled Device camera (ICCD: Princeton Instruments, 512x512 pixels) was placed at its exit port. Its spectral domain lies within 360 and 920 nm. The emitted photons were guided to the entrance slit of the V_MC spectrophotometer via a 3 m-long optical fibre (OF: UV-Silicon LG-455-020-3).

For VUV measurements the optical fibre (OF) and the visible spectrophotometer (V_MC)

were replaced by a 0.3 m VUV monochromator (VUV_MC, in the Czerny-Turner configuration as well) equipped with a fast solar blind photomultiplier tube (PMT). The PMT operated in the photon counting mode and its anode pulses were delayed through a delay line (DL) then processed by a constant fraction discriminator (CFD-Sig) and finally sent to a multi-channel analyzer (Stanford Research SR430). In order to classify these photons according to their emission instant, the time origin of the discharge needs to be unambiguously detected. For each micro-discharge this origin instant was defined on the leading edge of the fast current pulse via a fast amplifier and a constant fraction discriminator (CFD-Trig). No significant difference is observed for the electric and spectroscopic behaviours of the MF-DBD between the positive half-cycle and the negative one. So, the VUV emission spectrum was recorded by summing all the photons of the luminescence decays accumulated over a great number of discharges of only the positive half-cycle of the supply voltage, for each central exit wavelength of the monochromator, between 120 and 200 nm. For all VUV records, we verified that the probability for more than one photon to be recorded in a single channel was always less than 1 ‰.

All the emission spectra were performed on the first interference order of the grating. The exit spectral width Δλ exit and the wavelength step dλ step were equal. Our VUV, UV, visible and near IR optical emission spectra are given without any correction for the wavelength-dependent of the relevant detection device. For VUV measurements (130-200 nm: 2400 grooves/mm grating), the MgF 2 exit window was the high-pass filter whereas for UV emissions (200-400 nm: 2400 grooves/mm grating), air was a natural high-pass filter. For the 400-600 nm (2400 grooves/mm) and 600-900 nm (1200 grooves/mm grating) spectral ranges, either the Corning 0-53 (cutting wavelength: 340 nm) or the MTO J517a (cutting wavelength: 517 nm) high-pass filter was

Paper N 2 at P atm 20/12/10 9:04 Page 6/26 respectively used. The exit spectral widths were respectively 0.5 nm, 0.016 nm, 0.020 nm and 0.073 nm for these spectra.

ELECTRICAL DIAGNOSIS OF A MF-DBD

Fig. 2b shows the equivalent electrical circuit, which fairly describes the dynamic behaviour of an axially symmetric DBD [START_REF] Liu | Excitation of dielectric barrier discharges by unipolar submicrosecond square pulses[END_REF]. The filamentary discharge is represented by a non-linear dissipative current source i dis (t). The capacitor C die is the total capacity of both dielectric layers whereas C gap accounts for the inter-electrode region devoid of any discharge.

!

C cell = C gap " C die C gap + C die
represents the total cell's capacity. The capacitor C p represents the total stray capacitors of the driving circuit due to the electrical leads and probes. In this model, we assume that C die remains linear and that C gap keeps the same value whether a discharge occurs or not. These assumptions will be commented in section 4.1. C gap and C die are calculated from geometrical considerations:

! C gap = " 0 S d = 0.25 pF and ! C die = " 0 " R S 2 d die = 1
.00 pF . The stray capacitor C p was determined from u sup (t) and i sup (t) when no discharge occurs:

! C P = 0.44 pF .
Time origin is considered at the instant when the discharge voltage u dis (t) is maximum. This instant is obtained from our calculations. In our experimental conditions, the voltage drop u R (t) across the resistance R (50 Ω) was about (10.0 ± 0.1) V and was negligible compared to the amplitude of the supply voltage u sup (t). The voltage u cell (t) applied to the DBD cell was:

! u cell (t) " u sup (t) = U Max sin(2#ft + 0.150) (1)
When the voltage across the gap u dis (t) is high enough to induce a primary electronic avalanche, a single micro-discharge occurs for each half-cycle. The measured supply current i sup (t) is then a fast-rising pulsed current superimposed on a slow displacement current [START_REF] Merbahi | Luminescence of argon in a spatially stabilized mono-filamentary dielectric barrier micro-discharge: spectroscopic and kinetic analysis[END_REF]. The current i cell (t) flowing through the DBD cell can be evaluated:

Paper N 2 at P atm 20/12/10 9:04 Page 7/26

! i cell (t) = i sup (t) -i C p (t) = i sup (t) -C p du sup (t) dt " i sup (t) -C p du cell (t) dt (2)
In our experimental conditions, the current flowing through the stray capacitor i Cp (t) is so weak that i cell (t), calculated from equation ( 1), could hardly be distinguished from i sup (t): the amplitude of i Cp (t) was less than 0.7 mA. The current i cell (t) can also be considered as a fast and short current pulse i cell_Pulse (t) (this component exists only during the discharge phase)

superimposed on a slow weak displacement one: i cell_Displ (t) (itself due to the cell's capacity C cell ): (t) (3)

! i cell (t) = i cell_Pulse (t) + i cell_Displ
According to the electrical scheme given in Fig. 2b, the voltage drop u die (t) across both dielectric layers can be calculated from i cell (t). The voltage drop u dis (t) across the gap is then easily deduced.

In order to evaluate the electrical power injected in the micro-discharge, the discharge current i dis (t) should also be determined:

! i dis (t) = 1 + C gap C die " # $ % & ' i cell (t) -C gap du cell (t) dt (4)
And substituting equation (3) in equation ( 4), i dis (t) is found to be proportional to the pulsed component i cell_Pulse (t) of i cell (t):

! i dis (t) = 1 + C gap C die " # $ % & ' i cell_Pulse (t) (5)
i cell_Pulse (t) can be determined by subtracting the displacement component i cell_Displ (t) from i cell (t).

! i cell_Displ (t) = C cell du cell (t) dt (6)
The energy E 1 and the charge Q 1 injected into a single filament are respectively:

! E 1 = i dis (t) " u dis (t) dt 0 1 2 f # (7) ! Q 1 = i dis (t) dt 0 1 2 f " ( 8
)
Paper . We assume that our voltages and currents are all known with the above relative uncertainties.

In Fig. 3b the supply voltage and current are depicted with an expanded time scale corresponding to the discharge phase, from the ignition time (t B ) to the extinction time (t E ). In Fig. 4a, we represent the discharge voltage u dis and current i dis as well as the u cell and the voltage drop across both dielectrics u die , during the positive half cycle. The various currents and voltages of the circuit are displayed in Fig. 4b, with an expanded time-scale so that the different discharge phases can be followed. For the positive half-cycle, we can thus distinguish the following phases: the voltage drop across both dielectrics u die (t 0 ) is negative, due to the net charge accumulated on the dielectric surfaces during the preceding micro-discharge (negative half cycle). The initial voltage across the discharge itself is: electrical field across the gap is high enough to ignite the gas, at t = t B . During this phase i cell_Pulse and hence i dis are both equal to 0 A.

! u dis (t = t 0 ) = U 0 = q die (t = t 0 ) C die = (
! u dis = C die C gap +C die " # $ $ % & ' ' u cell +U 0 = 0.8 u cell +U 0 (9)
(ii) t = 0 ns: Gas ignition. The gas ignites (i dis starts) when the space charge is high enough to accelerate "seed electrons" present in the gas. At this instant t B the gap voltage attains its maximum value, the ignition voltage V B = u dis (t B ) = (15.752 ± 0.051) kV, which corresponds to a mean axial reduced field (E/N) of (644 ± 2) Td (where 1 Td = 10 -17 V cm 2 ). The cell and dielectric voltages are respectively: u cell (t B ) = (10.725 ± 0.035) kV and u die (t B ) = (-5.027 ± 0.016) kV.

(iii) 0 ns < t < 849 ns: Townsend stabilization phase or background charge phase. After ignition, i dis increases only slightly while u dis decreases very slightly around 15.7 kV. This time lag, which is about 0.8 µs here, is usually observed in nitrogen filamentary DBDs. According to Yurgelenas and Wagner [YurgelenasY V and Wagner H-E 2006] who performed a computational model of a barrier discharge in air (d = 1.2 mm) at atmospheric pressure N 2 /O 2 binary mixtures, residual charges play an important role in the micro-discharge (MD) formation. According to their calculations, in this first phase, which they labelled as a Townsend phase by stabilization, the "early" electrons are multiplied through ionization processes and then accumulated on the anode dielectric, thus leaving a net ionic density in the gap. It follows that ions dominate in the gap. The discharge current (and axial electron density) increases slowly until the charges accumulated on the dielectrics reduce the axial electric field. During this stabilization phase where background charges are accumulated [Capeillère J 2009], the axial field remains nearly constant along the gap's axis. It lasts several µs in air at atmospheric pressure, and the electron number density remains quite low.

Paper N 2 at P atm 20/12/10 9:04

Page 11/26

In other gases like argon and xenon this phase is generally skipped [START_REF] Merbahi | Luminescence of argon in a spatially stabilized mono-filamentary dielectric barrier micro-discharge: spectroscopic and kinetic analysis[END_REF] [ [START_REF] Merbahi | Electrical behavior and vacuum ultraviolet radiation efficiency of monofilamentary xenon dielectric barrier discharges[END_REF]. So it seems that, in nitrogen based DBDs ignition processes followed by the avalanche propagation should be somewhat different from the classical Townsend breakdown usually encountered at such high p.d values (76 Torr.cm) in noble gases.

(iv) 849 ns < t < 894 ns: Electronic avalanche propagation or space-charge-dominated phase. It then follows another phase characterized by an exponential growth of the charged particles (electrons and ions) in every position of the gap. The current i dis then increases much faster. The electrons attain the anode at about t = 894 ns, which corresponds to the first current peak around 30 mA. In the meanwhile u dis decreases slowly. A semi-log plot of i dis (not given here) shows that during the electronic avalanche i dis rises quasi-exponentially with a time constant of τ avalanche ≈ 100 ns, quasi-independent of pressure from 100 Torr to 760 Torr.

(v) 894 ns < t < t M = 923 ns: Propagation of the cathode directed ionization wave (IW) or filamentary phase. Following the preceding space-charge-dominated phase, a fast cathode directed ionization wave (streamer) propagates towards the cathode due to secondary electrons emitted by the cathode dielectric. During this phase (duration = 29 ns), i dis increases very rapidly.

When the ionization wave reaches the cathode, the discharge current rise slows down because of the additional charge deposited on the cathode layer. The usual characteristic "flagging" of the discharge current [START_REF] Merbahi | Luminescence of argon in a spatially stabilized mono-filamentary dielectric barrier micro-discharge: spectroscopic and kinetic analysis[END_REF] is not observed in our conditions probably because the streamer propagates too fast, the space charge being quite high at this pressure.

The charge deposited on both dielectrics t > t M , lowers the axial electric field. However, the radial electric field becomes strong enough to initiate a surface discharge in the vicinity of the cathode dielectric. The formation of a thin luminous cathode layer starts. Up to t = t M , this phenomenon gains over the lowering of the axial electric field.

Paper N 2 at P atm 20/12/10 9:04 Page 12/26

(vi) t M < t < t E =2000 ns: Cathode layer formation. When t = t M , i dis attains its maximum value, the charge deposited on both dielectrics predominates over the effect of the increase of the conducting cross-section available at the cathode. The cathode layer grows up while the discharge current i dis rapidly falls down. We verified that the current oscillations present near the end of this discharge extinction did not correspond to reflections of the pulsed current along the electrical leads.

(vii) t = t E : Discharge quenching. At t = t E when plasma conductivity is greatly reduced mainly due to accumulation of electrons and ions on the opposite surface of the dielectrics, the discharge stops: i dis = 0 mA. The gap voltage then attains its minimum value V E = (2097 ± 7) V (extinction voltage). The corresponding mean axial reduced field is (86.0 ± 0.3) Td. At this particular instant, the cell and dielectric voltages are respectively: u cell (t E ) = (11.365 ± 0.037) kV and u die (t E ) = (9.268 ± 0.030) kV.

(viii) t > t E : Post-discharge. The luminescence decay of the main VUV emission at 150 nm shows that this emission lasts more than 1 µs after ignition (section 4.2.2), while the current is completely quenched. Between t = t E and t = t 0 + T/2, no discharge exists again (i dis = 0 mA) and u dis is consistent with:

! u dis = C die C gap +C die " # $ $ % & ' ' u cell ( U 0 .
Then, the discharge of the negative half-cycle occurs, similarly to the previously described successive phenomena.

Characteristics of the mono-filamentary discharge

During the positive filamentary discharge, the cell voltage varies from 10.72 to 11.36 kV (Fig. 3b), showing that, between t B and t E , the supply voltage drop is about 5.8%, around 11.04 kV. Moreover, the current i gap flowing through C gap is negative (u dis decreases monotonically from V B to V E ). Subsequently, i dis > i cell , as predicted by equation ( 5). Due to the respective values The electric energy E 1 injected in a single discharge and its uncertainty

! "E 1 = E 1 "U Max U Max # $ % & ' ( 2 + "I Max I Max # $ % & ( 2
are also calculated:

! E 1 = (157.9 ± 2.0) µJ. Compared to argon [START_REF] Merbahi | Luminescence of argon in a spatially stabilized mono-filamentary dielectric barrier micro-discharge: spectroscopic and kinetic analysis[END_REF], krypton [Merbahi N et al 2010] and xenon [START_REF] Merbahi | Electrical behavior and vacuum ultraviolet radiation efficiency of monofilamentary xenon dielectric barrier discharges[END_REF], for which typical values of E 1 do not exceed 5 µJ, such a high value is not astonishing for nitrogen, in regard to both a high ignition voltage V B and discharge current amplitude, despite the short pulse duration. The electric charge Q 1 injected in one discharge (its uncertainty

! "Q 1 = Q 1 "I Max I Max ),
is:

! Q 1 = (17.72 ± 0.22) nC.
Much emphasis needs to be laid upon the fact that the displacement current of i cell perfectly fits the cell current during the whole capacitive voltage divider phase, meaning that C p is correctly determined. Besides, during the discharge phase, both C die and C gap seem to remain constant. As far as C die is concerned, this is not astonishing because the charges are deposited on the anode and cathode surfaces. However, even if the MD occupies a certain gap volume due to the filament, the funnel shape anode charges and the disc of the cathode layer, the gap capacitance C gap remains unaffected in the short duration of the discharge. Macroscopically, the gap behaves as a linear capacitor.

Fig. 4b shows that after discharge quenching, u dis never exceeds u die during the positive half-cycle. This excludes the occurrence of a second MD before the negative half-cycle. If wrong values of C die and C gap are used, the calculated waveform u dis can exceed V B in the post-discharge phase and most of the discharge parameters (mainly V B and V E ) will then be incorrectly determined. Indeed, the waveforms of u dis and i dis are very sensitive to C p , C gap and C die . So our results were crosschecked by calculating the injected charge by two different manners.

!

Q 1 = i dis (t) dt t B t E " = i cell (t) dt t B t E " # i gap (t) dt t B t E " = C die u die (t E ) -u die (t B ) { } + C gap V B -V E ( ) = 14.31+ 3.42 ( ) nC = 17.72 nC and ! Q 1 = C gap + C die ( ) V B -V E ( ) + C gap u cell (t E ) -u cell (t B ) { } = 17.08 + 0.64 ( ) nC = 17.72 nC
The last detailed estimation of Q 1 shows that the charge injected in a MD is merely (96%) the charge variation for the capacitor equivalent to two parallel capacitors C gap and C die . The injected energy is:

! E 1 " 1 2 C gap + C die ( ) V B 2 -V E 2 ( ) = 153 µJ . So, provided that t B and t E can be estimated by visualising ! u dis (t) = u cell (t) - i cell (t' ) dt' t 0 t " + q die (t 0 ) C die
, Q 1 and E 1 can be simply evaluated in the following manner, without the determination of i dis :

! V B = u dis (t B ) ! V E = u dis (t E ) ! E 1 " 1 2 C gap + C die ( ) V B 2 -V E 2 ( ) ! Q 1 = C gap + C die ( ) V B -V E ( ) + C gap u cell (t E ) -u cell (t B )
{ } a dominant emission (0,0) band head at 337.1 nm. Between 200 and 300 nm (Fig. 5b), a few weak double head characteristic emissions are observed and are attributed to the NOγ bands, even if laboratory 99.999% pure nitrogen is used. We believe that oxygen is present in the discharge, due to unavoidable impurity traces and sputtering of our dielectric material.

Spectroscopic results and discussion

UV spectrum

Visible spectrum

The visible spectrum recorded between 400 and 600 nm (Fig. 6) is mainly dominated by the SPS emissions. The weak emission at 391.4 (Fig. 6b), whose rotational blue wing looks different to SPS ones, is attributed to the (0-0) transition of the N 2 First Negative System (FNS:

N 2 + [B 2 Σ + u ] → N 2 + [X 2 Σ + g ]).
The upper level of this emission is very high (19 eV), can be reached by either likely step ionisation via a relay metastable molecule or by less likely direct electronic collisions, in the very short cathode directed streamer phase (814 ns < t' <923 ns). Subsequently, the FNS is expected to be very weak, and its existence is not astonishing since our discharge is

Paper N 2 at P atm 20/12/10 9:04 Page 16/26 filamentary. If any weak FNS (0-1) transition emitting at 427.8 nm is present it can't be distinguished from the characteristic SPS (1-5) band at 426.97 nm, bearing a characteristic SPS rotational blue wing. The SPS (2,5) band (band-head at 394.3 nm) is also present in our spectrum.

Another very weak molecular emission peaking at 557.7 nm is also present. It may correspond to

VUV emission spectrum

For the VUV emissions whose wavelengths are situated between 130 and 180 nm (fig. 8),

we observed a main narrow emission (Full Wave Half Maximum of about 1 nm) at 150.0 nm together with some other weak emissions between130 and 174 nm. Between 180 and 200 nm, did not record any emission because of the upper cut-off wavelength of our extremely solar blind

Paper N 2 at P atm 20/12/10 9:04 Page 18/26 photomultiplier tube at 180 nm. No significant emission was present between 120 and 130 nm.

These weak emissions are attributed to the Lyman-Birge-Hopfield System (LBHS) which arises from transitions issuing from the N 2 [a 1 Π g ] states towards the N 2 [X 1 Σ + g ] ones. The transitions and wavelengths of the observed emissions are given in table A.1 (appendix), where the reported wavelengths and relative intensities correspond to values given in reference [START_REF] Lofthus | Lotfhus A and Krupenie P H 1977 The spectrum of molecular nitrogen[END_REF]. A close look at these relative intensities suggests that the 0-2, 0-4, 1-0, 1-1, 1-4, 1-5 and 1-6 transitions of the LBH system are likely. If it occurs, the 0-3 transition is unexpectedly weak while the 0-5 one is not present in our spectrum. As specified in table A.1 and as expected due to the relative positions of the potential curves of the involved molecular nitrogen states [START_REF] Lofthus | Lotfhus A and Krupenie P H 1977 The spectrum of molecular nitrogen[END_REF], the 0-2 transition should be the most intense one.

The intense narrow emission peaking at 150 nm is therefore too strong to be attributed to the 0-1 LBHS transition. It is attributed to a nitrogen atomic line issuing from the N[2s 2 p 2 3s 2 P] triplet towards the N[2s 2 2p 3 2 D°] state. The N[2s 2 p 2 3s 2 P] excited atom can also undergo a radiative decay (175 nm) towards the N[2s 2 2p 3 2 P°]. The N[2s 2 p 2 3s 2 P] level has a very short lifetime of 2 ns. This means that if atomic nitrogen is produced, it can emit two emissions at 150 nm and 175 nm, even at 760 Torr. The temporal luminescence decays of these atomic emissions are depicted with linear scales in fig. 9a,

where the integrated values of the photon counts over the total emission width of each radiation are corrected for the quantum efficiency of the photomultiplier tube: 11% at 150 nm and 1.6% at 175 nm. The time lag of longer than 1 µs suggests that these emissions occur after the streamer propagation phase. They may follow dissociative recombination processes, such as for example reaction R12 of table 1. We can consider that N 2 + (X 2 Σ g + ) ground state ions lead to the N(2s 2 2p 2 3s 2 P) excited atomic state, which may emit both the 150 or 175 nm line. According to Rahman R et al [Rahman R et al 2004], N 2 (A 3 Σ u + ) created by electron impact excitation is mainly responsible for populating N(2P) via collisions with N( 4 S) atoms followed by electron impact of the N( 2 P 0 ) atomic by-product. Note that the dissociation of ground state N 2 dimers requires 9.8

eV [START_REF] Luo | Emission spectroscopic studies of plasmas-induced NO decomposition and water splitting[END_REF] which can be provided by the MF-DBD. To our knowledge, VUV emissions of nitrogen MF-DBDs have never been reported before.

Masoud N et al [START_REF] Masoud | VUV emission from a cylindrical dielectric barrier discharge in Ar and in Ar/N2 and Ar/air mixtures[END_REF] observed these emissions in a 300 Torr nitrogen radiofrequency (13.56 MHz) cylindrical dielectric barrier discharge and also attributed them to nitrogen atomic lines. In their spectrum where emissions from impurities were present, the peak intensity of the 175 nm line is about half the 150 nm one.

Rahman R et al [Rahman R et al 2004] also attribute these emissions recorded in a radiofrequency (rf) driven Ar/N 2 hollow slot micro-plasma, operating in open air, to nitrogen atomic lines. In their experiments the recorded emissions could arise either from the active rf discharge or from the downstream afterglow region. Ambient air species such as O 2 , H 2 O could also intervene. Consequently they also observed some other nitrogen atomic lines (120 nm, 124.3 nm), hydrogen (121.60) and oxygen (130.5 nm) ones, as well as the OH and NH bands at respectively 308 nm and 315.8 nm.

Occurrence of the observed VUV, UV, visible and near IR emissions of our pure nitrogen MF-DBD at 760 Torr may be partly explained by the non-exhaustive reactions listed in table 1.

Direct electronic excitation and ionisation are likely to occur during the electronic avalanche propagation and streamer phases where electrons are quite energetic. Metastable N 2 (A 3 Σ u + : 6.17 eV, τ rad = 13 s) molecules can then be populated through reaction R1, while N 2 + (X 2 Σ g + : 15.5 eV) molecular ions can be created indirectly by reaction R10 (+9.5 eV) or to a lesser extent directly by reaction R8. Note that according to Cesses [Cesses Y 2004], in the steady operation metastable N 2 (A 3 Σ u + ) molecules are accumulated in the neighbourhood of both electrodes. Direct electronic impact can also lead to the population of higher excited molecular states N 2 (B 3 Π g : 7.35 eV, τ rad = 13 ms), N 2 (a 1 Π g : 8.55 eV, τ rad = 110 µs), N 2 (C 3 Π u : 11.3 eV, τ rad = 38 ns) by respectively reactions R2, R3 and R4. These molecules can also be easily populated via electronic excitation processes stepping from the long-life metastable molecules N 2 (A 3 Σ u + ) through reactions R5 (+1

A full kinetic study of the post-excitation phase requires analysis of the pressure dependence of temporal decays of some visible emissions and is beyond the scope of this paper.

Conclusion

We report the electrical investigation of a high-pressure pure nitrogen MFDBD, together with its spectroscopic characteristics from the VUV to infrared emissions. As far as we know,

VUV emissions of nitrogen MF-DBDs have never been reported before.

The observed emission spectrum can be attributed to a single filamentary discharge and is dominated by the second positive system in the UV and blue spectral ranges. The Herman infrared and first positive systems are emitted between 650 and 850 nm. Inevitable weak UV emissions of the NO-γ system are observed as well and "traces" of O( 1 S)N 2 excimer emission at 557 nm. A weak emission of the (0-0) transition of the first negative system peaking at 391.4 nm is present and this is consistent with the visual observation of filamentary behaviour of our discharge. The corresponding ionic molecules are probably excited during the streamer propagation phase. Even at such a high pressure two significant nitrogen atomic lines are observed at 150 and 175 nm, lagging about 1 µs after discharge ignition, suggesting that the relevant exited nitrogen atoms likely result from recombination of ionic nitrogen molecules. Our emission spectra bear some discrepancies with the few results available for VUV and IR emissions. We believe that this is due to many combined reasons: the operating pressure, our operating discharge mode and possible impurities. The electrical analysis, performed with a macroscopic model, allows determination of the ignition and extinction voltages as well as the injected energy and electric charge. An empirical technique, based on the use of available digital oscilloscope, is proposed to rapidly evaluate all these quantities, without performing bulky calculations. The experimental results presented here provide valuable electrical and

Fig. 2a showsFig. 2 :

 2 Fig.2ashows the complete electrical driving circuit with the relevant electrical ratings. The

  behaviour of the nitrogen MF-DBDIn order to analyze the electrical behaviour of the MF-DBD, representative waveforms of u sup and of i sup were recorded and processed with the macroscopic model previously described so as to determine the discharge voltage u dis and current i dis . The supply voltage u sup and current i sup are given in fig.3afor one signal period T = 1/f. As explained, the sinusoidal cell voltage u cell was almost equal to u sup , and i cell could hardly be distinguished from i sup at f = 7.8 kHz.The recorded quantities (amplitudes, frequencies, rise-and fall-times) of our electric signals, denoted by x, correspond to mean values of the N = 101 samples. The expanded uncertainty Δx of each quantity was calculated, according to the general rules established by ISO in Guide to the Expression of Uncertainty in Measurement [ISO/IEC Guide 98: 1995], by multiplying the standard deviation σ X of each quantity x, by a coverage factor of two (Δx = 2 σ x ) which defines a level of confidence of approximately 95%. The standard deviation was obtained by the appropriate functionality of the digital oscilloscope.

Fig. 3a :

 3a Fig. 3a: The cell voltage and supply current at f = 7.8 kHz (one period)

Fig

  Fig. 4a: u cell , u dis , u die and i cell at f = 7.8 kHz (over a half period)

  7.500 ± 0.024) kV . u dis then increases to its maximum value, according to capacitor divider equation (9), until the axial

Fig. 5 :

 5 Fig. 5: Emission spectrum (Δλ exit = 0.016 nm): (a) 200 -400 nm (b) 200 -300 nm

  Fig. 7 shows the emission spectrum of the MF-DBD between 600 and 900 nm. The Herman IR System (HIRS: N 2 [C" 5 Π u ]→N 2 [A' 5 Σ g + ]) dominates the spectrum with emissions at 703.3 (3-

Fig. 7 :

 7 Fig. 7: Emission spectrum in the 600 to 900 nm range (Δλ exit = dλ step = 0.073 nm)

Fig. 8 :

 8 Fig. 8: Emission spectrum in the VUV range: 130 to 200 nm (Δλ exit = dλ step = 0.5 nm)

  Luminescence decays of the 150 and 175 nm emissions (5 ns resolution) with (a) linear (b) semi-log scales. The temporal luminescence decays of the nitrogen atomic lines are plotted in a semi-log scale in fig. 9b. Both emissions have similar time constants but the sharp maxima clearly suggest that the evolution laws of the photon counts are non-exponential This indeed provides evidence in favour of electronic dissociative recombination processes.

  The different characteristic times of the discharge current i dis are calculated for the electrical sample given here. The full wave half maximum (FWHM) of i dis and the duration

	of C gap and C die , the discharge's maximum current ((246.8 ± 3.0) mA) is 25% higher than the
	amplitude of i cell : (198.2 ± 2.4) mA.
	during which i dis remains above 10% of its maximum value are respectively t_half_cal = (21.6 ±
	2.1) ns and t_full_cal = (169.2 ± 6.2) ns. The rise-and fall-times of i dis are respectively (35.2 ±
	3.4) ns and (125.5 ± 4.6) ns.
	Paper N 2 at P atm	20/12/10 9:04	Page 13/26

Acknowledgements

This work was partly supported by the French National Research Agency (ANR) via the ANR-IPER project, Project manager: C. Laux. The authors would like to thank Dr. M. Yousfi, Dr. G. Ledru, Dr. O. Eichwald and Dr. A. Ricard for their useful help in this study.

eV), R6 (+2 eV) and R7 (+5 eV). During these phases, the highly excited N 2 (C 3 Π u ) molecule predominantly emits the SPS leading to N 2 (B 3 Π g ) which in turn emits the FPS. Furthermore, step-wise ionisation towards the N 2 + (X 2 Σ g ) can also take place from N 2 (A 3 Σ u + : 9.5 eV (R10)).

Further excitation leads to N 2 + (B 2 Σ u + ) which are responsible for the FNS.

Direct electronic excitation from the ground state N 2 molecule :

Step-wise electronic excitation from the metastable molecule :

Direct electronic ionisation from the ground state N 2 molecule :

Step-wise electronic ionisation from the metastable N 2 molecule :

Dissociative electronic recombination of the ground state N 2 molecular ion :

Radiative decay of nitrogen molecules and atoms: