MD Hélène Hermand

MD, PhD Jérémie H Lefèvre
email: jeremie.lefevre@aphp.fr

MD Conor Shields

MD Najim Chafai

MD Clotilde Debove

MD Laurent Beaugerie

MD, PhD Magali Svrcek

MD, PhD Yann Parc

Antoine Ibd

Prof Jérémie Lefevre

Post-operative diagnostic revision for Crohn Disease after subtotal colectomy for inflammatory bowel disease

Keywords: colitis, subtotal colectomy, Crohn's colitis, ulcerative colitis

Abstract:

Purpose: Subtotal colectomy (STC) is performed for severe acute and refractory colitis. The diagnosis can be difficult even after the surgery when colectomy specimen has overlapping features of Ulcerative Colitis (UC) and Crohn's disease (CD). The aim of this study was to evaluate the rate of post-operative diagnostic revision to CD after surgery and determine predictors factors.

Methods: Retrospective study of 110 patients who underwent STC (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018).

Results: pre-operative diagnosis comprised: UC=80(73%), CD=11(10%) and Unclassified Colitis (IBDU=19, 17%). Initial diagnosis of IBDU and UC was modified to CD in 6 patients (6 %) after STC. The final diagnosis after the follow-up of 10±6 years switched from CD for 8 patients (9%). The multivariate analysis showed that patients with a colitis evolving for less than 10 years and initial diagnosis of IBDU were the two independent factors associated with an increased risk of diagnosis change to CD (p=0.03; p=0.016). At the end of the follow up 15 patients (14%) had a definitive stoma.

Conclusions: In patients with IBD, attention must be paid to determine the right restorative strategy to patients with an evolution of the disease less than 10 years or with IBDU who are more at risk to have a diagnosis change to CD after STC.

INTRODUCTION:

The spectrum of inflammatory bowel disease (IBD) encompasses both Crohn's disease (CD) and ulcerative colitis (UC). While UC affects solely the rectum and the colon, CD may be found throughout the gastrointestinal tract, including the rectum and colon in the case of CD. [START_REF] Kalla | Crohn's disease[END_REF][START_REF] Ford | Ulcerative colitis[END_REF][START_REF] Price | Overlap in the spectrum of non-specific inflammatory bowel disease--'colitis indeterminate[END_REF][START_REF] Lee | Indeterminate colitis in the spectrum of inflammatory bowel disease[END_REF][START_REF] Wells | Natural history of indeterminate colitis[END_REF] In some cases, patients with chronic colitis clearly have IBD based on the clinical history but macroscopy and/or endoscopic biopsies show no definitive features of UC or CD; the term unclassified colitis (IBDU) is thus used for these patients. [START_REF] Magro | European consensus on the histopathology of inflammatory bowel disease[END_REF] Post-operative examinations of resections of such cases usually provide definitive evidence of UC or CD. The diagnosis can be difficult even after the surgery when colectomy specimen has overlapping features of UC and CD and, in those cases, pathologists use the term of Indeterminate Colitis (IC).

Accurately distinguishing between UC and CD is crucial as the surgical treatment for each differs considerably. While restorative proctocolectomy (RPC) is the usual approach in cases of UC [START_REF] Pemberton | Ileal pouch-anal anastomosis for chronic ulcerative colitis. Long-term results[END_REF][START_REF] Brunel | Restorative proctocolectomy for distal ulcerative colitis[END_REF][START_REF] Goudet | Changing referral patterns for surgical treatment of ulcerative colitis[END_REF], many surgeons are reticent to offer it to patients with CD. [START_REF] Turina | The J-pouch for patients with Crohn's disease and indeterminate colitis: (when) is it an option?[END_REF][START_REF] Panis | Ileal pouch/anal anastomosis for Crohn's disease[END_REF] Indeed, pouch failure is frequently accompanied by an alteration of the original diagnosis from UC to CD. [START_REF] Hartley | Analysis of the outcome of ileal pouch-anal anastomosis in patients with Crohn's disease[END_REF][START_REF] Sagar | Long-term results of ileal pouch-anal anastomosis in patients with Crohn's disease[END_REF] Therefore, strategies to minimise the need for post-operative revision of the diagnosis are critical.

There is a variety of indications for surgery in patients with colitis. The detection of dysplasia while the patient is under surveillance is observed most frequently in the setting of multiple biopsies, and prompts RPC. [START_REF] Peyrin-Biroulet | Endoscopy in inflammatory bowel disease: recommendations from the IBD Committee of the French Society of Digestive Endoscopy (SFED)[END_REF] However, surgery is also indicated in at least two other situations in which a long history and multiple biopsies are not always available: severe acute colitis and refractory colitis. These two situations now account for the majority of the indications for surgery. [START_REF] Katz | Medical and surgical management of severe colitis[END_REF][START_REF] Truelove | Medical management of ulcerative colitis and indications for colectomy[END_REF] It is more likely that it in such cases diagnosis uncertainty is more frequent, resulting in increased rates of pouch failure after RPC.

The aim of this study was to evaluate the rate of diagnosis change and to identify predictors factors of diagnosis revision to CD for patients who underwent STC for severe acute colitis or refractory colitis.

METHODS AND PATIENTS:

This retrospective observational study reports a monocentric experience in the management of IBD requiring surgical treatment in an expert center for IBD, Saint Antoine Hospital in Paris.

All patients who underwent STC at St Antoine's hospital between 2005 and April 2018 were included.

The diagnosis of colitis was based on a combination of clinical features, imaging modalities endoscopic findings, and histological interpretation of biopsies by pathologists expert in gastro intestinal pathology including IBD.

Severity of the colitis was evaluated with the Lichtiger's score. [START_REF] Lichtiger | Preliminary report: cyclosporin in treatment of severe active ulcerative colitis[END_REF] Complicating factors including colectasia or colonic perforation were derived from CT scans and endoscopy was performed when clinically indicated. There is two frequent indications for STC in patients with IBD: first patients with severe acute colitis defined according to Truelove and Witts' criteria or a Lichtiger's score higher than 10 who were admitted acutely and who failed to respond to maximum medical treatment or who had developed complications as a result of their pathology (mostly sepsis, colectasia or haemorrhage), and required emergency surgical intervention. The second indication is for patients with refractory colitis defined as steroid-dependent colitis (inability to reduce steroids below the equivalent of prednisolone 10mg/day within 3 months or relapse within 3 months of stopping steroids) or steroid-refractory colitis (active IBD despite prednisolone up to 0.75 mg/kg/day for 4 weeks) or immunomodulatory-refractory colitis (active IBD despite thiopurines or mercaptopurine for 3 months).

STC was performed by laparoscopy or laparotomy. [START_REF] Hor | Female sexual function and fertility after ileal pouch-anal anastomosis[END_REF] Ileostomy was always created in the right iliac fossa. Sigmoidostomy was created either suprapubic in the midline incision or with the ileostomy depending on surgeon's choice.

After STC patients were reclassified as CD, UC, or IC either after pathological analysis, or either during the follow up according to the guidelines proposed by the European Society of Pathology and the European Crohn's and Colitis Organization. [START_REF] Magro | European consensus on the histopathology of inflammatory bowel disease[END_REF][START_REF] Langner | The histopathological approach to inflammatory bowel disease: a practice guide[END_REF] Pathological diagnosis of IC on resected specimens relied on the presence of clear evidence of IBD, with "overlapping features" or the absence of a "clear diagnostic pattern", with insufficient criteria to make a definite diagnosis of UC or CD. [START_REF] Magro | European consensus on the histopathology of inflammatory bowel disease[END_REF] Before STC some patients were considered to have an unclassified colitis (IBDU) as the diagnosis was only made on disease history and endoscopic findings. For these patients, no histology was available before STC. A decision to attempt restore intestinal continuity was made only after definitive pathology results were available.

Statistical analysis

Descriptive analyses are presented as median (range) or mean ± standard deviation for quantitative data and as number of patients (percentage of patients) for categorical data. The correlation between diagnostic modification and initial diagnosis was studied by univariate analysis (chi-squared). All tests were two-sided, and a p value <0.05 was considered to be significant. The multivariate analysis was performed using a backward stepwise logistic regression model that included all variables with a p value <0.1 in univariate analysis.

RESULTS:

Patients:

One hundred and ten patients (mean age of 69±23 years) underwent STC for colitis. The mean time between the diagnosis of IBD and surgery was 10±6 years. Details of the patients can be found in Table 1. STC was performed for severe acute colitis in 58 patients (53%), of whom 23 (40%) exhibited an acute complication related to their disease and for refractory colitis in the remaining 52 patients (47%). 61 (55%) patients had received corticosteroid treatment within 6 months of surgery, 69 (63%) had immunosuppressive therapy within 6 months, while 67 (61%) had been treated with monoclonal TNF inhibitors.

Diagnosis revision (Figure 1):

The pre-operative diagnosis comprised UC (n=80, 73%), CD (n=11, 10%), and IBDU (n=19, 17%). The pathological analysis of the piece of STC allowed a rectification of the initial diagnosis for several patients. The previously IBDU cases (n=19, 17%) were reclassified mainly as UC (n=14, 74%), as CD (n=4, 21%) and IC (n=1, 5%). Among the patients initially diagnosed as UC (n=80, 73 %), 6 patients (8 %) had a change of diagnosis: 4 (5%) became IC and 2 (3%) CD. There was no change of diagnosis for patients initially diagnosed with CD.

Diagnosis modifications continued during the follow-up period of 10 years (±5 years), during which patients had their surgery to restore intestinal continuity.

Among the 80 patients initially diagnosed with UC, 69 (86%) didn't have a change of diagnosis at the end of the follow-up. Among the 19 patients initially diagnosed with IBDU only 1 patient stayed with an IC.

During the follow-up, 8 patients (9%) among those with UC and IC were deemed to have CD.

At the end, compared to the initial diagnosis, 14 patients (14%) classified UC, IBDU or IC were found to be CD.

Impact of diagnostic revision in the strategy of restoration of intestinal continuity:

The restorative continuity strategy was mainly guided by the pathological analysis of the STC specimen but also by risks factors of poor functional result as age, anal incontinence and severe perineal lesions. (Figure 2)

Excluding patients who were lost to follow up (n=2) or who died during the study (n=2), at the end of the follow up of 10 years (±5 years), 15 patients (14%) had a definitive stoma.

Among the 11 patients with an initial diagnosis of CD, 3 (27%) had a definitive stoma including 2 who never had an attempt to restore continuity according to their will.

Among the 14 patients with a diagnosis change to CD, 2 (14%) had a definitive stoma at the end of the follow up and 3 patients had a temporary ileostomy more than one year for the management of their CD. There was no difference of rate of definitive stoma between those two groups, p=0.623.

Among the 72 patients with a UC from the beginning, 7 patients (10%) had a definitive stoma.

Retrospectively, all histological samples of patients deemed to have CD were reanalysed to determine if features of CD existed. Details of those patients are given in supplementary table 1.

DISCUSSION:

Diagnostic revision after surgical treatment of UC is a quagmire for physicians. Many patients seek a RPC with the hope that they may be rendered stoma free. However, a resurgence of symptoms may prompt a reclassification of the diagnosis. The ileal pouch and the perineum are the most frequent sites of disease, which can lead to the construction of a diverting stoma which may become a permanent fixture. If the diagnosis had been accurate initially, the patient may have been spared restorative surgery, with the associated risks and dashed hopes. Even if one argues that an ileal pouch in the setting of CD may be appropriate in some cases, prior knowledge of the presence of CD would alter post-operative management and would colour both patient and clinician expectations.

Indications for surgery in UC can be divided into four categories: functional, prophylactic for cancer, resistance to treatment, and acute complications. Surgery for functional reasons and cancer prevention is most commonly seen in the context of a long disease history. In these cases, uncertainty over the diagnosis is unlikely. On the contrary, in the case of treatment resistance and especially in case of acute colitis and acute severe complications, the disease history may be short, with a corresponding increased risk of diagnostic revision after surgery.

In the present series, the delay between initial diagnosis and STC was only 6.5 years, which is quite short for a chronic disease. The aims of this study were then to evaluate the diagnostic revision rate to CD after pathologic examination of the specimen obtained after STC and the one observed after surgery to restore intestinal continuity and during the follow up with endoscopic biopsies.

In our series, before surgery 19 patients were considered to have IBDU. This must be regarded as a different entity to IC. An unclassified designation may result from a lack of material for pathological examination. In the absence of specific features of CD, a definitive diagnosis may prove elusive. If these patients had been deemed to have IC, and underwent subsequent RPC, 4 patients with CD, and one with an IC would have undergone pouch surgery. It can be argued that in the present series, 2 patients (14%) among the 14 who were considered to have UC after STC, while previously unclassified, ultimately were found to have CD. Same results are found in the literature as Zaghiyanand al. [START_REF] Zaghiyan | De novo Crohn's Disease after Ileal Pouch-Anal Anastomosis for Ulcerative Colitis and Inflammatory Bowel Disease Unclassified: Long-Term Follow-Up of a Prospective Inflammatory Bowel Disease Registry[END_REF] However, when comparing diagnostic revision rates between patients who had UC either after being consider to have one or an unclassified one, there was no difference. Moreover, the long-term diagnostic modification rate in these two situations is within the ranges reported in the literature. In fact, the risk of diagnostic modification was significantly higher among patients who had their initial diagnosis modified and especially among those who had their diagnosis modified from UC to IC. In this specific group of patients, the rate of secondary diagnostic modification reached 25%. Such a rate is higher than the one observed in cases of IC in the literature. [START_REF] Stewenius | Operations in unselected patients with ulcerative colitis and indeterminate colitis. A long-term follow-up study[END_REF][START_REF] Moum | Inflammatory bowel disease: re-evaluation of the diagnosis in a prospective population based study in south eastern Norway[END_REF][START_REF] Mcintyre | Indeterminate colitis. Long-term outcome in patients after ileal pouch-anal anastomosis[END_REF][START_REF] Kangas | Is "indeterminate colitis" Crohn's disease in the long-term follow-up?[END_REF][START_REF] Pezim | Outcome of "indeterminant" colitis following ileal pouch-anal anastomosis[END_REF][START_REF] Koltun | Indeterminate colitis predisposes to perineal complications after ileal pouch-anal anastomosis[END_REF] With this finding in mind, should these patients have had an operation other than restorative proctocolectomy? Probably not, as ileorectal anastomosis in CD is not always accompanied by a good result, and RPC is offered to only highly selected patients. Thus, those patients with a modified diagnosis of IC, after previously being classed as UC, are more likely to fulfil the criteria of a CD patient who would be considered for RPC. [START_REF] Panis | Ileal pouch/anal anastomosis for Crohn's disease[END_REF][START_REF] Hartley | Analysis of the outcome of ileal pouch-anal anastomosis in patients with Crohn's disease[END_REF] In the present series, one patient with CD underwent RPC without disease recurrence at the last follow up.

The term IC is used to describe colitis without a clear argument for UC or CD after colectomy, in our series the frequency was 5% just after STC and 4% after follow-up. These rates are rather low compared to those found in the literature between 3% and 10%. [START_REF] Matsui | Clinical features and pattern of indeterminate colitis: Crohn's disease with ulcerative colitis-like clinical presentation[END_REF] In our series, patients reclassified during the follow-up were patients with CD. In our study 14 patients (13%) classified UC, IBDU or IC were found to be CD after pathological examination of the piece of resections or during the follow up with the occurrence of perineal lesions or ileo-vaginal fistula.

Seven of them (50%) had a restorative protectomy and 4 (43%) developed severe complications of the ileal pouch witch required a diverting stoma for several years. These complications highlight the importance of a good classification of the IBD in the strategy of restorative continuity as shown in the study of Netz et al, the long-term quality of life for these patients is lower than in case of IRA. Taking into account the clinical impact, we analyzed individually the characteristics of the patients whose diagnosis was finally changed for CD: few studies have analyzed precisely the histological data of these patients, the Japanese study of Matsui et al. analyzed these features in their series of 735 patients with IBD with 23 patients whose diagnosis was modified (3%): patients initially UC whose diagnosis was rectified during follow-up for CD (n=8, 10%) have been done only after the detection of granulomas on biopsy material, the initial diagnosis of UC was based mainly on the diffuse appearance of colitis and the lack of cobblestones. In our series we have rectified the diagnosis of CD rather on a panel of histological arguments rather than on the exclusive presence of granulomas according to a methodology closer to the study of Henriksen and al. whose results are comparable to ours. [START_REF] Henriksen | Change of diagnosis during the first five years after onset of inflammatory bowel disease: results of a prospective follow-up study (the IBSEN Study)[END_REF] The univariate analysis realized, confirmed by an multivariate analysis to determine predictive factors of modification of the diagnosis found that disease evolution duration of less than 10 years and the diagnosis of IBDU were associated with an increased risk of modification of the diagnosis toward CD. This result enlightning the fact that for patients with a recent onset of the colitis, it might be necessary to take time to choose the right reconstruction strategy.

The confirmation of the original diagnosis by pathologic examination of the specimen obtained at STC allows the choice of subsequent appropriate surgeries, while particular heed needs to be paid to patients who have had a diagnosis of UC modified to IC after STC.

Statements

  We performed an univariate analysis between the group of patients diagnosed with UC, IBDU or IC with a change of diagnosis to CD during the study and the patient who stayed with the initial diagnosis to determine predictors factors to be reclassified as CD. (Table2) Patients with a colitis evolving for more than 10 years had less risk to have a change of diagnosis to CD, p=0.035. Patients with an initial diagnosis of IBDU were more likely to have a change of diagnosis to CD, OR= 4.08 IC 95% [0.9937; 16.1628] p=0.026. In multivariate analysis those two criterias were also associated with an increased risk of diagnosis change to CD with respectively p=0.03 and p=0.016 with an OR=5.81[1.62-18.54] for IBDU patients.

  List of collaborators of theSaint Antoine IBD Network: Lionel Arrivé, Laurent Beaugerie, Anne Bourrier, Marine Camus, Najim Chafai, Edouard Chambenois, Ulriikka Chaput, Clotilde Debove, Charlotte Delattre, Xavier Dray, Jean-François Fléjou, Guillaume Le Gall, Nadia Hoyeau, Julien Kirchgesner, Cécilia Landman, Jérémie H. Lefèvre, Philippe Marteau, Chloé Martineau, Laurence Monnier-Cholley, Isabelle Nion-Larmurier, Violaine Ozenne, Yann Parc, Philippe Seksik, Harry Sokol, Magali Svrcek, Emmanuel Tiret.

Figure legends Figure 1 .

 legends1 Figure legendsFigure 1. Evolution of the diagnosis. STC: Subtotal Colectomy

Figure 2 .

 2 Figure 2. Digestive continuity strategy after STC. STC: subtotal colectomy; IBD inflammatory Bowel Disease; IBDU inflammatory Bowel Disease Unclassified; CD: Crohn's disease; UC: ulcerative colitis; RPC: Restorative Proctocolectomy; IRA: ileorectal anastomosis; DS: Definitive Stoma; TS: Temporary Stoma; LTFU: Lost To Follow Up; APR: Abdomino-Perineal Resection

Table 1 .

 1 Patients' characteristics before STC and indication for surgery

	Characteristics

Availability of data and material: on request to the corresponding author

Acknowledgement: None

Statement of ethics: No ethical approval was needed as it was a retrospective study. All patients had given a written informed consent.

Declarations

Click here to access/download Supplementary Material Supp Table 1.docx


Philippe Roncin
email: philippe.roncin@u-psud.fr

Revisiting Atomic Collisions Physics with highly charged ions, A tribute to Michel Barat

Michel Barat passed away in November 2018 at the age of 80 after a rich career in atomic and molecular collisions. He had participated actively in formalizing to the electron promotion model, contributed to low energy reactive collisions at the frontier of chemistry. He investigated electron capture mechanisms by highly charged ions, switched to collision induced cluster dissociation and finally to UV laser excitation induced fragmentation mechanisms of biological molecules. During this highly active time he created a lab, organized ICPEAC and participated actively in the administration of research. This paper covers the ten years where he mentored my scientific activity in the blossoming field of electron capture by highly charge ions (HCI). In spite of an impressive number of open channels, Michel found a way to capture the important parameters and to simplify the description of several electron capture processes; orientation propensity, electron promotion, true double electron capture, Transfer ionisation, Transfer excitation, formation of Rydberg states, and electron capture by metastable states. Each time Michel established fruitful collaborations with other groups.

Introduction

In the 1960's he contributed to the birth of atomic collisions, he addressed an impressive number of different situations spanning from the construction of the electron promotion model in ion-atom collisions to low energy reactive collisions, collisions of atoms with ionic clusters, and fragmentation mechanisms of biological molecules induced by UV laser excitation.

Michel Barat always considered that if a problem is well-understood, theoretically or experimentally, then a simple minimalist model with a semiquantitative predictive power can be derived. In other words an extensive calculation or a match between results and qualitative prediction was not an explanation. To reach this goal, he was very good at making collaborations and he created the laboratoire des collisions atomiques et moléculaires (LCAM) as a place where both experimentalists and theoreticians could interact as often as possible. He was always completely open to collaborations around the world. I will try to describe this aspect by few examples, with apologies to the numerous other contributions that were made worldwide and that will not be quoted as they deserve, because this paper is more centered around the contributions of Michel Barat. The paper is organised as follow, the first section tries to recall briefly the context. The second one addresses specifically the technological challenges that we faced to investigate collisions with highly charged ions that can be skipped by readers only interested in scientific results. The following sections do not follow a chronological order but try to emphasize a specific problem to which Michel brought a significant contribution starting from single electron capture and followed by multiple electron capture. The last section describes succinctly a few examples of experimental realizations that can be traced to the legacy of Michel.

Collisions with highly charged ions, a brief context

One of the great world-wide technological and scientific challenges was, and still is, the construction of civil nuclear fusion devices. It was soon realized that impurities were present in the plasma as highly charged ions (HCI) and that many X-rays were emitted following electron capture by these HCI. This was identified as one of the factors limiting the temperature increase of the Tokamak plasma above a million degrees. In situ optical diagnoses of the plasma was an important activity involving academic groups, (e.g. F.J. De Heer) whereas beam foil spectroscopy where MeV ions undergo numerous electron impacts when passing though thin carbon foils was used to identify spectral lines. In both cases, quite a number of lines were present simultaneously and almost no experimental measurements of well defined collision processes were available to help modeling. Fortunately, the first ion sources able to produce highly charged ions were becoming available. The most impressive, technologically, were the electron beam ion source (EBIS) invented and developed by E.D. Donets from Dubna in Ukraine. Michel had started a collaboration with H. Laurent from the Orsay Institute of Nuclear Physics where J. Arianer was developing SILFEC, one of the first EBIS in the western world. In Grenoble, R. Geller from the commissariat à l'énergie atomique (C.E.A.), was investigating hot plasmas in a hexapolar magnetic structure heated by microwaves and demonstrated the presence of a broad range of highly charged ions. This was the beginning of a new generation of ion source known as electron cyclotron resonance ion source (ECR). S. Bliman, from Grenoble, and Michel managed to convince the C.E.A. to build a dedicated beam line and to open access to the academic community. Pioneering work had already started with ideas around the coulombic barrier model (see in fig. 2) originally proposed by N. Bohr and J. Lindhard (1954). Adapted to highly charged ions this model outlined the key role played by the very large electric field attached to the projectile and pointed to the region of interest for electron capture . The model clearly indicated that the situation was probably very different from that of singly charged ions where Michel had contributed to the development of the electron promotion model [START_REF] Barat | Extension of the electronpromotion model to asymmetric atomic collisions[END_REF]. Many groups around the world started to imagine experiments to probe these new exotic highly charged ions. The first international meeting of this growing community around highly charged ions took place in 1982 in Stockholm.

Technological challenges

Michel has always been interested in technological developments that could help his experiments and was not afraid of a challenge. I started my PHD in 1983, by assembling the electrostatic spectrometer (Fig. 3) for translational spectroscopy that Michel had designed. It was a simple but large 45 • parallel plate electrostatic analyser designed to host a 2D position sensitive detector. The only snag was that these detectors were not available at that time! Microchannel plates developed by the military industry for night vision application were indeed becoming available but quite expensive. Copying a design from D. de Bruyn of the F.O.M. Institute in Amsterdam [START_REF] Knibbeler | Novel two dimensional position sensitive detection system[END_REF], I started by constructing a 1D version based on the capacitive charge division technique : A set of 50 discrete copper anodes printed on fiberglass substrate and interconnected by capacitors. If the electron cloud of charge Q≈ 10 7 e -produced by the two MCP is narrow and falls entirely on the i th anode, then the charge Q will split according to the number of capacitors to be passed before reaching the leftmost or rightmost anode allowing identification of the anode i; i = 50×Q lef t (Q lef t +Q right ) . On the contrary, if the electron cloud is broad enough, the system rapidly points to the charge barycenter and provides a continuous localisation x = Q lef t (Q lef t +Q right ) . H. Laurent provided state of the art analog electronic modules; preamplifiers and linear pulse shaping amplifiers as well as specific modules to perform the analog addition and analog division required to obtain the position x of the impact. With help of L. Lilieby during a short visit from Stockholm, we managed to measure our first energy gain spectra with ions extracted from SILFEC. Michel immediately pushed me to develop a 2D version following a suggestion by D. De Bruyn (F.O.M. Amsterdam). It was made of two 1D arrays as above but with a triangular shape inter-penetrated anodes similar to a game of backgammon. We then faced the difficulty of performing so many analog additions and divisions; each step being sensitive to noise and to thermal drift. We also tried to perform division inside our brand new BFM186, the first 8086 personal computer at the lab, but even with integer divisions programmed in assembly language the acquisition rate was below a kHz. J.C. Brenot, from LCAM and expert in micro electronics convinced me that making a machine that would control all ADC's and perform the division would be simple. His argument being that a 12 bits division is nothing else that twelve subtractions, "if you hard wire the carry bit to a shift register and reload the result, it can be done in 12 clock cycles". I was naive, supported by Michel I started assembling (wrapping) a dedicated micro computer around a 12 MHz clock, an AMD2910 sequencer driving 5 chips of static RAM memory chips configured in 128 lines of 40 bits. Each line had 10 bits dedicated to the sequencer 3 bits for instructions plus 7 bits to drive the above 128 lines of memory address, few bits for an arithmetic and logic unit and few bits to manage the combined operation of the four ADC's. I had a hard time getting it to work properly but I learned to check the logic, the propagation delays, to track glitches and read documentation over and over... I learned micro electronic from the oscilloscope.

It is very difficult to imagine now, the acquisition program inside our PC was written in assembly language without connection to the newly created MS-DOS operating system. Our "micro-divider" was connected directly to the interrupt priority controller forcing the processor to execute the acquisition program at each event. No need to say that every tiny programming mistake usually resulted in the PC crashing.. But it worked and the acquisition was able to sustain 10 kHz rate including the generation of 2D plots written directly in the graphic memory mapping the green screen.

At this moment, other groups already succeeded to perform energy loss measurements : P. Hvelplund in Denmark, H. Tawara in Japan, B. Huber in Germany, L. Coke in the USA and R. Mc Cullough in U.K. We did not have the best resolution but the simple fact that we did not have to scan the energy or the scattering angle made our setup extremely efficient, a decisive advantage for the limited time available at external facilities. The large efficiency is also particularly useful for experiments where the recoil ion or the emitted electrons and photons have to be detected in coincidence because the overall efficiency is the product of both the individual detection and collection efficiency. Michel had chosen this technique because it is sensitive to the primary processes taking place at short inter-nuclear distance and is much less affected by the very complex decay processes of the highly excited states formed after electron capture.

In what sense is the captured electron rotating ?

In Denmark, N.O. Andersen had started to derive propensity rules for alignment and orientation in atomic collisions [START_REF] Andersen | Propensity rules for orientation by atom impact: I. three-state description of directs S→P transitions[END_REF] and D. Dowek, a former PHD student of Michel had also constructed a 2D detector to study these effects with laser excited targets [START_REF] Dowek | Strong effects of initial orbital alignment observed for electron capture in kev h + -na(3p) collisions[END_REF]. After discussions, Niels and Michel selected the B 3+ (1s 2 ) + He system to investigate the orientation rule ; would the B 2+ (1s 2 2p) electron rotate clockwise or anticlockwise? The scattering profile would give access to the impact parameter, the energy gain would identify the B 2+ (1s 2 2p) capture level and the circular polarisation of the 207 nm B 2+ (2p → 2s) decay photon would indicate the rotation sense of the captured electron. We designed and constructed a large aperture optical system mounted in our setup (fig. 3) to allow a triple coincidence measurement between the photon, the recoiled He + and the scattered B 2+ ions. Together with C. Adjouri (PHD), we were lucky to succeed on the first run in Grenoble [START_REF] Roncin | Observation of orientation propensity for electron capture in multiply-charged-ionatom collisions[END_REF]. The measured orientation displayed in fig. 4 shows that the electron is turning in the "intuitive" sense. The experiment indicated that it is easier to jump on a rotating merry go round if you run along side such that the relative velocity is reduced. This was was a bit puzzling because this should not hold in the molecular regime where the electron motion is much faster (here up to 20 times) than that of the nuclei. Our observations were first reproduced by J. P. Hansen, A. Dubois, and S. E. Nielsen [START_REF] Hansen | Partial cross sections and correlation effects in b 3+ -he collisions[END_REF] in a state of the art close coupling calculation. Then, V.N. Ostrovsky from Saint Petersbourg that Michel Lines are for an extensive theory [START_REF] Roncin | Differential cross sections and orientation propensities for electron capture in 0.75-12 keV b 3+ -he collisions: experiment and atomic basis theory[END_REF] and a simple collision model [START_REF] Adjouri | Quasimolecular analysis of orientation and alignment in b3 + -rare gas electron capture processes[END_REF][START_REF] Ostrovsky | Landau-zener model with inclusion of electron momentum transfer[END_REF].

had invited during the difficult period of the collapse of soviet union, derived an analytical model based on a Landau-Zener description adapted to circular states [START_REF] Ostrovsky | Landau-zener model with inclusion of electron momentum transfer[END_REF]. The explanation he had found was simple enough to be part of textbooks. In the genuine two-states Landau-Zener model electronic transitions take place at localised crossing between the energy curves with a probability p. In a collision, the crossing has to be passed twice, on the way in (while approaching the target) and on the way out giving two different possibilities ; capture on the way in and not on the way out with a probability p × (1 -p) or the reverse with an equal probabilities (1 -p) × p. A curve crossing indicates that both states have a different energy dependence so that the semi classical phase developed along these paths is different and evolves with the impact parameter producing the well-known Stuekelberg oscillations in the differential cross section of a simple system e.g. He + +He→ He +He + .

Here the situation is different, there are three B 2+ (1s 2 2p) states : 2p x , 2p y and 2p z . By symmetry only the linear combination 2p σ aligned on the inter nuclear (molecular) axis is available for electron capture. If it is strongly coupled to the molecular axis, then after rotation, the same 2p σ state will be available for capture (or not) on the way out and nothing is changed, Stuekelberg oscillations will be observed.

In contrast, in a diabatic situation where the 2p σ capture state is hardly forced to follow the rotation of the molecular axis, then the 2p σ state available on the way in, tends to stay fixed in space and, on the way out, it is likely to look like a 2p π state in the molecular frame, whereas symmetry still imposes capture into the 2p σ . The "Stueckelberg" phase difference now applies to different states as depicted in fig. 5 and produces an oscillation between linear and circularly polarized states.

Figure 5. schematic view of the B 3+ (1s 2 ) + He collision, the 2pσ state populated at the first crossing A tends to remain fixed in space so that it hardly interacts at the second crossing B. In this example, a phase difference of π/2 gives a circular state [START_REF] Ostrovsky | Landau-zener model with inclusion of electron momentum transfer[END_REF].

With this model in mind and after illuminating discussions with V. Sidis we could calculate electron capture coupling elements and build a three state model reproducing the observed polarization [START_REF] Adjouri | Quasimolecular analysis of orientation and alignment in b3 + -rare gas electron capture processes[END_REF]. The intuitive sense of rotation was indeed purely accidental. In a way, this corresponds to a situation where rotational coupling was quite strong whereas the quasi-molecular description often assumes an absence of rotational coupling.

Note that, in the mean time, alignment [START_REF] Dowek | Strong effects of initial orbital alignment observed for electron capture in kev h + -na(3p) collisions[END_REF] and orientation measurements with laser excited alkali targets had developed to establish propensity rules from the reversed process [START_REF] Salgado | State propensities in electron transfer processes from optically prepared states[END_REF].

Why do we observe higher n value?

The selective electron capture into a given n shell was well explained by the coulombic barrier model but both electron and photon spectroscopy provided evidence for electron capture into a higher lying n shell. These were particularly important for optical diagnostic of plasma because decay in the V.UV. range is more likely for large n values. Within the CBM, the crossing associated with direct electron capture corresponds to a tunneling situation through a very large barrier separating the projectile and target core (top curve in fig. 2). The cross section should be vanishingly small and the differential cross section peaked at very small scattering angle. With S. Ohtani, on leave from Nagoya, we observed the opposite : large cross sections peaking at large scattering angles [START_REF] Roncin | Electron capture by multiply charged ions on helium target a population mechanism for minor channels[END_REF]. Our data pointed to an indirect sequential process where an electron is first captured into the shell n predicted by the CBM on the way in, and then, at smaller inter-nuclear distance, transferred to the n + 1 shell. For O 8+ + He, this indicated a first capture into O 7+ (n = 4) followed by a coupling to O 7+ (n = 5) at shorter distance as depicted by the circle around R ≈ 3 -4 a.u. in fig. 2. It did not take long for Michel to recognize a situation that had not yet been observed, only predicted by the Fano-Barat-Lichten electron promotion model [START_REF] Fano | Interpretation of ar + -ar collisions at 50 kev[END_REF][START_REF] Barat | Extension of the electronpromotion model to asymmetric atomic collisions[END_REF]. As illustrated in fig. 2, at short distance, the separated atoms is not adequate because the excited electron evolves overs both the target and projectile core. The coulombic barrier model provides an adequate indication of the favorable capture distance but it only considers the inter-nuclear coulombic repulsion. Let us forget this contribution and focus on the weaker electronic terms for a one electron system as displayed in fig. 6. The general solution can be expressed in terms of elliptic states but one can distinguish two asymptotic quasi atomic regions, the separated atoms when the inter nucleardistance is larger than the size of the electronic wavefunction and the united atom when it is much smaller. At zero inter-nuclear separation, an excited electron would see a single nucleus of charge 9, and as the nuclei start to separated; the non spherical nuclear charge distribution gives rise to the quadrupolar splitting (left of fig. 6) slowly converging to stark states of the separated atoms when the recoiled H + ion polarizes the O 7+ (n = 5) ion at large internuclear distance. Since the number of states are different one state of the separated atoms has to be promoted to the n + 1 shell of the united atom.

Are the electrons captured simultaneously or one after the other?

Our doubly differential measurements were welladapted to address this naive question. First, for a given target atom, we measured rather similar energy gain and scattering profiles for projectiles with identical charge state C 6+ , N 6+ , O 6+ , F 6+ , or N 7+ , O 7+ , N e 7+ or O 8+ , N e 8+ [START_REF] Roncin | Differential crosssections for one-and two-electron capture by highly charged ions (n 7+ , o 7+ , o 8+ , ne 7+ , ne 8+ ) at low keV energies[END_REF][START_REF] Barat | Coincident energy gain spectroscopy of electron capture in multiply charged ions colliding with he, h2and heavy rare-gas targets[END_REF] indicating that the ion core structure does not play a key role in the primary capture mechanism whereas it has a drastic influence on the fate of the multiply excited ion after the collision; its final charge state, the emitted electron spectra as well as Xray or V.UV spectra. Focusing on double electron capture, we observed both one step and two step double electron capture so we reformulated the question differently. What is the driving mechanism in simultaneous double electron capture?

More precisely, does the simultaneous capture of two electrons at a curve crossing imply that the di-electronic 1/r 12 interaction is responsible? When the two electrons are captured one after the other, each crossing can be associated with a one electron exchange term but this is not the case in simultaneous double electron capture where a second order term has to be considered.

The question was therefore are second-order one-electron-exchange terms more important than first order di-electronic terms? With the help of V. Sidis, we decided to investigated this theoretical aspect on the C 4+ + He system considered as the ideal candidate because not only double electron capture takes place in one step but it is more important than single electron capture as measured e.g. by Okuno et al [START_REF] Okuno | Energy-spectroscopic studies of electroncapture processes by low-energy, highly stripped c, n, and o ions from he[END_REF] and confirmed theoretically by Kimura and Olson [START_REF] Kimura | Electron capture to (nl) states in collisions of c 4+ and c 6+ with he[END_REF]. By reproducing the measured angular dependence of all observed channels semi-quantitatively, we could show [START_REF] Barat | Single and double electron capture by c 4+ ions colliding with helium target[END_REF] that double electron capture can be explained by a second order effect without the need of di-electronic interaction as an electron capture mechanism. This early work was confirmed by later more extensive and more detailed studies [START_REF] Hansen | Dynamics of single-and double-electron capture in c 4+ -he collisions[END_REF][START_REF] Errea | Molecular calculation of total and differential charge transfer cross sections in c 4+ +he collisions[END_REF][START_REF] Gao | Single-and double-electron transfer in lowand intermediate-energy c 4+ + he collisions[END_REF]. The lesson is not that the di-electronic interaction is unimportant as will be discussed later in the study of the evolution of the multiply excited states, but that it is not the dominant driving force for electron capture [START_REF] Barat | Multiple electron capture by highly charged ions (electronnucleusversuselectron-electron interactions)[END_REF]. The importance of second order effects can be understood from the CBM model in fig. 2, if the inter-nuclear distance R CBM favorable for single electron capture is passed without capturing an electron, the level of the target electron is now above the saddle point and free to explore the projectile. Describing these states in an atomic basis requires handling a large basis set beyond the implicit first order perturbation theory. It is not completely surprising that a description in terms of well separated projectile and target wave functions is hardly relevant below R CBM [START_REF] Roncin | Transfer ionization in collisions involving multiply charged ions at low keV energy[END_REF].

The same quasi-atomic description with second order perturbation theory, was found to describe very well other two-electron process such as core changing electron capture by HCI, also called Transfer-Excitation, where two electrons are involved:

N e 7+ (1s Theses results were extended to multiple electron capture by L. Guillemot et al [START_REF] Guillemot | Critical study of the molecular coulombic barrier model for multiple electron capture by highly charged ions[END_REF][START_REF] Barat | Multiple electron capture by highly charged ions at keV energies[END_REF] during his PHD and can be also formulated in terms of characteristic times; the typical time τ e-e for di-electronic interaction can be as short as few hundred atomic time units (1 a.u. ∼ 2.42 10 -17 s), fast enough to have have some influence during the collision [START_REF] Stolterfoht | Evidence for correlated double-electron capture in low-energy collisions of o 6+ with he[END_REF] but much slower that the the time τ capt ∼ 1 a.u. needed for one electron to pass the barrier.

7. How do highly excited electrons manage to avoid autoionisation?

One of the puzzling results was that after multiple electron capture many of the multiply excited states managed to decay by photon emission, avoiding the much faster process of autoionisation. First, only measurements charge states measurements were available, by coincidence with doubly charged recoil target ions, one selects projectile ions that have captured two electrons, and the final charge states indicates if the projectile managed to keep them both (true double capture) or if one electron was released by autoionisation (auto-ionizing double electron capture). The ratio of true double capture was called stabilization ratio and was first expected to reflect an average branching ratio for radiative decay of the doubly excited states populated by the collision. With our spectrometer we could bring energy resolution and identify the principal quantum numbers involved in the primary collisional processes. With the group of A. Chétioui we performed triple coincidences with emitted X-rays to investigate the possibility that specific doubly excited states could be formed having a reduced Auger rate. Our results were compatible with branching ratios calculated by F. Martin but did not show sufficient contrast to derive any conclusion [START_REF] Chetioui | Doubly excited states populated in collisions of o 8+ ions with he and h2at 1.24 keV amu-1[END_REF]. We also tried a large core system such Ar 7+ (nl, n l ) [START_REF] Guillemot | Collisions of metastable he-like c 4+ ions on he and h 2 targets[END_REF] with the hope that so many different states would be populated that maybe some trend could appear by comparison with transition arrays calculated by E. Luc and J. Bauche [START_REF] Luc-Koenig | Radiative and non-radiative decays of doubly-excited configurations in the ar 7+ spectrum[END_REF]. Here again no clear result appeared, but we now understand that we made a mistake. After discussion with the theoreticians, the final step of massive configuration interaction between the thousands of doubly excited states was abandoned. We also had to face the profound contradiction that electron spectroscopy e.g. by A. Bordenave-Montesquieu et al [START_REF] Bordenave-Montesquieu | Two-electron capture into autoionising configurations n 4+ (1snln'l') with n=2, 3, 4 and n'¿n, observed by electron spectrometry in collisions of n 6+ (1s) with he and h 2 , at 4.2 keV amu-1[END_REF], M. Mack [START_REF] Mack | Electron spectroscopy in collisions with multiply charged ions[END_REF] or N.Stolterfoht et al [START_REF] Stolterfoht | Evidence for correlated double-electron capture in low-energy collisions of o 6+ with he[END_REF] was not reporting the same selectivity of the electron capture processes. They were observing only a comparatively weak population of the quasi-symmetrical doubly excited states (n, n n) that we were observing as primary populated states in agreement with coulombic barrier model. We have to mention that another successful interpretation of the coulombic barrier model by A. Niehaus [START_REF] Niehaus | A classical model for multiple-electron capture in slow collisions of highly charged ions with atoms[END_REF] stated that electrons are "molecularized" on the way in, at a given binding energy and redistributed among all possible states. The reaction window derived from this model was found to match the broad range of doubly excited states observed in electron spectroscopy [START_REF] Mack | Double electron capture by helike ions -collison energy dependence of the reaction window[END_REF].

The situation became clear when we investigated the systems N e 10+ + He, N e 8+ or O 8+ + N e, N 7+ or O 7+ + Ar where an important population of (4l, 4l ) and (4l, 5l ) doubly excited series was observed. The (4l, 5l ) series were all found to decay dominantly by auto-ionisation while the (4l, 4l ) series would decay mainly radiatively and therefore "stabilise" the two captured electrons on the projectile [START_REF] Roncin | Mechanism of true double electron capture by multiply charged ions[END_REF]. Obviously the naive interpretation that two electrons in the same shell interact more strongly (auto-ionize faster) than if they are in different shell was not the correct answer. [START_REF] Roncin | Differential cross sections and orientation propensities for electron capture in 0.75-12 keV b 3+ -he collisions: experiment and atomic basis theory[END_REF][START_REF] Roncin | Differential cross sections and orientation propensities for electron capture in 0.75-12 keV b 3+ -he collisions: experiment and atomic basis theory[END_REF]. The percentage indicated next to the series reports a typical stabilization ratio. Doubly excited states lying close and below the top of Rydberg series hardly auto-ionize [START_REF] Roncin | Mechanism of true double electron capture by multiply charged ions[END_REF].

By coincidence with doubly charged recoil target ions, we select projectile ions that have captured two electrons and the question is how many will manage to keep them both (true double capture) and how many will release one by autoionisation (auto-ionizing double electron capture). The ratio of true double capture was called stabilization ratio and is reported in fig. 7 Fig. 7 reports stabilization ratios observed for doubly excited series identified as initially populated for various systems. The largest stabilisation ratios are observed when the energy of the doubly excited series (n, n n) are lying below the top of an adjacent Rydberg series. Hence the following step wise decay, the N e 8+ (4l, 4l ) would dilute into the energy straddling N e 8+ (3l, n l ) series which is then believed to be sufficiently asymmetric to avoid auto-ionisation. The interaction diluting an initially populated N e 8+ (4l, 4l ) state is nothing else than the di-electronic 1/r 12 interaction responsible for dilution in the auto-ionisation continuum that would be available if the state would have been located above the N e 8+ (3l, n ∼ ∞) limit. In line with the quantum defect theory, we proposed with H. Bachau to calculate the decay width as if the autoionisation would be allowed [START_REF] Bachau | Stabilization of autoionizing states during ion-atom collisions[END_REF]. The matching in energy does not have to be precise because, due to incomplete shielding of the nuclear charge by the Rydberg electron, the relative energy position evolves with inter-nuclear distance in exactly the same way as it does for states autoionising at finite distance from the projectile so that the line profile is affected by the so called post-collisional interaction. The importance of a Rydberg series close to the initial capture levels was confirmed by other groups using ion [START_REF] Ali | On the radiative stabilization in slow double-electron capture collisions of highly charged ions with neutral atoms[END_REF], electron [START_REF] Bordenave-Montesquieu | Investigation by high-resolution electron-spectroscopy of the helium-like 3lnl' rydberg series in double capture processes at low collision velocity -auto transfer to rydberg states and electron stabilization[END_REF] and photon spectroscopy [START_REF] Martin | True double capture in collisions of bare and hydrogenlike ions with rare-gas atoms (z=7-13)[END_REF]. However, when trying to make this model quantitative by taking calculated partial transfer width to well-defined spectroscopic terms, we faced the difficulty that the populated N e 8+ (3l, n l ) series were not auto-ionizing fast enough to explain the observations [START_REF] Vanderhart | Radiative stabilization in double-electron capture to 4l4l' and 4l5l' states in o6+[END_REF]. This is because momentum conservation indicates a low l n value for the Rydberg electron, in other words, the N e 8+ (3l, n l ) Rydberg electron leaving the N e 8+ (4l, 4l ) core will return close to the n = 3 core after a Rydberg period and have a new chance to auto-ionize. Somehow, the orbit of the Rydberg electron has to be moved away from the core. Once again, we were lucky to have a skillful visiting scientist on leave from Saint Petersburg; A.K. Kazansky not only understood the problem before us, but proposed an analytic estimate of the stark mixing in the receding target field [START_REF] Kazansky | The rotation-stark mechanism of creating large-l rydberg states in double charge-exchange[END_REF]. In most situations, the angular momentum of the Rydberg electron was found to increase [START_REF] Kazansky | Stabilization of multiple electron capture by the post-collisional increase of angular momentum of rydberg electrons[END_REF], as expected from phase space considerations, explaining the observed quenching of auto-ionisation [START_REF] Roncin | Auto transfer to rydberg states and indirect stabilization following double capture[END_REF][START_REF] Roncin | Energy gain spectroscopy of ne 10+ + he collisions at keV energies[END_REF].

This also explained in more detail the stepwise δn = 1 Rydberg transitions from circular Rydberg states (i.e. the Yrast cascade) observed by S. Martin et al [START_REF] Martin | Rydberg transition emission after multielectron capture in low-energy collisions of ar 9+ with he, ne, and ar[END_REF] as well as the equivalent lines emitted by the receding excited target ion. Some of the Rydberg electrons ejected by the projectile were simply re-captured by the target as easily imagined from fig. 8 if the initial direction points to the target. We combined our observations with those of the Lyon group to better describe the population mechanism of double Rydberg series deriving from the dilution of quasi symmetrical triply excited series populated in collisions of highly charged C 6+ and N e 10+ ions [START_REF] Bernard | Double rydberg states in stabilized triple capture in bare ions (z= 6-10) on rare gas target collisions[END_REF].

Around 1994, we stopped our experiments in the gas phase with the satisfaction that we had managed to extract from the complexity of highly charged ions, a few examples of collisions that were even more simple than with singly charged ions. A with many students, I started my PHD with the impression that I would never be able to catch up with all the knowledge accumulated by generations of smart physicists. Thanks to Michel I had the feeling that, in our narrow field of HCI, I had revisited ion-atom collisions. In less than ten years Michel et al published more than 40 papers, always with great enthusiasm.

Then our scientific activity took different directions. Other topics had developed such as testing of quantum electrodynamics [START_REF] Indelicato | Effect of hyperfine structure on the 2 3 p 1 and the 2 3 p 0 lifetime in heliumlike ions[END_REF] particularly in Livermore where they had transformed the EBIS source into an EBIT trap [START_REF] Levine | The use of an electron beam ion trap in the study of highly charged ions[END_REF][START_REF] Beiersdorfer | Measurement of qed and hyperfine splitting in the 2s 1/2 -2p 3/2 x-ray transition in li-like 209 bi 80+[END_REF][START_REF] Dragani | High precision wavelength measurements of qed-sensitive forbidden transitions in highly charged argon ions[END_REF], the hollow atom formation at surfaces [START_REF] Briand | Production of hollow atoms by the excitation of highly charged ions in interaction with a metallic surface[END_REF][START_REF] Limburg | Probing hollow atom states formed during impact of highly charged ions on surfaces: N6,7+ and o7+ on al(110) and si(100)[END_REF][START_REF] Arnau | Interaction of slow multicharged ions with solid surfaces[END_REF][START_REF] Winter | Hollow atoms[END_REF], the guiding of highly charged ions through nano and micro capillaries [START_REF] Stolterfoht | Transmission of 3 kev ne 7+ ions through nanocapillaries etched in polymer foils: Evidence for capillary guiding[END_REF][START_REF] Ikeda | Production of a microbeam of slow highly charged ions with a tapered glass capillary[END_REF][START_REF] Skog | Evidence of sequentially formed charge patches guiding ions through nanocapillaries[END_REF][START_REF] Cassimi | Imaging dynamics of charge-auto-organisation in glass capillaries[END_REF][START_REF] Gruber | Temperature control of ion guiding through insulating capillaries[END_REF]. Many other groups were also equipped with their own ECR or EBIS ion source and by then, our setup was no longer unique.

A new generation of setups with large time sensitive imaging detectors to correlate the impacts of several fragments from a collision [START_REF] Moshammer | A 4π recoil-ion electron momentum analyzer: a high-resolution "microscope" for the investigation of the dynamics of atomic, molecular and nuclear reactions[END_REF][START_REF] Lafosse | Vector correlations in dissociative photoionization of diatomic molecules in the vuv range: Strong anisotropies in electron emission from spatially oriented no molecules[END_REF] was developing. The most efficient "collision microscope" named COLTRIMS took advantage of cold target atoms from supersonic expansion to measure the energy balance and scattering angles, not from the projectile as we were doing but from the target recoil momentum and continued revisiting the basic collisional processes [START_REF] Moshammer | A 4π recoil-ion electron momentum analyzer: a high-resolution "microscope" for the investigation of the dynamics of atomic, molecular and nuclear reactions[END_REF][START_REF] Mergel | He2+ on he: State-selective, scattering-angledependent capture cross sections measured by cold target recoil ion momentum spectroscopy (coltrims)[END_REF][START_REF] Ullrich | Recoil-ion momentum spectroscopy[END_REF][START_REF] Flechard | State-selective doubleelectron capture in low-velocity collisions studied by recoil-ion momentum spectroscopy[END_REF][START_REF] Jahnke | Experimental observation of interatomic coulombic decay in neon dimers[END_REF][START_REF] Yan | Direct evidence of interatomic coulombic decay in electron impact ionization of ne dimer[END_REF].

With J. Fayeton at LCAM, Michel started a new career in the emerging field of clusters by investigating atomic collisions with sodium clusters. Together with Y. Picard from LCAM and I. Ismail (PHD), they constructed a "zero dead time" position sensitive detector [START_REF] Ismail | A zero dead-time, multihit, time and position sensitive detector based on micro-channel plates[END_REF]. Ten years after Michel Barat started a last career on collision induced and laser induced dissociation of bio molecules.

Together with C. Jouvet from Laboratoire de photo physique moléculaire and J. Fayeton, Michel constructed "Arc en Ciel" to investigate the fate of large bio molecules after the impact fs laser or the collision with atoms [START_REF] Kang | Ultrafast deactivation mechanisms of protonated aromatic amino acids following uv excitation[END_REF]. Michel succeeded to have five fruitful careers, ion-atom collisions, reactive collisions, electron capture by HCI, collisons with cluster and bio-molecules, all related to collision physics with significant instrumental developments.

My legacy from Michel

Supported by Michel and J.P. Gauyacq, I switched to investigate atomic collisions at surfaces, in particular the impact of a single HCI had been found to convert their large potential energy into the emission of up to several hundred secondary electrons [START_REF] Kurz | Electron emission and imagecharge acceleration for the impact of very highly charged ions on clean gold[END_REF] during the decay of the hollow atom [START_REF] Briand | Production of hollow atoms by the excitation of highly charged ions in interaction with a metallic surface[END_REF][START_REF] Limburg | Probing hollow atom states formed during impact of highly charged ions on surfaces: N6,7+ and o7+ on al(110) and si(100)[END_REF][START_REF] Arnau | Interaction of slow multicharged ions with solid surfaces[END_REF][START_REF] Winter | Hollow atoms[END_REF]. I started designing a detector to collect the electrons emitted from the surface in coincidence with the scattered projectile, but once again, it was Michel who initiated discussions with Pr Leonas from I.P.M. Moscow. In close collaboration with V.N. Morosov and A. Kalinin from Moscow and Z. Szilagi (PHD), we constructed an ambitious 2π multi-detector [START_REF] Morosov | 2π spectrometer: A new apparatus for the investigation of ion surface interaction[END_REF]. The detector shown in fig. 9 was so efficient that, after calibration [START_REF] Villette | Calibration of a multiple microchannel plate detectors system by α-induced secondary electrons[END_REF] it was easy to correlate quantitatively the presence or absence of a specific feature such as the detection of a fast Auger electron with the number of slow electrons [START_REF] Morosov | Correlation between projectile auger electrons and slow secondary electrons emitted in collisions of highly charged ions with an insulator surface[END_REF]. We decided to apply this detector to investigate the basic electronic processes taking place during collisions at surfaces. With PHD students, J. Villette and J.P. Atanas, we started to address the paradox that, under ion impact, insulator surfaces tend to emit more electrons than metal surfaces in spite of a workfunction up to twice as high. Though each projectile undergoes several successive collisions with the surface atoms, we succeeded to observe individual inelastic processes and, with help of coincidences, we could identify the collisional processes governing electron emission in collisions of keV protons on a LiF (100) surface [START_REF] Roncin | Energy loss of low energy protons on lif(100): Surface excitation and hmediated electron emission[END_REF].

The incoming proton neutralizes before hitting the surface and then, at each alkali site, the H • atom has a chance to capture one electron by the ion-pair-like mechanism just discovered by C. Auth, A.G. Borisov and H. Winter [START_REF] Auth | High fractions of negative ions in grazing scattering of fast oxygen atoms from a lif(100) surface[END_REF]. We could show that when leaving the alkali site where capture took place, the H -ion has to cross the localised exciton level and that, most often, an H • atom would emerge because the captured electron was placed in this exciton level. The H -ions that could leave the alkali site with the captured electron were exposed to very efficient electron detachment when passing on top of the next alkali site, emitting a low energy electron and returning to the H • state from which new cycles of capture and loss could restart. With J. Villette, A. Momeni (PHD), H. Khemliche and A.G. Borisov both from LCAM, we could show that N e + ions neutralize on the LiF (100) surface via a dark-Auger process, without emitting electrons [START_REF] Khemliche | Electron bihole complex formation in neutralization of ne + on lif(001)[END_REF]. Just like the quenching of auto-ionisation by transfer to Rydberg series observed with Michel, the 1/r 12 di-electronic interaction was coupling the N e * excited states to a different continuum, the LiF conduction band. No electron was emitted because the excited N e * state is coupled to a different continuum, the conduction band. In the same spirit as the C 4+ + He system investigated with Michel [START_REF] Barat | Single and double electron capture by c 4+ ions colliding with helium target[END_REF], we could identify a simultaneous double electron capture by F + ions forming a F -or leaving a Trion on the surface, an excited state bound by two holes in the valence band [START_REF] Roncin | Evidence for Fformation by simultaneous double-electron capture during scattering of F + from a lif(001) surface[END_REF]. As P. Rousseau was exploring similar processes with protons impinging the surface of a NaCl crystal, an unexplained structure showed up in the scattering profile and I suspected that it could be due to the part of the trajectory where the proton evolves as a H • atom. We decided to prepare a pulsed neutral beam of H • with our charge exchange cell and we recorded the first diffraction of keV atoms at a crystal surface [START_REF] Rousseau | Quantum scattering of fast atoms and molecules on surfaces[END_REF]. Soon after the Berlin group of Pr. H. Winter also equipped with a position sensitive detector observed independently the diffraction of fast atoms [START_REF] Schüller | Diffraction of fast atomic projectiles during grazing scattering from a lif(001) surface[END_REF], and we participated in the rebirth of our discipline. So far, interactions of keV ions at surface were often considered as old physics since, apart from the atomic collisions itself, mainly classical mechanics was involved. By showing, as illustrated in fig. 10 that the crystal surface can act as a diffraction grating for the atomic wave several quantum features could be investigated, supernumerary rainbows [START_REF] Schüller | Supernumerary rainbows in the angular distribution of scattered projectiles for grazing collisions of fast atoms with a lif(001) surface[END_REF], multiple scattering, bound state resonances [START_REF] Debiossac | Transient quantum trapping of fast atoms at surfaces[END_REF], van der Waals interactions... Then, with M. Debiossac (PHD), after the identification of the elastic and inelastic diffraction, we managed to understand how a comparatively heavy projectile can bounce on the surface without transferring recoil energy to the surface atoms. The answer was in textbooks, in a sudden approximation an harmonic oscillator can absorb a recoil momentum p without changing energy as in the Lamb-Dicke effect of recoilless emission of a photon. We could therefore show that both the projectile and the target surface are quantum objects and that bouncing on a surface has similarities with the Lamb-Dicke regime needed to trap cold atoms in an optical lattice [START_REF] Roncin | Elastic and inelastic diffraction of fast atoms, debye-waller factor, and mössbauer-lamb-dicke regime[END_REF]. In hindsight, it is obvious that the ability to build and calibrate time and position sensitive detectors [START_REF] Barat | Absolute detection efficiency of a microchannel plate detector for neutral atoms[END_REF][START_REF] Villette | Calibration of a multiple microchannel plate detectors system by α-induced secondary electrons[END_REF][START_REF] Lupone | Note: A large open ratio, time, and position sensitive detector for time of flight measurements in uhv[END_REF][START_REF] Lupone | A large area high resolution imaging detector for fast atom diffraction[END_REF] adapted to specific needs was crucial.

I would like to conclude by drawing attention to a recent result where Y. Picard, a former PHD student of Michel played a key role. In the group of D. Comparat at laboratoire Aimé-Cotton, he was involved in the construction of a custom combination of time and position sensitive detector with a clever strategy ; create a population of cold Rydberg Xe * atoms and photo-ionize them at threshold with a laser. Then taking advantage of the impact location of the detected photo-electron to guess the position in space of the parent Xe + ion. They can then, apply active corrections to the deflector along the path of the extracted ion [START_REF] Lopez | Realtime trajectory control of deterministically produced ions[END_REF]. In principle, such an ion beam where each ion is taken from a known place and actively guided to its desired destination is not restricted to Liouville conservation of phase-space and could pave the way to a new generation of experiments.

conclusion

Michel never hesitated to change field, when a new subject attracted his interest, he would wonder what is needed to contribute and try to initiate collaborations. The examples are numerous were what he found was quite different from what was expected, but this is also part of the fun of doing Science, because sometimes, a bigger surprise was just waiting to be discovered. "Try to anticipate, but never think that you know everything!", in other words, it is not because you imagine numerous obstacles that you should not go and look at unexplored situations. Science is done by humans and it was a pleasure to do it with Michel and all his enthusiasm. Not that ha was always equal, certainly not, he had memorable explosions, but that was his way of expressing what was worrying him. An hour later, everything would return to a quiet state without any resent.

Figure 1 .

 1 Figure 1. Michel Barat played a key role to promote the physics of atomic and molecular collisions.

Figure 2 .

 2 Figure 2. The coulombic barrier model (CBM) considers the evolution of the coulombic barrier between target and projectile and points out the distance R CBM where the target electron can classically proceed toward the projectile. For the O 8+ + He system, the model indicates that capture should take place into the O 7+ (n = 4) shell, the O 7+ (n = 5) is too far away and separated by an very large barrier while the O 7+ (n = 3) corresponds to a quasi molecular situation.

Figure 3 .

 3 Figure 3. Scheme of the experimental setup. The electrostatic analyser selects the projectile final charge state and the P.S.D. records its energy gain and scattering angle in coincidence with recoil ions.

Figure 4 .

 4 Figure 4. Taking into account the polariser efficiency, the polarization P 3 = RHC-LHCRHC+LHC measured at 1.5 keV[START_REF] Roncin | Observation of orientation propensity for electron capture in multiply-charged-ionatom collisions[END_REF] is transformed into the angular momentum L ⊥ reaching one unit. Lines are for an extensive theory[START_REF] Roncin | Differential cross sections and orientation propensities for electron capture in 0.75-12 keV b 3+ -he collisions: experiment and atomic basis theory[END_REF] and a simple collision model[START_REF] Adjouri | Quasimolecular analysis of orientation and alignment in b3 + -rare gas electron capture processes[END_REF][START_REF] Ostrovsky | Landau-zener model with inclusion of electron momentum transfer[END_REF].

Figure 6 .

 6 Figure 6. schematic view of the O 8+ + H molecular orbitals at short inter-nuclear separation. The different shells of the separated atoms have to converge towards the energy levels of the united atom (R = 0). One σ state the 5gσ is promoted and crosses the 5dσ and 5f σ states correlated to the n = 5 shell of the separated atoms.

Figure 7 .

 7 Figure 7. Low keV energies collisions of N e 10+ ions with Xe, Kr, Ar, N e and He target indicate an initial population of several quasi-symmetrical double excited series ranging from (4, 4) to[START_REF] Roncin | Differential cross sections and orientation propensities for electron capture in 0.75-12 keV b 3+ -he collisions: experiment and atomic basis theory[END_REF][START_REF] Roncin | Differential cross sections and orientation propensities for electron capture in 0.75-12 keV b 3+ -he collisions: experiment and atomic basis theory[END_REF]. The percentage indicated next to the series reports a typical stabilization ratio. Doubly excited states lying close and below the top of Rydberg series hardly auto-ionize[START_REF] Roncin | Mechanism of true double electron capture by multiply charged ions[END_REF].

Figure 8 .

 8 Figure 8. Classical trajectory corresponding to a N e 8+ (3, n = 10) Rydberg electron emitted in the field an Ar 2+ receding target ion initially at a distance R=20 a.u.

Figure 9 .

 9 Figure 9. Top view of the 2π multi-detector designed and constructed by V.N. Morosov. 15 independant MCP detectors are facing the center where the target surface is placed. Collisions could be investigated at grazing or normal incidence.

Figure 10

 10 Figure 10. a) scattering profile of 460 eV helium atoms impinging the surface of a LiF crystal at an incidence of 1.57 deg. Diffraction is observed and b), d) isolate the elastic contribution on the Laue circle where all surface atoms appear frozen at equilibrium position while c) shows the inelastic contribution where binary collisions have excited phonons [81].

Acknowledgments

The authors is grateful to all former members of the LCAM for their support and to A.J. Mayne for careful corrections of this manuscript.


Cui Zhao-Feng

Yuan Shuang-Long

Yang Yun-Xia

François Cheviré

Franck Tessier

Chen Guo-Rong

Preparation and Photoluminescence Properties of Eu2+-Doped Oxyapatite-Type SrxLa10

come