Special Issue Preface pubs.acs.org/JPCA
Autobiography of Jean-Michel Mestdagh
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August 2008. Although he was in quite bad condition because of his disease, he gave me an idea that I never succeeded to exploit really. Let me mention it below, since it paves the way to perspectives in astrochemistry that might be of interest to the physical chemist community. As an astrophysicist, Pierre calculated intensively on the interactions between H2 and many molecules. He was quite fascinated by the calculation he did (but never published) on hydrogen clusters, and he showed me pictures of the wave function. Then, he suggested that such clusters certainly exist in the interstellar medium and could be responsible for an interesting new chemistry. His idea was that the H2 clusters could act in the interstellar medium as the argon clusters in the cluster-induced chemical reaction technique developed in my research group.2 Accordingly, radicals and molecules, which have been trapped collisionally by the same H2 clusters, may react if a barrierless reaction path exists. This mechanism changes drastically the expectations on neutral−neutral reactions in the interstellar medium since it affects the reaction cross section and can make it as large as the geometrical cross section of the cluster which traps the reactants. The idea that H2 clusters carrying embedded molecules might play a role in astrophysics was visionary in 2008. A recent publication, although carrying on a slightly different subject than astrochemistry, suggests that this might not be unrealistic.3 I was very lucky in 1976 when I was offered a permanent position at CNRS (“Centre national de la recherche Scientifique”, one of the major research agencies in France) as “attaché de recherche”. Since then, my scientific activity has focused on “reaction dynamics”. I foundand I still findthis fundamental approach of chemistry very exciting. In particular, I was quite fascinated by the idea of unraveling the subtle interplay between intermolecular movements, intramolecular deformations, and changes of electronic configurations as a chemical reaction proceeds. The way I see this field now is through a very strong relationship, almost a marriage, between experiment and theory, not only to establish solid conceptual basis to interpret the observed phenomena but also to develop and validate advanced calculation techniques so that theoretical approaches become predictive and reliable without being restricted to oversimple systems.
was born in Paris in 1952. My mother and father both were teachers, my mother in a nursery school and my father in a primary school. When I was 6−8 years old, I was a pupil in the same school where my father was teaching. During the lunch break or right after school I often went in his classroom, and there I discovered very simple experimental arrangements that he built to illustrate basic physical and chemical phenomena. Especially fascinating to me was a clever system of levers that he designed to transform the fairly small dilatation of a heated wired into a large amplitude rotation of a pointer. These setups had a great impact on me, and they definitely marked profoundly my way of addressing science. To appear indeed as interesting to me, an experiment or a calculation must be illustrative and give a convincing interpretation of a single, clearly identified question. Two people that I met during my initial formation have also marked my way of considering science. One was a math professor in my final year in high school, M. Coulomb. He was very concerned about our success at the baccalaureate, and in his counsels he was always speaking of observation. Observation in math? Yes, of course! His permanent advice was to observe and read carefully the math expressions that we have obtained or the theorems that we just demonstrated. There is good chance indeed that these observations cast invaluable indication on the correctness of what has just been written. The other was my physics professor, M. Burie, when I was preparing the exam to enter at Ecole Normale Supérieure. When facing a really difficult problem, he taught his students not to be fully satisfied by a single, necessarily superficial answer. On the contrary, he taught us how to go deeply to the root of the problem and how to look for alternative ways of getting the same answer. He considered, and I essentially agree with him, that it is the only way to be reasonably sure that the answer is correct. At that time I also realized that mathematics are absolutely essential in physics and that theory is the necessary step to make sure that physical phenomena are fully understood. In 1972−1976, I was student at Ecole Normale Supérieure, where I met my wife Hélène. She is a physical chemist also. I gained close personal friends there, and of course I had many terrific professors who marked me profoundly. Let me recall the memory of my friend Pierre Valiron, who passed away a few years ago. Many of you know Pierre since he was an astrophysicist in Grenoble doing high-level calculations and developing methodology in quantum chemistry. A tribute to him appeared in the EPJ Web of Conferences (http://www.epjconferences.org/articles/epjconf/abs/2012/16/contents/ contents.html1). We did a lot of mountaineering together and survived several stupid mistakes, which could have been dramatic! The point that marked everybody who met Pierre was his extensive culture and his amazing clarity in explaining physics (or anything elsebiology, music, etc.). Everything looked so simple and so obvious when he spoke. When I was with him, I always felt more clever than I am. Thank you, Pierre. One of the last times I saw him was in © 2015 American Chemical Society
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THE FIRST CROSSED BEAM MACHINE IN FRANCE A challenge in the mid 1970s was the detailed understanding of atom−atom collisions when non adiabatic energy transfers between electronic levels are at play. Of course the conceptual ground was existing long ago, but methodological development was still necessary to make calculations in practice, both to get the interaction potentials and to treat the collision dynamics. A solid experimental basis was necessary to validate these approaches. Crossed beam machines appeared as the dedicated Special Issue: Jean-Michel Mestdagh Festschrift Published: June 11, 2015 5903
DOI: 10.1021/acs.jpca.5b03439 J. Phys. Chem. A 2015, 119, 5903−5906
Special Issue Preface
The Journal of Physical Chemistry A
radioastronomy, where complex large-size equipment was used. I realized with Paul how powerful and useful it is to develop such software, even when dealing with fairly simple experimental setups. Several years later when Lionel Poisson joined my research group in Saclay, I gave him entire liberty to develop such fancy software because he seemed very smart at doing that. Actually, Lionel was successful in this activity since many of you, dear readers, are using the software he has designed to record and analyze electron and ion signals in velocity map imaging setups. Let me come back to reactive collisions and to my collaboration with Yuan Lee. Ph.D. students have commuted between our two groups, and chapters in the dissertations of Christian Alcaraz (France) and Arthur Suits (USA) report experiments on both sides of the Atlantic Ocean. They concern reactive scattering of electronically excited alkaline earth atoms. Again, a close coupling between experiment and theory was desired. However, the theoretical challenge was too strong at that time, and no detailed calculation could be performed. We simply used models, powerful of course, but qualitative only, like the single and double harpoon model or the persistence of the electronic configuration for electrons that do not participate directly in the reaction under study. This is a pity because experimental results of quite high quality were obtained in both laboratories. It might be useful to revisit these data since their full theoretical interpretation should be at hand now given the current explosion in the calculations capacities.
tools for this purpose. A number of them were existing in the USA and in Germany but none in France. Michel Barat (LCAM, Orsay), Jacques Baudon (LPL, Villetaneuse), and Jacques Berlande (CEA, Saclay) have worked actively to get the necessary funding from CNRS and CEA (“Commissariat à l’énergie atomique”, another major research agency in France) to build such a machine. I joined the CNRS precisely at that time. My Ph.D. project was to build this machine for exploring the energy dependence of the total cross section of fine structure changing collisions (np2P1/2↔3/2) when laser-excited alkali atoms are colliding rare gas atoms or simple unreactive molecules at thermal collision energies. I was purely an experimentalist at that time. Nevertheless, my three mentors, M. Barat, J. Baudon, and J. Berlande, were convinced of the need to associate experiment and theory in this field. Hence, my first paper was coauthored by Jean Pascale, one of the developers of the pseudopotential technique. The latter limits the electron problem in quantum chemistry calculations to the sole valence electrons. The benefit is considerable with alkalirare gas systems since the complex many-electron problem shrinks to a single-electron problem. Then, it was possible to calculate potential curves up to those correlating to Rydberg states of the alkali atom at large separation between the alkali and the rare gas atoms. J. Pascale was very active to validate such calculations using the experimental data provided by our research group. Those from the crossed beam machine were used to validate potential curves correlating to the first excited state of the alkali atom, whereas those obtained by Jean-Paul Visticot on far wings in the band profile of forbidden transitons documented curves correlating to 2S and 2D states of the alkali atom. Experimental work was also provided by François Gounand on Rydberg states.
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CLUSTER ISOLATED CHEMICAL REACTIONS (CICR) An intense activity appeared in the 80s to 90s concerning atomic and molecular clusters. The main reason is that clusters offer an elegant way of bridging the gap (a no man’s land, said J. Jortner) between molecular and condensed phase physical chemistry. This lead us, Jean-Paul Visticot and me, to propose that van der Waals clusters could provide a link between reactive collisions of isolated species that we used to study with our crossed beam machine and solution chemistry which is so important in practice. The year 1992 was the beginning of a great scientific adventure, which is continuing today. We used our crossed beam machine to collide the barium atoms of the primary beam with argon clusters carried by the secondary beam. N2O molecules were deposited on the argon clusters prior to crossing the Ba beam. The pick-up technique of G. Scoles 5 was used. We first interpreted the observed chemiluminescence as a reactive collision between Ba and a (N2O)Arn cluster;6 however, shortly after, we realized that the reaction proceeds in several steps. First, the Ba and N2O reactants are both picked up on the same cluster. Then, because the latter have enough mobility on the cluster, they collided each other. Eventually they reacted. Accordingly, the crossed beam configuration was useless except for reducing signals. We thus replaced the barium beam by a pick-up cell, and we got the most incredible increase in signal that I had ever experienced in my scientific life: more than 4 orders of magnitude! With my colleagues, we called these reactions Cluster Isolated Chemical Reaction (CICR), and we realized that the Poisson statistics which applies to describe the pick-up process can be used to determine the exact stoichiometry of the reaction under study.2 Similar experiments, with the same use of the Poisson statistics, were developed independently at about the same time in the group of Scoles, except that helium nanodroplets replaced the argon clusters.7 This gave rise to the widely used HElium NanoDroplet Isolation technique (HENDI).8
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REACTIVE COLLISIONS OF EXCITED ATOMS IN SHORT-LIVED ELECTRONIC STATES Several crossed beam machines were constructed in France in the early 80s for studying reactive collisions specifically: groups of M. Barat (LCAM, Orsay), G. Dorthe, M. Castes, C. Naulin and J. Joussot-Dubien (PPM, Bordeaux), J. C. Lehmann and J. Vigué (ENS, Paris), and R. Vetter (LAC, Orsay). The impressive work performed in the group of Yuan T. Lee in Berkeley was a strong motivation. Some might say that turning his scientific activity toward reactive collisions had become a great national cause in France at that time. (I am almost not joking!) After completion of my Ph.D., in concertation with my colleagues in Saclay, we decided to address reactive collisions, and given our experience on electronically excited alkali atoms, we thought that reactions of excited atoms in short-lived electronic states might be of interest. Actually, Yuan T. Lee (U. Berkeley, USA) had initiated a research program in that direction, starting with the Ph.D. thesis of M. F. Vernon, followed by that of Paul S. Weiss.4 I decided to join the group of Yuan for a 1 year postdoc in 1983. Actually, I stayed a bit longer in Berkeley, and I kept collaborating with Yuan for several years. I had a terrific time in Berkeley working with Paul S. Weiss, a very impressive Ph.D. student, and Hartmut Schmidt, a German collaborator of Yuan, coming from the group of I. V. Hertel (Freie Universität, Berlin). Paul had an incredible ease to develop complex software to perform data acquisition and data analysis. Using computers in this context was by no means usual in 1983−1984, except in 5904
DOI: 10.1021/acs.jpca.5b03439 J. Phys. Chem. A 2015, 119, 5903−5906
Special Issue Preface
The Journal of Physical Chemistry A
organic molecules, radicals, carbenes, biomolecules, van der Waals complexes, large clusters, nanoparticles, etc. The setup associates the femtosecond pump−probe technique with the velocity map imaging of the electrons and ions produced by the probe pulse. The experiments are conducted in a molecular beam, and several sources were designed to accommodate the variety of objects mentioned above. With this apparatus, we played our part to state that conical intersections are ubiquitous in molecular photochemistry. A lot of the current activity is performed by Lionel Poisson. Let me mention a recent work where a roaming wavepacket was observed about a conical intersection between the S1 and S2 potential energy surfaces of 2-hydroxypryridine. The experimental work was performed in Saclay, but its interpretation has needed very elaborated multiconfigurational quantum chemistry calculations performed by Martial Boggio-Pasqua (LCPQ, Toulouse).11
Let us consider the CICR technique in situations where an energetic barrier blocks the reaction between the ground-state reactants, which are deposited on the cluster. Then, the CICR technique appears as a way to configure geometrically the reactants prior the reaction. The latter can be turned on by electronic excitation of one of the reactants. With this perspective in mind, the CICR technique is mirroring the Transition State Spectroscopy (TSS) that Benoit̂ Soep had developed a long time before, in 1983, as he was still at Laboratoire de PhotoPhysique Moléculaire in Orsay (he joined our group in 2000).9,10 Accordingly, two reactants are frozen as a cold nonreactive 1:1 van der Waals complex. A reaction is turned on when a laser excitation projects the complex onto a reactive potential energy surface. Monitoring the reaction signal while scanning the excitation laser provides an action spectrum which informs very directly on the reactive potential surface in the transition-state region of the reaction. In particular, the presence of a vibrational structure in the action spectrum informs about deformation modes which lead to reaction, and the distinction between broad and narrow features carries information on the dynamics of the reaction, whether the excited mode is strongly coupled to the reaction coordinate or not. This is invaluable information in the context of reactions which proceed via a multidimensional dynamics. Applying the same technique in a CICR configuration on reactions which were already studied by the TSS technique helps document solvation effects. For instance, this was applied to the Ca(4s4p 1 P) − HBr → CaBr* + H reaction by Marc Briant as he was a Ph.D. student in our group. and he determined which reaction coordinates are still free and which one are blocked when the argon cluster is present. This work and many others on oxidation reaction of alkaline earth atoms gave me the opportunity to make my activity evolve from experiment to quantum chemistry calculation. Actually, this would not have been possible without the help and the incredible teaching qualities of Fernand Spiegelman (LCPQ, Toulouse). Fernand taught me everything I know in the very refined art of treating the electronic excitation in molecular assemblies where metal atoms are present.
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THE REACTION DYNAMICS GROUP AND CURRENT PROJECTS French research groups are not organized as in many other countries. They are formed about 3−10 staff scientists with permanent positions, some of them of a similar age. These groups are members of laboratories (in Universities or at CNRS) or research units (the services at CEA), which number 100 (sometimes up to 200 or 300) persons. For instance, the work done in Saclay that I have mentioned above was completed in the Reaction Dynamics Group, within the Service des Photons Atomes et Molécules (SPAM!!) at CEA-Saclay. SPAM (now LIDyL, Laboratoire Interaction, Dynamique et laser) is associated with the CNRS under the name laboratoire Francis Perrin. (The French organization of scientific research is quite complex with several agencies doing similar jobs with the hope (?!) that interferences between them could be constructive.) My colleague Jean-Paul Visticot was directing the Reaction Dynamics Group until 2004. I took his position when he left for higher responsibilities at CEA. Jean-Paul had organized the group on a very collective basis. I followed his action. I think indeed that such organization is the only one to be consistent with the structure of French laboratories with several staff scientists in the same group, some of them having their own international recognition and some others being still young scientists who need to develop their autonomy. Hence, most if not all of the works mentioned above do not result from the boss intuition. On the contrary, they result from ongoing discussions, very often at lunch time between the group members. I am about to hand over the group to Lionel Poisson with three ongoing research lines: DYNAFEMTO. This project addresses the ultrafast dynamics of electronically excited systems which are isolated in the gas phase: molecules, radicals, or carbenes and more recently nanosolvated species and nanoparticles. This project is associated with ATTOLAB, an attosecond laser facility which is currently under development at CEA-Saclay. This paves the way to handle electronic wave packets in physical chemistry. GOUTTELIUM. Here the spectroscopy and dynamics of species trapped in helium droplets will be addressed with a special attention to organic and bioorganic molecules either isolated or solvated by a controlled number of solvent molecules. FeC-CLUSTER. A project built on numerical simulation which aims at addressing important phenomena such as phase transition, chemical segregation, and self-organization in Fe−C systems.
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GAS-PHASE FEMTOCHEMISTRY In the early 90s no group in France was active in gas-phase femtochemistry, although a growing interest was emerging, motivated by the pioneering work of A. H. Zewail at Caltech (USA). With a few colleagues at CEA-Saclay (I. Dimicoli, M. Mons, P. d’Oliveira, F., Piuzzi, J.-P. Visticot) and LPPM-Orsay (C. Dedonder, C. Jouvet, B. Soep, and D. Solgadi), we decided to find the funding to build a beam machine and develop the necessary lasers to address femtochemistry on gas-phase species. We considered that this new equipment should be accessible by a large community, and from the beginning the femtosecond laser was shared with colleagues at CEA-Saclay: physicists working on multiphoton processes (B. Carré, C. Cornaggia, Ph. Martin) and physical chemists involved in solution chemistry (T. Gustavsson, J. C. Mialocq, S. Pommeret). This was the catalyst to what has become the Saclay Laser Interaction Centre (SLIC, http://iramis.cea.fr/ slic/), a laser facility which is accessible through the LASERLAB-EUROPE Web site (http://www.laserlab-europe. net/). The setup that we designed to perform gas-phase femtochemistry experiments is included as part of this access program (1 or 2 months/year). With it, it is possible to study the dynamics of a large variety of electronically excited systems: 5905
DOI: 10.1021/acs.jpca.5b03439 J. Phys. Chem. A 2015, 119, 5903−5906
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The Journal of Physical Chemistry A I cannot conclude these pages without two bitter considerations on the current scientific landscape. First, the situation of younger scientists, not only in France, is simply dramatic. They have to experience two, three, sometimes four postdoc positions before reaching a position with some stability. Many of them never reach a stable position, even if they are efficient and productive. This corresponds to an incredible waste of skills. Second, I observe a creeping evolution of the research politicsboth at the national and European leveltoward an industrialization of research, giving it as principal (exclusive?) objective to prepare the economic world of tomorrow. The document 2013/743/EU published in the Of f icial Journal of the European Union appears almost as a completion of this evolution. It defines the H2020 program and provides the framework for research and innovation in the European Union over the 2014−2020 period. The expression f undamental research appears a single time in this 76 page document. It is in the sentence, “The above listed areas will be underpinned by more f undamental research to address relevant biological questions as well as to support the development and implementation of Union policies and supported by adequate assessment of their economic and market potential” within a paragraph entitled, “Increasing production ef f iciency and coping with climate change, while ensuring sustainability and resilience”. After reading this document, we feel like we are being forced to lie about our true objectives when writing proposals in fundamental physical chemistry. This is just terrible!
J.-M. Mestdagh
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CNRS, IRAMIS, LIDyL, Laboratoire Francis Perrin, URA 2453 CEA, IRAMIS, Laboratoire Interaction Dynamique Laser
REFERENCES
(1) Ceccarelli, C.; Faure, A.; Wiesenfeld, L. EPJ Web Conf. 2012, 34, 00001. (2) Mestdagh, J. M.; Gaveau, M. A.; Gée, C.; Sublemontier, O.; Visticot, J. P. Int. Rev. Phys. Chem. 1997, 16, 215−247. (3) Bernstein, L. S.; Clark, F. O.; Lynch, D. K. Astrophys. J. 2013, 768, 84. (4) Vernon, M. F.; Schmidt, H.; Weiss, P. S.; Covinsky, M. H.; Lee, Y. T. J. Chem. Phys. 1986, 84, 5580−5588. (5) Gough, T. E.; Mengel, M.; Rowntree, P. A.; Scoles, G. J. Chem. Phys. 1985, 83, 4958. (6) Visticot, J. P.; Pujo, P. d.; Sublemontier, O.; Bell, A. J.; Berlande, J.; Cuvellier, J.; Gustavsson, T.; Lallement, A.; Mestdagh, J. M.; Meynadier, P.; Suits, A. G. Phys. Rev. A 1992, 45, 6371. (7) Goyal, S.; Schutt, D. L.; Scoles, G. Phys. Rev. Lett. 1992, 69, 933− 936. (8) Lewerenz, M.; Schilling, B.; Toennies, J. P. J. Chem. Phys. 1995, 102, 8191−8207. (9) Jouvet, C.; Soep, B. Chem. Phys. Lett. 1983, 96, 426−428. (10) Soep, B. J. Phys. Chem. A 2010, 114, 2956−2961. (11) Poisson, L.; Nandi, D.; Soep, B.; Hochlaf, M.; Boggio-Pasqua, M.; Mestdagh, J.-M. Phys. Chem. Chem. Phys. 2014, 16, 581.
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