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Autobiography of Benoît Soep I have always considered myself as born under a lucky star, although at the time of my birth, some dark clouds were sweeping over my country. Later, as a schoolboy I was attracted by experimental physics and my father introduced me to the initial experiments of Maiman, constructing the first laser in 1960, the ruby laser. I realized the mysteries of that invention and its magic. At the time I had not in mind to become a scientist, I had rather be a medical doctor for the delicate discernment you had to acquire to distinguish the sometimes subtle signs of a pathology. Yet, I followed my parent’s advices and became a chemical engineer. I felt quickly unfit for managing a chemical plant or an oil drill in the jungle of Borneo. After some skillful experiments in the French army where I spilled much gasoline on the ground, rather than in the tanks of very large gas-guzzlers, I decided to join a molecular spectroscopy group, a much more enticing perspective. This was a second chance, since science research was still a very open field and the overall attitude was very much more relaxed. I was offered a position after one year and even had the choice between a teaching assistant and a CNRS position. I felt too shy for teaching but perfectly fit for CNRS. There begun my career ... with a salary that my father mocked, but that I praised very much for the independence it gave us. When I started, the concept of an isolated researcher working independently was already out of date, if it had ever existed, since ideas do not merge directly out of nowhere but have a kind of immanent life before they are well formulated. Nevertheless, an enormous change has occurred since that time in the research environment, in terms of loss of independence. That comes from an industrialization of research and the intention of all governments to make it productive and profitable and to “prepare the world for the next century”. In the sixties, powerful professors or directors, who were divinities in their own realm, independently administrated research in physics or chemistry. They had discretional power (for the good and the bad), and we called these professors “Mandarins”. They were and acted as true feudal barons, very independent, and had little to report to the authorities. This often led to many abuses but had one advantage: they were free to investigate whatever field pleased them. It was before the notion of research policy, or administration of research, had become prevalent. The change that has come through the years is the result of the challenges for conducting scientific research. At these times, there were either purely scientific or military challenges: if you take, for example, reaction dynamics of the chemiluminescent reaction, this research was spurred by the possibility of generating ultrahigh power chemical lasers. Now, the challenge is an economic challenge, where the goal is to drive the economics via the outcome of fundamental and applied research, while before these were only the offsprings of space or military developments. To achieve these economic goals, research has to be administrated as an industrial company with short terms perspectives: goals, accomplishments, schedules of what you should find before you have found it, or not. Instead of local fostering of interesting results, a government or a European convergence plan is made, defining the lines that are to be followed. If you are away from these lines, there will remain little or no perspective for you, except if you have found a steady support or you are under a coVer operation. However, some
spirit is not lost since a new semipublic institution, “le Triangle de la Physique” supports research along purely scientific arguments. Starting with Photochemistry, Time-Resolved Laser Excitation My first thesis adviser was a true spectroscopist and I spent a short time learning infrared molecular spectroscopy. This was not my style, at the time, but he was also interested in a new theme: photochemistry as induced by multiphoton excitation with infrared photons, now “acronymed” IRMPD. He realized that this was a bit too much brute force, since many competing nonlinear processes were in action and he realized that pulsed Nd:glass lasers were coming to age (1967), which could directly excite electronic transitions via the harmonics of the laser, instead of vibrational transitions. In a dynamics rather than spectroscopic perspective, electronic excitation is merely interesting since the electrons can be readily shuffled in preparation for a reactive event: dissociation or collision. At the time, this was an entirely new perspective, to be named a decade later: laser chemistry. After thinking about possible systems, coincident with 266 nm, we realized that mastering the lasers was essential, since these were not off-the-shelf instruments. I stayed for 3 months, a delightful time, with the team of Pr Guy Mayer, one of the inventors of nonlinear optics. It was a great experience where I learned that intuition could lead to an experiment performed the same day and completed the next. One had just to think about any crazy thing, like transient induced gratings created by standing optical waves, and indeed, we published one of the first papers on Q switching a Nd laser by a transient grating in alcohol. This is now a convenient method for laser beam profile adjustment; I think it is called distributed feedback. Incidentally, I also discovered that eye safety was an issue with lasers: an employee was operating a vacuum coating system next to my laser, and I saw after a while that there were many Nd:glass laser impacts on the wall just above his head, fortunately while he had been sitting. I had long regrets not to continue in the domain of nonlinear and laser physics with this brilliant team. One of the members of this group F. Gires has been famous for the many optical setups he has designed (the Gires-Tournois interferometer). I am still amused of experiments they performed on organics (grease) dissolved in high pressure xenon at 1000 bar, akin to what we would do later in supersonic jets. Then 1968 passed and a soft revolution shook the spirits ... the University and the Society. The Era of Nonradiative Transitions I moved to Orsay in the laboratory created by Sydney Leach: Photophysique Mole´culaire, founded in one day, the name and the subject being a new concept. It was really a fantastic time, since there was an incredible freedom of thought, speech, and creation: what you could imagine, you could try to do. Let me say a few more words on how informal things were. The newly created laboratory was examined (audited) after a year’s existence: we assembled in a meeting room with no preparation of any kind. Each of us said a word about his work, giving its real motto. Such an exercise takes now a month or more to a lab.
10.1021/jp1001594 2010 American Chemical Society Published on Web 03/11/2010
J. Phys. Chem. A, Vol. 114, No. 9, 2010 2957 After some initial experiments on photoionization in glassy media and the construction of dye lasers with Lars Lindqvist, I was attracted by a field that was sparkling at the time, “nonradiative transitions”, under the supervision of Andre´ Tramer who was radiant with ideas on the theme. He was a refugee from Poland and eager to reinstall scientifically and personally in France. Nonradiative decay, the origin of the nonfluorescent properties of most polyatomic molecules, results from the decoherence of a bundle of true molecular states coherently excited by a light source. The brilliant idea of J. Jortner was to consider that, within the time scale of the experiment, the excitation reached an overlapping continuum of true molecular eigenstates with a width related to that time limit. Thus the decay could be treated by the Fermi Golden Rule. Relaxation acts in this view as the dissipation of wavepacket representing the nonstationary state over the quasi continuum of degenerate states. The idea of nonstationary states (not considering the excitation process) was shocking for the spectroscopic community and an effect on the medium was always expected. The theme was exciting through the debates it created. In the same way, much later, there was confusion over vibrational wavepackets as fabricated by femtosecond laser excitation. These views spurred experimental research aimed at understanding or bypassing electronic relaxation prior to photochemistry. A side effect of the model created by Bixon and Jortner, with equidistant (perturbing) levels, was the possibility of time recurrences. Thus there were experiments and speculations that in large aromatic molecules, the singlet-triplet intersystem crossing could be reversible. This was the true beginning of my Ph.D. I observed on times scales much prior to collisional relaxation in the gas phase, the irreversible growth of the triplet character in the S2 state of naphthalene and, at later times, the relaxation of this hot triplet into the vibrationless state T0 of naphthalene. The growth rate constant fitted that of fluorescence decay, and we could show that in this general case the decay led to well characterized intersystem crossing (by gas phase transient absorption) and not to some phantom state. I followed directly by the same experimental approach, internal conversion to the ground state: it was known to occur from the observation of low fluorescence quantum yields. However, there lacked direct methods to visualize this hot ground state, ill defined. Afterward, the interest of the physical chemistry community in these nonradiative processes declined since a semiquantitative picture could be given to fit all cases and the theory had given a global framework to depict all situations in terms of coupling and widths. In this, my advisor, A. Tramer had been quite active in digging up the nonexponential behavior of these decays and published, with Franc¸oise Lahmani and Catherine Tric, a very simple classical kinetics rationale for the time evolution. The quantitative calculation of the decay rested upon the calculation or measurement of a density of levels at a given excitation energy (a difficult task in RRKM for example!) and the possibility that in certain cases vibration rotation interaction had to be included to cope with strange selection rules or a much faster decay than expected. This domain went into dormancy for almost 20 years until the advent of femtosecond experiments where unexpectedly ultrashort decays were observed and faster than the molecular movements; then another dynamical explanation had to be found: conical intersections. Catherine Tric studied medicine and became a medical doctor, I was attracted too, but at that time more by experimental science. After the defense of my Ph.D. thesis, I was postdoctoral fellow at Columbia with Richard N. Zare. It was one of the
many glorious periods of his lab and there was a flurry of open subjects ranging from reaction dynamics and atomic and molecular spectroscopy to new methods for analytical chemistry. I chose the investigation of NO2. This was in continuity with my work at Orsay, because NO2 was a benchmark case for very strong vibronic coupling (between the ground and first excited state A). There resulted a dilution of the oscillator strength of the A state among the degenerate ground state levels, causing a lifetime lengthening. The perspective of Zare was to investigate this problem, not by dynamics (measurement of decay times of individual levels) but by spectroscopic methods. In his view the perturbations by the ground state are reflected in the rotational constants (including spin, NO2 is a radical!) of the electronically excited vibrational level under examination. The goal was to deperturb the accessed excited levels and thus disentangle the level structure. (This attempt was later successful on pyrazine with Jan Kommandeur.) We deciphered and analyzed with Dave Monts, a Ph.D. student, many rovibrational lines, but probably not in sufficient number within the A-X coupling width to reconstruct the coupling scheme. The experiment had been constructed by a series of personalities and had a past. I met R.Wallace the last Ph.D. student on the theme, a real character. We exchanged long discussions of many kinds; he was also an activist, picketing every week the Jason (Vietnam war) committee at Columbia. He was brilliant on many subjects and involved in the theory of the car flow in urban areas. Thus through its may aspects, this period was a free and happy one, where I discovered the beauties of reaction dynamics and the energy R. N. Zare had given to the domain with fluorescence detection of laser excited products. I admired his grace, his creativity, intelligence, and the way he endeavored to view problems, from the highest standpoint, leaving aside the many practicalities. I was attracted by reaction dynamics because a rather robust description of the reactivity of not-so-simple systems could be formulated and be predictive. At the same time, the enthusiasm of young and older American scientists was a revelation and a completely surprising attitude to me. I was accustomed to much more moderate expression. The penchant was a complete immodesty, which struck mesit is so distant from my education and I cannot still adopt this way; however, it is a fantastic force for the diffusion of knowledge. It is also a impetus for the young scientists. Molecular Clusters and Dynamical Studies Returning to France was a shock and an experience, since I had to invest in new perspectives. I had been attracted also by the work of the Chicago group, Smalley Wharton and Levy, who were pioneering the excited state spectroscopy of cold molecules and clusters, in supersonic jets. I praised the way, very systematic, they had treated the problem of coupling in NO2, picturing the extent of the A-X coupling through supersonic rotational cooling. However, the most appealing in the combination of tunable lasers and supersonic cooling, was the new chemical architecture of clusters that could be formed in the expansions. They could be characterized simply and efficiently by electronic excitation. This was a direct, spectroscopic way to approach intermolecular potentials and forces, simpler in my view than collisions (a subject of eternal discussion with Steven Stolte, a good friend), in experimental terms and in the theoretical description. Steven states that collision gives a global pattern of the potentials while spectroscopy gives a local bottom of potential perspective; true, but you can be cleverer than that! However, the spectroscopy of clusters provides a direct and accurate way to invert vibrational
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spacings into radial and angular potentials, but the practical reality is much more contrasted. I realized this later, when O. Roncero and co-workers constructed potential surfaces of the trimer HgAr2 from the inversion of data we had obtained on the Hg-Ar interaction; they needed an even greater precision than that obtained with our results. Then, I set up all the necessary equipment for such experiments, aimed at observing cold molecules and clusters: lasers, free jet expansion, and a molecular beam generously given by R. Campargue, then a great inventor at the CEA. Sydney Leach was the intercessor in this relation; I thank him for that. The detection of excited species was by emission of light, either phosphorescence in biacetyl or fluorescence in glyoxal. Once the tunability of the laser and its resolution were optimized, the excitation spectra appeared quickly, superb with textbook features on the nuclear spin statistics of the two hydrogen atoms of glyoxal. van der Waals complexes of glyoxal could easily be formed, and this led us to real dynamical studies. We made a flurry of complexes and then a student joined, who was full of intentions, ideas and energy, Christophe Jouvet. At the time, D. H. Levy et al. had studied the vibrational predissociation of iodine-He complexes by monitoring the excitation line width of the complex and its fragmentation channels. There energy, “locally deposited ”on iodine, was transferred by anharmonic coupling to the slow helium argon coordinate, thus leading to its rupture. Strict selection rules were observed with ∆V(I2) ) -1, whenever energetically allowed. J. A. Beswick and J. Jortner gave a simple and powerful quantum mechanical picture of this process, linking the rate of dissociation with an energy gap law. We had in mind with Tramer, to apply these results to polyatomic molecules, thus picturing in a different way vibrational energy deactivation by collisions. Glyoxal was a splendid molecule for dynamical studies in a polyatomic molecule, it was symmetric and had transitions in the visible range, and besides, we had a fair knowledge of its spectroscopy. This molecule was also especially attractive by its luminescent properties since it was in the small molecule limit for intersystem crossing that was induced by collisions and was not intramolecular. Glyoxal was lacking a sufficient level density for intersystem crossing, and clustering glyoxal with rare gases was a way to add a density of intermolecular modes. The most interesting was, however, the description in terms of half-collisions, used for molecular photodissociation. The molecular system was “dropped” by optical excitation onto an excited state half-collision (here glyoxal-argon), with an impact parameter close to zero. It was ready to predissociate on a 3Au triplet state, after excitation on a 1Au singlet state. Recording action spectra was a way to recognize the efficiency of some intermolecular argon-glyoxal modes in promoting the effect and to observe the effect of the collision geometry through the observation of different isomers of the complexes with different structures. Exciting the complex to a higher energy than the vibrational origin led us to observe competition between collisional intersystem crossing and collision induced vibrational relaxation. The experiment was at the edge, though with rather feeble lasers, and we preferred to pursue a simpler direction: intramolecular mode selective vibrational predissociation in glyoxal complexes. This was masterly accomplished by Nadine Halberstadt, who became after a full fledge theoretician. On that occasion, we discovered that, in the simple view of half-collision expressed above, we had forgotten an important relaxation partner, intramolecular vibrational redistribution, the companion of vibrational predissociation. If the separation of the partners was not sufficiently rapid
(as in glyoxal-argon) the local excitation on a vibronic mode could redistribute over the glyoxal modes, before it would flow into the glyoxal argon dissociation coordinate. We did a systematic study of complexes, and the system I preferred was the hydrogen-glyoxal complex where the dissociation was very rapid. We could identify selective dissociation modes preserving a minimum change in the vibrational quanta of the parent glyoxal. Besides, there were only glyoxal-(H2)1 complexes to be seen with a nice intermolecular vibrational structure that could be characterized by deuterium isotope effect. Transition State Studies in Complexes I realized that the half-collision approach had been applied to many molecular collision processes by the Chicago group. One essential process, chemical reaction had not been explored, except for photodissociation. Bimolecular processes had not been thought of in this way, which could bring a new dimension. The trend in reaction dynamics was on state to state experiments, where the reaction surface could be reconstructed from inverting the dynamics at the exit valley of reactions. Here, in contrast, as in transition state studies by J. C. Polanyi, one could access directly the transition state of reactions from electronically excited states. This would result from the absorption of the cold reagents prepared in the geometry of a van der Waals complex, to an electronically excited reaction surface. The electronic and vibrational structures observed from the cold complex on the excited state surface would reflect directly the potentials in the transitions state region together with dynamical information through the line width of transitions. I also realized that timeresolved experiments were possible where the swiftly traversed transition region could be observed in nascent pump/probe experiments, because, as opposed to collision experiments, a zero time could be given for the kinetics. I exposed these views to Christophe Jouvet and Andre´ Tramer. Experiments were not possible immediately since one essential element was missing: which system and what type of reaction! I was interested by addition reactions of chlorine atoms on ethylene where the twisting of the CdC bond was driven by the approach of the Cl atom and represented a coordinate to observe. There seemed, however, to be no way to detect the reaction products. A. Tramer suggested that we should examine chemiluminescent reactions of metal atoms since these reactions proceeded from charged transfer states and the products could easily be detected. These systems had great advantages: they proceeded at rather large distances determined by the covalent-ionic crossing of the surfaces. This was compatible with the long intermolecular separation of complexes ∼3 Å. Thus the transition region could be Franck-Condon illuminated from the ground unreactive state in chosen systems, in contrast to covalent reactions with short distances at the transition state. That was the beginning of a long story, but we were not ready until C. Jouvet had returned from a postdoctoral stay in Chicago, and we had made an essential technical leap forward in purchasing a Nd:YAG laser. Well-behaved technology can boost research! This allowed the construction of powerful dye lasers generating tunable UV. It took some time before we had success on Hg · · · Cl2* forming HgCl*. The Hg · · · Cl2 system is almost unreactive in the ground state with the Hg 6s2 closed valence shell, which then opens up in the 6s6p 3P states. There was a final experimental difficulty to solve: the complex should be formed from hot mercury and chlorine that could ultimately react within a millisecond in the given conditions. We designed, with the help of Philippe Ceraolo at the mechanical workshop, a fast mixer at the tip of the nozzle where the residence time would be minimal before expansion;
J. Phys. Chem. A, Vol. 114, No. 9, 2010 2959 however, the flow should be steady to avoid reactions in either feed gas tube. Success came on a quiet Saturday morning, in the fall of 1982 with a first action spectrum. A full action spectrum of the reaction was obtained and characterized soon. The spectrum, quasi continuous, could be described in a one-dimension Condon approximation by the projection of the Hg-Cl2 coordinate wave function (Gaussian), on a mixed covalent ionic transition region. This simple model described well the absorption and its cutoff and accounted for the mixed covalent ionic character of this region. Much later in a review paper with J-M. Mestdagh, we refined the description to include the angle dependent potential where the electron transfer occurs from the metal to the halogen only in a small angular range off 90° (owing to symmetry breaking). The new approach had thus been demonstrated but to be convincing demanded a benchmark example; C. Jouvet advised that it should involve hydrogen H2. With pump probe experiments success came quickly and C. J. was up in the air with these experiments. This led to a series of experiments with W. H. Breckenridge who came for sabbatical in Orsay. We all three had unending discussions on the dynamical implications of these experiments since W. H. Breckenridge had obtained great results on a dynamically similar system Mg + H2. There started a fruitful and longstanding friendship and collaboration on metal molecule complexes and reaction dynamics. It was a pleasure recently to publish a review on charge transfer in the gas phase with him. In the excitation spectra of the reactions driven in complexes, resonances were observed that represented intermolecular vibrations close to the transition region. Later, we also observed vibrations directly modified through the reaction. The experiments also revealed some features that we had not in mind at first time. The product Hg-H was detected by laser induced fluorescence; thus a complete vibrational and rotational energy distribution could be determined, from well specified initial locations of the potential energy surface, in the transition region. The second aspect was the orbital orientation, which was built in these experiments, since in presence of a perturber, the 6p orbital is no longer degenerate and leads to σ and π orientation with respect to the Hg-molecule axis and relates to different potential energies. Indeed, we observed two potential energy surfaces with different reactivities, i.e., reaction rates. Under the incentive of Jouvet, quantum chemical calculations were undertaken by Ph. Millie´ and A. Bernier, using two active electrons and a pseudopotential to represent the core effects. Calculations gave a nice confirmation of the reaction mechanism but did not inform on the possible selective branching from selective excited states. It was only later that, with Jean-Paul Visticot, we observed passive control of reactions by orbital orientation on the Ca-HBr system. These experiments, based upon collision observations of Rettner and Zare on Ca* + HCl, showed a selective preparation of the A2Π state from a π orientation of the 4p orbital of the calcium atom, while the B2Σ was formed from the σ orientation. At that time I met Ahmed Zewail in Oxford, at a Faraday discussion, and he proposed to me a visit in Pasadena asking for a project, I wrote a letter with some quick thoughts about possible experiments. Later he said, amused: how could I show your letter, handwritten in green ink, to the Dean! An offspring of such measurements was the observation of quasi-chemical bonding in excited complexes of mercury, with M. C. Duval. Mercury complexes were splendid systems to investigate, since there was already a wealth of experiments on sensitized reactions by gas phase mercury easily excited by
mercury lamps. In half-collisions only very few of the mercury systems investigated led to observable excited state reactions. Another decay channel was open, nonadiabatic relaxation would assist intramultiplet redistribution of locally excited mercury. This process efficiently competed with reactivity when the surfaces correlating with other multiplet states of mercury (3P0) were in the same range of Hg-molecule distances as the ground state complex, in the case of surface crossing. This favored this exit channel with respect to covalent reactive potentials crossing at much shorter distances ∼1-2 Å. However another situation was observed when a very deep Hg-molecule potential was accessed from the ground van der Waals state. A good example was found in Hg-NH3, where an extremely deep potential was accessed by excitation with a short equilibrium distance and modified emission properties. We could name this a quasichemical bond, with a Hg-N, 3 electron bond. There was no bond broken in the interaction, but a bond formed with an intermediate strength >1 eV and some characteristics of a chemical bond, a drastic change in the radiative lifetime. This was the result of a nice collaboration with T. Zwier, accomplished entirely by airmail. Time-Resolved Measurements in Clusters Thanks to J. Simons met at the stereodynamics initial workshop in Jerusalem, a joint application to a “Science” European grant was successful in 1989. This allowed our collaborating team with C. Jouvet and D. Solgadi to accomplish our dream of constructing an ultrashort laser, at that time subpicosecond. Since my early experiments, I had found that the time dependent approach to dynamical processes was powerful, where instead of measuring the forces between two interacting systems or within a single system, one could easily observe the kinematic effect of these forces. Specifically, the birth of a new chemical bond could be explored by subpicosecond lasers, as the group of Pr. Zewail had shown at Caltech in a van der Waals complex of CO2 with HI. For these experiments, in contrast, we chose systems where potentials were at longer distance, forces were weaker, and thus, the time stretch of the experiments could be longer and better adapted to the laser duration; nonetheless, the systems showed wavepacket motion along the soft coordinates. We also were supported by the visit of Pr. Qiu Peixia from the Shanghai Institute of Optics who was exceptionnal at running our subpicosecond laser homemade under the supervision of Y. Meyer and collaborators. I still have this masterpiece of equipment. I also acknowledge the gracious and generous help from Nguyen Dai Hung for running this complex picosecond laser. Pr. Hung now directs the Institute of Physics in Hanoi in a masterly way. Mercury complexes were ideal for these experiments and Lahouari Krim did splendid experiments, where, in addition to the real-time observation of the breaking of a van der Waals bond, he monitored irreversible and recurrent wavepacket movements corresponding to the slow van der Waals potentials and vibrations. This corresponds to observing the potential of the Hg-Ar system close to the dissociation limit. These experiments showed at the time, the complementarity between frequency and time-resolved observations: in transition state spectroscopy, high frequency vibrations of quasi-periodic motions in the vicinity of the transition region appear most readily, while low frequency movements in spectrally congested or inaccessible regions are best seen by time dependent experiments. In addition, time-resolved experiments allow observing non-Franck-Condon regions. We investigated several evolving
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mercury complexes, among which Hg-N2, where we saw the time signature of rotational resonances in the dissociation, which we had also observed spectroscopically. The system lingers in a quasi-bound state correlating to a rotational state of N2, before the anisotropy of the internal rotation of N2 kicks in at the inner turning point of the system out to a lower rotational state of N2, thus transferring rotational energy into translation. At the same time we saw the limits of the experiment in its actual form, designed to observe fluorescence or fluorescence excitation. This is limited to small molecules or atomic fragments, and if one wishes to unravel chemical mechanisms, a more universal detection has to be implemented that detects molecular systems, their mass, and eventually the change in the electronic distribution throughout a reaction path. Photoelectron spectroscopy already used by J. Syage seemed the ideal probe, since it senses not only the electronic distribution but also the vibrational distribution, indirectly via Franck-Condon distributions. Therefore we prepared for a new generation of experiments with electron and ion detection. A New Series of Transition State Studies Meanwhile, by 1987, we had undertaken with Jean-Paul Visticot and Arne Keller a new series of experiments on transition state spectroscopy with a promising system Ca* + HCl, in which beam-gas collision experiments by Rettner and Zare had shown the effect of orbital orientation. This was short after the Jerusalem workshop on stereodynamics named and organized by R. D. Levine, a real workshop with long discussions and a great occasion. I met many persons and discovered the field of alignment and orientation in collision processes as initiated by R. Bernstein. I met also C. Wittig who had done experiments similar to ours, but in the scope of prealigning the reactants and studied the reaction of H + CO2, from the CO2 · · · HCl complex. In addition, a very interesting system was mentioned during the meeting, the above-mentioned Ca* + HCl. There, aligned excited calcium atoms, colliding with HCl led preferentially to specific excited states of CaCl. The σ orientation of the 4p orbital in calcium with respect to the collision axis led, by orbital σ symmetry conservation, preferentially to the B2Σ state of CaCl and respectively in the π orientation to A2Π. As already mentioned, within the field of the van der Waals interaction, the Ca 4s4p atomic state splits into two components of Σ and Π symmetry, which are conveniently accessed at different energies, because Σ states are far less attractive. We set up a new source for Ca · · · HCl complexes based upon laser ablation, and this required long efforts but we were rewarded by splendid results. It proved interesting, beyond the aforementioned problem. We observed an intense vibrational structure that A. Keller cleverly analyzed as a progression of the Ca-HCl bend, merging into a quasi-free rotation of the HCl molecule. We made isotopic D isotope substitutions and hindered rotor simulations and probed several similar systems. J.-P. Visticot simulated the spectra by wavepacket calculations, using a realistic model potential. Calculations reproduced well the observations for the line positions and their widths. It substantiated the dynamical interpretation we had: the more the bending vibration was excited, the faster the system passed in the vicinity of the transition region, then missing it. Indeed, the width of the lines in the action spectra decreased with increasing vibrational excitation. I then met D. Neumark at a meeting in California, who had done similar experiments using negative ions instead of clusters as base complexes, to step onto the reaction surface. This experimental approach had the advantage
of shorter initial internuclear separations in negative ions that could also reach, by electron photodetachement, ground state neutral reactions. We have since maintained a longstanding and fruitful contact, through visits on both sides. A side interest of these experiments with metals and various oxidants was the production of radicals with interesting spectroscopic properties. In brief, the electronic excitation of these radicals is that of the calcium ion; therefore the excited states retain some local properties of this ion, angular momentum for example in a polyatomic radical. Two persons have done some marvelous work: Christopher Whitham postdoctoral fellow from J. Simons and R. Ławruszczuk, a student from Gdansk. More Complex Systems, Transition Metal Complexes We have initiated with Satchin Soorkia a new type of study where the full dimension of orbital selection in the van der Waals reaction could be used and we have thus selected transition metals as the chromophores, for their electronic diversity. In transition metals, internal d orbitals are close in dimension and in energy to the valence s outer shell, resulting in their well-known catalytic properties. We selected zirconium for its apparent simplicity and methyl fluoride as the reactant and we discovered a new world of electronic configurations. There were difficulties of all kinds, experimental with the difficult evaporation and the flurry of excited state of the product zirconium fluoride. We had very interesting results by using a depopulation detection method introduced by J. C. Polanyi, that comfronted two ideas: (1) there are reaction barriers in probably all channels of Zr + CH3F neutral ground and excited states and (2) these barriers can be overcome by electronic relaxation that places the reacting system high in energy above a lower lying electronic configuration. This hypothesis had already been brought by A. Gonzale`s-Uren˜a, but recently J.-M. Mestdagh and C. Sanz-Sanz showed unambiguously in calculations on the Ca + CH3F system, that this was the only way to bypass excited state barriers. Therefore the present conclusion is that orbital selectivity is partially lost by pervasive electronic relaxation, but facing the situation positively it is likely that in zirconium one reaches indirectly by relaxation the desired reactive and metastable electronic configuration 4d35s1, instead of reaching it directly by excitation. Also S. Soorkia was keen on pursuing the splendid spectroscopy of the radical product ZrF, revealing the delicate intricacy of 4d and 5s orbitals, well pictured by the ligand field model of R. W. Field. At this occasion the real clarification was given by the amazing high level calculations of J. Lievin who could show the hierarchy of the low lying electronic levels of the radical. Finally, I would like to mention that reaction dynamics in gas phase complexes should interestingly apply to interfaces where two nonmiscible reactants can be united, in the same way J.-C. Polanyi studied oxidants deposited at the surface of metals. A New Round of Time Dependent Studies at Laboratoire Francis Perrin It appeared that we could develop a new strategy for detecting short-lived transients in gas phase bimolecular collisions, using photoelectron spectroscopy. In this way we studied the dimer of azaindole. However, the initial intention (since 1992) was to probe the evolution of the electronic distribution of the reagents toward the products by monitoring with VUV photons, the photoelectron spectra. High energy, VUV photon electron can give a picture of the valence structure of the neutral and the
J. Phys. Chem. A, Vol. 114, No. 9, 2010 2961 corresponding ion along the reaction coordinate. This dreamed perspective seemed too difficult to implement and we limited our ambitions to 0-2 eV photoelectrons, which yield far less chemical information but are very sensitive to electronically excited state structures. This directed our efforts to nonadiabatic relaxation as with probably many groups in the field. In 2000 J.-P. Visticot announced that his department at CEA was creating a new joint structure CNRS/CEA and asked me to join this lab, named later Francis Perrin. I had enjoyed such fruitful relationship with him and J.-M. Mestdagh that I did not hesitate long before accepting. This also offered interesting conditions. I moved in 2001 and this has been a wonderful time since then. I specifically enjoy the discussions we have any time on a variety of subjects. At these occasions, we can address all scientific problems from the most practical to the most conceptual. The occasion is often lunchtime that I rarely miss. Besides, the group now directed by J.-M. Mestdagh, works on a very collective basis. This eases the development of subjects and favors the emulation for the young students. We really can try new ideas and projects. It has been since then a very productive time dedicated to time-resolved experiments and also to new, more elaborate experiments on transition state spectroscopy with more complex molecules and metals (transition metals). We could compare for the same chemical system Ca · · · CH3F, the free complex reaction and the same reaction deposited on large argon clusters as done by M.-A. Gaveau. We had tackled, by 1998, the dynamics of excited ethylene compounds at the femtosecond facility in Saclay, and to our surprise, it was extremely fast; still the electronic state description of ethylene lacked clarity. In the absence of a comprehensive picture of the excited state landscape, we stuck with Mulliken’s intuitions and views. I met J. Jortner, visiting Orsay, who immediately pointed to wavepacket motion through a conical intersection for the very rapid decay observed. This rationalized ours results, since the decay rates were correlated with the inertia of the moving chemical groups, thus to a true wavepacket motion. Then, with the Ph.D. students S. Sorgues and E. Gloaguen we used photelectron detection and electron (velocity map) imaging to follow the electronic decay dynamics of a very interesting system, a highly substituted ethylene allowing the formation of charge transfer states as in Mulliken’s views. My long known friend D. Parker and his colleague A. Eppink, from Nijmegen, guided us through the installation of their velocity map detection system that is ideal for timeresolved photoelectron spectroscopy with 100% collection efficiency and angular information on the electron distribution. I must say that we almost copied exactly their design, nut by nut and it gave immediate and conclusive results. Finally, concerning ethylene, the key question was the exact role of low lying Rydberg states; it is only in the recent calculations of Lischka that the size and energy of the ππ* and 3s orbitals are reasonably accounted for. I like our work on these substituted ethylenes because we can give experimental evidence for the mediation of Rydberg sates in the electronic relaxation, from the photoelectron fingerprint. With the position of Lionel Poisson at CNRS after his postdoctoral stay at Berkeley, the femtosecond experiment has taken a new dimension and several new themes have been developed. Among them, photoelectron spectroscopy at the surface of large clusters (argon, etc.) is starting to flourish with,
for example, the time-resolved solvation of Rydberg atoms and molecules at the surface of these clusters, a situation fundamentally different from the behavior in the bulk. Here, the Rydberg orbitals are free to quickly “adapt” to the surface through evolution in their many nlλ states and then, at picosecond times, the atoms rearrange at the surface. This type of experiment is aimed at studying some aspects of the conditions of solvation in condensed matter, since at the surface of the cluster several solvent molecules can be independently added to reactants. I am presently keeping an interest on nonadiabatic relaxation, especially on intersystem crossing in organic molecules, a subject that deserves revisiting by ultrafast observations, especially to monitor the energy flow in their higher triplet levels, irrigating the lowest reactive triplet state. This should be complemented by “ultraslow” experimentation to monitor directly the vibronic structure of such organic triplet states, in view of the spectroscopy of reacting triplet organic clusters. Also I have been collaborating with Niloufar Shafizadeh (whom I was fortunate to meet 20 years ago), for ten years on her investigations on the dynamics of large biomimetic metalloporphyrins, in the gas phase. This has recently led to interesting and unexpected observations on Hemin (the Heme building block) in the gas phase. Although we do not foresee the passage of blood cells to the gas phase, we should be able to produce significant subsystems by a new evaporation source. This will allow the observation of large and fragile such porphyrins with tailored protein substituents and ligands, to monitor the departure of ligands from these species and characterize their ligation energetics, largely known with a modest precision. The application of physical chemistry to specific local properties whenever possible of model biomolecules is an interesting emerging field. I am delighted to have been offered the opportunity to convey some impressions and some facts of my scientific life and acknowledge some of my friends. The achievements would not have been possible without the students and postdocs always excited and motivated whom you are happy to see ahead of you by the end of the thesis. Also, no experimental research exists without the inventive and dedicated people of the mechanical and electronical workshops and I thank here heartily Philippe Ceraolo, Fabien Lepetit, Christophe Pothier, and Marc Hilaire as well as “Bibi” Etcheverry. This life has been intertwined with my family life, the pleasure I share with Niloufar, my wife, to evoke any subject and my children. Moussou, our young son asked when he was 5, “Dad what kind of work are you doing?” “Well, I am teasing molecules, plucking electrons from them, and then I watch them how they react; these fowls do not like it very much and shake in all directions.” “Poor them,” he commented! I was also delighted by a recent and unexpected question at the telephone by my artist daughter living in China: “What are exactly quantum mechanics?” I liked to say that it was a different world from ours that should be seen with different, wavy eyes, ... but you all have the perfect answer to this still timely question!
Benoît Soep CNRS, Laboratoire Francis Perrin JP1001594
