M. Briant

Time resolved observation of the solvation dynamics of a Rydberg excited molecule deposited on an argon cluster-I: DABCO☆ at short times

This paper is a joint experimental and theoretical approach concerning a molecule deposited on a large argon cluster. The spectroscopy and the dynamics of the deposited molecule are measured using the photoelectron spectroscopy. The absorption spectrum of the deposited molecule shows two solvation sites populated in the ground state. The combined dynamics reveals that the population ratio of the two sites is reversed when the molecule is electronically excited. This work provides the timescale of the corresponding solvation dynamics. Theoretical calculation supports the interpretation. More generally, close examination of the short time dynamics (0-6 ps) of DABCO···Ar(n) gives insights into the ultrafast relaxation dynamics of molecules deposited at interfaces and provides hence the time scale for deposited molecules to adapt to their neighborhoods.

Spectroscopy and dynamics of calcium dimers deposited on large argon and neon van der Waals clusters

Laser-induced-fluorescence studies of calcium dimer deposited on large argon and neon clusters have been performed. The spectroscopy of the Ca2 A state is slightly perturbed by the cluster surface leading to shifts and broadenings of the order or less than 100 cm−1. An absorption has been evidenced in the 530–550 nm wavelength range that is tentatively assigned to the yet undocumented A ′1Πu state of Ca2 correlating to the Ca(1D)+Ca(1S) asymptotic limit. The excited calcium dimer dynamics are very different in neon and argon clusters. The argon cluster is much more efficient for electronic and vibrational relaxation of the excited dimer. Finally, excitation in the blue of the calcium atomic resonance line leads to a competition between dissociation of the dimer with ejection of an excited calcium atom out of the cluster and the relaxation of the dimer to lower excited levels.

Photoionization of Benzophenone in the Gas Phase: Theory and Experiment

We report on the single photoionization of jet-cooled benzophenone using a tunable source of VUV synchrotron radiation coupled with a photoion/photoelectron coincidence acquisition device. The assignment and the interpretation of the spectra are based on a characterization by ab initio and density functional theory calculations of the geometry and of the electronic states of the cation. The absence of structures in the slow photoelectron spectrum is explained by a congestion of the spectrum due to the dense vibrational progressions of the very low frequency torsional mode in the cation either in pure form or in combination bands. Also a high density of electronic states has been found in the cation. Presently, we estimate the experimental adiabatic and vertical ionization energy of benzophenone at 8.80 ± 0.01 and 8.878 ± 0.005 eV, respectively. The ionization energy as well as the energies of the excited states are compared to the calculated ones.

Fluorescence emission of Ca-atom from photodissociated Ca2 in Ar doped helium droplets. II. Theoretical.

The Ca(2) → Ca(4s4p(1)P) + Ca(4s(2)(1)S) photodissociation was investigated in a He droplet isolation experiment where the droplets are doped by Ar atoms. Fluorescence spectra associated with the Ca(4s4p(1)P → 4s(2)(1)S) emission were recorded as a function of the average number of Ar atoms per droplet. Three contributions were observed depending on whether the emitting Ca atoms are free, bound to helium atoms or bound to argon atoms. Moreover, the full Ca(4s4p(1)P → 4s(2)(1)S) fluorescence emission was recorded as a function of the wavelength of the photodissociation laser, hence providing the action spectrum of the Ca(2) → Ca(4s4p(1)P) + Ca(4s(2)(1)S) process. The latter spectrum suggests that in He droplets doped by argon, Ca atoms are attracted inside the droplet where they associate as Ca(2). Full analysis of the spectra indicate that the emission of Ca bound to a single Ar atom is redshifted by 94 cm(-1) with respect to the emission of free Ca.

Observation of a barium xenon exciplex within a large argon cluster

Spectroscopic measurements provide fluorescence and excitation spectra of a single barium atom codeposited with xenon atoms on argon clusters of average size approximately 2000. The spectra are studied as a function of the number of xenon atoms per cluster. The excitation spectrum with approximately 10 xenon atoms per cluster is qualitatively similar to that observed when no xenon atom is present on the cluster. It consists of two bands located on each side of the 6s6p (1)P-6s(2) (1)S resonance line of the free barium. In contrast, the fluorescence spectrum differs qualitatively since a barium-xenon exciplex is observed, which has no counterpart in xenon free clusters. In particular an emission is observed, which is redshifted by 729 cm(-1) with respect to the Ba(6s6p (1)P-6s(2) (1)S) resonance line.

Excited state reactions of metals on clusters: Full dynamics of the Ca*+HBr reaction on Ar2000

We report on the Ca*+HBr→CaBr*+H reaction when photoinduced within a Ca⋯HBr complex that is deposited at the surface of a large argon cluster (surface complex). The excitation that turns on the reaction is localized on the calcium atom. Information on the dynamics of the reaction is provided by observing the CaBr fluorescence while scanning the excitation laser across the calcium resonance line. This provides information on the access to the transition region of the reaction and helps to clarify how the argon cluster influences this access as compared to the gas phase experiment where the Ca⋯HBr complex is free (free complex). Chemiluminescence spectra were also recorded to characterize the output channel of the reaction. Not surprisingly, the presence of the cluster affects the dynamics of the reaction that proceeds at its surface. Several effects have been identified. Depending on which potential energy surface of the Ca⋯HBr complex is excited by the laser, the cluster acts passively or actively. When t...

Spectroscopy of the Ca Dimer on Argon and Helium Clusters by Laser Induced Fluorescence at 380 nm

Laser‐induced‐fluorescence studies of calcium dimer deposited on argon and helium clusters have been performed between 365 and 385 nm. Only the fluorescence of Ca(1P) has been observed both on argon and helium clusters, giving an evidence of the photodissociation of Ca2 after excitation. On argon clusters, a competition exists between ejection and solvation of Ca*: the emission spectrum exhibits the free Ca(1P←1S) line and a band on its red side assigned to solvated Ca* emission, while the latter is lacking for helium clusters. Excitation spectra of Ca(1P) fluorescence due to Ca2 photodissociation were recorded on argon and helium clusters. Both spectra present a fairly large band with a small side band blue shifted by 550 cm−1. The main band peaks at 26400 cm−1 for argon clusters and at 26520 cm−1 for helium clusters. These results can be interpreted in terms of the photoexcitation of Ca2 from its ground state up to two excited electronic states (calculated): A diexcited state corresponding to the small ...

Laser Induced Fluorescence Spectroscopy of the Ca Dimer Deposited on Helium and Mixed Helium/Argon Clusters

We study the laser induced fluorescence spectroscopy of the calcium dimer deposited on helium and mixed helium/argon clusters. In the wavelength range between 365 and 385 nm, the Ca dimer is excited from its ground state up to two excited electronic states leading to its photodissociation in Ca(1P)+Ca(1S): this process is monitored by recording the Ca(1P) fluorescence about 422.7 nm. These electronic excited states of Ca2 are respectively a diexcited one correlating to the Ca(4s 4p 3P)+Ca(4s 3d 3D) and a repulsive one correlating diabatically to the Ca(4s 4p 1P)+Ca(4s2 1S) asymptote, accounting for the dissociation of Ca2 and the observation of the subsequent Ca(1P) emission. On pure helium clusters, the fluorescence consists of the calcium atomic resonance line Ca(1S←1P) at 422.7 nm (23652 cm−1) assigned to ejected calcium, and a narrow red sided band corresponding to calcium that remains solvated on the helium cluster. Interestingly, the branching ratio to the ejection of Ca(1P) increases along with the...

Tribute to Jean-Michel Mestdagh.

A being tentatively seduced by astrophysics, Jean-Michel Mestdagh entered in the mid-1970s the realm of atomic collisions in the golden era of crossed beam studies, studying inelastic atomic collisions. Maybe his research direction was influenced by his passion, mountaineering. Indeed, Jean-Michel Mestdagh likes the summits, in many domains. He loves topological problems like climbing up to saddle points and in general potential energy surfaces. His scientific interests have naturally turned toward reaction dynamics. Reaction dynamics was an entirely new way to envision chemistry as Dudley Herschbach, John Polanyi, R. N. Zare, Yuan T. Lee, and many others imagined, through the examination of the forces that drive reactants to products over potential energy surfaces. It was certainly beyond the thermodynamic age, and the interaction between reactants could be directly probed through the recoil of the reaction fragments. The typical reactive systems were metal atoms with halogen molecules, and one could visualize the hooking of the metal by an electron jump to the halogen. This approach was generalized through the 60s by the success of the studies in crossed beam collisions describing the ballistics of the single reactive encounter. When Jean-Michel undertook postdoctoral research with Y. T. Lee at Berkeley in 1983−1984, the equivalent dynamics of reactions involving excited state species had just begun with the use of lasers; this was the opening of a new domain of investigation. Returning to Saclay, new molecular beam machines were constructed in the group of J. Berlande, and Jean-Michel was very active in the think tank of reaction dynamics in France initiated by J. P. Lehman and Raymond Vetter, the GdR “ Dynamique reáctionnelle”. Meanwhile he established many successful international collaborations with Y. Lee, A. Suits, J. Frey, and others. Then, combined with the interest of high-intensity laser physicists toward femtosecond lasers at the CEA, Jean-Michel and Jean-Paul Visticot saw a fantastic opportunity to have a facility fully dedicated to femtochemistry, the time-dependent view of reaction dynamics initiated in the 80s by A. H. Zewail, R. Hochstrasser, and others. This has led to the European facility “SLIC” (Saclay Laser-matter Interaction Center) that has hosted many fruitful experiments. Another innovative approach of reaction dynamics was developed together with Jean-Paul Visticot by Jean-Michel, in which large argon clusters could be used as nanoreactors to support chemical reactions at their surface. Compared with crossed beam experiments, clusters increase by several orders of magnitude the reaction cross-section as determined by their capture size. Thus, low-temperature chemiluminescence reactions were investigated on the epitomic electronically excited Ba+N2O system. This was the first on a series of new and successful experiments. When Jean-Michel took the lead of the reaction dynamics group in Saclay, he decided to leave the experimental side of his research only to advising. Being attracted by theoretical concepts and the need to model the experimental results, he naturally became a quantum chemist. Besides, this situation is the best for theoretical modeling since as an experimental actor he knew all the details of the experiments and their pitfalls and strengths. This situation provides a more intimate comprehension of phenomena. In some ways, as a piano player, JeanMichel anticipated the “keyboard-chemistry” age: Why perform unnecessary experiments if you can model them?! He thinks you cannot get away without both experiment and theory. Lately, following the redeployment of the research in chemical physics (and physical sciences) of the Orsay and Saclay campuses, he has been intimately involved in the organization of the future departments of the now “Paris-Saclay University”. He has struggled to keep effective links between the chemical and the physical departments. To anyone who has met Jean-Michel, his prodigious interest in understanding the scientific world is obvious, whatever the topic. He is keen at deciphering any problem, experimental, theoretical, or mathematical. It is always enriching to discuss with him confusing problems. Besides, often he will bring out the solution, as if you would have come to it by yourself. This is the sign of his unselfishness and open generosity. Scientists are actors of their research and have their egos. One must remember that the organization of research that

Reactivity of Ba and Ca atoms with N2O molecules deposited on van der Waals clusters and helium droplets

We have studied the chemical reactivity of the alkaline earth metal atoms Ba and Ca with N2O deposited on van der Waals (argon, nitrogen, methane, and neon clusters) in the case of Ba and on argon and helium clusters in the case of Ca. The first reaction forms BaO from 1 Ba and 1 N2O except at high Ba coverage where another channel is open yielding Ba2O. With Ca, the reaction 1:1 is hindered and the chemiluminescence comes from the reaction between 2 Ca and 2 N2O forming 2 CaO molecules.