Jean-Michel Mestdagh

Gas-Phase Dynamics of Spiropyran and Spirooxazine Molecules

The gas-phase dynamics of two classes of photochromic molecules, three spiropyrans and one spirooxazine, have been investigated here using both time-resolved mass spectrometry and photoelectron spectroscopy approaches. It is, to our knowledge, the first gas-phase experiment done of these kinds of molecules. The molecules are excited at 266 nm and probed at 800 nm. The comparison of the dynamics of these four molecules has been used to propose a sequential photoisomerization mechanism involving four steps occurring in the first 100 ps. Each of these steps is discussed and related to the observed condensed-phase dynamics and to theoretical calculations.

Two-electron pseudopotential investigation of the electronic structure of the CaAr molecule

The electronic structure of the Ca-Ar molecule is investigated using [Ca2+] and [Ar] core pseudopotentials complemented by core polarization operators on both atoms, considering the molecule to be a two-electron system. The electronic two-body problem is solved by achieving a full configuration interaction with extensive Gaussian basis sets. The potential energy curves and the molecular constants of all CaAr states dissociating into atomic configurations ranging between the ground state 4s2 1S and the doubly excited state 4p2 3P are determined. Spin–orbit coupling is also included in an atom-in-molecule scheme for states dissociating into the 4s4p and 4s3d configurations. The present theoretical results show good overall agreement with experimental data. They also help to clarify the very complicated spectroscopy of the CaAr system in the 38 000 cm−1 energy range where many states correlated with the 4s4d, 3d4p, and 4p2 atomic configurations interact with or cross one another. As a by-product of the prese...

Reaction geometry from orbital alignment dependence of ion pair production in crossed‐beam Ba(1P1)–Br2 reactions

Strong orbital alignment dependence was observed for Ba+ produced in crossed‐beam reaction of Ba(1P1) with Br2. The peak of this dependence varied strongly with scattering angle for alignment of the p orbital in the scattering plane, with the maximum flux seen for perpendicular alignment with respect to the relative velocity vector. The measured Ba+ was always favored by alignment of the orbital in the scattering plane, regardless of laboratory scattering angle. The experimental results suggest that this charge‐transfer process is dominated by large impact‐parameter collisions which achieve collinear nuclear geometry and Σ orbital alignment at the crossing point. Orbital locking is probably not important owing to the large internuclear distance of the crossing region.

Gas phase dynamics of triplet formation in benzophenone

Benzophenone is a prototype molecule for photochemistry in the triplet state through its high triplet yield and reactivity. We have investigated its dynamics of triplet formation under the isolated gas phase conditions via femtosecond and nanosecond time resolved photoelectron spectroscopy. This represents the complete evolution from the excitation in S2 to the final decay of T1 to the ground state S0. We have found that the triplet formation can be described almost as a direct process in preparing T1, the lowest reacting triplet state, from the S1 state after S2 → S1 internal conversion. The molecule was also deposited by a pick-up technique on cold argon clusters providing a soft relaxation medium without evaporation of the molecule and the mechanism was identical. This cluster technique is a model for medium influenced electronic relaxation and provides a continuous transition from the isolated gas phase to the relaxation dynamics in solution.

Unusual Quantum Interference in the S1 State of DABCO and Observation of Intramolecular Vibrational Redistribution

In this paper we report an experimental study of the time-resolved response of the molecule 1,4-diazabicyclo[2.2.2]octane (DABCO) to 266.3 nm electronic excitation of the S(1) state with a femtosecond laser. Rotational decoherence and vibrational oscillation within the S(1) state are observed. We performed state-of-the-art ab initio calculations on the ground and low electronic states of the neutral molecule and the cation, which assist in the assignment of the observed photoelectron signals. Using our theoretical and spectroscopic data, the experimental findings are interpreted in terms of an unusual quantum interference between two different vibrational modes, with only the nu = 1 level of each mode being populated.

Ultrafast Dynamics of Acetylacetone (2,4-Pentanedione) in the S2 State

The dynamics of the enolic form of acetylacetone (E-AcAc) was investigated using a femtosecond pump-probe experiment. The pump at 266 nm excited E-AcAc in the first bright state, S2(pi pi*). The resulting dynamics was probed by multiphoton ionization at 800 nm. It was investigated for 80 ps on the S2(pi pi*) and S1(n pi*) potential energy surfaces. An important step is the transfer from S2 to S1 that occurs with a time constant of 1.4 +/- 0.2 ps. Before, the system had left the excitation region in 70 +/- 10 fs. An intermediate step was identified when E-AcAc traveled on the S2 surface. Likely, it corresponds to an accidental resonance in the detection scheme that is met along this path. More importantly, some clues are given that an intramolecular vibrational energy relaxation is observed, which transfers excess vibrational energy from the enolic group O-H to the other modes of the molecule. The present multistep evolution of excited E-AcAc probably also describes, at least qualitatively, the dynamics of other electronically excited beta-diketones.

Direct mapping of recoil in the ion-pair dissociation of molecular oxygen by a femtosecond depletion method

Time-resolved dynamics of the photodissociation of molecular oxygen, O(2), via the (3)Sigma(u) (-) ion-pair state have been studied with femtosecond time resolution using a pump-probe scheme in combination with velocity map imaging of the resulting O(+) and O(-) ions. The fourth harmonic of a femtosecond titanium-sapphire (Ti:sapphire) laser (lambda approximately 205 nm) was found to cause three-photon pumping of O(2) to a level at 18.1 eV. The parallel character of the observed O(+) and O(-) images allowed us to conclude that dissociation takes place on the (3)Sigma(u) (-) ion-pair state. The 815 nm fundamental of the Ti:sapphire laser used as probe was found to cause two-photon electron photodetachment starting from the O(2) ion-pair state, giving rise to (O((3)P)+O(+)((4)S)) products. This was revealed by the observed depletion of the yield of the O(-) anion and the appearance of a new O(+) cation signal with a kinetic energy E(transl)(O(+)) dependent on the time delay between the pump and probe lasers. This time-delay dependence of the dissociation dynamics on the ion-pair state has also been simulated, and the experimental and simulated results coincide very well over the experimental delay-time interval from about 130 fs to 20 ps where two- or one-photon photodetachment takes place, corresponding to a change in the R(O(+),O(-)) interatomic distance from 12 to about 900 A. This is one of the first implementations of a depletion scheme in femtosecond pump-probe experiments which could prove to be quite versatile and applicable to many femtosecond time-scale experiments.

Reactions of laser-ablated zirconium atoms within a supersonic expansion: insertion versus radical mechanism.

In a laser ablation type microreactor followed by supersonic expansion, zirconium atoms have been reacted with methyl fluoride, CH(3)F (MeF), and mixtures of MeF and dimethylether, CH(3)-O-CH(3) (DME) seeded in He. With both mixtures, only a number of simple fluorinated products are formed, and they have been identified by one-photon ionization. All products can be linked to radical reactions either with F atoms, CH(3), or ZrF(1, 2, 3) radicals. No insertion products of the Grignard reagent type, F -Zr-CH(3) could be identified with or in the absence of DME. On the other hand, evidence has been found for the presence of organometallic compounds of the type ZrC(2)H(n=2, 4, 6), which could result from radical attack. Thus, even in conditions where intense solvation is at work, induced by clustering with polar DME molecules, which can act as stabilizing agents, a direct insertion mechanism into the C-F bond involving barrier suppression is not at work in our conditions. The reactivity due to radicals is very effective in this type of reactor, and the products that are efficiently formed can be quickly stabilized in the expansion. The radical attack supersedes, in the case of zirconium solvated by DME, the metastable mechanism with Zr(4d)(3)(5s)(1), that is certainly energetically impossible in the absence of strong reaction barrier suppression by a solvent. High level ab initio calculations performed at the CASPT2 level of theory are used for characterizing the electronic and geometric structure of the inserted products. They also reveal striking features of the reaction mechanism that support the absence of observation of inserted products within solvated clusters of zirconium.

Probing several structures of Fe(H2O)n+ and Co(H2O)n+ (n=1,...,10) cluster ions

Co(H2O)n≤10+ and Fe(H2O)n≤10+ cluster ions were generated in a source combining laser ablation and a supersonic expansion. The clusters were fragmented to get insight into their structure. Two questions were addressed: first, the arrangement of the water molecules about the metal ion, and second, the electronic properties of the solvated metal ion. Collision induced dissociation by helium was used to answer the first question, especially for the smallest clusters with n=2 and 3. This revealed the existence of filament structures where one water molecule lies in the second solvation shell about the metal ion although the first shell is not filled. The binding energies of second shell water in Co(H2O)2+ and Fe(H2O)2+ are 0.45±0.1 and 0.5±0.1 eV, respectively. The answer to the second question was provided by photofragmentation experiments where the cluster ions are illuminated at 532, 355 and 266 nm. The most striking effect is seen with cobalt ions where increasing the number n of water molecules above n=7 allows one to built up an absorption band that is known when Co+ is solvated in liquid water. The two fragmentation techniques appear as complementary.

Photochemistry of acetylacetone isolated in parahydrogen matrices upon 266 nm irradiation

The photochemistry of the chelated enol form of acetylacetone (AcAc) was investigated by UV excitation of the S(2) state at 266 nm in parahydrogen matrices, complemented by experiments in neon and normal hydrogen matrices. Infrared (IR) spectroscopy, combined with theoretical calculations, was used to identify the photoproducts. Isomerization towards various non-chelated forms (no intramolecular H-bond) of AcAc is the dominant channel whereas fragmentation is very minor. The isomerization kinetics is monitored by IR spectroscopy. Among the seven non-chelated conformers of AcAc, only three are formed in parahydrogen matrices, whereas four are observed in normal hydrogen matrices. This difference suggests that an active tunnelling process between conformers occurs in parahydrogen but is quenched in normal hydrogen where guest-host interactions are stronger. Fragmentation and isomerization of excited AcAc are discussed in the light of these new data. The role of the intermediate triplet state in the S(2)→ S(0) relaxation is confirmed, as the importance of phonons in the condensed phase.