Christophe Jouvet

Excited-state hydrogen detachment and hydrogen transfer driven by repulsive 1πσ* states: A new paradigm for nonradiative decay in aromatic biomolecules

The combined results of ab initio electronic-structure calculations and spectroscopic investigations of jet-cooled molecules and clusters provide strong evidence of a surprisingly simple and general mechanistic picture of the nonradiative decay of biomolecules such as nucleic bases and aromatic amino acids. The key role in this picture is played by excited singlet states of πσ* character, which have repulsive potential-energy functions with respect to the stretching of OH or NH bonds. The 1πσ* potential-energy functions intersect not only the bound potential-energy functions of the 1ππ* excited states, but also that of the electronic ground state. Via predissociation of the 1ππ* states and a conical intersection with the ground state, the 1πσ* states trigger an ultrafast internal-conversion process, which is essential for the photostability of biomolecules. In protic solvents, the 1πσ* states promote a hydrogen-transfer process from the chromophore to the solvent. Calculations for chromophore–water clusters have shown that a spontaneous charge-separation process takes place in the solvent shell, yielding a microsolvated hydronium cation and a microsolvated electron. These results suggest that the basic mechanisms of the complex photochemistry of biomolecules in liquid water can be revealed by experimental and theoretical investigations of relatively small chromophore–water clusters.

Orbitally selective chemical reaction in Hg–H2 van der Waals complexes

A new technique is described for probing the reaction dynamics of ‘‘half‐collisions’’ in systems where ‘‘full‐collision’’ chemical dynamics can also be studied. By selective laser excitation of an atom–molecule van der Waals complex, an electronically excited atom can be created at a known distance from, and with a known orbital symmetry with respect to, the reactive molecule. From spectra of the complex and from detection of nascent products in a state‐resolved fashion, not only can a great deal be learned about the dynamics of the half‐collision, but comparison can also be made with analogous full‐collision dynamical information. Reported here are initial results involving the Hg⋅H2 van der Waals complex. When the Hg (6s 1S0)⋅H2 ground‐state complex is excited to the Hg(6p 3P1)⋅H2 complex with frequencies near that of the Hg(6 1S0–6 3P1) free atom transition, the molecular product HgH(X 2Σ+) is readily detected. No fluorescence of the Hg(6p 3P1)⋅H2 complex is observed, nor is Hg(6p 3P0) detected as a ma...

The structure of several electronic states of the Hg–Ar complex as determined by laser double resonance in a supersonic jet

The electronic states of the Hg–Ar complex have been studied in a supersonic free jet expansion using a laser double resonance technique. Detailed observations were made of the states correlating with the mercury 6 3P levels 3P0, 3P1, and 3P2. The states correlating with the 3P0 and 3P2 metastable states had not been studied previously since they are optically inaccessible from the ground state. A model was developed which accounts for the structures of the various states. The binding energies can be related simply to the average orientation of the 6p mercury orbital with respect to the internuclear axis. In addition, the Hg(7 3S1)–Ar Rydberg state was reinvestigated and shown conclusively to exhibit a double minimum potential, with a deep well similar to the Hg–Ar+ ion and a shallow van der Waals minimum at larger internuclear distances.

Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: ππ*-πσ* energy gap propensity and ortho-substitution effect.

Lifetimes of the first electronic excited state (S(1)) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 0(0) transition and for the intermolecular stretching σ(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S(1) (ππ*) and S(2) (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S(1) optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a S(1)/S(2) conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the ππ* and πσ* states occurring through the out-of-plane distortion of the C-F bond.

Ultrafast deactivation mechanisms of protonated aromatic amino acids following UV excitation

Deactivation pathways of electronically excited states have been investigated in three protonated aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe). The protonated amino acids were generated by electrospray and excited with a 266 nm femtosecond laser, the subsequent decay of the excited states being monitored through fragmentation of the ions induced and/or enhanced by another femtosecond pulse at 800 nm. The excited state of TrpH+ decays in 380 fs and gives rise to two channels: hydrogen atom dissociation or internal conversion (IC). In TyrH, the decay is slowed down to 22.3 ps and the fragmentation efficiency of PheH+ is so low that the decay cannot be measured with the available laser. The variation of the excited state lifetime between TrpH+ and TyrH+ can be ascribed to energy differences between the dissociative pi sigma* state and the initially excited pi pi* state.

Internal conversion in highly excited benzene and benzene dimer: femtosecond time-resolved photoelectron spectroscopy

Abstract Photoelectron spectra of benzene molecules and dimers excited to the S2 electronic state by 140 fs laser pulses at a wavelength of 200 nm have been measured by combining the pump–probe technique with the coincidence detection of ions and electrons. Their time dependence allows one to directly follow the evolution of internal conversion processes in different electronic states on a fs time-scale. Analysis of the corresponding electron energy distribution reveals large variations in the geometry of the different electronic states in both benzene and in its dimer.

Photochemistry in van der Waals complexes: Observation of the intermediate state of the Hg*,Cl2 reaction

Abstract A novel method designed to observe the collision complex of a photochemical reaction is reported here. The reactants Hg, Cl 2 are frozen in a van der Waals complex (HgCl 2 ), and then promoted by an optical excitation (250 nm) to the reactive state. The broad complex action spectrum, presumably due to the Hg + -Cl 2 − intermediate, is monitored through the HgCl(B 2 Σ + ) fluorescence.

Intracluster hydrogen transfer followed by dissociation in the phenol–(NH3)3 excited state: PhOH(S1)–(NH3)3→PhO•+(NH4)(NH3)2

The study of the phenol–(NH3)3 cluster with two-color two-photon ionization shows that the main ion observed with delays between the lasers up to a few hundred nanoseconds is the (NH4)+(NH3)2 fragment, resulting from direct ionization of the (NH4)(NH3)2 product coming from the reaction: PhOH(S1)–(NH3)3→PhO•+(NH4)(NH3)2.

Experimental study of the cold mercury dimer

The mercury dimer is among the most weakly bound metal dimers and has been extensively studied.1 The ground state O+g dissociation energy has been considered to lie between 0.55 eV (440 cm−1) and 0.091 eV (730 cm−1). We report here a spectroscopic study of Hg2 in a supersonic jet. The first optical transition, 1u←O+g, was characterized by its fluorescence excitation spectrum and the binding energy of the ground state has been measured precisely through the threshold of collision induced dissociation of Hg2 1u to Hg(1S0)+Hg(3P0).The mercury dimer is among the most weakly bound metal dimers and has been extensively studied.1 The ground state O+g dissociation energy has been considered to lie between 0.55 eV (440 cm−1) and 0.091 eV (730 cm−1). We report here a spectroscopic study of Hg2 in a supersonic jet. The first optical transition, 1u←O+g, was characterized by its fluorescence excitation spectrum and the binding energy of the ground state has been measured precisely through the threshold of collision induced dissociation of Hg2 1u to Hg(1S0)+Hg(3P0).

Role of the charge-transfer state in the electronic absorption of protonated hydrocarbon molecules.

The vibrationally resolved electronic spectra of isolated protonated polycyclic aromatic hydrocarbons (PAHs)--naphthalene, anthracene, and tetracene--have been recorded via neutral photofragment spectroscopy. The S1←S0 transitions are all in the visible region and do not show a monotonic red shift as a function of the molecular size, as observed for the neutral analogues. Comparison with ab initio calculations indicates that this behavior is due to the nature of the excited state, which has a pronounced charge-transfer character for protonated linear PAHs with an even number of aromatic rings.