Andrée Lorquet

Intramolecular dynamics by photoelectron spectroscopy. I. Application to N+2, HBr+, and HCN+

The Fourier transform of an optical electronic spectrum leads to an autocorrelation function C(t) which describes the evolution in time of the wave packet created by the Franck–Condon transition, as it propagates on the potential energy surface of the electronic upper state. This correlation function is equal to the modulus of the overlap integral between the initial position of the wave packet and its instantaneous position at time. The original data resulting from an experimentally determined spectral profile must be corrected for finite energy resolution, rotational, and spin‐orbit effects. The behavior of the system can then be followed up to a time of the order of 10−13 s, i.e., during the first few vibrations which follow immediately the electronic transition. The method is applied to photoelectron spectra and the results are compared to the available information on potential energy surfaces of ionized molecules, in order to study their unimolecular dissociation dynamics. In the case of the X 2Σ+g, ...

Unimolecular dynamics from kinetic energy release distributions. V. How does the efficiency of phase space sampling vary with internal energy

A retarding field technique coupled with a quadrupole mass analyzer has been used to obtain the kinetic energy release distributions (KERDs) for the C2H3Br+→[C2H3]++Br dissociation as a function of internal energy. The KERDs obtained by dissociative photoionization using the He(I), Ne(I), and Ar(II) resonance lines were analyzed by the maximum entropy method and were found to be well described by introducing a single dynamical constraint, namely the relative translational momentum of the fragments. Ab initio calculations reveal the highly fluxional character of the C2H3+ ion. As the energy increases, several vibrational modes are converted in turn into large-amplitude motions. Our main result is that, upon increasing internal energy, the fraction of phase space sampled by the pair of dissociating fragments is shown to first decrease, pass through a shallow minimum around 75%, and then increase again, reaching almost 100% at high internal energies (8 eV). This behavior at high internal energies is interpre...

The Role of Long-Range Forces in the Determination of Translational Kinetic Energy Release. Loss of C4H4+ from the Benzene and Pyridine Cations

Kinetic energy release distributions (KERDs) for the benzene ion fragmenting into C 4H 4 (+) and C 2H 2 have been recorded by double-focusing mass spectrometry in the metastable energy window and by a retarding field experiment up to an energy of 5 eV above the fragmentation threshold. They are compared with those resulting from the HCN loss reaction from the pyridine ion. Both reactions display a similar variation of the kinetic energy release as a function of the internal energy: the average release is smaller than statistically expected, with a further restriction of the phase-space sampling for the C 5H 5N (+) dissociation. Ab initio calculations of the potential-energy profile have been carried out. They reveal a complicated reaction mechanism, the last step of which consists in the dissociation of a weakly bound ion-quadrupole or ion-dipole complex. The KERDs have been analyzed by the maximum entropy method. The fraction of phase space effectively sampled by the pair of fragments has been determined and is similar for both dissociations. Both reactions are constrained by the square root of the released translational energy, epsilon (1/2). This indicates that in the latter stage of the dissociation process, the reaction coordinate is adiabatically decoupled from the bath of the bound degrees of freedom. For the C 6H 6 (+) fragmentation, the analysis of the experimental results strongly suggests that, just as for a spherically symmetric interaction potential, the translational motion is confined to a plane. For the dissociation of the pyridine ion, the main dynamical constraint is also a restriction to a two-dimensional subspace. This dimensionality reduction of the translational phase space is due to the fact that the Hamiltonian of both weakly bound complexes contains a cyclic coordinate.

The Evolution of Electronically Excited Molecules

We describe a systematic way of investigating photochemical reactions which consists in the followings steps: (i) ab initio calculation of potential energy surfaces, (ii) calculation of nonadiabatic coupling matrix elements, (iii) calculation of semiclassical transition probabilities, (iv) average of these probabilities to obtain a microca-nonical rate constant. An alternative promising method consists in extracting photochemical information from experiment by Fourier transforming spectroscopic band profiles. This gives a correlation function which provides very direct information on the short-time behaviour of the molecule.