Jean-Claude Lorquet

Unimolecular decay paths of electronically excited species. II. The C2H+4 ion

The reaction paths of dissociation and the mechanisms of electronic relaxation of the ethylene cation have been calculated ab initio. Internal rotation is shown to bring about radiationless transition of states ? 2A and ? 2B2 to the ground state. Two competing channels are available for the first excited ? 2B3: it can either undergo internal conversion to the ground state ? 2B3 or lose a hydrogen atom by simple bond cleavage and give C2H+3+H fragments. The competition is governed by nonadiabatic interaction around a conical intersection. The coupling matrix element 〈?‖∂/∂q‖?〉 has been calculated and is shown to obey the linear model of nonadiabatic interaction quite well. The ground state ? 2B3 cannot eliminate directly a hydrogen molecule. It must first undergo rearrangement to a 2E state of a CH3CH+ structure via a bridged structure. Jahn–Teller interaction takes place at the doubly degenerate 2E state. Dissociation of CH3CH+ via 1,1‐hydrogen elimination is then allowed and leads to C2H+2+H2. There is a...

Dynamical study of non adiabatic unimolecular reactions: The conical intersection between the B 2B2 and à 2A1 states of H2O+.

The conical intersection connecting the B 2A′ and A 2A′ states of the H2O+ ion is studied. The two potential energy surfaces are calculated ab initio by the SCF/CI method within the CS point group. The nonadiabatic coupling matrix elements 〈A‖∂/∂q‖B〉 are computed for several cross sections throughout the potential energy surfaces. A transformation to the diabatic representation is performed. The linear model is found to be a good approximation in the region close to the apex of the cone. The global functions t(s) and T(S) governing the nonadiabatic transition probability are calculated; their shapes are those predicted by the Landau–Zener model (in the Nikitin bidimensional version). A dynamical study is undertaken by means of classical trajectory calculations on the upper adiabatic potential energy surface. An averaged transition probability Ptr is derived. Excitation of rotation or of the bending mode of H2O before photon impact has no influence on Ptr. Excitation of the symmetrical or antisymmetr...

Excited states of gaseous ions. V. The predissociation of the h2o+ ion

Abstract The B 2 state of H 2 O + is predissociated twice. First, by the a 4 B 1 state, giving OH + + H fragments via spinorbit coupling interaction. Secondly, by a 2 A state, giving H + OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B state of H 2 O + and D 2 O + is carried out. This provides the vibrational frequencies of the H 2 O + , D 2 O + and HDO + ions, as well as a vibrational assignment of the peaks. The H 2 O + ion in its B 2 B 2 state is found to have a OH bond length of 1.12 A and a valence angie of 78°. In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH + and OD + ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O + ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H 2 O + ions, and not to that of the OH + fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.

Excited states of gaseous ions. Transitions to and predissociation of the C2Σ+u state of n+2

Abstract A configuration interaction study of different electronic states of N + 2 has been performed. The position in energy and the relative intensity of the photoelectron bands of the 2 Σ + u states has been calculated and compared with experiment. The C 2 Σ + u state is predissociated by a 4 Π u state, as previously supposed. However, owing to the attractive nature of the 4 Π u state a double crossing occurs. Several predissociation mechanisms of the C state can therefore take place; their lifetimes have been calculated.

Unimolecular reaction paths of electronically excited species. IV. The C̃ 2Σ+g state of CO+2

The geometrical structure of the low‐lying states of CO+2 has been calculated ab initio. The C 2Σ+g/2A1 state is found to be slightly bent in its equilibrium geometry. A new assignment of the vibrational structure of the corresponding band in the photoelectron spectrum is suggested. State C is predissociated by two competitive channels. One of them leads to O++CO, the other to CO++O. The mechanism of these predissociations involves a slow, rate‐determining, intersystem crossing to a bent a 4B1 state. The population of state a has a choice between dissociating to O++CO fragments and undergoing a further, much faster, intersystem crossing to the ground state X which dissociates to CO++O. Since radiationless transitions between X and a are relatively rapid, the state which is lower in energy (i.e., X) has a much larger population than the other (i.e., a). Hence, the CO++O channel prevails as soon as it is energetically accessible. The rate‐determining step of both processes is the intersystem crossi...

Excited States of Gaseous Ions. II. Metastable Decomposition and Predissociation of the CH+ Ion

The slow fragmentation process (τ≃10−7sec) of the CH+ ion in a mass spectrometer can be ascribed to the superposition of several predissociation mechanisms. The potential energy curves of the CH+ ion have been calculated quantum mechanically, and for each intersection, the predissociation lifetime has been estimated on the basis of the Landau—Zener formula. Metastable ions are due to a spin‐forbidden predissociation process, allowed by the spin—orbit (Hso), spin—other‐orbit (Hsoo) and spin—spin (Hss) operators. These three operators are in some cases sufficient to obtain a lifetime much shorter than 10−7 sec. In order to give rise to a metastable fragmentation, the predissociation process must be slowed down, either by the tunnel effect, or because of a deviation from the Landau—Zener formula. The effect of the collisions on the predissociation processes is discussed. In some cases the mechanism of this effect involves energy transfer, while in others only a symmetry loss occurs. A strictly forbidden pred...

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, ...

On the use of distorted Franck—Condon factors in the interpretation of neutralization—reionization experiments

Neutralization by collision with an appropriate target gas is a vertical process. However, it takes place between two deformed potential energy surfaces, i.e., that of a polyatomic ion perturbed by an atomic collision gas (Hg or Xe) and that of the neutral polarized by the receding Hg+ or Xe+ ion. Hence, it is argued that the Franck—Condon factors which determine the yield of production of the neutral do not involve the overlap integral between the vibrational wavefunctions of the free species (ion and neutral). Instead, the quantity to be considered is |<0|φ0p(τ)⪢|2, where |0⪢ is the vibrational wavefunction of the unperturbed neutral, τ is the time during which the newly created polyatomic neutral is effectively polarized by the departing Hg+ or Xe+ ion, and |φ0p⪢ the vibrational wavefunction of the polarized neutral. A sequence of two consecutive relaxation processes induced by the neutralizing collision can, at least in favourable cases, bring about an increase in the yield of production of a marginally stable radical. These ideas have been applied to the controversial stability of the chloronium ylide. MNDO calculations reveal that the neutralization step is well represented by the Demkov model. This implies that the yield of neutralization is controlled not only by the energy difference Δ between the ionization energy of the neutralizing gas (Hg, Xe, Zn, Na) and the recombination energy of the polyatomic ion, as usually assumed, but also by an additional coupling element H12 whose value depends on the nature of the neutralizing gas.

Constructing approximately diabatic states from LCAO‐SCF‐CI calculations

We consider here two approaches which have been proposed in the literature to obtain diabatic states from ab initio calculations. First, by calculating explicitly the coupling vector g=〈ψ2‖∇ψ1〉 which describes the nonadiabatic interaction between two adiabatic states ψ1 and ψ2. Second, by some extrapolation process of the wave functions obtained at a particular reference point. The coupling vector g is the sum of three contributions: g=gCI+gLCAO+gAO. The first two represent the change in character of the adiabatic states in the region of nonadiabatic coupling due to the variation of the CI and LCAO coefficients, whereas gAO results from the translation of the atomic orbitals with the moving nuclear centers. Criteria have been given to recognize when it is possible to transform a set of CI wave functions into a pair of useful diabatic states. A particularly favorable situation is obtained when the interacting electronic states are doubly excited with respect to each other. Within the two‐state approximatio...

Isotopic effects in accidental predissociation. The case of the C2Σ+u state of N+2

Abstract The large isotopic effect recently found in the decay of the C 2 Σ + u state of N + 2 is interpreted by an indirect (accidental) predissociation mechanism. The C 2 Σ + u state interacts with a bound intermediate state, itself predissociated by a repulsive state. The competition between fluorescence and predissociation is controlled by the energy difference separating adjacent levels of the two bound states. This energy difference will be different for each vibrational level of each isotope. Fluorescence and predissociation rate constants are given for each level, and the position of the intermediate bound state is fitted to a Morse potential.