References of "Lorquet, Jean-Claude"
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See detailConstructing approximatively diabatic states from LCAO-SCF-CI calculations
Desouter-Lecomte, Michèle ULg; Dehareng, Dominique ULg; Lorquet, Jean-Claude ULg

in Journal of Chemical Physics (1987), 86(3), 1429-1436

We consider here two approaches which have been proposed in the literature to obtain diabatic states from ab initio calculations. First by calculating explicitely the coupling vector g which describes the ... [more ▼]

We consider here two approaches which have been proposed in the literature to obtain diabatic states from ab initio calculations. First by calculating explicitely the coupling vector g which describes the nonadiabatic interaction between the adiabatic states. Second, by some extrapolation process of the wavefunctions obtained at a particular reference point. The coupling vector is a sum of three contributions. The first two represent the the change in character of the adiabatic states in the region of nonadiabatic coupling due the variation of the CI and LCAO coefficients, whereas the third 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-states approximation, the first term, depending on the CI coefficients, is strictly irrotational and never gives rise to problems. One can expect situations where it is also true for the second term depending on the LCAO coefficients. However, the third term of the coupling can never be described as a rotation of two diabatic functions. Nevertheless, the latter contribution can frequently be neglected, at least when the coupling is strong. The theory of the electron transfer factors (ETF's) provides further insight into the problem and confirm our conclusions. [less ▲]

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See detailAvoided crossings: a study of the non-adiabatic transition probability
Desouter, Michèle ULg; Leyh-Nihant, Brigitte; Praet, Marie-Thérèse ULg et al

in Journal of Chemical Physics (1987), 86

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See detailAN ABINITIO STUDY OF THE ISOMERIZATION AND FRAGMENTATION OF CHO2+ IONS - AN EXAMPLE OF SPIN-CONTROLLED REACTIONS
Remacle, Françoise ULg; Petitjean, Serge; Dehareng, Dominique ULg et al

in International Journal of Mass Spectrometry and Ion Processes (1987), 77(2-3), 187-201

[CHO2+] ions exist in two isomeric forms, a quasi-linear carboxylic structure, OCOH+ (I), and a formate structure, O(CH)O+ (II). The latter is known to isomerise into the former with a low rate constant ... [more ▼]

[CHO2+] ions exist in two isomeric forms, a quasi-linear carboxylic structure, OCOH+ (I), and a formate structure, O(CH)O+ (II). The latter is known to isomerise into the former with a low rate constant (about 105 s−1). Configuration interaction ab initio calculations reveal that the ion can exist in three low-lying electronic states of different multiplicity and symmetry species, a singlet (Image 1A′) and two triplets (ã3A′ and Image 3A′'). The potential energy surfaces cross so that the nature of the lowest state varies as a function of the nuclear geometry. The singlet surface (Image 1A′) has its deepest minimum for structure I and a subsidiary minimum for structure II. The situation is reversed for the ã3A′ state which has its deepest minimum for structure II and which exhibits a shallow minimum for structure I. Thus, at low energies, the carboxylic and formate ions are respectively in a singlet and in a triplet state. These ions lose an oxygen atom on a microsecond time scale by a composite mechanism which is subject to a large isotope effect. One of the components of the metastable peak corresponds to reaction OCOH+(1A') -> O(CH)O+ (3A') -> HCO+ + O giving rise to a small kinetic energy release. The second component is due to a spin-orbit controlled direct predissociation process, viz. OCOH+ (1A') -> OCOH+ (3A') -> HOC+ + O. The probability of surface hopping varies as a function of the internal energy between 0 and a maximum value of ca. 0.008. The corresponding kinetic energy release is expected to be larger for the deuterated than for the hydrogenated compound. Thus, the two components are resolved in the deuterated compound, but hardly distinguishable in the hydrogenated species. [less ▲]

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See detailNonadiabatic Unimolecular Reactions of Polyatomic Molecules
Desouter-Lecomte, Michèle ULg; Dehareng, Dominique ULg; Leyh-Nihant, Brigitte et al

in Journal of Physical Chemistry (1985), 89

The nonadiabatic couplings which arise when two potential energy surfaces of a polyatomic molecule get close in energy can be classified as follows: (A) avoided crossings, (B) genuine intersections (Jahn ... [more ▼]

The nonadiabatic couplings which arise when two potential energy surfaces of a polyatomic molecule get close in energy can be classified as follows: (A) avoided crossings, (B) genuine intersections (Jahn-Teller and conical), (C) glancing intersections (Renner-Teller interactions). The characteristics of the potential energy surfaces in the adiabatic and diabatic representations are discussed for each case. The three coupling cases differ in the structure of the Hamiltonian matrix. When the latter is written in the diabatic representation, it is meaningful to retain the leading term only in its power series expansion. This gives rise to a so-called minimum-order model which is found to be surprisingly accurate (at least in a restricted zone of nuclear coordinates) when compared to the results of ab initio calculations. The characteristic features of each coupling case can only be understood in a two-dimensional configuration space, Le., when two nuclear degrees of freedom, often with different symmetry properties, are explicitly considered. A simple expression of the nonadiabatic transition probability between two electronic states can be worked out in the framework of the minimum-order models. Two-dimensional extensions of the Landau-Zener formula are obtained, which can be used to study the consequences of the anisotropic properties of the coupling. In the case of avoided crossings, only nuclear trajectories having a well-defined direction are able to bring about surface hopping, wheras there exists two active degrees of freedom for conical intersections. Hence, nonadiabatic processes which are controlled by genuine intersections are expected to take place faster than those controlled by avoided crossings. [less ▲]

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See detailThe isomerization and fragmentation of the CHO2+ ions: an example of spin-controlled chemistry?
Petitjean, Serge; Dehareng, Dominique ULg; Praet, Marie-Thérèse et al

in Advances in Mass Spectrometry (1985), X(B), 1129-1130

The article presents a study of the isomerization and fragmentation of the CHO2+ ions

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See detailUnimolecular Decay Paths of Electronically Excited Species. V. The A2B1 State of H2CO+
Barbier, Christian ULg; Galloy, Claudine; Lorquet, Jean-Claude ULg

in Journal of Chemical Physics (1984), 81

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See detailPotential Energy Surfaces and Theory of Unimolecular Dissociation.
Lorquet, Jean-Claude ULg; Leyh, Bernard ULg

in Almoster Ferreira, M. A. (Ed.) Ionic Processes in the Gas Phase. (1984)

Reaction mechanisms can be determined from ab initio calculations of potential energy surfaces. Their complicated nature explains the frequent success of statistical theories, but non-statistical ... [more ▼]

Reaction mechanisms can be determined from ab initio calculations of potential energy surfaces. Their complicated nature explains the frequent success of statistical theories, but non-statistical behaviour is also accounted for. A certain correlation between structure and reactivity can be established, subject to many qualifications. Information on the nuclear motion on the potential energy surfaces of an ionized molecule can be extracted from a photoelectron spectrum by a Fourier transform operation. [less ▲]

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See detailDynamical study of non adiabatic unimolecular reactions: The conical intersection between the B 2B2 and à 2A1 states of H2O+.
Dehareng, Dominique ULg; Chapuisat, Xavier; Lorquet, Jean-Claude ULg et al

in Journal of Chemical Physics (1983), 78(3), 1246-1264

The conical intersection conecting 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 ... [more ▼]

The conical intersection conecting 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 elements <A|d/dq|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 average 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 antisymmetrical valence mode of H2O lowers Ptr. The shape of ln(1-Ptr) as a function of time indicates the existence of two distinct regimes at short and intermediate time ranges, characterized by two different rate constants k1 and k2 respectively. The rate constants are of the order of 10exp(14) s-1. k1 exhibits a maximum as a function of the absorbed energy Eabs whereas k2 decreases as a function of Eabs. [less ▲]

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See detailA transition state theory of non-adiabatic unimolecular reactions controlled by a conical intersection
Desouter, Michèle ULg; Sannen, Christian; Lorquet, Jean-Claude ULg

in Journal of Chemical Physics (1983), 79

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See detailIntramolecular Dynamics of Molecular Ions.
Lorquet, Jean-Claude ULg; Lorquet, Andrée ULg; Dehareng, Dominique ULg et al

in Bulletin de la Société Chimique Belge (1983), 92(6/7), 609-611

The Fourier transform of an electronic spectrum gives a direct dynamic information on the nuclear motion of a wavepacket on the potential energy surface of the final state reached in the electronic ... [more ▼]

The Fourier transform of an electronic spectrum gives a direct dynamic information on the nuclear motion of a wavepacket on the potential energy surface of the final state reached in the electronic transition, at least during the first few molecular vibrations. The method is valid for both adiabatic and non-adiabatic situations. It has proved its usefulness in the study of molecular processes such as Duchinsky effect, fast predissociations and internal conversions. [less ▲]

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See detailIntramolecular dynamics by photoelectron spectroscopy. II. Nonadiabatic processes.
Dehareng, Dominique ULg; Leyh, Bernard ULg; Desouter-Lecomte, Michèle ULg et al

in Journal of Chemical Physics (1983), 79(8), 3719-3724

The Fourier transform of an electronic spectrum leads to an autocorrelation function which provides information on the propagation of the wave packet on the potential energy surface of the electronic ... [more ▼]

The Fourier transform of an electronic spectrum leads to an autocorrelation function which provides information on the propagation of the wave packet on the potential energy surface of the electronic state reached in the transition. The formula is valid even when nonadiabatic interaction is present, i.e., when the wave packet splits at a particular surface crossing with one part branching off to another potential energy surface. An explicit expression of the correlation function is given for a model of several discrete states interacting with a continuum. Closed-form solutions are given in the case of one and two resonances. A very simple formula valid in the strong coupling limit is also derived. The method is applied to the photoelectron spectrum of state A 2Sigma+ of HBr+, which is shown to correspond to the strong couplig case. [less ▲]

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See detailPotential energy surfaces and dissociation mechanisms of molecular ions
Lorquet, Jean-Claude ULg; Dehareng, Dominique ULg; Sannen, Christian et al

in Journal de Chimie Physique (1980), 77

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See detailNon-adiabatic interactions in unimolecular decay. V. Conical and Jahn-Teller intersections
Desouter, Michèle ULg; Galloy, Claudine; Lorquet, Jean-Claude ULg et al

in Journal of Chemical Physics (1979), 71

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See detailExcited states of gaseous ions. VI. Potential energy surfaces of CH2+
Galloy, C.; Lorquet, Jean-Claude ULg

in Chemical Physics (1978), 30(2), 169-176

Several low-lying excited states of the CH2 + ion have been studied by ab initio GTO-SCF-CL methods. Optimized geometries and energy differences are given for seven of them: B̃2B2, ã4A2, C̃2A2, D̃2A2, 4A2 ... [more ▼]

Several low-lying excited states of the CH2 + ion have been studied by ab initio GTO-SCF-CL methods. Optimized geometries and energy differences are given for seven of them: B̃2B2, ã4A2, C̃2A2, D̃2A2, 4A2(2), 2B1(2) and 4A2(3), in the order of increasing energies. Potential energy surfaces are given for the B̃2B2 and for the first three 4A2 states. These calculations enable us to discuss experimental results concerning the metastable decay (predissociation) of the CH2 + ion, as well as some of its ion-molecule reactions. © 1978. [less ▲]

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See detailNon-adiabatic interactions in unimolecular decay. II. Simplified formalism
Desouter, Michèle ULg; Lorquet, Jean-Claude ULg

in Journal of Chemical Physics (1977), 66

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See detailNon-adiabatic interactions in unimolecular decay. I.
Desouter, Michèle ULg; Leclerc, Jean-Claude; Lorquet, Jean-Claude ULg

in Chemical Physics (1975), 9

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See detailExcited states of gaseous ions. Transitions to and predissociation of the C2Σ+ u state of N2+
Lorquet, Jean-Claude ULg; Desouter, Michèle ULg

in Chemical Physics Letters (1972), 16(1), 136-140

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 2Σ+ u states has been ... [more ▼]

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 2Σ+ u states has been calculated and compared with experiment. The C2Σ+ u state is predissociated by a 4Πu state, as previously supposed. However, owing to the attractive nature of the 4Πu state a double crossing occurs. Several predissociation mechanisms of the C state can therefore take place; their lifetimes have been calculated. © 1972. [less ▲]

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