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See detailThe Role of Long-Range Forces in the Determination of Translational Kinetic Energy Release. Loss of C4H4+ from Benzene and Pyridine Cations.
Gridelet, E.; Locht, Robert ULg; Lorquet, Andrée ULg et al

in Journal of Physical Chemistry A (2008), 112

Kinetic energy release distributions (KERDs) for the benzene ion fragmenting into C4H4+ and C2H2 have been recorded by double-focussing mass spectrometry in the metastable energy window and by a retarding ... [more ▼]

Kinetic energy release distributions (KERDs) for the benzene ion fragmenting into C4H4+ and C2H2 have been recorded by double-focussing 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 C5H5N+ 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 kinetic energy, epsilon1/2. 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 C6H6+ fragmentation, the analysis of the experimental results strongly suggests that, just as for the symmetric interaction potential, the translational motion is confined 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. [less ▲]

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See detailHydrogen atom loss from the benzene cation. Why is the kinetic energy release so large?
Gridelet, E.; Lorquet, Andrée ULg; Locht, Robert ULg et al

in Journal of Physical Chemistry A (2006), 110(27), 8519-8527

The kinetic energy release distributions (KERDs) associated with the hydrogen loss from the benzene cation and the deuterium loss from the perdeuteriobenzene cation have been remeasured on the metastable ... [more ▼]

The kinetic energy release distributions (KERDs) associated with the hydrogen loss from the benzene cation and the deuterium loss from the perdeuteriobenzene cation have been remeasured on the metastable time scale and analyzed by the maximum entropy method. The experimental kinetic energy releases are larger than expected statistically, in contradistinction to what has been observed for the C-X fragmentations of the halogenobenzene cations. H(D) loss from C6H6+ (C6D6+) occurs via a conical intersection connecting the (2)A(2) and (2)A(1) electronic states. Two models are proposed to account for the experimental data: (i) a modified orbiting transition state theory (OTST) approach incorporating electronic nonadiabaticity; (ii) an electronically nonadiabatic version of the statistical adiabatic channel model ( SACM) of Quack and Troe. The latter approach is found to be preferable. It leads to the conclusion that the larger the energy stored in the transitional modes, which partly convert to the relative interfragment motion, the shorter the value of the reaction coordinate at which the adiabatic channels cross, and the larger the probability of undergoing the (2)A(2) -> (2)A(1) transition required for hydrogen loss. [less ▲]

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See detailGround and Excited State Dissociation Dynamics of Ionized 1,1-Difluoroethene
Gridelet, Evelyne; Dehareng, Dominique ULg; Locht, Robert ULg et al

in Journal of Physical Chemistry A (2005), 109(37), 8225-8235

The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment ... [more ▼]

The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment uses dissociative photoionization with the He(I) and Ne(I) resonance lines, providing the ions with a broad internal energy range, up to 7 eV above the dissociation threshold. The second experiment samples the metastable range, and the average ion internal energy is limited to about 0.2 eV above the threshold. In both energy domains, KERDs are found to be bimodal. Each component has been analyzed by the maximum entropy method. The narrow, low kinetic energy components display for both experiments the characteristics of a statistical, simple bond cleavage reaction: constraint equal to the square root of the fragment kinetic energy and ergodicity index higher than 90%. Furthermore, this component is satisfactorily accounted for in the metastable time scale by the orbiting transition state theory. Potential energy surfaces corresponding to the five lowest electronic states of the dissociating 1,1-C2H2F2+ ion have been investigated by ab initio calculations at various levels. The equilibrium geometry of these states, their dissociation energies, and their vibrational wavenumbers have been calculated, and a few conical intersections between these surfaces have been identified. It comes out that the ionic ground state (X) over tilde B-2(1) is adiabatically correlated with the lowest dissociation asymptote. Its potential energy curve increases in a monotonic way along the reaction coordinate, giving rise to the narrow KERD component. Two states embedded in the third photoelectron band ( (B) over tilde (2)A(1), at 15.95 eV and (C) over tilde B-2(2) at 16.17 eV) also correlate with the lowest asymptote at 14.24 eV. We suggest that their repulsive behavior along the reaction coordinate be responsible for the KERD high kinetic energy contribution. [less ▲]

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See detailKinetic Energy Release Distributions for Tropylium and Benzylium Ion Formation from the Toluene Cation.
Fati, D.; Lorquet, Andrée ULg; Locht, Robert ULg et al

in Journal of Physical Chemistry A (2004), 108

Hydrogen loss from the toluene molecular ion generates benzylium (Bz(+)) and tropylium (Tr+) ions via two competitive and independent pathways. The corresponding kinetic energy release distributions ... [more ▼]

Hydrogen loss from the toluene molecular ion generates benzylium (Bz(+)) and tropylium (Tr+) ions via two competitive and independent pathways. The corresponding kinetic energy release distributions (KERDs) have been determined under various conditions in the metastable time window for toluene and perdeuterated toluene and have been analyzed by the maximum entropy method (MEM). The isomeric fraction Tr+/Bz(+) is found to be equal to 0.9 +/- 0.3, in good agreement with the values obtained using photodissociation and charge exchange experiments. It is, however, in disagreement with the value 5 +/- 2 deduced by Moon, Choe, and Kim (J. Phys. Cheln. A 2000, 104, 458) from KERD measurements. The origin of the discrepancy is suggested to be the inadequacy of the orbiting transition state theory (OTST) for the calculation of KERDs in hydrogen loss reactions. For both channels, more translational energy is released in the reaction coordinate than would be expected on statistical grounds because of the presence of a barrier along the reaction path. For the Bz(+) channel, the barrier entirely results from centrifugal effects. Rotational energy is converted into translation as a result of angular momentum conservation. Deuteration is observed to reduce the importance of the rotational energy flow in the reaction coordinate. The Tr+ channel is characterized by the presence of a reverse activation energy barrier of electronic origin. The energy in excess of the dissociation asymptote can be partitioned into two components: the reverse barrier plus a nonfixed energy contribution. About 40% of the reverse barrier is converted into relative translational motion of the fragments. Here again, a lower fraction of the nonfixed energy flows into translation for the deuterated isotopomer. [less ▲]

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See detailHow ergodic is the fragmentation of the pyridine cation? A maximum entropy analysis
Gridelet, E.; Locht, Robert ULg; Lorquet, Andrée ULg et al

in International Journal of Mass Spectrometry (2003), 228(2-3), 389-402

The kinetic energy released to the C4H4+ and HCN fragments produced by the dissociation of the pyridine ion has been determined by a retarding field technique up to an internal energy of 4eV above the ... [more ▼]

The kinetic energy released to the C4H4+ and HCN fragments produced by the dissociation of the pyridine ion has been determined by a retarding field technique up to an internal energy of 4eV above the reaction threshold. This extends our previous study limited to the metastable domain [Int. J. Mass Spectrom. Ion Process. 185/186/187 (1999) 155]. Retarding potential curves resulting from dissociative photoionization using the He(I), Ne(I), and Ar(II) resonance lines have been analyzed by the maximum entropy method. The comparison between the experimentally measured curves and those calculated for the prior (i.e., most statistical) situation reveals the existence of dynamical constraints that prevent phase space from being fully explored. The "ergodicity index" F(E) that measures the efficiency of phase space sampling as a function of the internal energy E of the molecular ion is found to decrease steadily as a function of E and to level off at a value of about 50% when E greater than or equal to 2.5 eV At these high internal energies where phase space exploration no longer decreases, spontaneous intramolecular vibrational energy redistribution (i.e., resulting from the anharmonicity of the molecular vibrations) is thought to contribute to internal energy randomization to a limited extent only. When the lifetime is short, phase space exploration is believed to result instead from the relaxation of the electronic energy via a cascade of non-radiative transitions, which leads to a great diversity of initial conditions, and thus, contributes to statisticity. (C) 2003 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailHow ergodic is the Fragmentation of the Pyridine Cation ? A Maximum Entropy Ananlysis
Gridelet, E.; Locht, Robert ULg; Lorquet, Andrée ULg et al

in Anton, J.; Cederquist, H.; Larsson, M. (Eds.) et al 23rd International Conference on the Photonic, Electronic and Atomic Collisions: Book of Abstracts. (2003)

The experimental KER and the statistical distributions are compared by the Maximum Entropy Method. An Ergodicity Index F(E) is defined to measure the phase space sampling efficiency. This is applied to ... [more ▼]

The experimental KER and the statistical distributions are compared by the Maximum Entropy Method. An Ergodicity Index F(E) is defined to measure the phase space sampling efficiency. This is applied to the KERD of C4H4+ cation produced by the C5H5N+ -> HCN+C4H4+ fragmentation path. In this particular case the F(E) is found to decrease steadily with increasing internal energy. [less ▲]

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See detailDissociation of Difluoroetylene Cations: an ion-neutral complex along the fluoroethylidene+ + HF reaction path.
Leyh, Bernard ULg; Lorquet, Andrée ULg; Lorquet, Jean-Claude ULg et al

in Gelpi, E. (Ed.) 15th International Mass Spectrometry Conference: Book of Abstracts (2000)

The collision activated dissociation and dissociative photoionization of 1,1- and 1,2-C2H2F2 in the HF-loss channel are investigated. The measured KER ditributions are interpreted by the opening of two ... [more ▼]

The collision activated dissociation and dissociative photoionization of 1,1- and 1,2-C2H2F2 in the HF-loss channel are investigated. The measured KER ditributions are interpreted by the opening of two competitive reaction channels leading to two C2HF+ isomers. This hypothesis is checked by ab initio calculations. [less ▲]

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See detailUnimolecular Reaction Dynamics from Kinetic Energy Release Distributions: extent of Phase Space Sampling.
Hoxha, A.; Locht, Robert ULg; Lorquet, Andrée ULg et al

in Gelpi, Ed. (Ed.) 15th International Mass Spectrometry Conference: Book of Abstracts. (2000)

The dissociative photoionization of C2H3Br+ in the Br and HBr loss channels is reported. The KER distributions are analyzed by the Maximum Entropy Method (MEM). It can be concluded that at low and high ... [more ▼]

The dissociative photoionization of C2H3Br+ in the Br and HBr loss channels is reported. The KER distributions are analyzed by the Maximum Entropy Method (MEM). It can be concluded that at low and high internal energy the dissociations are statistical. At intermediate internal energies the excess energy is not completely randomized. [less ▲]

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See detailUnimolecular Dynamics from Kinetic Energy Release Distributions. V. How does the Efficiency of Phase Space Sampling vary with Internal Energy?
Hoxha, A.; Locht, Robert ULg; Lorquet, Andrée ULg et al

in Journal of Chemical Physics (1999), 111(20), 9259-9266

A retarding field technique coupled with a quadrupole mass analyzer has been used to obtain the kinetic energy release distributions (KERDs) for C2H3Br+->[C2H3]++Br dissociation as a function of the ... [more ▼]

A retarding field technique coupled with a quadrupole mass analyzer has been used to obtain the kinetic energy release distributions (KERDs) for C2H3Br+->[C2H3]++Br dissociation as a function of the 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 behaviour at high internal energies is interpreted as resulting from the conjugated effect of intramolecular vibrational redistribution (IVR) and radiationless transitions among potential energy surfaces. Our findings are consistent with the coincidence data of Miller and Bear, reanalyzed here, and with the KERD of the metastable dissociation. [less ▲]

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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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