Competing tunneling trajectories in a 2D potential with variable topology as a model for quantum bifurcations

We present a path - integral approach to treat a 2D model of a quantum bifurcation. The model potential has two equivalent minima separated by one or two saddle points, depending on the value of a continuous parameter. Tunneling is therefore realized either along one trajectory or along two equivalent paths. Zero point fluctuations smear out the sharp transition between these two regimes and lead to a certain crossover behavior. When the two saddle points are inequivalent one can also have a first order transition related to the fact that one of the two trajectories becomes unstable. We illustrate these results by numerical investigations. Even though a specific model is investigated here, the approach is quite general and has potential applicability for various systems in physics and chemistry exhibiting multi-stability and tunneling phenomena.Comment: 11 pages, 8 eps figures, Revtex-

Semiclassical model of ultrafast photoisomerization reactions

In this letter we propose a model which explains ultrafast and efficient photoisomerization reactions as driven by transitions between quasistationary states of one dimensional (1D) double well potential of an excited electronic state. This adiabatic potential is formed as a result of doubly crossing of a decay diabatic potential of the ground electronic state and a bound diabatic potential of the excited state. We calculate the eigenstates and eigenfunctions using the semiclassical connection matrices at the turning and crossing points and the shift matrices between these points. The transitions between the localized in the wells below the adiabatic barrier states are realized by the tunneling and by the double non-adiabatic transitions via the crossing points processes. Surprisingly the behavior with the maximum transition rate keeps going even for the states relatively far above the barrier (2 -4 times the barrier height). Even though a specific toy model is investigated here, when properly interpreted it yields quite reasonable values for a variety of measured quantities, such as a reaction quantum yield, and conversion time.Comment: 9 pages, 5 figures. accepted to Chem. Phys. Letters (2005

Structure de la transition électronique pure n-π* 1B 2g ← 1Ag de la p-benzoquinone en phase monocristalline

The structure of the origin band of the n-π* 1B 2g ← 1Ag transition has been measured by absorption at 4.2 °K using single crystals of p-benzoquinone-h4, -d 4 and isotopically mixed crystals. The pure electronic transition produces the first component, which appears as a magnetic dipole transition. The second component is a false crystal induced origine. The excitonic splitting of the bands is shown to be not greater than 0.3 cm-1.On analyse la structure de la bande origine de la transition n-π* 1B2g ← 1A g obtenue en absorption et à 4,2 °K pour des monocristaux de p-benzoquinone, hydrogénés, deutériés et isotopiquement mixtes. La transition électronique pure donne naissance, en tant que transition dipolaire magnétique, à la première composante. La deuxième composante est une fausse origine induite par le cristal. On montre que le dédoublement excitonique des bandes spectrales est au plus égal à 0,3 cm-1

THE EXCITED STATE OF CIS-THIOINDIGO

Dans le cadre de l'étude de la photoisomérisation cis-trans réversible du thioindigo, des mesures spectroscopiques à basse température des isomères cis et trans isolés en matrice ont été réalisées. C'est la première observation, pour l'isomère cis, de spectres ayant une structure vibrationnelle résolue. Différentes évaluations du rendement quantique de fluorescence de ce composé indiquent que la durée de vie de l'état excité est dans le domaine subpicoseconde.Spectroscopic measurements at low temperature of matrix isolated cis and trans-thioindigo are presented and are discussed in relation to the photoinduced reversible isomerization of this molecule. For the cis isomer these data represent the first vibrationally resolved spectra. Estimates of the emission quantum yield for this species show that the excited state decays on a subpicosecond timescale

Vapor-solvent shift of the lowest frequency vibration of p-benzoquinone and toluquinone and the consequences for the vibrational and electronic spectral assignments

Far-infrared (20–230 cm–1) absorption spectra of p-benzoquinone-h4,-d4 and of toluquinone have been measured. In the vapor phase, the lowest frequency vibration of these three compounds is found at 88.9, 87.5, and 82.3 cm–1, respectively. In the condensed phase the frequency increases by approximately 20 cm–1. These measurements clarify assignments, based on this vibrational mode, of the visible absorption spectrum as well as of other vibrations. It is proposed that intermolecular forces are responsible for the large frequency shift and that the observed width in solution (≈24 cm–1) is homogeneous, reflecting rapid relaxation processes of this large amplitude low frequency bending mode.

LASER INDUCED HYDROGEN MOTION IN ORGANIC CRYSTALS AT LOW TEMPERATURE

We study here a laser induced reaction leading to a rearrangement of the acid protons in benzoic acid crystals at very low temperature. This hydrogen motion results from the electronic excitation of pentacene molecules substitutionally included into a benzoic acid host matrix, protonated or deuterated. The growth of a deuterated benzoic acid crystal doped with pentacene leads to a partial deuteration of pentacene molecules, resulting in several origin bands in the absorption spectrum. The dynamics of the system following non selective and selective laser irradiations of the differently deuterated pentacene molecules is analyzed in terms of hydrogen rearrangements

A TRANSIENT GRATING STUDY OF ACOUSTIC PROPERTIES OF POLYMER MATERIALS

La technique des réseaux transitoires constitue une méthode très sensible pour l'étude de nombreux phénomènes de relaxation et de transport en phase condensée. Elle permet de produire, avec des impulsions lumineuses courtes, des phonons monochromatiques de haute fréquence, puis de suivre leur évolution. Elle offre ainsi une méthode adaptée à la mesure des propriétés acoustiques (propagation et atténuation) de matériaux liquides et solides. Dans ce travail cette technique est appliquée à l'étude d'un polymère (PMMA) au cours de transformations physiques ou chimiques.Transient grating techniques have been established as a very sensitive method to study numerous relaxation and transport properties in condensed phases. With short light pulses coherent ultrasonic phonons can be generated and monitored. Transient grating techniques afford thus a convenient method to measure and to follow acoustic (propagation and damping) properties of liquid and solid materials. In this work this technique is used to study the transformation of a polymer material (PMMA) undergoing physical and chemical transformations