Search Results - (Author, Cooperation:F. Remacle)

2018-07-25

National Academy of Sciences

0027-8424

1091-6490

Biology

Medicine

Natural Sciences in General

2015-10-24

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The formalism of the resonance states is used to derive approximate expressions of the unimolecular law of decay resulting from a specific excitation. These expressions contain no cross terms and wash out the quantum interferences. We propose a method to relate them to an experimentally observable quantity, viz., the autocorrelation function C(t) obtained as the Fourier transform of a spectral profile, which is available even when the spectrum is poorly resolved. For a specific excitation, the exact initial rate of decay (valid up to the dephasing time T1) is equal to the initial slope of ||C(t)||2. The subsequent time evolution can be obtained by averaging ||C(t)||2 over its oscillations. This generates a function ||C(t)||2av whose area (from time T1 onwards) is directly related to an average decay lifetime. At times t〉T1, a good approximation to the average decay curve Pav(t) can be derived by multiplying ||C(t)||2av by an appropriate constant. The method is exemplified on various diatomic and triatomic models. As an application to a real system, we study the B˜ 2B2 state of H2O+ which is coupled to the A˜ 2A1 state via a conical intersection. State B˜ is found to undergo an ultrafast intramolecular relaxation with a lifetime of (1.6±0.2) 10−14 s.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A direct method for the inversion of resonant Raman excitation profiles to the time domain is proposed. The inversion procedure is implemented within the maximum entropy (ME) formalism. The constraints used in the ME procedure are the values of the Fourier transform of the Raman excitation profile at a given set of values of times tr. It is shown that the ME functional form of the Raman cross section can be expressed in terms of a Raman amplitude, depending on the Lagrange multipliers and on the constraints. The Fourier transform of the Raman amplitude yields the time dependent cross correlation function. Another route to direct inversion, using a Fourier series expansion of the dispersion relation, is also considered. The analytical requirements that must be satisfied by the input excitation profile for a successful inversion to be possible are discussed. The optimum values of the times tr and of the Lagrange multipliers which determine the Raman amplitude are computed using a new algorithm (the min–max algorithm). The proposed ME numerical procedure is implemented for computed resonant Raman excitation profiles of the B˜ electronic state of the iodobenzene molecule and of a model anharmonic system. In addition, the analytical implications of the ME functional form of the excitation profile are discussed with special reference to the separation of time scales in the dynamics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

An analytical functional form for the distribution of intensities in an absorption spectrum is derived. Deviations from the purely statistical "Porter–Thomas'' distribution are shown to be directly related to finite time information on the dynamics in phase space. The predicted distribution is wider than the purely statistical one with a higher proportion of very low intensity transitions. The derivation is based on a maximum entropy form of the spectrum. The constraints used are the values of the survival amplitude at finite number of times. The amplitude is obtainable as the Fourier transform of an observed spectrum or as the result of a dynamical computation. The optimal choice of the time points which characterize the spectrum, is discussed and a numerical algorithm is provided. Extensive spectral fluctuations occur when more than one time scale is needed to characterize the dynamics. This separation of time scales is also manifested as a clump structure in the spectrum of maximal entropy. The formalism also provides the distribution of line spacings and the "correlation hole'' in the time autocorrelation function is discussed as an illustration.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The squared Fourier transform of the optical absorption spectrum provides a very useful characterization of the intramolecular dynamics. In practice, most of the information content is in that portion of the transform whose magnitude is of the order of (1/N) of its value for time=0, where N is the number of eigenstates in the zero order nonstationary bright state which is optically accessed. If the highly resolved spectrum manifests inherent structures ("clumps'') at lower levels of resolution then each clump can be regarded, for the purpose of the analysis, as a bright state with its own survival probability. This offers a significant advantage. We discuss theoretically and provide computational examples how this can be implemented within a maximum entropy formalism. We determine both the density of the region in phase space sampled up to time t and its entropy. Analytically and computationally it is shown that the evolution in phase space is sequential. Also discussed is the structure of the Hamiltonian matrix which can give rise to a nested inherent spectra. It is argued that each time scale is characterized by its set of good constants of motion which decrease in number upon the transition to the next time regime.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The dynamics of very high molecular Rydberg states, with special attention to the extreme long time behavior, is discussed using a quantum mechanical multichannel scattering theoretic formulation. Detailed computational results are provided for an electron revolving about a dipolar core in the presence of an external dc field. Two distinct effects are highlighted, trapping and dilution where the former is an origin of long time stability when there are very many bound states but much fewer states that are directly coupled to the continuum. Both trapping and dilution act to elongate the intermediate time decay of the high Rydberg states. The extent of dilution can be varied by changing the magnitude of the external dc electrical field. The formalism and specific results are discussed also towards the implications to other types of unimolecular processes. In particular it is argued that the study of molecular Rydberg states does suggest a possible route to mode selective chemistry. © 1996 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The classical limit is shown to provide a description exactly equivalent to the quantum mechanical one in the approximation where each electron is assigned to an orbital. Strictly speaking it is therefore not a limit but an alternative way of solving the problem. There are some merits of this reformulation, most notably in that it brings the phase of the orbitals to the forefront, on equal footing as the occupancies. This allows one to discuss, e.g., electron localization, in a clearer manner. But computationally the classical description is not superior. There will be a definite advantage for more realistic electronic Hamiltonians, i.e., for implementing configuration interaction, and/or when the nuclear motion is coupled to the electronic dynamics. In this paper we limit attention to a derivation and discussion of the simple orbital approximation. © 2000 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Using exact matrix elements for the coupling, the effect of the anisotropy of the core on high molecular Rydberg states is studied by quantum dynamics. It is found that on the time scale which can be probed by zero kinetic energy spectroscopy there is extensive interseries mixing. In particular, the long decay times are due to the sojourn in Rydberg series which are not directly effectively coupled to the continuum. These are series built on higher rotationally excited states of the core and a dynamical bottleneck controls the coupling to the bound series directly coupled to the ionization continuum. The computations are carried out for realistic molecular parameters and in the presence of a weak external dc field. The quadrupolar coupling is often more effective in interseries coupling than the dipolar anisotropy even though the latter has a far higher range. The external field exhibits the expected "dilution'' or "time stretching'' effect at short times (of the order of the Stark period) but enhances the interseries mixing at longer times. An incomplete l mixing is the origin of another dynamical bottleneck. The time evolution is described both by exact quantum propagation and by a reduced description where degenerate states (i.e., states which differ only in the magnetic quantum numbers) are taken to be equally populated, on the average. This grouping, valid at longer times, facilitates the quantal computations which include several series with the full complement of angular momentum states of the electron. Such computations are possible by taking advantage of the conservation of the (total projection) quantum number M. For higher values of M the coupling to the continuum is very much hindered and the bound Rydberg series exhibit extreme stability. The paper concludes by an analysis of the three bottlenecks which can give rise to longer decays. © 1996 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The qualitative physical aspects and the quantitative description of time and frequency resolved absorption spectroscopy of high molecular Rydberg states are discussed. The frequency is that of the excitation laser and the time is the independently variable delay before detection. The discussion allows for the presence of a weak external electrical field. The essential new ingredient is the finite slice of Rydberg states that are detected (=are in the detection window) and the variation of this population with time due to the coupling of the Rydberg electron with the molecular core. Line shapes are provided showing the effect of the depth of the detection window and the advantages and limitations imposed by the finite width of the excitation laser. The sharpening of the spectrum as the delay time to detection is increased is also illustrated. The quantitative theory is expressed in terms of the expectation value of a detection operator, describing the range of states that can be ionized by the delayed field, taken over a wave function. This wave function is the state of the system at the time of detection. However, even just at the end of the excitation stage, due to the interseries coupling, this wave function is not identical to the state that is directly optically accessed. The time correlation function of this wave function, obtained as a Fourier transform of the frequency resolved spectrum, is shown to provide further insight into the dynamics, the more so when the excitation laser has a narrow width in frequency. ©1997 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The translational kinetic energy release distribution (KERD) in the halogen loss reaction of the chloro-, bromo-, and iodobenzene cations has been experimentally determined in the microsecond time scale and theoretically analyzed by the maximum entropy method. The KERD is constrained by the square root of the translational energy, i.e., by the momentum gap law. This can be understood in terms of quantum-mechanical resonances controlled by a matrix element involving a localized bound state and a rapidly oscillating continuum wave function, as in the case of a vibrational predissociation process. The energy partitioning between the reaction coordinate and the set of the remaining coordinates is nearly statistical, but not quite: less translational energy is channeled into the reaction coordinate than the statistical estimate. The measured entropy deficiency leads to values of the order of 80% for the fraction of phase space sampled by the pair of fragments with respect to the statistical value. In the case of the dissociation of the chlorobenzene ion, it is necessary to take into account a second process which corresponds to the formation of the chlorine atom in the excited electronic state 2P1/2 in addition to the ground state 2P3/2. The observations are compatible with the presence of a small barrier (of the order of 0.12 eV) along the reaction path connecting the D˜ 2A1 state of C6H5Cl+ to the Cl(2P1/2)+C6H5+(X˜ 1A1) asymptote. © 1999 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A many-electron description of charge migration along a molecular backbone is discussed. Reference is made to site selective reactivity and the recent experiments of Weinkauf and Schlag on the dissociation of peptide ions following a localized ionization. The use of many-electron states allows a classification of the charge migration pathways through either covalent or ionic states. Electron correlation is introduced via Coulomb repulsion of electrons of opposite spins à-la Hubbard. Complete configuration interaction is implemented using the unitary group basis of Paldus. The primary factor determining charge migration is found to be the local ionization potential. It is shown that, at lower levels of excitation, the majority of possible initial states which describe localized ionization at one end of the chain lead to a preferential dissociation at the other end of the chain. © 1999 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The absorption spectrum of bound Rydberg states which can be detected by a delayed, pulsed field ionization is computed. The spectrum, measured for various delay times, provides information on the short and the longer time dynamics of high molecular Rydberg states. A quantitative dynamical theory, based on an effective Hamiltonian formalism is applied, illustrating the role of the Rydberg electron–core coupling and of an external electrical field in the delay-time dependent spectra. The sharpening of the spectra for longer delay times is reproduced by the dynamical computations. It is found that the overall intensity, as a function of the delay time before detection, is well described by a double exponential decay where the short lifetime is primarily a manifestation of the direct autoionization to the continuum, while the long lifetime is due to interseries coupling. Both lifetimes increase with the principal quantum number of the Rydberg states. The notion of trapped "reservoir states" is illustrated by the computational results, with special reference to a kinetic model analysis. The role of the initially optically accessed state(s) and of the depth of detection, in particular with regard to the intensity, is demonstrated. The effect of varying the strength of an external dc field in the time interval prior to the detection is illustrated by the dynamical computations, with respect to both the decay kinetics and the intensity of the spectrum. © 1997 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Finite state logic machines can be realized by pump–probe spectroscopic experiments on an isolated molecule. The most elaborate setup, a Turing machine, can be programmed to carry out a specific computation. We argue that a molecule can be similarly programmed, and provide examples using two photon spectroscopies. The states of the molecule serve as the possible states of the head of the Turing machine and the physics of the problem determines the possible instructions of the program. The tape is written in an alphabet that allows the listing of the different pump and probe signals that are applied in a given experiment. Different experiments using the same set of molecular levels correspond to different tapes that can be read and processed by the same head and program. The analogy to a Turing machine is not a mechanical one and is not completely molecular because the tape is not part of the molecular machine. We therefore also discuss molecular finite state machines, such as sequential devices, for which the tape is not part of the machine. Nonmolecular tapes allow for quite long input sequences with a rich alphabet (at the level of 7 bits) and laser pulse shaping experiments provide concrete examples. Single molecule spectroscopies show that a single molecule can be repeatedly cycled through a logical operation. © 2001 American Institute of Physics.

Electronic Resource

0301-0104

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0009-2614

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0009-2614

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0009-2614

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0009-2614

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0375-9601

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource