B. Fain

Two-dimensional spectroscopy: Theory of nonstationary, time-dependent absorption and its application to femtosecond processes

In this paper, we present a formalism for a generalized linear susceptibility applicable to femtosecond (fs) processes. As an application, we analyze the recent experimental results of the real‐time fs transition spectra of the photodissociation of NaI and NaBr reported by Zewail et al. In this connection we also investigate in general the behaviors of decaying and trapping of the molecule in an excited electronic state coupled to a continuum.

Application of the theory of two-dimensional spectroscopy to the real-time femtosecond transition state spectroscopy

Abstract This communication presents a theoretical treatment of the real-time femtosecond transition state spectroscopy which has been recently developed by Zewail and co-workers. An application of the theoretical results is presented.

Application of the density matrix method to multiphoton ionization of molecules

In this paper we shall apply the density matrix method to study the one‐photon ionization and two‐photon ionization of molecules. Fano’s results for one‐photon autoionization will be reproduced by this formalism. It will be shown that the existence of autoionization states will interfere with the determination of ionization thresholds of molecules by using the photoionization method if the autoionization state is located near the ionization threshold. It will also be shown that the density matrix method can treat two‐color photoionization spectroscopy and the excited state lifetime measurement by multiphoton ionization method.

Space--time coherences induced by ultrashort electromagnetic pulses

In femtosecond laser‐pulse experiments the pump pulse, with duration comparable or shorter than a typical period of intra‐ or intermolecular vibrations, creates a nonstationary wave packet. In this paper we use the density‐matrix method to analyze creation of space–time coherences by the pump pulse and their effect on the probe pulse. Expressions for the density‐matrix jumps, induced by the probe pulse, have been obtained in a general case. The material equations, determining propagation of the probe pulse, have been derived.

Laser‐induced explosive desorption

A theory of the explosive desorption, observed by Chuang and Domen [Phys. Rev. Lett. 59, 1484 (1987); J. Vac. Sci. Technol. A 5, 473 (1987); J. Chem. Phys. (90, 3318 1989; Chem. Phys. Lett. (with A. Modl). (154, 187 (1989)] is developed. When adsorbates are subjected to UV laser action, an explosive, molecular nonselective desorption takes place. This phenomenon is explained as follows: adsorbate molecules are excited by an UV laser to the higher electronic states. The electronic energy is delivered to the low energy vibronic states. The energy differences of these states are resonant to the corresponding phonon modes. This leads to the excitation of these phonons. When the energy input to the phonon modes exceeds their decay, phonon numbers start to increase exponentially. The phonon avalanche takes place. This in turn causes molecular nonselective desorption. The main result of the theory is the finding of the fluence and coverage dependent threshold conditions.

Theory of non-stationary, time-dependent emission and its application to ultrafast processes

Abstract This communication presents a theoretical treatment of femtosecond time-resolved emission spectroscopy. An application of the theoretical results to the observed time-resolved emission spectra of a dye molecule in water is reported.

Avalanche phenomenon in UV laser-induced desorption

Abstract A theory of explosive desorption observed by Chuang and Domen is developed. Adsorbate molecules are excited by a UV laser to higher electron states. The electronic energy is converted to the low-frequency vibronic states. Excited vibronic state interacting with crystal vibrations lead to the phonon “avalanche”. This causes non-selective molecular desorption. A main result of the present theory is the fluence- and coverage-dependent threshold conditions.

Theory of vibrational predissociation spectroscopy of weak complexes

Abstract The density matrix formalism has been applied to study the vibrational predissociation spectroscopy of weak complexes or clusters. The spectral bandshapes and the nature of the bandwidths are critically examined.

Laser-driven spontaneous emission. Calculation by fourth order approximation

Abstract The quantum theory of interference spectroscopy is developed. The intensity of the radiation is calculated up to the fourth order terms. It is shown that there exists a linear mechanism (depending on linear susceptibility only) which contributes to the fourth order radiation intensity. In this case, the intensity of the radiation is proportional to the square of the laser pulses amplitudes, and it does not vanish after the laser pulses are over.