Kai-Shue Lam

Semiclassical approach to spontaneous emission of molecular collision systems: A dynamical theory of fluorescence line shapes

A dynamical theory of spontaneous emission of molecular collision systems is presented. The theory makes use of specific nuclear trajectories in the time‐dependent Schrodinger equation, and hence can be described as semiclassical. Transitions between electronic states are considered. The description of the radiative interaction is effected by photon‐dressed electronic states (electronic‐field representation) in both the diabatic and adiabatic representations with respect to the field interaction Hamiltonian. In the diabatic representation, one has to deal with two coupled differential–integral equations [Eqs.(21)], whereas in the adiabatic representation, only one [Eq.(75)] is required. The solutions to these equations are sought for using perturbative–interative procedures. It is then found that the diabatic representation, coupled with the stationary‐phase approximation, allows an intuitive and pictorial interpretation of the nuclear dynamics involved, but is incapable of providing a tractable computati...

Penning ionization of Ar by He* (1s2s, 3S) in the presence of intense laser radiation: Pronounced laser‐modified collisional effects in the emitted‐electron energy spectrum

Results of calculations based on quantum‐mechanical theory are presented for ionizing collisions of He* and Ar in the presence of a strong electromagnetic field.

Semiclassical approach to collision-induced emission in the presence of intense laser radiation: An aspect in the study of cooperative chemical and optical pumping☆

Abstract Collion-induced emission in molecular systems in an intense laser field is studied using the semiclassical approach, with a view towards cooperative chemical and optical pumping in laser production. The formalism is developed with the electronic-field representation, which treats collision and radiative interaction on the same footing. Electronic-field surfaces can be regarded as forming spectra for spontaneous emission; and particular emission events can be accounted for by propagating classical trajectories on emission electronic-field surfaces. Pre-emission loss from the excited state is dealt with by propagating classical trajectories on a loss surface along a complex contour of emission branch points. This loss surface is derived on the basis of localized radiative couplings between electronic-field states and provides a framework to treat the general problem of discrete state-continuum interactions. The formalism is applied to a two-state, collinear exponential model to compute S -matrix elements and transition probabilities between asymptotic states.

Negative-ion formation from surface scattering and the Anderson correlation energy U

Abstract A theoretical investigation of negative-ion formation from positive-ion-surface scattering is presented from a unified point of view. Based on the time-dependent Anderson-Newns model, the correlation energy U is seen to play an important role in the two-electron transfer process. Calculations of the probability of negative-ion formation are in good agreement with experiments on the conversion of H + (D + ) to H − (D − ) by scattering from a cesiated W(100) surface.

Surface magnetic field/laser synergistic effects on the predissociation of adsorbed diatomic molecules

Synergistic effects of the surface magnetic field (SMF) and the laser on the predissociation of adsorbed diatomic molecules are studied theoretically. It is observed that the couplings between the laser‐dressed and SMF‐split molecular potentials create interference effects within the nuclear motion. These effects can cause enhancement or diminution in predissociation rate as compared to the analogous gas‐phase laser‐induced process. SMF/laser‐induced predissociation rates for a few specific examples are derived using the semiclassical S‐matrix formalism and results for the enchancement and diminution are discussed along with their dependence on laser power densities.

Theory Of The Interaction Of Laser Radiation With Molecular Dynamical Processes Occurring At A Solid Surface

Recent experiments indicate that laser radiation can have significant nonthermal effects on molecular dynamical processes occurring at a solid surface. These processes include unimolecular decomposition and desorption. It has also been suggested that the interaction with laser radiation involves multiphoton absorption. This is particularly interesting since the power density of the radiation is only 10 watts/cm2, which is orders of magnitude less than the power densities typically needed to induce multiphoton absorption in the gas phase. In an effort to understand the mechanisms for such processes and to further explore the novel area of heterogeneous catalysis with lasers, theoretical studies have been undertaken for several different types of processes occurring at a solid surface: 1) laser-stimulated surface phenomena (migration, recombination and desorption), 2) laser-controlled heterogeneous rate processes, and 3) atom-surface collisions in the presence of laser radiation. This last type includes diffractive scattering, energy transfer and collisional ionization of an adatom by a gas-phase projectile atom.

Effect of surface magnetic fields on laser‐induced predissociation of diatomic molecules

A semiclassical S‐matrix formalism is used to demonstrate a possible laser surface synergistic effect.(AIP)

Theory of collision-induced ionization of adsorbed species on solid surfaces in the presence of laser radiation

A formalism is proposed for treating the problem of ionization of adsorbed species on solid surfaces. The ionizing agents are taken to be impact atoms and laser radiation with frequency low compared to the inverse of characteristic collision times. The physical constraints of short collision times and low laser frequency then allow one to treat the adatom-surface-plus-field system under the quasi-static approximation (QSA) and the impact-atom-adatom-surface collision dynamics under the impulse approximation (IMA). The latter leads to a time-dependent ionization cross-section which is factorizable into the square of an electron-atom scattering matrix element and a spectral function describing the energy-momentum distribution of electrons in the adatom-surface-plus-field system. The formalism focuses on the spectral function which is shown to be derivable from a single-particle Greens function exactly calculable for the present problem.

A semiclassical approach to collisional ionization with application to the ArHe system

Abstract A semiclassical approach based on the propagation of classical trajectories on potential surfaces analytically continued into the complex plane, together with a discretization procedure, has been developed for the problem of collisional ionization. Based on Franck—Condon considerations the formalism is reduced to that of the two-state approximation. Preionization loss and tunnelling beyond turning points have not been considered. Calculated partial ionization cross sections for the ArHe system show good agreement with a fully quantum mechanical treatment.