Jui-teng Lin

Laser-Generated Electron Emission from Surfaces: Effect of the Pulse Shape on Temperature and Transient Phenomena.

Surface temperatures generated by gaussian, rectangular, and triangular laser pulses are determined by solving a heat diffusion equation. The dependence of the temperature on the pulse shape and, in turn, the dependence of the thermal diffusivity and absorptance on the temperature are investigated. The lifetime of an adspecies on a laser‐heated solid (e.g., Cs on W) is estimated in terms of the temperature, the desorption energy, and the coverage. The mechanism of laser‐generated electron emission from the adspecies is analyzed by means of the Richardson equation. A condition for generating an intense electron beam is that the laser pulse duration and the rise time of the temperature must be less than the lifetime of the adspecies.

Quantum–stochastic approach to laser‐stimulated desorption dynamics and population distributions of chemisorbed species on solid surfaces

A quantum–stochastic treatment is presented for multiphoton processes of chemisorbed species on solid surfaces. The average excitation is evaluated for different stochastic processes. In the Markovian fast modulation limit, the average excitation takes a Lorentzian form, while taking Gaussian in the slow modulation limit. The addition of the memory effects is calculated by Mori’s projection operator technique and is found to be a function of the correlation time and the frequency modulation of the stochastic processes. For low excitations, the average excitation is found to be linearly proportional to the laser intensity, and is saturated by the red shift of the anharmonicity for high excitations. The overall broadening of the infrared line shape is caused by the fluctuation of the adspecies‐field interaction energy, the excitation‐induced surface migration, the phonon‐dispersion‐induced dephasing, and the energy relaxation due to the adspecies–phonon coupling. The population distributions for the slow T1...

Quantum dynamical model of laser/surface-induced predissociation: Multiphoton—multiphonon processes and photon/phonon-dressed states

Abstract By the dynamical energy populations of a photon/phonon-dressed quantum system, laserstimulated surface predissociation (LSSPD) of an adsorbed diatomic molecule, characterized by the laser pumping rates, the phonon-induced damping factor (level width), the field detuning, and the phase detuning (Landau-Zener form), is studied. The optimal detunings for the enhancement of LSSPD are shown by the numerical results of a set of coupled generalized quantum equations (GQE). The important features of LSSPD and the possible predissociation of a multilevel multistate system via multiphoton—multiphonon processes are discussed.

On the synergistic effects of laser/phonon-stimulated processes: A master equation approach☆

Abstract The synergistic effects of an adspecies/surface system influenced by a coherent laser field and an incoherent thermal phonon field are theoretically investigated. A generalized master equation is derived in the Schrodinger-Markoff picture and the total stimulated transition rates are decomposed into three parts: thermal phonon, laser and the interference terms. The energy transfer dynamics is pictured via the evolution equation of the average excitation of the active mode. A random phase of the off-diagonal matrix elements induced by the laser-stimulated surface processes is introduced. Finally, possible applications of the laser/phonon-stimulated master equation are discussed.

Classical Stochastic Model of Laser-Stimulated Surface Processes and the Selective Nature of Laser Excitation via Multiphonon Couplings.

Abstract The dynamical behavior of laser-stimulated surface processes (LSSP) is studied by the generalized Langevin equation via the memory effects of the damping kernel, the dephasing kernel and mode-mode interactions. The temperature-dependent averaged energy absorption rate (which characterizes the line shape) is calculated by solving the Laplace-Fourier transform of the velocity correlation function. The features of the response function and its overall line broadening are discussed in terms of Markovian processes. The nature of LSSP and laser-selective effects are discussed in terms of the multiphonon coupling strength and the related “internal resonant” condition. It is shown that laser-selective bond breaking is possible for a slow intramolecular vibrational relaxation rate which is governed by a high-order multiphonon process or a far off “internal resonance”.

Multiphoton-multiphonon theory of laser-stimulated surface processes

A theoretical model for multiphoton laser-excitation and multiphonon (t1) and phase (T2) relaxation of molecules adsorbed on a solid surface is developed. The ensemble-averaged excitation number for the infrared-active adspecies is obtained within a Heisenberg-Markovian approximation. The selective nonthermal excitation is sensitive to the mass ratio of the surface atoms and the adspecies, the order of the multiphonon excitation and the duration of the laser pulse.

A classical model for stochastic multiphoton absorption processes of anharmonic molecules

Abstract Classical power absorption of an anharmonic molecule in intense infrared radiation is calculated. A generalized damping factor and an effective random phase are introduced for the stochastic field-molecule interaction. It is shown that the over-all broadening of the power response has a fundamental lorentzian, no matter what the source of line broadening is. Excellent agreement with the quantum-mechanical results is obtained by quantizing the classical model.

Quantum dynamical model of laser‐stimulated isotope separation of adsorbed species: Role of anharmonicity, coupling strength, and energy feedback from the heated substrate

A quantum model of a heterogeneous system consisting of a mixture of isotopes adsorbed on a solid surface and subjected to laser radiation is presented. The model system is described by a total Hamiltonian including direct and indirect (surface‐phonon‐mediated) couplings. The equations of motion are derived in the Heisenberg–Markovian picture in which the many‐body effects of the surface phonon modes and the adspecies are reduced to an overall broadening (damping factor) given by the sum of the energy (T1) and phase (T 2) relaxations. The effects of the dephasing and anharmonicity on the average excitation are investigated. The ‘‘bistability’’ feature with a red‐shifted optimal detuning is discussed in terms of the solution of a cubic equation. A diagonalization procedure is presented in a new basis which reveals the effects of the coupling strength, the frequency difference, and the level width of the isotopes on the total steady‐state excitation, which in turn reflects the surface spectrum of the model ...

Quantum model of laser excitation of isotopic species adsorbed on a solid surface

A quantum model of laser-excited interacting isotopic adspecies is presented. The bistability governed by a cubic equation of the active-mode-excitation is analyzed in terms of the nonlinear effects of the anharmonicity, where the transition point and critical laser intensity are exactly calculated. The isotopic selectivity governed by the frequency difference and the coupling strength of the active modes are investigated.

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.