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Physical review. B, Condensed matter

Thermal relaxation of adsorbed atoms in an intense laser field

Henk F. Arnoldus, Sander van Smaalen, Thomas F. George

Published Nov 15, 1986• 12 citations
Publisher

Abstract

Adsorbed atoms on the surface of a harmonic lattice are immersed in a strong laser field. The optical Bloch equations are derived, which include the thermal relaxation and the coherent excitation of the adbond. This is accomplished by a transformation to dressed states, which diagonalizes the interaction with the laser. The single-phonon couplings are then understood as transitions between dressed states. The radiative contributions for arbitrarily strong fields are obtained in the master equation, and it is shown that the coherences with respect to the dressed states decay exponentially, due to the phonon relaxation. General properties of the competing phonon-induced redistribution and optical excitation of the level populations are presented, and exemplified by an explicit elaboration of a three-level system. The results are amenable to analytical evaluation once the interaction potential is prescribed, and extensions of the approach to include multiphonon processes are straightforward.

DOI
10.1103/physrevb.34.6902

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Adsorbed atoms on the surface of a harmonic lattice are immersed in a strong laser field. The optical Bloch equations are derived, which include the thermal relaxation and the coherent excitation of the adbond. This is accomplished by a transformation to dressed states, which diagonalizes the interaction with the laser. The single-phonon couplings are then understood as transitions between dressed states. The radiative contributions for arbitrarily strong fields are obtained in the master equation, and it is shown that the coherences with respect to the dressed states decay exponentially, due to the phonon relaxation. General properties of the competing phonon-induced redistribution and optical excitation of the level populations are presented, and exemplified by an explicit elaboration of a three-level system. The results are amenable to analytical evaluation once the interaction potential is prescribed, and extensions of the approach to include multiphonon processes are straightforward.
The irradiation by infrared laser light of an atom adsorbed on the surface of a harmonic crystal is considered. The dynamic coupling between the atom (its motion) and the substrate degrees of freedom (phonon field) in the presence of a confining potential well (van der Waals bond between atom and crystal) gives rise to thermal relaxation of this adbond configuration. Both the atom and the substrate are assumed to be transparent, but the bond is allowed to have non-vanishing dipole-moment matrix elements, which couple the external field to the adsorbate. The equation of motion for the reduced adbond density operator is obtained with reservoir theory, and the relaxation constants are expressed in properties of the crystal. With a similar method, the spectral profile for absorption of weak radiation is derived. Subsequently, the illumination by a strong finite-linewidth laser field which is in close resonance with a single transition of the adbond is examined. The optical Bloch equations in operator form are derived and applied to study the process of laser heating of the crystal. It is pointed out how this mechanism can be understood as resulting from (multi)photon-phonon conversion reactions which are mediated by the adbond.
A coated surface of a crystal is irradiated by intense infrared light. The optically active atomic bonds absorb photons from the laser field, and the subsequent spontaneous decay goes together with emissions of phonons into the crystal. This photon phonon conversion results in an energy flux into the crystal. An equation for this flux is derived from the master equation for the level populations of the dressed atomic states. The saturation limit is discussed, and the general theory is illustrated with two examples. Furthermore, it is outlined with qualitative arguments that quantum yield of photodesorption is not sensitive to the laser power. Keywords: Laser heating; Transparent crystal; VIA Adsorbed atoms; Photon phonon conversion; Master equation; photodesorption.
A coated surface of a crystal is irradiated by intense infrared light. The optically active atomic bonds absorb photons from the laser field, and the subsequent spontaneous decay goes together with emissions of phonons into the crystal. This photon-to-phonon conversion results in an energy flux into the crystal. An equation for this flux is derived from the master equation for the level populations of the dressed atomic states. The saturation limit is discussed, and the general theory is illustrated with two examples. Furthermore, it is outlined with qualitative arguments that the quantum yield of photodesorption is not sensitive to the laser power.
The dynamics of an adsorbed atom irradiated by an infrared laser in resonance with the vibrational levels of the adbond can be described by the reduced density matrix of the adbond. A nonperturbative approach for the laser-adbond interaction is presented. Relaxation against the substrate degrees of freedom is described by second-order perturbation theory. Both pulsed lasers and cw lasers are considered. Expressions for the transition probabilities are derived in the low-intensity and in the high-intensity limits. It appears that the time evolution of the occupation probabilities is coupled to those of the coherences, i.e., a master equation cannot be obtained. The time evolution of all the elements of the reduced density matrix has to be taken into account. These expressions act as a basis for the study of physical processes as resonant de sorption, resonant heating, and laser-induced thermal desorption. Resonant desorption is described by the addition of loss terms into the equations of motion of the occupation probabilities. Expressions for the rate of desorption as well as the rate of energy transfer to the substrate are given. In particular, a comparison is made for the effect of a cw laser vs a pulsed laser on the desorption and resonant heating.
Recent experiments with single atomic ions isolated in electromagnetic traps have demonstrated abrupt changes in observed fluorescence intensities, which have been interpreted as evidence for quantum jumps as the atom changes energy levels. This presents the seeming paradox of discontinuously changing observables from a system whose dynamical evolution is governed by the continuous optical Bloch equations. In this paper I present an analysis of the dynamics of a model three-level atom interacting with two laser fields in terms of the dressed states of the atom-laser system. These dressed states are eigenstates of a Hamiltonian which includes laser-atom interactions, and thus they simplify the discussion of the time evolution of the combined laser-atom system. This dressed-state description is particularly useful in the regime in which both laser intensities are strong enough to saturate their respective transitions. I calculate the radiative lifetimes of the different species of dressed states and the branching ratios for decay to other dressed states, and demonstrate the consequences of these quantities for the observed fluorescence pattern. The calculated duration of bright and dark periods shows a pronounced dependence on the detunings of the lasers.
A derivation of surface-dressed optical Bloch equations, involving a treatment of surface-reflected photons and a surface plasmon resonance, is presented for a collision-damped two-level atom near or adsorbed on a metal surface. Effects of the laser bandwidth are included by means of a phase-diffusion model for the driving field. In the weak-field or large detuning limit, the population inversion and resonance fluorescence spectrum are obtained analytically. These quantities along with the surface-induced phase-decay constant of the adatom show strong oscillatory behavior as a function of the adatom-surface distance.