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Interaction of an adsorbed atom with a laser. Technical report

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

Published Dec 1, 1986• 0 citations
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Abstract

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.

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Theory of an infrared irradiated adbond

Sander van Smaalen, Thomas F. George

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.

International Laser Science Conference•Jan 1, 1986•0 citations

Thermal relaxation of adsorbed atoms in an intense laser field

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

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.

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Laser-heating of a transparent crystal via adsorbed atoms

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

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.

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Laser heating of a transparent crystal via adsorbed atoms

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

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.

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Laser-linewidth effects on the photon–phonon conversion rate at a gas–solid interface

Henk F. Arnoldus, Thomas F. George

An atom, bounded to the surface of a crystal; absorbs photons from a strong incident laser field and subsequently emits phonons into the solid. In this paper we study the influence of the laser linewidth on this photon–phonon conversion process. The appearance of coherences obscures the interpretation of the equation of motion in terms of transition rates and prohibits the derivation of a master equation in the transient region. The absorbed energy from the driving field, which is assumed to be resonant with a set of two levels only, results from the work done by the external field on the transition dipole. In the steady state, the energy absorption from the radiation field equals the energy emission in the phonon field. It is shown that in this long-time limit the coherences can be expressed in terms of the populations, and hence the system is again described by a masterlike equation. Specific aspects of the finite laser bandwidth are pointed out.

Journal of the Optical Society of America B•Feb 1, 1987•3 citations

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

Jui‐Teng Lin, Thomas F. George

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 system. Finally, the isotope selectivity given by the numerical results of the time-integrated excitation is discussed. It is shown that the optimal detuning for a weak coupling strength is further red-shifted for a strong isotopic coupling strength. Finally, energy feedback effects of the bath modes on the excitations of the active modes are investigated by combining a quantum excitation equation and a classical heat diffusion equation.

The Journal of Chemical Physics•Apr 15, 1983•5 citations
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.
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.
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.
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.
An atom, bounded to the surface of a crystal; absorbs photons from a strong incident laser field and subsequently emits phonons into the solid. In this paper we study the influence of the laser linewidth on this photon–phonon conversion process. The appearance of coherences obscures the interpretation of the equation of motion in terms of transition rates and prohibits the derivation of a master equation in the transient region. The absorbed energy from the driving field, which is assumed to be resonant with a set of two levels only, results from the work done by the external field on the transition dipole. In the steady state, the energy absorption from the radiation field equals the energy emission in the phonon field. It is shown that in this long-time limit the coherences can be expressed in terms of the populations, and hence the system is again described by a masterlike equation. Specific aspects of the finite laser bandwidth are pointed out.
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 system. Finally, the isotope selectivity given by the numerical results of the time-integrated excitation is discussed. It is shown that the optimal detuning for a weak coupling strength is further red-shifted for a strong isotopic coupling strength. Finally, energy feedback effects of the bath modes on the excitations of the active modes are investigated by combining a quantum excitation equation and a classical heat diffusion equation.