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AIP conference proceedings

Laser/surface-enhanced isotope separation of species adsorbed on solid surfaces

J. Lin

Published Jan 1, 1986• 0 citations
Publisher

Abstract

Isotope separation of adsorbed species is studied by the steady‐state total excitation spectra governed by the frequencies (detunings) of the isotope species, the direct dipole‐dipole interaction and the phonon‐mediated indirect coupling. Under certain conditions, theoretical prediction of infinite steady‐state excitation is presented.

DOI
10.1063/1.35790

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Isotope separation of adsorbed species is studied by the steady‐state total excitation spectra governed by the frequencies (detunings) of the isotope species, the direct dipole‐dipole interaction and the phonon‐mediated indirect coupling. Under certain conditions, theoretical prediction of infinite steady‐state excitation is presented.
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.
We propose a method of laser isotope separation based on the phenomenon of adiabatic inversion and illustrate the method in a multilevel system consisting of a single ground state and multiplet excited states. In this model, population may be transferred from the ground state to one of the excited manifolds with nearly 100% efficiency by using laser pulses, the envelope of which rises slowly and then decays rapidly on an internal time scale that can be precisely calculated. Adiabatic inversion in this model is strongly dependent on the laser frequency, thus permitting separation of isotopes by faking advantage of small shifts in molecular or atomic energy level spacings. The criterion that determines whether a laser pulse is adiabatic also depends strongly on the laser frequency and thereby enhances the separation. We demonstrate the effectiveness of the method with a solution of the time-dependent Schroedinger equation for a specific model of two isotopes.
Non-intrusive laser-spectroscopical methods for the diagnostics of the gas-surface interaction in the near-field of a dielectric surface have been developed both experimentally and theoretically. The techniques base on either two-photon evanescent wave excitation or combined evanescent-volume wave excitation. Spectra obtained for sodium atoms at a glass-vacuum interface are quantitatively reproduced by a rigorous theoretical approach. In the case of pure evanescent wave excitation the excitation conditions can be chosen such that either the velocity distribution of desorbing atoms is determined or that atom-surface interaction parameters (polarizability of the adsorbed atoms, desorption rate, sticking coefficient) are obtained from the Autler- Townes splitting of the two-photon lines. By the use of combined evanescent-volume wave excitation one is able to distinguish optically between different groups of atoms interacting with the surface and one can extract their two- dimensional velocity distributions. In addition, information is obtained about superelastic scattering of the atoms interacting with the dielectric substrate.
Adsorbed species on a solid-state surface interact with the large number of substrate modes, which gives rise to thermal relaxation. Commonly, the temporal evolution of the quantum state of the adsorbate is described by a master equation for the level populations (vibrational bond, internal modes, etc). It is pointed out that this approach does not necessarily give a correct account of the coupling to the solid when the effective level widths become comparable to the level separations, or larger. It is shown that the evolution of the populations does not decouple any more from the time evolution of the coherences (off-diagonal matrix elements), which implies that a random-phase approximation cannot be justified, and that the density matrix of the adsorbate is not determined only by the golden-rule transition constants. Especially, the line profiles turn out to be very sensitive to the coherence-coherence couplings. Although the coherences vanish in thermal equilibrium, their time-regression operator, and hence their mutual couplings and their couplings to the populations, which determine the absorption profile, do not. This information is lost in a master-equation treatment of relaxation.