Role of Coherences in the Relaxation of Adsorbates.
STATE UNIV OF NEW YORK AT BUFFALO DEPT OF CHEMISTRY
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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 anymore 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 only determined 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 determines the absorption profile, does not. This information is lost in a master-equations treatment of relaxation.
- Atomic and Molecular Physics and Spectroscopy
- Nuclear Physics and Elementary Particle Physics
- Quantum Theory and Relativity