We give the extension of our quantum-statistical theory of desorption for systems with many physisorbed bound states in the surface potentia1. Rate equations are set up and the desorption time t~is properly identified as the smallest eigenvalue of the matrix of transition probabilities. The latter are calculated exactly in second-order perturbation theory for a surface Morse potential. We show that desorption in weakly coupled systems with many bound states proceeds predominantly through one-phonon cascades. Two-phonon contributions are shown to be small. Desorption times are calculated for the He-LiF, He-NaF, He-graphite, H-NaCl, He-Ar, and Xe-W systems. The temperature regime over wiJich a Frenkei-Arrhenius parametrization ts =td exp(g/heT) can be invoked is given. Our theory which is essentially parameter-free produces prefactors td in the whole experimental range of physisorption from 10 ' to 10 '4 sec.

Considering the spin of a local magnetic atom as a quantum mechanical operator, we illustrate the dynamics of a local spin interacting with a ballistic electron represented by a wave packet. This approach improves the semi-classical approximation and provides a complete quantum mechanical understanding for spin transfer phenomena. Sending spin-polarized electrons towards a local magnetic atom one after another, we estimate the minimum number of electrons needed to flip a local spin.

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We develop an explicitly time-dependent theory for one-step resonant excitation-deexcitation processes of core electron states in diatomic molecules. Emphasis is placed on a conceptual picture demonstrating how the effective time of the formation of the spectra-which is influenced by the bandwidth of the exciting radiation, by the excitation of the molecule being resonant or off resonant ͑detuned͒, and by the actual core hole lifetime-changes the appearance of the deexcitation electron spectra. Explicit time-dependent model calculations for three final states each of 1s-hole excited N 2 ͑including one spectator decay͒ and C 1s -hole excited CO allow demonstration of the various consequences for the spectral shapes which derive from these influences. In particular, off-resonance excitation is shown to shorten the effective time of the spectrum formation below the lifetime of the core-excited state leading to the recently observed collapse of the vibrational structure in the spectrum. Our calculated spectra also demonstrate the influences of the relative positions and shapes of the potential curves involved. On resonance, the nodal structure of the vibrational wave functions of the core-excited state is reflected in the shapes of the spectator decay spectra of N 2 with a soft final state interatomic potential.