Surface to bulk relaxation

Section III deals with the surface excitations of the anthracene crystal, confined in the first (SJ, second (S2) and third (S3) (001) lattice planes. The experimental observations are briefly summarized. A simple model shows how the fast radiative decay arises and how the underlying bulk reflection modulates this superradiant emission, as well as why gas condensation on the crystal surface strongly narrows this emission, thus accounting for the observed structures. An intrinsic process is proposed to explain the surface-to-bulk relaxation at low temperatures, observed in spite of the very weak surface-to-bulk coupling for k 0 states. 

The position and the width of this dip, at about I Ocm"1 above the Raman peak, indicate the energy gap above which the intrasurface relaxation, assisted by acoustical-phonon creation, competes with the surface-to-bulk relaxation. If we figure 3 to 4cm -1 for the relaxation rate to the bulk (for K 0 wave vectors cf. Section 1II.A.3), we conclude that the intrasurface relaxation, at 10cm 1 above the emitting state, is comparable. This conclusion on the acoustical-phonon relaxation is consistent with the theoretical estimates121127 (cf. Section III.A.4) and the experimental values derived by KK analysis of the bulk reflectivity (Section II.C.3b). 

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