EFFECT OF TEMPERATURE ON THE INFRARED AND SUM-FREQUENCY GENERATION SPECTRA OF ADSORBATES

The dephasing and energy relaxation contributions to the line width in infrared (IR) and sum-frequency generation (SFG) spectra of adsorbate s are derived from the generalized master equation approach. Expressio n for the line shift is also obtained. The anharmonic interaction betw een the adsorbate and the substrate is expanded in a polynomial in ter ms of the adsorbate and phonon coordinates, and the dephasing is shown to be mainly due to two-phonon processes, while two-phonon, three-pho non or four-phonon processes can contribute to energy relaxation, depe nding on the relative values of the adsorbate vibrational and the phon on frequencies. The temperature-dependence data of the IR absorption f or C(111):H is found to be consistent with the theory, and the large l ine width for C(111):D can be accounted for by the efficient two-phono n energy relaxation process which is not available for C(111):H due to the higher adsorbate vibrational frequency for C(111):H. (C) 1997 Ame rican Institute of Physics.