Temperature dependence of the fundamental vibrational transition of CO on NaCl(100) single crystal surfaces has been measured between 4 and 55 K using Fourier-transform infrared spectroscopy. The absorption profiles at various temperatures are very nearly symmetric. At 4 K, the transition is centered at 2154.93 cm−1, and the full-width at half-maximum (FWHM) is 0.17 cm−1; at 55 K, the transition shifts to 2155.14 cm−1, and the FWHM broadens to 0.39 cm−1. The temperature-dependent frequency shift and broadening of the fundamental transition are interpreted by a vibrational phase relaxation process, in which the coupling between the stretching mode of CO and a low frequency CO-surface mode causes the dephasing of the excited state vibrational motion. The Boltzmann factor associated with the spectroscopic temperature dependence allows us to determine that the surface mode has a frequency of 40 cm−1 . We associate this mode with the frustrated translational motion of CO over the NaCl(100) surface. The observed temperature-dependent absorption band undoubtedly is a consequence of collective motions among the CO stretching vibrations within the monolayer. However, the current model, that assumes this collective motion is coupled by the CO transition dipoles, cannot explain the data. Further theoretical work will be required to understand the infrared spectroscopy of CO on NaCl(100). Below 10 K, the CO fundamental absorption becomes temperature independent. However, the limiting bandwidth and bandshape depend on the NaCl(100) crystal surface preparation. For one set of crystals, the 4 K bandshape was Lorentzian with a FWHM of 0.17 cm−1 . For another set, the 4 K bandshape was Gaussian with a FWHM of 0.11 cm−1. We believe these bandwidths, narrower than those reported previously for any adsorbate system, are limited by heterogeneities of the monolayer and/or substrate. The 0.11 cm−1 bandwidth provides a lower limit of 45 ps for the lifetime of vibrationally excited CO on NaCl(100).
Various approximations to the transition dipole moment matrix element (n'IMln) are compared with each other and to exact (numerical) values of this overlap integral for different n->n' transitions in a Morse potential with a linear dipole moment function. By partitioning the numerical integral into different contributions that involve the classically allowed and forbidden regions of each wave function, we have learned what conditions must be satisfied for validity of the different approximations. In particular, we consider the Landau approximation to the quasiclassical matrix element in which the exact wave function for the upper state is replaced by the Wentzel-Kramers-Brillouin (WKB) wave function in the classically allowed region of that state. We find that the Landau approximation is more accurate than might have been expected because of the compensation of the neglected tunneling contribution by the singular behavior of the WKB wave function in the classically allowed neighborhood of the turning point. Based on this study, we suggest an improved semiclassical approximation for transition dipole matrix elements that involve an arbitrary dipole moment function. This method is applied to the n'-0 transition of a Morse oscillator using a linear dipole moment function; it can reproduce the exact values of the transition dipole moment matrix element to better than 5% for n' = 1 to n' = 15. Under the condition that the dipole moment function is slowly varying or decreases monotonically with increasing internuclear separation, a simple expression is presented for estimating relative strengths of neighboring high overtone transitions.
Vibrational energy flow from CO physisorbed on NaCl(100) has been investigated. Infrared spectroscopy of monolayer CO reveals the narrowest linewidth reported for any surface-bound molecule. The vibrational band at 2154.98 cm -I has a full-width at half-maximum (FWHM) of 0.09 cm -1 at 4 K. We believe that this residuallinewidth is due to imperfections of the crystal face. Nevertheless it places a lower limit for an vibrational relaxation processes of 60 ps. The linewidth increases above 10 K, a behavior which can be accounted for by a vibrational phase relaxation mechanism. Attempts to photodesorb CO from its substrate by laser excitation into the vibrational absorption band reveal an exceedingly inefficient quantum yield of <1>,,;; 10 ... 7 • Finally laser excitations of a multilayer of CO on N aCl ( 100) at 22 K demonstrate that vibrational energy can be stored in the adsorbed CO long enough to allow fluorescence however relaxation to the substrate reduces the fluorescence lifetime from that of the bulk crystal. These measurements (bandwidth, vibrational desorption quantum yield, and fluorescence lifetime) of monolayer and multilayer CO on NaCl( 100) indicate that the vibrational motions are poorly coupled to this alkali halide substrate.
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