Coupling of H vibration to substrate electronic states in Mo(100)-p(1 x 1)H and W(100)-p(1 x 1)H: Example of strong breakdown of adiabaticity

The surface infrared reflectance spectra of Mo(100)-p(1 x 1)H and W(100)-p(1 x 1)H are both characterized by two vibrational absorptions in the 800--4000-cm/sup -1/ region: a broad band at 1016 cm/sup -1/ at T = 100 K for H on Mo(100) and at 1069 cm/sup -1/ at T = 300 K for H on W(100), corresponding to the symmetric stretch (ν1), and a narrow derivative-like feature at 1302 cm/sup -1/ on Mo(100) and at 1269 cm/sup -1/ on W(100), identified as the first overtone of the wag motion (2ν2). The physical origin of the line shapes, as well as the larger intensity of the 2ν2 than expected from adiabatic considerations, were investigated through phenomenological line-shape analysis of data obtained as a function of temperature and relative H/D and H/CO coadsorption concentrations. Inhomogeneous broadening is negligible, and dephasing processes contribute weakly to both ν1 and 2ν2 linewidths. The most distinctive feature of the spectra, the derivative or ''Fano shape'' of 2ν2, arises from a nonadiabatic coupling between the sharp 2ν2 vibration and the continuum absorption due to surface electronic transitions. The asymmetry parameter nu0tau, which gauges the 2ν2-continuum coupling strength, is temperature insensitive and does not exhibit an isotopic dependence, as predicted for a strong breakdown of adiabaticity. The narrow linewidths observed for 2ν2 on both surfaces at T = 100 K, 12.2 cm/sup -1/ on Mo(100) and 18.5 cm/sup -1/ on W(100), set a limit on the lifetime of these vibrational levels at T1≥0.9 ps and T1≥0.6 ps, respectively