The surface infrared reflectance spectra of Mo(100)-p(1×1)H and W(100)-p(1×1)H are both characterized by two vibrational absorptions in the 800–4000-${\mathrm{cm}}^{\mathrm{-}1}$ region—a broad band at 1016 ${\mathrm{cm}}^{\mathrm{-}1}$ at T=100 K for H on Mo(100) and at 1069 ${\mathrm{cm}}^{\mathrm{-}1}$ at T=300 K for H on W(100), corresponding to the symmetric stretch (${\nu }_1$), and a narrow derivative-like feature at 1302 ${\mathrm{cm}}^{\mathrm{-}1}$ on Mo(100) and at 1269 ${\mathrm{cm}}^{\mathrm{-}1}$ on W(100), identified as the first overtone of the wag motion (2${\nu }_2$). The physical origin of the line shapes, as well as the larger intensity of the 2${\nu }_2$ 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 ${\nu }_1$ and 2${\nu }_2$ linewidths. The most distinctive feature of the spectra, the derivative or ‘‘Fano shape’’ of 2${\nu }_2$, arises from a nonadiabatic coupling between the sharp 2${\nu }_2$ vibration and the continuum absorption due to surface electronic transitions. The asymmetry parameter ν${̃}_0$τ̃, which gauges the 2${\nu }_2$-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 2${\nu }_2$ on both surfaces at T=100 K, 12.2 ${\mathrm{cm}}^{\mathrm{-}1}$ on Mo(100) and 18.5 ${\mathrm{cm}}^{\mathrm{-}1}$ on W(100), set a limit on the lifetime of these vibrational levels at $T_1$≥0.9 ps and $T_1$≥0.6 ps, respectively.

Direct evidence for the electronic excitation continuum that interacts with 2${\nu }_2$ is obtained by analyzing changes in broadband reflectivity measurements on W(100)/H and Mo(100)/H as a function of coverage. Both H-saturated surfaces exhibit a strong absorption with x-y symmetry that can be related to optical excitation of surface states. Further support for surface-state participation in the nonadiabatic process is inferred from the strong attenuation of the 2${\nu }_2$ intensity upon CO co-adsorption.

• Received 26 April 1988

DOI:https://doi.org/10.1103/PhysRevB.38.3112