TRANSIENT VIBRATIONAL-MODE RENORMALIZATION IN DIPOLE-COUPLED ADSORBATES AT SURFACES

Dipole interactions among adsorbates at solid surfaces can strongly af fect the intensities, positions, and line shapes of vibrational resona nces. An understanding of these effects has been important in spectros copic investigations of surface structure. Here, the adsorbate dipole interactions are shown to create transient spectral intensity and reso nance position changes when vibrational modes are excited in ultrafast pump-probe laser experiments at surfaces. The spectral changes occur because the intensities and positions of vibrational resonances are de pendent upon the magnitude of interadsorbate dipole interactions, and vibrational excitation modifies the effective oscillator dynamic dipol es that determine these interactions. The vibrational modes are differ ent (renormalized) after excitation because of the change in coupling. The effects account for unusual spectral transients observed in recen t pump-probe experiments on the Si-H stretching modes of vicinal H/Si( 111) surfaces [K. Kuhnke, M. Morin, P. Jakob, N. J. Levinos, Y. J. Cha bal, and A. L. Harris, J. Chem. Phys. 99, 6114 (1993)]. The predicted effects serve as a novel time-resolved probe of the strength of dipola r interactions in adsorbate layers, and will arise in any adsorbate la yer where the vibrational dynamic dipole interactions are large enough to cause spectral intensity borrowing among different adsorption site s or different adsorbates.