Transient vibrational mode renormalization in dipole-coupled adsorbates at surfaces

Dipole interactions among adsorbates at solid surfaces can strongly affect the intensities, positions, and line shapes of vibrational resonances. An understanding of these effects has been important in spectroscopic investigations of surface structure. Here, the adsorbate dipole interactions are shown to create transient spectral intensity and resonance 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 dependent upon the magnitude of interadsorbate dipole interactions, and vibrational excitation modifies the effective oscillator dynamic dipoles that determine these interactions. The vibrational modes are different (renormalized) after excitation because of the change in coupling. The effects account for unusual spectral transients observed in recent 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. Chabal, and A. L. Harris, J. Chem. Phys. 99, 6114 (1993)]. The predicted effects serve as a novel time-resolved probe of the strength of dipolar interactions in adsorbate layers, and will arise in any adsorbate layer where the vibrational dynamic dipole interactions are large enough to cause spectral intensity borrowing among different adsorption sites or different adsorbates.