Investigation of one- and two-dimensional vibrational density of states using two-phonon infrared absorption spectroscopy

Abstract

The coherent excitation of two-phonon states using infrared absorption spectroscopy (IRAS) is employed to investigate band structure effects of vibrational modes. Examples representative of one-dimensional (1D), two-dimensional (2D) as well as mixed lateral coupling behavior are presented. It is demonstrated that each of these examples displays a characteristic signature which can be traced to its dissimilar spectral density of states. Momentum compensation of pairs of coherently excited adsorbate phonon modes is held responsible for the excitation of vibrational modes far from the Γ point using infrared absorption spectroscopy. The line shape of the resulting absorption feature (two-phonon continuum of states) then represents an average of all possible (νq,ν−q′) phonon pairs. It is shown that despite inherently lacking the ability to probe phonon dispersion curves, IRAS nevertheless is able to extract the vibrational density of states with a perfect average over the surface Brillouin zone. In particular, the two-phonon spectra contain sufficient information to derive the dimensionality of an adsorbate system and to extract the bandwidth of the contributing modes.