High-resolution PIRSS using a tunable infrared diode laser: the Davydov doublet of the antiferroelectric (2 × 1) monolayer CO adsorbed on NaCl(001)

Abstract

The temperature dependence of the Davydov doublet of the antiferroelectric (2 × 1) monolayer CONaCl(001) has been studied between 11 and 25 K by means of polarization infrared surface spectroscopy (PIRSS) using a tunable lead salt diode laser with a resolution better than 5 × 10 −3cm −1. At 11 K the linewidth of the symmetric vibrational exciton, polarized perpendicular to the surface, was determined to be 0.035 cm −1, the linewidth of the antisymmetric vibrational exciton, polarized parallel to the surface, to be 0.025 cm −1, the sharpest spectral line of an adsorbate observed. To obtain phase relaxation times T 2 in the low-temperature region, we fitted the parameters of the exchange dephasing model to the experimental data. A satisfactory fit for both the frequency and the linewidth is obtained at temperatures below 25 K. Above 25 K the antisymmetric vibrational exciton becomes extremely broad and finally disappears, and a successful fit was obtained only for the linewidth of the symmetric vibrational exciton. It is shown how the vibrational energy relaxation time T 1 may be obtained by varying the spectral power of the laser light.