HIGH-RESOLUTION PIRSS USING A TUNABLE DIODE-LASER - THE MULTIPLET OF THE COLLECTIVE V(2) BENDING VIBRATION OF THE P(2X1) MONOLAYER CO2 ADSORBED ON NACL(001)

Polarization infrared surface spectroscopy (PIRSS) has been used to in vestigate the multiplet of the collective nu(2) bending vibration of t he p(2 x 1) monolayer CO2 adsorbed on NaCl(001) in the spectral range 650-665 cm(-1) at temperatures between 10 and 90 K. Tunable diode lase r spectroscopy with a resolution of at least 5 x 10(-3) cm(-1) appears to be a powerful method to study the lineshape of extremely narrow ab sorptions observed for highly ordered adsorbates on insulator single c rystal surfaces. The experiments have been performed in transmission g eometry with a lead salt laser diode. Four polarized absorptions are e xpected in the spectral range of the bending vibration of CO2 adsorbed on NaCl(001) at monolayer coverage due to the combination of site sym metry and correlation field splitting. They have been observed and stu died in detail using four different laser modes. The continuously tuna ble range of a single mode is limited to about 3 cm(-1). All spectra h ave been recorded as a function of temperature. In the low-temperature limit, linewidths down to 0.04 cm(-1) have been observed, the bands b eing clearly asymmetric. One of the observed quarter components shows an unexpected doublet splitting under the extremely high resolution em ployed.