Spectroscopic manifestations of orientational phase transition in adsorbate monolayer

Abstract

In the framework of the virtual crystal approximation, the temperature dependences of frequencies and integral intensities of infrared absorption spectral lines are established for an adsorbate monolayer near the point of the order-to-disorder orientational phase transition. The results obtained are consistent with the experimentally observed temperature dependences of the vibrational spectra for the 2 × 1 monolayer CO/NaCl (100). For this system, the Davydov splitting does not vanish on the phase transition due to a nonzero value of the average dipole moment (unlike molecular crystals and orientationally planar monolayers in which the average dipole moment becomes zero at the phase transition temperature).

Keywords:

• orientational phase transition
• phase transition temperature
• adsorbate
• monolayer
• infrared spectroscopy

Acknowledgements

The authors would like to thank Dr V.D. Danchuk and Professor G.O. Puchkovska for fruitful discussions regarding the peculiarities of Davydov splitting in molecular crystals.