Raman Study of Hydrogen Bonding and Long-Wavelength Lattice Modes in an L-Alanine Single Crystal

Abstract

This paper presents a detailed polarization study of the Raman scattering spectrum from the lattice phonons of a single L‐alanine crystal, with and without the presence of an external de electric field from 120 to 350°K. The scattering experiments were made with the incident Ar ion laser beam propagating along the 〈100〉, 〈010〉, and 〈001〉 directions of the crystal. The Raman spectra of four different scattering configurations associated with each crystal orientation were measured. The experimental results have shown the presence of a large orientational anisotropy as well as polarization anomalies in the lattice Raman spectrum of the L‐alanine single crystal. By means of several different experiments (light transmission measurement, different excitation laser frequencies, temperature variations of the crystal, and de electric field study), several existing possibilities that might be associated with the polarization anomalies have been ruled out. To account for the observed effect, a picture of dynamic disorder due to the proton motion that exists within the hydrogen bond has been proposed. In addition to the spectral features associated with the lattice modes, the spectral bandwidth studies of the $–{\mathrm{NH}}_3^{+}$ libration and the $–\mathrm{N}–\mathrm{H}$ stretching also give a result consistent with the proposed picture. Due to the nature of charge fluctuations present in a hydrogen bond, it is suggested that this phenomena should be present in all hydrogen‐bonded molecular systems.