Vibrational frequency correlation and vibrational dephasing in aqueous nitrate solutions

Abstract

The vibrational frequency correlation functions

of the v ₁(A′₁) mode of NO₃ ^- ions in aqueous NH₄NO₃ solutions are calculated directly from isotropic Raman profiles, assuming that the process ω(t) is gaussian. The calculated correlation functions are oscillatory decaying functions with a time period of about 0·35 ps. A model based on the generalized Langevin theory is proposed and a theoretical correlation function, obtained by first-order truncation of Mori's continued fraction representation, is found to reproduce both the observed vibrational frequency correlation functions and the vibrational correlation functions. The observed short-time oscillation with a frequency of about 95 cm^-1 is attributed to an inter-molecular librational motion between a NO₃ ^- ion and surrounding water molecules and cations.