Analysis of perturbed H2O vibrations beyond Fourier transform

TL;DR

This paper introduces novel analysis techniques for vibrational dynamics in liquid water, revealing amplitude and frequency modulation effects beyond traditional Fourier analysis.

Contribution

It proposes and applies new demodulation and zero-crossing methods to analyze molecular vibrations without relying solely on Fourier transforms.

Findings

Vibrational modes are amplitude-modulated by libration and overtone frequencies.

Hydrogen bond variations cause frequency modulation of vibrational signals.

Demodulation techniques reveal detailed vibrational dynamics beyond standard spectra.

Abstract

New analysis methods for the vibrational dynamics from molecular dynamics are proposed and applied to liquid H$_{2}$O. The internal modes of H$_{2}$O are amplitude-modulated by Langevin dynamics with the frequency of the H$_{2}$O libration (~600 cm$^{-1}$) and its first overtone (~1200 cm$^{-1}$). The carrier signal is frequency-modulated by the variation of the hydrogen bond strength due to thermal motion and by dephasing collisions. The standard power spectra of the bond parameters yield broadened normal modes due to a superposition of several perturbations. Here, the oscillating bond lengths and angles are demodulated by the spline interpolation of the minima or maxima. Additionally, a zero-crossing method is described, which yields the spectrum of the frequency modulated carrier without Fourier transform by directly converting the oscillation periods.