Sample modulated Raman spectroscopy and frequency modulated visible light—resonance Raman spectrum of a polydiacetylene fiber

Abstract

Methods for obtaining sample modulated Raman (SMR) spectra and frequency modulated visible light are described. The sample modulated Raman method is one in which the frequency of the Raman scattered light is modulated by the sample which is responding to an externally applied periodic stress. The Raman active modes of the sample which are due to the vibrational motion of those molecular-scale units that change in response to the external stress are modulated and appear in the frequency-locked phase sensitively detected Raman spectrum. Those modes due to units that do not respond significantly to the stress do not appear. In principle the SMR method permits determination of those molecular units of a complex substance that are responsible for the substance’s response to an external stress. The Raman scattered light is frequency modulated, has the wave form ν=ν0−νi−(δν) sin(2πft), where ν0 is the incident monochromatic light, νi is the vibrational frequency of the statically stressed material, and (δν) is the magnitude of the frequency shift caused by the maximum stress applied at the frequency f.