Theory of Inverse Raman and Raman Gain Spectroscopy

Abstract

Conventionally Raman spectroscopy has been regarded only as a scattering process in which a portion of the monochromatic radiation incident on a molecular sample is scattered inelastically. The differential scattering cross-section, dσ/dΩ, relates to the power scattered into a given element of solid angle dΩ, σ measuring the rate at which energy is removed from the incident beam by scattering relative to the rate at which energy crosses a unit area perpendicular to the direction of propagation of the incident beam [1]. σ and dσ/dΩ are usually related to a single molecule and a unit wavenumber interval and the differential Raman scattering cross-sections relate to the relevant transition polarisability components [1]. The observation of the stimulated Raman effect by Woodbury and Ng [2] and the interpretation of the effect by Hellwarth [3] shortly after the construction of the first laser, the ruby laser [4], provided an indication of the benefits that were to accrue as a result of the power density available from these radical new light sources. Shortly thereafter Jones and Stoicheff [5] using a “giant pulse” ruby laser demonstrated the process to which they gave the name Inverse Raman Effect. This process occurs when two coincident beams of monochromatic radiation are incident on a Raman medium, the frequency difference of the two beams coinciding with a Raman active transition in the molecules under study.