Raman scattering and vacuum fluctuation: An Einstein-coefficient-like equation for Raman cross sections

Area of Science:

• Molecular Spectroscopy
• Quantum Electrodynamics
• Physical Chemistry

Background:

• Raman scattering (σRaman) is crucial for molecular characterization, but its theoretical connection to absolute stimulated Raman scattering cross sections (σSRS) is unclear.
• Existing frameworks primarily use σRaman, while σSRS offers an alternative interpretation using two coherent laser sources.

Purpose of the Study:

• To establish a theoretical link between spontaneous Raman scattering cross sections (σRaman) and absolute stimulated Raman scattering cross sections (σSRS).
• To decompose Raman cross sections into contributions from vacuum fields and intrinsic molecular response.

Main Methods:

• Derived an equation analogous to Einstein's A and B coefficients for Raman scattering, based on quantum electrodynamics.
• Utilized experimental measurements on methanol to validate the theoretical framework.

Main Results:

• Developed a theoretical equation [Eq. (16)] that relates σRaman and σSRS, decomposing σRaman into vacuum field and molecular response contributions.
• Experimental data supports the derived relation, showing σSRS as a vacuum-decoupled intrinsic molecular property.
• Demonstrated that the weak vacuum field in the Stokes channel accounts for the small observed σRaman values.

Conclusions:

• Stimulated Raman scattering cross sections (σSRS) provide a more intrinsic measure of molecular response compared to σRaman.
• The theoretical framework bridges the gap between spontaneous Raman and UV-Vis absorption cross sections by over 10^10 folds.
• This work clarifies the fundamental relationship between different Raman scattering regimes and their underlying physics.