Coherent anti-Stokes Raman spectroscopy in the presence of strong resonant signal from background molecules

TL;DR

This paper introduces a method using ro-vibrational coupling to selectively detect CO2 in the presence of strong oxygen signals in broadband femtosecond laser spectroscopy, avoiding complex Fourier analysis.

Contribution

It presents a novel approach leveraging silence windows from oxygen to isolate CO2 signals without requiring femtosecond time resolution or Fourier analysis.

Findings

Effective separation of molecular signals using silence windows.

Background-free detection of CO2 in mixed gases.

No need for femtosecond time-delay scanning.

Abstract

Optical spectroscopy with broadband femtosecond laser pulses often involves simultaneous excitation of multiple molecular species with close resonance frequencies. Interpreting the collective optical response from molecular mixtures typically requires Fourier analysis of the detected time-resolved signal. We propose an alternative method of separating coherent optical responses from two molecular species with neighboring excitation resonances (here, vibrational modes of oxygen and carbon dioxide). We utilize ro-vibrational coupling as a mechanism of suppressing the strong vibrational response from the dominating molecular species (O$_{2}$). Coherent ro-vibrational dynamics lead to long "silence windows" of zero signal from oxygen molecules. In these silence windows, the detected signal stems solely from the minority species (CO$_{2}$) enabling background-free detection and characterization of the O$_2$/CO$_2$ mixing ratio. In comparison to a Fourier analysis, our technique does not require femtosecond time resolution or time-delay scanning.