# Resonance enhanced two-photon cavity ring-down spectroscopy of vibrational overtone bands: a proposal

## TL;DR

This paper proposes a detailed theoretical analysis of resonance-enhanced two-photon cavity ring-down spectroscopy for detecting vibrational overtone bands, demonstrating its potential for highly sensitive and selective trace gas detection.

## Contribution

The paper derives comprehensive expressions for two-photon absorption rates in cavity ring-down spectroscopy, including polarization and degeneracy effects, and predicts significantly improved detection sensitivity and selectivity.

## Key findings

- Predicted detection limit of 32 ppq for CO2 using two-photon cavity ring-down.
- Most polyatomic molecules have sparse, Doppler-Free two-photon spectra, enhancing selectivity.
- The analysis shows higher sensitivity than traditional one-photon absorption methods.

## Abstract

This paper presents an analysis of near-resonant, ro-vibrational two-photon spectroscopy and the use of cavity ring-down spectroscopy for its detection. Expressions are derived for the photon absorption rate of a three-level system, correct to all orders and the simpler expressions that result from various approximations. The analysis includes the angular momentum projection degeneracies and linear or circular polarization of the exciting field. Expressions are derived for the rate of two-photon power loss for light inside a resonant cavity. Explicit calculations are made for excitation of the $\nu_3$ mode of $^{12}\textrm{C}^{16}\textrm{O}_2$ for which the two-photon excitation spectrum is dominated by a single $v_3 = 0 \rightarrow 2, Q(16)$ line at $\tilde{\nu} = 2335.826$\,cm$^{-1}$. This transition has an intermediate $v_3 = 0 \rightarrow 1,P(16)$ one-photon transition that is off resonance by 0.093 cm$^{-1}$ (2.8 GHz). At 1\,torr total pressure, the Q(16) two-photon transition has a calculated cross-section of $2.99 \cdot 10^{-38}$\,cm$^4$s per CO$_2$ molecule in the $J = 16$ state or $2.24 \cdot 10^{-39}$\,cm$^4$s per CO$_2$ molecule at 300\,K is calculated. Analysis of the sensitivity limits for 2-photon cavity ring-down spectroscopy predicts a theoretical detection limit of 32\,ppq ($10^{-15}$) Hz$^{-1/2}$ for $^{12}\textrm{C}^{16}\textrm{O}_2$, higher sensitivity than has been realized using one-photon absorption. The analysis predicts that most polyatomic molecules will have sparse, Doppler-Free two-photon absorption spectra, which will dramatically increase the selectivity of trace gas detection of samples with multiple components with overlapping absorption bands. This is demonstrated by the predicted mid-IR two-photon absorption spectrum of butadiene using theoretical spectroscopic constants.