Polarization-dependent Intensity Ratios in Double Resonance Spectroscopy

TL;DR

This paper extends the theoretical understanding of polarization-dependent intensity ratios in double resonance spectroscopy to strongly saturating fields, improving agreement with experimental data for complex molecular transitions.

Contribution

It provides new theoretical predictions for intensity ratios under strong saturation conditions, considering both homogeneous and inhomogeneous broadening, which were not addressed in previous weak field models.

Findings

Saturation reduces polarization anisotropy but does not eliminate it.

Large low-power anisotropy persists in inhomogeneously broadened lines.

Predictions align better with recent experimental measurements.

Abstract

Double Resonance is a powerful method spectroscopic method that provides an unambiguous assignment of the rigorous quantum numbers of one state of a transition. However, there is often ambiguity as to the branch ($\Delta J$) of the transition. The dependence of the intensity of the double resonance signal on the relative polarization of pump and probe radiation can be used to resolve this ambiguity and has been used for this in the past. However, the published theoretical predictions for this ratio are based upon a weak (i.e. non-saturating) field approximation. In this paper, we present theoretical predictions for these intensity ratios for cases where the pump field is strongly saturating, in the two limits of transitions dominated by homogeneous and inhomogeneous broadening. While saturation, as can be expected, reduces the magnitude of the polarization effect (driving the intensity ratio closer to unity), polarization anisotropy remains even with a strongly saturating probe field in most cases. For the case of an inhomogeneously broadened line, as when Doppler broadening linewidth dominates over even the power broadened homogeneous line width, a large fraction of the low pump power anisotropy remains. Results are presented for both the case of linear and circular pump and probe field polarizations. The present predictions are compared with experimental measurements on CH$_4$ ground state $\rightarrow \nu_3 \rightarrow 3\nu_3$ transitions recently reported by de Oliveira et al and found to be in better agreement than the weak field predictions.