Optical frequency comb double-resonance spectroscopy of the 9030-9175 cm$^{-1}$ states of ethylene

TL;DR

This study employs optical double-resonance spectroscopy with broadband and high-precision probes to measure and assign hot-band transitions in ethylene within the 3000-9000 cm$^{-1}$ range, providing new spectral data and improved frequency accuracy.

Contribution

First measurement of hot-band transitions in ethylene in this spectral range using dual-probe OODR spectroscopy, with detailed assignments and comparison to theoretical models.

Findings

Measured 90 hot-band transition frequencies and intensities

Assigned 28 transitions with theoretical comparison

Observed 18 sub-Doppler V-type transitions

Abstract

We use optical-optical double-resonance (OODR) spectroscopy to measure for the first time hot-band transitions of ethylene (C${_2}$H${_4}$) between states in the 3000 cm$^{-1}$ and 9000 cm$^{-1}$ energy ranges. A 3.2 ${\mu}$m continuous wave (CW) pump is used to populate selected states in the ${\nu}$${_9}$ vibrational mode. The sub-Doppler OODR transitions are then probed with two different cavity-enhanced probes tunable around 1.7 ${\mu}$m: a frequency comb probe that allows for broadband measurements and simultaneous detection of many OODR lines, and a CW probe that measures individual lines with higher signal-to-noise ratio and better frequency accuracy. We report center frequencies and relative intensities of 90 ladder-type hot-band transitions from three different states in the ${\nu}$${_9}$ vibrational mode. We exploit combination differences and measurements of polarization-dependent intensity ratios to determine the final state rotational quantum numbers J. Comparison to theoretical predictions from ExoMol allows tentative assignments for 28 transitions. We report improved center frequencies for the three pump transitions in the ${\nu}$${_9}$ band. Furthermore, we observe 18 sub-Doppler V-type transitions from the depleted ground state to the 6000 cm$^{-1}$ region and assign 14 of them to the variational line list of Mraidi et al. [J Quant Spectros Radiat Transfer. 2023;310:108734 doi:10.1016/j.jqsrt.2023.108734].