Measurement of Doppler effects in cryogenic buffer-gas cell

TL;DR

This paper investigates Doppler effects in a cryogenic buffer-gas cell using high-resolution spectroscopy of CaOH molecules, combining experimental measurements with Monte Carlo simulations to understand molecular dynamics at low temperatures.

Contribution

It provides the first detailed measurement and simulation of Doppler effects in buffer-gas cooled molecules, enhancing understanding of molecular behavior in low-temperature spectroscopy.

Findings

Doppler width decreases over time as molecules cool.

Doppler shift varies with molecular velocity distribution.

Monte Carlo simulations accurately reproduce experimental data.

Abstract

Buffer-gas cooling is a universal cooling technique for molecules and used for various purposes. One of its applications is using molecules inside a buffer-gas cell for low-temperature spectroscopy. Although a high-intensity signal is expected in the cell, complex molecular dynamics is a drawback for precise spectroscopy. In this study, we performed high-resolution absorption spectroscopy of low-J transitions in the $\tilde{A}^2\Pi (0,0,0)-\tilde{X}^2\Sigma ^+(0,0,0)$ band of calcium monohydroxide (CaOH). CaOH molecules were produced by laser ablation in a copper cell and cooled to $\sim$5\,K using helium buffer gas. We probed the Doppler effects in a buffer-gas cell by injecting counter-propagating lasers inside the cell. The time evolutions of the Doppler width and shift were simulated using a dedicated Monte Carlo simulation and compared with data.