Measurement of Doppler effects in cryogenic buffer-gas cell

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.