Measurement of Doppler effects in a 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 $\tilde{A}{}^2\mathrm{\Pi }(0,0,0)\text{−}\tilde{X}{}^2{\mathrm{\Sigma }}^{+}(0,0,0)$ band of calcium monohydroxide (CaOH). CaOH molecules were produced by laser ablation in a copper cell and cooled to $\sim 5\mathrm{K}$ using helium buffer gas. We probed the Doppler effects in a buffer-gas cell by injecting counterpropagating 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.

Figure 1

Experimental setup: Ablated CaOH molecules are cooled by collision with helium buffer gas in a cylindrical copper cell and extracted through the aperture. Counterpropagating lasers are injected inside the cell perpendicular to the expected molecular flow.

Figure 2

Example of transmittance trace (inset) and observed spectra of Doppler-limited absorption of CaOH $Q_1(3/2)$ transition probed by counterpropagating lasers at 0.3 ms after ablation. Dashed lines correspond to Gaussian fitting results.

Figure 3

Time evolution of CaOH translational temperature estimated from the Doppler width of absorption spectra of $Q_1(3/2)$ transition. The error bar on each data point corresponds to the standard error of multiple scan results. The solid area shows the simulated result assuming that the helium temperature evolution follows the fit parameters obtained from the data. The right axis corresponds to the square of the full width at half maximum of the spectra.

Figure 4

Time evolution of observed Doppler shift. Error bar on data points and the solid area correspond to the standard error of the multiple scan results and simulation result, respectively.

Figure 5

Time evolution of observed Doppler shift on multiple data sets. The data index is in order of data acquisition.