Spectroscopic characterization of the a$^3\Pi$ state of aluminum monofluoride

TL;DR

This study provides detailed spectroscopic data on the a$^3\Pi$ state of aluminum monofluoride, crucial for laser cooling applications, including lifetime, hyperfine structure, and potential energy benchmarks.

Contribution

It offers the first comprehensive spectroscopic characterization of the a$^3\Pi$ state of AlF, including lifetime, hyperfine parameters, and potential energy benchmarks, aiding laser cooling research.

Findings

Radiative lifetime of 1.89 ms for a$^3\Pi_1$, v=0, J=1 level.

Hyperfine splittings measured and compared with quantum chemistry.

Spectral line width of 1.27 kHz between hyperfine levels.

Abstract

Spectroscopic studies of aluminum monofluoride (AlF) have revealed its highly favorable properties for direct laser cooling. All $Q$ lines of the strong A$^1\Pi$ $\leftarrow$ X$^1\Sigma^+$ transition around 227~nm are rotationally closed and thereby suitable for the main cooling cycle. The same holds for the narrow, spin-forbidden a$^3\Pi$ $\leftarrow$ X$^1\Sigma^+$ transition around 367 nm which has a recoil limit in the micro Kelvin range. We here report on the spectroscopic characterization of the lowest rotational levels in the a$^3\Pi$ state of AlF for $v=0-8$ using a jet-cooled, pulsed molecular beam. An accidental AC Stark shift is observed on the a$^3\Pi_0, v=4$ $\leftarrow$ X$^1\Sigma^+, v=4$ band. By using time-delayed ionization for state-selective detection of the molecules in the metastable a$^3\Pi$ state at different points along the molecular beam, the radiative lifetime of the a$^3\Pi_1, v=0, J=1$ level is experimentally determined as $\tau=1.89 \pm 0.15$~ms. A laser/radio-frequency multiple resonance ionization scheme is employed to determine the hyperfine splittings in the a$^3\Pi_1, v=5$ level. The experimentally derived hyperfine parameters are compared to the outcome of quantum chemistry calculations. A spectral line with a width of 1.27 kHz is recorded between hyperfine levels in the a$^3\Pi, v=0$ state. These measurements benchmark the electronic potential of the a$^3\Pi$ state and yield accurate values for the photon scattering rate and for the elements of the Franck-Condon matrix of the a$^3\Pi$ $-$ X$^1\Sigma^+$ system.