Title: Rovibrational optical cooling of a molecular beam,
Abstract: Cooling the rotation and the vibration of molecules by broadband light
sources was possible for trapped molecular ions or ultracold molecules. Because
of a low power spectral density, the cooling timescale has never fell below
than a few milliseconds. Here we report on rotational and vibrational cooling
of a supersonic beam of barium monofluoride molecules in less than 440 $\mu$s.
Vibrational cooling was optimized by enhancing the spectral power density of a
semiconductor light source at the underlying molecular transitions allowing us
to transfer all the populations of $v''=1-3$ into the vibrational ground state
($v''=0$). Rotational cooling, that requires an efficient vibrational pumping,
was then achieved. According to a Boltzmann fit, the rotation temperature was
reduced by almost a factor of 10. In this fashion, the population of the lowest
rotational levels increased by more than one order of magnitude.