APS/Alan Stonebraker

Figure 1: Scheme for the simultaneous vibrational and rotational cooling of a molecule. A broadband femtosecond pulse, whose spectrum has been cut at the high-energy side, excites all vibrational states but the ground state ($v=0$, which remains “dark”). As the excited state spontaneously decays, the molecular population relaxes towards $v=0$ and, through many pumping cycles, it accumulates there and the sample cools vibrationally. Rotational cooling is achieved by pumping with a narrow-band, continuous-wave (CW) laser diode. The laser frequency is swept to excite in succession the lowest $10$ excited rotational states, leaving the rotational ground state unperturbed. Following laser excitation, the molecules relax towards $J=0$ through spontaneous decay.