Figure 1
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.