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.