In this paper, ultracold atoms and molecules in a dark magneto-optical trap （MOT） are studied via depumping the cesium cold atoms into the dark hyperfine ground state. The collision rate is reduced to 0.45 s-1 and the density of the atoms is increased to 5.6 × 1011 cm-3 when the fractional population of the atoms in the bright hyperfine ground state is as low as 0.15. The vibrational spectra of the ultracold cesium molecules are also studied in a standard MOT and in a dark MOT separately. The experimental results are analyzed by using the perturbative quantum approach.
The high precision two-photon excitation measurements for 5S1/2 （Fg = 2） to 5D5/2 （Fe = 4 to 1） of 87Rb are per- formed by using an optical frequency comb. The two counter-propagating femtosecond pulses （5S1/2 →5P3/2 at 780 nm, and 5P3/2→5D5/2 at 776 nm） act on 87Rb vapor, and the Doppler broadened background signal is effectively eliminated. The temperature and power dependences of the two-photon spectrum are studied in this paper.
