Vibrationally resolved lifetimes of the 2Σ state of Na.

Lifetimes of partially resolved ro-vibrational levels of the Na 2Σ double well state have been measured for the first time. Ground state sodium dimer molecules in a molecular beam are resonantly excited by the doubled output of a 10 ns pulsed dye laser in the range 333-357 nm. After being allowed to decay for a predetermined time interval, the surviving excited molecules are ionized by 532 nm photons from a delayed Nd:YAG laser and detected in a linear time-of-flight mass spectrometer. By appropriate tuning of the excitation laser and systematic variation of the probe laser delay, lifetimes are obtained for vibrational levels in the range from 22 to 57. At zero rotation, the three lowest vibrational quantum numbers that we have explored (22, 25, and 28) correspond to wavefunctions whose probability densities are appreciable only in the inner well. Levels with larger quantum numbers are located above the barrier, which, for the rotation-free case, lies between quantum numbers 33 and 34. Because of the congested nature of the excitation spectrum and the experimental resolution of about 0.2 cm available to us, our experimental results are only partially quantum state resolved. Nevertheless, we can discern a decrease in lifetime from about 50 to 40 ns for the inner well levels and a slight increase in lifetime with increasing quantum number for levels above the potential barrier. We have also performed lifetime calculations based on the LEVEL and BCONT programs made available by Le Roy, the latter of which was modified by McGeehan. When limited to bound-bound transitions, theoretical lifetimes for levels above the barrier are systematically larger than experimental values by a factor of almost two. With the addition of bound-free transitions, agreement between experiment and theory is, for the most part, within the experimental uncertainties.