Supersonic metal cluster beams of refractory metals: Spectral investigations of ultracold Mo2

Abstract

A novel technique involving pulsed laser vaporization of the bulk metal within a pulsed supersonic nozzle has been shown to successfully produce ultracold bare metal clusters of even the most refractory of metals, tungsten and molybdenum. Clusters of up to 25 atoms may be readily prepared using this technique. Mass-selective resonant two-photon ionization spectra of Mo 2 produced in this fashion show that the dimer is efficiently cooled in the expansion Ttrans, <6 K, Trot ∼5 K, and Tvib ∼325 K. We have rotationally resolved the A 1∑n+←X 1∑ g+ (0-0) band for 92Mo2 and determined the bond length in the ground and excited states to be 1.940±0.009 and 1.937±0.008 Å, respectively. This confirms and extends the analysis of Efremov et al. [J. Mol. Spectrosc. 73, 40 (1970)] who prepared 98Mo2 by flash photolysis of isotopically pure Mo(CO)6. We have also observed the (1-1), (2-2), and (3-3) sequence bands which together with the ground state data of Efremov et al. determine vibrational constants ω′e=449.0±0.2 cm-1 and ωex′e=2.3±0.2 cm-1 for the A 1∑u+ state. The lifetime of the A 1∑u+ ν=0 state of Mo2 has been measured to be 18±3 ns by time-delayed two-photon ionization. The ionization potential of Mo2 is found to be less than 6.42 eV (compared to 7.10 eV for atomic Mo) indicating a substantially stronger chemical bond in Mo2+ than in Mo2. A discussion of the electronic structure of Mo2 and the implications of these findings for bonding in other transition metal dimers is also presented. © 1983 American Institute of Physics.