Lifetime measurements of the A 4 P, B 4 P, and C 4 S 0 electronic states of the vanadium monoxide molecule were performed using population probing of resonant two-photon ionization in a molecular beam. These lifetimes were found to be 6 { 1 msec, 0.37 { 0.05 msec, and 73 { 2 nsec, respectively, for the £ Å 0 levels. Due to the very low rotational temperature, the first lines were observed for the A 4 P-X 4 S 0 transition and some consequences are discussed. ᭧ 1997 Academic Press
1. Introduction
has been described elsewhere (17, 18). Briefly, vanadium oxides were produced by focusing the fundamental output of a YAG-laser onto a 6-mm-diameter vanadium rod Molecular spectra are the dominant opacity source in M-(99.9% vanadium, Heraeus) in a supersonic nozzle. The type stars. Brett (1) has demonstrated the importance of laser energy was typically 5 -10 mJ in each pulse. These the near-infrared TiO and VO bands in model atmosphere pulses were synchronized with the opening of a rapid valve calculations. Earlier accurate lifetime measurements for excited states of these molecules were lacking.
providing argon pulses around 250 msec long. The backing Recently, Doversta ˚l and Weijnitz (2) have determined lifepressure was 8 atm. A cluster beam containing all sizes of times for the triplet states of TiO using laser vaporization V n clusters and their oxides formed as the vaporized mateand a subsequent delayed two-photon ionization technique. rial was forced through the nozzle by the inert gas pulse. This technique has been previously applied to Mo 2 (3) and Since no external oxygen was supplied, all oxygen present Cu 2 (4). We have now made corresponding measurements originated from the rod itself, the walls of the reaction for some excited quartet states of VO. The spectrum of the channel, or the supplied inert gas. The cluster beam was VO molecule shows three strong transitions between the skimmed; as it entered into a second vacuum chamber excited A 4 P, B 4 P, and C 4 S 0 states and the ground state where the pressure was kept at 10 07 Torr, it was met by a X 4 S 0 . The earliest rotational analysis on a transition in VO dye laser beam that was tuned into resonance with the VO was of the C-X system (5, 6) at 574 nm, which was first transitions. When the cluster beam passed the acceleration claimed to be a doublet transition.
field of a time-of-flight mass spectrometer, it was crossed The A-X and B-X systems known from stellar spectra at by an ArF excimer laser beam (Lumonics Excimer-500) 1055 and 790 nm, respectively, were not identified until the causing the vanadium oxide to ionize. laboratory spectrum of VO was photographed in the infrared
The dye laser (Lumonics HD-500), which was pumped (7, 8). The A-X system exhibits four distinct red-degraded by a Lumonics YAG laser HY-500, operated on Styryl 8, (0,0) bandheads at 1055, 1051, 1048, and 1046 nm. The B-Rhodamine 6G, Rhodamine 610, and Pyridin 1 dyes. The X system (0,0) heads are found at 785, 787, 795, and 798 nm, output was Raman-shifted when necessary. respectively. In a series of detailed analyses of the different In this way, R2PI spectra were first recorded around the transitions of VO, Merer and co-workers (9-14) studied the expected wavelengths with no delay time between the laser fine structure as well as the hyperfine structure of both doupulses. The upper A 4 P, B 4 P, and C 4 S 0 states were populated, blet and quartet systems. and this was confirmed by comparing with literature spectra. Recent stellar observations of VO by Brett (1) and Kirk-Depopulation of the upper states to the ground state occurs by patrick et al. (15) have once again stressed the need for good fluorescence. After a variable time delay, the excimer laser f values for the VO molecule (16).
pulse ionizes the molecules remaining in the excited states. Ions produced in this process are thereby deflected into the time-offlight mass-spectrometer, allowing them to be detected. The
2. EXPERIMENTAL
signal was recorded with a digital oscilloscope. The time delay between the dye laser pulse and the exci-Vanadium oxide clusters were prepared by a laser vaporization technique. The apparatus used in the present study mer laser pulse was varied in the ranges 0-16 msec for the 274