OBSERVATIONS OF MOLECULAR RYDBERG STATE DECAY FOR N=10-200

Time-resolved ZEKE (zero electron kinetic energy) spectroscopy has ena bled measurements of the rates of decay of molecular Rydberg states cl ose to (within a few cm-1 of) the ionization threshold. The ion yield as a function of photon energy in this region cannot be understood wit hout knowing both the ionization potential and the principal quantum n umbers, n, of the states involved. Unfortunately, these two unknowns a re linked by the equation hnu = IP - R/(n - delta)2; an independent me thod must be used to measure the ionization potential, IP. We have fou nd two molecules appropriate for these measurements, bis(benzene)chrom ium (BBC) and diazacyclooctane (DABCO), both of which have very long R ydberg series with resolved transitions up to n = 35 (BBC) and n = 70 (DABCO). Highly accurate values of the ionization potential (+/- 0.5 c m-1) were obtained by extrapolating these series. The line widths are shown to decrease with increasing n with a power of n of 2.5 +/- 0.3. These are the first measurements of the n dependence of the nonradiati ve rates over a large range of n for polyatomic molecules in Rydberg s tates. However, one cannot simply extrapolate the lifetimes (as determ ined from the widths) to the region near the ionization threshold wher e the n = 100-200 states are being excited. There the lifetimes as det ermined directly by ZEKE spectroscopy are longer by several orders of magnitude than the values obtained by extrapolation.