Observations of Molecular Rydberg State Decay for n = 10-200

Abstract

Time-resolved ZEKE (zero electron kinetic energy) spectroscopy has enabled measurements of the rates of decay of molecular Rydberg states close to (within a few cm-l of) the ionization threshold. The ion yield as a function of photon energy in this region cannot be understood without knowing both the ionization potential and the principal quantum numbers, n, of the states involved. Unfortunately, these two unknowns are linked by the equation hv = IP -R / ( n -a)*; an independent method must be used to measure the ionization potential, IP. We have found two molecules appropriate for these measurements, bis(benzene)chromium (BBC) and diazacyclooctane (DABCO), both of which have very long Rydberg series with resolved transitions up to n = 35 (BBC) and n = 70 (DABCO). Highly accurate values of the ionization potential (10.5 cm-I) were obtained by extrapolating these series. The line widths are shown to decrease with increasing n with a power of n of 2.5 1 0.3. These are the first measurements of the n dependence of the nonradiative rates over a large range of n for polyatomic molecules in Rydberg states. However, one cannot simply extrapolate the lifetimes (as determined from the widths) to the region near the ionization threshold where the n = 100-200 states are being excited. There the lifetimes as determined directly by ZEKE spectroscopy are longer by several orders of magnitude than the values obtained by extrapolation.

High Rydberg states of DABCO: Spectroscopy, ionization potential, and comparison with mass analyzed threshold ionization

“…The peak of the MATI spectrum lies at nϭ135 with a FWHM of 6 cm Ϫ1 (⌬nϭ60). On the low-energy side of the MATI peak the Rydberg series is clearly seen ͑as in previous ZEKE studies 20 ͒ and is shown in more detail in the inset in Fig. 5.…”

“…The highly symmetric caged amine DABCO ͑see Fig. 1͒ is an exception, 20 and states with n up to 80 are observed here.…”

“…45 For the lower n states of DABCO (nϽ30) the linewidths measured ͑Table I͒ also reflect fast decay processes. 20 Above nϷ40 a constant linewidth is measured, reflecting possibly the rotational transition profile of the ensemble of rotational states excited.…”

“…15 Our study follows the recent work of Even and co-workers 20 where ZEKE and double-resonance autoionization were compared. Our results are in good agreement with the previous studies but focus on more accurate energy calibrations and on higher Rydberg levels where the effects of applied static fields are most observable.…”

“…Efforts are made to increase the accuracy by detection of higher Rydberg levels and by improving the calibration of the transition wavelengths. Transition linewidths are also more accurately determined in light of the remarkable differences found previously 20 between Rydberg state lifetimes implied from linewidths and from ZEKE time delays. Furthermore, the effects of applied fields and time delays between excitation and pulsed field ionization on the observed Rydberg spectra are followed.…”