S. M. Beck

Jet‐cooled naphthalene. I. Absorption spectra and line profiles

Fluorescence excitation spectra have been recorded for the first 6000 cm−1 of the ultraviolet spectrum of h8‐ and d8‐naphthalene cooled in a supersonic free jet. Measured profiles of vibronic bands in these spectra display a monotonically increasing width as a function of vibrational energy (Ev) in the excited electronic state. The high fluorescence quantum yield and relatively constant fluorescence lifetimes in these spectral regions require the measured line broadening to be assigned to intramolecular vibrational relaxation (IVR) within the excited electronic state. The rate of this IVR process as measured by the width of the broadened profiles increases smoothly from 9×1010 sec−1 at Ev=3068 cm−1 to 7×1011 sec−1 at Ev=5200 cm−1 for h8‐naphthalene. Line profiles of d8‐naphthalene in this same spectral region are found to be ∼50% wider.

Rotational analysis of the 1B2u(ππ) ←1A1g, (610) band of benzene and helium–benzene van der Waals complexes in a supersonic jet

The fluorescence excitation spectrum of benzene at a rotational temperature of 0.3 K has been observed using both pulsed and continuously operated supersonic free jets. Individual lines of the 1B2u(ππ) ←1A1g, (610) band at 259 nm have been clearly resolved and analyzed. Rotational and Coriolis constants as derived from these resolved spectra are in agreement with those derived from previous band contour analyses of room temperature spectra. Rotational analyses of the corresponding spectral band for the He–benzene and He2–benzene van der Waals complexes are also reported. In both complexes the helium atoms are found to lie on the C6 symmetry axis approximately 3.5 A above (and/or below) the plane of the benzene ring. Sufficient rotational cooling is achieved in these supersonic expansions so that ≳95% of all benzene molecules are relaxed to the lowest rotational state available (consistent with nonrelaxation of nuclear spins).

Jet‐cooled naphthalene. II. Single vibronic level fluorescence spectra

Single vibronic level fluorescence spectra have been measured for naphthalene cooled in a supersonic free jet. Ten spectra in all have been recorded, the excitation energies ranging from 0 to 4029 cm−1 above the 1B3u(ππ*)←1Ag origin. These results reveal extensive intramolecular vibrational redistribution (IVR) occurring in the absence of collision on a time scale much faster than fluorescence even at vibrational energies as low as 2570 cm−1. Comparison of the observed spectra with model calculations shows that this IVR process involves most of the harmonic overtone/combination basis states of overall b1g symmetry having energies within the absorption line profile.

Fluorescence excitation spectrum of the 1Au (nπ) ←1Ag (0–0) band of oxalyl fluoride in a pulsed supersonic free jet

Oxalyl fluoride has been cooled to a rotational temperature of 0.17 K in a pulsed, supersonic jet. Individual rotational lines in the (0–0) band of the fluorescence excitation spectrum have been observed and assigned, and the rotational constants for both the 1Ag and 1Au states have been deduced. The pulsed nozzle used in these experiments can be operated with a far larger orifice diameter and at lower pressures than typically used in continuously operated nozzles, allowing greater cooling and higher jet densities while maintaining a low level of condensation.

Rotational and vibrational analysis of the ?←? system of XeF as observed in a supersonic free jet

The fluorescence excitation spectrum of XeF cooled in a supersonic expansion with helium has been observed for the wavelength region 2860 to 3205 A. The cold spectrum displays well‐resolved rotational structure for seven of the XeF isotopic species. Analysis of this rotational structure is reported for v′=16–21 in the ? (Ω=1/2) state and for v″=0 and 1 in the ? 2Σ+ state. Vibrational intervals have been measured for individual XeF isotopes with an accuracy of ±0.50 cm−1 for vibrational levels v′=11–22 and ±0.37 cm−1 for v″=0–2.