Rotational analysis of the 1B2u(ππ) ←1A1g, (61) band of benzene and helium–benzene van der Waals complexes in a supersonic jet

Abstract

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 ¹B_2u(ππ) ←¹A_1g, (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 Å 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).