Knowledge Bank

Supersonic expansion of a benzene/helium mixture prepares benzene in the vibrationless ground state with only the lowest $J=Kc$ rotational levels significantly populated. Laser excitation spectra are produced by crossing the free jet with a frequency-doubled, nitrogen laser-pumped dye laser that has a bandwidth of about 1 GHz in the ultraviolet. A comparison is made between molecular constants obtained by our study and those of the band contour analysis of Callomon, Dunn, and $Mills1$ Since spectra have been obtained using both continuous and pulsed supersonic expansion sources, a brief comparison of these techniques will be presented. A rotational analysis of the corresponding vibronic bands of the He-benzene and He2-benzene Van der Waals complexes will also be reported. In contrast to s-tetrazine, $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ the Van der Waals complexes of the excited state of benzene are less strongly bound than in the ground state.