Absorption of supersonically expanded cyclopentadiene in the vacuum-ultraviolet region

The absorption spectrum of cyclopentadiene (CPD) was measured over the wavelength region of 1680-1460 Å. The spectrum was investigated at room temperature and at low temperatures by supersonically expanding pure cyclopentadiene vapors. In the cold beam spectrum, a few of the absorption bands disappear. The assignment of the observed Rydberg transitions is aided by this simplification of the spectrum. The disappearance of these absorption bands is explained as being caused by the effect identified by Vaida (J. Phys. Chem. 1989, 93, 510) as the cluster-induced potential shifts (CIPS). The effect operates in cyclopentadiene contrary to what was observed in methyl iodide and acetone.