Vibrational structure in the photoelectron spectrum of cyclobutadiene as a probe of structure

The photoelectron spectrum of cyclobutadiene, c-C[sub 4]H[sub 4], is reported with partial resolution of vibrational structure. Model calculations of the Franck-Condon envelope in the spectrum, based on ab initio geometries and force constants for c-C[sub 4]H[sub 4][sup [center dot]+], find a surprisingly good agreement for a transition from the rectangular neutral to a rectangular, as opposed to a rhomboidal or square, radical cation. An adiabatic ionization potential of 8.16 [+-] 0.03 eV is also determined. The spectrum confirms that correlation between [sigma] and [pi] electrons is needed in ab initio calculations for C[sub 4]H[sub 4][sup [center dot]+] to obtain even a qualitatively correct geometry for the Jahn-Teller-distorted ion. 26 refs., 5 figs., 1 tab.