The effect of conformation on the ionization energetics of n -butylbenzene. II. A zero electron kinetic energy photoelectron spectroscopy study with partial rotational resolution

Rotationally resolved zero electron kinetic energy (ZEKE) spectra of n-butylbenzene have been investigated using a spectator orbital model to compare the ionization energetics of two molecular conformers. A new ZEKE electron detection scheme was employed to record ZEKE excitation spectra as a function of the S1←S0 excitation laser photon energy, with fixed photon energy of the ionization laser. These ZEKE excitation spectra are a sensitive probe of the rotational constants of all three states involved, the S0 and S1 of the neutral and the D0 of the cation. For gauche-conformer I, the rotational constants of the cation have been determined as A+=2330.6±2.3 MHz, B+=772.4±1.5 MHz, and C+=741.61±0.72 MHz. The rotational constants for the anti-conformer, V, were determined as A+=3410.4±3.3 MHz, B+=577.7±1.9 MHz, and C+=524.79±0.50 MHz. This is the first time that the geometric structures of two cationic conformers have been identified using a rotational analysis. The inertial defects derived from the rotational constants provide detailed information relating to the geometric changes experienced by the conformers upon S1←S0 excitation and D0←S1 ionization. Despite the relatively small difference in geometry between the two conformers, they are observed to display dramatically different ionization cross sections. The differences in ionization dynamics of the conformers are interpreted in terms of conformationally dependent electronic structure changes, with reference to excess charge delocalization from the aromatic ring to the side chain in the cations, resulting in higher angular momentum contributions in the spectator orbital of conformer V compared to conformer I. These higher spectator electron orbital angular momentum components result in a reduced ionization cross section for conformer V compared to conformer I.