The S1 neutral and D0 cationic states of fluorobenzene and fluorobenzene–argon probed by ZEKE spectroscopy with partial rotational resolution

Abstract

Rotationally resolved zero electron kinetic energy (ZEKE) spectra of fluorobenzene and fluorobenzene–argon have been investigated using a spectator orbital model to interpret the rotational structure. Unlike a frozen core model this model relates to the electronic structure in both final and initial states. 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. The rotational constants of the fluorobenzene cation were found to be A⁺ = 5406.8 ± 5.3 MHz, B⁺ = 2689.7 ± 3.5 MHz, and C⁺ = 1812.1 ± 1.8 MHz. The rotational constants for the argon complex were found to be A′ = 1773.3 ± 1.7 MHz, B′ = 1148.9 ± 1.4 MHz, and C′ = 996.83 ± 0.98 MHz in the intermediate S₁ state, and A⁺ = 1848.8 ± 1.8 MHz, B⁺ = 1076.6 ± 1.4 MHz, and C⁺ = 908.37 ± 0.89 MHz in the cationic D₀ state. Two structural parameters of the complex were derived from these rotational constants, the distance between the argon atom and the aromatic ring, r_z and the distance in the plane of the aromatic ring between the centre of mass and the argon atom r_x. These two parameters were determined as r_x = 0.78 Å and r_z = 3.33 Å in the S₁ state and r_x = 0.36 Å and r_z = 3.76 Å in the D₀ state.