High-Resolution Excited-State Photoelectron Spectroscopy of the Lower Rydberg States of Jet-Cooled C2H4 and C2D4

High-resolution (2+1) resonance-enhanced excited-state photoelectron spectroscopy on supersonic jet expansions of pure C2H4 and C2D4 has been employed to investigate the spectroscopy and dynamics of the lowest excited 1B3u(π,3s) Rydberg state, as well as the 11B1g(π,3p y ), 11B2g(π,3p z ), and 21Ag(π,3p x ) Rydberg states of ethylene. In combination with ab initio calculations, this approach has for the 11B3u(π,3s) Rydberg state confirmed the majority of previous assignments in the two-photon excitation spectrum but, at the same time, has demonstrated that the hypothesis of a vibronic coupling between this state and the Jahn−Teller distorted 11B2g(π*,3s) state should be discarded. The resonances on which this hypothesis was based are in the present study shown to arise from excitation of the ν3(ag) vibrational mode. For the 3p Rydberg manifold, the present study has for the first time been able to determine unambiguously the 0−0 transitions to all three (π,3p) Rydberg states and to disentangle the vibrational development of the practically degenerate 11B1g(π,3p y ) and 11B2g(π,3p z ) Rydberg states. Upon the basis of these assignments, it has been shown that the 21Ag(π,3p x ) Rydberg state is significantly more twisted around the central CC double bond upon excitation from the ground state than the other two (π,3p) Rydberg states. Although a priori argued to be difficult to detect, the present study does not give evidence for previous suggestions in the literature on the coupling of the 11B1u(π,π*) valence and the 11B3u(π,3s) or the 11B1g(π,3p y ) Rydberg states. However, the photoelectron spectra obtained for ionization via the lower Rydberg manifold of both C2H4 and C2D4 show unambiguously the absence of extensive coupling between the lower excited Rydberg states.