An MPI search for the π→3p Rydberg states of ethylene

Two-photon resonances in the 56 000–64 100 cm−1 (1780–1560 Å) region of the multiphoton ionization (MPI) spectra of ethylene-h4 and -d4 have been observed and interpreted as π→3s excitations, vibronically induced by the torsional modes ν4′ and ν4\. As in the one-photon spectrum, the excitation is rich in ν2′(C = C stretch), and double and quadruple quanta of the ν4′ torsion. An earlier suggestion that π→3p transitions are responsible for anomalies in the π→3s MCD spectrum of ethylene-h4 is not supported by this work, which assigns the anomalies to one-photon π→3s absorptions vibronically induced by a single quantum of ν4′. MPI spectra of ethylene-h4 and -d4 two-photon resonant in the 61 000–71 000 cm−1 region also have been recorded and the vibrational structure analyzed. Prominent electronic origins are found at 62 905 and 66 875 cm−1 in ethylene-h4, and at 63 115 and 67 030 cm−1 in the -d4 compound. These two transitions are assigned as π→3py and π→3px, respectively, where y is in-plane short axis, and x is out-of-plane. As in the π→3s one-photon and two-photon transitions, the two-photon transitions to 3p display long progressions of ν2′ vibrations, each member of which is accompanied by the 2ν4′ and 4ν4′ torsional doublets.