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-h ₄ and -d₄ have been observed and interpreted as π→3s excitations, vibronically induced by the torsional modes ν′₄ and ν″₄. As in the one-photon spectrum, the excitation is rich in ν′₂(C=C stretch), and double and quadruple quanta of the ν′₄ 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 ν′₄. 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→3p_y and π→3p_x, 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 ν′₂ vibrations, each member of which is accompanied by the 2ν′₄ and 4ν′₄ torsional doublets.