A new Rydberg state of acetylene has been identified as a three photon resonance in the four photon ionization spectra of both C2H 2 and C2D 2 . In both isotopic species the zero point level of this new electronic state is sufficiently long lived to support rotational structure. Spectral analysis, including use of the appropriate three photon rotational linestrength theory, enables characterization of the excited state symmetry as 10 u. Quantum defect considerations, and comparisons with recent ab initio calculations, encourage assignment of this 10 u state in terms of the electronic promotion n 3 3d69 ~-n u .~ Possible reasons for the dominance of this 10u-aY+g transition in the 3 + 1 MPI spectrum of acetylene are considered.

The complementary techniques of high-resolution vacuum ultraviolet absorption spectroscopy and resonance-enhanced multiphoton ionisation spectroscopy have been employed in a reinvestigation of the vertical electronic spectra of the molecules H2S and D2S. In accord with previous, lower-resolution, absorption studies of H2S the major part of the spectra of both isotopic species may be understood in terms of Rydberg series that arise from promotion of an electron from the highest occupied 2bl molecular orbital. The electronic symmetries of states involved in six Rydberg series have been established unambiguously through observation of the (partially resolved) rotational structure that accompanies their excitations. A number of additional resonances observed only in the multiphoton ionisation spectra are tentatively attributed to excitations involving Rydberg orbitals comprised predominantly o f f and/or g functions. Several of the absorption features (most notably those involving excited sites of B, symmetry) exhibit anomalous intensity distributions between the various rotational branches. L-uncoupling and/or rovibronic level dependent predissociation are suggested as contributory causes.

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