The vibronic spectra of jet-cooled propyne at 6.8–10.5 eV have been observed using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The ns (n=4–13), np (n=3–4), and 3dz2 Rydberg states of propyne have been identified, of which seven are newly discovered. The symmetries of the excited vibronic states have been determined directly from polarization-ratio experiments applying linearly and circularly polarized lasers. Under a C3V group, the observed s Rydberg series are of E symmetry and the p Rydberg states belong to A1 or E. Clear doublet splittings in the ns Rydberg states (n=4–9) are observed for the first time. The splittings, 306 cm−1 at 4 s, decrease with increasing n. The doublets of A′ and A″ symmetries, identified from the polarization-ratio measurement, are that due to CS molecular geometry, rather than C3V, for the ns Rydberg states. The term values for the ns Rydberg series (n=6–13) converge to an adiabatic ionization energy of 83 625±2 cm−1 with a quantum defect of δ=0.95. Comparing with one-photon absorption spectrum of propyne, the absence of π→π*, np (n⩾4) and nd (n⩾3, except 3dz2) Rydberg states in the REMPI spectra suggests a strong predissociation character for these states. Calculations for the vertical excitation energies of π→π*, ns, np, and nd (n=3,4) Rydberg states of propyne were performed using time-dependent density functional theory and ab initio methods to compare with experimental results and to test the computational accuracy.

The vibronic spectra of vinyl chloride at 7.3–10 eV have been studied using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The geometries and vibrational frequencies of vinyl chloride and its cation calculated with ab initio methods are utilized to compute the Franck–Condon factors. The transitions to the Rydberg states of π→ns, npσ, npπ, nd, nf, and nCl→3s are responsible for the observed REMPI spectra of vinyl chloride. The complex vibrational structures for the observed transitions are successfully identified with the aid of calculated Franck–Condon factors.

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2 þ 1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0-10.5 eV have been observed. The excited vibronic symmetry has been determined from polarizationratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to p* p, 3p p, 4s p, 4p p, and 4d p transitions. Vibrational progressions related to the CH 2 twisting ( 4 s770 cm À1 ) have been observed for several excited electronic states. Calculated Franck-Condon factors also confirm that CH 2 twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations.