Rydberg states of propyne at 6.8-10.5 eV studied by two-photon resonant ionization spectroscopy and theoretical calculation

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 3d(z2) 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 C-3V group, the observed s Rydberg series are of E symmetry and the p Rydberg states belong to A(1) 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 C-S molecular geometry, rather than C-3V, 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 delta=0.95. Comparing with one-photon absorption spectrum of propyne, the absence of pi -->pi*, np (n greater than or equal to 4) and nd (n greater than or equal to 3, except 3d(z2)) Rydberg states in the REMPI spectra suggests a strong predissociation character for these states. Calculations for the vertical excitation energies of pi -->pi*, 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. (C) 2000 American Institute of Physics. [S0021-9606(00)00417-7].