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Gas phase electronic transitions for the C 2B2←X 2A1 and D 2B1←X 2A1 band systems of T-shaped AlC2 (C2v) radical have been measured in the 345–475 nm range. Vibrational analyses of both band systems are reported. Simulation of several rotationally resolved bands confirms previously obtained rotational parameters for the C 2B2 state. The radical is produced by ablating an aluminum rod in the presence of acetylene gas. The resulting supersonic molecular beam is probed using both mass-selective resonant two-color two-photon ionization and laser induced fluorescence. Ab initio calculations and vertical electronic excitation energies help the assignment. Vibrational frequencies for the X 2A1, C 2B2, and D 2B1 states have been determined. Rotational analysis of a number of bands yields spectroscopic constants for one vibronic state in the C 2B2 manifold and the origin band of the D 2B1←X 2A1 system. [ABSTRACT FROM AUTHOR]

The A 2Π←X 2Π laser-excitation spectra of chloroacetylene and deuterochloroacetylene cations are reported. Analysis of the band systems yields values for all three stretching fundamentals (ν1–ν3) in the excited electronic state. Rotationally resolved spectra of the 000Ω =3/2 subbands have been recorded and analyzed. The derived rotational constants allow estimates of the H–Cl distances in both electronic states to be made. [ABSTRACT FROM AUTHOR]

The restricted coupled cluster with perturbative triples (RCCSD(T)) method has been used to calculate equilibrium geometries and excitation energies of the hitherto unknown quartet states in diacetylene, triacetylene and benzene radical cations. Spectroscopic data for the doublet states obtained with the same approach are found to be in good agreement with experiment. Whereas the 2Πu and 2Πg states in diacetylene and triacetylene cations form linear-linear Renner-Teller pairs, strongly bent minima are calculated for the quartet states correlating with the 4Πg and 4Πu linear structures. In both ions there are quartets that lie close to the minimum energies of the first electronically excited doublet states. The 4Au state of benzene radical cation lies below C2E1u and could be populated during the ultrafast nonradiative decay of this doublet state. [ABSTRACT FROM AUTHOR]

WITHDRAWN TO APPEAR IN SPECIAL ISSUE IN 2007 [ABSTRACT FROM AUTHOR]