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The B̃ 1 A 1 ←X̃ 1 A 1 electronic band system of the protonated diacetylene cation (H 2 C 4 H + ) is measured over the 230-295 nm range by photodissociating H 2 C 4 H + ions stored in a cryogenic ion trap and by photodissociating H 2 C 4 H + tagged with Ar and N 2 in a tandem mass spectrometer. The B̃ 1 A 1 ←X̃ 1 A 1 band system has an origin at 34 941 cm -1 for H 2 C 4 H + , 34 934 cm -1 for H 2 C 4 H + -Ar, and 34 920 cm -1 for H 2 C 4 H + -N 2 . The spectra of H 2 C 4 H + , H 2 C 4 H + -Ar, and H 2 C 4 H + -N 2 display similar vibronic structure, which is assigned using ab initio calculations to progressions in two symmetric a 1 C-C stretch vibrational modes (ν 6 and ν 4 ), with band spacings of 860 and 1481 cm -1 , respectively.

The C 1B1<--X 1A1 band system of the potential interstellar species Si3C has been recorded in a silane/acetylene discharge by resonant two-color two-photon ionization spectroscopy. The origin band is located near 24,925 cm-1 (3.09 eV). Several other features in the spectrum are assigned to progressions in the Si-Si stretching modes as well as to sequence and hot band transitions. The assignment was facilitated by ab initio calculations, which also indicate that this is the strongest electronic transition of Si3C in the visible region of the spectrum. Features in the spectrum are broadened considerably (ca. 10 cm-1), and suggest an excited state lifetime of a few picoseconds. Possible reasons for the short-lived nature of the excited state are discussed.

We report here a measurement of electric dipole moments in highly vibrationally excited HDO molecules. We use photofragment yield detected quantum beat spectroscopy to determine electric field induced splittings of the J=1 rotational levels of HDO excited with 4, 5, and 8 quanta of vibration in the OH stretching mode. The splittings allow us to deduce mua and mub, the projections of dipole moment onto the molecular rotation inertial axes. We compare the measured HDO dipole moment components with the results of quantitative calculations based on Morse oscillator wave functions and an ab initio dipole moment surface. The vibrational dependence of the dipole moment components reflect both structural and electronic changes in HDO upon vibrational excitation; principally the vibrational dependence of the O-H bond length and bond angle, and the resulting change in orientation of the principal inertial coordinate system. The dipole moment data also provide a sensitive test of theoretical dipole moment and potential energy surfaces, particularly for molecular configurations far from equilibrium.