We have detected a new carbon-chain molecule, CCO(3 sigma-), in the cold, dark molecular cloud TMC-1. The excitation temperature and the column density of CCO are, respectively, approximately 6 K and approximately 6 x 10(11) cm-2. This column density corresponds to a fractional abundance relative to H2 of approximately 6 x 10(-11). This value is two orders of magnitude less than the abundance of the related carbon-chain molecule CCS, and about half that of C3O. The formation mechanism for CCO is discussed.

The photochemical processes induced in sulfur dioxide by 193 nm excimer laser excitation have been investigated by examining vibration-rotation transitions of the reaction product SO with the use of tunable infrared diode-laser spectroscopy. Only X 3Σ− ground-state SO molecules were observed. About 70% of the nascent SO molecules were found to be in the v=2 state, about 20% in v=1, and some in v=5. The rotational distribution in each vibrational state differed significantly from the thermal distribution, and was shifted towards higher rotational levels in lower vibrational states. The electron spin was observed to be polarized in the v=1 and 2 states in a direction either parallel or antiparallel to the rotational angular momentum, i.e., the F1 and F3 spin sublevels are more populated than the F2 level in these states. This selective population was not observed for v=5. The origin of the spin polarization is discussed in terms of spin-orbit mixing between the C̃ state and a repulsive triplet state of SO2. The results obtained here emphasize the usefulness of high resolution, high sensitivity infrared laser spectroscopy in the study of primary processes in photochemistry.

A c-type band was observed at around 895 cm−1 by infrared diode laser kinetic spectroscopy combined with the excimer laser photolysis of vinyl halides at 193 nm and was assigned to the CH2 wagging mode of the vinyl radical. The band was found to consist of two component bands separated by 0.0541(11) cm−1. Both component bands showed clearly the statistical weight in an alternative way, that is, if one shows the weight 1:3 for even:odd Ka levels, the other exhibits 3:1, indicating that the radical is of C2v effective symmetry, executing a double-minimum motion probably associated with the C–H in-plane rocking vibration. The upper states of the two bands were found to be perturbed weakly, possibly by a Coriolis interaction with the first overtone state of the C–H rocking mode.

Five infrared bands of the CCH radical were observed by diode laser kinetic spectroscopy. Two Π–Π type bands were assigned to hot bands from the (0110) state in X̃ 2Σ+, namely to ν2+ν3−ν2 and 7ν12−ν12. The latter band was found to share the upper state with a Π–Σ type band, of which the lower state was the ground vibronic state. Two other Π–Σ type bands corresponded to transitions from the ν3 state in X̃ 2Σ+ to two vibronic Π states. Simultaneous analysis of all the observed bands yielded molecular parameters with high precision including the ν2 fundamental frequency, which was determined to be 371.6034 (3) cm−1. The parity of each rovibronic level was determined unambiguously because Π–Σ+ transitions were observed, allowing us to choose the signs of K-type doubling constants p and q uniquely.

The ν2 band of the silylene SiH2 molecule in X̃ 1 A1 was observed for the first time in the gas phase by using infrared diode laser kinetic spectroscopy. Silylene molecules were generated by the photolysis of phenylsilane at 193 nm. The observed spectrum was analyzed to determine the rotational and centrifugal distortion constants in the ground and v2 =1 states and the band origin ν0 =998.6241(3) cm−1 with one standard deviation in parentheses. The significance of the derived parameters is discussed in detail.

The 2P1/2–2P3/2 fine structure transitions in the ground (np5) state of the neon (n=2) and argon (n=3) atomic ions have been observed in absorption in the positive column of a liquid–nitrogen cooled discharge cell, by employing tunable infrared diode lasers as sources. The transition frequencies were measured to be 780.4240±0.0011 and 1431.5831±0.0007 cm−1 for Ne+ and Ar+, respectively. A few transitions have also been observed for the neutral Ne and Ar atoms.

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The vibrational distribution of CH 3 radicals produced by the 248 nm photodissociation of CH 3 I has been investigated by infrared diode laser kinetic spectroscopy. The V 2 bands of CH 3 , V 2 = 1 +-0 up to 4 ..... 3, were observed as functions of time for the sample pressure ranging from 9 to 100 mTorr. The observed Doppler line shape showed a clear correlation between the transition dipole moment and the recoil velocity (f-tv) in the photodissociation, and the observed linewidth indicated that CH 3 fragments were predominantly generated through the IeP I12 ) adiabatic dissociation channel. The relative population in each vibrational level was determined to be 0.66:0.26:0.08:0.004 for v = 0, 1,2, and 3, in sharp contrast to the inverted vibrational distribution reported previously. The present result suggests that the geometry of the CH 3 moiety in the precursor CH 3 I is relaxed gradually to its equilibrium planar structure before the cleavage of C-I bond takes place.