A summary is presented of recently published, critically evaluated experimental vibrational and electronic energy level data for neutral and ionic transient molecules and high temperature species possessing from three to sixteen atoms. Although the emphasis is on species with lifetimes too short for study using conventional sampling techniques, there has been selective extension of the compilation to include data for isolated molecules of inorganic species such as the heavy-metal oxides, which are important in a wide variety of industrial chemical systems. Radiative lifetimes and the principal rotational constants are included. Observations in the gas phase, in molecular beams, and in rare-gas and diatomic molecule matrices are evaluated. The types of measurement surveyed include conventional and laser-based absorption and emission techniques, laser absorption with mass analysis, and photoelectron spectroscopy. Not counting isotopic species, 904 molecules are surveyed, and 2696 distinct references are given.

Shifts in the ground-state vibrational fundamental frequencies of diatomic and small polyatomic free radicals, molecular ions, and other short-lived molecules upon trapping in neon and argon matrices are reviewed, with extensive documentation. Recent experimental and theoretical papers concerned with matrix shift phenomena are also discussed. The vibrational fundamentals of most molecules appear at somewhat lower frequencies in an argon matrix than in a neon matrix. Distributions of the matrix shifts have been obtained for stretching, hydrogen-stretching, and bending fundamentals. Neon-matrix shifts are typically smaller than argon-matrix shifts. There is a slight tendency for stretching vibrations observed in an argon matrix to lie below the gas-phase band centers.

The trapping of neutral and electrically charged molecular reaction intermediates in the solid rare gases and the characterization of these intermediates by vibrational and electronic spectroscopy are surveyed. Spectral data for reaction intermediates trapped in solid neon and argon are compared with the corresponding data obtained from gas phase studies and from quantum chemical calculations. Emphasis is placed on recent progress, including the production, stabilization, and spectroscopic study of highly reactive small molecular ions and the use of ab initio and density functional calculations for the identification of reaction intermediates.

Upon vacuum-ultraviolet photolysis of H2O in a CO matrix at 14°K, infrared absorptions of HCO, H2CO, HCOOH, and CO2 become prominent. Furthermore, new absorptions due to reactive product species appear at 615, 620, 1077, 1088, 1160, 1261, 1797, 1833, 3316, and 3456 cm−1. These absorptions diminish in intensity when the sample is subjected to radiation in the 2000–3000-Å spectral range. Detailed consideration of the processes which may occur in this system and extensive isotopic substitution studies support the assignment of these absorptions to the cis and trans stereoisomers of H–O–C=O, produced by the reaction of OH with the CO matrix. Valence force potentials having only small contributions from interaction terms have been found which correspond to a physically reasonable vibrational assignment and which satisfactorily reproduce the pattern of observed frequencies for the various isotopic species of both cis- and trans-HOCO. Evidence suggests that cis- and trans-HOCO photodecompose to produce H atoms and CO2.

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A summary is presented of recently published, critically evaluated experimental vibrational and electronic energy level data for approximately 1700 neutral and ionic transient molecules and high temperature species possessing from three to sixteen atoms. Although the emphasis is on species with lifetimes too short for study using conventional sampling techniques, there has been selective extension of the compilation to include data for isolated molecules of inorganic species such as the heavy-metal oxides, which are important in a wide variety of industrial chemical systems. Radiative lifetimes and the principal rotational constants are included. Observations in the gas phase, in molecular beams, and in rare-gas and diatomic molecule matrices are evaluated, and several thousand references are cited. The types of measurement surveyed include conventional and laser-based absorption and emission techniques, laser absorption with mass analysis, and photoelectron spectroscopy.

The experimentally detennined electronic energy levels of approximately 500 neutral and ionic transient molecules possessing from 3 to 6 atoms are tabulated, together with the associated vibrational structure, the radiative Iifeti~e, t?e principal rotational constants, and references to the pertinent lite.ratw: e . VIbrational a~d rotational data for the ground state are also given. ObservatIons In the gas phase, m molecular beams, and in rare-gas and nitrogen matrices are included .. The types ?f measurement surveyed include conventional and laser-based absorption and emISsion techniques, laser absorption with mass analysis, and ultraviolet photoelectron spectroscopy.A ZBl(lI) gas PF1 A-X 567-659 nm Predissociation into Si+ + HZ and into SiH+ + H was observed.CZ v B := 5.094(2); C :: 3.772 (4) PFI References 1M. C. Curtis, P. A. Jackson, P. J. Sarre, and C. J. Whitham, Mol. Phys. 56, 485 (1985). CH2 X 381 C2v Vib. No. Approximate cm-l sym. type of mode al 2 Bend 752.37 Ao = 37.787; t(B+C)o = 3.962; ~(B-C)o = 0.267 LMR18,22 Med. Type Refs. meas. gas LF 13 Med. Type Refs. meas. gas DL 19 a Value ~iven for 13CH Z ' b The a Al and 6 IBI states ~re perturbed by strong Renner-Teller interaction. 13 ,14 They are also strongly Qerturbed by interaction with the X 3Bl state. 27 ,ZS c Calculated value. 21 Rydberg series with members at 93054, 95753, 97193, and 98049, converging to NH2 (A lA 1 ) at 100410 (PI 19). i'\ 2Al(lll.l)a C2v Structure: ABl,4 To :: 11122.6 gas ABl,BLF6 A-~ 430-950 nm Ar,Kr,Xe b AB2,3,5 A-~ 344-790 nm 797 Ar IR 13 Ao = 4.857(2); Bo = 4.044(4); Co = 2.180(2) AB9 a The A 2A1 and K 2B1 states are perturbed by strong Renner-Teller interaction. L+ 3 BaD stretch 461.0(3) gas LF 4

Fourier transform absorption spectra have been obtained between 700 and 7900 cm−1 at a resolution of 0.2 cm−1 for Ar:C2H2 samples codeposited at 12 K with a beam of argon atoms that had been excited in a microwave discharge. Detailed isotopic substitution studies have confirmed that the predominant product species is HC2, which contributes not only the absorptions previously assigned to its two stretching fundamentals but also several weaker absorptions in the 2000–3600 cm−1 spectral region and a prominent, complicated pattern of absorptions between 3600 and 7800 cm−1. The previous assignment of the 3611 cm−1 HC2 absorption as the CH-stretching fundamental is reviewed, and the assignment of an absorption at 2104 cm−1 as ν2+ν3 of ground-state HC2 is discussed. The near infrared absorption band system has been assigned to the Ã 2Π–X̃ 2Σ+ transition of HC2, extensively perturbed by interaction with high vibrational levels of the ground state. The position of the transition origin could not be definitively established. The previous assignment of gas-phase absorptions of HC2 at 3786, 4012, and 4108 cm−1 to ground-state combination bands has been confirmed, and carbon-isotopic data have been obtained for these bands. Because all of the HC2 absorptions observed in an argon matrix must originate in the X̃(000) state and because the observed matrix shifts are relatively small, these studies may provide a map to facilitate more detailed gas-phase studies.