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When a Ne:HCOOH sample is codeposited at ca. 5 K with neon atoms that have been passed through a microwave discharge, new absorptions appear in the infrared spectrum of the resulting solid that can be assigned to trans-HOCO, trans-HCOOH ϩ , and HCO 2 Ϫ. The absorptions of trans-HOCO are readily identified by a comparison with those previously reported for that molecule trapped in solid argon. Preliminary assignments of infrared absorptions of HOCO ϩ , confirmed in studies using another experimental system, are also suggested. The identifications of trans-HCOOH ϩ and of HCO 2 Ϫ are aided by study of the photodestruction characteristics of these products when the deposit is exposed to various wavelengths of visible and ultraviolet radiation, by an analysis of the spectra obtained from isotopically substituted samples, and by a comparison with the results of ab initio and density functional calculations. Three previously unidentified vibrational fundamentals of trans-HCOOH ϩ have been assigned, as have been four vibrational fundamentals of HCO 2 Ϫ , in the inert, nonionic environment of solid neon. The CH-stretching fundamental of HCO 2 Ϫ appears at an exceptionally low frequency. The results of density functional calculations of the structures and vibrational fundamentals of trans-HCOOH ϩ , HCO 2 Ϫ , the W͑C 2v) structure of C͑OH) 2 ϩ , and cis-and trans-HCOOH Ϫ are given.

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When a Ne:CO2 mixture is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes absorptions assigned to CO2+, (CO2)2+, CO2−, and (CO2)2−. The lowest (μ 2∑u+) Renner component of the bending fundamental of CO2+ trapped in a neon matrix appears near the gas-phase band center, but other Renner components are undetectable. Absorptions of a photolabile product correspond to the recently identified CO-stretching fundamentals of (CO2)2+. Weak infrared absorptions at 1253.8 and 714.2 cm−1 are assigned to ν1 and ν2 of CO2−, respectively, and a moderately intense absorption at 2894.7 cm−1 is assigned to the ν1+ν3 combination band of that product. As in other recent argon- and neon-matrix studies, two weak infrared absorptions can be assigned to the two infrared-active OCO-stretching fundamentals of the D2d structure of (CO2)2−. Detailed isotopic substitution studies support all of these assignments. A weak absorption near the CO2 bending fundamental, for which isotopic substitution data are incomplete, may be contributed either by a second fundamental of (CO2)2− (D2d) with b2 symmetry or by a weakly interacting (CO2)n⋅⋅CO2− complex. Such ion–molecule complexes contribute other absorptions near ν3 of CO2 and of CO2−.