Potassium nitrate (KNO3) isolated in solid argon at 13 K was irradiated with emission from an ArF excimer laser at 193 nm. Recombination of the photofragments led to formation of both cis- and trans-potassium peroxynitrites (KOONO). The cyclic conformer, cis-KOONO, absorbs at 1444.5, 952.3, 831.6, and 749.1 cm-1, whereas trans-KOONO absorbs at 1528.4, 987.4, and 602.2 cm-1. The assignments are based on observed 18O- and 15N-isotopic shifts and comparison with similar compounds, cis-cis and trans-perp HOONO. Ab initio calculations using density functional theory at a Becke3LYP level predicted similar line positions and isotopic shifts for both conformers. Photoconversion among these three isomers was achieved at various wavelengths and periods of irradiation; cis-KOONO was photolyzed readily at 308 nm, whereas trans-KOONO increased slightly in intensity initially and was eventually transformed to KNO3 on prolonged irradiation. Similar results were obtained for LiNO3 and NaNO3; cis-LiOONO and cis-NaOONO absorb at (1423.4, 1422.0), 966.2, 874.2, 792.3 cm-1 and (1437.4, 1434.6), 961.4, 840.7, (770.9, 768.7) cm-1, respectively, whereas trans-LiOONO and trans-NaOONO absorb at (1581.6, 1580.4), (998.3, 995.6), 600.4 cm-1 and (1549.3, 1540.6), (996.3, 994.1), (609.4, 607.4) cm-1, respectively; the numbers in parentheses are due to line splitting. © 1995 American Institute of Physics.