A combination of matrix isolation and Fourier transform infrared (FTIR
) spectroscopy has been employed to investigate the low-temperature ph
otochemistry (4.2 K) of nitric acid (HONO2). UV photolysis of the matr
ix-isolated precursor in argon (MR 1/4000) at 184.9 and 253.7 nm led t
o the formation of peroxynitrous acid (HOONO), presumably via cage rec
ombination of OH + NO2. HOONO appeared to undergo secondary photolysis
to yield NO and HO2. A second primary photolysis channel produced the
novel species C-2v-HNO2, a thermodynamically less stable isomer of ni
trous acid; deuterated studies combined with group frequency arguments
and theoretical studies support this assignment. In nitrogen matrices
, N2O, cis-HONO, and trans-HONO were observed as additional products d
ue to efficient scavenging of O(D-1(2)) by the matrix material. Irradi
ation of HONO2 in oxygen-doped argon matrices resulted in photooxidati
on and produced primarily peroxynitric acid (HO2NO2) as well as ozone.