New selenium-nitrogen species were produced by condensing the effluent from an argon/nitrogen/selenium microwave discharge onto a 12 K substrate. Nitrogen-15 and selenium-76 and -80 isotopic substitutions, photolysis, and annealing provided basis for identification of the new product absorptions in the infrared spectrum. A weak band at 951 cm(-1) is assigned to SeN and the very strong 1019 cm(-1) band to v(3) of NSe2 radical with a 146 +/- 5 degrees valence angle. A new 1570 cm(-1) band in this system is due to the SeNO radical, and a 1771 cm(-1) absorption, also made from Se-2 and NO, is due to the (SeSe)(NO) complex. Higher-order SexNy (x > 1) species give rise to bands at 904, 736, 616, and 529 cm(-1). The argon/selenium discharge in quartz has also produced the SiSe, SiSe2, and SeSiO species. Structure and frequency calculations using density functional theory support the matrix infrared identification of these new selenium molecular species. The most stable (NSe)(2) structure is calculated to be the cis form analogous to (NO)(2), but by contrast, (NSe)(2) has a stronger N=N bond and Se-Se bonding.