We report results of new pair creation experiments using ~100 Joule pulses of the Texas Petawatt Laser to irradiate solid gold and platinum targets, with intensities up to ~1.9 × 1021 W.cm−2 and pulse durations as short as ~130 fs. Positron to electron (e+/e−) ratios >15% were observed for many thick disk and rod targets, with the highest e+/e− ratio reaching ~50% for a Pt rod. The inferred pair yield was ~ few ×1010 with emerging pair density reaching ~1015/cm3 so that the pair skin depth becomes < pair jet transverse size. These results represent major milestones towards the goal of creating a significant quantity of dense pair-dominated plasmas with e+/e− approaching 100% and pair skin depth ≪ pair plasma size, which will have wide-ranging applications to astrophysics and fundamental physics.

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.

Disulfur oxide (denoted as SSO) isolated in solid Ar at 13 K was irradiated with light at 308 nm from a XeCl excimer laser. New lines at 799.1 (797.0), 574.9, and 544.1 (545.6) cm−1, observed after photolysis, are assigned to cyclic S2O (denoted Cyc-S2O) with ∠S–O–S≅72.5±3.0° based on results of S34- and O18-isotopic experiments; lines in parentheses are associated with a minor matrix site and the broad line at 574.9 cm−1 may be deconvoluted to two lines at 575.4 and 574.6 cm−1. Secondary photolysis at 248 nm diminishes lines of cyc-S2O and produces SSO. Theoretical calculations using second-order Møller–Plesset theory with frozen core gradients and density-functional theories (Becke’s exchange functional with a correlation functional of Lee, Yang, and Parr) predict three stable isomers of S2O: cyc-S2O, SSO, and SOS, with the latter two having angular geometry. Relative energies, structures, vibrational wave numbers, and IR intensities were predicted for each isomer. According to calculations with Becke’s three-parameter exchange functional and the valence triplet-ζ basis set, cyc-S2O is bent with ∠S–O–S≅73.3° and has the S–S bond (2.058 Å) and both S–O bonds (1.724 Å) elongated relative to those of SSO (1.909 and 1.474 Å, respectively); it lies 41.3 kcal mol−1 above SSO. Isomer SOS, 62.0 kcal mol−1 greater in energy than SSO, has a S–O bond length 1.625 Å and ∠S–O–S≅128.5°. Calculated vibrational wave numbers, IR intensities, and isotopic shifts for cyc-S2O fit satisfactorily with experimental results. Two asymmetric transition states connecting SSO with SOS and cyc-S2O are characterized, yielding barriers for isomerization ∼104 and 122 kcal mol−1 (zero-point energy corrected), respectively. Photoconversion between angular SSO and cyc-S2O in a matrix cage is discussed. cyc-S2O might be responsible for some distinct features in thermal emission from the surface of Io, Jupiter’s moon.

The large shifts (−30.5 and +41.7 cm−1, respectively) of the IR absorption lines of nitric acid (HONO2) in a N2 matrix in the ν1 (OH stretching) and ν3 (HON bending) vibrational modes, relative to those observed for HONO2 in an Ar matrix, indicate a strong interaction between HONO2 and N2. Photolysis of HONO2 in solid Ar produced predominantly HOONO, whereas relatively intense lines of N2O and HONO were observed after photolysis of HONO2 in solid N2 with a mercury lamp, with the relative yield of HOONO decreased by more than a factor of 10. Possible photochemical processes are discussed.

Irradiation of an Ar matrix sample containing H2S and CO (or OCS) with an ArF excimer laser at 193 nm yields trans-HSCO (denoted t-HSCO). New lines at 1823.3, 931.6, and 553.3 cm(-1) appear after photolysis and their intensity enhances after annealing; secondary photolysis at 248 nm diminishes these lines and produces OCS and CO. These lines are assigned to C-O stretching, HSC-bending, and C-S stretching modes of t-HSCO, respectively, based on results of 13C-isotopic experiments and theoretical calculations. Theoretical calculations using density-functional theories (B3LYP and PW91PW91) predict four stable isomers of HSCO: t-HSCO, c-HSCO, HC(O)S, and c-HOCS, listed in increasing order of energy. According to calculations with B3LYP/aug-cc-pVTZ, t-HSCO is planar, with bond lengths of 1.34 A (H-S), 1.81 A (S-C), and 1.17 A (C-O), and angles angle HSC congruent with 93.4 degrees and angle SCO congruent with 128.3 degrees; it is more stable than c-HSCO and HC(O)S by approximately 9 kJ mol(-1) and more stable than c-HOCS by approximately 65 kJ mol(-1). Calculated vibrational wave numbers, IR intensities, and 13C-isotopic shifts for t-HSCO fit satisfactorily with experimental results. This new spectral identification of t-HSCO provides information for future investigations of its roles in atmospheric chemistry.