Esa Isoniemi

HONO in solid Kr: Site-selective trans↔cis isomerization with narrow-band infrared radiation

Narrow-band resonant IR radiation promotes trans↔cis isomerization of HONO in a Kr matrix when the O–H stretching or N=O first overtone modes are excited. The isomerization process is selective with respect to matrix sites, and the HONO concentrations in various matrix sites are unchanged, demonstrating the stability of the host cage under the resonant IR irradiation. The isomerization process is localized at the vibrationally excited molecule and possesses a high quantum yield, possibly approaching 1 for the cis to trans reaction. The present experimental results on IR-induced isomerization of HONO agree with the known molecular dynamics simulations. Our anharmonic ab initio calculations describe well the vibrational properties of both HONO isomers, including a number of observed overtone and combination bands. Especially remarkable in our opinion is the very good numerical agreement between the calculated and experimental IR absorption cross sections found for all the fundamental modes.

193 nm photolysis of H2S in rare-gas matrices: Luminescence spectroscopy of the products

The 193 nm photolysis of hydrogen sulfide (H2S) in solid rare gases is studied at 7.5 K. In order to get the most reliable data of the photolysis process, Fourier transform (FT) infrared and time-resolved luminescence methods are used in the same experiment. The 193 nm photolysis of H2S in Ar and Kr matrices was found to be very similar to the gas phase. A kinetic scheme of H2S photolysis, which is consistent with all the experimental features, was constructed. The major channel is formation of (H+SH) pairs, which are stabilized in the matrix. Then SH radicals decompose to (S+H) pairs, providing the main source for S atoms. No experimental evidence of a cage-induced reaction H+SH→S+H2 was observed in our study, which can be connected with high probability for hydrogen-atom exit from the parent cage, and/or with high probability of the recombination reaction H+SH→H2S. The available spectroscopic information for S atoms and SH radicals in Ar and Kr matrices is further specified, and new spectroscopic data o...

Low-temperature thermoluminescence in solid argon: Short-range mobility of atoms

The 193-nm photolysis of S2 (produced from H2S2) and SO2 in solid argon and the changes introduced by annealing are monitored by using Fourier-transform infrared spectroscopy and laser-induced fluorescence. The results highlight short-range atomic mobility distinguished from global long-range mobility. It is shown that under the 193-nm irradiation a small part of S2 and SO2 molecules undergo photodecomposition yielding quasi-isolated pairs where the fragments are in close vicinity to each other. In annealing, the short-range mobility results in recombination of these pairs at rather low temperatures (<15 K), and this known low-temperature thermoluminescence does not require long-range atomic mobility. Also, recombination of two sulfur atoms can be induced through phonon sideband absorption of the geminate photofragments, which introduces light-induced short-range atomic mobility. The simulations on two sulfur atoms in a static Ar lattice support this scenario, demonstrating that the photofragments can be ...

HSO2 isomers in rare-gas solids

Photochemistry of HSO2 isomers is studied by using FTIR spectroscopy in argon, krypton and xenon solids. Photodecomposition of H2S or H2O was used as a hydrogen atom source to study a diffusion controlled reaction of H and SO2. Mainly, the higher energy HSO2 isomer is formed in this reaction which is attributed to a low barrier for the HSO2 formation and a higher barrier for the formation of the lowest-energy species cis-HOSO. Photoexcitation of HSO2 at wavelengths below 580 nm promotes HSO2 → cis-HOSO interconversion. For this photoisomerisation two mechanisms are possible: direct isomerisation of HSO2 to cis-HOSO or a cage-induced reaction. The photodissociation threshold of cis-HOSO to H + SO2 is at 400 nm. The photon energies needed for these processes agree with the available computational data. To aid assignment of the experimental vibrational spectra, the equilibrium structures and the vibrational wavenumbers of HSO2, trans-HOSO, cis-HOSO and HXeOSO were calculated by ab initio methods.

Excited-state site effects in luminescence spectroscopy of SH radicals in krypton matrices: Experiment and simulations

The specific features in luminescence spectra of SH radicals in solid krypton differing remarkably from those of SH and OH species in various rare-gas matrices are discussed. Of special interest in Kr matrices are two bands appearing in the SH emission spectra under the A 2Σ+←X 2Π excitation, which center at 375 and 413 nm and possess very different lifetimes, 350 and 750 ns. Along with the experimental findings, we describe the results of the mixed quantum-classical molecular dynamics simulations of the emission spectra performed on the diatomics-in-molecules potential energy surfaces of the ground and excited states of the clusters SH@Krn (n⩾250). The simulations show that both bands in the emission spectra refer to the same (A 2Σ+,v′=0→X 2Π,v″) transition and originate from two different trapping sites in the excited state. The band at the shorter wavelength corresponds to the site with a relatively free movement of SH (A) inside the lattice while the adjacent more intense band at the longer wavelength...

Laser-induced fluorescence studies of S2+ in solid argon

Abstract The laser-induced fluorescence of S 2 + is studied in solid argon at 7.5 K. The S 2 + molecular ions were generated under 266 nm photolysis of a H 2 S 2 /Ar matrix. The A 2 Π u –X 2 Π g emission of S 2 + with a lifetime of 805 ns was detected while the matrix was excited in the 330–455 nm region. The assignment is based on the comparison of the measured spectroscopic parameters with the gas-phase values. The possible charge-transfer mechanism of the photogeneration of S 2 + is discussed. Our results indicate that S 2 + molecular ions constitute a minor channel in 266 nm photolysis of H 2 S 2 in an Ar matrix.