Jussi Ahokas

On photochemistry of water in solid Xe: Thermal and light-induced decomposition of HXeOH and HXeH and formation of H2O2

A photochemical study of water (H2 16O, H2 18O, D2 16O, and D2 18O) in solid Xe is described. The water–Xe samples were irradiated at 193 nm and then annealed at 40–50 K, which led to formation of various isotopomers of Xe-containing molecules, HXeOH and HXeH. This diffusion-controlled formation of HXeH and HXeOH consumes the main part of hydrogen atoms generated in the matrix during photolysis. Both photodecomposition profiles and ultraviolet (UV) absorption spectra of HXeOH and HXeH feature a broad absorption band of these species around 240 nm corresponding to the transition to the repulsive excited states. It is also found that HXeOH and HXeH molecules can be thermally destroyed in similar time scales of ∼10 min at about 54 and 66 K, respectively. This clear difference between the decomposition temperatures for HXeOH and HXeH suggests the intrinsic basis of the decomposition process, which possibly occurs over the barriers of the bending coordinates. The absence of strong H–D isotope effect in this th...

Theoretical analysis of alkali metal trapping sites in rare gas matrices

The rare gas (Ne, Ar, Kr, Xe)–alkali metal (Li, Na) ground-state pair interaction potentials and distance-dependent isotropic hyperfine coupling constants are evaluated by coupled-cluster approaches at the van der Waals region of the dimers. The computed properties are further utilized in classical molecular dynamics simulations of rare gas lattices doped with alkali atoms. Atomic trajectories and time averaged hyperfine constants are obtained from the simulations and exploited to provide theoretical insights into experimentally observed atomic trapping and dynamics of alkali metal atoms in rare gas matrices. The simulations support our previous electron paramagnetic resonance (EPR) data [Chem. Phys. Lett, 310, 245 (1999)], suggesting that alkali metal atoms, while generated by laser vaporization, do trap in single substitutional sites, whereas thermal atom sources yield trapping in multiple substitutional sites. In order to theoretically reproduce the EPR spectra for the latter case, more than six neighb...

Magnetic properties of atomic boron in rare gas matrices: An electron paramagnetic resonance study with ab initio and diatomics-in-molecules molecular dynamics analysis

The anisotropic boron atom electron paramagnetic resonance spectra measured in rare gas matrices (Ar, Kr, Xe) are interpreted with the aid of highly correlated ab initio calculations including spin–orbit coupling and diatomics-in-molecules (DIM) molecular dynamics simulations. The heavy-element and crystal field effects are inspected as they contribute to the electron g-shift. The DIM-simulated p-orbital splittings and lattice perturbed hyperfine coupling values provide a good starting point for spectral fitting and show the correctness of the guidelines given by purely synthetic generation of the spectra. The present combination of experiment and theory resulted in improved accuracy of the parameters measured in Ar matrix, new values are extracted for Kr matrix, and tentative assignment is also provided for the Xe matrix case.

On the formation mechanism of impurity–helium solids: evidence for extensive clustering

Optical emission studies of a discharged nitrogen–helium gas injected into superfluid helium near 1.5 K are described. Analysis of atomic (α group) and molecular Vegard–Kaplan transitions clearly indicates that the emitting species are embedded inside nitrogen clusters. Cluster formation is most efficient in the crater formed on the liquid surface. Model calculations based on the classical bubble model and density functional theory suggest that under the experimental conditions only clusters consisting of more than 1000 molecules have sufficient kinetic energy for stable cavity formation to occur inside liquid helium. The results obtained suggest that impurity–helium solids formation is a consequence of extensive clustering in the gas jet.

Visible light-driven chemistry of oxalic acid in solid argon, probed by Raman spectroscopy.

High-overtone induced chemistry of oxalic acid (OA) isolated in a low-temperature argon matrix was investigated using Raman spectroscopy. The Raman spectra of three conformers of OA are presented and discussed. Upon excitation of high overtone combination bands by 532 nm irradiation of the lowest energy cTc structure, the isomerization and unimolecular decomposition of OA were observed. Dissociation was induced presumably by absorption into the 5A(g) + B(u) vibrational state of the OH stretching mode of cTc. The photodecomposition leads to the formation of CO, CO(2), and H(2)O products. The experimental observations were supported by computational studies and vibrational anharmonic calculations.

Optical properties of atomic boron in rare gas matrices: An ultraviolet-absorption/laser induced fluorescence study with ab initio and diatomics-in-molecules molecular dynamics analysis

Boron atoms have been produced by laser ablation and trapped in solid Ar, Kr, and Xe matrices. In addition to stable doublet absorptions at 210–230 nm, broad temperature dependent bands less in blue have been recorded. With the aid of laser induced fluorescence measurements, ab initio pair potential calculations, and diatomics-in-molecules simulations these novel broad lines at 241 nm in Ar, 247 nm in Kr, and 254 nm in Xe were assigned to boron 3s(2S)←2p(2P) transitions. The thermal behavior of the broad absorption bands is similar to what was reported for boron atoms detected by electron paramagnetic resonance [J. Chem. Phys. 114, 7144 (2001)]. The revised interpretation based on the new spectroscopic observations is in good agreement with theoretical predictions.

Investigating isomerization reactions in solid state by using simultaneous high overtone pumping and Raman detection

Formation of the unstable cis-formic acid in solid argon matrix is induced by direct excitation of the 6<--0 transition of the nu(OH) vibration of the trans-formic acid. The experiment utilizes strongly focused laser beam that produces relatively high isomerization rate despite the low cross section of the absorption. Raman spectroscopy in a backscattering geometry is used for detection of the reactants and the products. This experimental arrangement allow us to use the same laser source for simultaneous pumping and Raman excitation, and it also guarantees that the excited and probed volumes overlap. The presented method has a high potential for solid state investigations of chemical reactions on the ground electronic state.

Host–guest charge transfer states: CN doped Kr and Xe

The host–guest charge transfer absorption of CN doped krypton and xenon matrices are identified through direct analogy with the previously assigned transitions of Cl/Kr and Cl/Xe. These intense, structured absorption bands appear with the onset at 245 nm in Kr and 360 nm in Xe. Excitation of the CN/Kr charge transfer band at 193 nm leads to emission over CN(A(2Π)→X(2Σ)) transition, indicating that an efficient curve crossing precludes the ionic state from radiating. No emissions were seen in CN/Xe when excited at 193 nm. The charge transfer absorption spectrum of CN/Kr is reproduced through an extended diatomics-in-ionic-systems treatment, using accurate ab initio pair potentials and transition dipoles as input, without further adjustment. The delocalized hole states are then analyzed in real-space, using atomic bases distributed over as many as eleven shells surrounding the CN− center. The ionic states are well described as J=1/2, 3/2 valence bands bound to CN−, with a substructure that cannot be exclusi...

On the Formation Mechanism of Impurity Helium Solids

In this study we present new data on cooling of seeded (N2 and NO) helium jets prior to penetration into the bulk helium. In this case, the fine structure originating from the molecular excited states is comparable to kT and therefore the corresponding ultraviolet and visible (UV/VIS) emissions can be used to estimate the temperature of the surroundings of the molecules. Cooling down of the clusters inside the bulk helium occurred slowly, presumably due to extra heat generated by atom‐atom recombination.

Raman spectroscopy and high-overtone driven isomerization of glyoxylic acid in solid argon

High-overtone induced chemistry of glyoxylic acid isolated in a low-temperature argon matrix was investigated using Raman spectroscopy. The Raman spectra of two most stable conformers of glyoxylic acid are presented. Upon excitation in high overtone vibrational bands by 532 nm irradiation of the lowest energy conformer most abundant in neat deposited sample, the isomerization of glyoxylic acid was observed. The process most plausible proceeds via absorption into the fifth vibrational overtone state of the OH group or its combination with the torsional vibrational transition. The assignment of the fundamental vibrational spectra was assisted by quantum chemical harmonic and anharmonic vibrational calculations.