Rare Gas Matrices Research Articles

Screening and the quantitative π-model description of the optical spectra and polarizations of phenyl based oligomers

The long standing problem of the inability of many semiempirical models to correctly predict the polarization of the higher dipole allowed optical transitions of phenyl based π-conjugated polymers and molecules is examined and related to the issue of internal and external screening of π–π electron Coulomb interactions within the molecules. Following a review of previous theoretical and experimental work, π electron only the Complete Neglect of Differential Overlap (CNDO) model is presented which, for the first time, is able to predict accurately the energies and symmetries of all the observed optical transitions of benzene, biphenyl and trans-stilbene, up to ∼8–10 eV. In so doing, it is demonstrated that the problem with previous calculations was the noninclusion of screening from outside the π electron system itself. By fitting separately the spectra in hydrocarbon based condensed phases, in the gas phase and in solid rare gas matrices, and comparing the resulting model parameters, we show that, while the effects of screening from the environment are certainly noticeable, the most important spectral features—in particular the ordering of dipole allowed transitions—come from effective screening by the σ electrons. We find that both of these effects can be adequately accounted for within a π electron only model by using a dielectric constant and appropriate parameter renormalization.

Ultrafast intramolecular relaxation of C 60

Transient absorption spectra of C 60 molecules trapped in cryogenic rare gas matrices and dissolved in room temperature toluene have been measured with a time resolution of 90 fs. An absorption build-up with a characteristic rise time of 200 fs is measured, which is solvent-, temperature- and probe wavelength-independent. It is attributed to the population of an intermediate state by the decay of the initially excited singlet state(s) by vibrational and electronic relaxation. The data are discussed in the light of results from the literature and a general picture of intramolecular relaxation in C 60 is proposed.

Vibrational overtones and rotational structure of HCl in rare gas matrices

The rotational structure in the vibrational transitions from v=0 to v=1, 2, 3, 4 of H35Cl and H37Cl is studied in Xe, Kr, and Ar matrices with high spectral resolution. A consistent set of rotational constants Bv for the vibrational levels v=0 to 4 is derived. B0 decreases with the tightness of the cage from 9.78 cm−1 in Xe to 8.83 cm−1 in Ar for H35Cl (gas phase 10.44 cm−1). The values for B0 to B4 decrease linearly with v due to the vibration-rotation-coupling constant α which increases from 0.37 cm−1 in Xe to 0.479 cm−1 in Ar (gas 0.303) according to the cage tightness. The splitting of the R(1) transition which originates from the hindering of rotation is analyzed in Xe using the T2g–T1u and T2g–Eg transition energies. A comparison with force field calculations yields a dominant contribution of the sixth spherical harmonic YA1g6 of the octahedral matrix potential. The modulation of the potential takes a value of K6/B=17 which corresponds to a barrier for the rotation of 160 cm−1. The splitting increases with the vibrational level v which can be interpreted as a weak admixture of the YA1g4 spherical harmonic. A large isotope effect and a reduction of the T1u–A1g transition energy [R(0)-transition] beyond the crystal field value are attributed to an eccentric rotation with a displacement of the center-of-mass of the order of 0.05 Å. The vibrational energies ωe show an opposite trend with matrix atom size and decrease with polarizability from 2970 cm−1 in Ar to 2945.4 cm−1 in Xe (gas 2989.9 cm−1) while the anharmonicity ωexe of the free molecule lies close to the Kr value and thus between that of Ar and Xe.

Photoinduced rotational isomerization mechanism of 2-chlorobenzaldehyde in low-temperature rare-gas matrices by vibrational and electronic spectroscopies

Rotational isomerization of 2-chlorobenzaldehyde in low-temperature rare-gas matrices has been investigated by vibrational and electronic spectroscopies with aids of the density functional theory (DFT) and configuration interaction single (CIS) calculations. Infrared spectrum of the less stable O-cis isomer, produced from the more stable O-trans isomer upon UV irradiation, is measured with an FT-IR spectrophotometer. The enthalpy difference between the O-cis and O-trans isomers is estimated to be 9.7±0.2kJmol−1 from the temperature dependence of the infrared band intensities. Analyses of the infrared and electronic absorption spectral changes after UV irradiation and the phosphorescence spectra measured at various excitation wavelengths suggest that the rotational isomerization occurs via the intersystem crossing from S1 to T1.

Perylene and terrylene in rare gas matrixes: spectroscopic and computational studies of inclusion sites

The low-temperature (20 K) electronic absorption and emission spectra of terrylene (C 30H 16) and perylene (C 20H 12) isolated in Kr and Xe matrixes are presented. The spectra of both chromophores in Kr matrix reveal similar multiplet structure, and in each spectrum, six different sites were identified. In order to understand the origin of observed site—splitting, the theoretical calculations of complex system—chromophore embedded into Kr matrix—were carried out by means of MM+ method. The optimization of system structure yielded six different types (sites) of embedding chromophore molecules into Kr crystal lattice. As calculations have shown, the nearest environment of each site distinguishes by its specific density of rare gas atoms, on which basis the relationship between theoretically calculated sites and observed spectral features was established.

Electronic structure and metal-insulator transition in SrTi 1 - x Ru xO 3

We studied the changes in the electronic structure of SrTi1-xRuxO3 across the metal-insulator transition. The parent compound, SrTiO3, is a well known diamagnetic insulator; whereas the doped compound, SrTi1-xRuxO3, becomes a ferromagnetic metal above x C = 0.35. The techniques used in the study were photoemission (PES) and O 1 s X-ray absorption (XAS) spectroscopy. The experimental spectra were analyzed in terms of band structure and Hubbard model calculations. The PES and XAS spectra of SrTi1-xRuxO3 show the Ru 4 d bands growing in the band gap of SrTiO3 . The analysis in terms of the Hubbard model indicates that the Ti 3 d and Ru 4 d bands are mostly decoupled. This suggests that the metal-insulator transition is a percolation transition like that of metals embedded in a rare gas matrix. Electron correlation effects are present in this system, but they do not seem to play a major role in the transition.

Infrared spectra and ab initio calculations of matrix isolated dimethyl sulfone and its water complex

The infrared spectrum of (CH3)2SO2 (dimethyl sulfone) trapped in several rare gas matrices is reported. Results are compared to previous gas phase and solid state spectra. In water-containing matrices, bands attributed to the ((CH3)2SO2)*(H2O) molecular complex are identified. Spectral results are discussed with reference to the bonding scheme suggested by theoretical ab initio calculations. The similarities to the previously recorded (H2SO4)*(H2O) mode frequencies shed light on the bonding scheme of both. The complex formation between H2O and dimethyl sulfone was studied by ab initio B3LYP computations using the aug-cc-pVTZ basis sets. The calculations show the existence of a stable 1∶1 complex of Cs symmetry, ΔfH = −21 kJ mol−1, with a single O⋯H bond of one water hydrogen bonded to one oxygen of the sulfone.

Transient Negative Species in Supercritical Carbon Dioxide: Electronic Spectra and Reactions of CO2 Anion Clusters

Transient absorption spectra following ionization of supercritical CO2 have been investigated using the pulse radiolysis technique. Absorption spectra measured from 400 to 800 nm suggest that at least two transient species absorb. We have previously reported that one species is (CO2)2+. In the near UV region, we observed a transient species of which the lifetime and reactivity are different from the dimer cation. We assign this species to a dimer anion, (CO2)2-, or an anion−molecule complex, (CO2-)(CO2)x. Comparison with the photobleaching of CO2 anion clusters in solid rare gas matrixes and their reactivity with H2 and O2 confirm the assignment. Theoretical calculations, in which solvation is taken into account, are consistent with these assignments. It is well-established that the adiabatic electron affinity of CO2 is negative, but the adiabatic electron affinity of CO2 dimer has been calculated to be 0.89 eV for D2d symmetry (CO2)2- in the gas phase. The calculations predict that CO2- in a model continuum solvent is stable to autodetachment.

The structure of the phototransformation product of monothiodibenzoylmethane

Infrared (IR) and electronic spectroscopy studies were carried out for monothiodibenzoylmethane isolated in rare gas matrices, low-temperature glasses, polymer films, KBr tablets (IR) and liquid solutions. This molecule is known to undergo light-induced conversion, but the nature of the product has long been a subject of discussion. Comparison of the experimental IR spectra with theoretical predictions (B3LYP/cc-pVDZ) obtained for various possible structures shows that the photoproduct corresponds to a non-chelated –SH exo-rotamer of the ( Z)-enethiol tautomeric form of monothiodibenzoylmethane. This conclusion is consistent with the experimentally determined energy of the activation barrier for the reverse ground-state process that brings the molecule back to the initial enolic, intramolecularly hydrogen-bonded structure.

Solvation of ion-pair states in nonpolar media: I2 in solid neon, argon and krypton

We report on the ion-pair state emission spectra of Iodine isolated in Neon, Argon and Krypton matrices in the spectral range from 300 nm to 700 nm, and present VUV-absorption spectra in the 50 000–60 000 cm−1 region. Spectroscopic analysis of the UV and visible emission bands of the lowest-lying ion-pair state D′(2g) in Ne and Ar provides unambiguous evidence for both vertical (energy) and horizontal shifts (equilibrium distance) due to solvation in a rare gas matrix, and suggests that the ion-pair states maintain their distinct character. In Krypton the previously reported broad emission band at 420 nm is shown to consist of two narrow bands, which have counterparts in the visible. They are assigned to emission from D′(2g) and a second ion-pair states of I2, most probably the δ(2u) state. Partial charge delocalization to the matrix can only play a minor role in Kr since very similar emission is also found for secondary sites in Ar. These findings should be highly relevant for a description of ion-pair states in nonpolar media. They are also useful for the interpretation of time-resolved experiments that use the ion-pair state emission as probe signal.

Matrix-isolation investigation of the diatomic anion radicals of aluminum and gallium (Al2− and Ga2−): An electron spin resonance and ab initio theoretical study

The diatomic aluminum and gallium anion radicals (Al2− and Ga2−) have been studied by electron spin resonance (ESR) as isolated ions in neon matrices near 4 K. Apparently no previous examples of purely metallic anions have been studied by the rare gas matrix isolation trapping technique with any type of spectroscopic method. Pulsed laser ablation of the metals was the experimental generation method which has enabled the first direct characterization of the ground electronic states of these anions. The X 4Σ assignment agrees with previous interpretations of photoelectron spectroscopic results. The analysis of fine structure and nuclear hyperfine interactions has yielded a detailed description of the electronic structure in these seven-valence electron anion radicals, with comparisons to other isovalent X 4Σ radicals presented. Experimental values of the nuclear hyperfine interactions (A tensors) were compared with theoretical results obtained from various ab initio computational methods, including configuration interaction and density functional theory. Reasonable agreement between these experimental results and theoretical predictions was observed.

Cage Structure and Long-Range Order in Solid Rare Gas Matrixes: A Combined FTIR and XRD Study

Infrared spectroscopy (FTIR) and X-ray diffractrometry (XRD) were applied in order to characterize solid rare gas matrixes containing SF6 or CH4 species as impurities. The two techniques may be considered as complementary insofar as FTIR probes the dynamics of the dopant species, which is more or less strongly influenced by the matrix environment, whereas XRD aims at the long-range order of the rare gas matrix, which is likely to depend on the type and amount of the dopant. In fact, the distortions induced in Ar matrixes by embedding perfectly isolated CH4 and SF6 monomer molecules are not seen by XRD since the respective contribution to the broadening of Bragg reflections is buried under the dominating contribution related to the crystallite size effect (Scherrer formula). The nonequilibrium conditions of the deposition process give rise to a mean crystallite diameter of less than 50 nm. From the trends observed for both position and width of the respective IR test bands in Ne, Ar, and Kr, it is concluded that the matrix cages for the CH4 monomer are single substitutional sites, whereas for the SF6 monomer the space required comprises that of six rare gas atoms in an octahedral arrangement. However, a tetrahedral cage consisting of four rare gas atoms may not be excluded definitely. The crystalline order of Ar is retained even at CH4 concentrations that are undoubtedly related to the previously reported miscibility gap and to significant dimer formation detected by FTIR. On the other hand SF6 dimer formation, again identified by FTIR, gives rise to a complete loss of coherent X-ray scattering of Ar and to the appearance of diffuse intensity in the XRD pattern attributed to an essentially amorphous Ar phase. The local Ar environment of an isolated monomer and of the monomer unit of an isolated dimer appear to be fundamentally different in the case of SF6 and resemble each other for CH4 as impurity. Obviously, the SF6 dimer formation in Ar immediately heralds in an efficient phase separation process already at 1 mol % SF6 even under the nonequilibrium conditions of matrix deposition. Under these conditions the phase separation in CH4/Ar mixtures appears to be strongly kinetically hindered.

Absorption Wavelengths and Bandwidths for Interstellar Searches of C60in the 2400–4100 A Region

Low-temp. gas-phase wavelengths and bandwidths for astronomical searches for C60 in the 2400-4100 .ANG. region are detd. from the absorption spectra of C60 in rare gas matrixes at 4 K, coupled with information on the dependence of the vibronic transition frequencies of this fullerene on the Lorentz-Lorenz polarizability parameter of matrixes and solvents. These bandwidths increase from 25 +- 5 cm-1 for the 1 1T1u-1 1Ag band at 4024.0 +- 0.5 .ANG. to 2300 +- 200 cm-1 for the 6 1T1u-1 1Ag band at 2488 +- 8 .ANG.. Spectral observations of astrophys. objects considered to be likely sites of C60 were examd. in the context of the wavelengths and bandwidths of bands of the allowed transitions of C60 which we recommend for searches of this species. No certain pos. assignment of a C60 band was made in the 2400-4100 .ANG. spectral region of these objects. The phys. origin of the obsd. C60 absorption bandwidths is analyzed and discussed in an appendix. [on SciFinder (R)]

Dimers of phenol in argon and neon matrices

The IR absorption spectra of phenol molecules in solid rare gas matrices of argon (10–12 K) and neon (4.5–5 K) are investigated at molar ratios of phenol:matrix of 1:1000 to 1:30 in the frequency range 400–4000 cm−1. Bands of dimers and larger complexes of phenol molecules are observed in the absorption spectrum of both matrices as the matrix ratio decreases. The first additional bands to appear in the spectral region of the stretching vibrations of the O–H group as the phenol concentration increases are two bands attributed to dimers with one and two hydrogen bonds. The absorption coefficients are determined for the bands of stretching vibrations of the O–H and C–O groups, O–H planar bending vibrations of monomers, and the stretching vibrations of the hydrogen-bonded O–H groups of the phenol molecules. The features of the formation of H-bonded complexes in low-temperature matrices are discussed. A model is proposed which permits calculation of the number of monomers, dimers, and larger complexes in argon and neon matrices for molecules which are close in size to the phenol molecule.

Absorption and emission spectroscopy of matrix-isolated benzo[g,h,i]perylene: An experimental and theoretical study for astrochemical applications

The absorption and emission spectra of benzo[g,h,i]perylene, a six ring polycyclic aromatic hydrocarbon molecule (C22H12), embedded in a rare gas matrix are reported. Time dependent emission shows that this molecule exhibits sharp phosphorescence in the red. Supporting theoretical calculations using the recently developed time-dependent density-functional response theory formalism (TD–DFRT) allow a tentative assignment for the observed transitions. The astrochemical significance of the results is briefly discussed.

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.

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.

IR and UV Absorption Spectrum of the Trifluoromethoxy Radical, CF3O·, Isolated in Rare Gas Matrices

The trifluoromethoxy radical is prepared in high yields by vacuum flash pyrolysis of CF3OC(O)OOC(O)OCF3 or CF3OC(O)OOCF3 highly diluted in inert gases, and subsequently isolated in inert gas matrices by quenching the product mixture at low temperatures. In the IR spectrum, six fundamentals have been observed and Cs symmetry is concluded for the matrix-isolated CF3O· radical by comparison with predicted band positions and band intensities from ab initio calculations. This finding is at variance with results from the analysis of the rotational resolved fluorescence excitation spectra and dispersed fluorescence spectra of gaseous CF3O· radicals, where C3v symmetry was found. Possible explanations for this discrepancy are given. The UV spectrum in the spectral region 28000−43000 cm-1 has been measured for the first time, and two different electronic transitions are observed. The structure of the low energy transition A2A1 ← X2E is in agreement with that in previous laser induced fluorescence spectra. Four vi...

Radiation Chemistry of Organic Molecules in Solid Rare Gas Matrices: 2. Selective Deprotonation of the Primary Radical Cations upon Irradiation of Oxygen-Containing Molecules in Xenon Matrices

The composition of radical products trapped after irradiation of various ethers (dimethyl ether, tetrahydrofuran, methylal, 1,3-dioxolane) or acetaldehyde in xenon matrices at 15–17 K in the presence of electron scavengers was studied by an ESR technique. It was shown that the primary radical cations give corresponding deprotonation products (carbon-centered radicals), rather than stabilize in xenon, under the given experimental conditions. The deprotonation process is characterized by extremely high selectivity; i.e., the only type of radicals resulting from deprotonation at maximum spin density position was observed in each of the cases. The possible mechanism of the reactions and the nature of their selectivity are discussed.

Formation of novel rare-gas-containing molecules by molecular photodissociation in clusters.

Recent work by Räsänen and coworkers showed that photolysis of hydrides in rare-gas matrices results in part in formation of novel, rare-gas-containing molecules. Thus, photolysis of HCl in Xe and of H2O in Xe result respectively in formation of HXeCl and HXeOH in the Xe matrices. Ab initio calculations show that the compounds HRgY so formed are stable in isolation, and that by the strength and nature of the bonding these are molecules, very different from the corresponding weakly bound clusters Rg...HY. This paper presents a study of the formation mechanism of HRgY following the photolysis of HY in clusters Rgn(HY). Calculations are described for HXeCl, as a representative example. Potential energy surfaces that govern the formation of HXeCl in the photolysis of HCl in xenon clusters are obtained, and the dynamics on these surfaces is analyzed, partly with insight from trajectories of molecular dynamics simulations. The potential surfaces are obtained by a new variant of the DIM (diatomics in molecules) and DIIS (diatomics in ionic systems) models. Non-adiabatic couplings are also obtained. The main results are: (1) Properties of HXeCl predicted by the DIM-DIIS model are in reasonable accord with results of ab initio calculations. (2) The potential along the isomerization path HXeCl-->Xe...HCl predicted by DIM is in semiquantitative accord with the ab initio results. (3) Surface-hopping molecular dynamics simulations of the process in clusters, with "on the fly" calculations of the DIM-DIIS potentials and non-adiabatic couplings are computationally feasible. (4) Formation of HXeCl, following photolysis of HCl in Xe54(HCl), requires cage-exit of the H atom as a precondition. The H atom and the Cl can then attack the same Xe atom on opposite sides, leading to charge transfer and production of the ionic HXeCl. (5) Non-adiabatic processes play an important role, both in the reagent configurations, and at the charge-transfer stage. The results open the way to predictions of the formation of new HRgY species.