Xe Matrices Research Articles

Matrix isolation and ab initio study of the hydrogen-bonded H2O2-CO complex.

The structure, energetics, and infrared spectrum of the H2O2-CO complex have been studied computationally with the use of ab initio calculations and experimentally by FTIR matrix isolation techniques. Computations predict two stable conformations for the H2O2-CO complex, both of which show almost linear hydrogen bonds between the subunits. The carbon-attached HOOH-CO complex is the lower-energy form, and it has an interaction energy of -9.0 kJmol(-1) at the CCSD(T)/6-311++G(3df,3pd)// MP2/6-311++G(3df,3pd) level. The higher-energy form, HOOH-OC, has an interaction energy of 4.7 kJmol(-1) at the same level of theory. Experimentally, only the lower-energy form, HOOH-CO, was observed in Ar, Kr, and Xe matrices, and the hydrogen bonding results in substantial perturbations of the observed vibrational modes of both complex subunits. UV photolysis of the complex species primarily produces a complex between water and carbon dioxide, but minor amounts of HCO and trans-HOCO were found as well.

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.

Electronic spectroscopy of B atoms and B2 molecules isolated in para-H2, normal-D2, Ne, Ar, Kr, and Xe matrices

We report spectroscopic observations on B atoms isolated in cryogenic parahydrogen (pH2), normal deuterium (nD2), Ne, Ar, Kr, and Xe matrices, and of B2 molecules in Ne, Ar, Kr, and Xe matrices. The 2s23s(2S)←2s22p(2P) B atom Rydberg absorption suffers large gas-to-matrix blue shifts, increasing from +3000 to +7000 cm−1 in the host sequence: Xe<Kr<Ar≈Ne≈nD2≈pH2. Much smaller shifts are observed for the 2s2p2(2D)←2s22p(2P) B atom core-to-valence transition. We assign pairs of absorption peaks spaced by ≈10 nm in the 315–355 nm region to the B2 (A 3Σu−←X 3Σg−) Douglas–Herzberg transition. We assign a much weaker progression in the 260–300 nm region to the B2 (2)3Πu←X 3Σg− transition. We report a novel progression of strong peaks in the 180–200 nm region which we suspect may be due to B2 molecules, but which remains unassigned. Ultraviolet (UV) absorption spectra of B/pH2 solids show two strong peaks at 216.6 and 208.9 nm, which we assign to the matrix perturbed 2s23s(2S)←2s22p(2P) and 2s2p2(2D)←2s22p(2P) B atom absorptions, respectively. This assignment is supported by quantum path integral simulations of B/pH2 solids reported in the following article in this journal [J. R. Krumrine, S. Jang, G. A. Voth, and M. H. Alexander, J. Chem. Phys. 113, 9079 (2000)]. Laser induced fluorescence emission spectra of B/pH2 solids show a single line at 249.6 nm, coincident with the gas phase wavelength of the 2s23s(2S)→2s22p(2P) B atom emission. The UV laser irradiation results in photobleaching of the B atom emission and absorptions, accompanied by the formation of B2H6.

Medium effects on the spectroscopy and intramolecular energy redistribution of C60 in cryogenic matrices

We review some of our works on the absorption, excitation and emission spectra of C60 embedded in rare gas matrices while stressing the role of the environment. The gas phase resonant two-photon ionization spectrum of C60 is reanalysed in the light of our previous work and the energy of the lowest three excited singlet states is determined with precision. In matrices, the visible absorption bands are red-shifted by an amount ranging from ∼30 cm−1 in Ne matrices to ∼330 cm−1 in Xe matrices. The observed reversal of state ordering of the lowest two singlets states (T1g and T2g) between Ne and Ar matrices (in emission) and in Ne matrices, between the absorption and emission spectra, is attributed to different Stokes shifts of the T1g and T2g states and to the small energy splitting (∼50 cm−1) between them. Finally, a detailed picture of the intramolecular energy redistribution processes is obtained thanks to a combination of picosecond fluorescence experiments and femtosecond pump–probe transient absorption experiments. The intramolecular relaxation processes among the pure electronic levels of the lowest three singlet states are found to be strongly medium dependent. Medium effects are manifest even on the very short time scale of the internal conversion in the singlet vibronic manifold.

Spectroscopy and energy relaxation processes of Hg-doped solid neon, argon, and xenon

Emission, absorption, and excitation spectroscopy has been used for a detailed analysis of the optical transitions of Hg2 trapped in cryogenic matrices. Upon excitation of electronic states correlating to the 3P1 or the 1P1 asymptote, fast nonradiative relaxation leads to emission from the lowest excited A0g+ state in all matrices, which decays monoexponentially in 1 ms in Ne, 280 μs in Ar, and 12 μs in Xe. In addition, electronically unrelaxed emission of Hg2 is reported in neon and in xenon matrices and attributed to the B1g state in neon and to the B1g state and the C0u− or A0g− states in xenon. The results are rationalized by assuming: (a) that population of the excited states occurs mainly close to the asymptotic limit, where branching is determined by nonadiabatic coupling and energetics, that are strongly environment dependent, and (b) that in Xe matrices the Hg2 states correlating to the 3P1 and 3P0 asymptotic limits are stabilized in different configurations, as a result of the very different solvation properties of the atomic 3P1 and the 3P0 state. Further emission bands are found in the vicinity of the dimer transitions, which we attribute to Hg3 and to site effects on Hg2. In particular, electronically unrelaxed emission from excited states of Hg3 is reported.

Trapping of laser-vaporized alkali metal atoms in rare-gas matrices

Abstract Alkali metal atoms prepared by laser ablation of solid Li and Na are trapped in Ar, Kr, and Xe matrices and studied by electron paramagnetic resonance spectroscopy (EPR) at 15 K. Evidence for tight trapping sites, not observed for atoms generated by conventional Knudsen oven techniques, is presented. The novel tight trapping sites are characterized by a large increase in the isotropic hyperfine coupling constant and a simultaneous decrease in the isotropic g -value. Based on the EPR data, it is suggested that the observed tight trapping corresponds to single substitution of lattice atoms in Ar, Kr, and Xe matrices.

Detection and spectroscopy of single molecules in rare gas matrices: dibenzanthanthrene in krypton and xenon

Fluorescence microscopy coupled with matrix isolation techniques have allowed the observation of dibenzanthanthrene molecules in rare gas matrices. Fluorescence excitation spectra of single molecules were measured in both Kr and Xe matrices. Spectral lines with Lorentzian shapes as well as irregular noisy profiles were observed.

Photogeneration of atomic hydrogen in rare gas matrices

Photodissociation of HCl and HBr upon excitation on their repulsive A 1Π states is studied in low-temperature Ar, Kr, and Xe matrices at photon energies of 5.0 and 6.4 eV. The dissociation is followed by fluorescence spectroscopy and electron paramagnetic resonance. In Ar matrix dissociation can be considered as a local event with simple first-order kinetics and 100% conversion efficiency of the precursor into isolated hydrogen atoms. In Kr matrix the conversion efficiency varies from 18% in 1:500 matrix to 100% in 1:8000 matrix. In Xe matrix the obtained H atom number density is extremely low and prevents detailed analysis of the photogeneration dynamics. The observed behavior is ascribed to long-range dissociation followed by efficient bimolecular reactive loss channels, and thus supports the previous findings by LaBrake, Ryan, and Weitz [J. Chem. Phys. 102, 4112 (1995)]. Molecular dynamics simulations based on a simplified model for dissociation are carried out. The initial 2.6 eV excess kinetic energy of the excited H atom is relaxed as local heating in Ar matrix, whereas in Kr and Xe matrices the excess energy is directed to long-range mobility with flight distances up to 40 Å.

193 nm photodynamics of NO in rare gas matrices: Fluorescence, thermoluminescence, and photodissociation

193 nm excited time gated emission spectra of a NO monomer isolated in Ar, Kr, and Xe matrices are presented. In the Ar matrix a 4Π→X 2Π, B 2Π→X 2Π, and A 2Σ→X 2Π band systems are completely separable. In solid Kr, both B 2Π→X 2Π and A 2Σ→X 2Π appear promptly from the laser pulse, and in the Xe matrix only Rydberg A 2Σ→X 2Π fluorescence is observed. Prolonged photolysis at 193 nm yields electron paramagnetic resonance signals attributed to isolated S4 nitrogen atoms. This is the first observation of condensed phase photodissociation of NO. Annealing of the extensively irradiated Ar matrix produces strong a 4Π→X 2Π and B 2Π→X 2Π thermoluminescence emissions due to N(4S)+O(3P) recombination. In the Kr matrix thermoluminescence is entirely due to a 4Π→X 2Π transition. No thermoluminescence is observed in Xe. Thermoluminescence is ascribed to short-range trapping of N and O fragments, and well separated atoms do not have significant contribution to recombination.

Photochemical Studies of Hydrogen Peroxide in Solid Rare Gases: Formation of the HOH···O(3P) Complex

UV photolysis of H2O2 in solid Ar, Kr, and Xe yields isolated OH radicals and a complex between a water molecule and a ground-state (3P) oxygen atom. The vibrational bands of the HOH···O complex are observed at 3730.1 and 3633.0 cm-1 in Ar, at 3718.6 and 3622.4 cm-1 in Kr, and at 3704.3 and 3607.3 cm-1 in Xe matrices. The assignment is based on isotopic substitution, selective photoinduced processes between H2O2 and HOH···O, and ab initio calculations. In Kr and Xe matrices, the complex is formed with much less efficiency when compared with Ar matrices, indicating the existence of other photolysis channels. Computational results on the complex geometry and vibrational frequencies are presented and compared with experimental data.

Lifetime lengthening of molecular Rydberg states in the condensed phase

We report on fluorescence lifetime lengthenings of molecular Rydberg states in condensed media in the case of the NO molecule trapped in inert gas matrices. In rare gas matrices, the fluorescence of the A 2Σ+ state originates from two types of sites, hereafter called red and main. The red site is considered to be a loose site with more than one vacancy in Ar, Kr, and Xe and an h.c.p. site in Ne matrices. It exhibits a lifetime lengthening with respect to the gas phase of 25% in Kr matrices and 100% in Xe matrices. The main site fluorescence stems from monosubstitutional sites. It exhibits lifetime enhancements of up to 100% when going from Ne to Xe matrices. When, however, the fluorescence quantum yields are taken into account, the lifetime increases from the gas phase value to up to two orders of magnitude in the sequence H2–Ne(D2)–Ar–Kr–Xe. Furthermore, this change in transition moment is not observed in the absorption spectrum. These results stress the influence of the solvent and its microscopic structure on molecular Rydberg lifetimes. Different mechanisms are discussed in relation with the observations.

On self-limitation of UV photolysis in rare-gas solids and some of its consequences for matrix studies

UV photolysis of small molecules embedded in rare-gas matrices is examined. We demonstrate that photolysis can be self-limited when products absorb the photolysing radiation. As a result of the rising absorption, in-situ detected luminescence of the photolysis product saturates faster than its concentration. In particular, the present study supports the conclusion that 193 nm photolysis of hydrogen-containing species in Xe matrices produces hydrogen atoms in amounts comparable with the other dissociating part of the precursor. Also, we show that 193 nm radiation activates mobility of hydrogen atoms in annealing, accelerating photochemical processes related to hydrogen mobility.

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 on the photolysis products in Ne and Xe matrices are reported. In particular, the luminescence data on SH radicals in solid rare-gas matrices (Ne, Ar, Kr, and Xe) were found to resemble the tendencies known for OH radicals. Also, the infrared absorptions of SH radicals in Ar and Kr matrices were identified to be at 2607 and 2594 cm−1, respectively, and a novel rare-gas molecule HXeSH with the Xe–H stretch at 1119 cm−1 was detected.

Photochemistry of formic acid in rare gas matrices: Double-doping experiments on the 193 nm induced photodecomposition

Photoinduced decomposition of formic acid has been investigated in low-temperature rare gas matrices. The photoproduct ratios were found to differ in Xe matrix from those in Ar and Kr matrices. In xenon, the H 2 + CO 2 dissociation channel was observed to be prominent, whereas in argon and krypton the formation of H 2O + CO was favored. The perturbation upon the photodecomposition mechanism produced by the matrix environment was studied by trapping the precursor in mixed Ar Xe matrices.

Matrix-isolated oxygen: Line-shapes and transition probabilities of the b 1∑ g+→X 3∑ g−, b 1∑ g+→a 1Δg and a 1Δg→X 3∑ g− transitions

The b 1∑ g +↔X 3∑ g −, b 1∑ g +→a 1 Δ g and a 1 Δ g→X 3∑ g − transitions of molecular oxygen isolated in Xe, Kr, Ar (2 sites) and Ne matrices have been studied in excitation and emission. The radiative lifetimes of the isolated monomers in these matrices are 9.5 ± 0.3, 14.5 ± 0.5, 24.5 ± 1 and 46 ± 1 ms in the b state, and 11.5 ± 0.5, 34 ± 1, 68 ± 2 and 146 ± 3 s in the a state. The vibrational relaxation rates of the a, ν′ = 1, level amount to 0.21, 0.18, 0.15 and ≥0.14 s −1. Dimers or otherwise perturbed oxygen molecules with shorter lifetimes have also been characterized. Relative intensity measurements of the involved electronic transitions show that the b→a and a→X radiative transition probabilities are enhanced up to four orders of magnitude due to a phonon-induced transition dipole moment, giving rise to intense phonon sidebands with similar local mode structure in both transitions. While the enhancement of the a→X transition is probability attributed to intensity borrowing from the b→a transition, the b→X transition is only mildly affected by the matrix. The ratio A b→a: A a→X of the Einstein coefficients for spontaneous emission in matrices compares well with the reported ratio for O 2 in CCl 4 solution, supporting the intensity borrowing mechanism.

IR Spectroscopic Study of H2O2, HDO2, and D2O2 Isolated in Ar, Kr, and Xe Matrices

The vibrational spectrum of hydrogen peroxide, released from an urea hydrogen peroxide adduct compound, is studied in Ar, Kr, and Xe matrices. The gaseous products of the thermal decomposition of u...

Dynamics of dissipation processes in the Ag–Xe complex

2ω–2ω bleaching measurements of the ground state absorption and 2ω–1ω fluorescence dip measurements were carried out on the s–p transitions of Ag atoms in Xe matrices by ps laser pulses with 2ω corresponding to about 320 nm and 1ω to 640 nm. The absorption spectrum is analyzed in terms of a dynamic Jahn–Teller (JT) effect with a depth of the bound −JT state of about 30 meV and a pseudorotation frequency of about 20 ps. The −JT state seems to be rather long lived with a depopulation time of 3(±1) ps and a small energy dissipation rate of about 1 phonon per ps. Fluorescence occurs after a significant static lattice deformation accompanied by an energy relaxation of 0.4 eV. This static deformation and the energy relaxation to the emitting states proceed on a similar time scale of 3 ps which is very fast if the involved energies and the dissipation rate of about 30 phonons per ps are considered. The different rates are related to the different coupling to the lattice in the pseudorotating and the statically deformed geometry. Transient absorption is only observed in the relaxed state with σ=1.6(±0.5)⋅10−16 cm−2.

Phosphorescence of C 60 in rare gas matrices

The phosphorescence of C 60 in Kr and Xe matrices is reported. It consists of a progression of bands of monotonically decreasing intensity which belong to the h g(1) mode but in Xe weaker contributions from the t 1u, t 2u, h u and g u modes are also present. In Xe matrices, the phosphorescence stems from a distribution of sites extending up to 150 cm −1 to the red of the main and most prominent one having its origin at 12714 cm −1. The phosphorescence lifetime is 16 ± 1 μs in Xe and 60 ± 5 μs in Kr. The phosphorescence is attributed to the 3T 2g state, whose gas phase energy we estimate to be 12925 ± 10 cm −1 from the gas-to-matrix shift.

Absorption by dissociative continua and Rydberg states in condensed matter: HCl in rare gas matrices

The absorption spectra of HCl in rare gas matrices have been recorded in the first continuum (A 1 ∏ ← X 1 Σ + ) region in Kr and Xe matrices and up to 100 nm in Ar matrices. Concentration and temperature effects have been used to distinguish between absorption from monomers and that from clusters. The first continuum band of the monomer is seen to shift red by ∼ 0.36 eV in Ar matrices, while it shifts blue by ∼ 0.3 and ∼ 0.08 eV in Kr and Xe matrices, respectively. Simultaneously, the oscillator strength of the A 1 ∏ state is seen to increase drastically in the sequence. ArKrXe. These trends can be interpreted by the fact that the excited state is solvated in Ar by dispersive and inductive interactions with the environments, while in Kr and Xe matrices, absorption is dominated by HCl − -Rg n + charge transfer states (Rg Kr, Xe). At higher energies and in Ar matrices, discrete absorption by the b 3 ∏, C 1 ∏, H 1 Σ + , K 1 ∏ and M 1 ∏ states occurs and their relative intensities reproduce well those of the gas phase. However, the b and C states are blue shifted with respect to their gas phase energies by ∼ 0.6 eV, in agreement with their predicted Rydberg character, while the H, K and M states are just slightly red shifted casting some doubt about their classification as Rydberg states.

Cage exit probability versus excess energy in the photodissociation of matrix-isolated HCl

Dissociation efficiencies for excitation of the repulsive A 1Π state of HCl were recorded in Xe, Kr, and Ar matrices for photon energies between 5 and 10 eV from the content of dissociation products and quantum efficiencies were derived with the absorption spectra. Influence of temperature and preparation conditions was investigated. The quantum efficiency rises monotonically in Xe from an excess energy of 1.4 eV above the gas phase dissociation energy on, saturates around 2.4 eV and remains then essentially constant up to 4 eV. In Ar and Kr, it saturates around 2 eV and in Ar an absolute efficiency of about 0.18 is determined at 3.7 eV. Results of molecular dynamics calculations and a statistical model agree qualitatively but the observed saturation at low excess energies is not well described and the absence of a temperature effect in Ar needs further consideration.