Kr Matrices Research Articles

The rotamerization of conformers of glycine isolated in inert gas matrices. An infrared spectroscopic study

The infrared spectra of glycine isolated in Ne, Ar and Kr matrices have been measured. The matrix-isolated glycine is shown to be in the molecular form. The spectral manifestations, both conformational and site splitting, are separated. Three different conformers of glycine have been identified experimentally for the first time. It is shown that during the deposition of the samples the substrate temperature must be lowered to 13 K — this is a decisive factor permitting fixation of a complete set of glycine conformers in an inert matrix. The relative energies of the three glycine conformers are estimated to be 0, 1.3–1.6 and 0.9–1.5 kcal/mol.

Luminescence of atomic magnesium in inert low temperature solids. I. Argon and krypton

Absorption and luminescence spectra have been recorded in the 200–700 nm range for atomic magnesium isolated in solid Ar and Kr at 12 K. Strong absorptions occurring in the near UV at 285 nm, showing a threefold splitting, are identified as the solid phase equivalent of the gas phase 3p 1P1←3s 1S0 Mg atom transition. Evidence of multiple site trapping of Mg atoms in Ar and Kr matrices formed at 12 K has been obtained from annealing studies in absorption, but especially in luminescence spectroscopy. The single emission band of Mg/Ar, centered at 297.6 nm, exhibits a radiative lifetime of 1.12 ns and is thereby assigned as singlet 3p 1P1→3s 1S0 Mg atom fluorescence. The luminescence exhibited by the Mg/Kr system is more complex than the Mg/Ar system in that a weak visible band at 472 nm occurs as well as several bands in the UV having nanosecond lifetimes. The richness of the Mg/Kr UV spectra has been examined with annealing and time-resolved measurements and identified as arising from multiple trapping site effects, with at least three spectrally distinct sites identified. Efficient resonant radiative energy transfer is demonstrated to be occurring between two of these sites and an average separation between the sensitizor and activator sites is calculated to be 60 nm at a Mg:Ar dilution ratio of 3:104. Annealing of Mg/Kr samples to 45 K was found to remove all but one site which exhibits emission at 297.6 nm and a very weak band at 472.6 nm. The former, having a radiative lifetime of 1.25 ns, is assigned as 3p 1P1→3s 1S0 Mg atom fluorescence; the latter with a radiative lifetime of 8.9 ms, is assigned as 3p 3P1→3s 1S0 Mg atom phosphorescence.

Photogeneration of ionic species in Ar, Kr and Xe matrices doped with HCl, HBr and HI

Photogeneration of Ar 2 H + , Kr 2 H + and Xe 2 H + and their deuterated counterparts is reported in H(D)Cl doped Ar, and H(D)X (X = Cl, Br or I) doped Kr and Xe matrices, respectively. The formation of these species is ascribed to optical access of the delocalized X/rare gas charge transfer states, and subsequent charge trapping processes. Once generated, the absorptions of the cations bleach thermally at low temperatures ( 12 K). The observed second-order decay kinetics is discussed in terms of charge-recombination involving a distribution of barrier heights. The deuterated cations bleach on a significantly longer time-scale. Bihalide ions, HX − 2 , of D ∞h symmetry are the other products formed in the photolysis. Annealing of the extensively irradiated HCl, HBr and HI doped xenon and HCl doped krypton yields strong infrared absorptions in the 1200–1600 cm −1 spectral region. These bands are tentatively ascribed to linear asymmetric (X-H—X) − ions perturbed by a nearby positive charge, or to (XeHX) + and (KrHX) + species.

Absorption by dissociative continua in condensed matter: H 2O in rare gas and ice matrices

The absorption spectrum of H 2O in Ne, Ar, Kr, Xe and ice matrices in the region of the first à 1B 1 continuum is reported. Substantial blue gas-to-matrix shifts are observed for the maximum, which increase with decreasing matrix cage size. The absorption thresholds show much weaker shifts. These features are interpreted in terms of Rydbergization of the (3s/4a 1) molecular orbital correlated to the à state. The photochemistry is strongly affected by the environmental effects on the excited state potential surface. An energy threshold to photodissociation is confirmed, consistent with a potential barrier due to the cage and a prompt exit of the H fragment. The photodissociation quantum yield shows a linear dependence with excess kinetic energy in Ar and Xe matrices, in agreement with recent MD simulations also favouring prompt exits as the dominant mechanism to permanent dissociation.

Absorption and fluorescence of Tl atoms in rare gas matrices

Tl containing Ar, Kr, and Xe matrices were produced by electrode sputtering in a self-igniting pulsed-discharge unit. The absorption and laser-induced fluorescence spectra of the thallium atoms have been investigated. The results are compared with our recent observations of gas phase diatomic thallium–rare gas (Tl–Rg) clusters. The observed linewidths, shifts, and relaxation dynamics are qualitatively interpreted on the basis of the pairwise Tl–Rg interaction potentials.

Spectra and relaxation of Hg atoms and molecules in low temperature matrices. I. CH4, CD4, and mixed CH4/Ar, CD4/Ar matrices

The absorption spectra in the 3P1←1S0 range and those of the prompt and long-lived emission components as well as their decay times are recorded for Hg atoms isolated in CH4, CD4, and mixed CH4/Ar matrices at T≂12 K using the dye-laser excitation. The results are compared to those obtained in the previous work for Ar, Kr, and Xe matrices. The rate of the 3P1■3P0 intramultiplet relaxation is only slightly increased, while that of the 3P0→1S0 emission shows an important increase due to the coupling to internal modes of methane molecules. The evidence of this coupling is the vibrational structure of the emission spectrum involving nontotally symmetric ν2(E), ν3(F), and ν4(F) normal modes of CH4 or CD4 molecules.

Experimental evidence to Rydbergization of antibonding molecular orbitals

Experimental evidence is presented for the process of molecular orbital Rydbergization postulated by Mulliken, whereby an antibonding orbital continuously changes its character from predominantly Rydberg to predominantly valence on its way to dissociation. By matrix-isolation spectroscopy of the à 1B 1 state of H 2O in Ar and Kr matrices and in ice, we show that the absorption threshold behaves like a valence transition while the maximum has a behaviour typical of Rydberg transitions in condensed matter, in agreement with the predicted changing character of the state.

The ν1+ν3 combination mode of C3 in Ar and Kr matrices: Evidence for a bent structure

The ν1+ν3 combination mode of laser-ablated C3 has been observed in Ar and Kr matrices for all six possible 12C/13C isotopomers. The ν1 totally symmetric mode frequencies have been determined by difference with the known ν3 asymmetric stretching frequencies, corrected for anharmonicity effects. A comparison of certain isotopomer pairs reveals that C3 is bent in these matrices. A normal coordinate analysis using four force constants resulted in an excellent fit of the 12 observed frequencies (ν1 and ν3 for six isotopomers). From the fit, a value for the ν2 bending mode frequency of 82 cm−1 was obtained. This is shown to be in good agreement with the value obtained from the C3 phosphorescence spectrum. Finally, the molecular bending angle which was determined to be 160.0° (in Ar and Kr) is shown to match very well with the predicted value from ab initio theory (161.6°) and from a fitting of a Morse oscillator–rigid bender potential to gas phase vibrational frequencies (162.5°). The influence of the matrix environment on the vibrational force constants and frequencies of C3 is discussed.

Matrix isolation infrared and ab initio studies on conformers of fluoroacetic and chloroacetic acid

The infrared spectra of matrix isolated fluoroacetic acid and chloroacetic acid were studied in solid Ar, Kr, and Xe matrices. The energies and spectra of possible conformers of fluoroacetic acid and chloroacetic acid were simulated by extensive ab initio calculations at the MP2/6-311G ** (energies, geometries), MP2/6-31G ** (barriers), and MP2/4-31G * (numerical frequencies) levels. The present calculations propose the existence of four distinct conformers. For fluoroacetic acid the two lowest energy conformers, i.e., Tt and Ct, were found to be present after deposition of the solids

Magnetic circular dichroism of matrix-isolated Mo atoms: spin-orbit coupling and Jahn-Teller activity

The magnetic circular dichroism and absorption spectra of molybdenum atoms isolated in Ar, Kr, and Xe matrices have been measured. A method of moments analysis has been applied to the spectral results and the z 7P excited state spin-orbit (SO) coupling constants determined. In Ar the SO constant is 126 cm −1, greater than the gas value of 101 cm −1, while in Kr and Xe it is smaller, 84.5 and 57.5 cm −1, respectively. These results compare favorably with the values predicted by a recent theory of SO coupling for metals in matrices. It is further shown that matrix-enhanced second-order SO coupling, which was found to be important in Cr in the rare gas matrices, is negligible in the present systems. In addition, the method of moments was used to determine the contributions of the noncubic rare gas cage vibrational modes to the observed bandwidths, an approach which has been used previously to identify excited state Jahn-Teller activity. It is shown that Mo atoms in all three gas matrices exhibit a strong Jahn-Teller effect on the z 7P state.

Spectroscopy of SH (A–X) transition in Ar and Kr matrices: The caging of predissociation

The A(2Σ+)–X(2Π) transition of SH isolated in Ar and Kr matrices is studied by laser induced fluorescence spectroscopy. The (0,0), (1,0), and (2,0) transitions are observed in excitation. These lines are structureless and homogeneously broadened (200–300 cm−1). The (0,0), (0,1), and (0,2) emission bands, are strongly Stokes shifted and further broadened (≥1000 cm−1). The spectra can be successfully interpreted in analogy with OH–Rg pair interactions. The SH(A)–Rg potential is deeply bound (∼1000 cm−1 in Ar and ∼2000 cm−1 in Kr) with nearly 1 Å compression along this coordinate. Franck–Condon analysis indicates that the equilibrium bond length of SH(A) is 0.055 Å longer than in the free radical. The A state, which is strongly predissociated in the free radical, relaxes radiatively in the matrix: τ=425 and 770 ns in Ar and Kr, respectively. Predissociation is treated as a two-dimensional tunneling process between two electronic surfaces. Poor Franck–Condon factors along the solvent coordinate, due to differential solvation of electronic states, leads to complete arrest of electronic predissociation.

VIS-VUV spectroscopic study of the reaction between Al and N 2O isolated in solid krypton

The reaction of Al with N 2 isolated in a krypton matrix has been studied by comparing the optical spectra of pure Al:Kr and mixed Al:N 2O:Kr matrices before and after photolysis with VUV light. Three reaction products, AlO, AlO 2 and Al 2O have been identified by absorption, emission and emission yield measurements in the 2–8 eV range using synchrotron radiation.

Theoretical description of interference effects in the absorption spectra of NO in rare-gas matrices

A theoretical model is presented to account for the Fano-like interference lineshapes on multiphonon continua in the electronic spectrum of NO in rare-gas solids. The model is based on the coupling of the optical transitions to one single local mode, while the latter is coupled to lattice modes. The model complements the original Fano theory by considering the finite lifetime of the continuum states. Lineshapes are simulated for the case of the interference between the B( v=12) valence level and the C( v=0) Rydberg level of NO in Kr matrices and good agreement with experimental lineshapes is obtained.

Vibrational matrix shift simulations of SO 2 in rare gases: importance of many-body effects

It is shown that a pair interaction potential approximation fails to reproduce the experimental tendencies in vibrational shifts for SO 2 in solid rare gases matrices of Ar, Kr, and Xe. Various approximations within a finite-size cluster model have been tested to estimate frequency shifts in SO 2. Pair potential parameters which allow reproduction of the structures of Ar-O 3 and Ar-SO 2 complexes lead to computed matrix shifts that are always positive and small compared to the experimental data. With the addition of the three-body interaction terms considerable improvements in calculation results are obtained.

Spectra and relaxation paths of Hg(3P1) in rare gas matrices

Absorption, fluorescence-excitation and fluorescence spectra, and lifetimes have been measured for 3P1↔1S0 and 3P0→1S0 transitions in Hg atoms isolated in Ar, Kr, and Xe matrices. In all systems, the 3P1 state decays mainly by fluorescence emission; the 3P1■3P0 relaxation, inducing the long-lived 3P0→1S0 emission, being very inefficient. The 3P1→1S0 emission in Ar and Kr is a mirror image of the absorption, while a strong redshift in Xe corresponds to the formation of Hg*Xe exciplexes. The results are discussed in a model assuming the additivity of interatomic potentials deduced from the spectroscopic studies of jet-cooled Hg–rare-gas complexes.

Spin-orbit splitting and Jahn-Teller effect in rare gas matrix-isolated Ag and Au atoms: a magnetic circular dichroism study

The magnetic circular dichroism (MCD) and absorption spectra of Ag and Au atoms isolated in Ar, Kr and Xe rare gas matrices have been measured. Moment analyses have been carried out on the 2P← 2S absorption and MCD bands for all six systems. The excited state spin-oribit (SO) coupling constants extracted for Ag are 783 cm −1 (Ar), 638 cm −1 (Kr) and 583 cm −1 (Xe) compared to the gas phase value of 614 cm −1. For Au, the SO constants found are 3354 cm −1 (Ar), 3310 cm −1 (Kr) and 2655 cm −1 (Xe) compared to the gas phase value of 2543 cm −1. These SO values are compared to a recent theory for SO splitting of the 2P state in matrix-isolated alkali and noble metals. Significant second-order SO coupling is found between the 2P 3 2 and 4P 3 2 states of Au in Ar and Kr but not Xe matrices. Quantitative details of the coupling, which is very likely the result of matrix- induced shifts of the Au 2P state, are determined. The reasons for the three observed peaks (two are observed in the gas phase) are studied in detail. It is shown that the distorted site model cannot account for both the absorption and MCD spectral band contours. From the moment analysis, is is further demonstrated that Jahn-Teller-active, nontotally symmetric vibrations of the lattice cage are active, contribute significantly to the bandwiths and are responsible, together with SO coupling, for the three-peaked appearance of the band contour. Computations by O'Brien confirm this conclusion for the Ag/Ar case.

Generation of SnO by photoproduction and -mobilization of oxygen atoms in krypton matrices doped with N2O and Sn

SnO molecules are photochemically formed by irradiation of a Sn: N2O mixture in solid krypton. A KrxO complex which is produced by VUV photolysis of N2O at 8.8 eV has been identified from a new absorption band at 6.9 eV. The mobilization of oxygen in Sn:N2O:Kr matrices, photoinduced by excitation of the KrxO species, was studied by trapping the oxygen at Sn atoms and by using the SnO generation as a monitor. Photolysis studies in Sn:N2O:Kr matrices as well as in Sn:Kr/N2O:Kr sandwich structures revealed an extraordinary high mobility of oxygen. A qualitative model of the mobilization scheme is discussed.

Vibrationally mediated selective excitation of matrix-isolated NO (B 2Π) applied to the study of intramolecular energy redistribution among the excited valence states

The B 2Π(ν′) vibronic levels of NO in Ar and Kr matrices are selectively populated by a sequential infrared-visible laser excitation. A significant fraction of the NO molecules are brought into high lying vibrational levels (up to ν=27) of the ground electronic state by IR laser initiated V-V pumping. A second laser operating in the visible range produces an efficient single photon excitation of the B 2Π (ν′ =0–6) levels. New time and energy resolved data on vibrationally relaxed and unrelaxed emissions from B 2Π(ν′) of 14N 16O and 15N 16O are presented. As previously reported by Chergui et al., the relaxation cascade among the B 2Π, b 4Σ + states is reasonably described in terms of the usual simple model taking into account the coupling between the various electronic states as well as the intramolecular and phonon Franck-Condon factors. The results are consistent with the idea that in solid Ne, Ar and Kr there is no heavy atom effect on the radiationless transitions in NO.

Photodissociation of molecular chlorine in xenon matrices

The photodissociation of Cl2 in crystalline Xe is examined using synchrotron radiation for all state of Cl2 from the C 1Π1u state to the region of Xe exciton absorption. Isolation of atomic chlorine in two dominant trapping sites is observed following photodissociation throughout the spectral region studied. The production efficiency of the thermally stable trapping site was found to increase significantly with temperature and was most pronounced in the region of the onset of the Xe2Cl excitation feature. Correlation between these two processes is explained in terms of a charge-transfer-induced dissociation mechanism involving the formation of a Xe+(Cl2)− intermediate. In contrast to Ar and Kr matrices, photodissociation of Cl2 in Xe appears to occur without a pronounced cage effect. This observation is rationalized in terms of the larger lattice parameters of Xe which allows isolation of Cl2 and Cl at single substitutional and octahedral interstitial sites, respectively. From this structural information, simple models are presented for the production of Cl in the two different kinds of trapping sites. Thus production of the thermally unstable site involves the symmetric dissociation of Cl2 with both Cl atoms occupying octahedral interstitial sites separated by a single lattice constant of Xe. The thermally stable site involves the isolation of one Cl atom at the substitutional site originally occupied by the Cl2 parent molecule and the other at an octahedral interstitial site.

Reactions and photochemistry of chromium and molybdenum with molecular hydrogen at 12 K

Here the authors describe the interactions of Cr and Mo atoms with hydrogen molecules in Kr and Ar matrices at 12K. Upon photolysis with UV light both Cr and Mo are found to be photoreactive with H{sub 2}, forming CrH{sub 2}, CrH{sub 3}, and MoH{sub n} (n=1-4). The authors report bond angles and stretching mode constants for the dihydride molecules.