Measuring diffusivity in supercooled liquid nanoscale films using inert gas permeation. I. Kinetic model and scaling methods

We describe in detail a diffusion model used to simulate inert gas transport through supercooled liquid overlayers. In recent work, the transport of the inert gas has been shown to be an effective probe of the diffusivity of supercooled liquid methanol in the experimentally challenging regime near the glass transition temperature. The model simulations accurately and quantitatively describe the inert gas permeation desorption spectra. The simulation results are used to validate universal scaling relationships between the diffusivity, overlayer thickness, and the temperature ramp rate for isothermal and temperature programmed desorption. From these scaling relationships we derive simple equations from which the diffusivity can be obtained using the peak desorption time or temperature for an isothermal or set of TPD experiments, respectively, without numerical simulation. The results presented here demonstrate that the permeation of gases through amorphous overlayers has the potential to be a powerful technique to obtain diffusivity data in deeply supercooled liquids.     

Formation of supercooled liquid solutions from nanoscale amorphous solid films of methanol and ethanol

Molecular beam techniques are used to create layered nanoscale composite films of amorphous methanol and ethanol at 20 K. The films are then heated, and temperature programed desorption and infrared spectroscopy are used to observe the mixing, desorption, and crystallization behavior from the initially unmixed amorphous layers. We find that the initially unmixed amorphous layers completely intermix to form a deeply supercooled liquid solution after heating above T(g). Modeling of the desorption kinetics shows that the supercooled liquid films behave as ideal solutions. The desorption rates from the supercooled and crystalline phases are then used to derive the binary solid-liquid phase diagram. Deviations from ideal solution desorption behavior are observed when the metastable supercooled solution remains for longer times in regions of the phase diagram when thermodynamically favored crystallization occurs. In those cases, the finite lifetime of the metastable solutions results in the precipitation of crystalline solids. Finally, in very thick films at temperatures and compositions where a stable liquid should exist, we unexpectedly observe deviations from ideal solution behavior. Visual inspection of the sample indicates that these apparent departures from ideality arise from dewetting of the liquid film from the substrate. We conclude that compositionally tailored nanoscale amorphous films provide a useful means for preparing and examining deeply supercooled solutions in metastable regions of the phase diagram.     

Experimental evidence for the existence of a Ramsauer—Townsend minimum in liquid CH4 and liquid Ar (Kr and Xe)

Experimental evidence for the existence of a Ramsauer—Townsend minimum in the electron scattering cross section for liquid CH₄ and liquid Ar (Kr and Xe) is provided from: (i) comparisons of thermal electron mobilities in gases with those in liquids, (ii) changes in the electron mobility with gas density, and (iii) the dependence of the electron mobility on temperature for liquids whose V₀, the energy of the electron state in the liquid, is ? 0 eV.     

Appearance of interatomic Coulombic decay in Ar, Kr, and Xe homonuclear dimers

Interatomic Coulombic decay (ICD) is observed in the rare gas homonuclear dimers Ar2, Kr2, and Xe2 with photoion spectroscopy techniques. Inner valence ionization of the outer ns shell of these systems is known to create a metastable state that dissociates to form a ground state ion and a neutral excited fragment. Inner valence ionization to form ns satellite states leads to similar dissociations, but the neutral fragment gets all the more excited as the internal energy of the ns satellite state increases. When enough excitation energy is transferred to reach the ionization potential, ICD occurs. ICD threshold is observed to coincide with the position of the A+A+ ground state in the Franck-Condon region.     

Modelling light diatomics trapped in rare gas matrices: H2, HD and D2 in Ar,Kr and Xe

The different models allowing the calculations of the rovibrational frequency shifts with respect to the free molecule of a molecular impurity embedded in a rare gas crystal are reviewed. It is shown that models which account for the translational motion of the impurity yield reliable results, the effects of the rare-gas translation seem to be less important. The different models currently used are described in detail and the computational procedures are discussed. Finally, it is shown that for heteronuclear diatomics, the translation-rotation coupling could play an important role.     

Noble gas-transition metal complexes: coordination of ScO+ by multiple Ar, Kr, and Xe atoms in noble gas matrixes

The combination of matrix isolation infrared spectroscopic and density functional calculation results provides strong evidence that the transition metal monoxide cation, ScO+, coordinates five noble gas atoms in forming the [ScO(Ng)5]+ (Ng = Ar, Kr, or Xe) complexes in noble gas matrixes.     

Theoretical investigation of rare gas hydride cations: HRgN2+ (Rg=He, Ar, Kr, and Xe)

Rare gas containing protonated nitrogen cations, HRgN(2)(+) (Rg=He, Ar, Kr, and Xe), have been predicted using quantum computational methods. HRgN(2)(+) ions exhibit linear structure (C(∞v) symmetry) at the minima and show planar structure (C(s) symmetry) at the transition state. The stability is determined by computing the energy differences between the predicted ions and its various unimolecular dissociation products. Analysis of energy diagram indicates that HXeN(2)(+) is thermodynamically stable with respect to dissociated products while HHeN(2)(+), HArN(2)(+), and HKrN(2)(+) ions are metastable with small barrier heights. Moreover, the computed intrinsic reaction coordinate analysis also confirms that the minima and the 2-body global dissociation products are connected through transition states for the metastable ions. The coupled-cluster theory computed dissociation energies corresponding to the 2-body dissociation (HN(2)(+) + Rg) is -288.4, -98.3, -21.5, and 41.4 kJ mol(-1) for HHeN(2)(+), HArN(2)(+), HKrN(2)(+), and HXeN(2)(+) ions, respectively. The dissociation energies are positive for all the other channels implying that the predicted ions are stable with respect to other 2- and 3-body dissociation channels. Atoms-in-molecules analysis indicates that predicted ions may be best described as HRg(+)N(2). It should be noted that the energetic of HXeN(2)(+) ion is comparable to that of the experimentally observed stable mixed cations, viz. (RgHRg')(+). Therefore, it may be possible to prepare and characterize HXeN(2)(+) ions in an electron bombardment matrix isolation technique.     

ChemInform Abstract: Multiphoton Spectroscopy of X-NO (X: Kr,Xe, CH4) van der Waals Molecules.

The spectra of the title species are investigated at wavelengths near the C2π ← X2π1/2 transition of nitric oxide by mass-resolved multiphoton ionization (MPI) spectroscopy.     

Vibrational overtone spectroscopy of saturated hydrocarbons dissolved in liquefied Ar, Kr, Xe, and N2

This article presents a collection of vibrational overtone spectra of hydrocarbons in cryogenic solutions. Vibrational overtone spectra of ethane and propane dissolved in liquid argon and n-butane and isobutane dissolved in liquid krypton were recorded between 5000 and 14,000 cm(-1). Spectral regions for the first four overtones were measured using a Fourier transform spectrophotometer. The fifth overtone (Deltaupsilon = 6) spectra were recorded with a double beam (pump-probe) thermal lens technique using concentrations as low as 10-3 mole fraction. We obtained the C-H (Deltaupsilon = 6) spectra of (a) liquid ethane at 100 K and ethane in solutions in liquid Ar at 92 K and liquid N2 at 85 K, (b) liquid propane at 148 K and propane in liquid Ar at 93 K, (c) n-butane in liquid Kr at 129 K, (d) n-pentane in liquid Xe at 160 K, and (e) isobutane liquid at 135 K and isobutane in liquid Kr at 130 K. Local-mode parameters were calculated for primary, secondary, and tertiary C-H oscillators in solution and compared with gas-phase local-mode parameters. The peak frequency shift (Deltaomega) from gas phase to solution is explained by the change in harmonic frequency and anharmonicity in solution with respect to the gas-phase values. The bandwidth (Deltaomega1/2) of the (Deltaupsilon = 6) C-H absorption bands of ethane in solution can be explained in terms of collisions with the solvent molecules.     

Radiative and nonradiative lifetimes in excited states of Ar,Kr and Xe atoms in a neon matrix

Synchrotron radiation with its intense continuum and its excellent time structure has been exploited for time resolved luminescence spectroscopy in the solid state. By selective excitation of n = 1, n′ = 2 exciton states of Xe, Kr and Ar atoms in a neon matrix we were able to identify the emitting states involved. Lifetimes within the cascade of radiative and radiationless relaxation between excited states as well as the radiative lifetimes for transitions to the ground state have been derived from the decay curves. Energy positions and radiative lifetimes of the emitting states correspond quite well with those of the free atoms. Radiative and radiationless relaxation processes take place within the manifold of excited states of the guest atoms. The rate constants for radiationless decay confirm an energy gap law. The order of the radiationless processes reaches in some cases extremely high values. Selection rules for spin and angular momentum are essential to understand the observed radiationless transition rates.     

Prediction of metastable metal-rare gas fluorides: FMRgF (M=Be and Mg; Rg=Ar, Kr and Xe)

The structure, stability, charge redistribution, bonding, and harmonic vibrational frequencies of rare gas containing group II-A fluorides with the general formula FMRgF (where M=Be and Mg; Rg=Ar, Kr, and Xe) have been investigated using second order M?ller-Plesset perturbation theory, density functional theory, and coupled cluster theory [CCSD(T)] methods. The species, FMRgF show a quasilinear structure at the minima and a bent structure at the transition state. The predicted species are unstable with respect to the two-body dissociation channel, leading to the global minima (MF2+Rg) on the singlet potential energy surface. However, with respect to other two-body dissociation channel (FM+RgF), they are found to be stable and have high positive energies on the same surface. The computed binding energy for the two-body dissociation channels are 94.0, 164.7, and 199.7 kJ mol(-1) for FBeArF, FBeKrF, FBeXeF, respectively, at CCSD(T) method. The corresponding energy values are 83.4, 130.7, and 180.1 kJ mol(-1) for FMgArF, FMgKrF, and FMgXeF, respectively, at the same level of theory. With respect to the three-body dissociation (FM+Rg+F) channel as well as dissociation into atomic constituent, they are also found to be stable and have high positive energies. The dissociation of the predicted species typically proceeds via MRgF bending mode at the transition state. The computed barrier heights for the transition states are 11.4, 32.2, and 57.6 kJ mol(-1) for FBeArF, FBeKrF, and FBeXeF, respectively, at the CCSD(T) method. The corresponding barrier heights for the Mg containing species are 2.1, 9.2, and 32.1 kJ mol(-1) along the series Ar--Kr--Xe, respectively. The M--Rg bond energies of the FMRgF species is significantly higher than the corresponding bond energies of the M+--Rg species ( approximately 53 and approximately 15 kJ mol(-1) for Be+--Ar and Mg+--Ar, respectively). The computed energy diagram as well as the geometrical parameters along with the AIM results suggest that the species are metastable with partial covalent character in the M--Rg bonding. Thus, it may be possible to prepare and to characterize these species using low temperature matrix isolation technique.     

Noble gas-actinide complexes of the CUO molecule with multiple Ar,Kr, and Xe atoms in noble-gas matrices

Laser-ablated U atoms react with CO in excess argon to produce CUO, which is trapped in a triplet state in solid argon at 7 K, based on agreement between observed and relativistic density functional theory (DFT) calculated isotopic frequencies ((12)C(16)O, (13)C(16)O, (12)C(18)O). This observation contrasts a recent neon matrix investigation, which trapped CUO in a linear singlet state calculated to be about 1 kcal/mol lower in energy. Experiments with krypton and xenon give results analogous to those with argon. Similar work with dilute Kr and Xe in argon finds small frequency shifts in new four-band progressions for CUO in the same triplet states trapped in solid argon and provides evidence for four distinct CUO(Ar)(4-n)(Ng)(n) (Ng = Kr, Xe, n = 1, 2, 3, 4) complexes for each Ng. DFT calculations show that successively higher Ng complexes are responsible for the observed frequency progressions. This work provides the first evidence for noble gas-actinide complexes, and the first example of neutral complexes with four noble gas atoms bonded to one metal center.     

Experimental measurement of the mobilities of atomic and dimer Ar, Kr, and Xe ions in their parent gases

A new experimental technique for measuring the mobilities of positive ions in their parent gases is presented. The technique was applied to the rare gases, Ar, Kr, and Xe, and, for pressures typically below 10 Torr, two different types of positive ions were observed. The reduced mobilities of these ions in their parent gases were measured as a function of E/N, the ratio of the electric field strength to the gas number density, at a temperature of 300±1 K. The results were compared with others available in the literature and the two ions were identified as being the atomic and the dimer rare gas ions. The results are in good agreement with those from other authors. Space charge and impurities effects are discussed.     

Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions

We report an experimental study of the rotovibrational fundamental PQR-band shapes in the IR absorption spectra of HCl dissolved in condensed rare gases in a wide range of temperatures. The effective vibrational frequencies are determined from analysis of the fine rotational structure partially resolved in the band wings. The central Q-branch components appear redshifted with respect to the effective vibrational frequencies, their shifts in different solvents found to match the HCl stretching mode shifts in binary Rg...HCl van der Waals heterodimers. Theoretical quasi-free rotor and modified rotor models are applied to describe evolution of the band profiles at changing thermodynamic conditions. Both models are shown to reproduce equally well the observed spectral density distributions in the band wings. However, the modified rotor formalism that accounts for depopulation of the lower-energy rotational solute states provides better agreement with the experiment in the range of the P- and R-branch maxima. We surmise that the Q branches separated from the measured spectral profiles are formed by transitions between rotationally hindered states of diatomic molecules coupled to the solvent by the local anisotropy of the interaction potential.     

Chemical reaction studies in CH4/Ar and CH4/N2 gas mixtures of a dielectric barrier discharge

Chemical reactions in a dielectric barrier discharge at medium pressure of 250-300 mbar have been studied in CH(4)/Ar and CH(4)/N(2) gas mixtures by means of mass spectrometry. The main reaction scheme is production of H(2) by fragmentation of CH(4), but also production of higher order hydrocarbon molecules such as C(n)H(m) with n up to 9 including formation of different functional CN groups is observed. Formation of C(2)H(2), C(2)H(4), and C(2)H(6) molecules has been investigated in some detail. Significant differences are noted in comparison to a theoretical estimate.     

The electronic spectrum of CUONg(4) (Ng = Ne, Ar, Kr, Xe): New insights in the interaction of the CUO molecule with noble gas matrices

The electronic spectrum of the CUO molecule was investigated with the IHFSCC-SD (intermediate Hamiltonian Fock-space coupled cluster with singles and doubles) method and with TD-DFT (time-dependent density functional theory) employing the PBE and PBE0 exchange-correlation functionals. The importance of both spin-orbit coupling and correlation effects on the low-lying excited-states of this molecule are analyzed and discussed. Noble gas matrix effects on the energy ordering and vibrational frequencies of the lowest electronic states of the CUO molecule were investigated with density functional theory (DFT) and TD-DFT in a supermolecular as well as a frozen density embedding (FDE) subsystem approach. This data is used to test the suitability of the FDE approach to model the influence of different matrices on the vertical electronic transitions of this molecule. The most suitable potential was chosen to perform relativistic wave function theory in density functional theory calculations to study the vertical electronic spectra of the CUO and CUONg(4) with the IHFSCC-SD method.     

Coordination of ScO+ and YO+ by multiple Ar, Kr, and Xe atoms in noble gas matrixes: a matrix isolation infrared spectroscopic and theoretical study

The combination of matrix isolation infrared spectroscopic and quantum chemical calculation results provide strong evidence that scandium and yttrium monoxide cations, ScO+ and YO+, coordinate multiple noble gas atoms in forming noble gas complexes. The results showed that ScO+ coordinates five Ar, Kr, or Xe atoms, and YO+ coordinates six Ar or Kr and five Xe atoms in solid noble gas matrixes. Hence, the ScO+ and YO+ cations trapped in solid noble gas matrixes should be regarded as the [ScO(Ng)5]+ (Ng = Ar, Kr, or Xe), [YO(Ng)6]+ (Ng = Ar or Kr) or [YO(Xe)5]+ complexes. Experiments with dilute krypton or xenon in argon or krypton in xenon produced new IR bands, which are due to the stepwise formation of the [ScO(Ar)(5-n)(Kr)n]+, [ScO(Kr)(5-n)(Xe)n]+ (n = 1-5), [YO(Ar)(6-n)(Kr)n]+ (n = 1-6), and [YO(Ar)(6-n)(Xe)n]+ (n = 1-4) complexes.     

Tests of a “free-volume” model of gas permeation through polymer membranes. II. Pure Ar,SF6, CF4, and C2H2F2 in polyethylene

Permeability coefficients for Ar, SF₆, CF₄, and C₂H₂F₂ (1,1-difluoroethylene) in polyethylene membranes were determined from steady-state permeation rates at temperatures from 5 to 50°C, and at applied gas pressures of up to 15 atm. The temperature and pressure dependence of the permeability coefficients was represented satisfactorily by an extension of Fujita's free volume model of diffusion of small molecules in polymers. The parameters required by this model were determined from independent absorption (diffusivity) measurements with the above gases in polyethylene rods. The present work confirms the results of previous studies with CO₂, CH₄ C₂H₄ and C₃H₈ in polyethylene.     

The solubility of the noble gases He,Ne, Ar,Kr, and Xe in water up to the critical point

The solubility of the noble gases Ar, He, Ne, Kr, and Xe in pure water was measured from 298 to 561°K. These data in turn were extrapolated to the critical point of water, thus providing a complete set of Henry's law constants from 274 to 647°K when combined with the existing literature data. Equations describing the behavior of the Henry's law constants over this temperature range are also given. The data do not confirm extrapolations of empirical correlations based on low-temperature solubility data.