Volume 215, Issue 6 - 6

The adsorption of hydrogen azide HN 3 on NaCl(100) has been studied by means of polarization Fourier transform infrared spectroscopy PIRS and low energy electron diffraction LEED. The well-defined surface was prepared in-situ under UHV by cleaving a single crystal with as few dislocations as possible. Ordered molecular adsorption of HN 3 has been observed. Multiple absorptions of all four molecular vibrations with frequencies between 4000 and 1000 cm -1 , all non-degenerate in the gas, are found at 120 K. In particular, a quartet of strongly polarized sharp absorption lines for the asymmetric stretching vibration ν 2 at 2162.8, 2154.0, 2149.2, and 2138.3 cm -1 is obtained, the line at 2162.8 cm -1 being extremely weak in s -polarization. Saturation of adsorption, indicating the monolayer, is observed both upon dosing HN 3 in the low coverage range, where island growth is inferred, and desorbing HN 3 from multilayers. No domain orientation appears to be preferred. An activation energy of desorption of 43±5 kJ/mol and a pre-exponential factor of 2.6×10 14±1.5  s -1 were determined. Isotopic mixture experiments with H 14 N 3 and H 14 N 15 N 14 N reveal that the four absorption lines of the ν 2 vibration are caused first by site splitting due to two energetically inequivalent HN 3 -species and second by Davydov splitting due to a strong correlation field between the dynamic dipoles. There are four molecules per adsorbate unit cell: two different pairs of energetically equivalent but translationally inequivalent molecules. Further, two different pairs of NH hydrogen bonds are observed. Tilt angles and intermolecular angles of the adsorbed molecules were determined. The translational symmetry of the adsorbate was derived by LEED. Simulation of the ν 2 quartet spectrum based on equations of classical electrodynamics was accomplished yielding the angles, lateral distances and dynamic vibrational couplings within the monolayer. Combining all results we propose a monolayer structure of angularly ordered zigzag chains of HN 3 molecules, where two molecules are linked by a hydrogen bond forming a dimer, while the other weaker hydrogen bond ties the dimers together or to a Cl – surface ion. Two HN 3 dimers make up the p (2×2) monolayer unit cell of 2D space group pg .

The problem of the ground state of hydrated electron is revisited with a focus on the effects due to nonlocal dielectric response of water. The standard variational analysis is performed. It takes into account – in addition to nonlocal polarization of nuclear modes – the electron repulsion from the closed shells of water molecules, a possibility to form a cavity around the electron and the interaction of the hydrated electron with the high frequency electronic degrees of freedom of water. The classical dielectric continuum limit, shown for a reference, gives the ground state hydration energy that is 2.5 times smaller than the experimental value. The situation alters dramatically if one accounts for the nonlocal dielectric response of water. If one takes literally the existing MD simulation data for the static wave-vector dependent dielectric response function (with an "over-screening" resonance at k ∼3 Å -1 ), the hydration energy becomes 3 times larger than the experimental one. Thus, an over-screening may have a dramatic effect on the formation of the hydrated electron. For a reduced height of the "over-screening peak", the ground state energy reduces to the measured value. At the same time, over-screening enhances the localization of electrons. The undamped resonance gives rise to an unphysically small localization radius. A reduced resonance, that provides the correct ground stage energy, is better in this respect but it still gives very compact localization: 2/3 of the Bohr radius. It is thus concluded that either the defect structure of water around the electron suppresses the resonance, or the models of bulk water, which predict a high peak in the response function, are inadequate. The study paves the way to future molecular or phenomenological multi-order parameter models, in which the density and polarization of molecular dipoles and charge density distributions of the solvated electron are considered on the same footing. Such models might reveal the reduction of over-screening near the excess electron.

Picosecond UV pump (248.5 nm)/IR probe spectroscopy has been applied to the study of the decomposition of several aromatic diacyl peroxides, peroxycarbonates, and of a peroxyester dissolved in CH 2 Cl 2 . Measuring the IR transient absorbance in the 2100 to 2450 cm -1 range allows to monitor the formation and vibrational relaxation of the photoproduct CO 2 via the asymmetric stretching mode (ν 3 ) with a time resolution of 1.8 ps. With each of the six peroxides a CO 2 molecule is released at delay times below 10 ps. The energy relaxation of the initially formed vibrationally hot CO 2 is followed over the time range up to 500 ps. Analysis of the transient IR spectra, via an anharmonic oscillator model proposed by Hamm et al.  [1], shows a monoexponential decay of internal energy. Irrespective of the type of peroxide a single relaxation time of 67±5 ps is found to adequately represent the cooling behavior of CO 2 in liquid CH 2 Cl 2 . The "initial" temperatures of vibrationally hot CO 2 at a delay time of 20 ps after applying the pump pulse differ considerably, between 1400 and 2700 K for the decompositions of tert -butyl benzoyl peroxide and tert -butyl benzoyl carbonate, respectively. These remarkably high temperatures are assumed to originate from energy release associated with structural relaxation of the bent OCO moiety to form the linear CO 2 molecule.

The ion–molecule reactions of state selected HCl + ( v,N ) ions with carbon dioxide and ethene have been investigated. For the reaction HCl + +CO 2 proton transfer leading to the HOCO + ion is the only reaction channel observed. For the reaction HCl + +C 2 H 4 very efficient charge transfer (leading to C 2 H 4 + +HCl) dominates at low particle densities. At higher densities consecutive reactions leading e.g. to C 2 H 2 + , C 2 H 3 + , and C 2 H 5 + take over. A kinetic model is presented which allows to derive rate constants for the relevant elementary reactions. Under the current experimental conditions the rate constants are found to be independent of the rotational angular momentum N of the HCl + ions.

The dynamics of formation of NHD + and ND 2 + in reaction between NH + and D 2 was investigated over the collision energy range of 0.5–3.0 eV (c.m.) using two different scattering machines. Scattering data show, in good agreement, formation of an intermediate complex NHD 2 + in this reaction. Decreasing forward-backward symmetry of the scattering pattern, and changes in the c.m. angular distribution over the investigated collision energy range indicate that the dominant reaction path goes via the excited state of the intermediate NHD 2 + ( 1 A ′), bound with respect to the reactants by about 4 eV. Low-energy parts and positions of maxima of the distributions of product relative translational energy are predicted well by the Klots model of translational energy release in statistical complex decomposition at collision energies 0.5 eV and 1.3 eV.

We compare the first jet-cooled two-photon fluorescence excitation spectrum of benzonitrile with its one-photon spectrum measured by resonance-enhanced multi-photon ionization (REMPI). Both spectra include the pure electronic transition and one vibrationally induced transition. The inducing mode is the ν 31 in the one-photon spectrum and the ν 27 in the two-photon spectrum. This is similar to benzene, where the corresponding vibration ν 6 induces a transition in the one-photon and the corresponding vibration ν 14 induces a transition in the two-photon spectrum. The one-photon spectrum shows no vibronic bands above 2160 cm -1 . This is ascribed to small Franck-Condon factors for the second progression members of totally symmetric vibrations. The two-photon fluorescence excitation spectrum has a cut off at the higher excess energy of 3000 cm -1 pointing to a nonradiative relaxation channel similar to benzene. No indication for a charge transfer process in the first electronic state of benzonitrile was found up to an excess energy of 3000 cm -1 .

Mixed clusters of benzene derivatives and trihalogenmethanes have been prepared in a supersonic beam and studied with infrared vibrational predissociation spectroscopy detected by resonant two photon ionization (IR/R2PI). We report the spectra of chloroform and fluoroform attached to the chromophores fluorobenzene, benzene, toluene, p -xylene and their deuterated analogues. Instead of the typical H-bonding induced red-shift of the CH stretching frequency a blue-shift is observed. From a comparison with ab initio calculations, we propose sandwich-like structures bonded by a C–H...π hydrogen bond exhibiting an unusual partial charge transfer.

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, 1 CH 2 (ã 1 A 1 ), from the vibrationally excited (0,1,0) level. (0,1,0) removal rate constants are reported for the 13 species N 2 , CH 4 , C 2 H 4 , C 2 H 6 , CH 2 CO, CH 3 Cl, CH 3 OC 2 H 5 , CH 3 OH, CH 3 CN, CH 3 F, CF 4 , C 2 F 4 , and C 2 F 6 . With the exception of the slowest three reacting species, CH 4 , CH 3 F and C 2 F 4 , for the species which are known to react from the (0,0,0) level the removal rate constants are essentially identical for the (0,0,0) and (0,1,0) levels. The removal rate constants for CH 4 , CH 3 F and C 2 F 4 increase by factors of 1.2, 2.2 and 1.45 respectively with vibrational excitation to (0,1,0). For the species N 2 , CF 4 , and C 2 F 6 , which do not appear to react from (0,0,0), the removal rate constants are increased in the (0,1,0) state. The increase is very large for N 2 , for which the factor is 4, and is substantial for CF 4 and C 2 F 6 , which show factors of 2.55 and 2.7 respectively. The increased removal rate constants for N 2 and for the fluorine-containing species are attributed to the formation of an association complex between 1 CH 2 (0,1,0) and the partner followed by dissociation to 1 CH 2 (0,0,0) and the partner. We refer to this as complex-mediated vibrational relaxation (CMVR). Removal rate constants for the (0,0,0) level are likely to be in error when experimental conditions allow the simultaneous production of 1 CH 2 (0,1,0) from the methylene precursor and where CMVR, which typically has comparable efficiency to collision induced intersystem crossing, provides a growth term for the (0,0,0) population. The species for which CMVR is likely to be significant are those for which reaction is either absent or a minor pathway and where there are electron-rich centres for the empty orbital of 1 CH 2 to "bind", allowing the formation of a long lived 1 CH 2 -collider complex.

The reaction kinetics of ground state Si atoms was studied behind reflected shock waves in the presence of excess CH 4 and CH 3 , respectively. The very fast Si atom formation by the thermal decomposition of SiH 4 and Si 2 H 6 was used as a reliable Si atom source at temperatures T >1550 K. CH 3 was formed by the thermal decomposition of different source gases like CH 3 Cl and CH 3 I. All test gases were highly diluted in argon. The atomic resonance absorption spectroscopy (ARAS) was applied for time-resolved measurements of Si and H atoms and it was found that the reaction of Si atoms with CH 4 (R1) is much slower than the reaction of Si atoms with CH 3 (R2) which seems to be an important reaction leading to the formation of very reactive intermediate species on its way to form SiC. The product channels are discussed and a temperature independent mean rate coefficient for (R2) was determined: Si+CH 3 ↔ SiCH+H 2 (with k 2 a ) (R2a) ↔ SiCH 2 +H (with k 2 b ) (R2b) with k 2 b / k 2 a <0.1 and a mean value for k 2 =2.0±0.69·10 14  cm 3  mol -1  s -1 . A reaction mechanism including the established GRI (Gas Research Institute) mechanism is presented which explains all measured Si and H concentration profiles obtained during this study.