High Vibrational Excitation Research Articles

High-resolution infrared spectroscopy of jet-cooled vinyl radical: Symmetric CH2 stretch excitation and tunneling dynamics

First high-resolution IR spectra of jet-cooled vinyl radical in the C-H stretch region are reported. Detailed spectral assignments and least squares fits to an A-reduction Watson asymmetric top Hamiltonian yield rotational constants and vibrational origins for three A-type bands, assigned to single quantum excitation of the symmetric CH(2) stretch. Two of the observed bands arise definitively from ground state vinyl radical, as rigorously confirmed by combination differences predicted from previous midinfrared CH(2) wagging studies of Kanamori et al. [J. Chem. Phys. 92, 197 (1990)] as well as millimeter wave rotation-tunneling studies of Tanaka et al. [J. Chem. Phys. 120, 3604 (2004)]. The two bands reflect transitions out of symmetric (0(+)) and antisymmetric (0(-)) tunneling levels of vinyl radical populated at 14 K slit-jet expansion temperatures. The band origins for the lower-lower (0(+)<--0(+)) and upper-upper (0(-)<--0(-)) transitions occur at 2901.8603(7) and 2901.9319(4) cm(-1), respectively, which indicates an increase in the tunneling splitting and therefore a decrease in the effective tunneling barrier upon CH(2) symmetric stretch excitation. The third A-type band with origin at 2897.2264(3) cm(-1) exhibits rotational constants quite close to (but at high-resolution distinguishable from) the vinyl radical ground state, consistent with a CH(2) symmetric stretch hot band built on one or more quanta of excitation in a low frequency vibration. The observed CH(2) symmetric stretch bands are in excellent agreement with anharmonically scaled high level density functional theory (DFT) calculations and redshifted considerably from previous low resolution assignments. Of particular dynamical interest, Boltzmann analysis indicates that the pair of 0(+) and 0(-) tunneling bands exhibits 1:1 nuclear spin statistics for K(a)=even:odd states. This differs from the expected 3:1 ratio for feasible exchange of the two methylenic H atoms but is consistent with a 4:4 ratio predicted for interchange between all three H atoms. This suggests the novel dynamical possibility of large amplitude "roaming" of all three H atoms in vinyl radical, promoted by high internal vibrational excitation arising from dissociative electron attachment in the discharge.

Kinematically complete reaction dynamics of slow ions

An experiment for crossed beam imaging has been developed for kinematically complete studies of the reaction dynamics of ion-molecule collisions. Here we report on this technique and on recent results regarding the charge transfer reaction of Ar+ + N2. We speculate that the experimental observation of N2+ product ions with unexpectedly high vibrational excitation may be caused by a Feshbach scattering resonance and propose further theoretical work on this reaction. Furthermore we discuss how future collision experiments of molecular ions and clusters will utilize a radiofrequency 22pole trap for internal cooling of the ions prior to the interaction.

Associative desorption of N 2 from Ru(0 0 0 1): A computational study

We have studied the associative desorption of N 2 from a Ru(0 0 0 1) surface, using a six-dimensional classical trajectory method and a potential energy surface based on density functional theory. The frozen surface and Born–Oppenheimer approximations were used, neglecting phonons and electron–hole pair excitation. Our results show, in agreement with experiments, that only 40% of the potential energy at the barrier is transferred to the translational and rotational degrees of freedom. Our results also show, in disagreement with experiment, high vibrational excitation of the desorbed molecules, suggesting the importance of non-adiabatic effects for this process.

Verification of long-lived molecular hydrogen anions (Hn−,Dn−,n=2,3) by secondary-ion mass spectrometry

The existence of long-lived (metastable) molecular hydrogen anions $\mathrm{H}_{2}{}^{\ensuremath{-}}$, $\mathrm{D}_{2}{}^{\ensuremath{-}}$, $\mathrm{H}_{3}{}^{\ensuremath{-}}$, and $\mathrm{D}_{3}{}^{\ensuremath{-}}$ is demonstrated. These anion species were produced by sputtering of $\mathrm{Ti}{\mathrm{D}}_{2}$ or $\mathrm{Hf}{\mathrm{H}}_{2}$ targets with ${\mathrm{Cs}}^{+}$ ions and were identified by secondary-ion mass spectrometry (SIMS). From the respective flight times through the spectrometer, minimum lifetimes for $\mathrm{H}_{2}{}^{\ensuremath{-}}$ and $\mathrm{D}_{2}{}^{\ensuremath{-}}$ of a few microseconds can be inferred. Correspondingly longer lifetimes can be expected for the triatomic hydrogen molecules. From the SIMS intensities, the following apparent abundance ratios are derived: $\mathrm{H}_{2}{}^{\ensuremath{-}}∕{\mathrm{H}}^{\ensuremath{-}}=3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $\mathrm{H}_{3}{}^{\ensuremath{-}}∕{\mathrm{H}}^{\ensuremath{-}}=1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$, $\mathrm{D}_{2}{}^{\ensuremath{-}}∕{\mathrm{D}}^{\ensuremath{-}}=3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, and $\mathrm{D}_{3}{}^{\ensuremath{-}}∕{\mathrm{D}}^{\ensuremath{-}}=7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$. The kinetic energy spectra of $\mathrm{D}_{2}{}^{\ensuremath{-}}$ and ${\mathrm{D}}^{\ensuremath{-}}$ demonstrate that these hydrogen anions are indeed formed by sputtering at the sample surface. According to recent theoretical calculations it is proposed that in the emission event molecular hydrogen species with high rotational and vibrational excitation are formed that are stable on the time scale of the experiment.

Atomic and molecular hydrogen densities in a plasma expansion

We treat the kinetics of the three dominant species in a hydrogen plasma expansion, which was studied using various laser spectroscopic techniques. Whereas the ground-state H2 molecules expand like a normal gas, the behaviour of H atoms and ro-vibrationally excited molecules is considerably altered as a consequence of their reactivity. The hydrogen atoms are primarily lost via surface association. In the source, H atoms are lost on the surface of the nozzle, reducing the atomic flux, and in the expansion, the H atoms diffuse out due to large H atom concentration gradients between the plasma and the background that exists as a result of surface association on the vessel walls. The loss process of H atoms in the nozzle, surface association, is the main production process of H2 molecules with high rotational and vibrational excitation. The behaviour of these ro-vibrationally excited H2 molecules in the expansion shows the relaxation process from the high temperature in the upstream region to the low temperature of the downstream region. The lower rotational levels (J < 5) adapt to the local temperature as expected, but the higher levels (J > 7) do not because of the large energy spacing between the hydrogen rotational levels. The consequence of this high rotational excitation is demonstrated in the dissociative attachment of electrons to H2.

Nitrogen vibrational excitation in a N2/He pulsed planar-ICP RF discharge

A pure inductive regime of pulsed RF discharge in N2–He mixtures at 0.2 Torr has been investigated by time resolved Langmuir probe and optical emission spectroscopy. A planar coil ICP with a Faraday shield has been used. Mixture compositions of 5–100% N2 and duty cycles, TON = 1–15 ms and TOFF = 15 ms, have been explored. The Langmuir probe analysis evidences a Maxwell electron energy distribution in the discharge and post-discharge, characterized by electron density and temperature that depend on both duty cycle and composition. The vibrational excitation of N2(C) state is inferred by the N2 second positive emissions. Its kinetic analysis reveals the formation of a high vibrational excitation of ground state nitrogen. A vibrational temperature higher than 10 000 K can be reached at the end of the 5 ms discharge pulse on the levels v = 0–3 of the X state. Such a high vibrational temperature slows the post-discharge relaxation of the electron temperature.

Dynamics of Bimolecular Reactions of Vibrationally Highly Excited Molecules: Quasiclassical Trajectory Studies

Excitation functions from quasiclassical trajectory calculations on the H + H2O --> OH + H2, H + HF --> F + H2, and H + H'F --> H' + HF reactions indicate a different behavior at low and high vibrational excitation of the breaking bond. When the reactant tri- or diatomic molecule is in vibrational ground state or in a low vibrationally excited state, all these reactions are activated; i.e., there is a nonzero threshold energy below which there is no reaction. In contrast, at high-stretch excited-states capture-type behavior is observed; i.e., with decreasing translational energy the reactive cross-section diverges. The latter induces extreme vibrational enhancement of the thermal rate consistent with the experiments. The results indicate that the speed-up observed at high vibrational excitation is beyond the applicability of Polanyi's rules in their common form; instead, it can be interpreted in terms of an attractive potential acting on the attacking H atom when it approaches the reactant with a stretched X-H bond.

Experimental and Theoretical Evidence for Long-Lived Molecular Hydrogen AnionsH2−andD2−

The existence of (metastable) molecular hydrogen anions H2(-), D2(-), and H3(-) is demonstrated. These anion species were produced by sputtering of TiH2 and TiD2 targets with Cs+ ions and were identified by accelerator mass spectrometry. From the respective flight times through the spectrometer, lifetimes for H2(-) and D2(-) of at least 3 micros and 4 micros, respectively, can be inferred. Theoretical calculations within the nonlocal resonance model predict the existence of highly rotationally excited anions with lifetimes in the micros range. It is proposed that in sputtering molecular hydrogen species with high rotational and vibrational excitation are formed that are stable on the time scale of the experiment.

A new mechanism for the production of highly vibrationally excited OH in the mesosphere: An ab initio study of the reactions of O2(AΣu+3andA′Δu3)+H

In an attempt to explain the observed nightglow emission from OH(v=10) in the mesosphere that has the energy greater than the exothermicity of the H+O(3) reaction, potential energy surfaces were calculated for reactions of high lying electronic states of O(2)(A (3)Sigma(u) (+) and A' (3)Delta(u)) with atomic hydrogen H((2)S) to produce the ground state products OH((2)Pi)+O((3)P). From collinear two-dimensional scans, several adiabatic and nonadiabatic pathways have been identified. Multiconfigurational single and double excitation configuration interaction calculations show that the adiabatic pathways on a (4)Delta potential surface from O(2)(A' (3)Delta)+H and a (4)Sigma(+) potential surface from O(2)(A (3)Sigma(u) (+))+H are the most favorable, with the zero-point corrected barrier heights of as low as 0.191 and 0.182 eV, respectively, and the reactions are fast. The transition states for these pathways are collinear and early, and the reaction coordinate suggests that the potential energy release of ca. 3.8 eV (larger than the energy required to excite OH to v=10) is likely to favor high vibrational excitation.

Picosecond pulsed laser deposition at high vibrational excitation density: the case of poly(tetrafluoroethylene)

The availability of tunable, picosecond free-electron lasers operating with high efficiency in the mid-infrared opens a materials-processing regime qualitatively distinct from that accessed by femtosecond Ti:sapphire lasers, one which is characterized by a high spatio-temporal density of vibrational, rather than electronic, excitation. As an example of this novel materials-processing regime, we present new results on pulsed laser deposition of thin poly(tetrafluoroethylene) films. Films of poly(tetrafluoroethylene) were deposited by resonant (4.2 and 8.26 μm) and non-resonant (7.1 μm) infrared picosecond laser ablation from either a pressed powder target or a commercial bulk target. The films were smooth and crystalline and largely free of particulates without annealing. Infrared and X-ray photoelectron spectra indicated that the films retained the chemical properties of the starting material. Observations of the film properties are consistent with a steady-state ablation mechanism, possibly enhanced by non-linear absorption due to the high photon flux in the free-electron laser micropulses.

Double ionization and dissociation of benzene-d6 induced by collision with H + and Ar 8+

A 15 keV H + or 120 keV Ar 8+ beam was collided with benzene-d 6 to induce multiple ionization and dissociation. The TOFs of the recoil ions were measured combined with the multiple coincidence technique and the position-sensitive measurement, which allow to identify various dissociation channels and to derive their kinetic energy releases (KERs). By qualitative analysis of TOF coincidence map, it is concluded that in the case of high vibrational excitation, doubly charged parent molecule dissociates into two molecular ions with equal number of carbon atoms in each of them. Comparison of coincidence maps obtained by collisions with H + and Ar 8+ indicates that the number of neutral D-substitution, which were missing from dissociation products, can be used as an indicator of vibrational excitation of parent molecule.

Access to vibrational population distributions via modelling of fluorescence spectra of small molecules

Abstract A model is devised that allows access to vibrational population distributions of small molecules in solution from their emission spectra. The model which is based on the harmonic oscillator approximation and the reflection principle shows that these spectra particularly hold information on high vibrational excitations. Comparison of simulations based on this model with experimental results on diiodide I 2 − shows that despite its simplicity the model captures all spectral features.

CARS investigations of a N2 planar-ICP discharge

A N2 pulsed ICP discharge has been investigated in the pressure range of (0.5÷10), Torr by CARS, and OES. In this paper we report some results on N2(X, v) vibrational excitation CARS measurements at 2 Torr indicate an increase of the rotational temperature during the discharge from 300 K up to 700 K, and a rise of the vibrational temperature up to 8000K. Such a high vibrational excitation on the lowv levels is in agreement with results obtained by the study of N2 (C, v) distribution inferred by the SPS emission.

Calculations on the vibrational level density in highly excited formaldehyde

Abstract The object of the present work is to develop a model that provides realistic estimates of the vibrational level density in polyatomic molecules in a given electronic state, at very high (chemically relevant) vibrational excitation energies. For S 0 formaldehyde (D 2 CO), acetylene, and a number of triatomics, the estimates using conventional spectroscopic formulas have yielded densities at the dissociation threshold, very much lower than the experimentally measured values. In the present work we have derived a general formula for the vibrational energy levels of a polyatomic molecule, which is a generalization of the conventional Dunham spectroscopic expansion. Calculations were performed on the vibrational level density in S 0 D 2 CO, H 2 C 2 , and NO 2 at excitation energies in the vicinity of the dissociation limit, using the newly derived formula. The results from the calculations are in reasonable agreement with the experimentally measured data.

Entrance channel localized states in ozone: Possible application to helium nanodroplet isolation spectroscopy

Accurate calculations of the bound ro-vibrational states of ozone are performed in the region of high vibrational excitation. Two unusual vibrational states localized in the far van der Waals region are discovered. They can be considered as the ground vibrational states of even and odd symmetries bound in the shallow van der Waals well. Properties of these states are presented and discussed, which opens possibilities for experimental realization of the helium nanodroplet isolation ro-vibrational spectroscopy of ozone.

Dynamics calculations for the LiH+H⇆Li+H2 reactions using interpolations of accurate ab initio potential energy surfaces

Complete active space self-consistent-field calculations followed by multireference configuration interaction calculations are used to generate potential energy surfaces for the reaction of LiH+H⇆Li+H2. For interpolation of the potential energy surfaces, a recently proposed interpolant moving least squares(IMLS)/Shepard scheme is used. Trajectory calculations for LiH+H→Li+H2 on the interpolated potential energy surface illustrate that the initial vibrational state of LiH plays little role probably due to the absence of the energy barrier for this reaction. The reactive cross section shows a maximum value corresponding to the initial collision energy as low as 0.03 eV. Trajectory calculations for Li+H2→LiH+H show that high vibrational excitation (ν⩾4) of H2 molecule is the most efficient way and the collision energy inefficiently induces the reaction.

Laser mass spectrometry at high vibrational excitation density

We describe a novel approach to infrared matrix-assisted laser desorption-ionization mass spectrometry using a tunable, picosecond pulse laser to selectively excite specific modes of a solid, thereby creating a high local density of vibrational quanta. The concept is based on recent results from our experiments employing a free-electron laser to explore ‘matrix-less’ mass spectrometry in which an infrared chromophore intrinsic to the sample, rather than an exogenous matrix, is excited by the laser. Examples from both environmental mass spectrometry and a proteomics-driven research project are presented, showing how the principle of selective vibrational excitation can be used to make possible novel and useful ion generation protocols. We conclude with an analysis of possible mechanisms for the phenomena of infrared desorption, ablation and ionization using very short laser pulses. Prospects for achieving similar results with more conventional laser sources are discussed.

Thermal Decomposition of Xenon Tetroxide at 490–780 K

The decomposition of XeO4 was studied over the temperature range 490–780 K. A reaction mechanism was proposed. A conclusion on the high vibrational excitation of O2 molecules formed in the elementary reaction O + XeO4 → O2 + XeO3 and the branched-chain character of the decomposition with branching in the reaction O + XeO3 → 2O + O2 + Xe was drawn.

Indirect evidence for strong nonadiabatic coupling in N2 associative desorption from and dissociative adsorption on Ru(0001)

This paper reports the simultaneous internal state and translational energy resolved associative desorption flux of N2 from Ru(0001) using two different experimental approaches. Both experiments show that the nascent N2 is formed with little vibrational excitation and that the total excitation in all N2 degrees of freedom accounts for only 13 of the barrier energy. Roughly 23 of the energy necessary to surmount the barrier is lost to the surface in desorption. This behavior, as well as the unusual behavior noted previously in direct measurements of dissociative adsorption, both imply strong vibrational quenching in reactive trajectories passing over the high exit channel (vibrational) barrier. Adiabatic quasiclassical dynamical calculations based on the ab initio potential energy surface and various models of coupling to the lattice are not qualitatively consistent with N2 vibrational damping to phonons. However, including a strong nonadiabatic coupling of the vibrational coordinate to electron–hole pairs in the dynamics does yield qualitative agreement between experiments and calculated dynamics, and we suggest this as indirect evidence for strong nonadiabatic coupling. We argue that the nonadiabatic coupling is strong in this case because of the high vibrational excitation necessary to pass over the high exit channel barrier in the reactive processes and the large charge transfer inherent in making or breaking π bonds. We believe that the same factors will be important in most activated dissociations of π bonded molecules on transition metal surfaces, e.g., for O2, NO, N2, and CO, and if this scenario is correct then nonadiabaticity should be important in the activated dissociation dynamics of these systems as well.

Photoexcitation of rare-gas neon and argon clusters doped with H 2 O

We have studied the fluorescence of electronically excited OH*, H* and H2O+* dissociation fragments after VUV excitation ( hν≥11.6eV) of rare-gas clusters (Rg = Ne, Ar) doped with H2O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH *(A2Σ+↦X2Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH*(A) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H2O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons.