Overtone Excitation Research Articles

Vibrational overtone spectroscopy of bound and predissociative states of hydrogen peroxide cooled in a supersonic expansion

The vibrational overtone excitation spectra of both bound and predissociative states of hydrogen peroxide molecules cooled in a supersonic expansion show features that are obscured otherwise. Spectra of predissociative states are measured by detecting the decomposition product following excitation of an overtone vibration. Spectra of bound states are obtained by a two-photon excitation technique in which a second photon excites the molecule from its bound vibrational overtone state to a dissociative state. The features in the bound state (4νOH) spectrum are 0.08 to 0.13 cm−1 wide, reflecting small inhomogeneous broadening, but those to the predissociative state (6νOH) are 1.5±0.3 cm−1 wide. This width, which corresponds to a lifetime of about 3.5 ps, reflects coupling into the dissociative continuum.

Proton transfer from nitromethane stimulated by C-H overtone excitation: evidence for vibrational photochemistry in solution

Vibrational overtone excitation of nitromethane in the C-H stretch (Δv = 3) band at 1144 nm enhances the rate of proton transfer to D2O solvent. Evidence is presented for vibrational photochemistry with quantum yield (3± 1 × 10−5.

Overtone spectroscopy of 3,3,3-trifluoropropyne

Intracavity dye laser photoacoustic spectra have been obtained for overtone excitation of the C-H stretch (deltanu/sub 1/ = 4, 5, and 6) of gas-phase CF/sub 3/C(triple bond)CH. Standard infrared techniques were used to obtain the fundamental and lower overtones (deltanu/sub 1/ = 1, 2, 3). The local-mode model is used to assign prominent peaks. For all transitions in the C-H stretching mode a band close in energy to the main absorption is observed and is assigned as a hot band (nu/sub 1/ = 0, nu/sub 7/ = 1) ..-->.. (nu/sub 1/ = nu, nu/sub 7/ = 1) for nu = 1-6, which shows the strong interaction between the C-H stretching mode (nu/sub 1/) and the C-H bending model (nu/sub 7/). Also the fundamental and lower overtones (..delta..nu/sub 2/ = 1, 2, 3) of the C(triple bond)C stretch are obtained. Harmonic frequencies (omega/sub i/) and anharmonicity constants (X/sub ii/) with i = 1 or 2 for C-H and C(triple bond)C bonds, respectively, are calculated. The off-diagonal local-mode-normal-mode anharmonicity constant X/sub 17/ derived from the interaction between modes nu/sub 1/ and nu/sub 7/ is also reported. Peak absorption cross sections are obtained for nu/sub 1/ = 1-5 of CH absorptions and nu/submore » 1/ = 1-3 of C(triple bond)C absorptions.« less

Methyl isocyanaide isomerization. Determination of collisional deactivation parameters following carbon-hydrogen overtone excitation

The isomerization of methyl isocyanide (CH/sub 3/NC) to acetonitrile (CH/sub 3/CN) was studied by excitation of the 5nu/sub C-H/ (726.6 nm) and 6nu/sub C-H/ (621.4 nm) overtone states, which lie about 1 and 8 kcal/mol, respectively, above the isomerization barrier. Products yields were measured as a function of pressure and collision partner. A Stern-Volmer plot (yield/sup -1/ vs. pressure) shows that (1) deactivation by collision with pure CH/sub 3/NC is more rapid than with C/sub 3/H/sub 6/, SF/sub 6/, or Ar, (2) the collisional deactivation efficiencies decrease in going from C/sub 3/H/sub 6/ to SF/sub 6/ to Ar, and (3) the single-collision deactivation approximation (strong collider approximation) fails for both the 6nu/sub C-H/ and 5nu/sub C-H/ data. With the use of a master equation solution, assuming an exponential down energy-transfer function, the average energy transferred in a deactivating collision, - /sub down/, is extracted from each data set, as well as the average energy transferred fer collision, - . It is concluded that the isomerization yield depends markedly on the collision partner and on the average energy transferred per collision, - , even though the single-collision deactivation approximation might have been expected to have its greatest validity in this energy regime.

Dynamics of overtone excitation processes for a polyatomic model

The excitation dynamics of a polyatomic system are presented in terms of both classical and quantum dynamics. These studies show that classical mechanics is qualitatively inaccurate in its description of the overtone excitation process. The quantum excitation is therefore an example of dynamic tunneling, which like most tunneling processes is slow. In a polyatomic molecule, intramolecular vibrational relaxation occurs much faster than does excitation, by several orders of magnitude. The disparity of time scales results in the excitation being very eigenstate specific in the small to medium molecule limit. We discuss the importance of these observations to photochemical experiments (possible preparation of long-lived localized excitations) and to theoretical interpretations of molecular wave functions (possible interference effects in the molecular state function).

Lifetimes of molecular negative ion resonances on surfaces

Publisher Summary This chapter discusses the lifetimes of molecular negative ion resonances on surfaces. The lifetime of the resonance of molecular O 2 physisorbed on graphite is qualitatively probed via the ratio of overtone to fundamental vibrational intensities in the resonant energy loss spectroscopy (EELS) spectrum as a function of surface coverage. Resonant vibrational excitation occurs because of nuclear motion in the potential of the temporary negative ion formed by electron capture. The impact energy is set near the observed peak in the cross-section for vibrational excitation, close to the peak assigned by gas phase workers to the ion state. The frequencies of the observed modes correspond to the fundamental and overtone excitations of gas phase O 2 . Future work is planned to focus on the coverage dependence of the resonance lifetime and also of shifts in the resonance energy.

Vibrational energy flow into a reactive coordinate: A theoretical prototype for a chemical system

A realistic model is developed to illustrate vibrational energy flow into a reactive coordinate. The isomerization of C≡N–H to H–C≡N is enhanced nonstatistically through overtone excitations of the H–N bond. The mechanism determined for this theoretical model is dominated by resonant energy transfer from the anharmonically suppressed H–N mode into a combination of the C≡N plus the bending reaction coordinate. Implications of this classical prototype to mode specific chemistry, in particular the prediction of possible quantum tunnelling enhancement, are discussed in terms of experimentally observable systems.

Energy transfer parameters from studies of unimolecular reactions induced by overtone excitation

Rates of cyclobutene isomerisation induced by excitation of the 6-0 CH stretching overtone at 16603 cm −1 have been measured as a function of the pressure of the bath gases He, Ar, CO 2, CH 4, SF 6 and cyclobutene. These data have been used to compare energy transfer parameters obtained from an approximate treatment of the effects of weak collisions with values from numerical solutions of the collisional master equation. Comparisons with data from previous photoactivation studies are also made. It is found that the combination of RRKM theory and an adequate treatment of inefficient intermolecular energy transfer can satisfactorily account for the observed results.

Intramolecular vibrational energy flow in model four-atom systems

Intramolecular energy transfer from local bond modes is studied by using quasiclassical trajectories. The energy in a local mode as a function of time is computed to determine the nature and time scale for the decay of local mode excitation. Variations of four basic models are examined: HCCC, HOOH, HNNH, and HC ≡ CH. Potential-energy surfaces for the models are sums of Morse functions for bond stretches and harmonic bending functions. The potential for HOOH torsional motion is represented by a truncated Fourier cosine series. The emphasis of the study is on H2O2. The decay of OH overtone excitation is characterized by slow, irreversible flow of energy out of the OH as a function of time. The rate of energy transfer is strongly dependent upon the OOH bending and the torsional motions.

Resonance vibrational excitation in electron-energy-loss spectroscopy of adsorbed molecules.

A mechanism is suggested which could lead to enhanced excitation of the low-frequency vibrational modes associated with the bond between a solid surface and a molecule adsorbed upon it or of hindered rotational modes with displacement components normal to the surface, as observed in electron-energy-loss spectroscopy. If the incident electron becomes trapped in a negative-molecular-ion shape resonance, manifesting itself in significant intramolecular overtone excitation, the molecule-surface potential-energy curve is augmented by the image potential of the negative ion. This allows displacement of the molecule from its equilibrium neutral position which in turn appears as vibrational excitation of adsorbate-substrate relative motions, upon return to the neutral curve. A simple theory for this effect is presented and numerical estimates of its magnitude are given.

On non-exponential unimolecular dissociation of molecules prepared by vibrational overtone excitation

The sensitivity of vibrational overtone excitation experiments to non-RRKM behavior is considered. It is proposed that the experimental results are not inconsistent with intrinsically non-RRKM unimolecular lifetime distribution.

A model study of overtone-induced isomerization: The role of non-linear resonances

Overtone-induced isomerization is studied in a simple model for unimolecular isomerization. Considerable reaction rate enhancement occurs for certain initial overtone excitations, and the reactivity patterns differ markedly from those expected from statistical ideas. The critical role of bend—stretch Fermi resonance in this model process is discussed.

Predominance of CF-stretch vibrational excitation in state resolved inelastic scattering of protons from 11 small fluorohydrocarbon molecules

Time-of-flight spectra of H+ scattered from all ten different fluorinated hydrocarbon molecules of type CHmFn and C2HmFn (m+n=4) as well as C2F6 have been measured for small scattering angles (θlab≤15°) at 9.8 eV collision energy. For the pure hydrocarbons the spectra show a complicated structure suggesting unresolved excitation of several modes and a significant effect of charge transfer collisions. In contrast all the spectra for fluorine containing molecules exhibit more or less sharp maxima indicating high overtone excitation of predominantly one vibrational mode with ωv≂1200 cm−1 corresponding to a CF-stretch vibration. At some scattering angles the overtone transition probabilities deviate somewhat from a Poisson distribution suggesting small unresolved contributions from additional vibrational modes. Over- all, however, there is good agreement for the low symmetry molecules, some of which have sizable dipole moments, with the predictions of the simple forced oscillator model previously tested only for the spherically symmetric molecules CF4 and SF6. The relative magnitudes of the observed energy transfers are consistent with absolute infrared intensities where these are available from the literature.

Adsorption of hydrogen and deuterium on Ru(001)

The adsorption of hydrogen and deuterium has been studied with high resolution electron energy loss spectroscopy (HREELS). At maximum coverage at 105 K the H adlayer exhibits two dominant loss features at 102 (ν1) and 141 (ν2) meV which show pronounced intensity variations as a function of the primary electron energy (maximum between 5 and 6 eV). The hydrogen is assigned to the threefold coordinated site of the Ru (001) surface leading to a C3v symmetry of the adsorption complex. The angular dependence of both loss peaks shows only a smooth intensity decrease in off-specular directions thus preventing the separation of dipole and impact scattering contributions. Three additional loss peaks with considerably lower intensities are identified with overtone and combination excitations. The anharmonicity coefficients derived from the respective energies are used to fit a two-dimensional potential function for the hydrogen atom located in a plane perpendicular to the surface with first order perturbation theory. The resulting differences in the potential gradient allows the assignment of the ν1 and ν2 fundamentals to the vibrational modes parallel and perpendicular to the surface, respectively. As a function of increasing coverage both ν1 and ν2 shift to higher frequencies with a concomitant decrease in the peak width, the shift of the parallel mode being distinctively larger. At low coverages a third loss peak is observed which is ascribed to a different adsorption state of hydrogen. For deuterium the two fundamentals and the lowest overtone are observed and exhibit reasonably accurately the expected isotopic shift. The adsorption properties are found to be similar.

State-resolved product detection in the overtone vibration initiated unimolecular decomposition of HOOH(6νOH)

Excitation of reactant overtone vibrations in combination with laser induced fluorescence detection of unimolecular decomposition products permits measurements of reactant vibrational overtone excitation spectra and product state distributions for selectively excited hydrogen peroxide and its partially deuterated analog. The hydrogen peroxide excitation spectrum in the region of the fifth OH overtone vibration (6νOH) shows a pure local mode feature along with several others that are apparently combinations of a local mode vibration with a normal mode vibration. The product state distributions show substantial energy release into rotation with some molecules having rotational excitation approaching the limit of the total energy available. A statistical calculation incorporating conservation of total angular momentum and energy describes the observed distributions quite well for excitation of the pure local mode vibration. The products from excitation of a combination transition are in a distribution of states that differs qualitatively from a statistical outcome, possibly indicating selective energy coupling on the time scale of the unimolecular reaction.

CH stretch excitation causes conformational interconversion in ground state methyl vinyl ether but not in methyl nitrite

Molecules exhibiting conformational isomerism have been studied by spectrally resolving the infrared laser induced fluorescence from CH stretch fundamental excitation of methyl vinyl ether and from CH stretch fundamental and N=O overtone excitation of methyl nitrite. By comparison of the resolved fluorescence spectra of methyl vinyl ether with its absorbance spectrum, we have observed the first case of state selected ground electronic state intramolecular conformational interconversion in an isolated molecule. The distribution of conformers is altered drastically by the deposition of energy. The cis/trans ratio of the laser excited molecules may also be sensitive to the laser excitation frequency. Excitation of the N=O overtones of both cis methyl nitrite and trans methyl nitrite, which are well separated in frequency, gives resolved fluorescence spectra which mimic the absorbance spectra for cis methyl nitrite and trans methyl nitrite, respectively, thereby placing a lower bound for the conformational interconversion energy barrier from trans to cis of 3325 cm−1. The experiments were performed by exciting either molecule cooled in a pulsed nozzle expansion with light from an infrared optical parametric oscillator. The fluorescence spectra were recorded with a cryogenic Michelson interferometer.

Mechanisms and rate constants for the vibrational relaxation of HD(v=4, 5, and 6) in collisions with HD, 4He, and D2

The relaxation of highly vibrationally excited HD with several collision partners is examined using direct overtone excitation and time-resolved photoacoustic detection of the subsequent change in translational energy of the gas. Analysis of such data for HD(v=4, 5 and 6) self-relaxation yields vibration-translation (V-T) and vibration–vibration (V–V) rate constants for collisions between HD(v=1 to 6) and HD(v=0). The V-T rate constant becomes larger than the V–V rate constant above v=4. Similarly, relaxation data for the HD(v=4 and 5)–4He systems is analyzed and the HD–4He V-T rate constants at v=4 and v=5 are presented. The differences in the quantum number scaling of the V-T rates for the HD–HD and HD–4He systems are discussed. Finally, the time-resolved photoacoustic data for HD(v=4) relaxing in collisions with D2 shows the effect of the rapid, near-resonant V–V energy transfer between HD in the v=1 through v=4 levels and D2 in v=0.

Site-selective liquid-prase vibrational overtone photochemistry of hydroxyhexadiene

Vibrational overtone excitation of 1,5-hexadien-3-ol in the liquid phase yields the rearrangement product 5-hexanal. It is shown that the product is only formed when a CH oscillator is pumped while the reaction does not proceed upon pumping of the OH oscillator.

New Approach to Many-State Quantum Dynamics: The Recursive-Residue-Generation Method

A new computational approach to time-dependent transition amplitudes in multistate quantum systems is developed. The method, which avoids explicit computation of matrix eigenvectors, is based upon the recursive generation of residues of Green's functions. The residues are computed from the eigenvalues of several tridiagonal matrices. Numerical results are presented for multiphoton excitation of overtones in a system with over 3000 states.

Vibrational excitation in molecule–surface collisions due to temporary negative molecular ion formation

Inelastic electron scattering from gaseous and physisorbed diatomic molecules results in greatly enhanced vibrational overtone excitation if the incident electron has the appropriate energy to form a shape-resonance-induced temporary negative molecular ion. It is proposed here that due to the image potential lowering of the electron affinity level of a diatomic molecule in interaction with a metal surface, somewhere outside the surface an incident molecule would find its affinity level degenerate with or lower than the substrate Fermi level at which point a substrate electron could hop onto the molecule, in analogy with gas phase harpooning processes. A negative molecular ion is thus formed which remains until the molecular ion reflects from the surface and the affinity level rises above the Fermi level, thus permitting reverse electron hopping back into the metal. The lifetime of the molecular ion can be controlled by varying both the kinetic energy of the incident molecule and also the substrate work function. In analogy with the electron scattering events, greatly enhanced vibrational excitation of overtones is expected in the molecules of the scattered beam. Induced fluorescence probing of the vibrational state distribution should then yield fundamental information pertaining to the dynamics of charge transfer reactions and nonadiabatic effects in molecule–surface interactions. A theory of this phenomenon is here presented together with the numerical consequences for a model system designed to simulate N2 or NO scattering from standard surface science metal surfaces.