Collisional Relaxation of Highly Vibrationally Excited Acetylene Mediated by the Vinylidene Isomer.

Collisional relaxation of highly vibrationally excited acetylene, generated from the 193 nm photolysis of vinyl bromide with roughly 23,000 cm-1 of nascent vibrational energy, is studied via submicrosecond time-resolved Fourier transform infrared (FTIR) emission spectroscopy. IR emission from vibrationally hot acetylene during collisional relaxation by helium, neon, argon, and krypton rare-gas colliders is recorded and analyzed to deduce the acetylene energy content as a function of time. The average energy lost per collision, ⟨ΔE⟩, is computed using the Lennard-Jones collision frequency. Two distinct vibrational-to-translational (V-T) energy transfer regimes in terms of the acetylene energy are identified. At vibrational energies below 10,000-14,000 cm-1, energy transfer efficiency increases linearly with molecular energy content and is in line with typical V-T behavior in quantity. In contrast, above 10,000-14,000 cm-1, the V-T energy transfer efficiency displays a dramatic and rapid increase. This increase is nearly coincident with the acetylene-vinylidene isomerization limit, which occurs nearly 15,000 cm-1 above the acetylene zero-point energy. Combined quasi-classical trajectory calculations and Schwartz-Slawsky-Herzfeld-Tanczos theory point to a vinylidene contribution being responsible for the large enhancement. This observation illustrates the influence of energetically accessible structural isomers to greatly enhance the energy transfer rates of highly vibrationally excited molecules.

Chemical activation through super energy transfer collisions.

Can a molecule be efficiently activated with a large amount of energy in a single collision with a fast atom? If so, this type of collision will greatly affect molecular reactivity and equilibrium in systems where abundant hot atoms exist. Conventional expectation of molecular energy transfer (ET) is that the probability decreases exponentially with the amount of energy transferred, hence the probability of what we label "super energy transfer" is negligible. We show, however, that in collisions between an atom and a molecule for which chemical reactions may occur, such as those between a translationally hot H atom and an ambient acetylene (HCCH) or sulfur dioxide, ET of chemically significant amounts of energy commences with surprisingly high efficiency through chemical complex formation. Time-resolved infrared emission observations are supported by quasi-classical trajectory calculations on a global ab initio potential energy surface. Results show that ∼10% of collisions between H atoms moving with ∼60 kcal/mol energy and HCCH result in transfer of up to 70% of this energy to activate internal degrees of freedom.

Collisional Energy Transfer from Highly Vibrationally Excited Radicals Is Very Efficient.

Although highly vibrationally excited (HVE) radicals are ubiquitous in natural environments, the effect of collisional energy transfer (ET) on their reactivity has yet to be fully characterized. We have used time-resolved IR emission spectroscopy to characterize the vibrational-to-translational quenching of a small HVE radical, ketenyl (HCCO), by inert gases. Photolysis of ethyl ethynyl ether at 193 nm provides HVE HCCO in the X̃(2)A″ electronic ground-state, with a nascent internal energy of 2.2 ± 0.6 eV. IR emission is monitored as an indicator of vibrational energy, and spectral modeling allows direct determination of the average energy lost per collision as a function of the internal energy. Collisional deactivation of HVE HCCO is shown to be minimally an order of magnitude more efficient than closed-shell molecules of comparable size. Schwartz-Slawsky-Herzfeld-Tanczos (SSHT) theory, modified for HVE molecules, suggests that this ET enhancement is due to a strong attractive intermolecular interaction.

Strong combination-band IR emission from highly vibrationally excited acetylene.

The nu(4) + nu(5) combination band, which appears relatively weak in the IR absorption spectrum, has been identified with exceptionally high intensity in the IR emission spectra from highly vibrationally excited acetylene, which is produced with approximately 71 kcal mol(-1) of vibrational energy from the 193 nm photolysis of vinyl bromide. The 'fundamental' transition of this combination band, from the (0,0,0,1(1),1(-1)) level to the zero point, occurs at 1328 cm(-1). The intensity and frequency of this band as well as the nu(3) and nu(5) bands, IR active but with lower emission intensity, as a function of the acetylene energy can be modeled accurately using the normal mode harmonic oscillator model with frequency anharmonicity corrections. Good fitting results are achieved even though the normal mode quantum numbers are no longer good for levels in the high energy region and the combination band is forbidden in the harmonic oscillator model. The identification of this intense combination band in emission, compared to its weak intensity in the absorption spectrum, highlights the necessity to include in consideration the combination bands for assignment of emission spectra in general and in particular emission from vibrationally hot acetylene which is ample in combustion, atmospheric, and interstellar environments.

Vibrational modes of the vinyl and deuterated vinyl radicals.

Following the initial report of the detection of fundamental transitions of all nine vibrational modes of the vinyl radical [Letendre , L. ; Liu , D.-K. ; Pibel , C. D. ; Halpern , J. B. ; Dai , H.-L. J. Chem. Phys. 2000 , 112 , 9209] by time-resolved IR emission spectroscopy, we have re-examined the assignments of the vibrational modes through isotope substitution. Precursor molecules vinyl chloride-d3, vinyl bromide-d3, and 1,3-butadiene-d6 are used for generating vibrationally excited vinyl-d3 through 193 nm photolysis. The nondeuterated versions of these molecules along with vinyl iodide and methyl vinyl ketone are used as precursors for the production of vinyl-h3. IR emission following the 193 nm photolysis laser pulse is recorded with nanosecond time and approximately 8 cm(-1) frequency resolution. A room-temperature acetylene gas cell is used as a filter to remove the fundamental transitions of acetylene, a photolysis product, in order to reduce the complexity of the emission spectra. Two-dimensional cross-spectra correlation analysis is used to identify the emission bands from the same emitting species and improve the S/N of the emission spectra. Isotope substitution allows the identification of several low-frequency vibrational modes. For C2H3, the assigned modes are the nu4 (CC stretch) at 1595, nu5 (CH2 symmetric bend) at 1401, nu6 (CH2 asymmetric + alpha-CH bend) at 1074, nu8 (CH2 + alpha-CH symmetric out-of-plane (oop) bend) at 944, and nu9 (CH2 + alpha-CH asymmetric oop bend) at 897 cm(-1). For C2D3, the modes are the nu5 (CD2 symmetric bend) at 1060, nu6 (CD2 asymmetric + alpha-CD bend) at 820, and nu8 (CD2 + alpha-CD symmetric oop bend) at 728 cm(-1).

Photodissociation of vinyl cyanide at 193 nm: Nascent product distributions of the molecular elimination channels.

The photodissociation dynamics of vinyl cyanide (H(2)CCHCN, acrylonitrile) and deuterated vinyl cyanide (D(2)CCDCN) at 193 nm are examined using time-resolved Fourier transform infrared emission spectroscopy. Prior photofragment translational spectroscopy studies [D. A. Blank et al., J. Chem. Phys. 108, 5784 (1998)] of the dissociation have observed the presence of four main dissociation channels; two molecular and two radical in nature. However, with the exception of a<0.01 quantum yield determined for the CN radical loss channel, the branching ratios of the remaining three elimination channels were not measured. The time-resolved emission spectra, including those from the deuterated samples, revealed the presence of acetylene, hydrogen cyanide (HCN), as well as the energetically less stable isomer hydrogen isocyanide (HNC). Acetylene is found in two distinct energetic distributions, suggesting that both three- and four-centered elimination reactions are occurring significantly in the dissociation. In contrast to prior ab initio studies that have suggested the dominant nature of the three-center elimination of molecular hydrogen (H(2)) and cyanovinylidene (:C=CHCN), we find this reaction channel to be of little importance as there is no evidence to support any significant presence of rovibrationally excited cyanoacetylene. Spectral modeling of the product distributions allows for the first experimental determination of the relative occurrence of the three-centered (resulting in HCN+vinylidene) versus four-centered (HNC+acetylene) elimination channels as 3.34 to 1.00, in contrast to the previously calculated value of 126:1. Rice-Ramsperger-Kassel-Marcus analysis depicts that the transition state energy of the four-centered reaction should be about 10 kcal mole(-1) lower than the three-centered reaction.