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  • 1
    Electronic Resource
    Electronic Resource
    Fast beam photodissociation spectroscopy and dynamics of the vinoxy radical (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 3049-3066 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spectroscopy and photodissociation dynamics of the vinoxy (CH2CHO) radical B˜(2A″) ←X˜(2A″) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B˜ state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: (1) CH3+CO, and (2) H+CH2CO, with a branching ratio of (approximate)1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH3+CO, and a considerably smaller barrier for H+CH2CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the A˜(2A) or C˜(2A) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, which satisfactorily reproduces the translational energy distributions. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473419
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  • 2
    Electronic Resource
    Electronic Resource
    Nonstatistical unimolecular dissociation over a barrier (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2448-2457 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general formulation is presented to model photodissociation processes in which internal conversion is followed by unimolecular dissociation over an exit barrier; this classification of dissociation mechanism results in a nonstatistical product state distribution. The energy available to products is divided into independent statistical and impulsive energy reservoirs. The statistical reservoir considers direct projections of a vibrational microcanonical ensemble at the transition state (TS) onto product quantum states, conserving vibrational adiabaticity and angular momentum. The impulsive reservoir represents the energy released in passing from the TS to products; this reservoir is treated assuming sudden dissociation of the zero-point TS wave function using a combination of Franck-Condon and impulsive models. We derive the statistical adiabatic impulsive model, which convolutes these two energy reservoirs, to predict the product translational energy distribution for nonstatistical dissociation over a barrier. Two test cases are modeled and compared with experimental data: unimolecular dissociation of acetyl radicals and photodissociation of vinoxy radicals via the B˜ 2A″-X˜ 2A″ band. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475627
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  • 3
    Electronic Resource
    Electronic Resource
    Photodissociation spectroscopy and dynamics of the HCCO free radical (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 10087-10098 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation spectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at 33 424 cm−1 has been identified. This band exhibits rotational resolution near the band origin, but the well-defined rovibronic structure is homogeneously broadened at higher photon energies. Based on the rotational structure this band is assigned to the B˜ 2Π←X˜ 2A′′ transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spin-forbidden CH(a 4Σ−)+CO channel dominates, while the ground state CH(X 2Π)+CO channel is the major channel at higher photon energies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversion dissociation mechanisms, with some evidence for nonstatistical dynamics in the CH(X 2Π)+CO channel. This work yields an improved heat of formation for HCCO, ΔHf,2980=1.83±0.03 eV. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474064
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  • 4
    Electronic Resource
    Electronic Resource
    Ultraviolet photodissociation of the HCCO radical studied by fast radical beam photofragment translational spectroscopy (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6078-6081 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultraviolet photolysis of jet-cooled mass-selected ketenyl radicals has been investigated using the technique of fast radical beam photofragment translational spectroscopy. The C˜2Π(2A″)–X˜2A″ photofragment yield cross section spans 33 400–48 000 cm−1 and exhibits resolved resonances and broad continua. Dissociation produces both ground and excited state CH radicals in association with ground state CO fragments; there is no evidence for H atom elimination. Analysis of the photofragment kinetic energy release spectra yield a value for the C–C bond dissociation energy and heat of formation of HCCO: D0(HC–CO)=3.14±0.03 eV (72.4±0.7 kcal/mol) and ΔHf,00(HCCO)=1.82±0.03 eV (42.0±0.7 kcal/mol). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472446
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  • 5
    Electronic Resource
    Electronic Resource
    Predissociation dynamics of the O2 B 3Σ−u state: Vibrational state dependence of the product fine-structure distribution (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2495-2508 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The predissociation of the O2B 3Σ−u state (υ=0–11) is investigated using fast beam photofragment translational spectroscopy. The energy resolution of the experiment, 7–10 meV, is sufficient to yield the correlated fine structure distribution P(j1,j2) for the two O(3Pj) fragments. These spin–orbit branching ratios depend markedly on the vibrational quantum number, providing detailed insight into a relatively unexplored facet of molecular dissociation dynamics. No less than four repulsive states are expected to mediate the predissociation of the B 3Σ−u state, primarily via spin–orbit coupling, and the couplings among these states at long range (R∼5–7 A(ring)) determine the final spin–orbit distributions P(j1,j2). We have attempted to model these distributions in both the adiabatic and diabatic limits, with neither limit proving very successful. A more phenomenological approach to fitting our data suggests that products with j1=j2=2 result from single transitions between adiabatic potentials at long range, whereas the populations in the other product states are determined by multiple transitions among the repulsive states. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469671
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  • 6
    Electronic Resource
    Electronic Resource
    Spectral and intensity dependence of spatially resolved two-photon conductivity defects on a GaAsP photodiode (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 626-633 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Femtosecond laser excitation through a near-field scanning optical microscope is used to study spatially resolved defects in the two-photon conductivity of a GaAs0.6P0.4 diffusion type photodiode. Two types of defects are observed when the photodiode is excited with femtosecond pulses below the bulk band gap energy. Photocurrent enhancement defects show a higher photocurrent than the surrounding areas and are driven by both one-and two-photon processes. These defects are often correlated with pits in the photodiode surface. Photocurrent depression defects have the normal two-photon power dependence and are not associated with surface pits. Based on the low measured coverage of both defect sites, the performance of GaAsP in auto- and cross correlators will be unaffected in most situations. A deviation in alloy stoichiometry, in which localized areas are rich in arsenic while poor in phosphorus, provides the best explanation for the origin of the enhancement defects. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1329351
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  • 7
    Electronic Resource
    Electronic Resource
    Vibrational predissociation and intramolecular vibrational relaxation in dimethyl-s-tetrazine–argon complexes (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2276-2283 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The one- and two-argon van der Waals complexes of dimethyl tetrazine (DMT) have been investigated by means of laser-induced excitation and dispersed fluorescence spectroscopy. The band shifts D‘0 − D'0 range from −23 to −31 cm−1 for DMT–Ar relative to DMT. The excited state dissociation energy of the DMT–Ar bond was estimated to be 270 ≤ D'0 ≤ 484 cm−1. In all levels with energy greater than the argon dissociation energy, predissociation is very fast. A dissociation rate constant kD = 3 × 109 s−1 has been determined for dissociation from the 6a1 level. This rate constant is an order of magnitude larger than that observed in tetrazine–argon. The dissociation rate constant of DMT–Ar2 from the 6a1 level is approximately 2 × 1010 s−1. Unlike aminotetrazine–Ar2, DMT–Ar2 shows no mode-selective variations in the branching ratios between the DMT and DMT–Ar photoproducts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463119
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  • 8
    Electronic Resource
    Electronic Resource
    Photodissociation spectroscopy and dynamics of the N2O−2 anion (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 5026-5039 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spectroscopy and dissociation dynamics of the N2O−2(C2v) anion have been investigated using the technique of fast ion beam translational spectroscopy. A newly developed pulsed supersonic discharge source is described for the production of internally cold N2O−2. A structured absorption band beginning near 580 nm is observed, and is assigned to the B˜(2A2)←X˜(2B2) transition with the aid of ab initio calculations. Two dissociation channels from the upper state are observed: (1) O−+N2O and (2) NO−+NO. Translational energy and angular distributions are measured for both channels at several excitation energies. The translational energy distribution for channel (1) at 570 nm shows resolved structure corresponding to N2O vibrational excitation. The translational energy distributions for channel (1) are reasonably well described by prior distributions, indicating this channel results from dissociation from the N2O−2 ground electronic state. In contrast, channel (2) appears to result from dissociation on a repulsive excited electronic state. From the translational energy distributions for channel (1), we obtain the bond dissociation energy and heat of formation (at 0 K) for N2O−2: D0(O–N2O)=1.40±0.03 eV and ΔH0f,0(N2O−2)=0.58±0.04 eV. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471132
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