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  • 1
    ISSN: 1520-6882
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7937-7950 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The internal-state and kinetic-energy distributions of the CO photofragments from the 266 and 355 nm photolysis of Mo(CO)6 have been measured under collision-free conditions using vacuum-ultraviolet laser-induced fluorescence. The rotational-state distributions for CO(v‘=0) and (v‘=1) are well represented by Boltzmann distributions with effective rotational "temperatures'' of Tr(v‘=0)=950±70 K and Tr(v‘=1)=935±85 K for 266 nm and Tr(v‘=0)=750±70 K and Tr(v‘=1)=1150±250 K for 355 nm photolysis. The CO(v‘=1/v‘=0) vibrational-state ratios for 266 and 355 nm photolysis are 0.19±0.03 and 0.09±0.02, respectively. The Doppler-broadened CO photofragment line shapes indicate that the translational energy distributions are isotropic and Maxwellian. There is no photolysis-laser wavelength or internal-state dependence to the extracted translational "temperatures.'' The observed energy partitioning and kinetic-energy distributions are inconsistent with an impulsive ejection of a single CO ligand. CO photofragment line shapes for 266 nm photolysis are not consistent with a mechanism involving the repulsive ejection of the first CO ligand, followed by the statistical decomposition of the Mo(CO)5 fragment. While phase-space theories do not predict quantitatively the energy disposal, the photodissociation mechanism appears to be dominated by statistical considerations. The results also suggest that the photodissociation of Mo(CO)6 at 266 and 355 nm involves a common initial "state'' and that similar exit channel effects are operative.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 250-261 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: NH stretching overtone and combination states in HN3 X˜ 1A' were excited by IR–visible double resonance pumping and by direct overtone pumping in the range 6ν1 (17 670 cm−1) to 7ν1 (20 070 cm−1). NH fragments in the a 1Δ and X 3Σ− states were detected by laser induced fluorescence with sub-Doppler resolution to determine branching ratios, correlated fragment rotational state and kinetic energy distributions, and fragment vector correlations. The spin-forbidden triplet channel was accessible to all states excited, while the threshold for the singlet channel was determined to lie in the range 18 190 to 18 755 cm−1. The measured energy release places limits on the HN–NN bond energy, and the heights of barriers to reaction. The barrier in the singlet exit channel is at least 540 cm−1. The singlet channel accessed by 7ν1 dissociation is characterized by a Boltzmann-like NH rotational distribution (〈J NH〉≈3.5), highly excited N2 rotations (〈JN2〉 ≥ 20), and total translational energy release peaked away from zero (〈ET〉≈1350 cm−1). Vector correlations and Λ-doublet propensities indicate that nonplanar dissociation processes influence the NH rotations, but become less important for higher NH rotational states. The principal correlations are a strong positive recoil anisotropy (β≈0.6), a weak positive v–J correlation (βvJ≈0.17), and a JNH-dependent Λ-doublet propensity. A model using parent vibrational motion projected onto fragment rotation is suggested to explain these observations. The triplet channel exhibits similar NH and N2 rotational state distributions, with most of the available energy (substantially greater than in the singlet channel) appearing as fragment kinetic energy.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5919-5922 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Doppler spectroscopy of the 16OH and 18OH products of the 16O(1D)+H218O reaction reveals marked c.m. reactive scattering anisotropy: 16OH scatters in the hemisphere containing the 16O-atom velocity vector. Internal energies of geminate OH fragments are correlated: fragments of high internal energy form with cofragments of low internal energy.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 2324-2338 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of a quantum-state-resolved study of the laser-induced desorption (LID) of NO from Si(111) 7×7 at a surface temperature of 100 K are reported. All aspects of the LID are found to be sensitive to the initial coverage. The coverage dependence indicates that there are two desorption mechanisms, one operative at low coverages that is quenched with increasing NO exposure, and one operative at high coverage. This report characterizes the low coverage channel. Most of the energy in the desorbed NO occurs as vibration and translation, with the rotations substantially cooler. The desorption is selective for production of the ground spin–orbit state. The energy partitioning shows strikingly little change as the desorption-laser wavelength was varied from 1907 to 355 nm. This, coupled with a quantitative study of the yield over the same photon energy range and selective coadsorption experiments, establishes that the desorption is specifically due to an interaction involving photogenerated holes in the rest-atom localized, intrinsic surface state of the 7×7 reconstructed surface. It is suggested that the surface state hole drives the desorption by neutralization of a NO−δ adsorbate.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5344-5345 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The internal state distributions of NO desorbed from a Pt(111) surface by visible and near-visible laser radiation (355, 532, and 1064 nm) were measured by laser-induced fluorescence. Non-Boltzmann rotational state distributions and inverted spin–orbit populations were observed and both were found to be relatively insensitive to the desorption-laser wavelength. It is suggested that the internal state distributions arise from the charge exchange processes occuring during desorption via a short-lived negative-ion resonance intermediate.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1378-1387 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Multiphoton vibrational excitation of deuterated hydrazoic acid, DN3, by a CO2 laser (I=10 GW/cm2) leads to dissociation forming DN in both X 3Σ− (spin forbidden) and a 1Δ (spin allowed) electronic states. Under collisionless conditions, the nascent DN fragments were probed via laser induced fluorescence, to determine initial product state distributions. The DN(X 3Σ−) molecules are formed predominantly in the symmetric F1 and F3 spin–rotation states with little population (≤6%) in the antisymmetric F2 levels. There is no significant population (〈3%) in excited DN(3Σ−) vibrational levels. The distribution of rotational states is Boltzmann-like, characterized by a rotational "temperature'' of about 920 K for the F1, F3 states and 500 K for F2 levels. Doppler profiles showed a large kinetic energy release of about 10 100 cm−1 total in the triplet channel. The DN(1Δ) products are formed preferentially in the symmetric Δ(A'), e-labeled lambda doublet levels: Δ(A')/Δ(A‘)=1.44. The DN(1Δ) is formed with no vibrational excitation (〈2%); the rotational states are populated Boltzmann-like with a rotational "temperature'' of 425 K. Doppler profiles give a total kinetic energy of about 1500 cm−1 in this channel. These observations give information about the distribution of energy in the reactant, the location of the barriers to dissociation, and the geometry of the transition states. Alexander, Werner, and Dagdigian (accompanying article) show that the observed DN(3Σ−) spin- and DN(1Δ) Λ-doublet selectivities reflect the symmetry properties of a planar transition state and that the low degree of DN(3Σ−) rotational and vibrational excitation is also expected from the transition state geometry.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 952-961 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational, vibrational and fine-structure state distributions for the reaction 16O(1D)+ H218O →16OH +18OH, triggered by 266 nm photolysis of O3, have been measured under conditions where less than 1% of the nascent fragments experienced collisions prior to detection. The distributions are qualitatively different than those reported earlier for 266 nm photolysis, which were evidently affected by collisions. The rotational and vibrational state distributions are similar to recent 248 nm photolysis experiments, but with differences attributed to collisional and/or energetic effects in those experiments. The "new'' 16OH is formed with vibrational populations in the ratio 0.39(v=0):0.29(v=1):0.3(v≥2). Gaussian rotational energy distributions peaked near N=12 give average rotational energies of 〈Erot〉 = 3440 and 2780 cm−1 for 16OH v=0 and v=1, respectively. The "old'' 18OH is much colder with vibrational populations 0.94(v=0):0.06(v=1) and a 18OH v=0 Gaussian rotational energy distribution characterized by 〈Erot〉 = 1920 cm−1.There is negligible rotational alignment of the 16OH fragments [βμJ' = β20(02) = 5A(0)2/4 = 0.06± 0.09], which is significantly less than expected for fragment rotations aligned with respect to the O+H2O relative velocity vector. The spin-orbit propensities deviate slightly from the statistical expectation and are characterized by [F2,N]/[F1,N]=(0.89±0.06) ×N/(N + 1). The Λ-doublet distributions for 18OH (all N) and 16OH (low-to-moderate N) fragments conform to an unconstrained-dynamics prior distribution. A slight propensity for Π(A') in excess of this expectation is seen for the 16OH (high N) fragments. These new results are discussed in terms of possible insertion and abstraction mechanisms for the reaction.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1824-1831 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The NO(v,J)-fragment population distributions and recoil energies were measured for the vibrational predissociation of NO⋅HF following excitation of the H–F stretch. Most of the available energy appears in NO vibration and/or HF rotation. There is little recoil momentum. All NO(v,J) fragments are formed with near δ-function recoil kinetic energies, correlated with HF fragments in a single rotational level. Two dissociation channels proceed with similar probability to produce NO(v=0) and NO(v=1). Only two rotational levels of HF are populated. One of the HF rotational states is correlated exclusively with NO(v=0) fragments, the other is paired with NO(v=1) fragments. Constraints on fragment angular momenta as well as energetics appear important for the dissociation dynamics. The presence of significant amounts of vibrationally excited NO fragments, in the absence of observable spectroscopic perturbations, implies that intramolecular vibrational redistribution proceeds as the dimer dissociates. The data support two possibilities for the NO⋅HF dimer bond energy: (1) D0=448±5 cm−1 with coincident pairs of fragments NO(v=0)+HF(J=12) and NO(v=1)+HF(J=8); (2) D0=1769±10 cm−1 with JHF=9 and 2, respectively.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1966-1976 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Picosecond and nanosecond lasers and pulsed molecular beam techniques have been used to measure the infrared photodissociation spectra, the product state distributions, and the predissociation lifetimes of vibrationally excited nitric oxide dimer (NO)2 . Results for the ν1 (v=1) symmetric NO stretching mode and the ν4 (v=1) antisymmetric NO stretching mode are presented. Predissociation lifetimes are determined by time-resolved laser induced fluorescence probing of the NO monomer product appearance rate. A dramatic mode dependence of the predissociation lifetimes is observed with the higher energy ν1 mode decaying in approximately 1 ns, and the lower energy ν4 mode decaying in approximately 40 ps. The mode dependence is independent of which product state is probed. The product state distributions show that 75% to 80% of the available energy is channeled into relative translational energy of the fragments for both modes. Rotational state distributions are Boltzmann-like with temperatures ranging from 71 to 112 K depending on both the initially excited mode and on the NO product spin–orbit state. Predissociation from ν1 produces NO fragments in the 2Π1/2 and 2 Π3/2 states with equal probability. Predissociation from ν4 exhibits a propensity for producing the lower energy 2 Π1/2 spin–orbit state. The observations are discussed in terms of various vibrational predissociation mechanisms, including vibrational potential coupling and electronically nonadiabatic predissociation.
    Type of Medium: Electronic Resource
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