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15 Dec 1989

Volume 91, Issue 12, pp. 7319-8003

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Vibrational spectroscopy of the hydrated hydronium cluster ions H3O+⋅(H2O)n (n=1, 2, 3)

L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee

J. Chem. Phys. 91, 7319 (1989); http://dx.doi.org/10.1063/1.457305 (12 pages) | Cited 194 times

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The gas phase infrared spectra of the hydrated hydronium cluster ions H3O+⋅(H2O)n(n=1, 2, 3) have been observed from 3550 to 3800 cm−1. The new spectroscopic method developed for this study is a two color laser scheme consisting of a tunable cw infrared laser with 0.5 cm−1 resolution used to excite the O–H stretching vibrations and a cw CO2 laser that dissociates the vibrationally excited cluster ion through a multiphoton process. The apparatus is a tandem mass spectrometer with a radio frequency ion trap that utilizes the following scheme: the cluster ion to be studied is first mass selected; spectroscopic interrogation then occurs in the radio frequency ion trap; finally, a fragment ion is selected and detected using ion counting techniques. The vibrational spectra obtained in this manner are compared with that taken previously using a weakly bound H2 ‘‘messenger.’’ A spectrum of H7 O+3 taken using a neon messenger is also presented. Ab initio structure and frequency predictions by Remington and Schaefer are compared with the experimental results.
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36.40.-c Atomic and molecular clusters
33.20.Ea Infrared spectra

The decay of triplet pyrazine in supersonic jets

Ofer Sneh, Dana Dünn‐Kittenplon, and Ori Cheshnovsky

J. Chem. Phys. 91, 7331 (1989); http://dx.doi.org/10.1063/1.457306 (9 pages) | Cited 31 times

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The decay rates of optically excited triplet states of pyrazine in supersonic expansion were measured by using three different methods. The excess energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecule was explored. Decay rates between 7×102 –2.5×104 s−1 were found in the 1500 cm−1 range of excess vibrational energy from the origin of the T1 state. The decay rates are free of mode specificity and rotational effects. The pure radiative lifetime in the measured range is rovibronic independent. The results support a model which suggests that certain vibrational modes, those which undergo large frequency changes in the excited state, control the strong vibrational energy dependence of the T1 →S0 intersystem crossing of pyrazine.
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33.50.Hv Radiationless transitions, quenching
33.70.Fd Absolute and relative line and band intensities
33.80.Be Level crossing and optical pumping

Electron scattering cross sections and negative ion states of silane and halide derivatives of silane

Hai‐Xing Wan, John H. Moore, and John A. Tossell

J. Chem. Phys. 91, 7340 (1989); http://dx.doi.org/10.1063/1.457307 (8 pages) | Cited 41 times

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The interaction of low‐energy electrons with silane and its halogenated derivatives is a fundamental step in many plasma chemical processes. The total scattering cross‐section for electrons in the 0.2–12 eV range are observed to range up to 10−14 cm2. Scattering resonances are assigned to temporary negative ion states on the basis of correlations with analogous inner shell excitation resonances.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
34.80.Gs Molecular excitation and ionization
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)

Water dimer Coriolis resonances and Stark effects

T. A. Hu and T. R. Dyke

J. Chem. Phys. 91, 7348 (1989); http://dx.doi.org/10.1063/1.457308 (7 pages) | Cited 14 times

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E±2 symmetry states for K=0, 1 of (H2O)2 are found to be perturbed and a Coriolis resonance model is used to give a satisfactory treatment of the observations. The effective b‐type Coriolis coupling constant ζ is 1409.4 MHz, implying appreciable vibrational angular momentum since ζ/2C≊1/8. The spacing Δ between the upper K=0 levels and the lower K=1 levels is 10 719 MHz. The rotational constants from the deperturbation analysis are now found to be in substantially better agreement with those from tunneling states of other symmetries and with the structure of the dimer. The Stark effects of the perturbed states have been analyzed. The a component of the electric dipole moment is well determined; the c component appears to be small, but the results are not completely consistent.
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36.40.-c Atomic and molecular clusters
33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.57.+c Magneto-optical and electro-optical spectra and effects

Tunable vacuum ultraviolet laser spectroscopy of the D(O+u) ion‐pair state of jet‐cooled I2

M. Bartels, R. J. Donovan, A. J. Holmes, P. R. R. Langridge‐Smith, M. A. MacDonald, and T. Ridley

J. Chem. Phys. 91, 7355 (1989); http://dx.doi.org/10.1063/1.457258 (6 pages) | Cited 7 times

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High‐resolution vacuum ultraviolet fluorescence excitation spectra of the D(O+u) ion‐pair state of I2 have been obtained using a pulsed supersonic jet. Tunable vacuum ultraviolet radiation in the region 184–200 nm was produced by Raman shifting (fifth and sixth anti‐Stokes) the output from a tunable excimer pumped dye laser. Vibrational constants valid up to v′=201 in the D(O+u) state have been determined.
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33.20.Ni Vacuum ultraviolet spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants

Conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 2‐fluoropropenoyl fluoride

J. R. Durig, Ai‐Ying Wang, T. S. Little, P. A. Brletic, and J. R. Bucenell

J. Chem. Phys. 91, 7361 (1989); http://dx.doi.org/10.1063/1.457259 (13 pages) | Cited 4 times

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The far infrared spectrum of gaseous 2‐fluoropropenoyl fluoride CH2CFCFO, has been recorded at a resolution of 0.10 cm−1 in the 350–35 cm−1 region. The asymmetric torsional fundamental of the more stable s‐trans (two double bonds oriented trans to one another) and the high energy s‐cis conformations have been observed at 84.1 and 68.5 cm−1, respectively, each with excited states falling to lower frequencies. From these data, the asymmetric torsional potential function governing internal rotation about the C–C bond has been determined. The potential coefficients are V1=−201±4, V2=2106±16, V3=382±6, V4=−41±5, and V5=−74±3 cm−1. The s‐trans to s‐cis and s‐cis to s‐trans barriers have been determined to be 2232 and 2124 cm−1, respectively, with an enthalpy difference between the conformations of 108±26 cm−1 (309±74 cal/mol). From variable temperature studies of the Raman spectrum, the conformational enthalpy difference has been determined to be 187±39 cm−1 (535±112 cal/mol) and 370±82 cm−1 (1058±234 cal/mol) for the gas and liquid, respectively. A complete assignment of the vibrational fundamentals observed from the infrared (3500–50 cm−1) spectra of the gas and solid and the Raman (3200–10 cm−1) spectra of all three physical states is proposed. All of these data are compared to the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing both the 3‐21G and 6‐31G∗ basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.
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33.20.Ea Infrared spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.20.Tp Vibrational analysis
33.20.Fb Raman and Rayleigh spectra (including optical scattering)

Singular value analysis and reconstruction of photon correlation data equidistant in time

R. Finsy, P. de Groen, L. Deriemaeker, and M. van Laethem

J. Chem. Phys. 91, 7374 (1989); http://dx.doi.org/10.1063/1.457260 (10 pages) | Cited 6 times

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The application of the algorithm of Zeiger and McEwen to the analysis of noisy photon correlation data is investigated. For the particular case where the data are sampled at equidistant time intervals a complete solution is given allowing reliable reconstruction of the spectrum of exponential decay rates without any a priori knowledge. A particular attractive feature of the method is that the singular value analysis of the Hankel matrix of autocorrelation functions offers a practical criterion for the decomposition of the data into a signal and a noise part. Some tests of the method are illustrated with experiments on monodisperse latices, gold sols, and binary mixtures of monodisperse latices. In the latter case comparable and even better results are obtained in significantly shorter computing time when compared to an analysis with Contin and the maximum entropy method. Since the present method does not require any a priori parameter setting, it is also complementary to these methods.
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82.70.Dd Colloids
78.35.+c Brillouin and Rayleigh scattering; other light scattering
66.10.C- Diffusion and thermal diffusion

The A 3Σ−–X 3Π transition of the SiC radical

C. R. Brazier, L. C. O’Brien, and P. F. Bernath

J. Chem. Phys. 91, 7384 (1989); http://dx.doi.org/10.1063/1.457261 (3 pages) | Cited 14 times

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The 0–0 band of the A 3 Σ−–X 3 Π system of SiC, analogous to the Ballik–Ramsay system of C2, has been observed in emission near 4500 cm−1. The internuclear separations (r0 ) were found to be 1.813 56 and 1.721 87 Å for the A 3 Σ− and X 3 Π states, respectively.
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33.20.Kf Visible spectra
33.20.Ea Infrared spectra
33.15.Dj Interatomic distances and angles
33.20.Sn Rotational analysis

Proton chemical shifts in some hydrogen bonded solids and a correlation with bond lengths

R. Kaliaperumal, R. E. J. Sears, Q. W. Ni, and J. E. Furst

J. Chem. Phys. 91, 7387 (1989); http://dx.doi.org/10.1063/1.457262 (5 pages) | Cited 5 times

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Using multipulse techniques, principal values of the proton nuclear magnetic resonance (NMR) chemical shift tensors were measured at room temperature for the monoclinic form of oxalic acid, potassium hydrogen oxalate, and potassium hydroxide. The isotropic and perpendicular shifts of the first two compounds, which are of medium hydrogen bond strength, were found to fit the linear correlation with O⋅⋅⋅O distance, RO⋅O, established by Rohlfing, Allen, and Ditchfield for medium and strongly O–H⋅⋅⋅O hydrogen bonded solids. Although KOH is very weakly hydrogen bonded, the shifts were also found to conform to the correlation, at least as well as does the other data, thus extending this to weakly hydrogen bonded solids. An empirical correlation of the isotropic and perpendicular shifts with exp(−RO⋅O/ρ), where RO⋅O is in Å and ρ is 0.94 Å, is given here which has better agreement with the data and has an interpretation in terms of a simple ionic model of an O–H⋅⋅⋅O bond.
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76.60.Cq Chemical and Knight shifts
76.60.Es Relaxation effects

Carbon‐13 nuclear magnetic resonance of ferroelectric liquid crystals with off‐magic‐angle spinning

Chi‐Duen Poon and B. M. Fung

J. Chem. Phys. 91, 7392 (1989); http://dx.doi.org/10.1063/1.457263 (7 pages) | Cited 22 times

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The orientational ordering of three ferroelectric liquid crystals 4′‐(3‐methyl‐2‐chloropentanoyloxy)‐4‐alkyloxybiphenyls has been studied by 2D carbon‐13 nuclear magnetic resonance (NMR) with variable angle sample spinning. The three homologs have hexyloxy (C6), heptyloxy (C7), and octyloxy (C8) chains, respectively, at the 4 position. They exhibit smectic A (SA) and chiral smectic C (S@B|C; ferroelectric) phases and have unusually large spontaneous polarization of the electric dipoles. In the SA phase, macroscopic alignment of the molecular director along the spinning axis can be achieved by cooling the sample rapidly from the isotropic liquid. For the S∗C phase, the molecular alignment is different for the three compounds. At a magnetic field of 7.05 T, the helical structures of C6 and C8 unwind and the molecular directors align along the spinning axis at a spinning rate of 1 kHz. However, under the same conditions, the helical structure of C7 is retained and its carbon‐13 NMR spectrum shows a partial powder pattern. On the other hand, the racemate of C7 has a normal smectic C (SC) phase without a helical structure, and the molecular director can be easily aligned for the NMR study. The racemate of C8 was also synthesized and studied for comparison. The 2D NMR technique used was separated local field (SLF) spectroscopy with an efficient proton–proton dipolar decoupling sequence BLEW‐48 in the evolution period. The carbon–proton dipolar coupling constants were determined and the order parameters for different molecular segments of the liquid crystal were calculated. The carbon‐13 chemical shifts were also measured as functions of temperature and the data were used to probe the temperature dependence of order parameters. The results show that the C–H bond at the first chiral center in these compounds has an unusually large negative order parameter. This could be due to restricted rotation of the molecular segment at the chiral center, which would be related to the high spontaneous polarization of these compounds.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
76.60.Es Relaxation effects

A study of some Rydberg states of CO2 by (3+1) multiphoton ionization spectroscopy

Ming Wu and Philip M. Johnson

J. Chem. Phys. 91, 7399 (1989); http://dx.doi.org/10.1063/1.457264 (9 pages) | Cited 20 times

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The MPI spectra of CO2 were recorded in the range 275 to 338 nm in both a conventional MPI cell and a molecular beam apparatus. The polarization ratio of each MPI peak was measured in order to assign the symmetry of excited states. The 3pσu 1Πu and 3pσu 1Πu110 Rydberg states were observed, while the 3pπu 1Σ+u Rydberg state was missing, and a new Rydberg state, 3pπu 1Δu was observed at 333.03 and 333.70 nm. In the nf (n=4,5) Rydberg series, evidence was found of interaction with other nearby Rydberg states. A mass selected pure CO+2 ion signal was measured for the 3pπu 1Δu state, but both CO+ and CO+2 ion signals were detected for the other Rydberg states. Predissociation and fragmentation in MPI processes are discussed. Lower limit lifetimes for the observed Rydberg states of CO2 are given.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Absorption spectra from high vibrational levels of He2

R. L. Brooks, J. L. Hunt, and D. W. Tokaryk

J. Chem. Phys. 91, 7408 (1989); http://dx.doi.org/10.1063/1.457265 (7 pages) | Cited 5 times

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Absorption spectra of dense helium gas at cryogenic temperatures has been acquired while the sample was irradiated using a 6.5 MeV proton beam. By chopping the proton beam, rather than the source lamp, we were able to achieve one part in 104 spectral sensitivity. The spectra showed six new bands in 4He2 and three in 3He2. These have been identified as transitions between high‐lying vibrational levels, with the strongest originating on the highest bound level of the a 3Σ+u potential. The temperature and pressure dependence of these features, as compared to low‐lying molecular and atomic features, offers some insight into the reaction dynamics of this fundamental system.
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36.40.-c Atomic and molecular clusters
33.20.Ea Infrared spectra
34.50.Gb Electronic excitation and ionization of molecules

Femtosecond real‐time probing of reactions. IV. The reactions of alkali halides

Todd S. Rose, Mark J. Rosker, and Ahmed H. Zewail

J. Chem. Phys. 91, 7415 (1989); http://dx.doi.org/10.1063/1.457266 (22 pages) | Cited 121 times

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The photodissociation dynamics of some alkali halides are explored via the method of femtosecond transition‐state spectroscopy (FTS). The alkali halide dissociation reaction is influenced by the interaction between the covalent and the ground state ionic potential energy surfaces (PES), which cross at a certain internuclear separation. Depending upon the adiabaticity of the PES, the dissociating fragments may be trapped in a well formed by the avoided crossing of these surfaces. Here, we detail the FTS results of this class of reactions, with particular focus on the reaction of sodium iodide: NaI∗→[Na‐‐‐I]°∗ →Na+I. As in our first report [T. S. Rose, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 88, 6672 (1988)], we observe the dynamical motion of the wave packet along the reaction coordinate and the crossing between the covalent and ionic surfaces. The studies presented here characterize the effects of various experimental parameters, including pump and probe wavelengths, on the dynamics of the dissociation and its detection. Comparisons of the results with classical and quantum mechanical calculations are also presented.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
78.47.-p Spectroscopy of solid state dynamics
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Femtosecond real‐time probing of reactions. V. The reaction of IHgI

M. Dantus, R. M. Bowman, M. Gruebele, and A. H. Zewail

J. Chem. Phys. 91, 7437 (1989); http://dx.doi.org/10.1063/1.457267 (14 pages) | Cited 77 times

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The dissociation reaction of HgI2 is examined experimentally using femtosecond transition‐state spectroscopy (FTS). The reaction involves symmetric and antisymmetric coordinates and the transition‐state is well‐defined: IHgI∗→[IHgI]‡@B|QS′Qa′q→HgI+I. FTS is developed for this class of ABA‐type reactions and recurrences are observed for the vibrating fragments (symmetric coordinate) along the reaction coordinate (antisymmetric coordinate). The translational motion is also observed as a ‘‘delay time’’ of the free fragments. Analysis of our FTS results indicates that the reaction wave packet proceeds through two pathways, yielding either I(2P3/2) or I∗(2P1/2) as one of the final products. Dissociation into these two pathways leads to HgI fragments with different vibrational energy, resulting in distinct trajectories. Hence, oscillatory behaviors of different periods in the FTS transients are observed depending on the channel probed (∼300 fs to ∼1 ps). These results are analyzed using the standard FTS description, and by classical trajectory calculations performed on model potentials which include the two degrees of freedom of the reaction. Quantum calculations of the expected fluorescence of the fragment are also performed and are in excellent agreement with experiments.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
78.47.-p Spectroscopy of solid state dynamics
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Photoionization spectra of CsnOm⋅(CO)x clusters

N. Malinowski and T. P. Martin

J. Chem. Phys. 91, 7451 (1989); http://dx.doi.org/10.1063/1.457268 (4 pages)

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One‐photon ionization spectra are reported for 82 clusters with the composition CsnOm⋅CO and CsnOm⋅2CO. These spectra are presented in the form of an energy distribution of ionizing transitions. Carbon monoxide apprears to react with CsnO clusters to form CO2−2.
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36.40.-c Atomic and molecular clusters
33.80.Eh Autoionization, photoionization, and photodetachment

One‐ and two‐photon‐resonant multiphoton ionization spectra of CS2 from 45 500 to 48 100 cm−1

D. J. Donaldson

J. Chem. Phys. 91, 7455 (1989); http://dx.doi.org/10.1063/1.457269 (6 pages) | Cited 16 times

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One‐ and two‐photon‐resonant multiphoton ionization spectra of the low energy region of the intense 1 B2 state of room temperature CS2 are reported. A comparison of these with one another, as well as with absorption spectra, yields a frequency of 46 247 cm−1 for the electronic origin of the transition. Most of the intense features in this region can be assigned as transitions to excited state bending levels. The frequency of this mode in the excited state is found to be 429 cm−1, in excellent agreement with results from the higher energy region of the spectrum. Two‐photon excitation is used to observe, for the first time, the excited state antisymmetric stretch mode. Its frequency is 1567 cm−1, close to the corresponding ground state value.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.20.Ni Vacuum ultraviolet spectra
36.40.-c Atomic and molecular clusters

A full calculation of multiconfiguration interaction effects up to 120 000 cm−1 (15 eV) on the ground configuration state levels of PrCl3. Zeeman effect interpretation

D. Garcia and M. Faucher

J. Chem. Phys. 91, 7461 (1989); http://dx.doi.org/10.1063/1.457270 (6 pages) | Cited 24 times

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A previous paper gave the results of a full calculation involving free ion and crystal–field interaction within the 4 f2 and 4 f15d1 configurations. Presently, the calculation is extended to 4 f16s1 and 4 f16p1 as well. As a result, the usual experimental/calculated discrepancy of the 1D2 multiplet is completely eliminated. A complete Hamiltonian, including the magnetic contribution, is utilized to reproduce the Zeeman effect in PrCl3.
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71.70.Ch Crystal and ligand fields
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
78.40.Ha Other nonmetallic inorganics

Ladder approximation for three‐ and four‐particle correlation functions

J. Blawzdziewicz, B. Cichocki, and G. Szamel

J. Chem. Phys. 91, 7467 (1989); http://dx.doi.org/10.1063/1.457271 (10 pages) | Cited 11 times

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An example of an application of a recently developed generalized Ornstein–Zernike formalism to the numerical evaluation of equilibrium three‐ and four‐particle correlation functions is given. Using a simple closure approximation leading to the ladder approximation we have numerically evaluated dipole–dipole‐interaction correlation functions for a polarizable nonpolar hard‐sphere fluid. These functions depend on the three‐ and four‐particle correlation functions and describe a correction to the Clausius–Mossoti formula for the dielectric constant and an integrated intensity measured in depolarized light scattering experiments. Qualitative agreement with computer simulation data was found for a wide range of densities up to the fluid–solid phase transition. For high densities the ladder approximation yeilds much better results than the Kirkwood superposition approximation, which becomes useless in this context at liquid‐state densities.
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61.20.Ne Structure of simple liquids
77.22.Ch Permittivity (dielectric function)
78.35.+c Brillouin and Rayleigh scattering; other light scattering

A theoretical study of transition state spectroscopy: Laser dressed potential energy surface and surface hopping trajectory calculations on K+NaCl and Na+KCl

Koichi Yamashita and Keiji Morokuma

J. Chem. Phys. 91, 7477 (1989); http://dx.doi.org/10.1063/1.457272 (13 pages) | Cited 7 times

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Spectroscopy during the chemical reactions, K+NaCl and its reverse, has been studied by surface hopping trajectory calculations. Laser absorption and emission processes are modeled as the transitions between the laser‐dressed ground and excited state potential energy surfaces (PESs), which are constructed from ab initio potential energy and transition dipole functions. The theoretical excitation spectrum measured by Na–D emission intensity as a function of laser wavelength agrees qualitatively with the experiment by Maguire et al. [J. Chem. Phys. 85, 844 (1986)]. The excitation spectrum is found to be very different from the absorption spectrum, because only a small portion of excited trajectories reach the Na∗ product due to the endothermicity of the excited state reaction. Therefore the excitation spectrum reflects only the excited state dynamics but not the transition state spectroscopy. The laser wavelength dependence of the spectra is well explained by a characteristic shift of the crossing seam accompanied with changes in laser wavelength. We have also predicted the absorption and excitation spectra for the reverse reaction and found that in this case the intensity of the product emission as a function of laser wavelength reflects to a large extent the true transition state spectroscopy.
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82.20.Kh Potential energy surfaces for chemical reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Classical mechanics of intramolecular vibrational energy flow in benzene. V. Effect of zero‐point energy motion

Da‐hong Lu and William L. Hase

J. Chem. Phys. 91, 7490 (1989); http://dx.doi.org/10.1063/1.457273 (8 pages) | Cited 38 times

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Zero‐point energy excitation has a profound effect on the relaxation of benzene CH and CD overtone states. Only adding a fraction of the zero‐point energy for each normal mode in the initial conditions results in smaller overtone relaxation rates. If no zero‐point energy is added to C6H6, the n=3 and 5 CH overtones do not relax within 1 ps. Adding zero‐point energy to different types of normal modes has nonequivalent effects on overtone relaxation. Zero‐point excitation of modes with HCC bend character is particularly effective in enhancing relaxation of the overtones.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.20.Tp Vibrational analysis

Photodissociation dynamics of acetone at 193 nm: Photofragment internal and translational energy distributions

Karen A. Trentelman, Scott H. Kable, David B. Moss, and Paul L. Houston

J. Chem. Phys. 91, 7498 (1989); http://dx.doi.org/10.1063/1.457274 (16 pages) | Cited 87 times

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The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum‐ultraviolet laser‐induced fluorescence was used to probe the CO fragment, and multiphoton ionization time‐of‐flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO@B: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K; CH3@B: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is divided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.20.Rp State to state energy transfer

The H+D2 reaction: Quantum‐state distributions at collision energies of 1.3 and 0.55 eV

Klaus‐Dieter Rinnen, Dahv A. V. Kliner, and Richard N. Zare

J. Chem. Phys. 91, 7514 (1989); http://dx.doi.org/10.1063/1.457275 (16 pages) | Cited 45 times

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We have studied the H+D2 →HD+D reaction using thermal D2 (∼298 K) and translationally hot hydrogen atoms. Photolysis of HI at 266 nm generates H atoms with center‐of‐mass collision energies of 1.3 and 0.55 eV, both of which are above the classical reaction barrier of 0.42 eV. The rovibrational population distribution of the molecular product is measured by (2+1) resonance‐enhanced multiphoton ionization (REMPI). The populations of all energetically accessible HD levels are measured. Specifically, we observe HD(v=0, J=0–15), HD(v=1, J=0–12), and HD(v=2, J=0–8). Of the available energy, 73% is partitioned into product translation, 18% into HD rotation, and 9% into HD vibration. Both the rotational and vibrational distributions are in remarkably good agreement with quasiclassical trajectory (QCT) calculations, though the calculated rotational distributions are slightly too hot. We discuss factors contributing to the success of the QCT calculations.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Rp State to state energy transfer

Formation of thermodynamically stable dications in the gas phase by thermal ion–molecule reactions: Nb2+ with small alkanes

James R. Gord, Ben S. Freiser, and Steven W. Buckner

J. Chem. Phys. 91, 7530 (1989); http://dx.doi.org/10.1063/1.457276 (7 pages) | Cited 15 times

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The gas‐phase reactions of Nb2+ with small alkanes at thermal energies are reported. For methane and ethane, dehydrogenation is a prominent reaction pathway. For propane and butane, charge transfer is virtually the only reaction pathway observed (>99%). NbCH2+2 and NbC2H2+2 formed in the reactions of Nb2+ with methane and ethane are thermodynamically stable with D(Nb2+–CH2)=197±10 kcal/mol, D(Nb+–CH+2)=107±10 kcal/mol, D(Nb2+–C2H2)≥74 kcal/mol, and D(Nb+–C2H+2)≥7 kcal/mol. The stability of these ions is most likely due to the charge‐stabilizing effect of the metal center. Collision‐induced dissociation of these ions results in charge‐splitting reactions as well as reactions in which both charges remain on the metal center. Hydride transfer is observed to be competitive in the primary reactions of Nb2+ with alkanes. The hydride‐ and charge‐transfer results are in qualitative agreement with a simple curve‐crossing model.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.60.Cx Enthalpies of combustion, reaction, and formation

A collocation approach for quantum scattering based on the S‐matrix version of the Kohn variational principle

Weitao Yang, Andrew C. Peet, and William H. Miller

J. Chem. Phys. 91, 7537 (1989); http://dx.doi.org/10.1063/1.457277 (6 pages) | Cited 10 times

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A collocation approach to quantum scattering is presented. The method is based on the S‐matrix version of the Kohn variational principle with a different linear expansion used for the two wave functions—one is a linear combination of basis functions and the other is a pointwise representation with proper asymptotic conditions imposed. The resulting equations are similar in structure to the usual version of the Kohn variational principle, however, in the present approach there are no integrals between the square integrable (L2) basis functions. In addition, the method does not require the knowledge of quadrature weights associated with the collocation points as was the case in a previous pointwise method for quantum scattering. This property means that the method is readily applicable to reactive scattering problems which use different sets of coordinates for reactants and products. Appliction to a simple inelastic test problem (collinear He–H2 vibrationally inelastic scattering) shows the accuracy of the approach to be comparable to that of the usual variatinal form of the S‐matrix Kohn method.
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34.50.Ez Rotational and vibrational energy transfer

Photodissociation of NCO(X 2Π) radicals

X. Liu and R. D. Coombe

J. Chem. Phys. 91, 7543 (1989); http://dx.doi.org/10.1063/1.457654 (7 pages) | Cited 11 times

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Photolysis of NCO(X 2Π) at 193 nm leads to the production of CN(X 2Σ+ )+O(3 P). The CN(X) was detected by monitoring the CN X 2Σ+ →B 2Σ+ laser‐induced fluorescence excitation spectrum. A spectral simulation calculation was used to obtain vibrational and rotational population distributions in the CN fragments. The fractional vibrational populations obtained in this manner are 0.43, 0.32, 0.21, and 0.04 for v=0, v=1, v=2, and v=3, respectively. The near‐nascent rotational distributions in the different CN(X) vibrational levels are not characterized by a Boltzmann rotational temperature, but rather are bi‐modal with maxima at both high and low N. The high N rotational excitation of the CN fragment suggests the existence of an excited dissociative state of NCO which is bent. A lower limit for the heat of formation of NCO, ΔHf >37 kcal/mol, is derived from the upper limit on the internal excitation of the CN(X) fragments. The spectrum of prompt emission produced by the 193 nm photolysis indicates the existence of a bound excited state of NCO which radiatively relaxes to the A 2Σ+ state. From the spectrum, this state is thought to be linear.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.20.Rp State to state energy transfer
82.33.Vx Reactions in flames, combustion, and explosions
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