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15 Dec 1982

Volume 77, Issue 12, pp. 5863-6350

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Second‐order cascading in third‐order nonlinear optical processes

Gerald R. Meredith

J. Chem. Phys. 77, 5863 (1982); http://dx.doi.org/10.1063/1.443859 (9 pages) | Cited 21 times

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Because cascaded second‐order processes make substantial qualitative and quanitative differences to the results of third‐order nonlinear optical experiments, a formalism for their treatment is presented. The symmetry dictates concerning the occurrence and relationships of magnitudes of cascading are tabulated for the higher symmetry crystal classes. Angular momentum considerations are applied to the situations allowing circularly polarized light waves.
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63.10.+a General theory

The EPR spectrum of Fe(CO)−5 in a single crystal of Cr(CO)6

S. A. Fairhurst, J. R. Morton, and K. F. Preston

J. Chem. Phys. 77, 5872 (1982); http://dx.doi.org/10.1063/1.443860 (4 pages) | Cited 4 times

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An anisotropic EPR spectrum observed at 20 K in γ‐irradiated single crystals of Cr(CO)6 doped with Fe(CO)5 is ascribed to the radical anion Fe(CO)−5. Measurements of the g‐, 57Fe‐, and 13C‐hyperfine tensors clearly show that the species is an acyl radical Fe(CO)−4CO in which the unpaired spin is principally located in Fe 3dz2 or 3dx2−y2 and in an sp2 hybrid of the unique carbon atom C(1). The Fe–C(1)−O(1) moiety is bent and its plane lies close to the ac crystallographic plane of Cr(CO)6.
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82.50.-m Photochemistry
76.30.Rn Free radicals

High orbital angular momentum states in H2 and D2

G. Herzberg and Ch. Jungen

J. Chem. Phys. 77, 5876 (1982); http://dx.doi.org/10.1063/1.443861 (9 pages) | Cited 59 times

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A characteristic group of lines observed in the infrared emission spectrum of H2 has been identified as representing the transitions between the 5g and the 4f group of levels. Because of nearly complete uncoupling of l from the molecular axis the appearance of this group of lines is very different from a normal band spectrum. An analysis was possible on the basis of ab initio computations of the spectrum using the knowledge of the vibrational levels, quadrupole moment, and the polarizability of the H+2 core. The validity of the assignments was further corroborated by agreement of the predicted spectra of D2 with the observed spectra. In addition, similar transitions corresponding to 4f–3d have been observed.
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33.20.Ea Infrared spectra
31.50.Df Potential energy surfaces for excited electronic states

The microwave spectrum, dipole moment, and barrier to internal rotation of 3‐methyl‐2‐butenenitrile

J. R. Durig, Y. S. Li, and J. J. Rizzolo@f@f

J. Chem. Phys. 77, 5885 (1982); http://dx.doi.org/10.1063/1.443862 (6 pages) | Cited 2 times

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The microwave spectrum of 3‐methyl‐2‐butenenitrile was investigated in the frequency range of 18.0 to 39.0 GHz, and the spectrum which has been obtained is characteristic of that of a nearly prolate rotor. Only a‐type transitions were observed and R‐branch assignments have been made for the ground vibrational state, as well as three vibrationally excited states. The rotational constants in the ground vibrational state were determined to be: A=8379.36±0.66, B=2184.87±0.01, and C=1769.96±0.01 MHz. With seven reasonably assumed structural parameters, a diagnostic least‐squares adjustment was used to fit the three rotational constants, and the five remaining structural parameters were obtained: cis, r(C–CH3)=1.503±0.024 Å; trans, r(C–CH3)=1.504±0.029 Å; cis, ∢C=C–CH3=120.6±0.1°; trans, ∢C=C–CH3=120.3±0.1°; and ∢C=C–CN=124.4±0.1°. The dipole moment components were determined from the Stark effect to be ‖μa‖=4.59±0.11 D, ‖μb‖=0.40±0.07 D, and ‖μt‖=4.61±0.13 D. Splittings due to the coupling of internal rotation of the cis methyl group with the overall rotation of the molecule were observed in one of the vibrational satellites, and a subsequent analysis yielded a barrier of 790 cm−1 (2.26 kcal/mol) for the cis methyl group. All of these results are compared to similar quantities in some corresponding molecules.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Structures and reactions of radical cations of some prototype alkanes in low temperature solids as studied by ESR spectroscopy

Kazumi Toriyama, Keichi Nunome, and Machio Iwasaki

J. Chem. Phys. 77, 5891 (1982); http://dx.doi.org/10.1063/1.443863 (22 pages) | Cited 76 times

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The structures and reactions of radical cations of prototype alkanes such as ethane, propane, isobutane, neopentane, etc. produced in SF6 and in other halocarbon matrices irradiated at 4 K have been extensively studied by ESR. The geometrical structures as well as the unpaired electron orbitals of these cations are unequivocally characterized based on the simple symmetry considerations as well as the INDO MO calculations. The ESR hyperfine coupling constants determined for these cations as well as their deuterated homologs give conclusive evidence for the interpretations. The unpaired electron in linear alkane cations is delocalized over the in‐plane C–H and C–C σ bonds forming delocalized σ radicals, whereas that in highly branched alkane cations is more confined to one of the C–C bonds forming localized σ radicals. The H1s spin density, giving a large hyperfine coupling in the linear alkane cations, is largely contributed from a direct participation of the C–H bond in the unpaired electron orbital, whereas that in the highly branched alkane cations is due to mainly hyperconjugation. The alkane cations undergo two types of reactions: one is deprotonation to form neutral alkyl radicals in SF6 and in CFCl2CF2Cl and the other is the H2 and CH4 elimination to form olefinic π cations in CFCl3. Based on the matrix effects on the structure of propane and isobutane cations, the site preference in forming alkyl radicals, and other observations, it is concluded that the alkane cations undergo reactions through the breakage of the bond in which the unpaired electron is highly populated.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
76.30.Rn Free radicals
33.35.+r Electron resonance and relaxation

Concentration dependence of the self‐diffusion coefficient of hard, spherical particles measured with photon correlation spectroscopy

M. M. Kops‐Werkhoven, C. Pathmamanoharan, A. Vrij@f@f, and H. M. Fijnaut

J. Chem. Phys. 77, 5913 (1982); http://dx.doi.org/10.1063/1.443864 (10 pages) | Cited 31 times

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The concentration dependence of the self‐diffusion coefficient Ds has been measured with photon correlation spectroscopy (PCS) from a mixture of two optically distinct hard spherical silica particles in cycloheptane. In the long wavelength limit (K=0), we have found that Ds is a linear function of the particle volume fraction ϕ in the range of 0<ϕ<0.10: Ds(K=0)=D0(1+kD,sϕ). Here, D0 is the diffusion coefficient at infinite dilution and the constant kD,s has an experimental value of −2.7±0.3. Away from the optical matching point of one of the components and close to that of the other component, we have shown that the theories of optical polydispersity can be applied in light scattering and it is found that the result of the PCS measurements is in accordance with that obtained from time‐averaged light scattering. With the ultracentrifugation techique, it was possible to obtain information about the sedimentation velocity of a few ‘‘tracer’’ particles in the presence of other particles. Here, we found that the experimental data are not consistent with the available theories.
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82.70.Dd Colloids

Interaction between molecular impurities trapped in rare gas crystals. I. Effective potential calculations

Claude Girardet and Daniel Maillard@f@f

J. Chem. Phys. 77, 5923 (1982); http://dx.doi.org/10.1063/1.443865 (18 pages) | Cited 7 times

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The effective potential felt by a dipolar dopant trapped in a harmonic monatomic matrix is studied by including the field induced in the crystal by either another molecular impurity or point defects. This potential is built up from the pair potential generally used in molecular interaction problems and its analytical expression is separated into contributions which characterize both the direct interaction between the dopant and the perturber and the indirect one through the perturbed matrix relaxation. New results are obtained on the validity of the ideal trapping in monatomic matrices by determining the minimum distance between one dopant and another defect (impurity or vacancy), which allows one to neglect the interdefect interaction. A direct application of these results is the study of the translational‐orientational dynamics of an hydracid monomer trapped in a realistic imperfect matrix in order to interpret experimental data on the Q‐branch behavior in the near infrared spectrum of HX in rare gas matrices (cf. paper II).
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33.20.Ea Infrared spectra
71.70.Ch Crystal and ligand fields
61.72.-y Defects and impurities in crystals; microstructure

Interaction between molecular impurities trapped in rare gas crystals. II. Interpretation of the Q branch in the near infrared spectrum of hydracids

Claude Girardet and Daniel Maillard@f@f

J. Chem. Phys. 77, 5941 (1982); http://dx.doi.org/10.1063/1.443813 (14 pages) | Cited 8 times

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For non‐negligibly low concentrations of hydracid dopant (20≤M/R≤500) in a rare gas matrix, the near infrared spectrum exhibits, besides the well‐known rovibrational structure of the monomer, a weak band in the absorption gap around the normally forbidden pure vibrational frequency of the hydracid. The intensity and the profile of this band is concentration and temperature dependent. We give here an interpretation of its structure, based on the calculations performed in the previous paper (paper I). The interactions between dopant monomers located in a wide range of positions (between a√2 and a√10, a nearest‐neighbor distance) are shown to be responsible for this absorption, which appears as a distribution of Q branches corresponding to the distribution of dopants inside the crystal. The shape of the sum band and its evolution with concentration and temperature are thus explained in terms of an inhomogeneous broadening process. More particularly, the three maxima observed at 2866.5, 2869.0, and 2870.6 cm−1 for HCl trapped in argon are, respectively, connected to the absorption of second and third nearest‐neighbor dopants and to the superposition of all other pair absorption in a frequency range about 1.5 cm−1.
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33.20.Ea Infrared spectra

Vibrationally adiabatic models for reactive tunneling

Rex T. Skodje, Donald G. Truhlar, and Bruce C. Garrett

J. Chem. Phys. 77, 5955 (1982); http://dx.doi.org/10.1063/1.443866 (22 pages) | Cited 103 times

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The approximation of vibrational adiabaticity in curvilinear natural collision coordinates is investigated for tunneling in three‐atom collinear reactions. A validity criterion is derived which limits the adiabatic approximation to systems with small reaction‐path curvature. A general formalism is developed for systems which satisfy this criterion. A one‐dimensional Schrödinger equation is proposed which is sufficiently flexible so as to be adaptable to many different models of tunneling. We present three new methods for including reaction‐path curvature effects on multidimensional tunneling in reactive systems: a method based on a quantum mechanical vibrational average (VA) over degrees of freedom transverse to the minimum‐energy path; a method (called DA for dynamical‐path vibrational‐ average) that includes internal centrifugal effects in the description of the transverse vibrational motion (in this method the vibrational average is approximated as a quantal vibrational average about the dynamical path along which the Born–Oppenheimer force cancels the internal centrifugal force); and a semiclassical optical potential (SOP) method based on the Feshbach formalism translated into an adiabatic representation with reaction‐path curvature providing the coupling mechanism between the explicit and implicit spaces. These models are compared, both formally and numerically, to each other and to four other methods that have been proposed previously, including the small‐curvature (SC) approximation that we have proposed in a recent communication. The VA and SOP methods are shown to provide generalizations of phase average (PA) and second‐order (SO) methods proposed earlier by Miller and co‐workers. The difference is that vibrations are treated quantum mechanically in the VA and SOP methods but classically and harmonically in the PA and SO methods; the quantum mechanical methods have the advantage that anharmonicity can be included more straightforwardly. The DA, SO, and SOP methods, although they include internal centrifugal effects more fully than the VA and PA methods, do not offer significant improvement in accuracy. The numerical results clearly support the physical interpretation of the collapse of the vibrational wave function about a least‐action path. The most successful methods are the Marcus–Coltrin path (MCP) and SC approximations. These methods, especially the SC approximation because it is more general, are recommended for future applications.
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82.20.Fd Collision theories; trajectory models
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Competition between atomic and molecular chlorine elimination in the infrared multiphoton dissociation of CF2Cl2

D. Krajnovich, F. Huisken, Z. Zhang, Y. R. Shen, and Y. T. Lee

J. Chem. Phys. 77, 5977 (1982); http://dx.doi.org/10.1063/1.443841 (13 pages) | Cited 56 times

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Infrared multiphoton dissociation of CF2Cl2 has been reinvestigated by the crossed laser‐molecular beam technique using a high repetition rate CO2 TEA laser. Both the atomic and molecular chlorine elimination channels were observed: (1) CF2Cl2→CF2Cl+Cl and (2) CF2Cl2→CF2+Cl2. No evidence was found for secondary dissociation of CF2Cl at laser energy fluences up to 8 J/cm2. Center‐of‐mass product translational energy distributions were obtained for both dissociation channels. In agreement with previous work, the products of reaction (1) are found to have a statistical translational energy distribution. The products of reaction (2) are formed with a mean translational energy of 8 kcal/mol, and the distribution peaks rather sharply about this value, indicating a sizeable exit barrier to molecular elimination. The product branching ratio was directly determined. Reaction (2) accounts for roughly 10% of the total dissociation yield in the fluence range 0.3–8 J/cm2. These results provide an additional test of the statistical theory of unimolecular reactions.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Hp Processes caused by visible and UV light
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Integral elastic‐scattering of Kr+ and Xe+ ions in collision with He atoms

H. Inouye and K. Tanji‐Noda

J. Chem. Phys. 77, 5990 (1982); http://dx.doi.org/10.1063/1.443842 (4 pages)

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Integral elastic‐scattering cross sections in collision with He atoms were measured for Kr+ and Xe+ ions with kinetic energies in the range 0.5–3.0 keV formed under various source conditions. Variation of source gas compositions and methods of ion formation had no effect on the cross sections. A possible explanation of the results is that the population distributions of the ions (2P3/2 and 2P1/2) produced in the ion source are quite similar.
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34.50.-s Scattering of atoms and molecules

Dynamics of the two‐photon photodissociation of NO2: A molecular beam multiphoton ionization study of NO photofragment internal energy distributions

Richard J. S. Morrison and Edward R. Grant

J. Chem. Phys. 77, 5994 (1982); http://dx.doi.org/10.1063/1.443843 (11 pages) | Cited 24 times

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Two‐photon photodissociation of NO2 is induced by the output of a pulsed dye laser tuned over the region from 455 to 425 nm. We characterize the dynamics of this process by recording the multiphoton ionization spectrum of the product NO: intensities of spectral features associated with math(2Σ+)←math(2π3/2,1/2) two‐photon resonance enhanced, four‐photon ionization of nascent NO reveal its distribution over accessible rovibronic states. A single laser pulse serves both as photodissociation source and probe. Over the wavelengths studied the dominant reaction pathway yields NO(math 2π) and O(1D). Its dynamics in all regions of the photodissociation spectrum, save one, are comparable to those observed for the loss of O(3P) in one photon photolysis at comparable available excess energy. In the region of 427 nm, however, the photodissociation dynamics are dramatically different. Here we find that the photoproduct NO rotational distribution is anomalously cold, apparently limited by the rotational temperature of our supersonic molecular beam, and that a product spin‐orbit state 2π1/2 is missing. We argue that this result suggests a linear or near‐linear dissociation geometry which imparts very little torque to the departing NO photofragment and places strict symmetry requirements on its spin‐orbit state. We offer an interpretation that traces the cause of this anomalous behavior to the participation at the two‐photon level of a theoretically predicted linear state of NO2.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes
82.50.Hp Processes caused by visible and UV light

Rainbows in rotationally inelastic scattering: A comparative study of different model potential surfaces and dynamical approximations

Reinhard Schinke, H. Jürgen Korsch, and Dirk Poppe

J. Chem. Phys. 77, 6005 (1982); http://dx.doi.org/10.1063/1.443844 (16 pages) | Cited 26 times

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Rainbow structures in rotationally elastic and inelastic differential cross sections in atom–diatom collisions are investigated by comparison of three model potential energy surfaces labeled I, II, and III which are represented by V(R,γ)=V0(R)+V2(R)P2(cos γ). The cross sections are calculated within the quantal infinite‐order‐sudden (IOS) approximation. The anisotropic part V2 is the same for all potentials and purely repulsive. The isotropic part V0 for potential I is also repulsive and the differential cross sections show the well‐studied rotational rainbow structures. Structural changes occur for collisions in potential II and III which have V0 terms being attractive at intermediate and large atom–molecule separations and having well depths of 10% and 25% of the collision energy, respectively. For example, the elastic cross section has no classical rainbow in the case of potential I but three in the case of potential III. The rainbow structures are analyzed within the classical and semiclassical versions of the IOS approximation and interpreted in terms of catastrophe theory. The quantitative comparison of the classical with the quantal IOS cross sections manifests possible quantum effects, i.e., tunneling into nonclassical regions and interference effects due to the superposition of several contributions (up to six in the present study). They can be very prominent and thus we conclude that much caution is needed if experimental data are compared with classical calculations. The accuracy of the IOS approximation is tested by comparison of classical IOS cross sections with cross sections obtained from exact classical trajectory calculations. The agreement is generally good with the exemption of the rainbow region and small angle, rotationally elastic scattering.
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34.50.Ez Rotational and vibrational energy transfer
34.50.-s Scattering of atoms and molecules

Experimental verification of the breakdown of the electric dipole rotational selection rule in electron impact ionization–excitation of N2 @fa@f)

S. P. Hernandez, P. J. Dagdigian@f@f, and J. P. Doering

J. Chem. Phys. 77, 6021 (1982); http://dx.doi.org/10.1063/1.443845 (6 pages) | Cited 26 times

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Variable energy (60–1500 eV) electron impact excitation of cold (∼10 K) N2 supersonic beams has been used to produce excited N+2 B 2Σ+u ions whose rotational energy distribution has been determined by spectroscopic examination of N2+ first negative system (B 2Σ+u→X 2Σ+g) emissions. For energies ≳800 eV, the rotational state distribution is found to be constant with electron energy. The distribution is non‐Boltzmann with a peak at the N′=1 rotational state and a long tail to higher energies. At high electron energies, the electric dipole selection rule (‖ΔJ‖=1) is expected to be obeyed in the ionization–excitation process. Above 800 eV, this rule is followed, and the non‐Boltzmann B 2Σu+ rotational distribution arises from the non‐Boltzmann rotational distribution in the neutral supersonic N2 beam. Below 800 eV, however, breakdown of the ‖ΔJ‖=1 selection rule occurs with ‖ΔJ‖=3 transitions observed first, followed by much larger ΔJ transitions at low electron energies (<100 eV). Implications of this effect for geophysics and supersonic beam diagnostics, as well as the theoretical questions involved, are discussed.
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34.80.Gs Molecular excitation and ionization

Geminate charge recombination in the photoionization of N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) in various solvents@fa@f)

H. T. Choi@f@f, D. S. Sethi@f@f, and C. L. Braun

J. Chem. Phys. 77, 6027 (1982); http://dx.doi.org/10.1063/1.443846 (13 pages) | Cited 39 times

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The photoionization of N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) dissolved in n‐hexane (HEX), 2,2,4‐trimethylpentane (TMP), and tetramethylsilane (TMS) is studied using measurements of single‐photon photoconductivity. Quantum yields for free carrier production—extrapolated to zero applied field—are reported for the energy range 4.4–7.5 eV. The applied electric field dependence of the quantum yields is measured up to fields approaching 2×107 V/m and interpreted via Onsager’s theory of geminate recombination. The low‐field, slope/intercept predictions of the theory are obeyed quantitatively for HEX and TMP as solvent but fail by 25% for TMS. Various distribution functions for the separations of geminate pairs are used to generate predicted field dependence curves, which are compared with the high‐field data. A broad distribution of thermalization lengths is definitely required to fit the TMP and TMS data, but the TMS result is suspect because of the observed failure of Onsager theory in that solvent. Thermalization lengths generally increase with increasing photon energy. Estimates of the photoionization (geminate pair) quantum yield are also made. For TMP, the geminate pair quantum yield exhibits maxima at 5.8 and 6.9 eV.
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72.40.+w Photoconduction and photovoltaic effects

Laser photolysis of benzene. V. Formation of hot benzene

Nobuaki Nakashima and Keitaro Yoshihara

J. Chem. Phys. 77, 6040 (1982); http://dx.doi.org/10.1063/1.443847 (11 pages) | Cited 59 times

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Flash photolysis of gaseous benzene has been carried out with a KrF laser (248.4 nm) as an excitation source. Two new transients have been detected, one of which has a structured absorption in the wavelength region from 235 to 260 nm. It has a rise time of 80 ns and a long decay time (≳25 μs). The precursor of this transient has also been detected. It appears as a shoulder at 225 nm and has a lifetime of 80 ns. This species is postulated to be a highly excited vibrational state of the ground electronic state, namely, a hot benzene molecule. This species is quenched by ground state benzene as well as by 12 different foreign gases, including oxygen and nitrogen, which have similar quenching rates. The decay of the hot benzene is accompanied by an increase in the temperature of the sample, which leads in turn to formation of the species with the structured absorption. The temperature of the equilibrated sample was determined by comparing the thermalized spectra in the nanosecond time scale with the steady‐state absorption spectra at higher temperatures. The temperature rises by about 50° when 20 Torr of benzene are excited with an energy of 33 mJ/cm2. This temperature increase indicates that the yield of hot benzene is 0.6±0.2. The Tn←T1 absorption spectrum has been measured and the intersystem crossing yield is estimated to be 0.2±0.1. The major nonradiative channel in benzene is internal conversion.
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82.50.Hp Processes caused by visible and UV light

Post barrier channel effects during unimolecular decomposition: A trajectory study of energy term variations

Kjell Rynefors

J. Chem. Phys. 77, 6051 (1982); http://dx.doi.org/10.1063/1.443848 (9 pages) | Cited 7 times

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Post barrier channel effects during the decomposition of a three‐atomic molecular complex to an atom and a two‐atom molecule have been investigated in a classical trajectory study. This was done by following the extent of energy exchange between the degrees of freedom outside the centrifugal barrier. An ideal dipole potential was used to model the potential energy between the atom and the molecule. In all runs the total energy used was approximately 4.66×10−20 J, corresponding to a temperature of close to 1000 K in a crossed molecular beam experiment, while total angular momentum Ltot has been varied in the runs. For large Ltot values the centrifugal barrier height can be a considerable fraction of the total energy. A recently developed statistical method (Holmlid, Rynefors 1981) has been used to generate the initial conditions at the top of the centrifugal energy barrier. The half‐reaction KNaCl→K+NaCl was chosen as a model decomposition process but the effects of light and heavy product atom masses have also been investigated. When the magnitude of Ltot was close to the upper bound compatible with this total energy, considerable exchange occurred between the degrees of freedom. The molecular rotation energy shifted 11%, in the meanwhile changes for individual molecules of 40% were not uncommon. A redistribution between the rotational degrees of freedom is the primary effect when the atomic masses are approximately equal. For systems with other mass ratios large shifts in the translational energy can also occur.
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82.20.Fd Collision theories; trajectory models
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Optical thickness effects in kinetic measurements using chlorine atom resonance fluorescence@fa@f)

M. J. Linevsky and N. deHaas

J. Chem. Phys. 77, 6060 (1982); http://dx.doi.org/10.1063/1.443849 (5 pages)

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The production of known concentrations of chlorine atoms by the laser photolysis of Cl2 allows the effect of the optical thickness on fluorescent intensity and base‐line chlorine atom decay rates to be evaluated. A simple absorption model leads to an average cross section of (2.8±0.3)×10−13 cm2 for the chlorine resonance transitions in the 134–140 nm region and a correction factor to the observed pseudo‐first‐order decay constant equal to [(2X0/eX0−1)−1]1/2, where X0 is the initial optical thickness.
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82.80.Dx Analytical methods involving electronic spectroscopy
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
32.50.+d Fluorescence, phosphorescence (including quenching)

Mechanisms of atom‐exchange reactions in rare gas atom–diatom collisions: Kr+NeAr, Ar+ArKr, Kr+Ar2, Xe+Ar2 @fa@f)

L. M. Raff and Donald L. Thompson

J. Chem. Phys. 77, 6065 (1982); http://dx.doi.org/10.1063/1.443850 (11 pages) | Cited 5 times

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Atom‐exchange and dissociation reaction cross sections, angular distributions, and product‐energy distributions have been computed for the Kr+NeAr, Kr+Ar2, and Xe+Ar2 van der Waals reactions using Monte Carlo quasiclassical trajectories. Angular and product‐energy distributions were also computed for the Ar+ArKr atom‐exchange reactions. Detailed collision mechanisms are suggested to explain the cross section ratios for the atom‐exchange branching for the two reactions. The trajectory results indicate that the branching ratio is the result of a competition between statistical factors, which tend to favor the more exothermic pathway, and mechanistic factors which tend to favor abstraction of the lighter atom of the reactant diatom through a stripping mechanism.
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34.50.Lf Chemical reactions
82.20.Hf Product distribution
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

Picosecond dynamics of twisted internal charge transfer phenomena. The role of the solvent

Ying Wang and K. B. Eisenthal

J. Chem. Phys. 77, 6076 (1982); http://dx.doi.org/10.1063/1.443851 (7 pages) | Cited 75 times

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To probe the role of the solvent in intramolecular charge transfer processes, and in particular, the origin of the well‐known dual fluorescence phenomena of p‐dimethylamino benzonitrile (DMABM), picosecond laser studies in mixed polar/nonpolar solutions were undertaken. The anomalous long wavelength emission is attributed to a complex formed between excited DMABN and butanol with a rate constant of (9.7±1.5)×108 M−1 s−1. The dominant stabilization of the twisted intramolecular charge transfer state is therefore concluded to be due to a short range specific interaction with a polar solvent molecule. A secondary solvent effect arises from a further stabilization of the complex by long range polarization interactions with solvent molecules. Evidence on the existence of ground state complexes between DMABN and butanol are also presented. Excitation of these ground state complexes leads to the rapid formation of the excited state complexes in 30 ps, which we have interpreted to be the time required for the complex to relax intramolecularly, presumably a rotational motion along the C–N bond of DMABN, to achieve the final twisted charge transfer geometry.
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33.50.Dq Fluorescence and phosphorescence spectra
31.70.Dk Environmental and solvent effects

Ab initio dipole moment functions for the X 2Σ+ and B 2Σ+ states of AlO

B. H. Lengsfield and B. Liu

J. Chem. Phys. 77, 6083 (1982); http://dx.doi.org/10.1063/1.443852 (7 pages) | Cited 25 times

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The dipole moment function for the X 2Σ+ state of AlO has been determined by a convergent sequence of MCSCF and CI calculations. It is nearly constant between R=3.0 and 4.0a0 because of a gradual transition from Al+O− to Al++O−−. This leads to a small vibrational band oscillator strength of f01=(4.0±3.0)×10−7. Several MCSCF models were tested for their ability to describe the charge transfer process, and the best model was used to study the B 2Σ+ state AlO.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.V- Electron correlation calculations for atoms, ions and molecules

Conditions for the definition of a strictly diabatic electronic basis for molecular systems

C. Alden Mead and Donald G. Truhlar

J. Chem. Phys. 77, 6090 (1982); http://dx.doi.org/10.1063/1.443853 (9 pages) | Cited 268 times

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A strictly diabatic electronic basis is defined as one for which all components of the nuclear momentum coupling vanish. We examine the possibility that such a basis may exist, and we find that, in general, it does not. The only important exception is for diatomic states of the same symmetry. We also consider some conditions for the definition of an approximately diabatic electronic basis. For molecular systems with three or more nuclei, one can obtain useful approximate diabatic basis sets if the transverse (solenoidal) part of the coupling is negligible; this may occur, for example, if the part of the coupling due to the internuclear‐distance dependence of the configurational wave functions is negligible as compared to that due to the internuclear‐distance dependence of the configurational coefficients. We derive a criterion showing that such approximations may be useful and accurate if the role of the coupling is important over regions of sufficiently small linear dimensions.
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31.70.-f Effects of atomic and molecular interactions on electronic structure

A simple CI model for the valence CI bands in CO+2 and isovalent ions. Ionization potentials of CO2 in the 20–30 eV range@fa@f)@fb@f)

Walter B. England

J. Chem. Phys. 77, 6099 (1982); http://dx.doi.org/10.1063/1.443854 (4 pages) | Cited 8 times

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The qualitatively correct Mulliken–Walsh Aufbau rules are used to derive a simple CI model for the energy differences between the valence CI bands and the valence MO bands in CO+2 and isovalent ions. The essence of the model is that the valence CI bands form multiplet terms, and at least one member of the multiplet has the same symmetry as, and interacts weakly with, at least one MO ion level. In this case, a common set of orbitals may be derived and modest CI calculations simultaneously provide accurate term splittings and at least one accurate energy separation between one term and one known lower energy MO ion state. CI calculations which include roughly 2000 or fewer spin and space adapted configurations are used to determine all valence CI band ionization potentials of CO2 in the 20–30 eV range. Agreement with observed peaks is typically very good.
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31.15.V- Electron correlation calculations for atoms, ions and molecules

Isomeric structures of CH2LiF, the prototype carbenoid

Mark A. Vincent and Henry F. Schaefer

J. Chem. Phys. 77, 6103 (1982); http://dx.doi.org/10.1063/1.443855 (6 pages) | Cited 9 times

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Molecular electronic structure theory has been used to investigate the geometrical and energetic characteristics of CH2LiF the simplest halolithiocarbenoid. The structures of three isomers were predicted using self‐consistent field (SCF) theory in conjunction with a basis set of better than double zeta plus polarization quality, namely, C(9s5p1d/4s2p1 d), H(4s1p/2s1p), Li(9s4p/4s2p), and F (9s6p1d/4s3p1d). At this level of theory there are three minima on the potential energy hypersurface: I, which may be roughly described as the ion pair H2CLi+⋅⋅⋅F−; II, for which the description H2C⋅⋅⋅LiF is suitable; and III, where the picture H2C⋅⋅⋅FLi (and to a lesser degree H2CF−⋅⋅⋅Li+) is pertinent. Vibrational frequencies have been predicted for each of these structures. The part of the potential surface connecting structures II and I is initially extremely flat, but a transition state connecting them has been located. Explicitly correlated wave functions (22 366 configurations) confirm the SCF prediction that I is the lowest energy isomer with II lying at ∼27 kcal and structure III at ∼25 kcal.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.A- Ab initio calculations

Update methods in multiconfigurational self‐consistent field calculations

Jeppe Olsen and Poul Jørgensen

J. Chem. Phys. 77, 6109 (1982); http://dx.doi.org/10.1063/1.443856 (22 pages) | Cited 11 times

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We have demonstrated how Hessian update methods may easily be implemented into an MCSCF calculation. We have shown that the Hessian update methods constitute the most efficient iterative methods for obtaining local convergence of an MCSCF calculation and we have further shown that Hessian update methods, when implemented with step size and sign control, have very promising global convergence properties.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
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