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Volume 104, Issue 17, 01 May 1996

Nonphotochemical hole burning in hyperquenched glassy films of water: A pronounced deuteration effect
View Description Hide DescriptionThe effects of deuteration on hole burning of aluminum phthalocyanine tetrasulfonate (APT) in glassy films of water is reported. Deuteration has no effect on the zero phonon hole width of the APT electronic transition, but a large effect on the hole burning kinetics. These effects are discussed in terms of the two level systems of glassy water.

Spectroscopy of TiH: Rotational analysis of the ^{4}Γ→X ^{4}Φ (0,0) band at 530 nm
View Description Hide DescriptionEmission bands of the TiH radical in the region 18 000–20 000 cm^{−1} have been recorded both with gratingspectrograph and Fourier transform techniques. The rotational structure of the 530 nm band, which is the strongest one of the three observed bands, has been analyzed and molecular parameters have been derived for the electronic states involved. The electronic assignment of this band is confirmed as ^{4}Γ→X ^{4}Φ, with a heavily perturbed upper state. The principal parameters (cm^{−1}) of the analyzed states are X ^{4}Φ (v=0): A=33.083(29), B=5.362 06(19), and γ=0.1823(27). ^{4}Γ (v=0): T=18 878.10(47), A=41.54(10), B=5.7408(22), γ=−4.33(12), and λ=0.77(14). The derived first order spin–orbit parameter values of X ^{4}Φ and ^{4}Γ are shown to be consistent with the electronic configurations δπσ and δππ′, respectively. The local perturbations in the ^{4}Γ state are discussed and arguments are given to suggest that the dominating perturber is of ^{4}Φ symmetry.

Symmetry‐breaking perturbations in the ν_{2}+3ν_{3} rovibrational manifold of acetylene: Spectroscopic and energy‐transfer effects
View Description Hide DescriptionTime‐resolved infrared–ultraviolet double resonance (IR‐UV DR) spectroscopy is used to characterize complex rovibrational levels in the highly perturbed ν_{2}+3ν_{3} region (∼11 600 cm^{−1}) of gas‐phase acetylene, C_{2}H_{2}. Here, very few of the known rovibrational levels have appreciable Franck–Condon factors linking them to accessible excited rovibronic levels, as is needed in the fluorescence‐detected IR‐UV DR excitation scheme; rovibrational levels that are ‘‘IR‐bright’’ tend to be ‘‘UV‐dark’’ and vice versa. The rovibrational states detectable by IR‐UV DR in this region are strongly perturbed, such that IR‐bright (but UV‐dark) vibrational basis states are coupled to other states with more favorable Franck–Condon factors. The characterization of these perturbed rovibrational states (and their associated dynamical properties) is facilitated by a novel IR‐UV DR technique in which the UV and IR laser frequencies are simultaneously scanned in opposite directions, with their sum held constant. From the observed IR‐UV DR spectra, it is inferred that local perturbations tend to break symmetries and spoil quantum numbers (such as l, J, and possibly I) that are usually regarded as ‘‘good’’ in the C_{2}H_{2} molecule. The most remarkable case entails an apparent collision‐induced breaking of g/u symmetry that gives rise to rovibrational energy transfer with odd ΔJ (rather than the usual even‐ΔJ situation). This observation is consistent with IR‐UV DR kinetic measurements of collision‐induced state‐to‐state energy transfer that are also briefly described. The supposed mechanism relies on Coriolis coupling to cause strong rovibrational perturbations by basis states with dominant bending character, such that the resulting perturbed state is then susceptible to dynamical breaking of g/u symmetry, with odd‐ΔJ rovibrational transfer a direct consequence. Other possible mechanisms imply that excitation of C_{2}H_{2} to a particular perturbed rovibrational level might cause facile interconversion of the ortho and para nuclear‐spin modifications. One such interpretation of g/u symmetry‐breaking in C_{2}H_{2} invokes a combination of Coriolis coupling and nuclear hyperfine interaction, thereby mixing basis states that have a very close accidental coincidence in energy. Another (but energetically unlikely) possibility is that g/u symmetry is spoiled photochemically by intramolecular state‐mixing involving the vinylidene isomer, thereby destroying the molecule’s center of symmetry.

Estimation of fundamental frequencies of perhalogenated ethylenes in terms of molecular fragment contributions
View Description Hide DescriptionA model, in which a molecule is subdivided into a number of subunits, is used to calculate approximately fundamental frequencies of perhalogenated ethylenes in terms of contributions of subunits and clusters of subunits. Different levels of approximation are defined by a subunit additivity scheme, a linear subunit pair and subunit triple scheme as well as by a nonlinear coupled subunit pair scheme. Algebraic dependencies of the approximate formulas for distinct ethylene derivatives are used as criteria for testing the different schemes. The method is applied to the fundamental frequencies of normal vibrations which, with respect to symmetry, correlate uniquely in the series of substituted ethylenes considered, otherwise the method is applied to corresponding sums of frequencies. For the fundamental frequencies of the 27 ethylene derivatives C_{2}F_{ n }Cl_{ m }Br_{4−n−m }, n+m≤4, completely known from a normal coordinate analysis, the main results are: (a) The out‐of‐plane frequencies correlate uniquely in these molecules, three of the in‐plane frequencies and two sums formed from the remaining ones correlate as well. (b) The simple additivity scheme with three parameters for each set of correlating frequencies yields a sufficiently exact fit of the out‐of‐plane frequencies; their mean absolute deviation is about 5 cm^{−1}. (c) The linear subunit pair approximation with twelve parameters for each set is sufficiently exact for all the correlating frequencies and sums of frequencies, respectively; their mean absolute deviation is about 1 cm^{−1}.

Fourier transform emission spectroscopy of new infrared systems of LaH and LaD
View Description Hide DescriptionThe electronic emission spectra of LaH and LaD have been investigated in the 3 μm–700 nm spectral region using a Fourier transformspectrometer. The molecules were excited in a lanthanum hollow cathode lamp operated with neon gas and a trace of hydrogen or deuterium. The bands observed in the 1 μm–3 μm region have been assigned into two new electronic transitions; A ^{1}Π–X ^{1}Σ^{+} and d ^{3}Φ–a ^{3}Δ. The LaH bands with origins at 4533.5593(8) cm^{−1} and 4430.1916(13) cm^{−1} have been assigned as the 0‐0 and 1‐1 bands of the A ^{1}Π–X ^{1}Σ^{+} transition. The rotational analysis of these bands provides the following principal molecular constants for the ground X ^{1}Σ^{+} state, B _{ e }=4.080 534(80) cm^{−1} and α_{ e }=0.077 39(10) cm^{−1} and r _{ e }=2.031 969(20) Å. To higher wave numbers, three subbands of LaH with origins at 5955.8568(16) cm^{−1}, 6238.3768(8) cm^{−1}, and 6306.6757(15) cm^{−1} have been assigned as the ^{3}Φ_{2}–^{3}Δ_{1}, ^{3}Φ_{3}–^{3}Δ_{2}, and ^{3}Φ_{4}–^{3}Δ_{3} subbands of the d ^{3}Φ–a ^{3}Δ electronic transition. The rotational analysis of the 0‐0 and 1‐1 bands of the ^{3}Φ_{2}–^{3}Δ_{1} and ^{3}Φ_{4}–^{3}Δ_{3} subbands and the 0‐0, 1‐1, and 2‐2 bands of the ^{3}Φ_{3}–^{3}Δ_{2} subband has been obtained and effective equilibrium constants for the spin components of the d ^{3}Φ and the a ^{3}Δ states have been extracted. Magnetic hyperfine structure was also observed in the a ^{3}Δ state. The rotational analysis of the corresponding LaD transitions has also been carried out and equilibrium constants for the ground and excited states have been determined. The singlet–triplet interval between the X ^{1}Σ^{+} state and the a ^{3}Δ state is not known but on the basis of ab initio calculation and by comparison with LaF and YH, we believe that the ground state of LaH is a ^{1}Σ^{+} state.

Photodissociation spectroscopy of MgCH^{+} _{4}
View Description Hide DescriptionThe photodissociation spectroscopy of MgCH^{+} _{4} has been studied in a reflectron time‐of‐flight mass spectrometer. MgCH^{+} _{4} molecular absorption bands are observed to the red of the Mg^{+}(3 ^{2} P _{ J }←3 ^{2} S _{1/2}) atomic ion resonance lines. The photofragmentation action spectrum consists of a broad structureless continuum ranging from 310 nm to 342 nm, and peaking near 325 nm. In this spectral region, both the nonreactive (Mg^{+}), and two reactive fragmentation products (MgH^{+} and MgCH^{+} _{3}) are observed, all with similar action spectra. The product branching is independent of wavelength, Mg^{+}:MgCH^{+} _{3}:MgH^{+}∼60:33:7. The absorption is assigned to the transition (1 ^{2} E←1 ^{2} A _{1}) in C _{3v } symmetry (with η_{3} coordination), followed by a geometrical relaxation of the complex toward states of ^{2} B _{1} and ^{2} B _{2} symmetry in C _{2v } geometry (with η_{2} coordination). Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg^{+} product angular distribution to be isotropic, with an average translational energy release which increases slightly from E _{ t }∼370±150 cm^{−1} at 332.5 nm to E _{ t }∼520±180 cm^{−1} at 315 nm. These values are less than 2% of the available energy and are well below statistical expectations. Analogous experiments on MgCD^{+} _{4} show the kinetic energy release in the nonreactive channel to be significantly larger for the CD_{4} case, ranging from E _{ t }∼540±180 cm^{−1} at 332.5 nm to E _{ t }∼830±200 cm^{−1}. These results clearly demonstrate that the dissociation is nonstatistical. Preliminary ab initiopotential surface calculations suggest a possible dynamical mechanism to explain these unusual results.

Photodissociation dynamics of Ã state ammonia molecules. I. State dependent μ‐v correlations in the NH_{2}(ND_{2}) products
View Description Hide DescriptionThe H(D) Rydberg atom photofragment translational spectroscopy technique has been applied to a further detailed investigation of the photodissociation dynamics of NH_{3} and ND_{3} molecules following excitation to the lowest two (v _{2}=0 and 1) vibrational levels of the first excited (Ã ^{1} A _{2} ^{″}) singlet electronic state. Analysis of the respective total kinetic energy release spectra, recorded at a number of scattering angles Θ [where Θ is the angle between the ε vector of the photolysis photon and the time‐of‐flight (TOF) axis], enables quantification of a striking, quantum state dependent, μ‐vcorrelation in the NH_{2}(ND_{2}) products. The spatial distribution of the total flux of H(D) atom photofragments is rather isotropic (β_{lab}∼0). However, more careful analysis of the way in which the TOF spectra of the H(D) atom photofragments vary with Θ reveals that each H+NH_{2}(D+ND_{2}) product channel has a different ‘‘partial’’ anisotropy parameter, β_{lab}(v _{2},N), associated with it: The H(D) atom ejected by those molecules that dissociate to yield NH_{2}(ND_{2}) fragments with little rotational excitation largely appear in the plane of the excited molecule (i.e., perpendicular to the transition moment and the C_{3} axis of the parent, with β tending towards −1). Conversely, the H(D) atoms formed in association with the most highly rotationally excited partner NH_{2}(ND_{2}) fragments tend to recoil almost parallel to this C_{3} axis (i.e., β→+2). Such behavior is rationalized in the context of the known potential energy surfaces of the Ã and X̃ states of ammonia using a classical, energy and angular momentum conserving impact parameter model in which we assume that all of the product angular momentum is established at the ‘‘point’’ of the conical intersection in the H–NH_{2}(D–ND_{2}) dissociation coordinate. We conclude by reemphasizing the level of care needed in interpreting experimentally measured β parameters in situations where there is averaging over either the initial (parent) or final (product) quantum states.

Photodissociation dynamics of Ã state ammonia molecules. II. The isotopic dependence for partially and fully deuterated isotopomers
View Description Hide DescriptionThe technique of H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the photodissociation dynamics of the mixed isotopomers NH_{2}D and NHD_{2} following the excitation to the v _{2} ^{′}=0 and 1 levels of their lowest lying Ã ^{1} B _{1} (C_{2v }) excited electronic states. Peaks in the resulting total kinetic energy release (TKER) spectra are assigned to levels of the NH_{2}, NHD, or ND_{2} fragments with a wide range of quantum numbers K _{ a } for rotation about their a inertial axes, and with N=K _{ a }, N=K _{ a }+1, or N=K _{ a }+2 as appropriate. These data provide the first measurements of high rotational levels for the ground electronic state of the NHD radical. The least squares fitting of all these spectra, and those resulting from NH_{3} and ND_{3}, to the best calculated NH_{2}, NHD, and/or ND_{2} rotational term values provides accurate estimations of the respective N–H and N–D bond dissociation energiesD ^{0} _{0} across the whole series. These values are D ^{0} _{0}(H–NH_{2})=37 115±20 cm^{−1} (4.602±0.002 eV); D ^{0} _{0}(H–NHD)=37 240±50 cm^{−1}; D ^{0} _{0}(H–ND_{2})=37 300±30 cm^{−1}; D ^{0} _{0}(D–NHD)=37 880±60 cm^{−1}; and D ^{0} _{0}(D–ND_{2})=38 010±20 cm^{−1}. The differences between these values are fully consistent with differences in zero‐point energies and lead to a mean value of D _{ e }=40 510±25 cm^{−1}. Dissociation of NH_{2}D or NHD_{2} through their (Ã−X̃) 2^{1} _{0} bands to give an NHD product leads to TKER spectra with a much higher statistical character than those leading to an NH_{2} or ND_{2} product, and to those obtained following excitation through the 0^{0} _{0} bands. This is rationalized in a semiquantitative manner in terms of a varying contribution to the dissociation rate of the parent molecules from internal conversion (IC) to high levels of their respective ground states. Nuclear permutation symmetry appears to play an important role both for the IC rates and for the subsequent branching between product channels.

Low‐energy electron scattering by halomethanes: Elastic and differential cross sections for CF_{4}
View Description Hide DescriptionLow‐energy quantum calculations are carried out for electrons scattering by CF_{4} molecules in their ground electronic states. The corresponding elastic cross sections (rotationally summed) are obtained as integral quantities and as angular distributions, i.e., differential cross sections (DCS), over a range of collision energies from ≊3 eV up to 35 eV. The exact static exchange (ESE) results compare well with experiments and with previous calculations. The inclusion of a modelpolarization potential is shown to generally improve results, especially at low collision energies and in the small‐angle scattering region.

Reaction‐path dynamics in curvilinear internal coordinates including torsions
View Description Hide DescriptionWe describe a procedure for calculating generalized normal mode vibrational frequencies along a reaction path in curvilinear coordinates for a polyatomic system involving torsions. We apply the method to calculate generalized normal mode frequencies and rate constants for the OH+H_{2}→H_{2}O+H reaction using variational transition state theory with a multidimensional semiclassical tunneling approximation for the transmission coefficient. The results are compared to rate calculations carried out in rectilinear Cartesian coordinates to test the sensitivity of the calculated thermal rate constants to the choice of coordinate.

Interrogation and control of condensed phase chemical dynamics with linearly chirped pulses: I_{2} in solid Kr
View Description Hide DescriptionThe effect of linearly chirped pulses in condensed phase ultrafast pump–probe experiments is investigated by classical simulations for the model system of I_{2} isolated in a Kr matrix. The central frequency of the probe laser is selected to monitor exclusively the event of first collision and recoil of atoms from the host cage. It is shown that a chirped probe pulse enables characterization of the magnitude and sign of the momentum of the evolving trajectory flux. This can be understood by transforming the frequency–time profile of the probe pulse to coordinate–time space, and noting that the observable signal is a function of the relative group velocities of the traveling wave packet and the traveling window function. The effect of the pump pulse chirp, is a measure of the controllability of the evolving dynamics. In the particular case studied, breaking and remaking of the I_{2}bond near the dissociation limit of the bare molecule, it is shown that the memory of the system outlasts the collision with the cage. Negatively chirped pulses produce a more tightly focused wave packet during recoil, leading to a stronger population coherence in the subsequent dynamics.

Collisional vibrational energy transfer of OH (A ^{2}Σ^{+}, v′=1)
View Description Hide DescriptionVibrational energy transfer (VET) and quenching of the v′=1 level of A ^{2}Σ^{+} OH have been studied using laser‐induced fluorescence in a discharge flow cell at room temperature. VET cross sections (Å^{2}) are N_{2}, 30.1±2.8; O_{2}, 2.8±0.3; Ar, 0.56±0.05; H_{2}O, 8.6±0.6. The rotational energy distribution in v′=0 following the VET event was determined for nine colliders. It is nonthermal, generally populating high rotational levels. There are three broad categories of colliders that cause varying degrees of vibrational to rotational energy transfer; H_{2}, D_{2}, and CH_{4} show the least; N_{2}, CO_{2}, CF_{4}, and N_{2}O more; and O_{2} and Ar the most, with about one‐third of the vibrational energy appearing as OH rotation.

An accurate multireference configuration interaction calculation of the potential energy surface for the F+H_{2}→HF+H reaction
View Description Hide DescriptionA three dimensional potential energy surface for the F+H_{2}→HF+H reaction has been computed using the internally contracted multireference configuration interaction (MRCI) method with complete active space self‐consistent field (CASSCF) reference functions and a very large basis set. Calibration calculations have been performed using the triple‐zeta plus polarization basis set employed in previous nine‐electron full CI (FCI) calculations of Knowles, Stark, and Werner [Chem. Phys. Lett. 185, 555 (1991)]. While all variational MRCI wave functions yield considerably larger barrier heights than the FCI, excellent agreement with the FCI barrier height and the exothermicity was obtained when the Davidson correction was applied (MRCI+Q). The convergence of the barrier height and exothermicity, spectroscopic constants of the HF and H_{2} fragments, and the electron affinity of the fluorine atom with respect to the basis set has been carefully tested. Using the largest basis sets, which included 5d, 4f, 3g, and 2h functions on fluorine, a linear barrier height of 1.84 kcal/mol and an exothermicity of 31.77 kcal/mol (exp. 31.73 kcal/mol) was obtained. The true saddle point has a bent structure and the barrier height is predicted to be (1.45±0.25) kcal/mol. About 700 points on the three‐dimensional potential energy surface have been computed using a slightly smaller basis set, which yield F–HH barrier heights of 1.92 kcal/mol (linear), 1.54 kcal/mol (bent), and an exothermicity of 31.3 kcal/mol. The barrier height for the H+FH→HF+H exchange reaction is predicted to be 41.2 kcal/mol. In the entrance channel cuts through the three potentials correlating with F(^{2} P _{3/2,1/2})+H_{2}(^{1}Σ^{+} _{ g }) have been computed, and the effect of spin–orbit coupling is investigated. It is found that the spin–orbit coupling increases the barrier height relative to the asymptotic F(^{2} P _{3/2})+H_{2}(^{1}Σ^{+} _{ g }) ground state by about 0.35 kcal/mol, leading to final estimates for the effective collinear and bent barriers of (2.18±0.25) kcal/mol and (1.80±0.25) kcal/mol, respectively. An accurate global analytical fit of the potential (without the effect of spin–orbit coupling) has been obtained using the method of Aguado and Paniagua. Our new ab initio potential is compared to various potentials used so far in dynamics calculations for the F+H_{2}reaction.

Quantum mechanical angular distributions for the F+H_{2} reaction
View Description Hide DescriptionQuantum mechanical integral and differential cross sections have been calculated for the title reaction at the three collision energies studied in the 1985 molecular beam experiment of Lee and co‐workers, using the new ab initiopotential energy surface of Stark and Werner (preceding paper). Although the overall agreement between the calculated and experimental center‐of‐mass frame angular distributions is satisfactory, there are still some noticeable differences. In particular, the forward scattering of HF(v′=3) is more pronounced in the present calculations than it is in the experiment and the calculations also predict some forward scattering of HF(v′=2). A comparison with the quasiclassical trajectory results of Aoiz and co‐workers on the same potential energy surface shows that the forward scattering is largely a quantum mechanical effect in both cases, being dominated by high orbital angular momenta in the tunneling region where the combined centrifugal and potential energy barrier prevents classical trajectories from reacting. The possible role of a reactive scattering resonance in contributing to the quantum mechanical forward scattering is also discussed in some detail.

A numerical test of activated rate theories for cusped and smooth potentials
View Description Hide DescriptionA numerical study of the effect of dissipation on the radiationless transition rate in the adiabatic and solvent‐controlled limits is presented. For light particle reactions, the nonlinearity of the potential surface in the vicinity of the barrier top is important, and the potential may be approximated as a cusped double well potential, provided that the nonadiabatic coupling is small compared to the thermal energy. Three different theoretical approaches for calculation of the thermally activated rate are analyzed and compared with exact numerical results. We find that Variational Transition State Theory (VTST) with a planar dividing surface, as well as the approach of Calef and Wolynes (CW), provide a good description of the rate of symmetric reactions. A rate expression suggested by Dekker is found to be the least accurate. The CW approach is most accurate in the strong damping regime, while VTST is better in the weak damping regime. The accuracy of both methods improves as the potential is smoothed. VTST and the CW expression are also found to give a reasonable description of asymmetric reactions, provided that the asymmetry is not too large.

Molecular dynamics study of solvation effects on acid dissociation in aprotic media
View Description Hide DescriptionAcidionization in aprotic media is studied using molecular dynamics techniques. In particular, models for HCl ionization in acetonitrile and dimethylsulfoxide are investigated. The proton is treated quantum mechanically using Feynman path integral methods and the remaining molecules are treated classically. Quantum effects are shown to be essential for the proper treatment of the ionization. The potential of mean force is computed as a function of the ion pair separation and the local solvent structure is examined. The computed dissociation constants in both solvents differ by several orders of magnitude which are in reasonable agreement with experimental results. Solvent separated ion pairs are found to exist in dimethylsulfoxide but not in acetonitrile. Dissociation mechanisms in small clusters are also investigated. Solvent separated ion pairs persist even in aggregates composed of rather few molecules, for instance, as few as 30 molecules. For smaller clusters or for large ion pair separations cluster finite‐size effects come into play in a significant fashion.

Ab initio study of van der Waals interaction of CO_{2} with Ar
View Description Hide DescriptionThe ab initiopotential energy surface of the ArCO_{2} cluster is calculated using the supermolecular Mo/ller–Plesset perturbation theory (S‐MPPT) and dissected into its fundamental components; electrostatic, exchange, induction, and dispersion energies. The surface contains a single minimum for the perpendicular approach of Ar toward the C atom which has a well depth of ∼210 cm^{−1} at R=6.5 a _{0}. This value is obtained using an extended basis set supplied with the bond functions and the fourth order supermolecular Mo/ller–Plesset calculations, and is expected to be accurate to within ±5%. The areas of the surface corresponding to the collinear approach of Ar to CO_{2} contain an extended plateau. The saddle point in this region for R=9.0 a _{0} is stabilized by 117 cm^{−1}. The analytical pair potential for Ar–CO_{2} obtained by fitting to the individual interaction components is provided. The three‐body effects in the related cluster, Ar_{2}CO_{2}, are examined for two configurations of the Ar_{2}CO_{2} cluster. The overall nonadditivity is dominated by the three‐body dispersion effect; however, the exchange nonadditivity is the most anisotropic. The sources of this anisotropy are discussed.

Generation of fullerenes and metal–carbon clusters in a pulsed arc cluster ion source (PACIS)
View Description Hide DescriptionBy combining a pulsed arc cluster ion source with a reflection time‐of‐flight mass spectrometer, we have produced and detected in situfullerene cations as well as vanadium–carbon cluster cations. The cluster arrival time distributions favor the ‘‘fullerene road’’ mechanism for fullerene growth. The formation of fullerene cations was found to be strongly dependent on the discharge and nozzle conditions. Fullerene C^{+} _{60}/C^{+} _{70} were preferentially produced by optimizing these conditions. Vanadium–carbon cluster cations were produced and detected by using a composite electrode (V_{2}O_{5}/graphite). While the metallocarbohedrene V_{8}C^{+} _{12} could be produced with a reasonable intensity, it was not dominant under our experimental conditions.

State‐selective multireference coupled‐cluster theory: In pursuit of property calculation
View Description Hide DescriptionIn this work, we examine the efficiency of the recently developed [P. Piecuch et al., J. Chem. Phys. 99, 6732 (1993)] state‐selective (SS) multi‐reference (MR) coupled‐cluster (CC) method for calculation of molecular properties. In our earlier papers, we demonstrated that the SSMRCC method with inclusion of single, double, and internal and semi‐internal triple excitations [SSCCSD(T) approach] is capable of providing an accurate description of the ground‐state potential energy surfaces. In this paper, we present the dipole moment and polarizability values of the HF molecule at equilibrium and stretched geometries calculated using finite field technique and SSCCSD(T) ansatz. The calculations use double zeta quality basis sets with and without polarization functions. Molecular orbital basis sets include both relaxed and nonrelaxed orbitals.

Spin‐unrestricted time‐dependent Hartree–Fock theory of frequency‐dependent linear and nonlinear optical properties
View Description Hide DescriptionA self‐consistent time‐dependent unrestricted Hartree–Fock (TDUHF) theory of linear and nonlinear optical properties is presented. Expressions are derived to calculate the elements of the linear polarizabilitytensor α, the first‐hyperpolarizability tensor β, and the second‐hyperpolarizability tensor γ, in terms of spin‐adapted perturbed density matrices. For the hyperpolarizability tensors, β and γ, expressions are also derived from the lower‐order solutions to the TDUHF equations. A novel feature of the present formulation is that it automatically allows for the separation of the contributions to (hyper)polarizability tensors from individual spin. Results obtained from the calculations of α(ω_{σ};α_{ a }) for H, C, N, O, F, Si, P, S, Cl, O_{2}, NO, and OH and of β(−ω_{σ};ω_{ a },ω_{ b }) corresponding to various second‐order nonlinear optical processes for NO and OH radicals are presented. The present results for α show excellent agreement with the literature data. The calculated result for β_{∥} in the case of the NO radical is too small and has a different sign in comparison to the published experimental data. For both NO and OH, the β tensor for different second‐order nonlinear optical processes show the order: β(−2ω;ω,ω)≳β(−ω;0,ω) ≡β(0;ω,−ω)≳β(0;0,0).