Index of content:
Volume 103, Issue 11, 15 September 1995

Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane‐d _{11}: I. Solvent‐dependence
View Description Hide DescriptionWe examine the gas‐to‐liquid frequency shifts and isotropic Raman linewidths for the axial and equatorial CH stretching vibrations of cyclohexane‐d _{11} in 13 different solvents. The ‘‘perturbed hard‐fluid model’’ of Ben‐Amotz and Herschbach provides a consistent description of the observed shifts in terms of average repulsive and attractive solute–solvent interactions. The attractive part of the shift is dominated by dispersive interactions, with a small contribution from inductive forces. This analysis also reproduces the observed ratio of equatorial to axial attractive shift components if certain details of the cyclohexane‐d _{11} geometry are accounted for when the repulsive part of the shift is calculated. The ratio of equatorial to axial peak linewidths is nearly the same for all solvents and is consistent with inhomogeneous broadening by attractive force fluctuations. A version of the Schweizer–Chandler theory of attractive force broadening, modified to incorporate interactions between the solute vibrational dipole and solvent dipoles, successfully describes the dependence of the observed linewidths on solventdipole moment. However, the remaining portion of the linewidth does not correlate with solvent parameters characterizing dispersive force fluctuations in the manner predicted by the theory.

Theory of nonlinear optical experiments with harmonic oscillators
View Description Hide DescriptionWe develop a theory of photon‐echo phenomena in harmonic vibrational modes. Although classical harmonic oscillators cannot produce any nonlinear optical signal in the linear response limit, we demonstrate that quantum harmonic oscillators that are coupled to any physically reasonable bath can give rise to novel nonlinear optical behavior, even in the perturbative limit. We show that photon echoes in high‐frequency vibrational modes are strongly affected by both population relaxation and pure dephasing. The time dependence of the echo signal is shown to be highly sensitive to the amount of inhomogeneous broadening in the vibrational line. As an example, we develop the simple model of population relaxation resulting from linear coupling to the bath and pure dephasing resulting from quadratic coupling to the bath. Counter to the classical picture, echo signal is present when the only coupling to the bath is linear, but absent when the only coupling is quadratic.

Many‐body correlations in the interaction‐induced light scattering from liquid CS_{2}
View Description Hide DescriptionAdditional information concerning the interaction‐induced contribution to the intensities in depolarized light scatteringspectra of liquid carbon disulfide is presented here. This was obtained from molecular dynamicscomputer simulations of relevant time‐correlation functions (TCFs) along the gas‐liquid coexistence curve. The interaction‐induced part of the polarizability was evaluated assuming a first‐order dipole‐induced dipole mechanism. The simulated spectral TCFs for this system show coupled rotation‐translation dynamics that are due in part to the nonspherical molecular shape of CS_{2} and in part to the orientation dependence of the dipole‐induced polarizabilityΠ. By writing Π in the usual way as the sum of a diagonal orientation‐independent term plus a second‐rank tensor in the polarizabilityanisotropy, the TCFs can be split into terms with varying degrees of orientation dependence. In addition, the collective TCFs were broken down into their component 2‐, 3‐ and 4‐body parts. The component parts of both the pure interaction‐induced (II) and the cross (CR) TCFs between the II polarizabilities and the orientation (OR) variable were evaluated. At the highest densities, it is shown that the cancellation of II TCFs at short times is not as complete as for other simple molecules even though there is nearly a perfect cancellation of the 2‐ and 4‐body correlations by negative 3‐body terms in the long time limit.

Investigation of the Ã ^{3} E↔X̃ ^{3} A _{2} system of methylnitrene radical by laser spectroscopy
View Description Hide DescriptionThe vibrationally resolved laser‐induced fluorescence spectra of the Ã ^{3} E↔X̃ ^{3} A _{2} system of CH_{3}N radical were obtained experimentally. The dispersed spectra at different vibrationally excited states confirm that the system studied belongs to the CH_{3}N radical. From the vibrational progressions in the excitation spectrum, following vibrational frequencies of the Ã ^{3} E state were determined: ν_{1} ^{′}=2943 cm^{−1}, ν_{2} ^{′}=1239 cm^{−1}, ν_{3} ^{′}=759 cm^{−1}, and ν_{5} ^{′}=1500 cm^{−1}. In addition, the lifetimes of the Ã ^{3} E state at four vibrational states (ν_{3} ^{′}=0, 3, 4, and 6) were measured, which are, respectively 292.8±1.2, 203.4±1.2, 188.2±0.5, and 157.2±0.5 ns at a total pressure of 1.7 Torr. Our experiments show that the Ã ^{3} E state is bound at least up to 4800 cm^{−1} and is relatively free from photochemistry.

Autoionizing Rydberg and zero electron kinetic energy states in Ar
View Description Hide DescriptionWe present spectroscopic data for the autoionizing npRydbergmanifold of Ar atoms together with the lifetimes of its Zeke states (Rydbergs of very high n’s studied by time resolved ZEKE spectroscopy). The lower nRydberg states have finite widths, indicating a fast autoionization process that diminishes with increasing n (as n ^{3}). This trend cannot be used to predict the behavior of the high Zeke states whose lifetimes are almost 2 orders of magnitude longer than the lifetimes extrapolated from the low n values.

The dynamics of high autoionizing Rydberg states of Ar
View Description Hide DescriptionIn this paper we present a theoretical study of the autoionization dynamics of high ^{2} P _{1/2} np′[3/2]_{1}Rydbergs (with the principal quantum numbers n=100–280) of Ar in weak homogeneous electric fields (F=0.01–1.0 V/cm), which were experimentally interrogated by time‐resolved zero‐electron kinetic energy (ZEKE) spectroscopy [M. Mühlpfordt and U. Even, J. Chem. Phys. 103, 4427 (1995)], and which exhibit a marked dilution (i.e., ∼2 orders of magnitude lengthening) of the lifetimes relative to those inferred on the basis of the n ^{3} scaling law for the spectral linewidths of the np′ (n=12–24) Rydbergs. The multichannel effective Hamiltonian (H _{eff}) with several doorway state(s) (for excitation and decay) and pure escape states (for decay) was advanced and utilized to treat the dynamics of the mixed Stark manifold of the ZEKE Rydbergs.H _{eff} of dimension 2n−1 is then constructed for a nRydbergmanifold using independent experimental information on the (l dependent) quantum defects δ(l) and the (l, K, J dependent) decay widths, which are of the form Γ_{0}(lKJ)/( n−δ(l)) ^{3}, with Γ_{0}(lKJ) being the decay widths constants. Here, l, K, and J are the azimuthal, the electronic and the total electronic angular momentum quantum numbers, respectively. Two coupling ranges are distinguished according to the strength of the reduced electric fieldF̄(n,p′)=(F/V cm^{−1})n ^{5}/ 3.4×10^{9}[δ(p′)(mod1)].
Range (A); The onset of the effective coupling of the doorway and escape states, i.e., 0.7≤F̄(n,p′)≤2. Range (B); The strong mixing domain F̄(n,p′)≥3. The lifetimes in range (B) can be well represented by a nearly democratic mixing of all the doorway and escape states (lKJ), with the average value 〈τ(n)〉≂〈τ_{SM}(n)〉= 2n ^{4}ℏ/[J(lJK)Γ_{0}(lJK)]. In range (B) 〈τ(n)〉 increases with increasing n and is only weakly F dependent. Range (A) is characterized by a hierarchy of two time scales for the decay, with a short decay component, which manifests the residue of the doorway state, and a distribution of very long lifetimes with an average value 〈τ_{LONG}(n)〉≂η(n)〈τ_{SM}(n)〉, where η(n)≂2–5. In range (A), 〈τ_{LONG}(n)〉 decreases with increasing n and decreases with increasing F, manifesting the enhancement of mixing. We identified range (B) for n=150–280, where a semiquantitative agreement between the experimental ZEKE lifetimes and spectra and our theory was obtained. A tentative identification of range (A) for lower n (=100–150) values was accomplished. We have also performed a theoretical study of the Ar autoionization dynamics via the ^{2} P _{1/2} nd′[3/2]_{1} doorway state, which was experimentally studied by Merkt [J. Chem. Phys. 100, 2623 (1994)].
The onset of range (A) was identified in the region n=70–80, with the estimated lifetimes near the onset being in agreement with experiment. Our analysis explains the higher n onset for the np′ doorway state mixing (n≂100 and F≂0.1 V/cm) than for the np′ doorway state mixing (n′=70–80 for F≂0.1 V/cm). Experimental values of 〈τ_{LONG}(n)〉 (around n≂90) in range (A), excited via the ^{2} P _{1/2} nd′[3/2]_{1} doorway state, are considerably longer than those predicted by our theory for l mixing. The discrepancy may be due to (lm _{ l }) mixing, which presumably originates from Rydberg–ion collisions.

Vibronic structure of the cyclopentadienyl radical and its nonrigid van der Waals cluster with nitrogen
View Description Hide DescriptionFluorescence excitation and two color mass resolved excitation spectroscopy are employed to study the D _{1}(^{2} A _{2} ^{″})←D _{0}(^{2} E _{1} ^{″}) vibronic transitions of the cyclopentadienyl radical (cpd) and its van der Waals cluster with nitrogen. The radical is created by photolysis of the cyclopentadiene dimer and cooled by expansion from a supersonic nozzle. The cpd(N_{2})_{1} cluster is generated in this cooling process. Mass resolved excitation spectra of cpd are obtained for the first 1200 cm^{−1} of the D _{1}←D _{0} transition. The excitation spectrum of cpd(N_{2})_{1} shows a complicated structure for the origin transition. With the application of hole burning spectroscopy, we are able to assign all the cluster transitions to a single isomer. The features are assigned to a 55 cm^{−1} out‐of‐plane van der Waals mode stretch and contortional (rotational) motions of the N_{2} molecule with respect to the cpd radical. Empirical potential energy calculations are used to predict the properties of this cluster and yield the following results: (1) the N_{2} molecular axis is perpendicular to the cpd fivefold axis and parallel to the plane of the cpd ring with the two molecular centers of mass lying on the fivefold ring axis; (2) the binding energy of cpd(N_{2})_{1} is 434 cm^{−1}; and (3) the rotational motion of the N_{2} molecule is essentially unhindered about the cpd fivefold axis. The molecular symmetry group D _{5h }(MS) is applied to the nonrigid cluster, and optical selection rules exclude even↔odd transitions (Δn=0, ±2, ±4,... allowed) between the different contortional levels. Tentative assignments are given to the observed contortional features based on these considerations. The barrier to internal rotation is also small in the excited state. The results for the cpd(N_{2})_{1} van der Waals cluster are compared to those for the benzene (N_{2})_{1} and benzyl radical (N_{2})_{1} clusters.

Efficient absorption line shape calculations for an electron coupled to many quantum degrees of freedom: Applications to an electron solvated in dry sodalite and halo‐sodalites
View Description Hide DescriptionWe present quantum mechanical calculations of the absorption line shape of an electron ‘‘solvated’’ in several sodalites.Photon absorption by the electron modifies the forces acting on the nuclei, setting the counterions in motion. This nuclear motion causes broadening and gives vibrational structure to the absorptionspectrum of the electron. The major effort in the computation of the absorptionspectrum is directed toward the evaluation of an overlap integral that evolves in time because of nuclear motion. The systems considered here have a very large number of nuclear degrees of freedom, and this makes a brute‐force quantum mechanical calculation of the overlap impossible. Good results can be obtained with a method that exploits the fact that in a system with many degrees of freedom the overlap integral decays rapidly to zero, and can therefore be evaluated accurately and efficiently by short‐time methods. The short‐time method that seems most advantageous is the Gaussian wave packet (GWP) procedure proposed some time ago by Heller. This simplifies the nuclear dynamics and also substantially diminishes the number of electron energy calculations needed for determining the forces acting on the nuclei. When the GWP method is used, the electronic wave function is calculated only for a small number of nuclear configurations along the classical trajectory on which the center of the nuclear wave packet evolves. The present calculation is the first use of this method to compute the absorptionspectrum of a complex system. We study the absorption line shape for an electron solvated in a dry sodalite, and in chloro‐, bromo‐, and iodo‐sodalite. We find that the homogeneous linewidth due to the nuclear motion is narrower than that observed experimentally. This implies that the measured linewidth is due to inhomogeneous broadening. For the dry sodalite the main inhomogeneity is the disorder in the position of the counterions, and for halo‐sodalites, the presence of defects introduced during synthesis. Our results imply that a careful synthesis can improve the contrast in displays based on the cathodochromic effects in zeolites.

A study of the Ã ^{2} B _{2}–X̃ ^{2} A _{1} band system of CaNH_{2} employing molecular beam optical Stark spectroscopy
View Description Hide DescriptionThe 0^{0} _{0} Ã ^{2} B _{2}–X̃ ^{2} A _{1} visible band system of a supersonic molecular beam sample of calcium amide, CaNH_{2}, recorded in the presence of a variable static electric field has been studied using high resolution laser induced fluorescence spectroscopy. The determined spectroscopic parameters are (in cm^{−1}), origin=15 464.366 9(8); A″=13.057 4(9); B″=0.300 47(5); C″=0.292 88(5); ε_{ bb } ^{″}=0.001 1(3); ε_{ cc } ^{″}=0.001 1(3); A′=11.448 7(2); B′=0.307 1(1); C′=0.299 2(1); ε_{ aa } ^{′}=8.238(2); ε_{ bb } ^{′}=0.005 2(6); ε_{ cc } ^{′}=0.055 7(6); Δ^{ s } _{ k }=−0.060 8(5). The rotational parameters are interpreted to give an approximate r _{0} structure. The large spin‐rotation parameter for the Ã ^{2} B _{2} state is discussed in terms of a second order spin–orbit interaction with other low‐lying electronic states. Stark splitting of the ^{ PP } _{11}(1.5)+^{ PQ } _{12}(0.5) branch feature was analyzed to produce a ground state permanent electric dipole moment, μ″, of 1.74(1) D. A comparison of μ″ with other monovalent calcium compounds is given.

The electronic spectroscopy of jet‐cooled difluorocarbene (CF_{2}): The missing Ã‐state stretching frequencies
View Description Hide DescriptionThe Ã(^{1} B _{1})←X̃(^{1} A _{1}) electronic transition of difluorocarbene (CF_{2}) has been studied by laser‐induced fluorescence spectroscopy in a supersonic free jet and by ab initiotheoretical methods. The radical was formed by pyrolysis of C_{2}F_{4} at ∼1000 °C with a heating chamber at the tip of the nozzle just prior to the gas expansion. Fairly complete rotational cooling, but incomplete vibrational cooling allowed the identification of several new hot band transitions. Ab initio calculations for the X̃(^{1} A _{1}), ã(^{3} B _{1}), and Ã(^{1} B _{1}) electronic states were performed using the CASSCF method in conjunction with Dunning’s cc‐pVTZ basis set, as well as two smaller sets. The calculations allow us to distinguish between several possible assignments of the observed transitions, and hence determine the vibrational frequencies of the two previously unassigned Ã‐state stretching frequencies: ν_{3} ^{′}=1180±2 cm^{−1} and ν_{1} ^{′}=1011±2 cm^{−1}. The ordering of these modes is different from that in the ground state (ν_{1} ^{″}≳ν_{3} ^{″}) and mechanisms for the switching are discussed.

Density dependence of the ionization potential of CH_{3}I in krypton and of the quasi‐free electron energy in krypton
View Description Hide DescriptionField ionization has been employed to determine the ionization potential of CH_{3}I doped in krypton for krypton densities between the gas and the triple point liquid. The ionization potential is correlated to the different configurations of the host atoms around the dopant at the moment of excitation and is, therefore, represented by a distribution. The ion‐medium polarization energy distribution was calculated, from the first and second moments, at the densities at which the experiments were performed. The calculated widths are in very good agreement with those of the experimentally determined ionization potentials. By combining the experimental results with those of the calculations, the density dependence of the quasi‐free electron energy in Kr was determined. These results are compared to those obtained in previous experiments and with recent theoretical calculations.

Vibrational and rotational energy distribution of ClO produced in reactions of O(^{1}D) atoms with HCl, CCl_{4}, and chlorofluoromethanes
View Description Hide DescriptionNascent state‐resolved ClO(X ^{2}Π) radicals produced in reactions of O(^{1}D) with HCl, CCl_{4}, CFCl_{3}, CF_{2}Cl_{2}, and CF_{3}Cl have been measured, using vacuum‐ultraviolet laser‐induced fluorescence of the ClO(C ^{2}Σ^{−}–X ^{2}Π) transition. Rotational temperatures of ClO (X ^{2}Π_{3/2}, v=0 and 1) are 900–1000 K for chlorofluoromethanes and 2050±60 K (v=0) and 1770±180 K (v=1) for HCl. The rotational distribution for the O(^{1}D)+HCl system is interpreted by the orbital‐to‐rotational angular momentum transfer mechanism. Vibrational level populations of the ClO(v=0–3) products are inverted for CCl_{4} and chlorofluoromethanes, but not for HCl. Vibrational excitations in the v=0–3 levels are higher for the reactions with the larger available energies.

Dynamics of the N(^{4} S)+NO(X ^{2}Π)→N_{2}(X ^{1}Σ^{+} _{ g })+O(^{3} P) atmospheric reaction on the ^{3} A″ ground potential energy surface. III. Quantum dynamical study and comparison with quasiclassical and experimental results
View Description Hide DescriptionA detailed reactive–infinite‐order sudden approximation (R‐IOSA) study of the reactivity of the N+NO→N_{2}+O system has been carried out in the 0.0038 to 1.388 eV translational energy range and the results have been compared with the existing quasiclassical trajectory(QCT) and experimental data available. The general features already observed in the previous QCT studies are reproduced qualitatively in the quantum study, even though some differences arise in the product vibrational distributions and state‐to‐state opacity functions in the low energy range. The observed differences have been justified in terms of the anisotropy of the potential energy surface and the vibrational barriers to reaction at fixed angles. A strong vibrational adiabaticity is observed quantally in the low translational energy range, disappearing at moderately high collision energies (around 0.3 eV), where a simple Franck–Condon type model is capable of describing the evolution of the vibrational distribution with translational energy. The vibrational distributions at fixed angles have been discussed within the context of Polanyi’s and Light’s correlation between products vibrational excitation and the features of the potential energy surface. The validity of extending the conclusions drawn from collinear to three‐dimensional (3D) collisions is discussed. Finally, the detailed reaction mechanism is examined in light of the vibrational matrix elements of the close‐coupling interaction matrix.

Rotational autoionization dynamics in high Rydberg states of nitrogen
View Description Hide DescriptionThe decay dynamics of the high Rydberg states of N_{2} converging on the first few rotational levels (N ^{+}=0,1,2,3) of the ground vibronic X ^{2}Σ^{+} _{ g } (v ^{+}=0) state of the N^{+} _{2} cation have been investigated by delayed pulsed field ionization (PFI) following two‐photon enhanced (2+1′) three‐photon excitation via the a″ ^{1}Σ^{+} _{ g } (v′=0) state of N_{2}. The experiments were carried out in the presence of a weak homogeneous dcelectric field and at typical ion densities of 200–2000 ions/mm^{3}. All Rydberg states in the range of principal quantum number n=140–200 exhibit extreme stability against autoionization and predissociation and some have lifetimes which exceed 30 μs. The decay of the highest Rydberg states beyond n=200 is induced by external perturbations (field ionization and collisional ionization) and no Rydberg states beyond n=350 can be observed by delayed PFI. The Rydberg states which converge on the N ^{+}=0 and 1 rotational levels of the ion, and which therefore are not subject to rotational autoionization, decay into neutral products (by a process presumed to be predissociation) in less than 7 μs in the range n<100. The importance of predissociation is greatly reduced beyond n=100 and becomes negligible on our experimental timescale (30 μs) above n=140. The decay of the Rydberg states converging on the N ^{+}=2 and 3 rotational levels of the ion is more complex. Below n=100, only 30%–40% of the Rydberg population decays by fast rotational autoionization whereas 60%–70% decays by predissociation.
The importance of predissociation decreases rapidly above n=100 and becomes negligible beyond n=140. The decay by rotational autoionization can be observed at all n values but becomes noticeably slower beyond n=100. In the range n=140–200 it exhibits a marked biexponential decaying behavior with 30% of the population decaying within a few microseconds and 70% displaying long term stability (τ≳30 μs). The branching between predissociation and autoionization is explained by the effect of the dcelectric field which mixes strongly the optically accessible pRydberg series with the high lmanifold beyond n=100. The long lifetimes observed experimentally indicate that m _{ l } mixing becomes important as soon as l mixing sets in.

Transverse diffusion measurements and Monte Carlo simulation studies of Rb^{+} ions in Kr and Xe
View Description Hide DescriptionThe ratio of the transverse diffusion coefficient to the mobility (D _{ T }/K) for Rb^{+} ions drifting in Kr and Xe gases at about 303 K has been measured at electric field to neutral gas density ratio (E/N) values ranging from 5 to 500 Td. Calculations of the reduced mobility (K _{0}) and the ratios of the longitudinal and transverse diffusion coefficients to mobility (D _{ L }/K and D _{ T }/K, respectively) for both Rb^{+}–Kr and Rb^{+}–Xe systems were made using a Monte Carlo simulation(MCS) technique and the interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)]. Furthermore, D _{ L }/K and D _{ T }/K values were derived from K _{0} values obtained from the present MCS calculations and from experimental results reported in the literature, using the generalized Einstein relations which are based on the three‐temperature theory. For the Rb^{+}–Kr system, the experimental values of D _{ T }/K were found to be 2%–4% higher than those from MCS calculations for E/N values above 120 Td. For the Rb^{+}–Xe system, the experimental D _{ T }/K values were similarly higher than the MCS values in the range 90–220 Td. Given that the standard error of the experimental measurements of D _{ T }/K is estimated to be better than 3% and that of the MCS calculations is better than 2.5%, the agreement between the present experimental D _{ T }/K data and those from MCS calculations can be considered to be fairly good. The interaction potential of Koutselos, Mason, and Viehland therefore appears to represent well the actual potential for both Rb^{+}–Kr and Rb^{+}–Xe systems.

Avoided resonance overlapping beyond the energy independent formalism. I. Vibrational predissociation
View Description Hide DescriptionThe interaction of overlapping resonances is studied in the case of a vibrational predissociation in a linear molecule. The energy independent approach rests on the neglect of the variation of the coupling elements with the energy in the continuum. It predicts the formation of trapped and short‐lived decay modes when resonances overlap. The relevance of this prediction is discussed on the basis of results obtained from two different methods going beyond this energy independent formalism. The first one consists in searching the solutions of the implicit equation derived from the Feshbach partitioning method, taking into account the full energy dependence of the local effective Hamiltonian H _{eff}(E). The second one is the variational complex coordinate method (CCM), implemented in a discrete variable representation (DVR). The results demonstrate that the energy dependency enhances the trapping effect. The distribution of the resonance energies in the complex plane is different from the prediction of the energy independent formalism. The fast modes are found to be slower than expected. Their position is also completely shifted. The mean width is not constrained by a sum rule which is characteristic of the energy independent approximation.

Collisional enhancement of Rydberg lifetimes observed in vibrational wave packet experiments
View Description Hide DescriptionEvidence for lifetime enhancement of Rydberg states by interactions with surrounding ions is obtained from femtosecond pump–probe experiments on the vibrational wave packet dynamics of the I_{2}(B)‐state, in which either detection of I^{+} _{2} ions or zero‐kinetic energy (ZEKE) electrons was used. The use of the ZEKE technique leads to the appearance of new frequency components in the Fourier transform power spectra of pump–probe time delay scans, as well as the observation of enhanced peak‐to‐valley ratios in these scans. These observed frequency components do not correspond to any energy level spacings in the molecule; they correspond to a sum of two energy level spacings, suggesting a nonlinear detection mechanism in the ZEKE technique (i.e., interaction with ions). Additional evidence for our interpretation is presented through experiments in which additional ions were formed through nonresonant multiphoton ionization of I_{2} with a picosecond 266 nm laser, as well as experiments in which both the decay time of the ZEKE signal and the appearance of the pump–probe time delay scans as a function of the pulsed field time delay were studied. Theoretical wave packet calculations which support the conclusions are presented.

Interaction of the Fe^{+} cation with heavy noble gas atoms
View Description Hide DescriptionThe diatomic iron–noble gas complexes FeAr^{+}, FeKr^{+}, FeXe^{+}, and FeRn^{+} have been theoretically investigated by means of quantum‐chemical calculations including an extensive treatment of electron correlation. Potential energy curves and spectroscopic constants for the lowest ^{4}Δ and ^{6}Δ states are derived from an open‐shell coupled‐cluster approach and the relative energies of all seven low‐lying electronic states are evaluated by the multireference configuration interaction method. While in FeAr^{+} the lowest quartet and sextet states are found to be energetically almost degenerate, the heavier Fe^{+}–noble gas molecules are predicted to exhibit ^{4}Φ ground states. From a qualitative point of view bonding in these species is shown to be electrostatic in origin with intrinsically higher interaction energies for the quartet as compared to the sextet states. For calibration purposes, also an accurate calculation of the ^{4} F(4s ^{0}3d ^{7})–^{6} D(4s ^{1}3d ^{6}) energy difference in the atomic Fe^{+} cation is provided.

An ab initio calculation of magnetic structure factors for Cs_{3}CoCl_{5} including spin–orbit and finite magnetic field effects
View Description Hide DescriptionSpin–orbit interaction plays a significant role in determining the magnetic density in some transition metal complexes. We present a new ab initio technique, based on an extension of unrestricted Hartree–Fock theory, which includes nonperturbatively these spin–orbit effects, and simultaneously also the effects of a finite magnetic field. We also present a new and efficient method for calculating magnetic structure factors, based on the current density rather than magnetic dipole moment density, for a crystal composed of noninteracting molecular fragments. These structure factors are directly comparable to polarized neutron diffraction experiments. Results for the Cs_{3}CoCl_{5} crystal are compared with experiment and previous studies. Without one‐electron spin–orbit coupling terms, the magnitudes of the predicted structure factors are on average 10–15 % too low, whereas, with the spin–orbit terms, the magnitudes are 25–30% too high. Using an effective nuclear charge for Co in the spin–orbit term brings the results into much better agreement, and suggests that the two‐electron spin–orbit shielding terms omitted in the present work are important. For over one quarter of the reflections studied, the magnetic contribution to the structure factors is more than 20% of the nuclear contribution.

Gaussian basis sets for use in correlated molecular calculations. V. Core‐valence basis sets for boron through neon
View Description Hide DescriptionThe correlation‐consistent polarized valence basis sets (cc‐pVXZ) for the atoms boron through neon have been extended to treat core and core‐valence correlation effects. Basis functions were added to the existing cc‐pVXZ sets to form correlation‐consistent polarized core‐valence sets (cc‐pCVXZ) in the usual pattern: Double zeta added (1s1p), triple zeta added (2s2p1d), quadruple zeta added (3s3p2d1f), and quintuple zeta added (4s4p3d2f1g). The exponents of the core functions were determined by minimizing the difference between all‐electron and valence‐only correlation energies obtained from HF+1+2 calculations on the ground states of the atoms. With the cc‐pCVXZ sets, core, core‐valence, and valence correlation energies all converge exponentially toward apparent complete basis set (CBS) limits, as do the corresponding all‐electron singles and doubles CI energies. Several test applications of the new sets are presented: The first two ionization potentials of boron, the ^{3} P–^{5} S separation in carbon, and the X̃ ^{3} B _{1}–ã ^{1} A _{1} state separation in CH_{2}. As expected, correlation effects involving the core electrons of the first row atoms, B–Ne, are small but must be included if high accuracy is required.