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Volume 102, Issue 5, 01 February 1995

Magnetic modulation of hyperfine quantum beats
View Description Hide DescriptionWeak magnetic fields are observed to produce significant alterations in the temporal evolution of fluorescence emitted from cyanogen following laser excitation of the vibronically allowed 4^{1} _{0} Ã(^{1}Σ^{−} _{ u } )←X̃(^{1}Σ^{+} _{ g }) transition near 219 nm. Magnetically induced modulations of molecular quantum beats are found to arise from Zeeman splittings among the hyperfine levels of spin–orbit coupled molecular eigenstates. These magnetic modulations are accurately described by a simple model in which fluorescence interferences that arise from coupling between the Zeeman components of the molecular eigenstates are analogous to the interference of light passing through the equally spaced slits of a transmission diffraction grating. Landé g factors derived from the magnetically induced modulations are in good agreement with the values directly measured from the Zeeman broadened envelopes of quantum beat frequency components.

Multiphoton ionization of Ag_{2}: Assessment of a new excited electronic state and resolution of the Ag_{2} ionization potential
View Description Hide DescriptionA previously unknown excited electronic state of Ag_{2} has been observed using mass selected resonant two photonionization. The initial rovibrational level of this state observed via a forbidden transition near 46 870 cm^{−1} lies in near perfect double resonance with the lower energy A ^{1}Σ^{+} _{ u }(v’=3)←X ^{1}Σ^{+} _{ g }(v’=0) transition. The double resonance leads to an anomalously large single color ionization signal near the A(v’=3)←X(v‘=0) transition wavelength (426.7 nm). Symmetry selection rules allow an identification of the new state symmetry as 1_{ g } or 0^{+} _{ g } [Hund’s case (c)]. The presence of this state is demonstrated to have a significant impact on previous measurements of the ionization potential of Ag_{2}. Two color resonant two photonionization spectroscopy of the Ag_{2} A ^{1}Σ^{+} _{u} state is implemented using both excimer laser and Nd:YAG laser fourth harmonic ionization. The results of these experiments yield a new measurement of the ionization potential of Ag_{2}, while demonstrating the importance of field ionization effects.

Two‐photon spectroscopy of HI in the 69 600–73 600 cm^{−1} region
View Description Hide DescriptionSeveral new electronic states in HI have been identified from resonance‐enhanced multiphoton ionization (REMPI) spectra in the 69 600–73 600 cm^{−1} region. These results have been combined with previously unpublished single‐photon absorption data and, in selected wavelength regions, with REMPI spectra of DI to provide a more complete description of the excited electronic structure of HI up to ∼9.2 eV above the ground state.

Hyperfine structure effects in probing atomic alignment
View Description Hide DescriptionWe have performed an analysis of the effect of hyperfine structure in an experiment in which an aligned excited state is probed using linearly polarized light. The state is born with a given electronic alignment resulting from a collisional process. It is probed after a time which is short in comparison to its radiative lifetime but much longer than ω^{−1} _{hfs}, where ℏω_{hfs} is the hyperfine splitting. This means that hyperfine structure has ample time to ‘‘develop.’’ It is therefore essential to take into account both the hyperfine structure of the level and the manner in which the alignment is affected by recoupling to the new basis. Ratios of fluorescent intensities for different laser polarization configurations contain information on the alignment of the system. We show how these may be calculated for a system with several isotopes, some of which have hyperfine structure, and where many components are probed with appreciable strength. A comparison is made with our experimental results for the Hg(6 ^{3} P _{1}) level probed on the 6 ^{3} P _{1}–7 ^{3} S _{1} transition. The presence of hyperfine structure has a considerable influence on the measurements. It is important to understand such effects if one is to obtain information on the nascent electronic alignment by observation of the fluorescent intensities.

Spectroscopy of mixed early–late transition metal diatomics: ScNi, YPd, and ZrCo
View Description Hide DescriptionResonant two‐photon ionization spectroscopy has been employed to investigate the spectra of the jet‐cooled transition metal diatomics ScNi, YPd, and ZrCo, which are isovalent species which possess (or are thought to possess) an X ^{2}Σ^{+}ground state. Several electronic band systems have been observed for these species in the near infrared, and the analysis of these systems is reported. Ground state vibrational intervals of ΔG _{1/2} ^{‘} = 334.5 ± 1.0, 264.4±0.2, and 357.7 cm^{−1} have been determined for ScNi, YPd, and ZrCo, respectively. The spectroscopic results obtained for ScNi and YPd are compared to theoretical calculations performed by other researchers, and a discussion of the chemical bonding in these species is presented.

Nanoscale shock wave generation by photodissociation of impurities in solids: A molecular dynamics study
View Description Hide DescriptionThe dynamics of shock wave generation, propagation, and decay in an Ar lattice following photodissociation of an I_{2} impurity are studied using molecular dynamics simulation. A two‐dimensional model is treated to allow the modeling of shock wave propagation over larger distances than easily accessible in full three‐dimensional calculations. The shock waves are created on atomic length scales by binary collisions between the nascent photofragments and adjacent lattice atoms, and propagate long distances through the crystal in a highly directed, quasi‐one‐dimensional manner. As a consequence of the I/Ar mass ratio, the I fragments undergo multiple collisions with the adjacent Ar atoms situated along the I–I bond axis, generating pulse trains of shock waves, each with a characteristic initial energy, velocity, and decay rate. The dynamics of the system are interpreted using a simple one‐dimensional hard sphere model.

Collisional redistribution in Sr–He spin‐changing energy transfer collisions: Final‐state alignment
View Description Hide DescriptionWe have measured the product alignment resulting from the collisional redistribution of polarized light in Sr–He inelastic spin‐changing energy transfer collisions. The experimental results are in good agreement with the predictions of an analytic theoreticalmodel, based on a standard orbital‐locking and following approximation, but generalized to this inelastic collision process. The good agreement indicates a clear understanding of the energy transfer dynamics in this case, and suggests that this simple analytic approach may be generalized to a much broader range of collisional phenomena.

Two‐dimensional imaging of metastable CO molecules
View Description Hide DescriptionDirect time and spatially resolved detection of metastable CO molecules, prepared in selected quantum states via pulsed laser excitation, is experimentally demonstrated in a molecular beam machine. Characterization of the molecular beam in terms of parallel and perpendicular velocity distributions and rotational temperatures is performed. A direct two‐dimensional (2D) demonstration of the mass‐focusing effect in binary gas mixtures is given. Two‐dimensional imaging of the spatial distribution of the metastable a 3Π CO molecules in the beam after passage through a hexapole field is used to study hexapole focusing performance. Structured 2D images demonstrate the dependence of the focusing characteristics on the magnitude of the Λ‐doubling and on the angular dependence of the focusing force in a hexapole consisting of cylindrical rods.

A vibrationally adiabatic theory of molecular Penning ionization
View Description Hide DescriptionBased on available theoretical and experimental information on the Penning ionization of molecules by metastable helium and neon atoms at thermal energies, an approximate theoretical approach for incorporating the vibrational degrees of freedom is developed. The electronically excited, metastable atoms have diffuse outer orbitals, giving rise to relatively soft intermolecular repulsion in nonbonded excited state potentials. A low‐energy ionizing collision is then near‐adiabatic in respect to its effect on the vibrations of the molecule under attack. In addition for the great majority of experimentally studied molecules, nearly vertical vibrational populations in the Penning molecular ion are observed in Penning ionizationelectron spectroscopy (PIES). In the simplest limit of vibrational adiabaticity, the bond oscillator remains unperturbed by the collision, and small deviations from verticality may then be interpreted as a reflection of the bond‐length dependence of the discrete‐continuum coupling that gives rise to ionization. The theory presented may be cast into an approximate but simple form that allows the ready extraction of such information from a complete set of vibrational populations. Recent experimental studies of the He*+H_{2} system provide both justification and an example of the application of the theory to reduction of population data.

Production of OH by dissociating ozone–water complexes at 266 and 355 nm and by reacting O(^{1} D) with water dimers
View Description Hide DescriptionIn the present study the production of OH was monitored when ozone water complexes were dissociated at 266 and 355 nm and when O(^{1} D) atoms were reacted with water dimers. The results indicate that the absorption of ozone at 355 nm is enhanced by two orders of magnitude when the ozone is complexed with water. In addition it is shown that the rotational energy distribution of the OH product is very similar when it is produced in an intracluster process, or by reaction of O(^{1} D) with water dimers. The results are rationalized by complex induced potential energy surfaces shift. The shifts may strongly depend on the relative conformation of the water and the ozone in the complex.

Multiconfiguration time‐dependent Hartree studies of the Cl_{2}Ne vibrational predissociation dynamics
View Description Hide DescriptionThe vibrational predissociation of a van der Waals complex (Cl_{2}Ne) is studied using a method based on the multiconfiguration time‐dependent Hartree approximation. The three‐dimensional wave function is first expanded to the time‐independent Cl_{2} vibrational bases and the Hartree approximation is then imposed on the channel wave functions. The wave packets are propagated for a few picoseconds and five configurations are found to give convergent results. The decay lifetimes, product state distributions and the wave packet dynamics are compared with exact results and the agreement is found to be generally satisfactory. It is found that the decay depends sensitively on the quality of the initial resonance wave function and the single configuration TDH gives only a crude approximation of the dissociation dynamics.

Radiationless decay of the 1,2,3 ^{3}Π_{ g } states of Al_{2}: A fully first principles treatment using adiabatic and rigorous diabatic states
View Description Hide DescriptionThe decay mechanisms of the metastable 2,3 ^{3}Π_{ g } states of Al_{2} are investigated. Both nonadiabatic radiationless decay to the dissociative 1 ^{3}Π_{ g } state and radiative decay to the ground X ^{3}Π_{ u } state are considered. The 1,2,3 ^{3}Π_{ g } states are described using state averaged multiconfiguration self consistent field/configuration interaction wave functions [ψ^{ a } _{ m }(r,Q)]. The derivative couplings f ^{ a } _{ mn }(Q)≡〈ψ^{ a } _{ m }(r, Q)‖(d/dQ)ψ^{ a } _{ n }(r,Q)〉_{ r } are determined and used to construct a rigorous diabatic basis for this strongly interacting three state problem. The 2 ^{3}Π_{ g } state and somewhat surprisingly the 3 ^{3}Π_{ g } state are rapidly predissociated by the dissociative 1 ^{3}Π_{ g } state. The lifetimes for nonradiative decay of the vibrational levels of the 2 ^{3}Π_{ g } state are on the order of picoseconds while those of the 3 ^{3}Π_{ g } state are on the order of nanoseconds being reduced from the direct coupling (3 ^{3}Π_{ g }∼1 ^{3}Π_{ g }) rate of milliseconds by indirect coupling through the 2 ^{3}Π_{ g } state, (3 ^{3}Π_{ g }∼2 ^{3}Π_{ g }∼1 ^{3}Π_{ g }). Radiative decay is found to be on the order of 10^{2} and 30 ns for the 2 ^{3}Π_{ g } and 3 ^{3}Π_{ g } states, respectively, so that radiationless decay is principal decay mechanism. Significant variation in the lifetimes of the individual vibrational levels of the 2,3 ^{3}Π_{ g } states is expected. This is attributed to the mechanism of the predissociation which involves nonadiabatic interactions near the ‘‘inner walls’’ of the 1,2 ^{3}Π_{ g } states. Although avoided crossings strongly affect the properties of the 1,2,3 ^{3}Π_{ g } states the adiabatic basis is preferred over the diabatic basis both conceptually and computationally.

Deactivation of two‐photon excited Xe(5p ^{5}6p,6p’,7p) and Kr(4p ^{5}5p) in xenon and krypton
View Description Hide DescriptionLifetimes and bimolecular quenching rate constants have been determined for two‐photon laser excited states of Xe*(5p ^{5}6p,5p ^{5}6p’,5p ^{5}7p) and Kr*(4p ^{5}5p) in krypton and xenon buffer gases. Collisional mixing between Kr*5p[5/2]_{2} and Kr*5p[5/2]_{3} in krypton is observed and analyzed using a coupled two‐state model to obtain the rate of mixing. The measured rate constants for quenching of Xe*(6p’,7p) by krypton are 15%–20% smaller than those measured previously in xenon while bimolecular rates for the Kr*(5p) states are an order of magnitude larger in xenon than those in a krypton buffer. Measurements of state‐to‐state rate constants for deactivation and excitation transfer are also reported for these states in krypton and xenon buffer gases.

Rotational predissociation dynamics of OH–Ar (A ^{2}Σ^{+}) using the finite range scattering wave function method
View Description Hide DescriptionPredissociative resonances of OH–Ar are computed up to 300 cm^{−1} above the Ar+OH (A ^{2}Σ^{+}, v=0, j=0) asymptote for total (rotational) angular momentum states J≤10. The energies, lifetimes, and OH A ^{2}Σ^{+} product rotational distributions of the predissociative resonances are calculated using a numerical method based on the ‘‘energy independent integral’’ finite range scattering wave function (FRSW) [J. Chem. Phys. 99, 1057 (1993)]. The FRSW method involves evaluation of the scattering matrix and its energy derivative, both of which are only parametrically dependent on energy. The energy independent matrices are determined from the discrete eigenvectors of the L^{2} Hamiltonian matrix H, which is obtained in discrete variable representation, and an exact (analytical) eigenfunction of the asymptotic Hamiltonian operator Ĥ_{0}. Many long‐lived (≳1 ps) resonances are identified for OH–Ar in J=3 with projections of J onto the intermolecular axis of K=0–3. The resonances are characterized with approximate bend and stretch quantum numbers based on the nodal structure of the wave functions. The predissociative states decay by Coriolis coupling to a lower K state and/or through mixing of OH rotor levels induced by the anisotropy of the interaction potential. States that predissociate by Coriolis coupling are identified by their J‐dependent lifetimes and the OH product rotational levels accessed. The influence of potential anisotropy on the predissociative resonances is explored by changing the average intermolecular bond length and degree of intermolecular bending excitation. A comparison of the theoretically calculated resonances with those observed experimentally provides a guide for refinement of the adjusted semiempiricalpotential energy surface [J. Chem. Phys. 98, 9320 (1993)] used in the computations.

Potential energy surfaces for the low‐lying ^{2} A‘ and ^{2} A’ States of HO_{2}: Use of the diatomics in molecules model to fit ab initio data
View Description Hide DescriptionA DIM (diatomics in molecules) model utilizing a large basis set (34 ^{2} A ^{‘} and 32 ^{2} A ^{’} states) was used to obtain the potential energy surfaces relevant to the chemical reaction H+O_{2}→OH+O. The ground state, 1^{2} A ^{‘}, surface was fitted to 910 accurate ab initio points of Walch et al. [J. Chem. Phys. 94, 7068 (1991)]. The resulting fit accurately describes the C _{2v } conical intersection in the regions for which ab initio data are available, and the linear conical intersection is accurately described in the H+O_{2} region. It is also an accurate global fit with an rms deviation of 0.096 eV (2.22 kcal/mol). The behavior of the low‐lying excited states, 1^{2} A ^{’}, 2^{2} A ^{‘}, and 2^{2} A ^{’}, appears to be qualitatively correct everywhere and quantitative near the low‐lying conical intersections. The DIM formulation allows the computation of the gauge potential relevant for the description of the geometric phase and non‐adiabatic effects in multi‐surface reactive scattering calculations.

Spin‐adapted open‐shell state‐selective coupled cluster approach and doublet stability of its Hartree–Fock reference
View Description Hide DescriptionThe performance of the unitary group based state‐selective coupled cluster approach, in both linear and quadratic approximations, is examined for the case of doublet ground states of the first two alkaline earth hydrides in the whole range of internuclear separation. It is shown that the doublet instability of the restricted open‐shell Hartree–Fock wave functions is responsible for the singular behavior of the linear coupled cluster potential energy curves, as well as for slight discontinuity in these curves when the bilinear terms are included. The effect of using different types of orbitals is investigated and the results are compared with full or very precise limited configuration interaction results as well as with the coupled cluster results employing the unrestricted Hartree–Fock reference, which is free of the instability problems in open shell systems.

Relativistic and correlation effects in CuH, AgH, and AuH: Comparison of various relativistic methods
View Description Hide DescriptionThe effects of relativity on the bond lengths, dissociation energies, and harmonic vibrational frequencies of the ^{1}Σ^{+} electronic ground states of the group IB hydrides CuH, AgH, and AuH have been evaluated with a variety of ab initio methods. These properties were investigated with moderately‐sized basis sets at the self‐consistent field Hartree–Fock (SCF‐HF) level and with second‐order Mo/ller–Plesset (MP2) perturbation theory for electron correlation. Comparisons were made between all‐electron results using the nonrelativistic Hamiltonian, perturbation theory (PT‐MVD) at first‐order with only the one‐electron nonfine‐structure terms of the Breit–Pauli Hamiltonian, the spin‐free Douglas–Kroll (DK) transformed Dirac Hamiltonian and the untransformed Dirac Hamiltonian, and results using two sets of relativistic effective core potentials (RECPs). The expected trends of bond length decrease, dissociation energy increase, and harmonic vibrational frequency increase with both relativity and correlation are found. Both sets of RECPs are shown to give good results, if accompanied by a reasonable basis set. The DK method is demonstrated to be an inexpensive, reliable approximation to the DHF method.

Benchmark calculations with correlated molecular wave functions. VII. Binding energy and structure of the HF dimer
View Description Hide DescriptionThe hydrogen bondenergy and geometry of the HF dimer have been investigated using the series of correlation consistent basis sets from aug‐cc‐pVDZ to aug‐cc‐pVQZ and several theoretical methods including Mo/ller–Plesset perturbation and coupled clustertheories. Estimates of the complete basis set (CBS) limit have been derived for the binding energy of (HF)_{2} at each level of theory by utilizing the regular convergence characteristics of the correlation consistent basis sets. CBS limit hydrogen bondenergies of 3.72, 4.53, 4.55, and 4.60 kcal/mol are estimated at the SCF, MP2, MP4, and CCSD(T) levels of theory, respectively. CBS limits for the intermolecular F–F distance are estimated to be 2.82, 2.74, 2.73, and 2.73 Å, respectively, for the same correlation methods. The effects of basis set superposition error (BSSE) on both the binding energies and structures have also been investigated for each basis set using the standard function counterpoise (CP) method. While BSSE has a negligible effect on the intramolecular geometries, the CP‐corrected F–F distance and binding energy differ significantly from the uncorrected values for the aug‐cc‐pVDZ basis set; these differences decrease regularly with increasing basis set size, yielding the same limits in the CBS limit. Best estimates for the equilibrium properties of the HF dimer from CCSD(T) calculations are D _{ e }=4.60 kcal/mol, R _{FF}=2.73 Å, r _{1}=0.922 Å, r _{2}=0.920 Å, Θ_{1}=7°, and Θ_{2}=111°.

Theoretical investigation of the lowest singlet and triplet states in poly(paraphenylene vinylene)oligomers
View Description Hide DescriptionUsing the semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian in combination with configuration interaction techniques, we calculate the optical and photoinduced absorption spectra of poly(paraphenylene vinylene) oligomers containing from two to five phenyl rings; the evolutions with chain length of the singlet–singlet and triplet–triplet excitation energies as well as the values extrapolated for the polymer are in good agreement with experiment. The geometry relaxation phenomena in the first one‐photon allowed singlet excited state and in the lowest triplet state are modeled on the basis of either bond‐order/bond‐length relationships or the formation of (bi)polaron‐type defects; the results are compared to those of direct geometry optimizations in the excited state. The different methods consistently lead to more pronounced bond‐length modifications in the triplet state than in the singlet state.

The accuracy of the pseudopotential approximation. I. An analysis of the spectroscopic constants for the electronic ground states of InCl and InCl_{3} using various three valence electron pseudopotentials for indium
View Description Hide DescriptionSpectroscopic constants for InCl and InCl_{3} are determined by a coupled cluster procedure using relatively large basis sets and an energy‐consistent semilocal three valence electron pseudopotential for indium. Possible errors within the pseudopotential approximation are discussed in detail by comparison of available pseudopotentials adjusted through different techniques. Core‐polarization corrections and the deviation from a point core approximation are discussed. These corrections, however, do not lead to more accurate bond distances as compared to the experimental results. Differently adjusted three valence electron pseudopotentials yield quite different results for the bond distances of InCl and InCl_{3}. The single‐electron adjusted energy‐consistent pseudopotential of Igel‐Mann et al. [Mol. Phys. 65, 1321 (1988)] yields the best results and therefore, this pseudopotential has been chosen for all further investigations on molecular properties. The Dunham parameters for InCl are calculated by solving the vibrational‐rotational Schrödinger equation numerically. A finite field technique is used to determine the dipole moment and dipole‐polarizability of diatomic InCl. The dependence of several molecular properties on the vibrational quantum state is determined by calculating the expectation value P _{ n }=〈n‖P(R)‖n〉, where P(R) is the distance dependent molecular property. The P(R) curves show strong linear behavior and therefore, the shape of the P _{ n } curve is mostly determined by anharmonicity effects in the InCl potential curve. For the vibrational ground state, ‖0〉, the calculated propertyP _{0} deviates only slightly from the property determined directly at the equilibrium distance, P _{ e }. There is in general satisfying agreement of our calculated values with available experimental results. However, it is concluded that in order to obtain very accurate spectroscopic constants a small core definition for indium has to be preferred.