Volume 102, Issue 19, 15 May 1995
Index of content:

Intermolecular potentials and rovibrational energy levels of the Ar complexes with HCN and HCCH
View Description Hide DescriptionThe intermolecular potentialsurfaces for ArHCN and ArHCCH are computed by Mo/ller–Plesset perturbation theory at the fourth‐order approximations (MP4) with a large basis set containing bond functions. Rovibrational energies and spectroscopic constants of the two systems are computed from the intermolecular potentials using the collocation method. The intermolecular potential for ArHCN at the MP4 level has a single minimum at the collinear Ar−H−C−N configuration (R=4.56 Å, θ=0°) with a minimum potential energy of V _{ m }=−135.9 cm^{−1}. The bending frequencies, rotational constants, and centrifugal distortion constants of ArHCN and ArDCN calculated using the MP4 potential are in good agreement with experiment. Rovibrational energies with J=0 through 6 arising from j=0 and j=1 levels of HCN are calculated and compared with the experimental transition frequencies. The intermolecular potentialsurface for ArHCCH has a symmetric double minimum near the T‐shaped configuration. The minimum positions at the MP4 level are (R=4.05 Å, θ=60° and 120°) and the minimum potential energy is V _{ m }=−110.9 cm^{−1}. The rotational constants and bending frequency of ArHCCH arising from the MP4 potential are calculated and compared with experiment. The anisotropy of the MP4 potential is slightly underestimated. The effects of monomer bending vibration on the ArHCN and ArHCCH potentials are studied by additional calculations. The potential anisotropy of ArHCN decreases, whereas that of ArHCCH increases as the monomer vibration is taken into account. This might be partially responsible for the discrepancies between the theoretical predictions and experiment.

Determination of the structure of HBr OCS
View Description Hide DescriptionThe structure of the HBr OCS van der Waals complex has been studied by Fourier transformmicrowave spectroscopy. The ground state complex is hydrogen bound and quasilinear with SCO–HBr atomic ordering. Spectra from five different isotopomers were observed and assigned. The wide amplitude bending angle of the hydrogen bromide was calculated from the nuclear quadrupole coupling constant χ_{ aa } to be 25.2°. Second order quadrupole effects, centrifugal distortion in the nuclear quadrupole coupling constant D _{χ}, a spin–rotation interaction, C _{Br}, and a spin–spin interaction, D _{ aa }, were all included in the Hamiltonian. The following spectroscopic constants have been determined for the H^{79}Br OCS isotopomer: B̄=488.7948(4) MHz; D _{ J }=2.167(6) kHz; H _{ J }=1.19(3) Hz; χ_{ aa }=387.14(1) MHz; D _{ x }=6.79(14) kHz; C _{Br}=0.55(13) kHz; and D _{ aa }=7.7(1.2) kHz.

Line mixing and nonlinear density effects in the ν_{3} and 3ν_{3} infrared bands of CO_{2} perturbed by He up to 1000 bar
View Description Hide DescriptionWe present high density experimental and theoretical results on CO_{2}–He absorption in the ν_{3} and 3ν_{3} infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. Computations are based on an impact line‐mixing approach in which the relaxation operator is modeled with the energy corrected sudden (ECS) approximation. Comparisons between experimental and calculated results demonstrate the accuracy of the ECS approach when applied to band wings and band centers at moderate densities. On the other hand, small but significant discrepancies appear at very high pressures. They are attributed to a number of reasons which include nonlinear density dependence due to the finite volume of the molecules, neglected contributions of vibration to the relaxation matrix, and incorrect modeling of interbranch mixing.

The vibrationally resolved participator Auger spectra of selectively excited C 1s(2σ)^{−1}2π^{1} vibrational states in carbon monoxide
View Description Hide DescriptionThe fully vibrationally resolved participator Auger spectra originating from the decay of the C 1s(2σ)^{−1}2π^{1}resonance in CO are presented. The C 1s(2σ)^{−1}2π^{1} v’=0 resonance has been excited with a 75 meV monochromatorbandpass, i.e., in Auger resonant Raman conditions, and the participator Auger spectrum observed. The C 1s(2σ)^{−1}2π^{1} v’=1 resonance is also excited and the corresponding participator Auger spectrum observed with a monochromatorbandpass slightly larger than the inherent width. The results are compared to theoretical simulations using coherent lifetime‐vibrational interference theory which accounts for the details of the spectrum. We have observed an interference shift on the transitions to different vibrational sublevels in the final state. A high resolution C 1sphotoelectron spectrum of CO is also presented. The lifetime width of the C 1s core–hole state is determined to be 97(10) meV, whereas the C 1s(2σ)^{−1}2π^{1}resonance is measured to have a width of 86(10) meV.

Infrared vibrational spectra of matrix‐isolated cyclic Li_{2}F_{2}, Li_{3}F_{3}, and Li_{4}F_{4} isotopomers
View Description Hide DescriptionInfrared (IR) spectra of the ^{6,7}Li isotopomers of the LiF species isolated in rare gas matrices are interpreted with the input of molecular geometries and vibrational spectra computed using ab initio molecular orbital (MO) theory at the MP2/6‐31G* level. The MO computations, together with IR matrix‐isolation observations, support firm species assignments and vibrational assignments for the IR‐active modes of the title molecules. The study presents the first evidence for the existence of matrix‐isolated Li_{4}F_{4} cyclic tetramer, and resolves contradictory species/vibrational assignments for the smaller oligomers. Most of the matrix‐isolated cyclic trimer and tetramer population is formed by coalescence of smaller species during the matrix deposition process or during subsequent matrix annealing/warming processes. There appears to be no evidence in the available matrix‐isolation data for the existence of a matrix‐stabilized Li_{2}F_{2} linear dimer.

Matrix trapping sites and interactions with LiF monomer
View Description Hide DescriptionInfrared spectra of the LiF monomer isolated in rare gas and RG/X mixed‐matrices are interpreted with the guidance of ab initio molecular orbital (MO) computations performed on FLiAr_{ n } clusters and on FLiX van der Waals complexes (X=Ar, N_{2}, CO, O_{2}, and CH_{4}). The work suggests there are two distinct trapping subsites for LiF in a single‐substitutional vacancy of the Ar fcc crystal. In the primary subsite LiF lies on a tetragonal crystal axis and makes four near‐optimal LiAr contacts; in the metastable subsite it lies on a trigonal axis and makes three near‐optimal LiAr contacts. The model is supported by its account of the large Ar‐induced vibrational frequency shift, spectral doublet, and temperature‐dependent infrared (IR) absorption profile of the LiF vibrational fundamental. IR observations of LiF isolated in several RG/X mixed matrices support the existence of FLiX van der Waals complexes isolated in Ar double‐substitutional sites of the lattice.

Far‐wing excitation study on the transit region of Hg ^{3} P _{1}→^{3} P _{0} intramultiplet process in collisions with N_{2} and CO
View Description Hide DescriptionLaser‐pump and probe approach has been applied to the far wings of Hg^{3} P _{1}−^{1} S _{0} resonance line broadened by collisions with N_{2} and CO to measure excitation spectra for the formation of Hg(6 ^{3} P _{0}) and Hg(6 ^{3} P _{1}). The excitation spectra are highly asymmetric with the red wing being much more extended than the blue wing. The absolute ratio of nascent yields of Hg^{3} P _{0} to ^{3} P _{1} is determined as a function of the excitation wave number. From these measurements, it is found, commonly for Hg–N_{2} and Hg–CO systems, that (a) the nascent product ratio, Hg(^{3} P _{0})/Hg(^{3} P _{1}), grows on the red‐wing surface (the Ã state) with increasing shift, Δν, of the excitation wave number from the line center and finally surpasses unity; (b) the blue‐wing surface (the B̃ state) gives mostly Hg(^{3} P _{1}) but has a small chance to give Hg(^{3} P _{0}). Time constant τ_{0} for the Ã→^{3} P _{0} process of Hg–N_{2} is found to change from 17 to 35 ns as the absorption distance R _{ c } between Hg and N_{2} changes from 3.6 to 4.7 Å. From these values of τ_{0}, the transition probability P(Ã→^{3} P _{0}) for a single approach of Hg–N_{2} to the turning point region is estimated to be about 3.7×10^{−5}. The transition probability P(B̃→^{3} P _{0}) is about 270 times larger than P(Ã→^{3} P _{0}).
CO is about 20 times more effective than N_{2} for the B̃→^{3} P _{0} process. The R _{ c } dependence of τ_{0} can be qualitatively explained by the vibrational frequencies of the bound Ã state and the Franck–Condon factor between the bound Ã state and the free (repulsive) ã state arising from Hg(^{3} P _{0})+N_{2}. These findings suggest that the direct Ã→ã transition is realized in these Hg–N_{2} and Hg–CO collisions. This gives a remarkable contrast to Hg‐atom collisions, where the A→^{3} P _{0} process is parity‐forbidden due to the 0^{+} and 0^{−} characters of the A and a states, respectively. The coupling mechanisms for the Ã→ã and B̃→ã transitions in Hg–N_{2} collisions are discussed in detail. The theoretical estimate of the Ã→ã transition probability is made to be compared with the experimental value.

Very large zero field splittings in the triplet state of an asymmetric top: Rotational analysis and Zeeman effects in the 820 nm ã ^{3} A _{2}– X̃ ^{1} A _{1} band system of selenoformaldehyde
View Description Hide DescriptionHigh‐resolution laser induced phosphorescence spectra of the 820 nm band system of selenoformaldehyde have been recorded with Doppler‐limited resolution. Rotational analysis of the 0^{0} _{0} bands of D_{2}C^{80}Se, H_{2}C^{78}Se, and H_{2}C^{80}Se revealed only P, Q, and R branches obeying ΔK _{ a }=0 selection rules with no evidence of spin splittings. However, these bands show pronounced Zeeman broadening which is linearly dependent on the upper state J value. These results are interpreted as the consequences of case (ab) coupling in the ã ^{3} A _{2} state, in which there is a large zero field splitting due to substantial mixing of two of the spin components with the nearby ^{3} A _{1}(π,π*) state. The only other asymmetric top molecule known to exhibit case (ab) coupling in the triplet state is carbon disulfide.

The electronic spectroscopy of the Ba^{+}–Ar complex: Potential surface and dissociation energies
View Description Hide DescriptionBa^{+}–Ar open‐shell ionic complexes were produced in a pulsed free‐jet expansion. The dispersed emission and both the low and high resolution A ^{2}Π–X ^{2}Σ^{+} excitation spectra of the Ba^{+}–Ar complex are reported. The data obtained were used to construct potentials for the ground and excited states. A simple quantum mechanical model was introduced in order to simulate the experimentally measured potentials. The model potential is used to estimate the dissociation energy of the ground ^{2}Σ^{+} state. This value, when combined with the spectral red shift, allows the dissociation energies of the two components of the excited ^{2}Π state to be determined. The same electrostatic interaction model also explains the observed angular momentum coupling scheme as well as the much stronger binding in the excited ^{2}Π state.

Renner–Teller effect and Rydberg‐valence mixing in the N and O K‐edge photoabsorption spectra of N_{2}O
View Description Hide DescriptionWe have measured high‐resolution angle‐resolved ion‐yield spectra of the O and N 1s excited N_{2}O molecules (N_{ t }–N_{ c }–O) and investigated them with the help of ab initio quantum chemical calculations. The peak width of the N_{ c } 1s→π* transition is larger than that of the N_{ t } 1s→π* one. This mainly arises from different populations of bending vibrations excited through the Renner–Teller effect, which breaks the degeneracy of the 1s→π* excited states by bending the linear molecule. The angular distribution of fragment ions emitted after the Auger decay is also affected by the Renner–Teller effect. That is, fragment ions significantly lose the information of the Π symmetry of the linear molecule. For the 1s→Rydberg excitations no Renner–Teller effect is observed and the angular distribution is directly related to the Σ and Π symmetries. Furthermore, it is found that the peak intensities of the O and N_{ t } 1s→nsσ Rydberg transitions are unusually large. The N_{2}O molecule has two σ* orbitals, σ_{ s } ^{*}(8σ) and σ_{ p } ^{*}(9σ), below and above the ionization threshold and the 1s→σ_{ s } ^{*}excited states are mixed with the O and N_{ t } 1s–nsσ Rydbergexcited states; on the other hand, the N_{ c } 1s→σ_{ s } ^{*} transition is almost dipole‐forbidden because of the orbital symmetry.

The structure of small nickel clusters. II. Ni_{16}–Ni_{28}
View Description Hide DescriptionThe molecular adsorption of nitrogen on nickelclusters is used to probe the clusters’ geometricalstructures. The application of nitrogen binding rules derived from earlier studies of both larger and smaller nickelclusters allows a determination of structure from nitrogen uptake patterns. In the 16‐ and 28‐atom size region clusterstructure is dominated by local pentagonal symmetry, a consequence of a preference for close packing of atoms on clusters with curved surfaces. In most cases, the structures that result can be derived from the 13‐atom icosahedron, the polyicosahedral 19‐, 23‐, and 26‐atom clusters, and the 55‐atom icosahedron, by adding or removing atoms. Icosahedral and polyicosahedral clusters often have substantial surface strain, which in some cases is relieved by deviations from the ideal geometry.Structures are proposed for all clusters in the Ni_{16} to Ni_{28} size range, with the exception of Ni_{27}. Generally, there is no evidence for structural changes as a consequence of nitrogen binding, so that the proposed structures are those of the bare as well as the nitrogenated clusters. Where possible, comparison with existing theoretical calculations of nickelclusterstructure is made.

Spectral projection approach to the quantum scattering calculations
View Description Hide DescriptionA new method of implementing scattering calculations is presented. For the S‐matrix computation it produces a complete set of solutions of the wave equation that need be valid only inside the interaction region. For problems with small sizes the method is one of several that are practical in the sense that it involves merely a real symmetric Hamiltonian represented in a minimal L^{2} basis set. For more challenging larger systems it lends itself to a very efficient time independent iterative procedure that obtains results simultaneously at all energies. A modified Chebyshev polynomial expansion of (E−Ĥ)^{−1} is used. This acts on a set of energy independent wave packets located on the edge of the interaction region. The procedure requires minimal storage and is shown to converge rapidly in a manner that is uniform in energy.

Quantum dynamics study of the reaction HD+OH→H+DOH, D+HOH
View Description Hide DescriptionAccurate time‐dependent (TD) quantum wavepacket calculations are reported for the combustionreaction HD+OH. Due to the lack of symmetry, the HD+OH reaction has roughly twice the number of channels of the corresponding H_{2}+OH reaction and produces two distinguishable products–HOH and HOD. In order to make the TD calculation possible on workstations with limited memories, we employed a normalized quadrature scheme in the wavepacket propagation by the split‐operator propagator. The normalized quadrature scheme eliminates the need to store large matrices during the wavepacket propagation while preserving the unitarity of the split‐operator propagator and producing numerically stable results. This approach made TD dynamics calculations possible on small‐memory workstations for the title reaction and for other polyatomic reactions.Reaction probabilities, cross sections, rate constants, and reaction branching ratios are reported in this paper for the title reaction. The observed strong dependence of the reaction probabilities on the reactive HD rotation and the relative weak dependence on the nonreactive OH rotation are explained in terms of a steric effect. The isotope effect in the branching ratio is examined and physical explanation is given for the observed branching ratio at low and high kinetic energies.

On the ‘‘direct’’ calculation of thermal rate constants
View Description Hide DescriptionWe present a new approach for the direct (and correct) calculation of thermal rate constantsk(T) (‘‘direct’’ meaning that one avoids having to solve the state‐to‐state reactive scattering problem, and ‘‘correct’’ meaning that the method contains no inherent approximations). The rate constant is obtained from the long time limit of the flux‐position correlation function,C _{ f,s }(t), whose calculation is made efficient by taking advantage of the low rank of the flux operator. Specifically, the trace required to obtain C _{ f,s }(t) is evaluated by a Lanczos iteration procedure which calculates only the nonzero eigenvalues. The propagation in complex time, t _{ c }=t−iℏβ/2, is carried out using a Chebychev expansion. This method is seen to be both accurate and efficient by application to the Eckart barrier, the collinear H+H_{2}reaction, and the three‐dimensional D+H_{2} (J=0) reaction.

Temperature dependent quenching of A ^{2}Σ^{+} NO between 215 and 300 K
View Description Hide DescriptionCollisional quenching of the v’=0 vibrational level of the A ^{2}Σ^{+} state of nitric oxide has been studied over the temperature range 215 to 300 K for the collision partners CO_{2}, O_{2}, H_{2}O, NH_{3}, H_{2}S, and NO itself. The pressure dependence of the time decay of laser‐induced fluorescence furnished the quenching cross sections σ_{ Q }. The temperature is measuredin situ. For all colliders, σ_{ Q } is large and increases as the temperature decreases; this includes NO and O_{2} which do not show a temperature dependence above 300 K. The temperature dependence of σ_{ Q } for each collider can be adequately described by the simple attractive force model of Parmenter and co‐workers.

Highly efficient collisional stabilization and the symmetry constrained dynamics of high temperature complex formation
View Description Hide DescriptionThe highly efficient collisional stabilization of high temperature complexes of some considerable spatial extent is demonstrated. A series of near single collision and well defined multiple collision (following paper) chemiluminescent and laser induced fluorescent studies extending over six decades of pressure demonstrate the stabilization of electronically excited group IIA dihalide collision complexes via a radiative three body recombination process (R3BR) operative at microTorr pressures. Over the pressure range 1×10^{−6}–5×10^{−4} Torr, a comparative study of the emission from M (M=Ca, Sr, Ba)–X_{2} (Cl_{2},Br_{2},I_{2}) and M–XY (ICl,IBr) reactive encounters identifies a symmetry constrained dynamics associated with the formation of the dihalide product complexes. The onset of the monitored R3BR process at 1×10^{−6} Torr signals an extremely large stabilization cross section (σ_{ S }≳3000 Å^{2}) which may not be readily explained within the RRKM framework. Comparisons between the highly ionic dihalides and the isoelectronic CO_{2} molecule are noted as they affect excited state dynamics. The pressure dependence of the light emission from these complexes in the near single collision pressure range displays a striking correlation with the periodicity of dihalide molecular electronic structure and the resultant nature of the low‐lying dihalide electronic transitions. The absence of a difluoride emission associated with the M(Ca,Sr,Ba)–F_{2} reactive encounters signals an important periodic trend in these systems. A simple first order model within the electron jump framework is presented to explain the qualitative trends inherent to these reactions.

Confirmation of long‐range collision complex stabilization through the controlled relaxation of high internal excitation
View Description Hide DescriptionA series of controlled multiple collision chemiluminescent and laser induced fluorescent studies confirm the long‐range collisional stabilization of high temperature group IIA dihalide complexes of some considerable spatial extent. The relaxation process demonstrates that the pseudocontinuum emissions observed under near single collision conditions [J. Chem. Phys. 102, 7425 (1995)] correspond to the overlap of a closely spaced, highly excited, rovibronic distribution. Controlled relaxation reveals the first vibrationally resolved electronic emission for the dihalides. The vibronic structure of the observed emission spectra correlates well with expectations based on the molecular electronic structure of the ground and low‐lying electronic states of the dihalides. The vibronically resolved emission from the Sr+ICl^{He} _{→}SrICl* and mixed halogen Sr+Cl_{2}, Br_{2} _{→} ^{He}SrCl_{2} ^{*}+SrBr_{2} ^{*}reactions provides strong support for the formation of a collisionally stabilized dihalide complex. These results, correlated with near single collision studies, form a basis for the discussion of (1) the kinetics of formation of the dihalide complexes and (2) the implications of long‐range collisional stabilization. Current theories may not accurately model these observations. Dihalide complex formation as it influences the energy partitioning to metal monohalide excited states may account for the discrepancies between those monohalide bond strengths determined by mass spectrometry and chemiluminescent techniques.

Theory of nonadiabatic transition for general two‐state curve crossing problems. II. Landau–Zener case
View Description Hide DescriptionNew accurate and compact formulas are established for general two‐state curve crossing problems in the Landau–Zener case, in which the two diabatic potentials cross with the same sign of slopes. These formulas can cover practically the whole range of energy and coupling strength, and can be directly applied to various problems involving the curve crossing. All the basic potential parameters can be estimated directly from the adiabatic potentials and nonunique diabatization procedure is not required. Complex contour integrals are not necessary to evaluate the nonadiabatic transition parameter; thus the whole theory is very convenient for various applications. The compact formula for the Landau–Zener transition probability, which is far better than the famous Landau–Zener formula, is proposed. Now, together with the previous paper [Zhu and Nakamura, J. Chem. Phys. 101, 10 630 (1994)], the present semiclassical theory can present a complete set of solutions of the the two‐state curve crossing problems.

Classical analysis of diatomic dissociation dynamics in intense laser fields
View Description Hide DescriptionThe dissociation of a diatomic ion in an intense laser field is studied using a one‐dimensional model with a Morse function representing the nuclear interaction potential, and coupling to a linear dipole moment representing the interaction with the laser field. A perturbative treatment is generally not possible because the field strengths employed are large enough to significantly distort the potential surface. Instead, classical trajectories are used to investigate some qualitative features of the dissociation process, with the goal of introducing some simple models to explain these features. A modified barrier suppression model is proposed which predicts the field strength at which trajectories first start to dissociate, and a ‘‘wagging tail’’ model is proposed which predicts the maximum kinetic energy of the dissociation products. Both these models provide physical insight into the dissociation process, and can be used to qualitatively understand experimental results.

Charge renormalization at the large‐D limit for N‐electron atoms and weakly bound systems
View Description Hide DescriptionWe develop a systematic way to determine an effective nuclear charge Z ^{ R } _{ D } such that the Hartree–Fock results will be significantly closer to the exact energies by utilizing the analytically known large‐D limit energies. This method yields an expansion for the effective nuclear charge in powers of (1/D), which we have evaluated to the first order. This first order approximation to the desired effective nuclear charge has been applied to two‐electron atoms with Z=2–20, and weakly bound systems such as H^{−}. The errors for the two‐electron atoms when compared with exact results were reduced from ∼0.2% for Z=2 to ∼0.002% for large Z. Although usual Hartree–Fock calculations for H^{−} show this to be unstable, our results reduce the percent error of the Hartree–Fock energy from 7.6% to 1.86% and predicts the anion to be stable. For N‐electron atoms (N=3–18, Z=3–28), using only the zeroth order approximation for the effective charge significantly reduces the error of Hartree–Fock calculations and recovers more than 80% of the correlation energy.