Volume 104, Issue 8, 22 February 1996
Index of content:

Photoelectron spectroscopy and zero electron kinetic energy spectroscopy of germanium cluster anions
View Description Hide DescriptionAnion photoelectron spectra of Ge^{−} _{ n }, n=2–15, have been measured using an incident photon energy of 4.66 eV. In addition, the spectra of Ge^{−} _{2}, Ge^{−} _{3}, and Ge^{−} _{4} have been measured at photon energies of 3.49 and 2.98 eV. From these spectra the electron affinity of the corresponding neutral cluster has been determined. Vibrational frequencies and term values for several electronic states of Ge^{−} _{2} and Ge^{−} _{3} have been determined. Vibrational structure in the ^{3} B _{3u }excited state of Ge_{4} has been resolved using zero electron kinetic energy (ZEKE) photoelectron spectroscopy. The assignment of the spectra of Ge^{−} _{3} and Ge^{−} _{4} is facilitated by a comparison to the similar spectra of Si^{−} _{3} and Si^{−} _{4}, respectively. The spectra of the larger clusters, Ge^{−} _{ n }, n=5–15, are characterized by many broad structureless features which indicate the presence of multiple electronic transitions. Several of these were assigned based on comparison with previous ab initio calculations on germanium and silicon clusters.

Assignment of the ^{3}Π_{ u }←X ^{3}Σ^{−} _{ g } bands of O_{2} observed in the region 1040–1200 Å
View Description Hide DescriptionA comprehensive vibronic assignment of the ^{3}Π_{ u } states in the energy region approaching the first ionization threshold of O_{2} is presented for the first time. Measurements of seventeen ^{3}Π_{ u }←X ^{3}Σ^{−} _{ g } bands in the 85 800–93 000 cm^{−1} region of the ground‐state photoabsorption spectra of ^{16}O_{2} and ^{18}O_{2} at 79 K are reported, including six bands in the spectrum of ^{18}O_{2} for the first time. Irregularities in vibrational spacings and in the extent of broadening by predissociation are shown to result from Rydberg‐valence interactions. The observed transitions have been assigned, using semiempirical modeling of the ^{3}Π_{ u } Rydberg‐valence interactions based on the coupled‐channel Schrödinger equations technique, to two vibrational progressions with upper states which we call F ^{3}Π_{ u } and F′ ^{3}Π_{ u }. The F and F′ states are formed from the avoided crossings of the npσ_{ u } (n=3–5) Rydberg potential‐energy curves by the lowest repulsive ^{3}Π_{ u } valence potential‐energy curve.

Vibrational spectrum and structure of silicon trioxide SiO_{3}: A matrix isolation infrared and density functional theory study
View Description Hide DescriptionMolecular silicon trioxide has been isolated in argon matrices at low temperature (i) as a secondary reaction product within Si+2(O_{2}) aggregates or (ii) as a primary product in the direct reaction of silicon monoxide with oxygen molecules after UV excitation at 5.17 eV. The silicon trioxide molecule has been characterized through the observation of five fundamental vibrations for at least eight different isotopic species, namely ^{28}Si^{16}O_{3}, ^{29}Si^{16}O_{3}, ^{30}Si^{16}O_{3}, ^{16}OSi^{18}O_{2}, ^{18}OSi^{16}O^{18}O, ^{16}OSi^{16}O^{18}O, ^{18}OSi^{16}O_{2}, and Si^{18}O_{3}. From the experimental data, a structure of the C _{2v } symmetry can be deduced with a Si=O double bond much shorter than two equivalent Si–O single bonds. These findings are fully supported by DFT calculations, which moreover predict a large binding energy of about 60 kcal/mol with respect to the separate fragments (SiO and O_{2}). The possible reaction pathways are also discussed.

Cavity ringdown laser absorption spectroscopy and time‐of‐flight mass spectroscopy of jet cooled platinum silicides
View Description Hide DescriptionThe cavity ringdown technique (CRLAS) has been employed to measure the gas phase absorptionspectrum of the platinum silicide molecule in the 350 nm region. All nine of the measured rovibronic bands are assigned to a single ^{1}Σ–^{1}Σ electronic transition, with a ground state vibrational frequency of ω_{ e } ^{″}=549.0(3) cm^{−1}, and a bond length of r _{0} ^{″}=2.069(1) Å. The results of this study are compared with experimental data for the coinage metal silicides. Additionally, time‐of‐flight mass spectrometric results indicate that a variety of polyatomic metal silicides are formed in our molecular jet expansion.

Resonance enhanced multiphoton ionization spectroscopy of the PF radical
View Description Hide DescriptionPF radicals in both their ground (X ^{3}Σ^{−}) and metastable (a ^{1}Δ) electronic states have been produced by the gas phase reaction of F atoms with phosphine in a discharge flow reactor and detected by mass selective resonance enhanced multiphoton ionization (REMPI) spectroscopy in the wavelength range 410–225 nm. Analysis of the longer wavelength end of this spectrum (λ≳328 nm) has enabled identification and spectroscopic characterization of five hitherto unknown Rydberg states of this radical. These we label the h ^{1}Σ^{+} (T _{0}=57 324.9 cm^{−1}), D ^{3}Δ (T _{0}=58 223.0 cm^{−1}), E ^{3}Σ^{−} (T _{0}=58 690 cm^{−1}), i ^{1}Δ (T _{0}=59 881.6 cm^{−1}), and j ^{1}Σ^{−} (T _{0}=61 873.9 cm^{−1}) states, respectively. [Origins, relative to the lowest rovibrational level of the ground (X ^{3}Σ^{−}) state are indicated by the numbers in brackets.] Quantum defect arguments suggest that all five states derive from the electronic configuration [^{2}Π]4pπ^{1}. Within the shorter wavelength region we identify two additional sets of poorly resolved resonances which we associate with a further two Rydberg states belonging to series converging to the ground stateionization limit, followed by two extensive progressions of bands. Possible assignments for these two progressions are considered.

Spectroscopic properties of the bound n=3 states of H^{+} _{2}
View Description Hide DescriptionSpectroscopicproperties are determined for the six bound electronic states of H^{+} _{2} that correlate as nuclear separation R → ∞ with a proton and a hydrogen atom in an n=3 state. Two antibonding orbitals 5gπ_{ g } and 6hσ_{ u } have significant potential wells and an infinite number of bound vibrational states due to strongly attractive pole–dipole leading multipole terms. Vibrational–rotational eigenenergies are reported for three angular momentumJ values for each bound state. Coefficients up to a _{6} are listed for Dunham power‐series expansions of the vibrational potentials V(R) about the equilibrium separations R _{ e }. The higher a coefficients come almost entirely from the nuclear Coulomb repulsion only for the 3dδ_{ g } state, the one state that is lowest of its symmetry type and is therefore not perturbed from below. Dunham Y _{ ij } are determined also; the leading vibrational Y _{10}, rotational Y _{01}, and anharmonicity ‖Y _{20}‖ constants are very small due to the very broad potential wells and large values of R _{ e } for all six n=3 electronic states. For five of the six states, Y _{00} is negative and therefore the vibrational quantum number v _{ e } at the potential minimum is ≳−1/2. The wavelength range of the bound–bound (n=3)↔(n=2)H^{+} _{2} transitions is 3800–9700 Å; many unidentified diffuse interstellar bands have been reported in this range.

Effects of vibronic coupling on the local Frenkel states in doped naphthalene crystals
View Description Hide DescriptionEffects of vibronic coupling on charge transfer induced local Frenkel exciton states (CTILFES) in doped naphthalene crystals are discussed. Two simple coupling models are proposed, one suitable for neat and the other for doped crystals. The results suggest that the difference between the stabilization energy of the local state and its vibrational replica may be attributed to the vibronic interaction with charge transfer states.

All optical triple resonance spectroscopy of the A ^{1}Σ^{+} _{ u } state of ^{7}Li_{2}
View Description Hide DescriptionThe A ^{1}Σ^{+} _{ u } state of ^{7}Li_{2} has been studied using all optical triple resonance spectroscopy. Vibrational levels v=27–62 and rotational levels ranging from J=0 to 27 have been observed. This represents a region between previous data from single photon experiments and new data from cold atom photoassociative spectroscopy. Our data was fit to a Dunham expansion and the resulting molecular constants reproduce most observed energies to within 0.050 cm^{−1}. A Rydberg–Klein–Rees potential curve has also been constructed and the calculated eigenvalues are in good agreement with observed energies.

Vibrational spectra of penta‐atomic silicon–carbon clusters. III. Pentagonal Si_{3}C_{2}
View Description Hide DescriptionFourier transform infrared measurements on the spectra of the products of the vaporization of silicon/carbon mixtures trapped in argon at ∼10 K combined with the results from a published ab initio study and from new theoretical calculations carried out in the present work, have resulted in the first observation of a spectrum for the pentagonal Si_{3}C_{2} cluster. Three vibrational fundamentals have been assigned: the ν_{2}(a _{1}) symmetric ‘‘breathing’’ vibration at 681.1 cm^{−1}, the ν_{7}(b _{2}) Si_{α}–C–C–Si_{α} stretching deformation at 956.7 cm^{−1}, and the ν_{8}(b _{2}) C–Si_{β}–C antisymmetric stretching vibration at 597.8 cm^{−1}. The observed frequencies, relative intensities, and ^{13}C, ^{29}Si, and ^{30}Si isotopic shifts are in excellent agreement with the theoretical predictions.

Thermal rate constants of the N_{2}+O→NO+N reaction using ab initio ^{3} A″ and ^{3} A′ potential energy surfaces
View Description Hide DescriptionTheoretical determinations of the thermal rate constants and product energy distributions of the N_{2}+O→NO+N reaction, which plays a crucial role in hydrocarbon air combustion and high temperature air chemistry, are carried out using a quasiclassical trajectory method. An analytical fit of the lowest ^{3} A′ potential energy surface of this reaction based on the CCI ab initio data is obtained. The trajectory study is done on this surface and an analytical ^{3} A″ surface proposed by Gilibert et al. [J. Chem. Phys. 97, 5542 (1992)]. The thermal rate constants computed from 3000 to 20 000 K are in good agreement with the available experimental data. In addition, the dependence of the rate constant on the N_{2} internal state is studied. It is found that a low vibrational excitation can reduce the rate constant of this reaction by a factor of 3. Also, we investigate the effect of the N_{2}vibrational state on the product NO vibrational distribution, and it is found that at low N_{2}vibrational states, the NO vibrational distribution is nearly Boltzmann. However, at N_{2}(v≳10), the product distribution is almost uniform at low energy levels.

Reaction path Hamiltonian based on a reaction coordinate and a curvature coordinate
View Description Hide DescriptionWe propose a reaction path‐based Hamiltonian in terms of the reaction coordinate, the curvature coordinate, the remaining 3N−8 transverse normal coordinates (whose directions are orthogonal to the path tangent and curvature vectors), and their conjugate momenta, for an N atomic reaction system. The 3N−8 transverse vibrational modes are independent of the motion along the reaction path, although they have coupling terms with the curvature direction in the harmonic approximation. A two‐dimensional plane determined by the path tangent and curvature vectors is termed the ‘‘reaction plane.’’ We introduce a function that estimates changes of the reaction plane along the reaction path, and analyze the reaction path based on this function for an abstraction reaction, CH_{3}+H_{2}→CH_{4}+H. The scheme proposed here should be effective when a reaction path has a sharply curved region.

The planar reaction OH+H_{2}→H_{2}O+H: A quasiclassical trajectory study
View Description Hide DescriptionWe report a quasiclassical trajectory(QCT) study of the reaction OH+H_{2}→H_{2}O+H performed on the same model as that used in a recent exact quantum treatment. The reaction is constrained to occur on a plane, with the OH bond frozen. We used Clary’s modification to the Schatz–Elgersma potential energy surface. Initial state selected reaction probabilities and cross sections were calculated at several energies between 0.31 and 0.65 eV. The effect of rotational excitation of H_{2} and OH on reaction probabilities was studied and QCT results were compared with exact quantum results for the same model. We found very good quantitative agreement between quantum and QCT results. Furthermore, QCT calculations confirm the qualitative effects predicted by the quantum treatment; exciting the rotation of H_{2} decreases reactivity while the rotation of OH has almost no effect. Similarities and differences between the present QCT results with 2D and 3D quantum and classical results by other authors are discussed.

Classical/quantal method for multistate dynamics: A computational study
View Description Hide DescriptionWe discuss a classically‐motivated method for modeling ultrashort laser pulse optical excitation. The very same method can be used to treat the breakdown of the Born–Oppenheimer approximation. The results are compared to numerically‐exact quantum mechanics for a model problem representing excitation from the X (ground) state to the B (excited) state of molecular iodine. Expectation values and final B state populations are predicted quantitatively. The method provides a new way to simulate pump–probe experiments in particular and multistate dynamics in general. The method appears extendible to multidimensional problems. We argue that the increase of effort with dimensionality will be similar to that encountered in classical mechanical simulations as opposed to the exponential scaling of numerically‐exact quantum mechanical propagation techniques.

Pressure dependence and metastable state formation in the photolysis of dichlorine monoxide (Cl_{2}O)
View Description Hide DescriptionThe photodissociation of dichlorine monoxide (Cl_{2}O) was studied using broadband flash photolysis to investigate the influence of variations in the photolysis wavelength domain, bath gas pressure and bath gas identity on the yield and temporal dependence of the ClO product. ClO yields were independent of bath gas pressure when the photolysis spectral band extended to 200 nm (quartz cutoff) but for photolysis restricted to wavelengths longer than about 250 nm, ClO yields decreased with increasing bath gas pressure and there was a pressure‐dependent delay in the formation of ClO. Under these conditions, a weak, highly structured absorptionspectrum was observed in the range 16 600–26 000 cm^{−1} with a lifetime on the order of 500 ms. A portion of the spectrum could be analyzed (22 000–26 000 cm^{−1}) which showed progressions having differences of 283, 443, and 505 cm^{−1}. Ab initio calculations were performed to evaluate vertical excitation energies and oscillator strengths from the lowest‐energy singlet (X ^{1} A _{1}) or triplet (1 ^{3} B _{1}) states to various excited states. The calculations indicated that the 2 ^{3} A _{2}←1 ^{3} B _{1} transition has an unusually large oscillator strength. The transition energy, 3.05 eV, is consistent with the observed metastable spectrum. The observed pressure dependence of ClO formation could be modeled using a mechanism which assumed that Cl_{2}O excitation at wavelengths longer than about 300 nm leads to rapid intersystem crossing to two metastable states in the triplet manifold. These states undergo competitive dissociation to ClO+Cl and collisional relaxation to the ground state. The dynamics of Cl_{2}O may serve as a model for other molecules of importance in the earth’s lower stratosphere such as ClONO_{2} where filtering of the solar spectrum by ozone restricts photolysis to the weak tail of the absorption continuum.

Hydrogen transfer in vibrationally relaxing benzoic acid dimers: Time‐dependent density matrix dynamics and infrared spectra
View Description Hide DescriptionWe employ time‐dependent density matrix theory to characterize the concerted double‐hydrogen transfer in benzoic acid dimers—the ‘‘system’’—embedded in their crystalline environment—the ‘‘bath.’’ The Liouville–von Neumann equation for the time evolution of the reduced nuclear density matrix is solved numerically, employing one‐ and two‐dimensional models [R. Meyer and R. R. Ernst, J. Chem. Phys. 93, 5528 (1990)], the state representation for all operators and a matrix propagator based on Newton’s polynomials [M. Berman, R. Kosloff, and H. Tal‐Ezer, J. Phys. A 25, 1283 (1992)]. Dissipative processes such as environment‐induced vibrational energy and phase relaxation, are accounted for within the Lindblad dynamical semigroup approach. The calculation of temperature‐dependent relaxation matrix elements is based on a microscopic, perturbative theory proposed earlier [R. Meyer and R. R. Ernst, J. Chem. Phys. 93, 5528 (1990)]. For the evaluation of the dissipative system dynamics, we compute (i) time‐dependent state populations, (ii) energy and entropy flow between system and bath, (iii) expectation values for the hydrogen transfer coordinate, (iv) characteristic dephasing times and (v) temperature‐dependent infrared spectra, determined with a recently proposed method by Neugebauer et al. Various ‘‘pure’’ and ‘‘thermal’’ nonequilibrium initial states are considered, and their equilibration with the bath followed in time.

State‐to‐state dissociation of OClO(Ã ^{2} A _{2}ν_{1},0,0)→ClO(X ^{2}Π_{Ω},v, J)+O(^{3} P)
View Description Hide DescriptionApplying the two‐photon laser‐induced fluorescence technique for nascent state resolved ClO(X ^{2}Π_{Ω},v,J) detection, the photofragmentation dynamics of OClO in the (Ã ^{2} A _{2}11,0,0) and the (Ã ^{2} A _{2}18,0,0) state is investigated at fixed photolysis wavelengths of 351 nm and 308 nm. In both experiments the product fragments are formed in their electronic ground states, namely ClO(^{2}Π_{Ω}) and O(^{3} P). A complete product state analysis proves the ClO radicals originating from the OClO(Ã ^{2} A _{2}11,0,0) dissociation at 351 nm to be formed in v=0–4 vibrational states with a spin–orbit ratio of P(^{2}Π_{3/2}):P(^{2}Π_{1/2})=3.8±0.5. The ClO fragment shows moderate rotational excitation. The obtained ClO product state distributions and the relatively high translational energy of the fragments can be explained by predissociation of the (Ã ^{2} A _{2}ν_{1},0,0)‐excited parent molecule in the course of which the initial symmetric stretch motion (ν_{1}) of OClO is transferred into the dissociative asymmetric stretching mode (ν_{3}). ClO line profile measurements indicate a dissociation time of less than 0.5 ps. Resulting from the OClO(Ã ^{2} A _{2}18,0,0) dissociation at 308 nm ClO is generated in very high vibrational states. The rotational excitation is comparable to that of the 351 nm photolysis study.

Electron transfer reactions in clusters: The effect of polar solvents on the (2p3s) Rydberg state of azabicyclo‐octane
View Description Hide Description(1+1) mass resolved excitation spectra are reported for the (2p3s)←(2p)^{2}Rydberg transition of azabicyclooctane (ABCO) van der Waals clusters. The solvent molecules employed in this study are mostly polar. The polar solventcluster spectra are red shifted from those of the bare molecule ABCO by more than 500 cm^{−1} in most cases. This large increase in the interaction energy of the ABCO molecule Rydberg state in polar solvent clusters with respect to that of the ground state ABCO cluster is due to an exchange delocalization or electron transfer interaction for the excited state cluster. The ABCO Rydberg state electron is delocalized into the available (virtual) orbitals of the polar solvent molecule. Relaxation dynamics are measured for the generation of the electron transfer state of the cluster. This behavior is similar to that characterized for other cyclic amines in polar solvent clusters.

The Hessian biased singular value decomposition method for optimization and analysis of force fields
View Description Hide DescriptionWe present methodology (HBFF/SVD) for optimizing the form and parameters of force fields (FF) for molecular dynamics simulations through utilizing information about properties such as the geometry, Hessian, polarizability, stress (crystals), and elastic constants (crystals). This method is based on singular value decomposition (SVD) of the Jacobian describing the partial derivatives in various properties with respect to FF parameters. HBFF/SVD is effective for optimizing the parameters for accurate FFs of organic, inorganic, and transition metalcompounds. In addition it provides information on the validity of the functional form of the FF for describing the properties of interest. This method is illustrated by application to organic molecules (CH_{2}O, C_{2}H_{4}, C_{4}H_{6}, C_{6}H_{8}, C_{6}H_{6}, and naphthalene) and inorganic molecules (Cl_{2}CrO_{2} and Cl_{2}MoO_{2}).

A periodic orbit analysis of the vibrationally highly excited LiNC/LiCN: A comparison with quantum mechanics
View Description Hide DescriptionBy constructing continuation/bifurcation diagrams of families of periodic orbits of LiNC/LiCN system the spectroscopy and dynamics for this species are deduced and compared with accurate quantum mechanical calculations up to 13 000 cm^{−1}. The interesting phenomenon of the appearance of gaps in the continuation diagram of the principal family that corresponds to the bend motion is shown to occur in both isomers, LiNC and LiCN. Through semiclassical quantization a one to one correspondence of specific periodic orbits to certain eigenstates is demonstrated. One interesting example is the case of periodic orbits that are generated from a saddle‐node bifurcation and describe rotations of the Li^{+} ion around the CN^{−} fragment. The correspondence of these periodic orbits to regular rotating type eigenfunctions is shown, thus, demonstrating in a clear way the importance of the saddle‐node bifurcations in locating localized wavefunctions in highly energized molecules.

On the role of conical intersections in photodissociation. IV. Conical intersections and the geometric phase in the 2 ^{3} A″ and 3 ^{3} A″ states of CH_{2}
View Description Hide DescriptionThe 2 ^{3} A″–3 ^{3} A″(1 ^{3} A _{2}–2 ^{3} B _{1}) conical intersection seam in CH_{2} is characterized directly, that is without prior determination of the potential energy surfaces. Nuclear configurations in the vicinity of the seam are characterized in terms of a set of natural polar coordinates defined directly from the seam mapping procedure. Using these coordinates the geometric, or Berry phase, effect is demonstrated and the topology and interstate couplings in the vicinity of the seam are analyzed. The techniques used to study this surface of intersection are readily applied to larger polyatomic systems.