Volume 106, Issue 9, 01 March 1997
Index of content:

Resonant xray scattering beyond the Born–Oppenheimer approximation: Symmetry breaking in the oxygen resonant xray emission spectrum of carbon dioxide
View Description Hide DescriptionAlthough resonantxray scattering of molecules fulfills strict electronic symmetry selection rules, as now firmly proven by spectra of diatomic molecules, the accumulated body of data for polyatomic molecules indicates that an apparent breaking of these rules represents the common situation rather than the exception. The CO_{2} molecule provides a good example of symmetry breaking, with the oxygen xray emission spectra showing strong intensity for transitions that are forbidden by the parity selection rule. We present timeindependent and timedependent theories for frequencydependent resonantxray scattering beyond the Born–Oppenheimer approximation in order to explore under what circumstances one can anticipate symmetry breaking in the spectra of polyatomic molecules. The theory starts out from the Kramers–Heisenberg dispersion relation and is generalized for vibrational degrees of freedom and for nonadiabatic coupling of the electronic (vibronic) states, including the frequency dependency of the scattering cross section. Different limiting cases and fewlevel models are considered. The symmetry breaking is proven to be the result of pseudoJahn–Tellerlike vibronic coupling between neardegenerate coreexcited states. Thus vibronic interaction over the antisymmetric vibrational mode between the “bright” 1σ_{g} ^{−1}2π_{u} ^{1} and “dark” 1σ_{u} ^{−1}2π_{u} ^{1} intermediate states of CO_{2} allows transitions otherwise forbidden. The measurements and theory demonstrate that the symmetryselective character of the resonant xray emission is strongly frequency dependent. The strong intensity of “dipoleforbidden” transitions in the π^{*} oxygen Kspectrum at resonance is reduced monotonically with the detuning of the excitation energy from resonance, and the spectra become “symmetry purified.” Simulations with full vibronic coupling predict this feature of the xray scattering experiment and a fewlevel model explains the energy dependence of the symmetry selection and the symmetry purification at large detuning energies in the limit of narrowband photon excitation.

Reactions of laserablated Zn and Cd atoms with O_{2}: Infrared spectra of ZnO, OZnO, CdO, and OCdO in solid argon
View Description Hide DescriptionLaser ablatedZn and Cd atoms contain sufficient excess energy to react directly with O_{2} to produce the subject molecules. A zinc isotopic triplet at 769.2, 766.8, 764.5 cm^{−1} gave a doublet with statistical isotopic oxygen confirming the identification of ZnO. A previous resonance Raman fundamental at 769 cm^{−1} is reassigned to ZnO, which suggests a green visible absorption for the zinc oxide molecule. A zinc isotopic triplet at 748.2, 744.4, 740.9 cm^{−1} produced a triplet with statistical isotopic oxygen and isotopic shifts appropriate for the linear OZnO dioxide molecule. Cadmium is less reactive than zinc; infrared absorptions at 645.1 and 626.6, 625.4 cm^{−1} exhibit proper oxygen isotopic behavior for assignment to CdO and OCdO and the latter showed partially resolved cadmium isotopic splittings.

Photoion spectra of C molecules at resonance excitation and ionization energies near the C edge
View Description Hide DescriptionPhotoionization dynamics of gasphase C molecules after selective excitation with synchrotron radiation has been studied with timeofflight mass spectrometry. The energy level scheme deduced from previous photoelectron and Auger electron spectra is used to analyze the photoion data. At the vicinity of preedge resonances the fragmentation of multiplycharged ions varies strongly as a function of photon energy. The C ion yield decreases slowly after threshold indicating that the slow photoelectrons can get recaptured. Somewhat further above the threshold the enhanced relative fragmentation of the parent C ions suggests excitation of the giant plasmon resonance due to 20 eV photoelectrons.

Tunneling splitting due to weak coupling between methyl rotators in acetylacetone
View Description Hide DescriptionIn acetylacetone the energy level matching spectrum of nuclear Zeeman and tunnelingenergies of CH_{3} groups in the rotating frame of their proton spins demonstrates resonant transfers of population characteristic of a 100 KHz range tunneling splitting. However, in this material two A to Etunneling splittings, of 10.4 GHz and 1 GHz, were observed by inelastic neutron scattering. Therefore the small energy splitting must be a result of a weak torsion–torsion interaction, which has not been resolved in neutron spectroscopy. This small splitting is investigated through the dependencies of its energy level matching spectra on mixing time, tilt angle in the magnetic field, temperature, and the strength of the main dc field H _{0}. Upon heating, the level matching resonances begin to broaden at 10 K and average out to zero at 25 K. The spectrum does not depend on H _{0}. These observations are explained by Zeemantunneling level matching transitions within manifolds of EEEsymmetry states and AEEsymmetry states, both of which are split by the interaction among three CH_{3} groups. Based on resonances detected at ν_{z}=nΔ, where n=1/4, 1/3, 1/2, and 1, this splitting Δ is determined to be 116±2 KHz at 10 K.

Polarization of emission in asymmetric rotors. I. The effects of elastic collisions, electron and nuclear spins
View Description Hide DescriptionWe report measurements of the linear and circular polarization ratios as a function of rotational state for the asymmetric rotor NH_{2}. This molecule displays fine structure splitting from its unpaired electron and hyperfine structure from coupling with the nuclear spins. We present a theory of polarized emission for this molecule which includes the effects of fine and hyperfineinteractions. These have a marked effect on the polarization ratios and are well described by a theory in which the effect of electron and nuclear spin are introduced as timeindependent perturbation coefficients. We find that theory predicts different values of polarization ratio according to the manner of coupling of the proton nuclear spins. The best fits to experimental data are obtained when the coupling follows a physically intuitive scheme rather than that usually adopted. When all intramolecular couplings due to electron and nuclear spins are properly accounted for; there is no depolarization that may be attributed to the effect of elastic collisions. Thus, as in the case of diatomic molecules, orientation and alignment show a marked stability to change by collision.

Charge shifting in the ultrafast photoreactions of ClO^{−} in water
View Description Hide DescriptionThe reaction dynamics of ClO^{−} in water following femtosecond ultraviolet photolysis is investigated by measuringtimeresolved absorption and anisotropy.Ab initio calculations show that light absorption induces charge shifting from the O^{−} atom to the Cl atom. Molecular dynamics simulations predict that the charge shift is followed by the destruction of the solvent structure around the O atom and its formation around newly formed negative charge on the Cl atom. An ultrafast (∼60 fs) transient absorption change is observed and likely corresponds to the inertial part of the destruction of the solvent structure around the newly formed neutral O atom of the excited state OCl^{−}. The early time anisotropy of −0.13±0.05 decays on the 230 fs time scale and is attributed to the dissociation along a new reaction path toward that is seen independently through the evaluation of the spectrum of Cl. The remaining anisotropy decays within 6 ps due to rotational diffusion of the ion. Probe wavelength dependence of the longer time dynamics (1.3–7 ps) is proposed to be the vibrational relaxation of the vibrationally hot ground state of ClO^{−}, the generation of which can be described as electron transfer from the excited state OCl^{−} to the ground state ClO^{−}.

Millimeterwave spectroscopy of FeF (X ^{6}Δ_{i}): Rotational analysis and bonding study
View Description Hide DescriptionThe pure rotational spectrum of the FeF radical in its ^{6}Δ _{i} ground electronic state has been recorded using millimeter/submillimeter direct absorption techniques. Transitions arising from all six spinorbit components have been observed in the v=0, 1, and 2 vibrational levels of ^{56}FeF, the main isotopic species, and also in the less abundant ^{54}Fe isotopomer. Hyperfine splittings, arising from the ^{19}F nuclear spin of I=1/2, were resolved in the majority of transitions recorded, and lambdadoubling interactions were observed in the Ω=3/2, 1/2, and −1/2 spinorbit ladders. The complete data set has been analyzed using a ^{6}Δ Hamiltonian, and rotational, spinorbit, spin–spin, lambdadoubling, and hyperfine constants determined. This study has conclusively demonstrated that the ground electronic state of FeF is ^{6}Δ _{i} . It also suggests that FeF has more covalent character to its bonding than alkaline earth or alkali metal counterparts.

Femtosecond Stokes shift in styryl dyes: Solvation or intramolecular relaxation?
View Description Hide DescriptionTransient absorption and gain spectra of the styryl dye LDS750 in solution have been studied by the pump/supercontinuum probe (PSCP) technique with excitation at 530 nm. The pump/probe intensity correlation width was 70 fs, providing a time resolution of 40 fs. Spectra were detected in the range 400–800 nm with 1.5 nm resolution. Before 70 fs, prominent spectral structure is observed due to resonant Raman scattering from a 1500 cm^{−1} active mode of the chromophore. At later time, the gain spectrum undergoes an ultrafast redshift and change of shape, with time constants of ∼200 and ∼600 fs for acetonitrile and chloroform solutions, respectively. At high pumping energy (1.2 μJ), the final emitting state is reached by internal conversion from higher electronic states without a further essential Stokes shift. The emitting state is assigned to an excited isomeric form of the molecule. At low pumping energy (0.3 μJ), the first excited electronic state isomerizes in an ultrafast process followed by a slower process, the dynamics of which is controlled by the solvent. The geometrical and electronic nature of these processes and their coupling to the solvent needs further clarification.

On the electronic states and photochemistry of simple alkyl amines
View Description Hide DescriptionThis paper extends recent work on the excited states of methyl amine to some other simple amines. Jetcooled massresolved multiphoton ionizationspectroscopy is reported for methyl amine, dimethyl amine, trimethyl amine, ethyl amine, diethyl amine, and triethyl amine. A set of highlevel calculations for methyl amine supports the assignment of S _{1} as a 3sRydberg state by predicting the experimental transition energy to within about 1000 cm^{−1} after including vibrational zeropoint energies for each potential energy surface. Perturbations observed in the experimental S_{1}←S_{0} spectrum prompt a series of calculations which implicate a dissociative 3sRydberg state in both the perturbation in the spectrum and in the photodissociation of methyl amine at these energies. A series of excited state calculations performed at the ground state geometry for methyl amine, dimethyl amine and trimethyl amine indicates the relative energies of the bound 3s and 3pRydberg states as well as the first dissociative 3sRydberg state. Taken together, this work presents a picture of the S _{1} and S _{2}excited states in which dissociative lowlying Rydberg states play a significant role in both the spectroscopy and photochemistry of the simple amines.

Geometric phase effects in the resonance spectrum, statetostate transition probabilities and bound state spectrum of HO_{2}
View Description Hide DescriptionThe general vector potential (gauge theory) approach for including geometric phaseeffects in accurate 3D quantum scattering calculations in hyperspherical coordinates is applied to lowenergy H+O collisions using our new more accurate DIM (Diatomics In Molecules) potential energy surface. The newly developed hybrid DVR/FBR (Discrete Variable Representation/Finite Basis Representation) numerical technique is used to include geometric phaseeffects due to the conical intersection in HO. The scattering results for zero total angular momentum computed both with and without the geometric phase show significant differences in the resonance energies and lifetimes. Significant differences in the statetostate transition probabilities are also observed. The results indicate that geometric phaseeffects must be included for H+Oscattering even at low energies. All 249 vibrational energies of HO are computed both with and without the geometric phase. Due to the localized nature of the bound state wavefunctions, no geometric phaseeffects are observed in the vibrational energies even in the highlying states near dissociation.

Rate constants for the reactions of O^{+} with N_{2} and O_{2} as a function of temperature (300–1800 K)
View Description Hide DescriptionWe have studied the rate constants for the reaction of O^{+} with N_{2} over the temperature range 300–1600 K and the reaction of O^{+} with O_{2} over the range 300 to 1800 K. The results are in good agreement with previous measurements made up to 900 K. The rate constant for the O^{+}reaction with N_{2} shows a minimum in the temperature range 1100–1300 K. The increase above this temperature is due to N_{2} v=2 becoming populated. The rate constant for O^{+}+O_{2} shows a minimum in the 800–1100 K range. Comparing to previous drift tube measurements allows the rate constant for O_{2} (v>0) to be derived. The v>0 rate constant is approximately five times larger than the v=0 rate constant.

The gasphase acidity of H_{3}PO_{4}
View Description Hide DescriptionThe gasphase acidity (free energy of deprotonation, ΔG ) of H_{3}PO_{4}, orthophosphoric acid, was determined both experimentally and theoretically for the first time. Ionmolecule bracketing experiments yielded the result 1372 kJ mol^{−1}>ΔG (H_{3}PO_{4})>1331 kJ mol^{−1}. The most reliable theoretical result, ΔG H(H_{3}PO_{4})=1343 kJ mol^{−1}, is in agreement with experiment and was obtained using Brueckner methods with large basis sets.

Quantum scattering studies of collisional energy transfer from highly excited molecules: Classical/quantum comparisons for collinear He+CS_{2}
View Description Hide DescriptionWe present the results of an accurate quantum scattering study of collisional energy transfer in the collinear He+CS_{2} system, considering energies up to 75 kcal/mol. These results are generated using a coupled channel calculation, with vibrational eigenfunctions obtained from a discrete variable representation method. Detailed comparisons with the results of classical trajectory calculations are performed so as to assess classical/quantum correspondence for energy transfer moments, and for the energy transferprobability distribution function. We find very good agreement of the energy averaged first moments over a wide range of molecular vibrational energies provided that the translational energy is not too low (translational temperatures significantly below 300 K). The second moments, as well as 〈ΔE〉 _{up} and 〈ΔE〉 _{down} show less quantitative agreement, especially at low temperatures. The energy transferdistribution functions show considerable modespecific behavior, but the overall envelope is approximately exponential in ΔE except for a spike near ΔE=0. Only weak dependence of the energy averaged results on the strength of intramolecular coupling is noted.

Connecting quantum state resolved scattering data directly to chemical kinetics: Energy transfer distribution functions for the collisional relaxation of highly vibrationally excited molecules from state resolved probes of the bath
View Description Hide DescriptionAn energy transferprobability distribution function, P(E,E), for the collisional relaxation of a highly vibrationally excited donor molecule (C_{6}F_{6}, pyrazine) is constructed for the first time from experimental data on the bath (CO_{2}) energy gain. A prescription for mapping bath quantum state resolved data onto P(E,E) is described in detail. Analysis of earlier experimental data allows a calculation of the high ΔE=E−E region (−7000 cm^{−1}<E−E<−1500 cm^{−1}) of P(E,E) for the above systems. Comparison of the P(E,E) functions reveals that C_{6}F_{6} is a more efficient donor molecule than pyrazine, in agreement with previous experiments and trajectory calculations. In addition, resonance like structures in the P(E,E) functions arising from long range force mediated, V–V excitation of the carbon dioxide ν_{3} mode are discussed. These results indicate that accurate P(E,E) functions can be determined from experiments involving probes of the bath energy gain. This technique can be expected to provide stringent tests of current energy transfertheory and can, in principle, be used in conjunction with measurements of thermal kinetics to obtain energy dependent unimolecular rate constants, k_{E} .

Potential of mean force and reaction rates for proton transfer in acetylacetone
View Description Hide DescriptionThe intramolecular proton transfer in the enol form of acetylacetone is investigated at various temperatures both classically and quantummechanically using computer simulations. The potential energy surface is modeled using the empirical valence bond (EVB) approach of Warshel and fitted to the results of ab initio calculations. Quantumstatistical results are obtained via discretized Feynman path integral simulations. The classical and centroid potential of mean force for the reaction coordinate is obtained using umbrella sampling. The proton transfer rate is calculated based on classical and on Feynman path integral quantum transition state theory. For the classical system, the transmission coefficient is obtained from activated dynamics. Two different reaction coordinates are compared, the first one involving explicitly the transferring proton and the second one involving only heavy atoms in the molecules. The influence of isotopic substitutions is investigated by considering a fully deuterated version of acetylacetone. It is observed that there are significant differences between classical and quantummechanical calculations caused mainly by the lack of tunneling effects in the former. The quantum fluctuations of heavy atoms are found to have a considerable influence on the magnitude of the proton transfer rate.

Alteration of the lifetimes of autoionizing Rydberg states by a circularly polarized microwave field
View Description Hide DescriptionAutoionizing Rydberg states in zero field exhibit lifetimes scaling as the cube of the principal quantum number, n ^{3}, but in a linearly polarized static or microwave field the lifetimes scale as n ^{4} due to lmixing. To mimic the effects of collisions with charged particles we have examined the effect of a circularly polarized field on the decay rate and find longer lifetimes than in a linearly polarized field, but not an n ^{5} dependence.

A theoretical study of the HCHO+CCl_{2} reaction: Cycloaddition or ylide formation?
View Description Hide DescriptionThe chemical reaction between HCHO and CCl_{2} was studied using a high level of theory. Geometry optimizations were performed at the complete active space selfconsistent field (CASSCF) level with the 631G^{*} basis set, and single point calculations were performed using the coupledcluster with single, double, and perturbative inclusion of triple excitation [CCSD(T)] method. Additivity approximation of the correlation energy was used, so we have obtained an effective CCSD(T)/6311G(2d1f,2p)reaction and activation energies. Our results show that the ylide exist as a stable species, having a transient character. Its lifetime in relation to ring closure was estimated to be 1 ms at 298.15 K, and it can react backward forming the HCHO and CCl_{2} fragments. The cycloaddition reaction is highly competitive with ylide formation, and that is the dominant process. The reaction rate of cycloaddition is about two and a half times higher than the ylide formation at 298.15 K.

Fluctuations around nonequilibrium steady states of electrochemical reaction systems and concentration polarization: A formalism of stochastic thermodynamics of concentration polarization
View Description Hide DescriptionAt mesostatistical level a stochastic thermodynamics of the concentration polarization in nonequilibrium steady state electrode processes is presented. In this formalism a simplified stochastic model of irreversible electrode process is suggested. Based on this model the fluctuations around the nonequilibrium steady states of electrochemical reaction systems are discussed. It turns out that the nonequilibrium departure of electrode potential of the steady electrode process with hesitative diffusion step consists of two pasts; the concentration polarization following the Nernst formula of order V ^{0}(V, volume of the electrochemical reaction system) and an additional contribution (of order V ^{−1} in the Gaussian regime of the distribution) due to the nonequilibrium fluctuations. Furthermore, the nonequilibrium nonideality of the electrode solution is analyzed. An explicit relation between the coefficient of the electrolytic ions at nonequilibrium steady state and corresponding nonequilibrium correlation has been derived.

The ν_{CC}+3ν_{CH} rovibrational manifold of acetylene. I. Collisioninduced statetostate transfer kinetics
View Description Hide DescriptionInfrared–ultraviolet double resonance (IR–UV DR) spectroscopy is used to measure collisioninduced, rotationally resolved statetostate energy transfer in the ν_{CC}+3ν_{CH} vibrational manifold of gasphase acetylene, C_{2}H_{2}. Attention focuses on three sets of vibrational eigenstates spectroscopically labeled (0 1 3 0 0), (0 1 3 0 0), and (4 0 3 3), with vibrational term energies in the region 11 585–11 600 cm^{−1}. IR–UV DR spectroscopy identifies the channels of Jchanging rotational energy transfer (RET) and intermode vibrational (V–V) transfer. The secondorder statetostate kinetics of these channels is measured by scanning the IR–UV pulse delay with the IR PUMP and UV PROBE lasers tuned to particular spectroscopic features. There is a clear propensity for evennumbered changes ΔJ of rotational quantum number in the observed RET and V–V transfer, consistent with conservation of ortho or para nuclearspin symmetry in the stateselected C_{2}H_{2} molecule, but there are some notable exceptions as unusual symmetrybreaking processes result in oddΔJ V–V transfer. Anomalies of this type have been investigated in a preceding paper [A. P. Milce and B. J. Orr, J. Chem. Phys. 104, 6423 (1996)]. A detailed IRUV DR study is made with the UV PROBE laser monitoring the (0 4 0 3 3), J=12 rovibrational level; RET is measured when the IR PUMP laser prepares molecules in various Jstates of (0 4 0 3 3), while V–V transfer is monitored when initial Jstates of (0 1 3 0 0) or (0 1 3 0 0) are prepared by the IR PUMP. The corresponding IRUV DR kinetic curves are fit to a detailed rateequation model in which empirical exponentialgap fitting laws are used to describe evenΔJ channels of RET and V–V transfer. It is remarkable that the kinetics of symmetrybreaking oddΔJ V–V transfer between the (0 1 3 0 0)_{I} ^{0}, J=5 and (0 4 0 3 3), J=12 rovibrational levels is well fit by the same model. The dynamical implications of these results are discussed.

Accurate analytical representations of the coreelectron densities of the elements 3 through 118
View Description Hide DescriptionThe coreelectron density in a molecule is defined as a sum of perfectly transferable, spherically symmetrical atomic contributions ρ_{AC} (Z,N,r). Analytical functions can be fitted to ρ_{AC} (Z,N,r) with a chargeconserving algorithm. The relativistic coreelectron densities of the elements 3 through 118, obtained from numerical multiconfigurational Dirac–Fock calculations, are accurately represented by linear combinations of 50 stype Gaussian primitives arranged in eventempered basis sets. These representations are well suited for the augmentation of valenceelectron densities produced by semiempirical methods and approaches involving effective core potentials. Calculations of the electronic properties of atoms in the TiCl_{4} and CdH_{2} molecules that employ such augmentation are presented.