Volume 104, Issue 5, 01 February 1996
Index of content:

Ultraviolet photoelectron spectroscopy of molybdenum and molybdenum monoxide anions
View Description Hide DescriptionThe 351 nm photoelectron spectra of Mo^{−} and MoO^{−} have been measured. The electron affinity of atomic molybdenum is 0.748(2) eV and that of molybdenum monoxide is 1.290(6) eV. The term energies of several MoO electronic states not previously observed are obtained and compared with ab initio predictions. The ground state of MoO is confirmed to have ^{5}Π symmetry and the term energy of the ^{3}Π excited state, 10 179(20) cm^{−1}, closely matches calculations. The ground state of MoO^{−} is a ^{4}Π state with a vibrational frequency of 810(40) cm^{−1}. The first excited state of molybdenum monoxide is tentatively assigned as a ^{3}Δ state with T _{0}=621(50) cm^{−1}. At least one state, possibly a ^{5}Σ^{−} state, lies 8000(500) cm^{−1} above the ground state, and a ^{5}Σ^{+} state is observed at 11 590(60) cm^{−1} above the ground state. The separations of spin–orbit levels for the MoO X ^{5}Π, ^{3}Π, and ^{3}Δ states are 169(30), 410(20), and −720(20) cm^{−1}, respectively. The vibrational frequencies of the ^{3}Π and ^{3}Δ states are found to be 600(20) and 1000(20) cm^{−1}, respectively. These observations give new insight into the Mo–O bond.

The anomalous behavior of the Zeeman anticrossing spectra of Ã ^{1}A_{ u } acetylene: Theoretical considerations
View Description Hide DescriptionP. Dupré, R. Jost, M. Lombardi, P. G. Green, E. Abramson, and R. W. Field have observed anomalous behavior of the anticrossing density in the Zeeman anticrossing (ZAC) spectra of gas phase Ã ^{1}A_{ u } acetylene in the 42 200 to 45 300 cm^{−1} energy range. To best explain this result, they hypothesize a large singlet–triplet coupling due to the existence of a linear isomerization barrier connecting a triplet‐excited cis‐ and trans‐acetylene in the vicinity of the studied energy range (∼45 500 cm^{−1}). Theoretically such a linear stationary point, however, must have two different degenerate bending vibrational frequencies which are either imaginary or exactly zero. Neither case has yet been experimentally detected. Here, we have studied the two lowest‐lying linear triplet‐excited‐state stationary points of acetylene, ^{3}Σ^{+} _{ u } and ^{3}Δ_{ u }, to see if they fit Dupré et al.’s hypothesis. We have completed geometry optimization and harmonic vibrational frequency analysis using complete‐active‐space self‐consistent field (CASSCF) wave functions as well as determined energy points at those geometries using the second‐order configuration interaction (SOCI) method. Harmonic vibrational analyses of both stationary points reveal two different doubly degenerate vibrational modes with imaginary vibrational frequencies (or negative force constants) indicating that they are indeed saddle points with a Hessian index of four. At the DZP SOCI//CASSCF level of theory with zero‐point vibrational energy (ZPVE) correction, the ^{3}Σ^{+} _{ u } stationary point lies 35 840 cm^{−1} above the ground state of acetylene. This is much too low in energy to contribute to the ZAC spectral anomaly. At the same level of theory with ZPVE correction, the ^{3}Δ_{ u } stationary point lies 44 940 cm^{−1} above the ground state consistent with Dupré et al.’s hypothesis. Several solutions to the anomalous ZAC spectra are discussed. We propose that the anomaly may also be due to coupling with a nearly linear structure on the T _{3}surface of acetylene.

An extended x‐ray absorption fine structure study by employing molecular dynamics simulations: Bromide ion in methanolic solution
View Description Hide DescriptionX‐ray absorption spectroscopy is widely employed in the structuralanalysis of disordered systems. In the standard extended x‐ray absorption fine structure(EXAFS)analysis the coordination of the photoabsorber is usually defined by means of Gaussian shells. It is known that this procedure can lead to significant errors in the determination of the coordination parameters for systems which present anharmonic thermal vibrations or interatomic asymmetric pair distribution functions. An efficient method has been recently employed in the study of the hydration shells of bromide and rubidium ions and brominated hydrocarbon molecules in diluted aqueous solutions. According to this method, pair distribution functions [g(r)] obtained from molecular dynamics simulations can be used as relevant models in the calculation of the EXAFS signals. Moreover, asymmetric shells modeled on the g(r) first peaks, have been employed in the EXAFSanalysis and the parameters defining the asymmetric peaks have been optimized during the minimization procedure. In the present paper this new procedure has been used to investigate the coordination of Br^{−} in methanol. The analysis of this system is particularly interesting due to the presence of three well separated coordination shells. We show that the inclusion of the hydrogen signal is essential to perform a reliable analysis. A comparison of the analysis with asymmetric and Gaussian shells shows how the accuracy of the EXAFSdata analysis is improved by using asymmetric shells.

A combined theoretical and experimental determination of the electronic spectrum of acetone
View Description Hide DescriptionA combined ab initio and experimental investigation has been performed of the main features of the electronic spectrum of acetone. Vertical transition energies have been calculated from the ground to the n _{ y }→π*, π→π*, σ→π*, and the n=3 Rydberg states. In addition, the ^{1} A _{1} energy surfaces have been studied as functions of the CO bond length. The ^{1} A _{1} 3p and 3d states were found to be heavily perturbed by the π→π* state. Resonant multiphoton ionization and polarization‐selected photoacoustic spectra of acetone have been measured and observed transitions were assigned on internal criteria. The calculated vertical transition energies to the n _{ y }→π* and all Rydberg states were found to be in agreement with experiment. This includes the 3s‐, all three 3p‐, and the A _{1}, B _{1}, and B _{2} 3d‐Rydberg states. By contrast, there is little agreement between the calculated and experimental relative intensities of the A _{1} and B _{2} 3d‐Rydberg transitions. In addition, anomalously intense high vibrational overtone bands of one of the 3p‐Rydberg transitions have been observed. These results confirm the strong perturbation of the 3p‐ and 3d‐Rydberg states by the π→π* state found in the theoretical calculation and support the calculated position of this unobserved state.

Magnetic interactions between the reduced bacteriopheophytin and quinone electron acceptors in reaction centers of the photosynthetic purple bacterium rhodopseudomonas viridis: An X‐band and Q‐band electron paramagnetic resonance study
View Description Hide DescriptionThe electron paramagnetic resonance(EPR) signal associated with the photo‐accumulated radical anion of the primary electron acceptor (I ^{⋅−}, a bacteriopheophytin radical) in reaction centers (RCs) of the photosynthetic purple bacterium rhodopseudomonas viridis shows a characteristic splitting of about 14.0 mT. This splitting has been attributed to an exchange interaction between I ^{⋅−} and the reduced complex of the second electron acceptor, a quinone molecule, and a divalent, high‐spin (S=2) Fe‐ion, [Q ^{⋅−} _{ A }Fe^{++}]. The magnetic structure of the three‐spin complex, Fe^{++} (S=2), Q ^{⋅−} _{ A } (S=1/2), and I ^{⋅−} (S=1/2), is assessed by Q‐band (34.8 GHz) and X‐band (9.2 GHz) EPR spectroscopy. The EPR spectrum of [I ^{⋅−} Q ^{⋅−} _{ A }Fe^{++}] is simulated accurately for the first time, using the magnetic parameters for the quinone‐iron complex [W. F. Butler, R. Calvo, D. R. Fredkin, M. Y. Okamura, and G. Feher [Biophys. J. 45, 947 (1984)]. A largely isotropic interaction between I ^{⋅−} and Q ^{⋅−} _{ A } is required (J _{ IQ }=−7.5 mT), together with an anisotropic interaction between I ^{⋅−} and the Fe^{++}‐ion, whose y‐component, C _{ IFe,y }, is −3.5 mT. The simulations were insensitive to the magnitude of the x,z‐components, C _{ IFe,x/z }. The experimental magnetic interactions correspond very well with values calculated from the distances in the RC crystal structure. Thus, the interaction between I ^{⋅−} and Q ^{⋅−} _{ A } is largely isotropic (exchange), whereas the interaction between I ^{⋅−} and Fe^{++} has a purely dipolar character. This result is used to determine the principal directions of the magnetic interaction tensors of the Fe^{++}‐ion.

Observations of rotational magnetic moments in the ground and some excited vibrational Σ states of C_{2}H_{2}, C_{2}HD, and C_{2}D_{2} by magnetic vibrational circular dichroism
View Description Hide DescriptionMagnetic vibrational circular dichroism (MVCD) spectra of acetylene and its deuterated isotopomers have been recorded for the following Σ symmetry combination and overtone bands of C_{2}H_{2}: ν_{4}+ν_{5}; C_{2}HD: ν_{4}+ν_{5}, 2ν_{4}, 2ν_{5}; C_{2}D_{2}: ν_{4}+ν_{5}, the ν_{3} fundamental for C_{2}HD and C_{2}D_{2}; and the ν_{4}→2ν_{4}+ν_{5} and ν_{5}→ν_{4}+2ν_{5} hot bands for C_{2}H_{2}. For a Σ_{ g }→Σ_{ u } transition, the MVCD A terms observed must arise primarily from the rotational Zeeman effect. These negative A _{1}/D _{0} values for low J″ transitions confirm that the sign of the rotational g‐value for acetylene is positive. The rotational magnetic moments in both the lower and upper vibrational states were determined by comparison of moment analyses of experimental and simulated MVCD spectra obtained with a model Hamiltonian for acetylene. The g _{ J } values in all the excited bending combination and overtone vibrational levels observed are smaller than those in the ground and the first excited stretching vibrational levels. This observation has been confirmed by theoretical simulation of the MVCD spectra of the ν_{4}+ν_{5} combination band of C_{2}H_{2}. From these MVCD results, for C_{2}H_{2}, g _{ J }(ground)=+0.0535±0.0033 and Δg(ν_{4}+ν_{5})=−0.0061±0.0004; for C_{2}D_{2}, g _{ J }(ground)∼g _{ J }(ν_{3}) =+0.0363±0.0048, Δg(ν_{4}+ν_{5})=−0.0052±0.0031; and for C_{2}HD, g _{ J }(ground)∼g _{ J }(ν_{3})=+0.0409 ±0.0069. These are the first quantitative, MVCD determinations of nondegenerate excited stateg values distinctly different from the ground state. The decrease in g value correlates with off‐axis deformation of the linear C_{2}H_{2} rotation.

Mass‐resolved multiphoton ionization spectroscopy of jet‐cooled Cl_{2}. I. Bound‐free‐bound spectroscopy
View Description Hide DescriptionSpectroscopic constants, obtained using two‐color optical double resonance via repulsive intermediate states, are presented for four ion‐pair states of Cl_{2}; i.e., the E(0^{+} _{ g }), β(1_{ g }), f(0^{+} _{ g }), and G(1_{ g }) states. One‐color excitation, also via a repulsive intermediate state, has been used to further extend the vibrational data for the β(1_{ g }) state. The same pumping scheme has been used to extend a vibrational progression in the [^{2}Π_{1/2}]_{ c }4s; 1_{ g }Rydberg state. The absence of perturbations when the [^{2}Π_{1/2}]_{ c }4s; 1_{ g }Rydberg and the β(1_{ g }) ion‐pair states cross, together with the key role played by the intermediate C(1_{ u }) state in accessing both singlet and triplet final states, are discussed in terms of the changes in spin–orbital coupling schemes that are required on bond stretching.

Mass‐resolved multiphoton ionization spectroscopy of jet‐cooled Cl_{2}. II. The (2+1) REMPI spectrum between 76 000 and 90 000 cm^{−1}
View Description Hide DescriptionThe (2+1) resonance‐enhanced multiphoton ionization (REMPI) spectrum of Cl_{2} has been recorded between 76 000 and 90 000 cm^{−1}. The origins of twenty Rydberg states are located, with the s series (4s−8s) generally exhibiting four Ω components and the d series (3d−5d) two components for each value of n. Vibronic coupling between the β(1_{ g }) ion‐pair state and the 3d and 4d (Ω=1) states is pronounced, necessitating the use of mass‐resolved REMPI for the analysis of these vibronically mixed states.

Time‐resolved infrared diode laser spectroscopy of the ν_{5} band of the cyanomethyl radical (H_{2}CCN)
View Description Hide DescriptionThe infrared spectrum of the cyanomethyl radical (H_{2}CCN) generated by the 193 nm excimer laserphotolysis of chloroacetonitrile was observed by time‐resolved diode laser spectroscopy. About 50 lines, involving those split into doublet due to the spin–rotation interaction, were assigned to rovibrational transitions in the ν_{5} (CH_{2}‐wagging) band of cyanomethyl. The molecular constants in the ν_{5}vibrational state were derived from the analysis of the observed wave numbers, resulting in the rotational constants,A=9.095 03(21) cm^{−1}, (B+C)/2=0.335 363 4(41) cm^{−1}, and (B−C)=0.011 503 6(71) cm^{−1}, and the spin–rotation interaction constant ε_{ aa }=−2.143(47)×10^{−2} cm^{−1}, where the figures in parentheses are 2.5 standard deviations to be attached to the last digit, the constants in the ground state being fixed to the reported values from microwave spectroscopy. The band origin determined ν_{0}=663.793 98(85) cm^{−1} is consistent with the value derived from the photo‐detachment spectroscopy of the H_{2}CCN^{−} anion. Large changes in the rotational constantA and the centrifugal distortion constant Δ_{ K } on vibrational excitation from the ground state to the ν_{5} state are accounted for by the a‐type Coriolis interaction of the ν_{5}vibrational state with the ν_{8} (CH_{2}‐rocking) and ν_{9} (in‐plane ∠CCN‐bending) vibrational states.

Core level binding energy shifts and polarization screening: A combined experimental and theoretical study of argon clusters
View Description Hide DescriptionPhotoelectron spectra of the argon 2p core level for free argon clusters of up to 4000 atoms are compared to detailed calculations. The comparison shows that the size‐dependent shifts of the core level binding energy can be explained in a pure polarization‐screening model. Important differences arise between the shifts for the bulk (interior) and the surface atoms. The agreement between experiment and theory allows the extrapolation of the cluster data to the ‘‘infinite’’ solid. In this way we obtain the shifts of the core level binding energy between the free atom, the surface atom and the bulk of argon. The relation between these shifts and those of the first ionization potential is discussed.

Low energy electron scattering from CH_{3}Cl
View Description Hide DescriptionDifferential cross section measurements for the elasticscattering of electrons from CH_{3}Cl at energies from 0.5 to 9.5 eV are reported for scattering angles of 30° and 100°. The angular scattering dependence is determined at selected energies over this range. At energies below 1.0 eV, the cross sections are in excellent agreement with calculations using the Born dipole approximation. At large angles and higher energies, the scattering is dominated by a ^{2} A _{1} temporary negative ion state near 3.5 eV. Energy loss data at 3.5, 5.0, and 8.5 eV are reported and the relative contributions of various vibrational modes determined. Differential cross sections for vibrational excitation of the υ_{3}(a _{1})^{C–Cl} and υ_{4}(e)^{CH} stretching modes have been measured. The latter reveals a broad shape resonance of ^{2} E symmetry peaking near 5.5 eV. Angular distributions for excitation of these same modes are also reported. Using fittings to the vibrational excitation functions, the resonance parameters have been extracted and used in a mixed semiempiricalab initio calculation to compute the relative strengths of the vibrational energy loss peaks. These compare favorably with the results of the experiment. Elastic cross sections integrated over angle are reported for low energies. They are substantially larger than the results from recent measurements of the total cross section.

Quantum phase space theory for the calculation of v⋅j vector correlations
View Description Hide DescriptionThe quantum state‐counting phase space theory commonly used to describe ‘‘barrierless’’ dissociation is recast in a helicity basis to calculate photofragment v⋅jcorrelations. Counting pairs of fragment states with specific angular momentum projection numbers on the relative velocity provides a simple connection between angular momentumconservation and the v⋅jcorrelation, which is not so evident in the conventional basis for phase space state counts. The upper bound on the orbital angular momentum,l, imposed by the centrifugal barrier cannot be included simply in the helicity basis, where l is not a good quantum number. Two approaches for an exact calculation of the v⋅jcorrelation including the centrifugal barrier are described to address this point, although the simpler helicity state count with no centrifugal barrier correction is remarkably good in many cases. An application to the photodissociation of NCCN is consistent with recent classical phase space calculations of Klippenstein and Cline. The experimentally observed vector correlation exceeds the phase space theory prediction. We take this as evidence of incomplete mixing of the K states of the linear parent molecule at the transition state, corresponding to an evolution of the body‐fixed projection number K into the total helicity of the fragment pair state. The average over a thermal distribution of parent angular momentum in the special case of a linear molecule does not significantly reduce the v⋅jcorrelation below that computed for total J=0. Predictions of the v⋅jcorrelations for the unimolecular dissociation of NCNO and CH_{2}CO are also provided.

‘‘False tunneling’’ and multirelaxation time nonexponential kinetics of electron transfer in polar glasses
View Description Hide DescriptionClassical electron transfer in polar glasses is described by a theory based on a model microscopic Hamiltonian which includes the discreteness and randomness of the glassy polar modes with distinct orientation. When configurational dynamics is fast, the reaction is described by exponential kinetics with a rate constant of non‐Arrhenius type. The temperature dependent rate constant resembles the tunneling rate, despite the classical transfer of the electron. This effect is called ‘‘false tunneling.’’ In this limit the possibility of a self‐acceleration of the reaction is pointed out. When configurational dynamics is very slow the reaction kinetics are nonexponential with multirelaxation time behavior. The reaction is shown to be almost insensitive to temperature change pointing out on a possible explanation of a broad temperature‐independent range in the ‘‘rate constant’’ in an electron transfer in cytochrome coxidation in chromatium. At short times, the reaction accelerates compared to the exponential behavior, while at long times it becomes slower. For strongly exothermic reactions the kinetics are always slower than an exponential decay.

Toeplitz matrices within discrete variable representation formulation: Application to collinear reactive scattering problems
View Description Hide DescriptionRecently, a new approach based on the features of the Toeplitz matrix was introduced for reactive scattering problems. So far these features were used only along the reagents translational coordinate (either for Eckart‐type models or for collinear scattering). In this work, we show how to employ the Toeplitz features for the two asymptotic regions of the collinear system.

A selected ion flow tube study of the reactions of NO^{+} and O^{+} _{2} ions with some organic molecules: The potential for trace gas analysis of air
View Description Hide DescriptionA study has been carried out using our selected ion flow tube apparatus of the reactions of NO^{+} and O^{+} _{2} ions in their vibronic ground states with ten organic species: the hydrocarbons, benzene, toluene, isoprene, cyclopropane, and n‐pentane; the oxygen‐containing organics, methanol, ethanol, acetaldehyde, acetone, and diethyl ether. The major objectives of this work are, on the one hand, to fully understand the processes involved in these reactions and, on the other hand, to explore the potential of NO^{+} and O^{+} _{2} as chemical ionization agents for the analysis of trace gases in air and on human breath. Amongst the NO^{+}reactions,charge transfer, hydride‐ion transfer, and termolecular association occur, and the measured rate coefficients, k, for the reactions vary from immeasurably small to the maximum value, collisional rate coefficient, k _{ c }. The O^{+} _{2}reactions are all fast, in each case the k being equal to or an appreciable fraction of k _{ c }, and charge transfer producing the parent organic ion or dissociative charge transfer resulting in two or three fragments of the parent ion are the reaction processes that occur. We conclude from these studies, and from previous studies, that NO^{+} ions and O^{+} _{2} ions can be used to great effect as chemical ionization agents for trace gas analysis, especially in combination with H_{3}O^{+} ions which we now routinely use for this purpose.

Fragmentation of fullerenes in collisions with atomic and molecular targets
View Description Hide DescriptionFragmentation cross sections of fullerene cations have been measured as a function of collision energy under single collision conditions. The ions are produced by laser desorption from a C_{60}/C_{70} substrate. Collision products are analyzed with a linear time of flightmass spectrometer. The collision energy ranges from 5 eV up to more than 150 eV in the center of mass reference frame. A combination of molecular dynamics calculations and statistical RRKM theory has been applied to help understand the nature of the fragmentation dynamics. The results of collisions using different fullerenes as projectiles (C^{+} _{56}, C^{+} _{58}, C^{+} _{60}, C^{+} _{70}) as well as collisions with a range of target gases (Ne, Ar, O_{2}, CO_{2}, (CH_{2})_{3}, C_{3}H_{6}, SF_{6}) give additional insight into the fragmentation mechanisms. The energy dependence of the fragmentation cross sections can be used to identify exothermic reactions between projectile and target as is shown for the case of molecular oxygen.

Selective production of photofragments by monitoring the shape of asymmetric resonances in OH photodissociation: Dependence on initial vibrational states
View Description Hide DescriptionQuantum mechanical analysis is presented on the vibrational state dependence of the total dissociation cross sections and the branching ratios of O(^{3} P _{ j }, j=0,1,2) in the predissociation of OH. Two transformation matrices, each of which describes the relation between an atomic term limit and the correlating molecular states, are constructed and incorporated in the close coupling calculations. The branching ratios of O(^{3} P _{ j }, j=0,1,2) depend very sensitively on the vibrational levels (v=0–4) of the initial X ^{2}Π state. The variations of the spin–orbit distributions as a function of the excitation energy near the asymmetric resonances change markedly depending on the vibrational levels. The variations are either redshifted or blueshifted from the resonance position, depending on the degree of asymmetry of the resonances. The widths of the variations tend to increase with increasing vibrational quantum number of the initial state, suggesting the possibility of choosing the proper linewidths in the experiments to selectively produce the photofragments in one‐photon process. Discussion is presented on the applicability of the theoretical scheme to analyze the recent measurements by Neumark and co‐workers [J. Chem. Phys. 103, 2495 (1995)] on the product fine structure distributions in the predissociation of O_{2}.

Reaction–diffusion description of biological transport processes in general dimension
View Description Hide DescriptionWe introduce a reaction–diffusion system capable of modeling ligand migration inside of proteins as well as conformational fluctuations of proteins, and present a detailed analytical and numerical analysis of this system in general dimension. The main observable, the probability of finding the system in the starting state, exhibits dimension‐dependent as well as dimension‐independent properties, allowing for sharp experimental tests of the effective dimension of the process in question. We discuss the application of this theory to ligand migration in myoglobin and to the description of gating fluctuations of ion channelproteins.

The dynamics of Rydberg states of molecules in the intermediate regime: The role of the vibrations
View Description Hide DescriptionThe coupling of a Rydberg electron to the vibrational motion is discussed in the intermediate regime in which the orbital period is long on the scale of the vibrational motion but is still considerably faster than the rotation of the core. Two dimensionless variables characterize the dynamics: the ratio of time scales and the action exchanged between the electron and the core, per one revolution. The classical dynamics are reduced to a map which provides a realistic approximation in the limit when the action exchanged is larger than ℏ. There are two distinguishable time regimes, that of prompt processes where the corresponding spectrum is so broad that individual Rydberg states cannot be resolved and a much slower process, where the electron revolves many times around the core before it ionizes. The overall spectrum is that of a Rydberg series, where the lines are broadened by (the delayed) vibrational autoionization superimposed on a broad background. The semiclassical dynamics is quantitatively more accurate in the typical situation when the action exchanged is comparable or smaller than ℏ. Explicit analytical expressions are obtained for the width for vibrational autoionization including for the case when resonances are possible. The presence of resonances is evident in Rydberg lines which are broader. For low Rydberg states the present approach recovers the Herzberg–Jungen approximation in the weak coupling limit.

Tunneling in the H_{2}S+O(^{3} P)→HS+OH reaction: A theoretical study
View Description Hide DescriptionTitle reaction has been investigated by a quantum mechanical reactive scattering method. A potential energy surface has been constructed on the basis of ab initio calculations at the MP2(fc)/6‐311G(3df,3pd) level of theory. The reaction probabilities have been calculated under an assumption of a collinear atom‐diatom collision. It has been found that OH(v=1) is mainly produced in the reaction at room temperature. The rate constants evaluated from the reaction probabilities were 2 orders of magnitude higher than those calculated by the transition‐state theory, implying that quantum mechanical tunneling plays an important role in this reaction even at room temperature.