Volume 100, Issue 11, 01 June 1994
Index of content:
Non‐Markovian optical dephasing dynamics in room temperature liquids investigated by femtosecond transient absorption spectroscopy: Theory and experiment100(1994); http://dx.doi.org/10.1063/1.466832View Description Hide Description
This paper examines the electronic dephasingdynamics of the dye molecule HITCI in an ethylene glycol solution. Degenerate transient absorption data are presented as a function of laser pulse width and detuning from the absorption maximum. The dephasingdynamics revealed by the experimental results are shown to be non‐Markovian. The experimental data cannot be quantitatively described by stochastic theories that assume a single relaxation time for the solvent. A model is presented that includes contributions from both fast and slow modulations to the absorption line shape. Using this approach, quantitative agreement is observed between theory and experiment. These results clearly indicate that multiple time scales for liquiddynamics contribute to the absorption line shape of solute molecules. The solvent parameters obtained are discussed in light of results from complementary experiments designed to measurecharacteristicrelaxation times.
Electron spin resonance investigation of small magnesium cluster cation radicals, Mg+ N , in neon and argon matrices at 4 K: Comparison with ab initio calculations100(1994); http://dx.doi.org/10.1063/1.466833View Description Hide Description
A series of cationic magnesium radicals (Mg+ N , N=1–6) was studied by electron spin resonance(ESR)spectroscopy. The ESR data indicate that all Mg nuclei are equivalent on the ESR time scale in each of these clusters. The nuclear hyperfineinteractions obtained for 25Mg+ N (N=1–3) in neon and argon matrices from ESR measurements were compared with ab initio calculations. The minimum energy structures for Mg+ 2 and Mg+ 3 and the transition‐state structure for Mg+ 3 were optimized using complete active space self‐consistent field (CASSCF) wave functions. The minimum energy structure for Mg+ 3 was determined to be linear and the barrier for the pseudorotation between equivalent minima was calculated. Good agreement between theory and experiment is obtained if averaging between the center and end position of Mg+ 3 is assumed to occur in the ESR experiment in spite of the 6 kcal/mol calculated energy barrier.
Transient changes in polarizability for centrosymmetric linear molecules interacting at long range: Theory and numerical results for H2...H2, H2...N2, and N2...N2100(1994); http://dx.doi.org/10.1063/1.466834View Description Hide Description
Transient, collision‐induced changes in polarizability Δα on the subpicosecond time scale affect Rayleigh and rototranslational Raman scattering by diatomic molecules in dense gases and liquids, induced birefringence, impulsive stimulated scattering, and dielectric and refractivity virial coefficients. For pairs of D ∞h molecules, this work gives the long‐range contributions to Δα complete through order R −6 in the intermolecular separation R, including the first‐ and second‐order dipole–induced‐dipole (DID) interactions, higher‐multipole induction, effects of the nonuniformity in the local field, hyperpolarization, and dispersion. We have used spherical tensor methods to cast Δα into the symmetry‐adapted form needed for spectroscopic line shape analysis.
The polarization mechanisms included here give rise to isotropic rototranslational Raman scattering and to simultaneous rotational transitions on two interacting molecules; both are collision‐induced phenomena. Transitions with ΔJ up to ±4 are produced by the R −5 and R −6polarization mechanisms treated in this work (and the isotropic part of the pair potential). For the pairs H2...H2, H2...N2, and N2...N2, we have used ab initio results for permanent multipoles and susceptibilities to evaluate the classical induction contributions to Δα. For the dispersion contributions, we have derived exact results in the form of integrals of the polarizability and γ hyperpolarizability over imaginary frequencies, and we have approximated these numerically in terms of the static α and γ values, together with van der Waals energy coefficients. For the pairs and configurations studied, the first‐order DID terms give the dominant contributions to Δα; the agreement between these terms and the full long‐range results tends to be better for the anisotropic collision‐induced polarizability Δα M 2 than for the isotropic part Δα0 0, particularly for T‐shaped pairs.
The relative contributions to Δα from the other polarization mechanisms reach as high as 29% for second‐order DID terms, 27% for E‐tensor terms (higher‐multipole induction and local field nonuniformity), 21% for dispersion, and 8% for hyperpolarization effects, for R values ∼0.5–1.0 a.u. outside the isotropic van der Waals minimum.
100(1994); http://dx.doi.org/10.1063/1.466835View Description Hide Description
We describe a new class of methods for measuring the properties of particles suspended in fluids. We have named these methods modulated dynamic light scattering (MDLS) methods. We present a detailed description of the apparatus and the theoretical foundations for a few examples of MDLS methods in which measured signals are compared to predictions calculated using Brownian motion theory to infer one or more of the particle mass, diffusion coefficient, velocity, shape category, and other properties. The direct measurement of particle mass is novel. MDLS methods can thus be used in characterizing particles with respect to both mass and surface area, size and shape, or other combinations of properties. MDLS measurements of many individual particles provide the distributions of particles over one, two, or more property values, singly or jointly. To be widely used in particle measurements, the method must measure several thousand in a few minutes. We therefore investigate precision and measurement speed and discuss size range of particle characterization by MDLS. We derive an expression for the precision of an MDLS measurement. Provision of both a measured mass or size of a single particle and an estimate of the measurement precision is also novel. The capability to measure mass, size, and other properties of individual particles at known precision should make MDLS useful in characterizing particles of unknown shape including thin flake‐shaped and elongated rod‐shaped particles.
100(1994); http://dx.doi.org/10.1063/1.466836View Description Hide Description
In an earlier paper we described the theoretical foundations of modulated dynamic light scattering (MDLS) and indicated how these methods can be implemented in the characterization of particles. In this paper we illustrate the range of application of MDLS methods by describing eight types of MDLS experiments we have performed. In addition to illustrating the range of application of MDLS methods, the results of these experiments validate our theoretical predictions. The experiments were done with an MDLS apparatus in which the requisite modulation is produced by frequency‐biasing two intersecting laser beams with acousto‐opticmodulators.Measurements were made by introducing a single sample particle into the intersection volume of the two beams and processing the scattered light signal from the particle. Data from these experiments typically provide detailed records of the translational and/or rotational Brownian motion for one or more single particles suspended in a fluid medium. Dynamic particle properties are inferred by comparing measured results with theoretical results based on models of Brownian motion. Described MDLS method experiments include (a) simultaneous measurement of the translational diffusion coefficient and velocity component of particles suspended in air; (b) simultaneous measurement of the translational and rotational Brownian motion of hematite ellipsoids and spindles with submicrometer dimensions suspended in water; (c) determination of the diameters and slip correction factors of micrometer‐sized spheres suspended in air; (d) determination of the precision of the present MDLS apparatus for finding sphere diameters; (e) precise measurement of sphere diameters; (f) simultaneous measurement of the mass and friction coefficient of a particle suspended in air and verification of predicted effects of particle inertia on Brownian translation and rotation; (g) measurement of translational Brownian displacement distributions; and (h) determination of precision vs measurement time for an MDLS method.
100(1994); http://dx.doi.org/10.1063/1.466837View Description Hide Description
Methane’s 3ν1+ν3 vibrational overtone spectrum has been recorded at room temperature, 100 K, and approximately 15 K using laser intracavity photoacoustic and molecular beam techniques. Although rotational congestion renders the room temperature spectrum undecipherable, the 100 K data suggest possible rovibrational assignments that are confirmed in the 15 K spectrum by their individual temperature dependences. Molecular beam Stark spectroscopy is used to unambiguously identify the E symmetry components of the R(2), P(2), and Q(2) transitions. These results indicate the presence of several other bands whose intensities are derived through interactions with the 3ν1+ν3 vibrational overtone transition. Quantitative analysis of these interactions suggests a bright state origin of 11 277.0 cm−1.
High resolution Fourier transform emission spectrum of the (2) 3Π g –A 3Π u transition of the B2 molecule100(1994); http://dx.doi.org/10.1063/1.466838View Description Hide Description
The (2) 3Π g –A 3Π u system of B2 has been characterized for the first time. A full rotational analysis of the 0–0, 0–1, 1–0, and 1–3 bands has been performed. The molecular parameters are compared with recent high level theoretical calculations and show good agreement. An extensive perturbation of the upper (2) 3Π g state by the (1) 3Π g state was observed. This was most clearly evident for v=0 of the (2) 3Π g state for the 10B11B isotopomer, which is split into two components by the interaction with v=7 of (1) 3Π g . The perturbation interaction exhibited a significant rotational dependence, which was interpreted as arising from a change in the Franck–Condon factors as the rotational contribution to the potential changed. A weak local perturbation was observed in v=1 of the A 3Π u state from a crossing with v=2 of the a 5Σ− u state.
100(1994); http://dx.doi.org/10.1063/1.466839View Description Hide Description
The polarized infrared spectrum of naphthalene single crystal has been studied between 800 and 1600 cm−1. The dependence on temperature of the linewidth of eight Davydov components of internal modes has been studied in the 10–240 K temperature range. The deconvolution of the experimental line shapes allows us to extract a constant inhomogeneous contribution to the line broadening, due probably to crystal defects, over all the temperature range. The bandwidth data are interpreted on the basis of elementary relaxation mechanisms. In particular, at low temperature, the vibrational decay is dominated by three‐phonon depopulation processes. Above 80 K, higher order dephasing mechanisms generally contribute to the vibron dynamics. The calculated decay compares well with experimental results. The infrared results are compared with the relaxation data presented for the Raman modes in previous papers.
100(1994); http://dx.doi.org/10.1063/1.466840View Description Hide Description
Weakly bound complexes of the form Mg+–RG (RG=Ar, Kr, Xe) are prepared in a pulsed nozzle/laser vaporization cluster source and studied with mass‐selected photodissociationspectroscopy. The chromophore giving rise to the molecular spectra in these complexes is the 2 P←2 SMg+ atomic resonance line. A 2Σ+ground state and 2Σ+ and 2Π excited states are derived from this atomic transition. Vibrationally resolved spectra are measured for each of these complexes in the A 2Π←X 2Σ+ electronic transition. These systems are redshifted from the atomic resonance line, indicating that each complex is more strongly bound in its excited 2Π state than it is in the ground state. Extended vibrational progressions allow determination of the respective vibrational constants: Mg+–Ar, ω e ’ = 272 cm−1; Mg+–Kr, ω e ’ = 258 cm−1; Mg+–Xe, ω e ’ = 258 cm−1. Extrapolation of the excited state vibrational progressions, and combination with the known atomic asymptotes and spectral shifts, leads to determination of the respective dissociation energies: Mg+–Ar, D 0 ‘= 1281 cm−1 (3.66 kcal/mol; 0.159 eV); Mg+–Kr, D 0 ‘ = 1923 cm−1 (5.50 kcal/mol; 0.238 eV); Mg+–Xe, D 0 ‘ = 4182 cm−1 (11.96 kcal/mol; 0.519 eV). The spin–orbit splitting in the 2Π1/2,3/2 state for all complexes is larger than expected by comparison to the Mg+ atomic value. This larger splitting in the complexes, which is attributed to configuration mixing with states on the rare gas atoms, increases for the series Ar, Kr, Xe, and decreases linearly for higher vibrational states of each complex.
100(1994); http://dx.doi.org/10.1063/1.466841View Description Hide Description
The nfRydberg levels of 14N2 converging to the X 2Σ+ g ground state of N+ 2 have been studied from n=4–9 in the high‐resolution absorption spectra of supersonically expanding nitrogen at wavelengths ranging from 843.2 to 802.6 Å (118 600–124 600 cm−1). The best experimental results, achieving rotational temperatures on the order of 20 to 40 K and a resolution of 0.5 cm−1, were obtained by photographing the jet absorption against the background continuum from a synchrotron radiationsource. Complementary data for 14N2 and 15N2 come from the analyses of spectra recorded under equilibrium conditions at 70 K with a resolution of 1 cm−1, using the He continuum as background source. The observations are interpreted with the help of multichannel quantum defect calculations and lead to the conclusion that, to varying degrees and with the exception of 4f(v=0), all the complexes studied here show the effects of interactions with core excited d and sRydberg levels built on the A 2Π u first excited state of N+ 2. Also, the 9f(v=0) complex is shown to be perturbed by 10p(v=0), and the strength of the p–f interaction has been determined.
Exploiting the properties of line‐shape singularities in orientationally selected electron spin echo envelope modulation spectra of Cu2+ −(15N‐imidazole)4 for the determination of hyperfine coupling with the remote imidazole nitrogen100(1994); http://dx.doi.org/10.1063/1.466790View Description Hide Description
Orientationally selected two‐, three‐, and four‐pulse electron spin echo envelope modulation (ESEEM) spectra from remote 15N nuclei in a Cu2+‐(15N–imidazole)4 complex have been obtained in the microwave frequency region 9.12–9.82 GHz. This range of frequencies has allowed us to reach the conditions corresponding to S and ν⊥ singularities of the lineform of basic transitions. The development of these lineform singularities in orientationally selected spectra is discussed. The knowledge of the magnetic fields corresponding to the singularities was used to find the hyperfine coupling parameters for the remote nitrogen as a=2.44 MHz and T ⊥=−0.39 MHz as well as to determine the angle between the axis of the complex and the direction of the largest principal hyperfine tensor element as 40°. Additional geometrical information about the ligand orientation has been obtained from the analysis of shifts of the sum combination harmonic in proton four‐pulse ESEEM spectra.
One‐ and two‐color two‐photon resonance enhanced multiphoton ionization spectroscopy of the d 1Σ+ state of NH100(1994); http://dx.doi.org/10.1063/1.466791View Description Hide Description
The ionization and dissociation processes occurring after two‐photon excitation of the d 1Σ+ state of NH have been investigated using one‐ and two‐color multiphoton ionizationspectroscopy in combination with mass‐resolved ion and kinetic energy‐resolved electron detection. The photoelectron spectra obtained for ionization of the molecule via the d 1Σ+ state (v’=0,1) reveal a one‐photon ionization process to all energetically accessible vibrational levels of the X 2Π ground ionic state, at variance with a one‐configuration description of the two states. Moreover, electrons are observed that derive from a one‐photon ionization of excited neutral nitrogen and hydrogen atoms. Two‐color excitation experiments of the d 1Σ+ state show that the influence of discrete states at the overall three‐photon level on the dissociation dynamics is insignificant. Using the results of ab initio calculations it is concluded that these experimental observations can be interpreted consistently if two competing processes are assumed to take place from the d 1Σ+ excited state: a molecular one‐photon ionization which is forbidden in zeroth order, and a two‐photon nonresonance enhanced excitation to neutral (pre)dissociative states.
Vibrational circular dichroism intensities by ab initio second‐order Mo/ller–Plesset vibronic coupling theory100(1994); http://dx.doi.org/10.1063/1.466792View Description Hide Description
The vibronic coupling theory (VCT) of Nafie and Freedman for evaluating vibrational circular dichroism (VCD) intensities has been implemented at the level of ab initio second‐order Mo/ller–Plesset perturbation theory (MP2). This method, in conjunction with the MP2 force field, is applied to calculate VCD spectra of (S,S)‐2,3‐dideuteriooxirane and (S)‐NHDT. Several basis sets, namely 6‐31G*, 6‐31G*(0.3), vd/3p, and vd/3p(u) are used in the calculations. Comparisons with the experimental results for oxirane and alternate high level calculations in the case of NHDT show that VCT/MP2 with a near‐Hartree–Fock limit basis set will predict nearly quantitatively values of vibrational rotatory strengths. The inclusion of electron correlation at the MP2 level into calculation of the atomic axial tensors is important for obtaining reliable vibrational properties for the conventional 6‐31G* basis set. The 6‐31G*(0.3) basis set, which has previously proved to be highly successful in predicting VCD spectra with the VCT‐SCF method, yields only modest improvement at the MP2 level.
100(1994); http://dx.doi.org/10.1063/1.466793View Description Hide Description
Tl containing Ar, Kr, and Xe matrices were produced by electrodesputtering in a self‐igniting pulsed‐discharge unit. The absorption and laser‐induced fluorescence spectra of the thallium atoms have been investigated. The results are compared with our recent observations of gas phase diatomic thallium–rare gas (Tl–Rg) clusters. The observed linewidths, shifts, and relaxation dynamics are qualitatively interpreted on the basis of the pairwise Tl–Rg interaction potentials.
Reactions of thermal hydrogen atoms in ethane and propane at 10 K: Secondary site selectivity in hydrogen abstraction from propane100(1994); http://dx.doi.org/10.1063/1.466794View Description Hide Description
The reactions of hydrogen atoms, generated by ultraviolet photolysis of hydrogen iodide, with solid ethane and propane at 10 K are investigated using electron spin resonance(ESR) to identify the reaction products. Although the results are generally similar to those found by others for these tunneling hydrogen abstraction reactions in dilute solutions of the alkanes in xenon, there also are significant differences, most notably, evidence that only the secondary hydrogen of propane is abstracted in its tunnelingreaction with an H atom. Also notable is the high degree of crystalline order in the ethane matrix, which is preserved in the ethyl radical product of the H atom–ethane reaction.
A direct ab initio dynamics approach for calculating thermal rate constants using variational transition state theory and multidimensional semiclassical tunneling methods. An application to the CH4+H↔CH3+H2 reaction100(1994); http://dx.doi.org/10.1063/1.466795View Description Hide Description
We present a new methodology, called ‘‘direct ab initio dynamics,’’ for calculations of thermal rate constants and related properties from first principles. The new method is based on full variational transition state theory plus multidimensional semiclassical tunnelingtransmission coefficients with the potential energy information to be calculated from an accurate level of ab initioelectronic structuretheory. To make this approach practical, we propose the use of a focusing technique to minimize the number of electronic structure calculations, while still preserving the accuracy of the dynamical results. We have applied this method to study detailed dynamics of the hydrogen abstraction reaction, CH4+H↔CH3+H2, and obtained excellent agreement with the available experimental data for both the forward and reverse rate constants for a range of temperatures from 300 to 1500 K. In these calculations, the potential energy surface was calculated at the quadratic configuration interaction including single and double excitation (QCISD) level of theory using the triple‐zeta plus polarizations 6‐311G(d,p) basis set.
100(1994); http://dx.doi.org/10.1063/1.466796View Description Hide Description
We present experimental and theoretical differential cross sections for the fine structure changing process Ar + O(3 P 2)→Ar + O(3 P jm j ) with (j,m j )=(1,1), (1,0), and (0,0). The measured cross sections refer to a collision energy of 3.2 kcal/mol, and were obtained from Doppler line shapes associated with (2+1) resonance‐enhanced multiphoton ionization of O(3 P jm j ) after scattering in a crossed‐beam apparatus. The theoretical results are based on 3Σ− and 3Π potential curves obtained from high quality ab initio calculations, and on quantum coupled‐channel calculations. The calculated differential cross sections show strong Stuckelberg oscillations, with similar magnitudes and phases for all three final states. With slight adjustment (0.03 Å) of the hard wall on the 3Σ− curve, the calculated angular distributions match up well with the corresponding experimental results, much better than is found using previously derived empirical potentials. At the same time, the integral total cross sections obtained from the ab initiomeasurements are in satisfactory agreement with previous measurements.
100(1994); http://dx.doi.org/10.1063/1.466797View Description Hide Description
Production of electronic ground state NO2 (2 A 1) from 248 nm photolysis of HNO3 was detected by laser induced fluorescence(LIF). A growth in the LIF signal was observed following the photolysis and has been interpreted as the relaxation of NO2 through the higher vibrational levels of the X(2 A 1) state; an energy region where the probe laser photodissociates the NO2 instead of inducing fluorescence. The rate coefficients for NO2 relaxation through these high vibrational levels were determined by fits of time resolvedLIF signal to a stepladder kinetic model. The results of the kinetic analysis suggest that the observed relaxation begins at the 2 B 2 threshold near 9500 cm−1 and extends downward through approximately 5 vibrational levels of the ground electronic surface. The derived quenching rate coefficients (in units of 10−12 cm3 molecule−1 s−1) are 0.51±0.05, 1.0±0.1, 1.4±0.2, 2.6±0.6, and 8.7±1.1 for Ar, He, N2, O2, and CO2 collision partners, respectively. The discrepancies between these coefficients and previous literature values are rationalized in terms of a dependence of the vibrational relaxation rate on total internal energy.
100(1994); http://dx.doi.org/10.1063/1.466798View Description Hide Description
The threshold photoelectron (PE) spectrum for nascent SH formed in the ultraviolet photodissociation of H2S has been measured using the nonresonant two‐photon pulsed field ionization (N2P‐PFI) technique. The rotationally resolved N2P‐PFI‐PE spectrum of SH indicates that photoionizationdynamics favor the rotational angular momentum change ΔN≤0 with the ΔN values up to −3, an observation similar to that found in the PFI‐PE spectra of OH (OD) and NO. The ionization energy for SH(X 2Π3/2) is determined to be 84 057.5 ± 3 cm−1 (10.4219 ± 0.0004 eV). The spin–orbit splitting for SH(X 2Π3/2,1/2) is 377 ± 2 cm−1, in agreement with the literature value. This study illustrates that the PFI‐PE detection method can be a sensitive probe for the nascent internal energy distribution of photoproducts.
100(1994); http://dx.doi.org/10.1063/1.466799View Description Hide Description
The photodissociation of dichlorine monoxide (Cl2O) at 308, 248, and 193 nm was studied by photofragment translational energy spectroscopy. The primary channel upon excitation at 308 and 248 nm was Cl–O bond fission with production of ClO+Cl. A fraction of the ClO photoproducts also underwent spontaneous secondary dissociation at 248 nm. The center‐of‐mass translational energy distribution for the ClO+Cl channel at 248 nm appeared to be bimodal with a high energy component that was similar in shape to the 308 nm distribution and a second, low energy component with a maximum close to the threshold for the 2Cl+O(3 P) channel. Observation of a bimodal distribution suggests that two pathways with different dissociation dynamics lead to ClO+Cl products. The high product internal energy of the second component raises the possibility that ClO is formed in a previously unobserved spin‐excited state a 4Σ−. Following excitation at 193 nm, a concerted dissociation pathway leading to Cl2+O was observed in addition to primary Cl–O bond breakage. In both processes, most of the diatomic photofragments were formed with sufficient internal energy that they spontaneously dissociated. The time‐of‐flight distributions of the Cl2+O products suggest that these fragments are formed in two different channels Cl2(3Π)+O(3 P) and Cl2(X 1Σ)+O(1 D).