Volume 89, Issue 2, 15 July 1988
Index of content:

Infrared diode laser spectroscopy of SO in the a ^{1}Δ state generated by the photolysis of Cl_{2}SO at 193 nm
View Description Hide DescriptionThe SO radical in the a ^{1} Δ state was produced by photolyzing Cl_{2} SO at 193 nm, and its vibration–rotation spectrum was observed for the bands up to v=6–5 by infrared diode laser kinetic spectroscopy.Analysis of the observed spectrum yielded precise Dunham coefficients, which were used to calculate an RKR potential energy curve for the a ^{1} Δ state. The branching ratio of the two Cl_{2} SO photolysis processes yielding SO in a ^{1} Δ and X ^{3} Σ^{−}, respectively, was estimated to be 1:4, in sharp contrast with the value of smaller than 0.01 for the photolysis of SO_{2} at 193 nm.

Collisionally induced population transfer effect in infrared absorption spectra. I. A line‐by‐line coupling theory from resonances to the far wings
View Description Hide DescriptionThe line‐by‐line coupling for a pressure broadened rovibrational band is formulated in the far‐wing limit. The present quasistatic theory assumes that the wave frequency is displaced from the line centers by an amount that is large compared with both the reciprocal duration of a typcial binary collision and frequency separation between strongly coupled lines. This theory generalizes that of Rosenkranz [J. Chem. Phys. 8 3, 6139 (1985)] where the Fano’s relaxation operator was reduced to a scalar parameter through a band average. The present approach permits computation of far‐wing absorption more specifically tailored to individual lines. Such a line‐by‐line approach is needed for rovibrational bands where some far lines contribute significantly to the total absorption. In order to obtain a qualitative picture of the line coupling as a function of the frequency displacement, calculations for collisions of CO_{2} with Ar have been performed for some lines. The results are compared with the corresponding variation of the diagonal relaxation matrix element.

Excited state relaxation in bichromophoric rotors: Time‐resolved fluorescence of 1,3‐di(N‐carbazolyl) propane: A three‐state analysis
View Description Hide DescriptionThe transient fluorescence profiles of 1,3‐di(N‐carbazolyl)‐propane, DCP, were reinvestigated as a function of temperature in toluene as solvent. Typically t h r e e‐e x p o n e n t i a l patterns, both for the low‐energy, red edge fluorescence of the excimer F _{ E }(t) and, in part, for the monomerfluorescenceF _{ M }(t) were observed in the moderate temperature range −15≤t/°C≤55, whereas at temperatures t/ °C>55 profiles were found to be approximately biexponential, within the limitations of time resolution. On the premises given in Sec. IV of this work, data were analyzed in terms of a discrete three‐state model which assumes t w o monomeric conformers (X _{1}=t t, X _{2}=t g ^{±}) and a single excimer‐forming conformation (X _{3}=g ^{∓} g ^{±}) interconverting in an open, linear scheme. Starting from a generalized treatment of n‐particle interaction, the analytical δ‐pulse solutions to the fluorescence evolutions X _{1}(t), X _{2}(t), and X _{3}(t) were formulated in terms of 18 amplitudes A _{ i j }(k) (i, j=1,2,3) and 3 eigenvalues τ_{ j } =−1/T _{ j } ( j=1,2,3) for two different, initial boundaries (k=1,2). For reasonable choices of fluorescence rate constants, the simulated parameters proved useful (a) to recover satisfactorily the experimental subnanosecond (T _{1}) and nanosecond time constants (T _{2},T _{3}), (b) to rationalize the b i e x p o n e n t i a l rise of excimer fluorescence at moderate temperatures, and (c) to explain the pseudo‐Birks behavior in the high‐temperature regime. Results from both experiments and computation allow to specify the time scales of rotating carbazole chromophores, and they strongly indicate that the rapid conformational equilibrium hypothesis is not valid in DCP. The limitations of the minimal model have been addressed and the potential problem encountered in analyzing the data by a discrete set of multiexponentials has been discussed.

Perturbations of the A ^{1}Σ^{+} _{ u } and b ^{3}Π_{ u } states of Na_{2} and the effects on the transition intensity and the line splitting
View Description Hide DescriptionDoppler‐free polarization spectra of the A ^{1}Σ^{+} _{ u } (v=8, J=0–15) and b ^{3}Π_{0u } (v=14, J=0–15) states were observed. By analyzing the energy shifts, the spin–orbit interaction between the A ^{1}Σ^{+} _{ u } (v=8) and b ^{3}Π_{0u } (v=14) states was studied. The magnitudes of the hyperfine splittings were different in corresponding lines of the two states. In addition to the energy shifts, the line intensities and the line shapes were found to change remarkably near the perturbation. These are useful to study the mixing of the perturbing states.

Experimental and molecular dynamics study of the pressure dependence of Raman spectra of oxygen
View Description Hide DescriptionThe pressure dependence of Raman spectra of gaseous O_{2} at 300 K has been studied experimentally and by molecular dynamics (MD) simulation. Experimental spectra are reported for the pressure range of 40–3000 bar and MD spectra for four thermodynamic states in the pressure range of 130–3000 bar. The MD trajectories are calculated using the Lennard‐Jones atom–atom intermolecular potential. The interaction‐induced effects on the system polarizability are evaluated using the first order dipole–induced dipole (DID) approximation. In the case of depolarized Raman scattering, the experimental line shapes and time correlation functions agree very well with the MD results. The density dependence of the experimental second spectral moment is also in excellent agreement with the MD predictions. The MD results indicate that the relative contribution of the interaction‐induced polarizability to the depolarized spectrum increases with increasing density, but remains small within the density range considered, and that the spectrum is dominated by orientational relaxation of the molecular polarizability. The experimental depolarized Raman and MD orientational time correlation functions are compared to the results of J‐diffusion and Steele models of relaxation. We find that neither of these models can account for single molecule reorientation in oxygen gas over the entire range of pressures. At high pressures, the experimental results for the frequency‐dependent depolarization ratio η differ significantly from the ‘‘classical’’ value of 3/4 over most of the accessible frequency range. Similar behavior is found for the corresponding Rayleigh depolarization ratios. The MD calculations predict a much smaller deviation of η from the value of 3/4, suggesting that induction mechanisms other than DID are needed to explain the experimental data. The experimentally observed pressure dependence of the Q branch of the Raman spectrum is explained using the motional narrowing model of Brueck.

Infrared diode laser spectra of MgH and MgD (X ^{2}Σ^{+})
View Description Hide DescriptionInfrared absorption spectra of magnesium hydride in its ^{2}Σ^{+} ground state have been measured at Doppler limited resolution in a dc discharge of hydrogen over magnesium. Most of the transitions arise from the fundamental bands of ^{2} ^{4}MgH and ^{2} ^{4}MgD, but, in addition, a few lines attributable to ^{2} ^{5}Mg and ^{2} ^{6}Mg species have been detected, as well as v=2←1 hot band transitions in ^{2} ^{4}MgH and ^{2} ^{4}MgD. Most of the lines are characterized by a doublet splitting due to the electron spin–rotation interaction (γN⋅S) which helps to distinguish them from the broader Rydberg transitions of atomic magnesium observed in the same spectral region. The spectra were analyzed in a combined fit with the FIR pure rotational transitions which leads to an accurate set of molecular parameters either for each vibrational level or as Dunham coefficients. The mass dependence of the molecular parameters is examined in terms of possible breakdown of the Born–Oppenheimer approximation.

Vibration–rotation interactions and ring‐puckering potential function in trimethylene sulphone
View Description Hide DescriptionThe microwave spectrum of trimethylene sulphone has been studied in the ground and five vibrational excited states of the ring‐puckering vibration. Very large deviations from the rigid rotor behavior were observed for the lowest pair of states. These perturbations resulted from a strong Coriolis coupling between them and have been analyzed using a reduced axis system Hamiltonian. The energy separation between the v=0 and v=1 levels has been determined from this analysis to be of 0.9106±0.0004 cm^{−} ^{1}. Small deviations from rigid rotor spectrum were observed for the v=2 to v=5 ring‐puckering states that were found to fit semirigid rotor theory where the quartic centrifugal distortion constants account for the vibration–rotation interaction effects. From the variation of the rotational constants and the vibrational separation between v=0 and v=1 states, a reduced ring‐puckering potential function has been determined to be V(X)=6.62(X ^{4}−9.22X ^{2}), which gives a barrier to the planar configuration of 140±35 cm^{−} ^{1}. With this potential function the vibration–rotation interaction effects observed for states v=2 to v=5 have been interpreted. Considerations about the ring conformation and the dynamics of the ring‐puckering motion have also been made. The μ_{ a } component of the electric dipole moment has been determined to be 4.8 D.

A kinetic theory of collision‐induced depolarized light scattering
View Description Hide DescriptionThe collision‐induced depolarized light scatteringspectrum arising from the dipole–induced dipole (DID) interactions in atomic fluids has been calculated using kinetic theory methods. The theory contains no adjustable parameters and requires only the temperature, density, and collision diameter as input. Above 25 cm^{−} ^{1}, the calculated line shape was found to be in excellent agreement with the spectrum obtained from a hard‐sphere molecular dynamics simulation, with the same model for the interaction‐induced polarizability. The domain of applicability of the approximations used in reducing the theory to a tractable form was carefully evaluated. The methods developed should be applicable to other intermolecular correlation functions, such as those observed in vibrational relaxation and interaction‐induced infrared spectroscopy.

Rotary resonance recoupling of dipolar interactions in solid‐state nuclear magnetic resonance spectroscopy
View Description Hide DescriptionA new resonance effect in solid‐state nuclear magnetic resonance(NMR) is described. The effect involves a combination of magic‐angle sample rotation with irradiation of a heteronuclear spin system at the Larmor frequency of one of the spin species. If the irradiation intensity is such as to establish a match between spin nutation and sample rotation, it is shown that the heteronuclear dipolar spin interaction is selectively reintroduced into the spectrum. This allows small dipolar coupling constants to be measured in the presence of large shielding anisotropies. Applications are anticipated for determination of internuclear distances in materials lacking long‐range order, such as polycrystalline materials, polymers, and surfaces.

The properties of the hydrogen‐bonded dimer (CH_{3})_{3}N⋅⋅⋅HCN from an investigation of its rotational spectrum
View Description Hide DescriptionThe ground‐state rotational spectra of the three symmetric‐top isotopomers (CH_{3})^{14} _{3}N ⋅⋅⋅HC^{1} ^{4}N, (CH_{3})^{14} _{3} N⋅⋅⋅HC^{1} ^{5}N, and (CH_{3})^{14} _{3} N⋅⋅⋅DC^{1} ^{5}N of a hydrogen‐bonded dimer formed from trimethylamine and hydrogen cyanide have been investigated by pulsed‐nozzle, Fourier‐transform microwave spectroscopy. The spectroscopic constants B _{0}, D _{ J }, D _{ J K }, χ(^{1} ^{4} N _{1}), and χ(^{1} ^{4} N _{2}) have been determined from analyses of the spectra and for the isotopomer (CH_{3})^{14} _{3} N⋅⋅⋅HC^{1} ^{4}N take the values 1373.6236(2) MHz, 0.694(10) kHz, 98.52(2) kHz, −5.061(14) MHz, and −4.533(16) MHz, respectively. The spectroscopic constants have in turn been interpreted with the aid of a model of the dimer to yield the following molecular properties for the species (CH_{3})^{14} _{3} N⋅⋅⋅HC^{1} ^{4}N: r(N⋅⋅⋅C)=3.1035(6) Å, the subunit oscillation angles θ_{av}=13.4(2)°, and φ_{av}=9.1(4)° for the (CH_{3})_{3}N and HCN subunits, respectively, and the hydrogen bond stretching force constant k _{σ}=14.7(2) N m^{−} ^{1}. The opportunity has been taken to determine a more accurate value χ_{0}(^{1} ^{4}N)=−5.5024(25) MHz for the ^{1} ^{4}N‐nuclear quadrupole coupling constant of free (CH_{3})^{14} _{3} N. The properties of the family of dimers R_{3}N⋅⋅⋅HX, where R=CH_{3} or H and X=CN or CCH, are compared.

Third order a b i n i t i o calculations of the f↔f transition amplitudes for ions across the lanthanide series
View Description Hide DescriptionThe behavior of various contributions to the f↔f transition amplitude defined up to the third order in perturbation theory is analyzed. Main attention is paid to the third‐order contributions which are due to electron correlation inside the rare earth ion and arise from the static and dynamic models. The discussion is based on the numerical results of exact a b i n i t i o calculations performed for Pr^{+} ^{3}, Nd^{+} ^{3}, Eu^{+} ^{3}, Gd^{+} ^{3}, Tb^{+} ^{3}, and Tm^{+} ^{3}ions.

The Raman spectra of polycrystalline orthorhombic YF_{3}, SmF_{3}, HoF_{3}, YbF_{3}, and single crystal TbF_{3}
View Description Hide DescriptionThe Raman spectra of the orthorhombic form of polycrystalline YF_{3}, SmF_{3}, TbF_{3}, HoF_{3}, and YbF_{3} have been recorded. Room‐temperature, polarized Raman spectra of a single crystal of orthorhombic TbF_{3} have also been obtained. Based on these polarized Raman spectra, symmetry assignments have been made for 23 of the expected 24 Raman active phonon vibrations in TbF_{3}. By analogy to the results from single crystal TbF_{3}, tentative symmetry assignments of the observed phonon vibrations were also made for the other compounds included in this work.

The vibrational spectra (100–1600 cm^{−} ^{1}) and scaled a b i n i t i o STO‐3G and 3‐21G harmonic force fields for norbornane, norbornene, and norbornadiene
View Description Hide DescriptionThe vibrational spectra in the region of 100–1600 cm^{−} ^{1} of norbornane and norbornadiene are reinvestigated, and those of norbornene reported for the first time. On the basis of 3‐21G a b i n i t i o force fields, evaluated for each molecule and scaled using overlay refinements of 12 scaling factors, the spectra are assigned and the observed transition frequencies reproduced with an overall average error of 6.2 cm^{−} ^{1}. STO‐3G force constants, modified to correct for deficiencies in the stretch/bend interaction force constants but otherwise scaled analogously to 3‐21G, reproduce the final set of assignments to within 10.1 cm^{−} ^{1} on average. The STO‐3G basis is shown to be suitable as a less costly alternative to split‐valence basis sets, particularly for describing the vibrational dynamics of low frequency ring bending and torsional modes.

Static and dynamic hyperfine coupling of FE(II) in the FeS_{4} unit. Results of Mössbauer spectroscopy on a [P(C_{6}H_{5})_{4}]_{2}[Fe(SC_{6}H_{5})_{4}] monocrystal
View Description Hide DescriptionThe hyperfine coupling parameters of Fe(II) in the FeS_{4} unit of the complex anion [Fe(SC_{6}H_{5})_{4}]^{2} ^{−} have been determined from an analysis of Mössbauer spectra of a monocrystal in the temperature range of 1.5 to 20 K and in magnetic fields of 0 to 6.7 T. Dynamic spin fluctuation effects have been included in the analysis by a stochastic model with spin–lattice relaxation.Anisotropy in the relaxation matrix element was of moderate significance and apparent only at low fields. The use of a monocrystal allows more precise determination of hyperfine parameters, because it avoids the averaging procedures which are inherent in studies on polycrystalline material. The set of parameters is essentially identical with that reported for reduced rubredoxin and indicates that the electronic ground state of [Fe(SC_{6}H_{5})_{4}]^{2} ^{−}, predominantly of [3z ^{2}−r ^{2}] character, simulates very well the ground state of the protein FeS core.

Trajectory studies of unimolecular reactions of Si_{2}H_{4} and SiH_{2} on a global potential surface fitted to a b i n i t i o and experimental data
View Description Hide DescriptionThe unimolecular decomposition dynamics of Si_{2}H_{4} have been investigated using classical trajectory methods on a global potential‐energy surface fitted to the results of a b i n i t i o calculations and the available experimental data. The required phase‐space averages are computed using Metropolis sampling techniques. It is found that unless the parameters of the Markov walk are adjusted for each different type of atom present, extremely long Markov walks are required to adequately cover the phase space of the system. Microcanonical rate coefficients for the decomposition of Si_{2}H_{4} into all open channels are reported at energies in the range 5.0<E<9.0 eV. The most important dissociation channel over this energy range is three‐center elimination of molecular hydrogen leading to H_{2} Si=Si. At energies below 7.0 eV, the other channels are, in order of importance, Si–Si bond rupture, four‐center H_{2} elimination, and simple Si–H bond rupture. At or above 8.0 eV, four‐center H_{2} elimination replaces Si–Si bond rupture as the second most important decomposition channel. The energy dependence of the rate coefficients is well described by an RRK expression. Three‐center H_{2} elimination involves a simultaneous rupture of both Si–H bonds whereas the four‐center elimination is found to proceed by a hydrogen atom transfer process followed by H_{2} elimination. Except for a small propensity to form H_{2} with excess rotational energy, the energy partitioning among the products is nearly statistical. A comparison study of the decomposition of Si_{2}H_{4} complexes formed by the recombination of two SiH_{2} molecules shows that the rates for both three‐ and four‐center H_{2} elimination are in agreement with those computed using a statistical distribution of the same internal energy. The rate for Si–Si bond rupture, however, is significantly larger for Si_{2}H_{4} complexes formed by SiH_{2} recombination than for Si_{2}H_{4} molecules with the same internal energy randomly distributed. The decomposition dynamics of SiH_{2} on the global surface are also reported.

Coupling schemes for atom–diatom interactions and an adiabatic decoupling treatment of rotational temperature effects on glory scattering
View Description Hide DescriptionThe quantum mechanical theory for scattering of a particle by a rigid rotor is formulated in five alternative diabatic representations, corresponding to alternative coupling schemes. Use is made of a recently introduced procedure for obtaining discrete representations by artificial quantization. In order to develop an efficient computational scheme for obtaining information on the interaction potential from atom–diatom scattering experiments, decoupling approximations are developed. An adiabatic representation in the coupled states framework is applied to the computation of integral cross sections and nonadiabatic couplingeffects are analyzed. The approach provides an accurate description of the experimentally observed dependence of glory scattering from the rotational temperature of the diatom.

Frictional effects on barrier crossing in solution: Comparison with the Kramers’ equation
View Description Hide DescriptionIn our efforts to examine the validity of the Kramers’ equation, the rate constants of the excited stateisomerization of 1,1’‐binaphthyl in n‐alkane solvents were measured at room temperature using picosecond spectroscopy. These data, and data measured previously in n‐alcohols, were compared with Kramers’ model using two forms for the friction. When a hydrodynamic model for the friction was used, good agreement was found for the alcohol data only. When the isomerizationfriction is assumed to scale linearly with the friction for overall reorientational motion, we find excellent agreement for both the alcohol and alkane solvents. In addition, the friction in alkanes is found to be considerably larger than that of alcohols of comparable viscosity. This provides a direct indication that the molecular aspects of the solute–solvent interaction play a role in the barrier crossing process.

Theoretical studies of collisional energy transfer in highly excited molecules: The importance of intramolecular vibrational redistribution in successive collision modeling of He+CS_{2}
View Description Hide DescriptionIn this paper we present a new method for studying the collisional relaxation of highly excited molecules in low density gases known as the redistributed successive collisions (RSC) method, and we apply it to the relaxation of CS_{2} by He at 300 K in the vibrational energy range E=32 640–3180 cm^{−} ^{1}. The RSC method involves calculating sequences of collisions, subject to the assumption that rapid vibrational redistribution occurs between each collision. As a result, initial conditions for each trajectory in a sequence are sampled from a microcanonical ensemble that is defined by the final energy and angular momentum of the previous trajectory. The application to He+CS_{2} leads to 〈ΔE〉’s that vary linearly with E over the entire energy range considered. The agreement of these 〈ΔE〉’s with measured values is good, but there is a qualitative difference in the E dependence of 〈ΔE〉 over part of the range of E’s. We also examine a second successive collision method that is more appropriate for high‐density gases in which the internal coordinates and momenta are c o n s e r v e d (i.e., not redistributed) between collisions (CSC method). We find that a substantial fraction of the CSC ensembles (∼50%) exhibit extremely slow relaxation which in some cases is not complete even after 80 000 collisions. This unphysical result appears to be a classical artifact, and it leads to very small 〈ΔE〉’s at medium to low E and a stronger dependence of 〈ΔE〉 on E (close to quadratic) at high E. Omission of these slowly relaxing ensembles from the CSC ensemble average leads to CSC 〈ΔE〉’s which are nearly identical to those from the RSC calculation. An analysis of the distribution of energy among vibrational modes in the CSC calculations indicates that the slow relaxation arises from energy becoming frozen in the asymmetric stretch of CS_{2}. The influence of the CS_{2} intramolecular dynamics on the collisional relaxation is considered, and we find evidence of abrupt collision induced intramolecular energy redistribution due to nonlinear resonance formation.

Metastable decay of photoionized niobium clusters: Clusters within a cluster?
View Description Hide DescriptionThe photoinduced metastable ion fragmentation of niobium clusters has been found to closely correlate with the laser vaporization source conditions. The experimental evidence indicates the clusters agglomerate in the supersonic expansion. These agglomerated clusters exhibit rather unique behavior for a transition metal system. Both evaporation and fission fragmentation are observed. Two color MPI studies reveal the existence of long lived neutral intermediate states. Also, the kinetic energy release is extremely small, <10 meV, and independent of the excitation energy. The concept of clusters within a cluster, i.e., agglomerated clusters, is introduced to rationalize many experimental findings.

Quantum theory and collisional propensity rules for rotationally inelastic collisions between polyatomic molecules (NH_{3} and CO_{2}) and an uncorrugated surface
View Description Hide DescriptionWe present the general quantum theory of collisions of a symmetric top molecule with an uncorrugated surface. The similarities between the description of collisions of a molecule with a structureless atom and a flat surface allow us to exploit earlier gas‐phase results. We then derive several collisional propensity rules: (1) In experiments in which both inversion states in the initial J,K doublets of para‐NH_{3} are equally populated, both inversion states of all collisionally excited levels must also be equally populated. If, however, the initial inversion level can be state selected, then unequal populations will be observed in collisionally excited inversion doublets. (2) For transitions from the J=0 level of ortho‐NH_{3} into rotational levels of the K=3 stack, a strong propensity will exist toward conservation of the inversion symmetry for transitions into levels with J’o d d, but toward a change in the inversion symmetry for transitions into levels with J’e v e n. (3) If the odd terms in the angular expansion of the potential dominate, then for transitions out of rotational levels with J>0 in the K=0 stack of ortho‐NH_{3} into rotational levels of the K=3 stack, a strong propensity will exist toward population of the u p p e r level of the inversion doublet if the initial state has e v e n J, and toward population of the l o w e r level if the initial state has o d d J. Using the similarities between the wave functions of a symmetric top and those of a linear polyatomic molecule with degenerate bending modes, we derived several propensity rules for the specific case of collisions of CO_{2} (00^{0}0) with an uncorrugated surface. In collisions which excite the low‐lying (01^{1}0) bending vibration, if the initial rotational quantum number is small, then we predict that the probability of transition into a final state with J’ odd will be much larger than for transition into a final state with J’ even.