Volume 102, Issue 4, 22 January 1995
Index of content:

Slow motion electron spin resonance line shapes of lyotropic liquid crystals in hexagonal phase
View Description Hide DescriptionIn this paper the slow motion electron spin resonance(ESR)line shape theory is extended to hexagonal mesophases. The stochastic Liouville equation is applied to the dynamic description of both local molecular reorientation and azimuthal surface translational diffusion of the lipid around the cylinder axis. The established ESRline shapemodels allow a separation of contributions to the electron spin relaxation from the two motions, if macroscopically aligned hexagonal spectra are simulated. Such simulations can yield, not only the dynamic parameters such as local ordering parameter, local motional rate, and azimuthal surfacediffusion coefficient, but also a disorder parameter accounting for residual cylinder disorder and the disorder effects due to finite cylinder length and curved surface along the cylinder axis. To test these models, they are applied to the analyses of X‐band ESR spectra of the hexagonal phase of the sodium dodecyl sulphate (SDS)/decanol/water ternary system. The obtained surfacediffusion coefficient and disorder parameter are in good agreement with previously reported values in a deuterium nuclear magnetic resonance(NMR) study. The local dynamic properties, which are not readily available by NMR method, are compared with those obtained for the micellar phase of the SDS/water binary system. Such a comparison reveals that while the local orderings are similar, the local dynamics is much slower in the hexagonal phase than in the micellar phase.

Electronic spectroscopy of the Mg^{+}–N_{2} complex: Evidence for photoinduced activation of N_{2}
View Description Hide DescriptionThe ion–molecule complex, Mg^{+}–N_{2} is formed in a supersonic expansion and studied with mass‐selected photodissociationspectroscopy. The lowest energy bands observed in the electronic excitation spectrum are redshifted more than 12 000 cm^{−1} from the Mg^{+} (^{2} P←^{2} S) atomic transition at 280 nm. The red‐shift, resulting from differential bonding in the ground and excited states of the complex, is much larger than the shifts observed in previously studied Mg^{+}–ligand complexes. Resolved vibronic structure is observed extending for more than 5000 cm^{−1}. The observation of spin–orbit multiplet structure indicates that the complex is linear and that the electronic transition is ^{2}Π←X ^{2}Σ^{+}. The spin–orbit splitting of 46 cm^{−1} is significantly less than that observed for other Mg^{+}–L complexes. Vibronic intervals of about 1000 and 500 cm^{−1} are assigned respectively to a stretching mode and to double quanta in a bending mode. The study of isotopically substituted complexes indicates that the best assignment for the stretch progression is the N–N mode, with a frequency far below that in free N_{2}. The vibrational activity, frequency shift, and spin–orbit splitting are all signatures for N_{2} activation by the excited metal ion. However, the degree of this interaction is greater than that predicted by ab initio calculations.

Fourier‐transform microwave spectroscopy of triplet carbon monoxides, C_{2}O, C_{4}O, C_{6}O, and C_{8}O
View Description Hide DescriptionRotational spectra of C_{ n }O with n=2, 4, 6, and 8 have been observed by using a Fabry–Perot type Fourier‐transform microwave spectrometer cooperated with a pulsed discharge nozzle. The molecules have been generated by an electric discharge of carbon suboxide diluted in Ar, and adiabatically cooled to ≊2 K in a subsequent supersonic expansion. All the observed spectra for these species are characterized as linear molecules in the ^{3}Σ^{−} electronic ground state. Since all the three spin sublevels have been detected even in the free‐jet condition, the spin–spin coupling constants have been determined precisely as well as other spectroscopic constants. The coupling constants show rapid increase as n becomes larger, indicating smaller energy gaps between the excited ^{1}Σ^{+} state and the ^{3}Σ^{−}ground state for the longer species. Along with the recent observation of singlet C_{ n }O (n=5, 7, and 9) [Ogata, Ohshima, and Endo, J. Am. Chem. Soc. (submitted)], the present study has established the existence of a complete set of the linear carbon‐chain series C_{ n }O up to n=9 in the gas phase. The effective C=C bond lengths evaluated from the rotational constants decrease gradually to a converging value of ≊1.28 Å as n becomes larger. No apparent quasilinearity has been observed in the centrifugal‐distortion constants of all the members, in contrast to the relevant series of the pure carbon clusters, C_{ n }, some of which (n=3 and 7) have shown substantial nonrigidity for the bending vibration.

The microwave spectrum and structure of the pyridine–CO complex
View Description Hide DescriptionRotational spectra of five isotopomers of the pyridine–CO complex with D, ^{13}C, ^{15}N, and ^{18}O have been measured over the 8–18 GHz frequency range. Rotational constants, centrifugal distortion constants, and ^{14}N quadrupole coupling constants have been fitted to the measured transition frequencies of each isotopomer. Two different structures have been found to be compatible with the moments of inertia of all isotopomers. Both have CO lying in the plane of pyridine approaching simultaneously N and ortho‐H but with two different orientations of CO.

Electronic absorption spectra of cyano‐substituted polyacetylene cations in neon matrices
View Description Hide DescriptionThe A ^{2}Π←X ^{2}Π transitions and vibrational frequencies in the excited electronic states have been determined for the cyanopolyacetylene cations H–(C≡C)_{ n }–CN^{+} (n=2–6) and NC–(C≡C)_{ n }–CN^{+} (n=1–5). The spectroscopic information was derived by absorption measurements after cations from a mass selected beam were condensed together with excess of neon to form a 5 K matrix. Spectra have been also obtained for ions with odd number of π‐centers and are tentatively assigned to H–C_{2n−1}–CN^{+} (n=3–6) and NC–C_{2n−1}–CN^{+} (n=3–5). The assignments are based on trends observed for the homologous series of these cations, known gas phase data on three of the smaller species, and the free electron molecular orbital model.

Resonance enhanced multiphoton ionization spectroscopy of the NF molecule: ^{1,3}Φ 3d and 4d Rydberg states
View Description Hide DescriptionThe (3dδ)^{1,3}Φ and (4dδ)^{1,3}Φ Rydberg states of NF have been investigated by multiphoton ionization (MPI) spectroscopy. These states were observed as two‐photon resonances in an overall (2+1) MPI process from NF a ^{1}Δ produced from the F+N_{3}reaction.Ab initio calculations performed at the multireference double excitation configuration interaction level showed that the excited Φ states were of Rydberg character with configurations of ...1π^{4}5σ^{2}2π^{1}(3dδ)^{1} and ...1π^{4}5σ^{2}2π^{1}(4dδ)^{1}. The ^{3}Φ←a ^{1}Δ two‐photon transitions were found to derive their intensities from spin–orbit and spin‐uncoupling interactions in the ^{1}Φ_{3}, ^{3}Φ_{4}, ^{3}Φ_{3}, and ^{3}Φ_{2} upper states. Analysis of the rotationally resolved bands, using a model which includes these factors, allowed the spin–orbit constant, a _{π}, for the 2π valence orbital to be derived as (159.0±1.0) cm^{−1}. Rotationally resolved envelopes recorded for the (3dδ)^{1}Φ, v’=0, 1←a ^{1}Δ, v‘=0 and (3pπ)^{1}Σ^{+}, v’=0←a ^{1}Δ, v‘=0 (2+1) NF MPI bands, could be simulated reasonably well assuming a Boltzmann rotational distribution for the a ^{1}Δ state at ≊180 K. Experiments showed, however, that this was not a true measure of the initial state distribution because of predissociation or perturbation effects in the resonant intermediate state. Assignments for other two‐photon resonant NF MPI bands observed in this work are also suggested.

Time‐resolved laser control of vibrational excitations in molecules
View Description Hide DescriptionWe show that, on a short time scale, the dynamics of vibrational excitations in multimode ground‐state molecular systems, linearly coupled to a laser field, can be expressed as a simple functional of the laser pulse area. The dependence of the vibrational system’s dynamics on a field area leads to simple algebraic equations for this area, in the formulation of the inverse problem associated with the time‐resolved control (tracking) of vibrational excitations. The control equation to be solved is quadratic in the area, when the object of the time‐resolved control is the total vibrational energy, and linear when the object to be controlled is an average elongation (position tracking), or the average energy of a remotely coupled mode. This yields a control algorithm which requires no iteration and is easy to implement. Numerical tests of the algorithm are performed on the energy and position trackings in simple one‐dimensional modelsystems. An excellent analytical, approximate description of the laser‐driven dynamics of these systems is obtained using the concept of Lewis invariant. This analytical description is used as a reference with which the field numerically generated by solving the inverse control problem, using the aforementioned algorithm, can be compared.

Spectral diffusion of individual pentacene molecules in P‐terphenyl crystal: Stochastic theoretical model and analysis of experimental data
View Description Hide DescriptionWe propose a microscopic theoretical model to explain recent experiments involving the spectraldiffusion of individual pentacene molecules embedded in p‐terphenyl crystal. The experimental spectraldiffusion trajectories are analyzed in terms of three stochastic characterizations: The time autocorrelation of transition frequency fluctuations, the time‐dependent distribution of spectral jumps, and the equilibrium distribution of frequencies. The observed spectraldiffusion is well described by our model, and we determine detailed quantitative information about the localized excitations that are responsible for the fluctuations in the pentacene transition frequency. We find that the spectraldiffusion of the pentacene transition is caused by the reorientation of the central phenyl ring in the p‐terphenyl molecule, and that this reorientation can only occur at a wall between domains of different central phenyl ring ordering. Furthermore, we find that only those pentacene molecules that reside within a few lattice spacings of these domain walls exhibit spectraldiffusion.

Ground and excited states of Xe^{+} _{2} observed by high resolution threshold photoelectron spectroscopy of Xe_{2}
View Description Hide DescriptionThreshold photoelectron spectra of the xenon dimer have been observed with a resolution of 2 meV in the wavelength range 920–945 Å and 1022–1112 Å using the penetrating field technique and synchrotron radiation. Threshold photoelectron bands associated with transitions to the A ^{2} Σ^{+} _{1/2u }, B ^{2} Π_{3/2g }, C ^{2} Π_{3/2u }, C ^{2} Π_{1/2u }, and D ^{2} Σ^{+} _{1/2g } states of Xe^{+} _{2} have been identified. Vibrational structure associated with the C ^{2} Π_{1/2u } state has been observed for the first time and a new value of the D ^{2} Σ^{+} _{1/2g } state ionization potential is reported.

Scattering angle dependence of electron impact excitation: Intensity variation within a vibrational progression
View Description Hide DescriptionIntensity distributions of electronic transitions in O_{2} and CO within a vibrational progression resulting from electron impact excitation are studied theoretically and experimentally. The multireference single‐ and double‐excitation configuration interaction (MRD‐CI) method is used to elucidate details of selected electronic transitions. In particular, the adiabatic MRD‐CI approach can account for the variation of the Franck–Condon envelope with scattering angle that has been reported for the B ^{1}Σ^{+}←X ^{1}Σ^{+} transition in CO and also was recently observed in the B’ ^{3}Σ^{−} _{ u }←X ^{3}Σ^{−} _{ g } transition of O_{2}. This behavior contrasts with the relative stability of the intensity distribution observed within the CO A ^{1}Π←X ^{1}Σ^{+} vibrational progression. In the former cases the excited state undergoes changes with internuclear separation because of the presence of an avoided crossing. Since a transition from the zeroth vibrational level in the ground electronic state to an individual vibrational level in the excited electronic state tends to select a particular internuclear distance (R centroid), each vibrational band may behave as a transition to a separate electronic level. This happens because the excited‐state wave function undergoes a compositional change with internuclear separation between the adiabatic partners of the avoided crossing.

Field dependence of the magic angle spinning nuclear magnetic resonance line shapes of paired spin‐1/2 nuclei in solids
View Description Hide DescriptionProceeding from the average‐Hamiltonian theory, the effects caused by nonsecular terms of the spin Hamiltonian in magic angle spinning nuclear magnetic resonance spectra of a coupled pair of rare spin‐1/2 nuclei are described. Using doubly rotating coordinates with the Larmor frequencies (including isotropic screenings) measured in units of the rotation frequency, while the small frequency offsets between the actual Larmor frequencies and multiples of the rotation frequency were treated as small amplitudes within the Hamiltonian, a general formalism could be established that provides a universal description of the field‐dependent line shifts, broadenings, and splittings. Analytical expressions are given for a continuous range of experimentally accessible sample rotation rates, including the rotational‐resonance conditions where the Floquet theory fails. The second approximation includes the nonsecular terms and is sufficient to adequately describe the spectral features found. The anisotropic screening cross terms with the dipolar and J couplings generate in the high resolution spectrum of each single crystal at most four lines in the heteronuclear and up to eight lines in the homonuclear case. The positions and relative intensities of these lines depend both on the strength of the external magnetic field and on the mechanical sample rotation rate. In the powder pattern of a homonuclear pair, a broadened Pake‐like doublet appears over a limited magnetic field range. This splitting persists even at the highest attainable rotation rates. The splittings and line shapes can provide useful data about the relative orientations of the two nuclear screening tensors.

Rydberg state dynamics of rotating, vibrating H_{3} and the Jahn–Teller effect
View Description Hide DescriptionWe extend a recently developed multichannel quantum defect theory (MQDT) of electron half‐collisions with a triatomic ion. The calculations reproduce much of the observed photoabsorption spectrum of H_{3} near its lowest ionization thresholds. The method utilizes a rovibronic frame transformation that accounts simultaneously for vibrationally and rotationally inelastic collisions (preionization) and for l‐uncoupling effects. Jahn–Teller interactions among degenerate Rydberg states play a crucial role in the formulation. Although H_{3} is a weak Jahn–Teller molecule, this interaction is responsible for major resonance features seen in the experimental photoabsorptionspectra. Calculations over an extended photonenergy range for photoabsorption from the H_{3} (1s ^{2}3s ^{2} A _{1} ^{’}, v _{ i }=0, N _{ i }=1, K _{ i }=0) initial state into final states with total angular momentumN _{ f }=0,1,2 are given for parallel and perpendicular (M _{ i }=0, ‖M _{ f }‖=0,1) polarization schemes for the excitation. In spectral regions where experiments have been carried out, semiquantitative agreement with experiment is obtained. The limitations of the present theory are briefly discussed.

Reaction path zero‐point energy from diffusion Monte Carlo calculations
View Description Hide DescriptionA general diffusionquantum Monte Carlo method is described for accurately calculating the zero‐point energy of the vibrations orthogonal to a reaction path in a polyatomic system. The method fully takes into account anharmonic and mode–mode coupling effects. The algorithm is applied to the OH+H_{2}→H_{2}O+H reaction and the results are compared with a more approximate calculation. The technique will have many useful applications to kinetic and spectroscopic problems involving polyatomic molecules.

On the effects of molecular fluctuations on models of chemical chaos
View Description Hide DescriptionIn this work the effects of molecular or intrinsic fluctuations on some models of coupled chemical reactions exhibiting low‐dimensional deterministic chaos are investigated. The study is performed by considering the system at the mesoscopic level, namely by stochastically simulating the corresponding chemical master equation. Two specific models are studied: the isothermal three‐variable autocatalator of Peng et al. and a chemical version of Rössler’s model of spiral chaos. The main conclusions are that the corresponding strange attractors obtained in these models are robust against fluctuations, although when the system is near the onset of chaos the presence of fluctuations may anticipate the appearance of chaos.

Polyexponential kinetics of chemical reactions in condensed media within the quasiclassical approximation
View Description Hide DescriptionPolyexponential kinetical behavior typical for condensed phase reactions in highly viscous media is studied on a simple example of one‐dimensional diffusion equation with a sink modeling a chemical conversion of reactants. The corresponding polyexponential regime is demonstrated to have a thorough analogy with the quasiclassical approximation of one‐dimensional quantum mechanics and a relevant approximation for the Green’s function is developed. The asymptotic short‐ and long‐time kinetics are examined at the analytical level. Contrary to the frozen medium approximation according to which the slow diffusion motion of the medium is entirely ignored, the present quasiclassical model is fit for a qualitative description of the total time interval covering the reaction events from the initial moment up to the ultimate steady‐state monoexponential evolution. The range of validity of the quasiclassical approach is discussed. Numerical tests expose some peculiarities of the present treatment for equilibrium and nonequilibrium initial distributions. The work presents a qualitative development of the theory of nonexponential kinetics pioneered by papers of Agmon and Hopfield, Sumi and Marcus, and Nadler and Marcus.

Dynamics and kinetics of molecular high Rydberg states in the presence of an electrical field: An experimental and classical computational study
View Description Hide DescriptionThe effect of an electrical field on the dynamics and decay kinetics of a high Rydberg electron coupled to a core is discussed with special reference to simulations using classical dynamics and to experiment. The emphasis is on the evolution of the system within the range of Rydberg states that can be detected by delayed pulsed ionization spectroscopy (which is n≳90 for both the experiment and the computations). The Hamiltonian used in the computations is that of a diatomic ionic core about which the electron revolves. The primary coupling is due to the anisotropic part of the potential which can induce energy and angular momentum exchange between the orbital motion of the electron and the rotation of the ion. The role of the field is to modulate this coupling due to the oscillation of the orbital angular momentuml of the electron. In the region of interest, this oscillation reduces the frequency with which the electron gets near to the core and thereby slows down the decay caused by the coupling to the core. In the kinetic decay curves this is seen as a stretching of the time axis. For lower Rydberg states, where the oscillation of l is slower, the precession of the orbit, due to the central but not Coulombic part of the potential of the core, prevents the oscillation of l and the decay is not slowed down. Examination of individual trajectories demonstrates that the stretching of the time axis due to the oscillatory motion of the electron angular momentum in the presence of the field is as expected on the basis of theoretical considerations.
The relation of this time stretch to the concept of the dilution effect is discussed, with special reference to the coherence width of our laser and to other details of the excitation process. A limit on the principal quantum number below which the time stretch effect will be absent is demonstrated by the computations. The trajectories show both up and down processes in which the electron escapes from the detection window by either a gain or a loss of enough energy. Either process occurs in a diffusive like fashion of many smaller steps, except for a fraction of trajectories where prompt ionization occurs. The results for ensembles of trajectories are examined in terms of the decay kinetics. It is found that after a short induction period, which can be identified with the sampling time of the available phase space, the kinetics of the decay depend only on the initial energy of the electron and on the magnitude of the field, but not on the other details of the excitation process. The computed kinetics of the up and down channels are shown to represent competing decay modes. A possible intramolecular mechanism for long time stability based on the sojourn in intermediate Rydberg states is discussed. The available experimental evidence does not suffice to rule out nor to substantiate this mechanism, and additional tests are proposed. The theoretical expectations are discussed in relation to observed time resolved decay kinetics of high Rydberg states of BBC (bisbenzenechromium) and of DABCO (1,4‐diazabicyclo[2.2.2]octane).
The experimental setup allows for the imposition of a weak (0.1–1.5 V/cm) electrical field in the excitation region. The role of the amplitude of the time delayed field, used to detect the surviving Rydberg states by ionization, is also examined. The observed decay kinetics are as previously reported for cold aromatic molecules: Most of the decay is on the sub‐μs time scale with a minor (∼10%) longer time component. The decay rate of the faster component increases with the magnitude of the field. Many features in such an experiment, including the absolute time scales, are similar to those found in the classical trajectory computations, suggesting that the Hamiltonian used correctly describes the physics of the faster decay kinetics of the high Rydberg states.

Investigation of rare gas–oxygen exciplexes
View Description Hide DescriptionMixtures of rare gases (Rg) with oxygen donors at high pressures were excited by an electron beam. The fluorescence spectra in the ultraviolet were investigated. Besides the well‐known continua of KrO (180 nm) and XeO (235 nm) several new transitions were identified. They are attributed to the decay of RgO and possibly of Rg_{2}O. The study of the reaction kinetics yielded the rate constants for the most important buildup and decay reactions of KrO and ArO. The inverse lifetime of KrO was determined to be 3 × 10^{6} s^{−1} for ArO to be <3 × 10^{6} s^{−1}.

Photodissociation dynamics of state‐selected resonances of HCO X̃ ^{2} A’ prepared by stimulated emission pumping
View Description Hide DescriptionMetastable resonances on the ground electronic state of the HCO radical have been prepared by stimulated emission pumping. The resonances have energies 5000 to 10 000 cm^{−1} above the dissociation limit of HCO and can be assigned by their vibrational and asymmetric‐top rotational character. The transition linewidths of the resonances and the rotational and vibrational distributions of the CO dissociation products have been measured. The linewidths show a strong dependence on the vibrational character of the resonance rather than a monotonic dependence on energy, and thus provide an important example of nonstatistical behavior. CO(v=2) was produced in the decay of all six resonances studied, while only the three highest energy resonances produced measurable amounts of CO(v=3). CO rotational distributions with population in low‐J states, which often showed nonstatistical structure, were characteristic of the products from all the resonances studied. The rotational distributions depend both on the vibrational character of the parent state and on the rotational state prepared in the HCO. The experimental results are compared and contrasted with previous quantum mechanical calculations and analyzed in the context of a modified Franck–Condon model for the dissociation.

A new photon angular momentum approximation for molecular collisions in intense nonresonant laser fields
View Description Hide DescriptionWe here explore a new scheme for dealing with the photonangular momentum effects which arise in the analysis of collisions in intense nonresonant laser fields. The scheme involves approximating the system total angular momentum so as to create a reduced set of dynamical equations depending parametrically upon the relative orientation of laser field axis and system transition dipole moment. The equations are solved as a function of orientation and results are collected using numerical quadrature. We examine the scheme in application to a model collision problem. Comparing with the results of exact calculations we find that the new scheme is very effective in determining the magnitudes of S‐matrix elements but not the corresponding phases. The scheme is also compared with an earlier one which involves predynamical orientation averaging of laser–particle coupling elements. We find that the new scheme is at least as accurate as the earlier and is much more tractable computationally.

Vibrational relaxation in the ^{3} B _{1} state of SO_{2}
View Description Hide DescriptionThe vibrational relaxation rate constants of the four low‐lying vibrational levels in the ^{3} B _{1} state of the SO_{2} molecule have been investigated by a laser induced phosphorescence method in pure SO_{2}. The relaxation rate constants for the ^{3} B _{1}(0,1,0) level have also been studied in Ar, O_{2}, N_{2}, and CO_{2}. The low‐lying vibrational levels of the ^{3} B _{1} state are populated from ground state SO_{2}(X ^{1} A _{1}) by direct pumping, and the phosphorescence emissions from the laser excited as well as the collisional product levels are monitored. The state‐to‐state relaxation rate constants for the upper two levels are assigned by the kinetic simulations of the phosphorescence time profiles observed from each level. The magnitudes of the relaxation rate constants are in the range of 2.3–3.0×10^{−11} cm^{3} molecule^{−1} s^{−1} in SO_{2} and 0.26–1.3×10^{−11} cm^{3} molecule^{−1} s^{−1} in other gases.