Volume 105, Issue 6, 08 August 1996
Index of content:

High resolution near‐infrared electronic spectroscopy of HCBr
View Description Hide DescriptionThe rotationally resolved spectrum of the HCBr Ã ^{1} A″(0,2,0)←X̃ ^{1} A′(0,0,0) K _{ a }=0←1 transition between 12760 and 12850 cm^{−1} was obtained for the first time at Doppler‐limited resolution using a transient frequency‐modulation absorption technique. Rotational structure of HC ^{79}Br and HC ^{81}Br was identified and analyzed. The analysis shows R″(C–Br)=1.85_{2} Å and R′(C–Br)=1.74_{9} Å. The observed band indicates a linear–bent transition. This yields an upper limit of approximately 1600 cm^{−1} for the barrier to linearity above the zero‐point energy for the Ã ^{1} A″ state. Perturbations caused by singlet–triplet interactions were also found in the observed spectrum. The analysis of these perturbations indicates a very low‐lying ã ^{3} A″ state.

Resonance enhanced multiphoton ionization spectroscopy of carbonyl sulphide
View Description Hide DescriptionRydbergexcited states of the OCS molecule in the energy range 70500–86000 cm^{−1} have been investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet‐cooled sample of the parent molecule. Spectral interpretation has been assisted by companion measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. The present study supports the earlier conclusions of Weinkauf and Boesl [J. Chem. Phys. 98, 4459 (1993)] regarding five Rydberg origins in the 70500–73000 cm^{−1} energy range, attributable to, respectively, states of ^{3}Π, ^{1}Π, ^{3}Δ, ^{1}Δ and ^{1}Σ^{+} symmetry arising from the 4pλ←3π orbital promotion. We also identify a further 21 Rydberg origins at higher energies. These partition into clumps with quantum defects ca. 3.5 and 4.5, which we associate with the orbital promotions npλ←3π (n=5,6), and others with near integer quantum defect which are interpretable in terms of excitation to s,d and (possibly) fRydberg orbitals. We also identify MPI resonances attributable to CO(X ^{1}Σ^{+}) fragments and to S atoms in both their ground (^{3} P) and excited (^{1} D) electronic states. Analysis of the former resonances confirms that the CO(X) fragments resulting from one photondissociation of OCS at excitation wavelengths ca. 230 nm are formed with a highly inverted, bimodal rotational state population distribution, whilst the latter are consistent with previous reports of the wavelength dependence for forming ground and excited state S atoms in the near uv photolysis of OCS.

Matrix isolation study of the interaction of excited neon atoms with O_{3}: Infrared spectrum of O^{−} _{3} and evidence for the stabilization of O_{2}⋅⋅⋅O^{+} _{4}
View Description Hide DescriptionWhen a Ne:O_{3} sample is codeposited at approximately 5 K with neon atoms that have been excited in a microwavedischarge, the most prominent infrared absorptions of the resulting solid are contributed by trans‐ and cyc‐O^{+} _{4} and by O^{−} _{3}. The failure to detect infrared absorptions of O^{+} _{3} is consistent with the initial formation of that species in one or more dissociativeexcited states. The ν_{3} absorption of O^{−} _{3} appears at 796.3 cm^{−1}, close to its position in earlier argon‐matrix experiments in which photoionization of an alkali metal atom provided the electron source and in which diffusion of the atomic cation would result in the stabilization of appreciable M^{+}O^{−} _{3}. The identification of O^{−} _{3} isolated in solid neon is supported by observations of O^{−} _{3} generated from isotopically substituted Ne:O_{2}:N_{2}O samples, also codeposited with excited neon atoms. An upper bound of 810 cm^{−1} is estimated for the gas‐phase band center of ν_{3} of O^{−} _{3}. Infrared absorptions which grow on mild warmup of the sample are tentatively assigned to an O_{2}...O^{+} _{4} complex.

Rubidium ion hydration in ambient and supercritical water
View Description Hide DescriptionX‐ray absorption fine structure (XAFS) measurements and analyses are presented for Rb^{+} in supercritical water solutions. The structure of the first hydration shell at ambient conditions is compared to that in the supercritical region at a temperature of 424 °C and pressures from 382 to 633 bar. For all reported studies, RbBr at a concentration of 0.5 molal was used. XAFS results show that there is a well‐defined hydration shell around the cation even at 424 °C but at these high temperatures the extent of hydration of the Rb cation is reduced by about 40%. A slight contraction of this first shell distance by about 0.10 Å is also observed under supercritical conditions. The reduction in the number of water‐ion bonds is analogous to the reduction in the amount of water–water hydrogen bonding that has been observed by others under supercritical conditions. The reduction in waters‐of‐hydration under supercritical conditions may also be in part due to formation of contact‐ion pairs.

Photofragmentation spectrum of the Sr^{+}Ar complex
View Description Hide DescriptionThe photofragmentation spectrum of the weakly bound Sr^{+}Ar is measured over the 418–448 nm wavelength region. Two vibrational progressions are observed and are attributed to transitions from the X ^{2} Σ_{1/2} electronic ground state to vibrational levels of the excited A ^{2}Π_{1/2} and A ^{2}Π_{3/2} states. Isotope‐resolved measurements of several of the observed transitions are performed to obtain the absolute vibrational numbering. From these, the spectroscopic constants of the involved states are deduced. We obtain ω_{ e } ^{′}=120.8 cm^{−1}, ω_{ e }χ_{ e } ^{′}=1.67 cm^{−1}, and D _{0} ^{′}=2303±232 cm^{−1} for the A ^{2}Π_{1/2} state and ω_{ e } ^{′}=122 cm^{−1}, ω_{ e }χ_{ e } ^{′}=1.6 cm^{−1}, and D _{0} ^{′}=2575±256 cm^{−1} for the A ^{2}Π_{3/2} state. For the ground X ^{2}Σ_{1/2} state the observed hot bands yield ΔG _{1/2} ^{″}=48 cm^{−1}. Furthermore a value of 803±244 cm^{−1} is estimated for D _{0} ^{″}. Finally the potential constants are compared with pseudopotential calculations from literature and the trends of binding energies for group II cation–argon complexes are discussed.

Resonance coherent anti‐Stokes Raman spectroscopy of iodine in solution
View Description Hide DescriptionCoherent anti‐Stokes–Raman scattering (CARS)‐excitation profiles (CEPs) of the fundamental vibrational mode of I_{2} dissolved in cyclohexane and dibromomethane are determined with excitation wave numbers ranging from 17 000 to 23 000 cm^{−1}. The experiments were performed at room temperature and with laser beams polarized parallel to each other. Calculations were performed for both the absorptionspectrum and the CEPs of iodine in both solvents and compared with experimental results. Good qualitative agreement of the major features of the CEPs was found using gas phase potentials for the excited states^{1}Π_{1u } and ^{3}Π_{0+ u } and the ground state^{1}Σ^{+} _{ g }. The excited state potential functions were slightly shifted to match the measured absorption spectra.

Electron‐impact excitation of low‐lying preionization‐edge n→σ* and Rydberg transitions of CHF_{2}Cl and CHFCl_{2}: Absolute generalized oscillator strength measurement
View Description Hide DescriptionAngle‐resolved electron energy loss spectroscopy has been used to determine the absolute generalized oscillator strengths (GOSs) of valence‐shell electronic transitions of difluorochloromethane (CHF_{2}Cl) and dichlorofluoromethane (CHFCl_{2}) as functions of energy loss and momentum transfer at an impact energy of 2.5 keV. Absolute GOS profiles of the prominent low‐lying preionization‐edge energy loss features of CHF_{2}Cl and CHFCl_{2} were determined and found to be consistent with the previous assignments of the underlying transitions made by VUV photoabsorptionspectroscopy. In particular, the lowest‐lying features at 8.0 eV in CHF_{2}Cl and at 7.5 eV in CHFCl_{2} have been attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals to the C–Cl σ* antibonding orbital, in good accord with single‐excitation configuration interaction (CI) excited‐state calculations. The corresponding GOS profiles of these n(Cl 3p)→σ*(C–Cl) (HOMO→LUMO) transitions revealed an interesting trend of increased dipole character with increasing Cl content, i.e., from an essentially quadrupole‐dominated profile, characterized by a maximum at K ^{2}∼0.9 a.u., in CHF_{2}Cl to a mixed dipole‐quadrupole profile in CHFCl_{2} and CHCl_{3}. The CI calculations further showed that some of the underlying n(Cl 3p)→σ*(C–Cl) transitions in CHF_{2}Cl, CHFCl_{2}, and CHCl_{3}, like the other chlorofluorocarbons: CF_{3}Cl, CF_{2}Cl_{2}, CFCl_{3}, and CCl_{4}, could lead to dissociation of the C–Cl bond. In addition, the GOS profiles of the remaining low‐lying preionization‐edge features at 9.8 and 11.2 eV in CHF_{2}Cl and at 9.4, 10.7, and 11.6 eV in CHFCl_{2} were also determined. These features have been previously assigned as Rydberg transitions originated from the nonbonding HOMOs. In particular, these experimental GOS profiles were found to be dominated by a strong maximum at K=0, which is indicative of strong dipole interactions. The weak secondary maxima observed at K ^{2}∼2.8–3.5 a.u. could be interpreted qualitatively in terms of the spatial overlaps between the initial‐state and final‐state orbital wave functions. Together with our earlier work on CHF_{3} and CHCl_{3}, the present work on the remaining members of the CHF_{ m }Cl_{3−m } (m=0–3) series, CHF_{2}Cl and CHFCl_{2}, provides further evidence for the empirical trends on the preionization‐edge structures observed in the CF_{ n }Cl_{4−n } (n=0–4) series.

Matched two‐pulse electron spin echo envelope modulation spectroscopy
View Description Hide DescriptionThe theory of nonideal microwave pulses acting on electron–nuclear spin systems is extended and applied to optimize the two‐pulse electron spin echo envelope modulation (ESEEM) experiment. A superoperator approach for a computationally efficient simulation of experiments involving non‐ideal pulses is introduced and the corresponding unitary transformation superoperator is given analytically for a system consisting of one electron spin S=1/2 and one nuclear spinI=1/2. Density operator single‐element transfers are divided into allowed and forbidden ones and are classified according to their functioning in pulse ESR. By increasing the efficiency of forbidden transfers by Hartmann–Hahn matching during prolonged pulses, the sensitivity of the conventional two‐pulse ESEEM experiment may drastically be improved and discrimination between basic, hyperfine, and combination frequencies becomes possible. The implications of the theory for spin systems with an arbitrary number of nuclear spins 1/2 are investigated by deriving and discussing a general condition for Hartmann–Hahn matching of forbidden transitions. It is shown that the product rule valid for two‐pulse echo modulations caused by more than one nucleus does not hold for nonideal pulses. A method is developed that allows one to reduce the thus arising large dimensionality of the diagonalization problem in numerical simulations. The theoretical conclusions are verified by experiments on two transition metal complexes in single crystals and on a spin‐label‐doped polymer sample.

^{13}C isotope effects for pentacene in p‐terphenyl: High‐resolution spectroscopy and single‐spin detection
View Description Hide DescriptionWeak satellites in S _{1}←S _{0} excitation spectra of natural‐abundance pentacene in p‐terphenyl due to position‐specific ^{13}C isotope shifts were observed. An assignment of these satellites to the various carbon positions is derived. The extremely narrow inhomogeneous linewidth allowed selective excitation of pentacene molecules with ^{13}C in specific positions. Spectra of the magnetic‐resonance transition between triplet sublevels of such ensembles, and of individual pentacene molecules showed position‐specific ^{13}C hyperfine broadening.

Small angle scattering from porous mass fractal solids
View Description Hide DescriptionThe small angle scattering of radiation or particles from many porous solids may be described in terms of fractal geometry. To date most examples have been surfacefractals but a few studies have suggested that some porous solids may be mass fractals. We give a derivation of the scattering law from such a solid and present an experimental example of such scattering behavior.

Vibronic analysis of fluorescence spectrum of NO_{2} D̃ ^{2} B _{2}(0,0,0) in the region of 250–550 nm
View Description Hide DescriptionThe dispersed fluorescencespectrum of NO_{2} D̃ ^{2} B _{2}(0,0,0) was measured and analyzed in the spectral range of 250–550 nm. The strong fluorescence bands in 250–350 nm correspond to D̃ ^{2} B _{2}(0,0,0)→X̃ ^{2} A _{1}(n _{1}=0–9, n _{2}=0–5, n _{3}=0) with a Franck–Condon maximum at n _{1}=4 and n _{2}=0. The weak and broad bands in 350–410 nm are built on a progression of bending frequency, 710 cm^{−1}. The lower state responsible for this fluorescence was interpreted as admixture ^{ ev } B _{2} levels generated by a vibronic coupling between a _{1}‐vibrational levels on Ã ^{2} B _{2} and highly excited b _{2} levels on X̃ ^{2} A _{1}. The medium‐intensity bands in 410–550 nm were assigned to D̃ ^{2} B _{2}(0,0,0)→C̃ ^{2} A _{2}(n _{1}=0–2, n _{2}=0–5, n _{3}=0–2) with a Franck–Condon maximum at n _{1}=0, n _{2}=2, and n _{3}=0. The vibrational frequencies of C̃ ^{2} A _{2} are 1010 cm^{−1} for symmetric stretch (ω_{1}), 740 cm^{−1} for bending (ω_{2}), and 250 cm^{−1} for antisymmetric stretch (ω_{3}). The simple Franck–Condon calculation for D̃ ^{2} B _{2}(0,0,0)→C̃ ^{2} A _{2}(n _{1},n _{2},n _{3}) gives the approximate geometry of the C̃ ^{2} A _{2} state as r(N–O)∼134 pm and θ∼108°. The partial rotational structure of C̃ ^{2} A _{2}(0,0,0) was analyzed using an optical–optical double resonance measurement, which confirms the A _{2} vibronic symmetry. The origin of NO_{2} C̃ ^{2} A _{2} (T _{0}) was determined to be around 16 234 cm^{−1}.

Multiconfigurational molecular dynamics with quantum transitions: Multiple proton transfer reactions
View Description Hide DescriptionWe present the new method ‘‘multiconfigurational molecular dynamics with quantum transitions’’ (MC‐MDQT) for the simulation of processes involving multiple proton transferreactions. MC‐MDQT is a mixed quantum/classical molecular dynamics method that allows the quantum mechanical treatment of the nuclear motion of multiple hydrogen atoms and accurately describes branching processes (i.e., processes involving multiple channels or pathways). MC‐MDQT is based on the surface hopping method MDQT, which has already been applied to single proton transferreactions in solution, where the nuclear motion of only the hydrogen atom being transferred is treated quantum mechanically. The direct extension of MDQT to multiple proton transferreactions, where many hydrogen atoms must be treated quantum mechanically, is not computationally practical. In MC‐MDQT a multiconfigurational self‐consistent‐field method is combined with MDQT to allow the quantum mechanical treatment of multiple hydrogen atoms while still including the significant correlation. The adiabatic states are expanded in a basis set of single configurations, which are products of one‐particle states calculated using effective Hamiltonians derived from the occupied adiabatic state. Thus the one‐particle states and the multiconfigurational adiabatic states must be calculated self‐consistently. Both the MC‐MDQT and the full basis set expansion MDQT methods are applied to a model system comprised of two quantum protons moving in double well potentials and one classical harmonic solvent degree of freedom. The results show that MC‐MDQT incorporates the significant correlation and accurately describes branching processes. The MC‐MDQT method is also used to study model systems comprised of three quantum protons and one classical solvent degree of freedom.

Semiclassical calculations on the energy dependence of the steric effect for the reaction Ca(^{1} D)+CH_{3}F(jkm=111)→CaF+CH_{3}
View Description Hide DescriptionIn a previous article [A. J. H. M. Meijer, G. C. Groenenboom, and A. van der Avoird, J. Chem. Phys. 101, 7603 (1994)] we investigated the energy dependence of the steric effect of the reaction Ca (^{1} D)+CH_{3}F (jkm=111)→CaF (A ^{2}Π)+CH_{3} using a quasiclassical trajectory method. It was found that we could not reproduce the experimental results for this reaction [M. H. M. Janssen, D. H. Parker, and S. Stolte, J. Phys. Chem. 95, 8142 (1991)]. In this article, we reinvestigate this reaction using a semiclassical method, in which the rotation of the molecule and the electronic states of the interacting atom and molecule are treated quantum mechanically. For the chemical reaction we use a model which correlates the projection of the electronic orbital angular momentum of the Ca atom on the intermolecular axis with the projection of the electronic orbital angular momentum of the CaF product on the diatomic axis [M. Menzinger, Polon. Phys. Acta A 73, 85 (1988)]. This model is applied to examine the CaF (A ^{2}Π, B ^{2}Σ^{+}, A ^{′ 2}Δ) exit channels separately. We conclude that we can reproduce the experimental results for the steric effect using this model. The improvement with respect to the classical trajectory results is shown to be due primarily to the extended reactionmodel rather than to the semiclassical description of the dynamics. We find trapping and reorientation in the semiclassical calculations, as in the previous classical trajectory results, but also non‐adiabatic effects are present. The latter do not affect the reactive cross sections very much.

Solvent–solute reaction path curvature effects on energy transfer corrections to the solute reaction rate
View Description Hide DescriptionWe present a new rate theory which accounts for anharmonicities (nonlinearities) in the solute potential (force) over the complete range of solvent damping. The theory is based on a new method for calculating energy diffusion rates which incorporates anharmonicity‐induced solvent–solute reaction path curvature and is thus valid throughout the intermediate to large damping regimes. This energy diffusion factor is combined with the microcanonical variational transition state theory spatial diffusion correction factor. The new theory is applied to the case of a cubic solute potential coupled to a long time scale bath and shown to be significantly more reliable than the turnover theory of Pollak, Grabert, and Hänggi [J. Chem. Phys. 91, 4073 (1989)] in the Kramers turnover regime.

Recovering a full dimensional quantum rate constant from a reduced dimensionality calculation: Application to the OH+CO→H+CO_{2} reaction
View Description Hide DescriptionTwo reduced dimensionality theories are used to calculate the thermal rate constant for the OH+CO→H+CO_{2}reaction. The standard theory employs energy‐shift approximations to extract the full six degree‐of‐freedom quantum rate constant for this reaction from the previous two degree‐of‐freedom (2‐DOF) quantum calculations of Hernandez and Clary [M.I. Hernandez and D.C. Clary, J. Chem. Phys. 101, 2779 (1994)]. Three extra bending modes and one extra ‘‘spectator’’ CO stretch mode are treated adiabatically in the harmonic fashion. The parameters of the exit channel transition state are used to evaluate the frequencies of those additional modes. A new reduced dimensionality theory is also applied to this reaction. This theory explicitly addresses the finding from the 2‐DOF calculations that the reaction proceeds mainly via complex formation. A J‐shifting approximation has been used to take into account the initial states with non‐zero values of total angular momentum in both reduced dimensionality theories. Cumulative reaction probabilities and thermal rate constants are calculated and compared with the previous quasiclassical and reduced dimensionality quantum calculations and with experiment. The rate constant from the new reduced dimensionality theory is between a factor of 5 and 100 times smaller than the statistical transition state theory result, and is in much better agreement with experiment.

Vibrational coherence in electron transfer: The tetracyanoethylene–pyrene complex
View Description Hide DescriptionCoherent vibrational wave packet motion is created in the excited charge‐transfer state of the electron donor–acceptor complex between tetracyanoethylene (TCNE) and pyrene by an ultrashort (40 fs) 810 nm pump pulse. Observations of the dynamics of the TCNE–anion transient absorption and the disappearance of the bleach of the ground state absorption show that the electron‐transfer reaction back to the ground state of the complex occurs on a 250 fs–1.5 ps time scale. The bleach recovery signal shows clear oscillations and both impulsive stimulated Raman scattering in the ground state and coherent repopulation of the ground statesurface due to a vibrationally coherent electron transferreaction were considered as the cause. Vibrational coherence has also been monitored by observing quantum beats in the stimulated emission from the charge transfer state back to the ground state in the near‐ir. This observation strongly suggests that the electron transferreaction is indeed vibrationally coherent and that the reaction rate is modulated by this coherence. This interpretation is corroborated by a classical Monte Carlo simulation of vibrationally coherent reactions in the inverted regime.

Photofragmentation of I_{2} ^{−}⋅Ar_{ n } clusters: Observation of metastable isomeric ionic fragments
View Description Hide DescriptionWe report the 790 nm photofragmentation of mass‐selected I^{−} _{2}⋅Ar_{ n } clusters, n=1 to 27. We determine the I^{−}+I caging efficiency as a function of the number of solvent Ar atoms and compare these results with I^{−} _{2} in CO_{2} clusters. Caging is much less effective with Ar. In addition to ‘‘normal’’ caged photoproducts (I^{−} _{2}⋅Ar_{ m }, where m<n), the evaporation process following photoexcitation produces ‘‘solvent‐separated’’ (I^{−}...I)⋅Ar_{ m } photofragments, where the I^{−} _{2} bond has not reformed. These metastable species comprise ∼55% of the photofragment yield for precursor clusters for n≥14 and have lifetimes ≳5 μs. This unusual photofragment exists either as a trapped excited electronic state or as a solvent‐separated pair at an internuclear separation of ∼5.5 Å. The photofragmentation data also exhibit the existence of two distinct isomeric forms of the precursor I^{−} _{2}⋅Ar_{ n }, for n≤14. These forms are evaporatively distinct in that one isomer displays highly nonstatistical fragmentation, probably arising from a cluster in which the I^{−} _{2} resides on the surface, rather than in the interior. The photofragmentation distribution of the other form exhibits statistical behavior, consistent with the evaporation of an I^{−} _{2} solvated inside the cluster.

Approximate quantum scattering studies of the CN+H_{2} reaction
View Description Hide DescriptionReduced dimensionality quantum scattering calculations have been carried out for the H_{2}+CN→HCN+H reaction. A new potential energy surface, which has recently been developed on the basis of extensive ab initio molecular orbital calculations, has been employed. In order to study the effect of H_{2}CN complex‐formation on the hydrogen abstraction, three active degrees of freedom have been considered in the scattering calculations: the H‐H internuclear distance, the H‐G_{CN} distance (where G_{CN} is the center of mass of CN) and the angle between H‐H and H‐G_{CN}. This reduces the problem to the usual atom–diatom scattering calculation for H_{2}+A, where A represents a pseudoatom. A hyperspherical coordinate coupled‐channel method has been used to solve the Schrödinger equation. The reaction probabilities calculated show that H_{2}CN complex‐formation mechanism is not important for the hydrogen abstraction channel in the energy range considered in the present calculations. On the other hand, complex‐formation is important for inelastic processes such as H+HCN(ν,j)→H+HCN(ν′,j′), where ν and j are the C–H local vibrational and rotational quantum numbers of HCN. This is consistent with previous full‐dimensional quasiclassical trajectory calculations. The reaction probabilities, final vibrational distributions, and thermal rate constants calculated with the present reduced dimensionality theory have been critically compared with those calculated using quasiclassical trajectories and with other approximate quantum scattering methods including the adiabatic‐bend approximation and the rotating‐bond approximation.

Hydride exchange via two‐dimensional vibrational tunneling
View Description Hide DescriptionWe obtain analytic expressions for the zero temperature and finite temperature exchange interaction between two hard sphere fermions bound in a symmetric double‐well potential in 1, 2, and 3 dimensions and discuss the effect of dimensionality on the model. The three‐dimensional model (due to Zilm/Landesmann) and the two‐dimensional model presented here are of relevance to the strongly temperature dependent exchange interaction that has been observed between hydrides bound in metal polyhydride complexes.

Kinetics of diffusion‐influenced reversible reaction A+B ⇌ C in solutions
View Description Hide DescriptionReversible diffusion‐influenced pseudo first order reactionA+B ⇌ C with static particles in excess is rigorously studied. Under most general assumptions, the problem of the reversible reaction kinetics is reduced to the consideration of the effective irreversible reaction studied by conventional methods. In the framework of the average t‐matrix approximation (ATA) we reproduce some results derived earlier and establish their applicability limits. Rigorous investigation of the kinetics behavior at long times shows that the t ^{−3/2} law predicted earlier and reproduced by ATA has a different concentration‐dependent amplitude. On the basis of diagrammatic summation, providing correct long‐time asymptotics, a modified theory has been developed. The range of validity of the modified theory is much wider than that of ATA and similar theories.