An inelastic neutron scattering study of hydrogen bonding in potassium hydrogen dichloromaleate

The inelastic neutron scattering (INS) spectrum (350–2000 cm^?1) of potassium hydrogen dichloromaleate (solid slate) has been obtained. Two of the normal modes of vibration of the hydrogen bond [γ(OHO) and δ(OHO)] were observed and assigned. No INS band v_as(OHO) was observed in the region 500–1300 cm^?1. This conflicts with expectations from infrared data.     

Reorientation,polarization and scaling laws in rotational transfer experiments

Reorientation of angular momentum in molecules excited by polarized light, following rotationally inelastic collisions results in depolarization of the light emitted by these molecules. We show here how this effect can be quantitatively predicted from degeneracy, i.e. orientation-averaged rotational transfer rates, as usually measured in cell experiments. For this we first develop a classical phenomenology of rotational relaxation of diatomic molecules in the ¹Σ state, based on a vector model for angular momenta. Different simple scaling relationships are then given and applied to analyze the Li₂

+He system for which both orientation-averaged rate constants and polarization measurements for rotationally inelastic collisions already exist. It is found indeed that the latter data set agrees quite well with what can be predicted from the former data set. Finally we show that a propensity for conserving 0, the orientation of the angular momentum with respect to a laboratory-fixed axis results generally from geometrical reasons and is fairly independent of the collision dynamics.     

Inelastic neutron scattering and Raman spectroscopies and periodic DFT studies of Rb(2)PtH(6) and Rb(2)PtD(6)

The present work has provided a complete set of assignments for the vibrational spectrum of Rb(2)PtH(6) and Rb(2)PtD(6). To confirm the assignments, a periodic density functional theory (DFT) code has been applied to the analysis of the inelastic neutron scattering (INS) spectrum of an ionic material for the first time. The work has also provided an explanation for the unusual infrared spectrum of the potassium salt. The most significant aspect of the work is the use of the momentum transfer information provided by an INS chopper spectrometer. The straightforward method employed for the analysis of the data is applicable to any molecular system (organic or inorganic) and demonstrates the potential of these instruments for chemistry. Periodic DFT was also used to study the other A(2)PtH(6) (A = alkali metal) including, the at present, unknown Li salt, which is found to be stable. The DFT studies have also highlighted the crucial role of the cation in removing charge from the transition metal and "hydride" ligand. It is suggested that this is a general occurrence.     

Inelastic neutron scattering studies of polyethylene and N-alkanes

High quality inelastic neutron scattering (INS) spectra of randomly oriented polycrystalline polyethylene, perdeuteropolyethylene and highly oriented polyethylene are presented. The instrumental resolution was significantly better than previous work and has revealed increased detail in the 0 − 600 cm⁻¹ region. For the polycrystalline sample, comparison with the best available dispersion curves shows that these qualitatively reproduce the INS spectrum, apart from the energy of the maximum in the in-plane C-C bending mode v5. For the oriented samples, comparison with calculated INS spectra show fair, but not exact, agreement with the experimental spectra.     

Assignment of Metal-Ligand Modes in Pt(II) Diimine Complexes Relevant to Solar Energy Conversion

This work describes a comprehensive assignment of the vibrational spectra of the platinum(II) diimine bisthiolate and chloride complexes as a prototype structure for a diversity of Pt(II) diimine chromophores. The dynamics and energy dissipation pathways in excited states of light harvesting molecules relies largely on the coupling between the high frequency and the low frequency modes. As such, the assignment of the vibrational spectrum of the chromophore is of utmost importance, especially in the low-frequency region, below 500 cm(-1), where the key metal-ligand framework modes occur. This region is experimentally difficult to access with infrared spectroscopy and hence frequently remains elusive. However, this region is easily accessible with Raman and inelastic neutron scattering (INS) spectroscopies. Accordingly, a combination of inelastic neutron scattering and Raman spectroscopy with the aid of computational results from periodic-DFT and the mode visualizations, as well as isotopic substitution, allowed for an identification of the modes that contain significant contributions from Pt-Cl, Pt-S, and Pt-N stretch modes. The results also demonstrate that it is not possible to assign transition energies to "pure", localized modes in the low frequency region, as a consequence of the anticipated severe coupling that occurs among the skeletal modes. The use of INS has proved invaluable in identifying and assigning the modes in the lowest frequency region, and overall the results will be of assistance in analyzing the structure of the electronic excited state in the families of chromophores containing a Pt(diimine) core.     

The role of angular momentum in collision-induced vibration-rotation relaxation in polyatomics

Vibrational relaxation of the 6(1) level of S(1)((1)B(2u)) benzene is analyzed using the angular momentum model of inelastic processes. Momentum-(rotational) angular momentum diagrams illustrate energetic and angular momentum constraints on the disposal of released energy and the effect of collision partner on resultant benzene rotational excitation. A kinematic "equivalent rotor" model is introduced that allows quantitative prediction of rotational distributions from inelastic collisions in polyatomic molecules. The method was tested by predicting K-state distributions in glyoxal-Ne as well as J-state distributions in rotationally inelastic acetylene-He collisions before being used to predict J and K distributions from vibrational relaxation of 6(1) benzene by H(2), D(2), and CH(4). Diagrammatic methods and calculations illustrate changes resulting from simultaneous collision partner excitation, a particularly effective mechanism in p-H(2) where some 70% of the available 6(1)-->0(0) energy may be disposed into 0-->2 rotation. These results support the explanation for branching ratios in 6(1)-->0(0) relaxation given by Waclawik and Lawrance and the absence of this pathway for monatomic partners. Collision-induced vibrational relaxation in molecules represents competition between the magnitude of the energy gap of a potential transition and the ability of the colliding species to generate the angular momentum (rotational and orbital) needed for the transition to proceed. Transition probability falls rapidly as DeltaJ increases and for a given molecule-collision partner pair will provide a limit to the gap that may be bridged. Energy constraints increase as collision partner mass increases, an effect that is amplified when J(i)>0. Large energy gaps are most effectively bridged using light collision partners. For efficient vibrational relaxation in polyatomics an additional requirement is that the molecular motion of the mode must be capable of generating molecular rotation on contact with the collision partner in order to meet the angular momentum requirements. We postulate that this may account for some of the striking propensities that characterize polyatomic energy transfer.     

Methyl rotational tunneling dynamics of p-xylene confined in a crystalline zeolite host

The methyl rotational tunneling spectrum of p-xylene confined in nanoporous zeolite crystals has been measured by inelastic neutron scattering (INS) and proton nuclear magnetic resonance (NMR), and analyzed to extract the rotational potential energy surfaces characteristic of the methyl groups in the host-guest complex. The number and relative intensities of the tunneling peaks observed by INS indicate the presence of methyl-methyl coupling interactions in addition to the methyl-zeolite interactions. The INS tunneling spectra from the crystals (space group P2(1)2(1)2(1) with four crystallographically inequivalent methyl rotors) are quantitatively interpreted as a combination of transitions involving two coupled methyl rotors as well as a transition involving single-particle tunneling of a third inequivalent rotor, in a manner consistent with the observed tunneling energies and relative intensities. Together, the crystal structure and the absence of additional peaks in the INS spectra suggest that the tunneling of the fourth inequivalent rotor is strongly hindered and inaccessible to INS measurements. This is verified by proton NMR measurements of the spin-lattice relaxation time which reveal the tunneling characteristics of the fourth inequivalent rotor.     

An infrared and inelastic neutron scattering spectroscopic investigation on the interaction of eta-alumina and methanol

The industrially important interaction of methanol with an eta-alumina catalyst has been investigated by a combination of infrared spectroscopy (diffuse reflectance and transmission) and inelastic neutron scattering (INS) spectroscopy. The infrared and INS spectra together show that chemisorbed methoxy is the only surface species present. Confirmation of the assignments was provided by a periodic DFT calculation of methoxy on eta-alumina (110). The thermal conversion of adsorbed methoxy groups to form dimethylether was also followed by INS, with DFT calculations assisting assignments. An intense feature about 2600 cm(-1) was observed in the diffuse reflectance spectrum. This band is poorly described in the extensive literature on the alumina/methanol adsorption system and its observation raised the possibility of a new surface species existing on this particular catalyst surface. INS measurements established that the 2600 cm(-1) feature could be assigned to a combination band of the methyl rock with the methyl deformation modes. This assignment was reinforced by an analysis of the neutron scattering intensity at a particular energy as a function of momentum transfer, which confirmed this particular adsorbed methoxy feature to arise from a second order transition. Similar behaviour was observed in the model compound Al(OCH3)3. The anomalous infrared intensity of the 2600 cm(-1) peak in the diffuse reflectance spectrum is a consequence of the different absorption coefficients of the C-H stretch and the combination mode. The implications for catalyst studies are discussed.     

A compact model system for electron–phonon calculations in discotic materials

Electron–phonon coupling underlies the unwanted rapid relaxation of electrically excited states in potential organic solar-cell materials. A compact model for the vibrational dynamics of 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT6) is derived from the combined use of inelastic neutron scattering (INS) spectroscopy and first-principles calculations. Because this model reproduces the essential features of the vibrational dynamics and electronic structure on the aromatic core of HAT6 it can be used as a basis for future calculations of the relaxation mechanisms of the electronically excited states.     

Molecular Spin Clusters: New Synthetic Approaches and Neutron Scattering Studies

We review our recent work in the field of molecular spin clusters and single-molecule magnets, showing how inelastic neutron scattering (INS) can be used to determine magnetic exchange interactions and anisotropy splittings. A general introduction to neutron scattering precedes selected examples, building upon the first determination of exchange coupling in a transition metal complex using INS, through anisotropic exchange in cobalt(II ) spin clusters to the determination of exchange interactions in a dodecanuclear nickel(II ) wheel. The strength of INS for the accurate determination of anisotropy splittings in single-molecule magnets is revealed. Not only can one determine the axial zero-field splitting parameter D, which plays a key role in single-molecule magnet behavior, but also higher-order terms important in understanding the quantum tunneling behavior. Finally, we review two of our synthetic approaches towards new single-molecule magnets based on nickel, manganese, and iron.     

Spectroscopic Studies of the Magnetic Excitation and Spin-Phonon Couplings in a Single-Molecule Magnet

Large separations between ground and excited magnetic states in single-molecule magnets (SMMs) are desirable to reduce the likelihood of spin reversal in the molecules. Spin-phonon coupling is a process leading to magnetic relaxation. Both the reversal and coupling, making SMMs lose magnetic moments, are undesirable. However, direct determination of large magnetic states separations (>45 cm⁻¹) is challenging, and few detailed investigations of the spin-phonon coupling have been conducted. The magnetic separation in [Co(12-crown-4)₂](I₃)₂(12-crown-4) ( 1 ) is determined and its spin-phonon coupling is probed by inelastic neutron scattering (INS) and far-IR spectroscopy. INS, using oriented single crystals, shows a magnetic transition at 49.4(1.0) cm⁻¹. Far-IR reveals that the magnetic transition and nearby phonons are coupled, a rarely observed phenomenon, with spin-phonon coupling constants of 1.7–2.5 cm⁻¹. The current work spectroscopically determines the ground–excited magnetic states separation in an SMM and quantifies its spin-phonon coupling, shedding light on the process causing magnetic relaxation.     

Molecular dynamics in liquid crystals by dielectric relaxation and neutron scattering

Abstract

Detailed dielectric permittivity and relaxation investigations have been performed on compounds having different liquid-crystalline phases. At the smectic B–smectic A as well as the smectic G–smectic B transitions definite jumps were found in the dielectric relaxation times associated with rotation of the molecules around their short axis. For the interpretation of the large jumps in the relaxation times the change of the phonon spectra at the two dimensional crystal-two dimensional liquid phase transition was assumed. To verify this idea an inelastic neutron scattering study was performed. The measurements have proved the good orientation of the smectic A and smectic B phases. The values of the layer spacing, and the appearance of libron peaks for the smectic B phase at different momentum transfer were determined.     

Low-energy electron distributions from the photoionization of liquid water: a sensitive test of electron mean free paths

The availability of accurate mean free paths for slow electrons (<50 eV) in water is central to the understanding of many electron-driven processes in aqueous solutions, but their determination poses major challenges to experiment and theory alike. Here, we describe a joint experimental and theoretical study demonstrating a novel approach for testing, and, in the future, refining such mean free paths. We report the development of Monte-Carlo electron-trajectory simulations including elastic and inelastic electron scattering, as well as energy loss and secondary-electron production to predict complete photoelectron spectra of liquid water. These simulations are compared to a new set of photoelectron spectra of a liquid-water microjet recorded over a broad range of photon energies in the extreme ultraviolet (20–57 eV). Several previously published sets of scattering parameters are investigated, providing direct and intuitive insights on how they influence the shape of the low-energy electron spectra. A pronounced sensitivity to the escape barrier is also demonstrated. These simulations considerably advance our understanding of the origin of the prominent low-energy electron distributions in photoelectron spectra of liquid water and clarify the influence of scattering parameters and the escape barrier on their shape. They moreover describe the reshaping and displacement of low-energy photoelectron bands caused by vibrationally inelastic scattering. Our work provides a quantitative basis for the interpretation of the complete photoelectron spectra of liquids and opens the path to fully predictive simulations of low-energy scattering in liquid water.

Our study reveals the detailed influence of elastic and inelastic mean-free paths on the complete photoelectron spectra of liquid water, including the low-energy electron distributions and the reshaping of the primary photoelectron bands.     

The effect of kinematic parameters on inelastic scattering of glyoxal

The effect of kinematic parameters (relative velocity v(rel), relative momentum p(rel), and relative energy E(rel)) on the rotational and rovibrational inelastic scatterings of 0(0)K(0)S(1) trans-glyoxal has been investigated by colliding glyoxal seeded in He or Ar with target gases D2, He, or Ne at different scattering angles in crossed supersonic beams. The inelastic spectra for target gases He and D2 acquired with two different sets of kinematic parameters revealed no significant differences. This result shows that kinematic factors have the major influence in the inelastic scattering channel competition whereas the intermolecular potential energy surface plays only a secondary role. The well-defined exponential dependence of relative cross sections on exchanged angular momentum identifies angular momentum as the dominant kinematic factor in collision-induced rotationally and rovibrationally inelastic scatterings. This is supported by the behavior of the relative inelastic cross sections data in a "slope-p(rel)" representation. In this form, the data show a trend nearly independent of the target gas identity. Representations involving E(rel) and v(rel) show trends specific to the target gas.     

Quasiclassical trajectory study of the vibrational quenching of hydroxyl radicals through collision with O atoms

The collisional removal of vibrationally excited OH radicals by O atoms is studied by the quasiclassical trajectory method. To evaluate the effect of different topological features on the scattering processes two different global potential energy surfaces, DMBE IV and TU, are used. Results for reactive, exchange, and inelastic scattering probabilities are reported for central collisions (with zero total angular momentum) with a fixed relative translational energy for vibrational levels of OH ranging from nu=1 to v=8. Vibrational state distributions of product molecules are also compared on the two potential energy surfaces. Both surfaces predict higher probabilities for reaction than for exchange or inelastic scattering. The vibrational state distributions of the product diatomic molecules are different on the two surfaces. In particular, the two surfaces give substantially different probabilities for multiquantum OH vibrational relaxation transitions OH(v)+O-->OH(v')+O.     

Inelastic neutron scattering study of water in hydrated LTA-type zeolites

The vibrational dynamics of water molecules encapsulated in synthetic Na-A and Mg-exchanged A zeolites were studied versus temperature by inelastic neutron scattering (INS) measurements (30-1200 cm(-1)) as a function of the induced ion-exchange percentage by using the indirect geometry tof spectrometer TOSCA at the ISIS pulse neutron facility (RAL, UK). The experimental INS spectra were compared with those of ice Ih to characterize the structural changes induced by confinement on the H2O hydrogen-bonded network. We observed, after increasing the Mg2+ content, a tendency of water molecules to restore the bulklike arrangements together with more hindered dynamics. These results are confirmed by the analysis of the evaluated one-phonon amplitude-weighted proton vibrational density of states aimed, in particular, to follow the evolution of the water molecules librational mode region.     

Studies on Nuclear Resonant Scattering of Synchrotron Radiation by 40K

The studies on nuclear resonant scattering by ⁴⁰K using synchrotron radiation are reviewed. Brilliant and high pure synchrotron radiation permitted us to observe the nuclear resonant forward scattering by ⁴⁰K in a powdered KCl sample, the excitation of which is impossible with ordinary radioactive sources. Furthermore, nuclear resonant inelastic scattering of synchrotron radiation by ⁴⁰K in the KCl sample at room temperature has been measured using a high-resolution monochromator. Adding to these, from the excitation experiments of ⁴⁰K, the energy and lifetime of the first excited state of ⁴⁰K were confirmed. These measurements clearly show that the studies on the electronic states through hyperfine interactions and the dynamical properties of potassium atoms, which are very important in material science and biology, are possible. It should be noted that ⁴⁰K is the natural isotope of potassium and weakly radioactive. Our observation of forward and inelastic scattering of the radioactive nuclide ⁴⁰K will lead to further studies on other radioactive nuclides the resonant forward and inelastic scattering of which are not observed to date.     

Inelastic X-ray scattering and vibrational effects at the K-edges of gaseous N2, N2O, and CO2

We report non-resonant inelastic X-ray scattering experiments of several gaseous samples in the inner-shell excitation energy range. The experimental near-edge spectra from all the K-edges of N(2), N(2)O, and CO(2) including the momentum transfer dependence are presented. The results are analyzed using density functional theory calculations that accurately reproduce the experimental spectral features. We observe vibrational effects in the measured spectrum and in the calculations the atomic motion is modeled using the Franck-Condon approximation and the linear coupling model. Our findings show that vibrational effects cannot be neglected in the analysis of high resolution inelastic X-ray scattering spectroscopy. The results also support the validity of the transition potential approximation for calculating core excited state potential energy surfaces.     

Inelastic scattering from glyoxal: collision kinematics rather than the interaction potential dominates rotational channel selection

Relative cross sections have been obtained for the rotationally and rovibrationally inelastic scattering of S1 trans-glyoxal (CHO-CHO) in its zero point level with K' = 0 from the target gases H2, D2, and He. Emphasis is placed on using crossed molecular beam conditions that provide several choices of collision kinematics (center-of-mass collision energy, relative velocity, center-of-mass collision momentum) for each collision pair. The cross sections define the state-to-state competition among numerous rotational channels involving destination states with DeltaK' ranging from 1 to >15 for collisions with each target gas and under every kinematic condition. They also resolve a similar rotational competition among rovibrational channels where the torsion nu7' is collisionally excited. The cross section sets also allow the relative overall magnitudes of the two types of scattering to be compared. The primary motivation of these experiments concerns the rotationally inelastic scattering. Earlier studies with rare gases and fixed kinematics demonstrated that the distribution of rotational cross sections is remarkably similar from one collision pair to another. The new data show that the competition among rotational channels actually has a small but distinct dependence on kinematic conditions. Data analysis shows that the dependence is a systematic function of the available collision momentum and entirely unrelated to the identity of the target gases, including the heavier rare gases used in earlier studies. The competition among the rotational energy transfer channels and its kinematic heritage is discussed in the context of a classical hard ellipse model of linear momentum to angular momentum conversion much used with room temperature systems. When adapted to our beam conditions, the resulting account of the rotational scattering is accurate and provides insight into the collisional details.