Vibrational spectroscopy inelastic neutron scattering

Supplementary to other vibrational spectroscopies, inelastic neutron scattering (INS) spectroscopy is a very useful technique for studying organic molecules as it is extremely sensitive to the vibrations of hydrogen atoms. INS spectroscopy has been used to analyze the molecular dynamics of the energetic compound ANTA 5 <2005CPL(403)329>. 

M.H. Herzog-Cance, D.J. Jones, R.E1. Mejjad, J. Roziere J. Tomkinson (1992). J. Chem. Soc. Faraday Trans., 88, 2275-2281. Study of ion exchange and intercalation of organic bases in layered substrates by vibrational spectroscopy. Inelastic neutron scattering, infrared and Raman spectroscopies of aniline inserted alpha and gamma zirconium hydrogen phosphates. 

Vibrational spectroscopy provides detailed infonnation on both structure and dynamics of molecular species. Infrared (IR) and Raman spectroscopy are the most connnonly used methods, and will be covered in detail in this chapter. There exist other methods to obtain vibrational spectra, but those are somewhat more specialized and used less often. They are discussed in other chapters, and include inelastic neutron scattering (INS), helium atom scattering, electron energy loss spectroscopy (EELS), photoelectron spectroscopy, among others. 

Kearley, G. J., Pressman, H. A. Slade, R. C. T. (1986). The geometry of the HjOJ ion in dodecatungstophosphoric acid hexahydrate, (HjOJ)j (PWjjOfo), studied by inelastic neutron scattering vibrational spectroscopy. Journal of the Chemical Society Chemical Communications, 1801-2. 

Vibrations in molecules or in solid lattices are excited by the absorption of photons (infrared spectroscopy), or by the scattering of photons (Raman spectroscopy), electrons (electron energy loss spectroscopy) or neutrons (inelastic neutron scattering). If the vibration is excited by the interaction of the bond with a wave... 

The final chapters of this book review the progress of three recently developed techniques that provide information about the vibrational states of adsorbed molecules. Perhaps the most important of these techniques is electron energy loss spectroscopy that, despite its inherent low resolution, gives valuable information on vibrational modes that are either inactive in the IR, or inaccessible because of experimental difficulties. The applications of this technique are discussed in two chapters by Somorjai and Weinberg. The review of new experimental techniques concludes with presentations on inelastic electron tunneling spectroscopy and neutron scattering by Kirtley and Taub. 

As we shall discuss below, it is also more straightforward to calculate the relative intensity of vibrational modes observed by inelastic neutron scattering than in electron-energy-loss and optical spectroscopies. The relative intensity of the modes, as well as their frequency, can then be used to identify the atomic displacement pattern or eigenvector of the mode. We shall also see through examples of model calculations how the relative intensity of surface vibratory modes is sensitive to the orientation of the adsorbed molecule and the strength and location of its bond to the surface. 

The cross-section in Eq. (1 illustrates another distinguishing feature of inelastic neutron scattering for vibrational spectroscopy, i.e., the absence of dipole and polarizability selection rules. In contrast, it is believed that in optical and inelastic electron surface spectroscopies that a vibrating molecule must possess a net component of a static or induced dipole moment perpendicular to a metal surface in order for the vibrational transition to be observed ( 7,8). This is because dipole moment changes of the vibrating molecule parallel to the surface are canceled by an equal image moment induced in the metal. 

II. Vibrational Spectroscopy of Adsorbed Molecules by Inelastic Neutron Scattering... 

The shifts in vibrational frequencies between the LS and HS species are responsible for the major parts of the entropy changes AS in spin transitions [13]. This has been repeatedly confirmed by infrared [13-15], Raman [14-17], and inelastic neutron scattering spectroscopies [18]. Since the observed entropy change 13.8 J K-1 mol-1 [12] in the present compound is not explained adequately solely by the entropy of the spin multiplicity (R ln(5/3) = 4.25 J K-1 mol-1). However, we cannot assign the extra entropy change to a vibrational origin, since the Raman spectra of the two phases differ only in intensities. 

Volume 50 of Advances in Catalysis, published in 2006, was the hrst of a set of three focused on physical characterization of solid catalysts in the functioning state. This volume is the second in the set. The hrst four chapters are devoted to vibrational spectroscopies, including Fourier transform infrared (Lamberti et al.), ultraviolet Raman (Stair), inelastic neutron scattering (Albers and Parker), and infrared-visible sum frequency generation and polarization-modulation infrared rehection absorption (Rupprechter). Additional chapters deal with electron paramagnetic resonance (EPR) (Bruckner) and Mossbauer spectroscopies (Millet) and oscillating microbalance catalytic reactors (Chen et al.). 

Adsorbed hydrogen on metal surfaces is of particular interest from both theoretical and experimental points of view. Vibrational spectroscopy data on hydrogen adsorbed from the gas phase have been obtained from IR reflection-absorption experiments as well as from electron energy loss spectroscopy and inelastic neutron scattering techniques [39-41]. In UHV, absorption bands for the M-H bond have been reported in the mid- and far-infrared regions [41, 42],... 

K. Prassides et ai, Vibrational Spectroscopy of Superconducting KsCgo by Inelastic Neutron Scattering, Nature 354, 462-463 (1991). 

Vibrational spectroscopy of superconducting KaCeo by inelastic neutron scattering... 

Vibrational spectroscopy with neutrons is a spectroscopic technique in which the neutron is used to probe the dynamics of atoms and molecules in solids. In this introductory chapter we provide a descriptive account of the discovery and properties of the neutron, the development of neutron scattering, how inelastic neutron scattering spectroscopy compares with infrared and Raman spectroscopy and the benefits of using the neutron as a spectroscopic probe. 

In order to introduce the theory of vibrational spectroscopy in inelastic neutron scattering, we make some simplifications that will help us to understand the concepts. First we shall deal with the vibrational modes of molecules in a vacuum or in a dilute gas phase. Note, however, that in INS experiments the sample is cooled to ca 20 K, therefore the molecules are part of an extended solid. However, because the forces that keep the atoms in the molecule are often larger than the forces that molecules experience from other molecules in the condensed phase, isolated molecule calculations can be good models. 

In this chapter we describe some applications of inelastic neutron scattering in surface chemistry, more particularly in studies of catalysts and adsorbed species [1]. Our emphasis will be on the spectroscopy. The subject matter is arranged broadly according to the type of catalyst metals ( 7.3), oxides ( 7.4), zeolites and microporous materials ( 7.5) and sulfides ( 7.6) and, within each group, according to the reactant molecules. We start ( 7.1) with a general discussion of surface vibrations. 

J. Howard, T.C. Waddington C.J. Wright (1978). Chem. Phys. Lett., 56, 258-62. The vibrational spectrum of hydrogen adsorbed on palladium black measured using inelastic neutron scattering spectroscopy. 

---