Neutron scattering spectroscopy

In this paper we discuss how neutron scattering spectroscopy can be applied to the study of the structure and dynamics of adsorbed molecules. Since reviews of elastic and inelastic neutron scattering from adsorbed films have recently appeared (1.-3), our purpose here is not to present a comprehensive survey of every adsorbed system investigated by neutron scattering. Rather, we shall be concerned primarily with two questions which are basic to the characterization of adsorbed species on catalysts and which have been central to the discussion of this symposium. These are the extent to which the neutron scattering technique can be used to determine 1) the orientation and position of an adsorbed molecule and 2) the strength and location of the forces bonding a molecule to a surface. 

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. 

These speciation concepts are illustrated in Fig. 3 for the idealized basal-plane surface of a smectite, such as montmorillonite. Also shown are the characteristic residence-time scales for a water molecule diffusing in the bulk liquid (L) for an ion in the diffuse swarm (DI) for an outer-sphere surface complex (OSQ and for an inner-sphere surface complex (ISC). These time scales, ranging from picosecond to nanosecond [20,21], can be compared with the molecular time scales that are probed by conventional optical, magnetic resonance, and neutron scattering spectroscopies (Fig. 3). For example, all three surface species remain immobile while being probed by optical spectroscopy, whereas only the surface complexes may remain immobile while being probed by electron spin resonance (ESR) spectroscopy [21-23]. 

Inelastic neutron scattering spectroscopy is characterized by completely different intensities because the neutron scattering process is entirely attributable to nuclear interactions [110] Each atom features its nuclear cross section, which is independent of its chemical bonding. Then the intensity for any transition is simply related to the atomic displacements scaled by scattering cross sections. And because the cross section of the proton is about one order of magnitude greater than that for any other atom, the method is able to record details of quantum dynamics of proton transfer. 

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. 

Phonon wings are probably the most important band shaping processes in inelastic neutron scattering spectroscopy and this theme is developed in later chapters. The intensity arising from the vth internal transition and remaining at the band origin, coq, is termed the zero-phonon-band intensity, often found in the literature as Sq. From Eq. (2.62), for R = 0... 

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M.G. Taylor, S.F. Parker P.C.H. Mitchell (2003). J. Mol. Struc., 651/653, 123-126. A study by high energy transfer inelastic neutron scattering spectroscopy of the mineral fraction of ox femur bone. 

D. Lennon, D.T. Lundie, S.D. Jackson, G.J. Kelly and S.F. Parker (2002). Langmuir, 18, 4667-4673. Characterisation of activated carbon using X-ray photoelectron spectroscopy and inelastic neutron scattering spectroscopy. 

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