Neutron specular reflectivity

X-ray and neutron specular reflection Molecular structure across the interface, laterally averaged over the beam area X-rays electron density profiles neutrons scattering length density profiles of the nuclei of atoms). With neutrons different parts of the monolayer can be studied independently by selective H/D substitution. See ellipsometry. 

For some time, specular scattering has been successfully applied to multilayer polymer films. Many examples were given by T. R. Russel and here we have selected one of the earliest, shown in Figure 3. The layered stmcture of the diblock polymer film was revealed in neutron specular reflection as shown in the inset of Figirte 3. 

Though a powerfiil technique, Neutron Reflectivity has a number of drawbacks. Two are experimental the necessity to go to a neutron source and, because of the extreme grazing angles, a requirement that the sample be optically flat over at least a 5-cm diameter. Two drawbacks are concerned with data interpretation the reflec-tivity-versus-angle data does not directly give a a depth profile this must be obtained by calculation for an assumed model where layer thickness and interface width are parameters (cf., XRF and VASE determination of film thicknesses. Chapters 6 and 7). The second problem is that roughness at an interface produces the same effect on specular reflection as true interdiffiision. 

Thin films of block copolymer melts, and block copolymers adsorbed at the liquid-liquid interface, have been investigated using specular reflectivity (largely neutron reflectivity due to the ability to vary the scattering contrast). Off-specular reflection is, in principle, a powerful method for determining in-plane structure in block copolymer films but is not yet widely used. 

The specular reflectivity of neutrons, like the analogous light or X-ray reflectivity, from a surface or interface provides information about the neutron refractive index gradient or distribution in the surface region and in a direction orthogonal to the plane. This can often be simply related to a composition or concentration profile in the direction orthogonal to the surface, to provide directly information about adsorption and the structure of the adsorbed layer. 

Consider a beam of neutrons in medium 1 incident on a perfectly smooth surface of a medium 2 at an angle 0. Some of the beam is specularly reflected (angle of reflection = glancing incident angle), and some is refracted into medium 2 with an angle 02 (Fig. 4). A neutron refractive index can be defined for medium 2 as... 

Bayerl, T. M., Thomas, R. K., Penfold, J., Rennie, A. and Sackmann, E. (1990). Specular reflection of neutrons at phospholipid monolayers. Changes of mono-layer structure and headgroup hydration at the transition from expanded to the condensed phase. Biophys. J. 57 1095-1098. 

Penfold, J. and Thomas, R. K. (1990). The application of the specular reflection of neutrons to the study of surfaces and interfaces. J. Phys. Condens. Matter 2 1369-1412. 

Johnson SJ, Bayerl TM, McDermott DC, Adam GW, Rennie AR, Thomas RK, Sackmann E. Structure of an adsorbed dimyris-toylphosphatidylcholine bilayer measured with specular reflection of neutrons. Biophys. J. 1991 59 289-294. 

In an X-ray or neutron reflection experiment the specular reflection, R (the ratio between the reflected and incident intensities), is measured as a function of the scattering vector q q = - fc.. (where Jc, and arc the incident and reflected wave... 

T.P. Russell, X-ray and Neutron Reflectivity for the Investigation of Polymers. Mater. Sci. Reports 5 (1990) 171-271. (Basic theoretical and experimental concepts of specular reflection, and application to polymer science,)... 

Like many other in situ coupled techniques, the ideal experimental conditions for the collection of specularly reflected neutrons are not ideal for maintaining electrochemical control. This mandates the improvisation of new cell designs. As detailed above, measurements performed in the presence of electrolyte should use an inverted cell geometry. The cell needs to be constructed so that the neutrons... 

As mentioned earlier, simple specular reflection profiles can yield detailed interfacial structural information. X-ray and neutron reflectometers have been developed specifically to investigate the liquid surfaces. The problem is to be able to study the buried interfacial region by x-ray methods. 

Thin film polymer bilayers and confined liquids have been investigated by x-ray specular reflectivity. First, it is shown that x-ray scattering on polymer bilayers can be superior to neutron scattering if special methods to analyze the data is applied, although neutrons are usually preferred. Second, confined liquids are addressed. The experimental setup for x-ray scattering is presented. I eliminary results are shown, which suggest quantized film thicknesses of the confined liquid. 

However, the low intensity of the neutron beam and the coarse resolution compared to x-rays often result in quite large error bars. The flux of a neutron beam is typically more than 5 orders of magnitude smaller than the photon flux of a synchrotron radiation source with up to 10 photons/sec on an area of (0.2x2)mm. From Eq. 1 it is known that the specularly reflected intensity at least falls with 1/ /. Thus, the low intensity of a neutron beam restricts the acc sible -range (a neutron reflectivity almost never exceeds a -value of qz,muF 0.2A" ). This means a relatively coarse resolution Az in real space via Azoc27i/ niax- In contrast, an x-ray reflectivity easily covers about 9 orders of magnitude which corresponds to a range of 0.7A to l.OA This yields a 4 times better resolution in real space if x-rays are used. 

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