Grating constant

Gitter-konstante, /. lattice constant grating constant, -loch, n. (Cryst.) lattice hole, lattice void, -masche, /. grid mesh, -span-nung, /. (Elec.) grid voltage, -spektrograph,... 

Where in this expression a and b are the width and spacing between the slits of the grating, and hence d (= a + b) is defined as the grating constant. Here d and X are measured in the same units of length. The diffraction order is given by p, an integer value. 

For the moment, it is sufficient to know that the sample response, which is the time dependent diffraction efficiency after switching off the optical grating, contains at least a fast contribution from heat and a slow one from mass diffusion. The corresponding diffusion time constants depend on the grating constant and are typically of the order of 10 /rs and 100 ms, respectively. 

Some constraints apply for the measurement of mass and thermal diffusion by TDFRS, which originate from excessive sample heating at high laser powers, the resulting onset of convection, and the need to avoid boundary effects at the cuvette windows when the grating constant becomes comparable to the sample thickness. The problem of optimization of the experimental boundary condi-... 

Since the length scales associated with the thermal lens are on the order of 10 to 1000 times the grating constant, their characteristic time scale interferes with polymer diffusion within the grating. Such thermal lensing has been ignored in many FRS experiments with pulsed laser excitation [27,46] and requires a rather complicated treatment. A detailed discussion of transient heating and finite size effects for the measurement of thermal diffusivities of liquids can be found in Ref. [47]. 

The work by M. Arndt el al. starts with the description of far-field diffraction experiments with fullerene molecules Cm. Since for larger objects the far-field observations become much more challenging, the authors also study the feasibility of near-field Talbot Lau interferometry with 6Vo. This particular technique allows them to work with a spatially incoherent beam and thus with a much increased count rate. Moreover it has a better wavelength scaling in the grating constant and can potentially be applied to much smaller wavelengths at a reasonable grating constant. 

We can now calculate the deflection angle to the first diffraction order in the small angle approximation as the ratio of the wavelength and the grating constant,... 

Figure 6 Interference fringes in C70 Talbot-Lau interferometry. The periodicity of the signal corresponds to the grating constant [Hornberger 2003 (a)].

Fig. 18. Spectral dispersion at a diffraction grating, a Grating constant, / Blaze angle, t/tp. angle of incident radiation, 2. angle of diffracted radiation with wavelength X.

Photoplates, films and photo multipliers are used as detectors. Normally, gratings are used at low orders (m < 4) and they have a small grating constant (1/3600 mm < a < 1/300 mm). The different orders can be separated by using special photomultipliers. For instance, with a solar-blind photomultiplier only radiation with a wavelength below 330 nm can be detected. This allows separation of the 1st order radiation at 400 nm from the 2nd order radiation at 200 nm. This can, for example, be applied in polychromators to double the practical resolution. 

Fig. 17 Left schematic representation of a sinusoidally shaped SRG with the modulation height Ad, initial film thickness do, and the grating constant A. Right rotating such a sample by 90° and superimposing a second SRG yields an egg-crate structure...

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