Asphericity shifts

Detectable asphericity shifts are not, however, restricted to H and can be significant for elements of the first row of the periodic table. Recently a systematic pairwise analysis of X and N bond lengths showed X—N differences across a wide range of bonding situations - 0.0096(9) for C=C (ethylenic), - 0.0052(9) for C=C (aromatic), - 0.0078(16) for C=N, etc. The data of Table 3a show clearly that a similar effect operates for C=C as well. For Csp —C=C—Csp it is reasonable to assign a Csp —Csp separation of 2 X 1.461 +1.209 = 4.131 A in the gas phase. This separation is 2 x 1.474+1.184 = 4.132 A across 44 X-ray crystal structures. The principal geometrical difference between these two cases lies in an inward movement of the two Csp atoms by - 0.0125 A each. This X—G value is entirely consistent with the X—N differences described earlier ... 

Simple systems such as that shown in Fig. 5.11 yield an excellent optical efficiency, and are almost free of pulse dispersion and wavelength-dependent pulse shift. Another benefit is that the detection path is almost free of polarisation effects compared to monochromator-based systems. The high numerical aperture of the light eollection system further reduces the influence of the rotational relaxation see Fig. 5.9. With aspheric lenses an NA of around 1 can be achieved, and at an NA this high the polariser in the detection path can often be omitted. In the setup in the left graphic of Fig. 5.11, residual depolarisation effects can be removed by slightly tilting the polarisation direetion of the laser. 

Since polymers can be easily molded into aspheric lenses with high productivity and low cost, they have been applied positively. In order to diminish shift of focal distance... 

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