• Amar Nath Nigam

Articles written in Proceedings – Section A

• Field theoretical effects in X-ray spectra

The LI-LII screening doublet separation in the relativistic approximation up to Z20 has been compared with that obtained experimentally by the transitions MIVLII (Lβ1) and MIVLI (Lβ10). The experimental and theoretical values deviate considerably. If one retains terms up to Z4, an apparently close fit with the experimental values is obtained. It is shown that such a fit disappears on applying the field theoretical corrections and this stresses the need of retaining the terms up to Z20. The justification of neglecting the still higher powers of Z lies in the smallness of their magnitude when compared to the experimental errors. The field theoretical corrections modify the relativistic curve that contains terms up to Z20 in such a way that one again gets a good fit with the experimental values. This shows how the various field theoretical corrections and the contributions from relativistic terms with powers of Z greater than four compensate each other.

• X-Ray K-absorption edges of cobalt in two recently prepared chelates

K-absorption edges of cobalt in two chelates,viz., Co (II) bis S-ethyl 1-amidino 2-thiourea and Co (III) isonitroso acetylacetonate have been recorded photographically. The results have been discussed in the light of available magnetic measurements and favour a tetragonal structure for the first and an octahedral one for the second chelate.

• Thickness effect in the X-ray K absorption edge of nickel

Effect of absorber thickness on the X-ray K absorption edge of nickel has been recorded photographically. The extended structures show a variation in their relative intensities. In agreement with the earlier observations of Parratt and Sawada the structures in the immediate neighbourhood of the edge show a decrease in their absorption intensities and the fluctuations following them up to 140 eV. remain unaltered. In addition, it is found that the structures lying beyond 150 eV. from the edge become more pronounced on the increase of absorber thickness.

• Relativistic estimates of X-ray K-line intensity ratios: Calculations including retardation and screenings

X-ray K-line intensity ratiosα2/α1,β3/β1 andβ2II/β2I have been calculated following the method due to Payne which accounts for the retardations. In addition screening has been included. It is found thatα-line intensity ratios are given best by Sommerfeld screening but that Slater screening is better for theβ-lines. Field theoretically corrected energies have also been used and it is found that the agreement is less favourable. The necessity of including the potentials corresponding to these effects as perturbations to the wavefunctions has been pointed out.

• Effect of field theoretical corrections on X-ray spectra

The wavelengths of$$K_{\alpha _1 } , K_{\alpha _2 } , K_{\beta _1 } , K_{\beta _2 } , K_{\beta _2 } I and K_{\beta _2 } II$$ lines of heavy elements of atomic number 70–92 and the associated energy levels, K, LII, LIII, MII, MIII, NII and NIII have been calculated using a modified form of the relativistic energy formula of Sommerfeld, incorporating in it both the total and internal screening constants in a closed form. All the field theoretical corrections in the second and the fourth order as well as the finite nuclear size correction have been included.

The relative intensities$$K_{\alpha _2 } /K_{\alpha _1 } , K_{\beta _2 } / K_{\beta _1 } , and K_{\beta _2 } II/K_{\beta _2 } I$$ have been calculated with relativistic wavefunctions. In addition to the retradation and screening, field theoretical corrections have also been used. A logical extension of the one electron-model with application of the field theoretical corrections improves the calculated results of not only the energy levels but also the relative intensities of X-ray lines.

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