The polarization of atomic line radiation excited by electron impact

The Royal Society - Tập 251 Số 990 - Trang 113-138 - 1958
I C Percival1, M. J. Seaton1
1Department of Physics, University College, London

Tóm tắt

The dipole radiation emitted by an atom excited by a unidirectional electron beam has a nonuniform angular distribution which is simply related to the percentage polarization P of the radiation emitted perpendicular to the beam. P was first calculated using the OppenheimerPenney (O.-P.) theory. In this theory the probability of excitation of an upper quantum state and the probability of subsequent emission of a polarized photon from such a state are considered independently. P is finally expressed in terms of the cross-sections QM for excitation of states of definite component of angular momentum along the direction of the electron beam. In general, P is dependent on detailed numerical calculations of QM j, but the selection rule A = 0 removes this dependence at threshold. In the O.-P. theory allowance may be made for fine structure and hyperfine structure, but the theory is ambiguous when the f.s. or h.f.s. separations are comparable with the line width. A theory is therefore developed which is based on the calculation of the probability of a polarized photon being emitted by the complete system of atom + electron. The ambiguity of the O.-P. theory is removed by integration over line profiles, but the expressions reduce to O.-P. expressions when the f.s. or h.f.s. separations are much smaller or much larger than the line width. The Lyot line of hydrogen is an intermediate case for which the line widths and the h.f.s. separations are comparable. Assuming the validity of the Born approximation, a simple expression is obtained which allows the QM to be calculated from the angular distribution of the scattered electrons. Theoretical predictions are compared with experimental results. For the Na£) lines the predicted polarization is small enough to escape experimental detection. Polarizations observed by - Skinner & Appleyard in 1927 for various Hg lines rise to maxima with decreasing electron energy, and then tend to values close to zero at threshold. These experimental results at low energies appear to be inexplicable in terms of the reactions considered, but if the polarization curve above the maximum is extrapolated to threshold, the theory and experiment are found to be in reasonable agreement. Further experimental work is thought to be desirable

Từ khóa


Tài liệu tham khảo

Baranger E. & Gequoy E. 1958 Proc. Phys. Soc. 72 326.

Bates D. R. Fundaminsky A. Leech J. W. & Massey H. S. W. 1950 Trans. A 243 93.

Bethe H. A., 1933, Hand, Physik. 24, part, 1, 508

Condon E. U. & Shortley G. H. 1951 The theory of atomic spectra. (Reprint of 1st ed. 1935.) Cambridge University Press.

10.1103/PhysRev.103.1125

10.1103/PhysRev.28.1151

10.1103/PhysRev.27.31

Fite W. L. & Brackmann R. T. 1958

10.1007/BF01341743

Heitler W. 1954 The quantum theory of radiation (3rd ed. chap. 5). Oxford University Press.

10.1088/0370-1328/71/4/305

Kossel W. & Gerthsen C. 1925

10.1103/PhysRev.105.559

Lamb W. E. & Maiman T. H. 1957

Massey H. S. W., 1931, Ann. Phys. Lpz., 77, 273

Thông tin
Thông tin xuất bản

The Royal Society

Tập 251 Số 990

113-138

Thông tin tác giả