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Observation of Na I inner shell-excited quartet terms
Volume 97A, number 9
PHYSICS LETTERS
26 September 1983
OBSERVATION OF Na I INNER SHELL-EXCITED QUARTET TERMS R. FRöHLING Institut für Atom- und Festkörperphysik der Frelen Universitdt Berlin, Arnimalle~14, D-1000 Berlin 33, West Germany
and HJ. ANDRA Institut für Kern physik der Universitdt Münster, Domagkstra~3e71, D-4400 Münster, West Germany Received 11 July 1983
Stokes parameters of Na I spectral lines after ion-beam surface interaction at grazing incidence were measured. The as yet unidentified line at 507.1 nm showed opposite polarization to that normally found in such experiments. Measurements after ion—atom collisions with charge state separation indicate that this line is a member of the neutral Na I-spectrum. Together with an unpublished calculation by Weiss of inner shell-excited term energies of Na atoms the analysis of the polari4D 4P zation suggests the line to stem from a transition between the inner shell-excited terms 3s3p 712 — 3s4s 512 of neutral sodium.
During studies ofthe ion—surface interaction mechanism we were measuring the polarization of Na spectral lines excited at grazing incidence on a copper target. We observed two striking lines with special values of circular polarization. One is at 388.3 nm, which was identified by Berry and coworkers [1] as originat4D ing from4Fthe doubly excited Na I transition 3s3p 712 912. TheInother at 507.1 nm remains yet3s3d unidentified. orderline to make sure, that these as spectral lines orginate from transitions in the neutral Na-system, we also excited them in ion—atom coffisions with helium gas, made a charge state separation before the optkal observation, and could thus prove that both lines are members of the neutral Na system. Since the normal Na I spectrum is well known [2], it seems likely that the line at 507.1 nm stems also from a transition in the inner shell-excited quartet system. The incidentions were produced by an electrostatic accelerator with an energy range from 50 to 300 keV. They were directed onto a target of 25 mm length at extremely small angles of incidence varying between 0.2°and 0.6°.The optical observation region started directly behind the target and was 25 mm long. The direction of observation was perpendicular to the surface normal and to the beam direction. The measure—
0.031-9163/83/0000—0000/s 03.00 © 1983 North-Holland
ments of the Stokes parameters were carried out with an experimental arrangement described earlier [3], adapted here to UHV-conditions. Because Mu and C/I are always small and vary little with energy only S/I, the normalized fraction of circular polarization is shown in fig. 1 for both lines. In addition the S/I of the resonance at shown. 589.6 nm 2P 4d line 2D) is Theand the line at 568.8 nm (3p remarkable feature of the 388.3 nm line is the high degree of circular polarization. No other measured line in the Na I spectrum exhibits such a strong S/I. Na II lines show a maximum S/I of 65%. In earlier measurements [3] we found comparable values only for spectrally resolved fine structure components and only for those components with a maximum of total angular momentum. Berry and coworkers suggested,that the line at 388.3 nm stems from a transition between levels with maximum allowed total angular momentum, because these are most stable against mixing with doublet 1evels, which tend to decay via autoionisation. Their identification is consistent with our interpretation of the polarization measurementof the 388.3 nmline,which is done with a model [3] describing the Stokes parameters of the light emitted by ion beams excited at graz—
375
Volume 97A, number 9
S/I 70
PHYSICS LETTERS
at 500.7 nm which fulfils the conditions with respect
%
to the polarization. Independent of the spectral reso-
3882 nm
lution such a whole multiplet is expected to yield negative S/I because the total orbital angular momentum of the upper term is smaller than the lower one [6]. For certain multiplet components described above this negative S/I is even more pronounced than the
50 568,8 nrii
30
10 0
589.6 nm
~—I-—---4_$_4
•a
0 2°
o ~
0 4° 6°
A ~
~
averaged multiplet. We therefore suggest I 4 D the Na 4P
507.1 nm line to stem from the 3s3p
Na’—~—Cu
I
100
512
momentum of the upper term of about 0.5 (see ref. I
200
712—3s4s
multiplet component. Under the assumption of a standard positive orientation of the orbital angular
______________________________
-10
—
300 E (key)
[7]) we calculate aS/1 —0.4 for this transition. Within this multiplet this is the compenent with the largest negative value of S/I and as such in reasonable agree-
507mm
30 Fig. 1. S/I of 4 Na I spectral lines for different angles of incidence as function ofenergy.
ing incidence on a metal surface. Within this model only lines stemming from spectrally resolved fme structure multiplets and from levels with maximum total angular as shownmomentum in fig. 1. can exhibit such strong S/I values In contrast to the Na I line at 388.3 nm, the one at 507.1 nm exhibits a negative S/I. In the past negative signs of the S/I were only observed in two cases: first, components of spectrally resolved multiplets exhibit the minus sign if the angular momentum number of the upper level is smaller than that of the lower level, And second, hydrogen-like ionic systems may show a negative S/I, because the surface electric field of the target causes transitions via Stark effectbetween states •of different parity [4]. But in this case the S/I should increase with increasing energy and angle of incidence. This increase should be most pronounced for large angles of incidence in contrast to fig.l. We therefore adopt the first case and search for an identification of the 507.1 nmline which fulfils the condition that the upper level has smaller total angular momentum than the lower one. Fortunately Weise has calculated already term energies of various inner shell-excited Na quartet terms and corresponding transition probabilities. We are grateful to him for having communicated to us this as yet unpublished work [5]. In the wavelength region of interest he obtained only one transition 3s3p4D 3s4s4P —
376
26 September 1983
ment with the observations. Our line identification is in accordance with the arguments of Berry et al. [1], that only levels with highest angular momenta are stable against autoionization.
Although our line identification is the first example for which a measurement of the polarization in cornbination with ion—atom and ion—surface collision yields important information there remain ambiguities con-4P. 5 3s4s nectedthe with the structure ofthe excited term 2p is Since anisotropy in ion—surface interaction known to stem from an orbital angular momentum effect [3] it is obvious that in this special case only an initial 2p5 core-orientation can be responsible for the polarization observed [8]. For such core-orientations only quite_indirect hints exist so that our assumption of (L )/~/((L2>)has further to be justified in future work. This work is supported by the Sonderforschungsbereich 161 der Deutschen Forschungsgemeinschaft. Lii H.G. Berry, R. Hahn, R. Sjödin and M. Gaillard, Phys. Lett. 50A (1974) 191. [2] M. Born and E. Wolf, Principles of optics (Pergamon,
New York, 1975). [3] H.J. AndrA, R. Frohling and H.J. Plöhn, in: Inelastic ion—surface coffision, eds. N.H. Tollc, J.C. Tully, W. Heiland and C.W. White (Academic Press, New York, 1977). Frohling and H.J. Andrä, to be published.
[41 R.
[5] A.W. Weiss, private communication of unpublished work. [6] Burgdörfer, Freie (1981). [7] J. H.J. Andrã, R. Thesis, Frohling, H.J.Universität Pldhn andBerlin J.D. Silver, Phys. Rev. Lett. 18(1976)1212. [8] H. Schrdder, Z. Phys. A284 (1978) 125.
PHYSICS LETTERS
26 September 1983
OBSERVATION OF Na I INNER SHELL-EXCITED QUARTET TERMS R. FRöHLING Institut für Atom- und Festkörperphysik der Frelen Universitdt Berlin, Arnimalle~14, D-1000 Berlin 33, West Germany
and HJ. ANDRA Institut für Kern physik der Universitdt Münster, Domagkstra~3e71, D-4400 Münster, West Germany Received 11 July 1983
Stokes parameters of Na I spectral lines after ion-beam surface interaction at grazing incidence were measured. The as yet unidentified line at 507.1 nm showed opposite polarization to that normally found in such experiments. Measurements after ion—atom collisions with charge state separation indicate that this line is a member of the neutral Na I-spectrum. Together with an unpublished calculation by Weiss of inner shell-excited term energies of Na atoms the analysis of the polari4D 4P zation suggests the line to stem from a transition between the inner shell-excited terms 3s3p 712 — 3s4s 512 of neutral sodium.
During studies ofthe ion—surface interaction mechanism we were measuring the polarization of Na spectral lines excited at grazing incidence on a copper target. We observed two striking lines with special values of circular polarization. One is at 388.3 nm, which was identified by Berry and coworkers [1] as originat4D ing from4Fthe doubly excited Na I transition 3s3p 712 912. TheInother at 507.1 nm remains yet3s3d unidentified. orderline to make sure, that these as spectral lines orginate from transitions in the neutral Na-system, we also excited them in ion—atom coffisions with helium gas, made a charge state separation before the optkal observation, and could thus prove that both lines are members of the neutral Na system. Since the normal Na I spectrum is well known [2], it seems likely that the line at 507.1 nm stems also from a transition in the inner shell-excited quartet system. The incidentions were produced by an electrostatic accelerator with an energy range from 50 to 300 keV. They were directed onto a target of 25 mm length at extremely small angles of incidence varying between 0.2°and 0.6°.The optical observation region started directly behind the target and was 25 mm long. The direction of observation was perpendicular to the surface normal and to the beam direction. The measure—
0.031-9163/83/0000—0000/s 03.00 © 1983 North-Holland
ments of the Stokes parameters were carried out with an experimental arrangement described earlier [3], adapted here to UHV-conditions. Because Mu and C/I are always small and vary little with energy only S/I, the normalized fraction of circular polarization is shown in fig. 1 for both lines. In addition the S/I of the resonance at shown. 589.6 nm 2P 4d line 2D) is Theand the line at 568.8 nm (3p remarkable feature of the 388.3 nm line is the high degree of circular polarization. No other measured line in the Na I spectrum exhibits such a strong S/I. Na II lines show a maximum S/I of 65%. In earlier measurements [3] we found comparable values only for spectrally resolved fine structure components and only for those components with a maximum of total angular momentum. Berry and coworkers suggested,that the line at 388.3 nm stems from a transition between levels with maximum allowed total angular momentum, because these are most stable against mixing with doublet 1evels, which tend to decay via autoionisation. Their identification is consistent with our interpretation of the polarization measurementof the 388.3 nmline,which is done with a model [3] describing the Stokes parameters of the light emitted by ion beams excited at graz—
375
Volume 97A, number 9
S/I 70
PHYSICS LETTERS
at 500.7 nm which fulfils the conditions with respect
%
to the polarization. Independent of the spectral reso-
3882 nm
lution such a whole multiplet is expected to yield negative S/I because the total orbital angular momentum of the upper term is smaller than the lower one [6]. For certain multiplet components described above this negative S/I is even more pronounced than the
50 568,8 nrii
30
10 0
589.6 nm
~—I-—---4_$_4
•a
0 2°
o ~
0 4° 6°
A ~
~
averaged multiplet. We therefore suggest I 4 D the Na 4P
507.1 nm line to stem from the 3s3p
Na’—~—Cu
I
100
512
momentum of the upper term of about 0.5 (see ref. I
200
712—3s4s
multiplet component. Under the assumption of a standard positive orientation of the orbital angular
______________________________
-10
—
300 E (key)
[7]) we calculate aS/1 —0.4 for this transition. Within this multiplet this is the compenent with the largest negative value of S/I and as such in reasonable agree-
507mm
30 Fig. 1. S/I of 4 Na I spectral lines for different angles of incidence as function ofenergy.
ing incidence on a metal surface. Within this model only lines stemming from spectrally resolved fme structure multiplets and from levels with maximum total angular as shownmomentum in fig. 1. can exhibit such strong S/I values In contrast to the Na I line at 388.3 nm, the one at 507.1 nm exhibits a negative S/I. In the past negative signs of the S/I were only observed in two cases: first, components of spectrally resolved multiplets exhibit the minus sign if the angular momentum number of the upper level is smaller than that of the lower level, And second, hydrogen-like ionic systems may show a negative S/I, because the surface electric field of the target causes transitions via Stark effectbetween states •of different parity [4]. But in this case the S/I should increase with increasing energy and angle of incidence. This increase should be most pronounced for large angles of incidence in contrast to fig.l. We therefore adopt the first case and search for an identification of the 507.1 nmline which fulfils the condition that the upper level has smaller total angular momentum than the lower one. Fortunately Weise has calculated already term energies of various inner shell-excited Na quartet terms and corresponding transition probabilities. We are grateful to him for having communicated to us this as yet unpublished work [5]. In the wavelength region of interest he obtained only one transition 3s3p4D 3s4s4P —
376
26 September 1983
ment with the observations. Our line identification is in accordance with the arguments of Berry et al. [1], that only levels with highest angular momenta are stable against autoionization.
Although our line identification is the first example for which a measurement of the polarization in cornbination with ion—atom and ion—surface collision yields important information there remain ambiguities con-4P. 5 3s4s nectedthe with the structure ofthe excited term 2p is Since anisotropy in ion—surface interaction known to stem from an orbital angular momentum effect [3] it is obvious that in this special case only an initial 2p5 core-orientation can be responsible for the polarization observed [8]. For such core-orientations only quite_indirect hints exist so that our assumption of (L )/~/((L2>)has further to be justified in future work. This work is supported by the Sonderforschungsbereich 161 der Deutschen Forschungsgemeinschaft. Lii H.G. Berry, R. Hahn, R. Sjödin and M. Gaillard, Phys. Lett. 50A (1974) 191. [2] M. Born and E. Wolf, Principles of optics (Pergamon,
New York, 1975). [3] H.J. AndrA, R. Frohling and H.J. Plöhn, in: Inelastic ion—surface coffision, eds. N.H. Tollc, J.C. Tully, W. Heiland and C.W. White (Academic Press, New York, 1977). Frohling and H.J. Andrä, to be published.
[41 R.
[5] A.W. Weiss, private communication of unpublished work. [6] Burgdörfer, Freie (1981). [7] J. H.J. Andrã, R. Thesis, Frohling, H.J.Universität Pldhn andBerlin J.D. Silver, Phys. Rev. Lett. 18(1976)1212. [8] H. Schrdder, Z. Phys. A284 (1978) 125.