Line identification problems in beam-foil spectroscopy

NUCLEAR

INSTRUMENTS

AND

METHODS

90 (197o) 15-23; ©

NORTH-HOLLAND

PUBLISHING

CO.

LINE IDENTIFICATION PROBLEMS IN BEAM-FOIL SPECTROSCOPY M. D U F A Y *

Laboratoire de Physique de l'AtmosphOre, Equipe de Recherche associde au C.N.R.S., Facultd des Sciences de Lyon, 69 - Villeurbanne, France A review o f the present day contributions of the beam-foil method to atomic energy level spectroscopy is given. Different experimental settings which try to overcome the fundamental limitations of the m e t h o d are discussed and further improvements suggested. The validity range of the hypothesis currently made for charge state identification purposes is established. Examples

are taken from a recent investigation o f sodium ions which clearly illustrates the interest o f the comparison o f results obtained by beam-foil spectroscopy in a given isoelectronic sequence. N e w research directions of astrophysical interest are suggested in the field o f line identification.

1. Introduction The originality of the beam-foil spectroscopy technique comes from the fact that the observed light is emitted by moving particles which undergo in vacuum spontaneous transitions to lower quantum states. Hence, the decay of the observed lines along the beam is connected to the lifetimes of the excited states in a more or less complicated way depending on the importance of cascade phenomena. The problem of identification of the observed lines is vital to give physical significance to the study of such decay curves. However, Doppler broadening and Doppler shift of the lines due to the motion of the emitting particles put strong limitations to spectrogram resolution. They also introduce uncertainty in wavelength measurements, and make identifications difficult. On the other hand, scattering of energetic ions through a solid target yields a large amount of excited ions in high states of charge. So one of the fields opened by beam-foil spectroscopy is the study of spectra generated by ions in high state of charge otherwise difficult to observe in the laboratory. In that case again the identification problem is of prime interest.

the problem. Such a device has been designed and experimented by Stoner2) in Tucson, and simultaneously by Gaillard and Desesquelles in Lyon. The conclusion was the same in both cases, namely, that unexpected limitations arise from the angular scattering of the beam by the foil. Indeed, in the axicon system the resulting width of the lines has a first order dependence on the angular width of the ion beam (whereas in paraxial observation the dependence is only of second order).

2. Experimental aspeet In the identification process, the main difficulties of experimental origin are the following: a. poor spectrogram resolution due to Doppler broadening; b. low accuracy of absolute wavelength measurements due to the width of the lines and their displacement by Doppler shift. In this field progress made since the first Tucson meeting has been slow. The "axicon" of Meinel and Bashkin ~) (fig. 1C) was expected to solve drastically

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* Presented the paper.

15 I. G E N E R A L I N T R O D U C T I O N

16

M. D U F A Y

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Fig. 2. A part of a recording with 500 keV Na + ions. Axial view (Spex 1700 spectrometer, photomultiplier EMI 6256 S). Consequently, the best spectral resolution has been obtained with optical systems using paraxial observation of the beam (figs• 1D and E). We used both settings for the study of nitrogena), neon 4' s), argon6), and sodium7). Fig. 2 shows a set of sodium spectra at maximum obtainable resolution• One can distinguish two lines 1.5 ~ apart, which is equivalent to a resolution of about 1500• Similar studies have been made by Bakken et al.8), on carbon ions, and Pinnington 9) used a rotating lateral mirror for lifetime measurement purposes. One should remark that for a given resolution, the observation along the axis of the beam offers marked improvement over the transverse observation from the luminosity point of view• It will consequently be preferred for the observation of weak signals. However, the sensitivity to the optical setting may alter the reproductibility of the results, and more specifically lines with short lifetimes might escape observation for only slight misadjustment of the optics. The main disadvantage of the paraxial observation arises from the large Doppler shift of the observed lines. Accurate wavelength measurements then require precise knowledge of ion velocity (which in any case

would become necessary for lifetime calculations). The magnetic analysis of the beam after the foil could provide at the same time a measure of the electrical composition of the beam, of the energy loss in the foil and of the speed distribution. This procedure becomes rapidly unrealistic at high energies due to the prohibitive size of the required magnet• It is then much cheaper to rely on the identification a priori of a few lines of the spectrum. The Doppler shift of the other lines can then be calculated by interpolation. This leads to an accuracy of about 0.3 ,~ [Bakken et al.S)] in the absolute wavelength measurements. Despite all the inconvenience of the paraxial observation mentioned above, one must notice that the theoretical limit of resolution which can be achieved in that manner is very low. For the time being, one is limited in fact by the sensitivity of the detectors. If further gain in that domain could be obtained by use of such devices as the electronic camera or the light amplifier, then the best experimental setting from the point of view of the resolution would become the setting displayed in fig. 1F. In that case, the influence of scattering would become a minimum and the line width would arise only from the dispersion of the energy losses in the foil. An improvement of the resolution would make easier experiments on Zeeman or Stark effect particularly useful from the point of view of transition identification as they allow determination of the excited level multiplicity. A very interesting attempt in that direction has been made by Bashkin et al. t°) at Canberra University. Neither of the devices described above can be used in vacuum ultraviolet• Fortunately, in that region, Doppler broadening is less important and a transverse observation gives usually satisfactory results (fig. 1B). Let us note, to end this list of experimental difficulties, that impurity lines in BSF may also appear in spite of the advantages of the method on this point over standard spectroscopic sources. These lines may be generated by unwanted ions in the beam escaping magnetic analysis. They can be generally predicted and are easy to detect. If necessary, in order to avoid accidental mass coincidence one uses more complicated ionic species of ions, such as H H F + ions for the study of fluorine 11). Under the impact of the beam, the target, the residual gas and even tile collimators will emit strong characteristic lines. Berry et al.lZ) have examined recently the occurrence of lines emitted by different targets• With carbon foils, two lines of C I at 1558 ~ and 1657 A are observed with various intensities depending on the

17

LINE I D E N T I F I C A T I O N PROBLEMS

to short lifetimes are beyond observation. At least the line of sight being perpendicular to the ion beam, the Doppler broadening is important and the obtained resolution is inadequate. Nevertheless a certain number of errors in the determination of the state of charge in the case of nitrogen have been detected by Fink is) using this technique. A less direct method of more frequent use has been suggested by Kay 16) in 1965. It consists in correlating the line intensities with the velocity v of the ions. The Lyon group has used this method modified in order to widen the range of velocity and allow an easier comparison of the intensities with the charge composition of the beam. For a given target we define a normalised light intensity In = Iov/(iAx) where Io is the intensity of the observed transition at the target surface (x = 0, x axis of the beam), i the intensity of the incident ion beam, Ax the length of the portion of the beam which is imaged on the slit of the spectrometer. The introduction of the v/Ax factor relates the light received by the detector to the light emitted during a fixed time of flight. In characterises, on an arbitrary scale, the efficiency of the target for a given line. In depends also on cascade effects. On fig. 3 we have plotted the variations of I, for several well identified transitions occurring in C, N, O as a function of the velocity for each state of charge. The curves have a similar shape and can be characterized by a value vm corresponding to a maximum of I,. In fig. 4, we have compared these curves in the neon case with the ionic composition of the beam. The similarity of the results is very striking.

aging process in the foil. Sometimes, L~, He, HI3 also appear emitted, by adsorbed hydrogenic compounds on the surface of the target. At high beam intensities Cu lines are emitted by Cu collimators. All these lines may turn to be useful as references for wavelength measurements. 3. Determination of the state of charge

Preceding the spectroscopic analysis proper, a magnetic or electrostatic charge state analysis of the beam is useful to help to spectral identification by a first hand determination of the most probable states of charge. This analysis is necessary for the understanding of the variation of a complex spectrum with the accelerating voltage, specially when one is faced with a totally unknown spectrum. Such was the case during the experiment recently made at Orsay on C, N, O, Ne and A ions at 1 MeV/nucleon (a more detailed account of this experiment is given in the communication by J. Desesquelles). The spectroscopic identification of the observed lines then proceeds in two steps: a. determination of the state of charge; .~: b. identification of the transitions for a given state of charge. In order to proceed with the first step, a radical method is to separate the different states of charge produced at the target by means of a suitable electric or magnetic field'3-15). As shown by Fink14), this method gives rise to technological difficulties due to the likelihood of sparking generated by the use of high voltage. Moreover, weak lines and lines corresponding Jn

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18

M. DUFAY

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Kay's method has been used qualitatively by many experimenters and gives obviously very valuable information. As an example fig. 5 shows for the sodium spectrum transitions of Na I( and Na III at 500 keV and 800 keV. Comparison of the two spectra clearly indicates that the two intense lines observed at 2388 /~ and 2396 • attributed formerly to Na II unidentified lines behave in fact like Na lII lines (fig. 5). A more precise study has shown (as will be seen later) that they belong to a Na III doublet 17). Bashkin and Malmberg tg) raised strong criticism against the first proposition by Kay. One of the difficulties lies in the poor resolution of the spectrum, as in order to minimize the lifetime effects, one has to use the transversal observation. So when there is a blending of lines which belong to different states of charge the intensity variations are perturbed. A more important difficulty lies in the fact that Kay's method cannot be applied rigorously, because there is no theoretical reason why the excited level population should remain the same for various energies. To check the limits of validity of the method the lithium case is particularly convenient. The emitted spectrum is simple, the number of states of charge is small and these states can be obtained easily for energies below 2 MeV. Li [I levels spread from 66 eV to 81 eV above the ground state of Li I. The curve of fig. 6 [Gaillard2°)] plots the variation

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LINE IDENTIFICATION

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identification of the transitions generated by BFS. The information obtained this way can also be useful to determine the states of charge of lines observed in other sources.

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PROBLEMS

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4. Spectroscopy The basic documents in spectroscopic analysis are the tables of energy levels of Moore often improved by recent studies made at Lund and Uppsala. We should keep in mind that data provided by standard spectroscopic techniques are limited in the domain of high charge state ions, since the observation of high temperature plasmas with heavy spectrographs becomes rapidly unfeasible. Another limitation appears in the accuracy of the tabulated line intensities because in standard sources complex secondary effects impair strongly the simplicity of the excitation phenomena. Hence the data which have to be used to identify BF spectra are not often in proper tabulated form. Spectroscopic results obtained by BF are included in a limited number of recent publications: Bakken et al. s) (C II to C IV), Poulizac et al. 2) (C II to C VI), Desesquelles 3) (N II to N VI), Lewis et al. 22) (O II

Fig. 6. Variations of relative intensities of s o m e Li I, Li H, Li III transitions with energy (after ref. 20), T h e intensity of t h e 3 d - 4 f line is taken as a reference.

of some selected line intensities with respect to the 3d-4f (4672-4678 A) line taken as a reference. Among Li II transitions no relative intensity variation can be detected within the experimental error limit (15%) for those coming from 4p and 4d levels in the neighbourhood of the 4f reference level. However, strong variations of the 2s3S-2p3P transition intensity show that as energy increases 4s and 4p level population outweigh the 2p one. Notice that the 2p level is located about 11 eV beyond the 4f level. Similar examples could be given for other kinds of ions. For example, we recently observed strong variations between the relative intensities of two N V transitions (2s-2p and 4f-5g) on spectra taken at 1 MeV, 2 MeV and 16 MeV energies (fig. 7). Again, the population of the high lying levels are strongly favoured at high energies. As one would expect, Kay's method is not rigorous and its unreliability increases as the relative distance of the levels under consideration becomes larger. In the case of heavy ions highly accelerated, when the number of possible charge states is high, this method provides only a lower limit to the probable ionisation stage of the observed transition. In all cases, a qualitative study of relative intensities is absolutely necessary if one wants to be sure of the

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INTRODUCTION

20

M. DUFAY

to O V), Andersen et al.23) (A1), Whaling et al. 2.) (Fe). One has also to point out the complete study made recently at the University of Arizona 25) on sulphur spectra (S I to S VI) which was poorly known and had not been reinvestigated up to then; and also the work of Denis et al. *' 2) on the multiionized neon (N II to Ne VII) which, unfortunately contains a large number of unidentified lines. Berry et al. j J) are about to publish a fairly complete study of the fluorine spectrum. The results can be classified under the following headings: a. Wavelengths for transitions between known levels already tabulated. This may carry new information only if a correct evaluation of the intensities is given simultaneously. b. Wavelengths for transitions between known levels not included in the standard tables (generally because they are difficult to obtain in the laboratory). In that case, BFS brings interesting confirmation to the existing classification. c. Wavelengths for unidentified transitions but for known charge state (or at least with a limit on the charge state). d. Wavelengths for unidentified transitions of unknown charge state. We shall see now what we could get from the results I Nell

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by beam-foil spectroscopy (O+, 0.5 MeV; Ne +, 1 MeV; Na +, 1.8 MeV). To clarify the figure, lines belonging to the Ne V spectrum have been omitted. A: 3s 4p-3p 4D°; B: 3s' 2D-3p' ~F; C: 3s" 6S-3p" 6p0; D: 3s 4P-3p 4p0; E: 3s' ZD-3p' 2D; F: 3p" 6P-3d" 6D; G: 3p' 2F-3d' ZG; I-I: 3p 4D-3d 4F.

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Fig. 8. Comparison of Ne II and Na III spectra obtained by beam-foil spectroscopy (Ne +, 0.4 MeV; Na +, 0.8 MeV). A: 3s ~P-3p 2D°; B: 3s 4P-3p 4p0; C: 3s' 2D-3p' 2F°; D: 3s 2p_ 3p 2S°; E: 3s 4p-3p aDO; F: 3s 2P-3p ~po; G: 3p' 2F°-3d' ~G.

classified under the c and d headings. In several cases, the analysis of the data has been made on computer. One compares the line to be identified with all combinations of lines computed from energy levels extrapolated along isoelectronic sequences. When a close agreement is reached the extrapolated level is corrected and can be used for further computations and identifications. Such a use of numerical analysis usually gives limited results due to the great uncertainties in the absolute measurements of the wavelengths. A less elaborate version of the same method relies on the comparison of spectra of elements of a given isoelectronic sequence obtained in fixed experimental conditions. In most cases, the corresponding transitions have comparable relative intensities, because the populations of the corresponding levels are similar as long as

LINE IDENTIFICATION

transition probabilities vary slowly when one goes from one element to another in the same sequence. An interesting example is given by the comparison of Ne II and Na III spectra (fig. 8). It provides us with an additional argument to identify the two lines at 2388 A and 2396 • with the multiplet of Na [II 3s'ZD-3p'2F °. Similarly comparison of O I [ , Ne IV and Na V spectra (fig. 9), permits identification of several multiplets of Ne IV and Na V in the uv region. A last example, also very significant, has been provided by an experiment still under way at 1 MeV/nucleon energy which will be detailed in the communication by Desesquelles. 5. Recent contributions of BFS and possible developments in atomic spectroscopy The BFS contribution to atomic spectroscopy is still small. One can, however, characterize several fields of research. 5.1. STUDY OF HIGHLYEXCITEDSTATES It is now well established that excitation processes occurring during the interaction of the incoming ion with the carbon foil leads to excited states whose energy is close to the corresponding ionisation potential. Therefore, even in the case of light elements in a low state of charge (C, N, O), transitions can be obtained which otherwise would be difficult to generate. 1D

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21

PROBLEMS

As an example, in the course of our study of Na, we repeated the observations previously made on Na II by Brown et al.26). We have been able to identify a great number of transitions generated in the 35004200 • range to be the 3d-4f lines. These lines are weak in other sources and were previously observed by Po Chuan Tsui et al.27). In addition, extrapolation of results obtained by Persson and Minnhagen 2s) on Ne I[ have permitted identification of several lines of Na I[I mixed with the 3d-4f lines of Na II. 5.2. STUDY OF MULTIPLE EXCITATIONS It is also well known that multiple excitation occurs in BFS. Transitions between autoionizing quartet states have been observed in Li [ [Buchet et al.27), Bickel et al.2S)]. The line intensities, particularly high at low velocity, lead to the conclusion that autoionization plays a major role in BF excitation and should allow study of this phenomenon by an optical method. Moreover theoretical calculations of doubly excited states have found their experimental check. 5.3. SPECTROSCOPIC STUDY OF HIGHLY IONIZED IONS

An exciting opportunity offered by BFS lies in the study of highly ionized ions. But, for the time being their investigation has hardly begun. The use of small accelerators (below 2 MeV) allows the study of a res{ricted number of elements mostly 3F

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INTRODUCTION

22

M. DUFAY

among the light, several-times-ionized elements. Among those, the study of ionized sodium is typical due to the large number of gaps in former studies. Studies with standard sources had been up to now restricted to the resonance lines of Na IV and Na V located in the far uv region (wavelengths below 1000 A). We have been able to excite lines in the 1100-2500 ~ spectral range with energies between 1 and 2 MeV. Figs. 10 and 11 show the diagrams of Na IV, Na V multiplets which have been identified. Identical study on K + ions is under way and similar results are expected. The study of multiply ionized heavy elements, extremely important in astrophysics, demands powerful accelerators (tandem or linear accelerators). In order to get ions of Si, Fe, Ni,... of astrophysical interest, it is required to develop suitable sources. Although the problem has been solved for small accelerators with the "universal" source of Aarhus which is able to produce ions up to the mass 235, the difficulties of adapting such devices on energetic machines is considerable. The high cost of these investigations is justified by their physical importance which spreads out beyond the field of pure spectroscopy (quantum electrodynamics and the hydrogenic ions). It is well known that the needs of astrophysicists in spectroscopic data are important. However, apart from the work of Whaling et al.24) on iron spectrum, BFS applications are scarce. Particularly, it would be 2S

2p

2D

2F

2G

interesting to extend our measurements to the red and infrared, in order to make better use of the large amount of spectroscopic data obtained by terrestrial astrophysics. Such is tb.e case for the observation of hot stars (Wolff-Rayet) emitting numerous C n+, O n+, N n+ lines in the red. Due to the weak intensities of the BFS sources and to the poor efficiency of the detectors in this wavelength range, few measurements have been reported at the present time (carbon up to 6000 A in ref. 21 ; infrared N I lines by use of a photomultiplier with Cs-Ag-O cathode in ref. 3).

6. Conclusion Beam-foil experiments have been carried out to such a point that the possibilities and limits of the method are by now clearly exhibited. For the ions obtained from conventional rf accelerator sources and the lower charge states excited at Van de Graaff energies, the BFS results are or minor interest to the classical spectroscopists. The only advantage lies in the availability of a spectroscopic source which has almost no impurity and an excitation mechanism virtually perturbation free. The charge state identification is often easier in the beam than in the usual discharges. Finally the excitation of high lying levels is clearly favored, a fact which might lead classical spectroscopists to improve their classification in that domain. It is only at high energies for ions of high state of

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Fig. 11. Diagram of the identified transitions in Na V. The leveIs indicated by a dot are already tabulated.

LINE I D E N T I F I C A T I O N PROBLEMS

charge that one can take full advantage of the BF excitation process. Although one can easily observe such ionic species in hot plasma machines, the spectral identifications are often complicated in that case by complex secondary effects due to the high temyeratures and high densities. The beam-foil source on the contrary is of a great simplicity and for example, the low density in the beam rules out all interionic Stark effects. Although little progress has been made in that direction during the past two years numerous exciting developments are to be expected in a near future from the use of high energy accelerators for spectral line identification purposes. References 1) S. Bashkin, in Beam-foil spectroscopy, vol. 1 (ed. S. Bashkin; Gordon and Breach, 1962) p. 3. 2) j. O. Stoner, Jr., Appl. Opt. 9 (1970) 53. 3) j. Desesquelles, Thesis (Lyon, 1970). d) A. Denis, J. Desesquelles and M. Dufay, J. Opt. Soc. Am. 59 (1969) 976. ~) A. Denis, P. Ceyzeriat and M. Dufay, J. Opt. Soc. Am. 60 (•970) 1186. 6) A. Denis, unpublished results. v) M. Galliard, unpublished results. s) G. S. Bakken, A. C. Conrad and J. A. Jordan, Jr., J. Phys. B2 (1969) 1378. 9) E. H. Pi1~nington, Proceedings of this conference. 10) S. Bashkin and G. W. Carriveau, Phys. Rev. A1 0970) 269. ll) H. G. Berry, I. Martinson, R. M. Schectman, W. S. Eickel and H. P. Palenius, J. Opt. Soc. Am. 60 0970) 1461. 1~) H. G. Berry, I. Martinson and J. Bromander, Phys. Letters 31A (1970) 521. 13) p. R. Malmberg, S. Bashkin and S. G. Tilford, Phys. Rev. Letters 15 (1965) 98. :4) U. Fink, Appl. Opt. 7 (1968) 2373. 15) U. Fink, J. Opt. Soc. Am. 58 (1968) 937. 16) L. Kay, Proc. Phys. Soc. 85 (1965) 163.

23

iv) M. Gaillard, P. Ceyzeriat, A. Denis and M. Dufay, Compt. Rend. Acad. Sci. Paris 269 (1959) 526. is) M. Druetta, M. C. Poulizac and J. Desesquelles, J. Opt. Soc. Am. 60 (1970) 1463. 19) S. Bashkin and P. R. Malmberg, Proc. Phys. Soc. 87 (1956) 589. 20) F. Galliard, Thesis (Lyon, 1969). 21) M. C. Poulizac, M. Druetta and P. Ceyzeriat, J. Quant. Spectr. Radiative Transfer, to be punished. 22) M. R. Lewis, F. S. Zimnoch and G. W. Wares, Phys. Rev. 178 (1969) 48. 2a) T. Andersen, K. A. Jessen and G. Sorensen, J. Opt. Soc. Am. 59 (1969) 1197. -24) W. Whaling, R. B. King and M. Martinez Garcia, Astrophys. J. 158 (1969) 389. 25) H. G. Berry, R. M. Schectman, I. Martinson, W. S. Bickel and S. Bashkin, J. Opt. Soc. Am. 60 (1970) 335. 26) L. Brown, W. Kent Ford, Jr., V. Rubin, W. Trachslin and W. Brandt, in Beam-foil spectroscopy, vol. 1 (ed. S, Bashkin; Gordon and Breach, 1969) p. 45. 27) Po Chuan Tsui, Shao-Chi-Ma and Chen-Ming Wu, Chinese J. Phys. 3 (1965) 127. 2s) W. Persson and L. Minnhagen, Arkiv Fysik 37 (1968) 273. zg) j. p. Buchet, A. Denis, J. Desesquelles and M. Dufay, Phys. Letters 28A (1969) 529. 80) W. S. Bicke!, I. Bergstrom, R. Buchta, L. Lundin and I. Martinson, Phys. Rev. 178 (I969) 118.

Discussion MINNI-IAGEN:The previously published Na lli analysis has been revised and corrected. The earlier given 3d-levels are generally wrong, except ~D. Also did you see the 3d-4f transitions in Na 1It? DurAv: Yes, we observed some of them. GAILLARD: They were weak in the beam-foil spectra, but we saw them at 800 keV. MINNHAGEN: We can now give you the wavelengths of the 3p-3d transitions. KAY: A blended line, consisting of multiplets from different charge states can often be analyzed by plotting the intensity versus beam velocity.

I. G E N E R A L I N T R O D U C T I O N