A renewed study of singly ionized boron

593

Nuclear Instruments and Methods in Physics Research B9 (1985) 593-597 North-Holland, Amsterdam

A RENEWED STUDY OF SINGLY IONIZED BORON S. BASHKIN

‘) L.C. MCINTYRE

‘), H. v. BUTTLAR

*) J.O. EKBERG

3, and I. MARTINSON

3,

‘) Department of Phisics, University of Arizona, Tucson, AZ 85721, USA ‘) Experimentalphysik III, Ruhr - Uniuersitiit, D 4630 Bochum I, W. Germany *’ Department of Physics, University of Lund S - 22362 Lund, Sweden

Some energy levels and lifetimes in B II have been studied using classical emission spectroscopy and beam-foil spectroscopy. Revisions are suggested for the excitation energies of the 2~3s ‘S and 2s5p ‘PO levels. The 2~3s ‘P” and 2s6p ‘P” terms have also been located. Lifetimes have been measured for the 2s2p ‘PO, 2p* ‘S, 2s3d ‘D and 2p2 ‘P levels. The results are compared with theoretical data.

1. Introduction There exist several experimental and theoretical investigations of radiative transitions, energy levels and oscillator strengths in singly ionized boron (B II). The information about energy levels is largely based on the high-resolution spectroscopic work of Glme [l] which has later been complemented by a beam-foil study [2]. Several authors (see, e.g. [3-51) have also used the beam-foil technique to determine lifetimes and f-values in B II. Theoretical excitation energies originate from the superposition-of-configurations (SOC) calculations of Weiss [6], the polarized frozen-core data of Markiewicz et al. [7] and the multiconfiguration calculations of Luke [8]. These three theoretical investigations and numerous others (listed below) have also provided f-values for B II. However, a detailed comparison between the available experimental and theoretical results reveals the existence of a number of unsolved problems and inconsistencies. For example, the experimental energy of the 2~3s “S level, based on the classification of a line at 1607.76 A as the 2s2p ‘PO-2~3s ‘S transition, differs substantially from theories [6-81, which predict a much shorter wavelength. These calculations further imply that the f-value of the 2s2p ‘PO-2~3s ‘S transition is very low (because of mixing between the 2~3s and 2p2 ‘S levels) and thus in poor agreement with beam-foil results [3-51, obtained from decay measurements for the 1607 A line. Since the theoretical data are in good agreement with experimental excitation energies in most other cases, the possibility of an erroneous classification should be examined. Furthermore, the situation concerning f-values is not entirely satisfactory in B II. A notable example is the 2s2 ‘S-2s2p ‘PO resonance line, for which theoretical analyses yield f-values that are 0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

about ’ 20-30% higher than those obtained from beam-foil lifetime measurements [3-51. It has been suspected that the disagreement is largely due to the rather complicated cascade situation which makes decay curve analyses by curve-fitting methods quite difficult and thus calls for more elaborate analysis techniques such as the ANDC method of Curtis et al. [9]. Indeed the latter was successfully applied to the same resonance transition in Be-like N IV and 0 V [lo] and the results so obtained were in excellent accord with those of elaborate theoretical calculations. In an attempt to clarify such problems we have re-examined the spectrum of B II, using the beam-foil excitation method. Some spark spectra of B II have also been recorded which complement and extend the beam-foil results. We have finally also reinvestigated some beam-foil data registered earlier [2-41.

2. Experiment The beam-foil investigation was performed at the University of Arizona, using the 2 MV Van de Graaff accelerator. With BCl, in the rf ion source we obtained good and stable beams of B+ ions which were accelerated to 0.5, 0.75 or 1.0 MeV and directed through a carbon foil. The spectra (1200-2000 A) were recorded with a McPherson 235 0.5 m Seya-Namioka monochromator, equipped with an EMR 541F photomultiplier at the exit slit. We also measured the decay times for a number of spectral lines. The spark spectra were obtained at the University of Lund using a sliding spark light source and a 3 m normal-incidence vacuum spectrograph. The plate factor of this instrument was 2.77 A/mm in the first order.

IV. BEAM-FOIL

& SURFACE INTERACTIONS

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-c

595

S. Bashkrn et al./ Renewedsru~r oJB II

Our decay data for the 2p2 ‘S, 2s3d ‘D and 2p2 ‘P levels were analyzed with the DISCRETE program. The cascading effects could be handled in a comparatively straightforward way. Our results are presented in table 2. together with previously published experimental data. Only one set of theoretical results (by Weiss [6]) has been included for comparison. a detailed compilation of theoretical data being given in table 3. The present result for the 2s2p ‘P” level agrees with the early phase-shift measurement of Lawrence and Savage [15] but the uncertainty has now been significantly reduced. However. our lifetime is much shorter than the results of Martinson et al. [3] and Kernahan et al. [5]. We have compared our present decay data with those of ref. [3] and conclude that the difference is largely caused by a much better spatial resolution along the excited beam (and thus time resolution) than in the early experiment. Also, less powerful computer programs were used in [3]. For the other three terms our results agree with those obtained by Kernahan et al. [5]. In all cases agreement is good with the theoretical data of Weiss [6]. The j-values for An = 0 transitions, computed from our experimental lifetimes are compared to theoretical results in table 3. A large number of theoretical data (in addition to those already mentioned [6-S]) are available for B 11. Most modern calculations include the electron correlation effects in a detailed way. For a discussion of theoretical problems and results in this spectrum we recommend the comprehensive article by Day and Larson [ 161 and the recent review by Crossley [ 171. Of particular interest are the calculations of Sims

be complemented by additional work. preferably over a larger wavelength range. Furthermore it is not quite satisfactory that some transitions, for example those at 1604.5 and 1616.0 A, remain unclassified. 3.2.

Liferimes and/-i*alues

We measured the decay times of the 2s2p ‘P”, 2p2 ‘S, 2s3d ‘D and 2p2 ‘P terms in the present experiment. Particular emphasis was placed on the 2s2p ‘P” lifetime. in view of the problems discussed above. The decay curves were analyzed using the powerful multiexponential fitting program DISCRETE [12] and the ANDC method, by means of the computer program CANDY (131. Fig. 2 shows a decay curve of the 1362 A line in B II (2s’ ‘S-2s2p ‘P”) and the main cascading into 2s2p ‘P”. from the levels 2p2 ‘S, 2p2 ‘D and 2s3d ‘D. The measured decay curves for these levels (for 2p2 ‘S and 2s3d ‘D from the present experiment and for 2p2 ‘D from an unpublished investigation [14]) were used as inputs to the ANDC/CANDY study of the 2s2p ‘PO lifetime. Interestingly enough, the results obtained by DISCRETE and CANDY were quite similar in the present case whereas larger differences - with the CANDY analysis giving much better agreement with theory - were established for the 2s2p ‘P” term in N IV and 0 V [lo]. This result may indicate that cascading from 2p2 ‘S into 2s2p ‘P” (which is very difficult to correct for by multiexponential fitting techniques) is less significant in beam-foil studies of B 11 than in higher members of the Be I isoelectronic sequence.

102

1 0

I

I 20

I 60

I

I 60

Distance

I (ChMnel

I 80

I

I 100

I

I 120

140

number1

Fig. 2. Decay curve of the 2s2p ‘PO level in B II. The main cascading levels and transitions. included in the present analysis, are indicated.

also

IV. BEAM-FOIL & SURFACE INTERACTIONS

S. Bashkin et al. / Renewed study of B II

596 Table 2 Lifetimes

of excited levels in B II Wavelength

Lifetime (ns)

(A)

This work

Other experiments

Theory

2s2p ‘PO

1362

0.86 (7)

0.816

2p2 ‘s 2s3d ’ D 2p* 3P

1842 1230 1625

0.72 (5) 0.80 (7) 1.16 (6)

0.9 (2) b); 1.15 (10) c) 0.99 (5) d); 1.17 (7) e, 1.04 (8) =); 0.84 (5) e); 0.96 (6) ‘); 0.77 (4) ef 1.38 (10) ‘); 1.29 (6) ‘) 1.24 (7) e,

Level

a) b, ‘) d, ‘)

a)

0.738 0.71 1.14

Weiss [6]. Lawrence and Savage [15]. Martinson et al. [3]. Bromander [4]. Kernahan et al. [5].

and Whitten [18] which provide upper and lower bounds for the f-values of the 2s2 ‘S-2s2p ‘PO transition in Be I, C III and 0 V. Day and Larson 1161 interpolate these data for B II, obtaining f=0.984,0.010, in good agreement with the result of their own elaborate calculations. Table 3 shows that our new f-value for the 2s’ ‘S-2s2p ‘P“ line is in good agreement with most theoretical results [6-8; 16-251. The previous discrepancy has thus been removed. The number of theoretical results is somewhat smaller for the other transitions. Here also we agree with the theoretical data. In view of previous experiments for the Be I sequence, in particular Be I [27], N IV and 0 V [lo], our B II results are hardly surprising. However, they should be of value by

Table 3 Oscillator

strengths

We are grateful to A.W. Weiss for providing us with his unpublished B II data, for valuable advice and interesting discussions. The experimental material, provided by H.G. Berry and W.S. Bickel has been very valuable. We also acknowledge enlightening discussions with T.M. Luke, R.P. McEachran, A. &me and W.H. Parkinson. Two of the authors (H. v B. and I.M.) are grateful for the hospitality extended to them at the University of Arizona. Support from Deutsche Forschungsgemein-

in B II Wavelength

Transition

further demonstrating that the beam-foil method can provide reliable f-values also in intricate cascading situations and by illustrating the importance of combining atomic spectroscopy and lifetime measurements.

Oscillator

strength

(A)

This work

Theory

1PO

1362

0.98 (8)

2s2p ‘PO-2p2

‘s

1842

0.24 (2)

2s2p 3P0-2pz

3P

1624

0.34 (3)

1.02 a); 1.03 b); 0.942 c); 1.01 d); 1.00 et 1.01 O; 0.93 &I;0.985 ht; 1.01 “; 1.02 jt 1.14 kf; 1.08 0 0.230 @; 0.241 b); 0.231 d); 0.161 @; 0.21 h); 0.229 ‘); 0.236 ‘) 0.348 @; 0.330 s); 0.343 i); 0.363 ‘)

2s2 ‘S-2szp

a>Weiss (61. W N&&ides Cl dl Cl 0 8) h) it

jl kl 1)

et al. [19]. Banyard and Taylor [20]. Hibbert [21]. Stewart [22]. Moser et al. [23]. Victorov and Safronova [24]. Day and Larson [16]. Laughlin et al. 1251. Ganas and Green 1261. Markiewicz et al. [7]. Luke [8].

597

S. Bashkin et al. / Renewed study of B II

schaft (H.v.B.), the U.S. Department of Energy (S.B.), and the Swedish Natural Science Research Council (J.O.E. and I.M.) is also gratefully aeknowl~ged.

References [l] A. &ne, Physica Scripta 1 {1970) 256. (21 H.G. Berry and J.L. Subtil, Physica Scripta 9 (1974) 217. [3] 1. Martinson, W.S. Bickd and A. &me, J. Opt. Sot. Am. 60 (1970) 1213. [4] J. Bromander, Physica Scripta 4 (1971) 61. [5] J.A. Kemahan, E.H. Pinnington, A.E. Livingston and D.J.G. Irwin, Physica Scripta 12 (1975) 319. [6] A.W. Weiss, private communication (1973). [7] E. Markiewin, R.P. McEachran and M. Cohen, Physica Scripta 23 (1981) 828. [S] T.M. Luke, Physica Scripta 23 (1981) 1066. [9] L.J. Curtis, H.G. Berry and J. Bromander, Phys. Lett. 34A (1971) 169. [lo] L. Engstrijm, B. Denne, J.O. Ekberg, K.W. Jones, C. Jup&, U. Lit&n, Weng Tai Meng, A. Trigueiros and I. Martinson, Physica Scripta 24 (1981) 551. [ll] R.A. Roig and G. Tonddlo, J. Phys. B 9 (1976) 2373.

WI

S.W. Provencher, J. Chem. Phys. 64 (1976) 2772. 1131 L. EngstrGm, Nucl. Instr. and Meth. 202 (1982) 369. material (1973). 1141W.S. Bickel and I. Martinson, unpublish~ I151G.M. Lawrence and B.D. Savage, Phys. Rev. 141 (1966) 67. [I61O.W. Day and E.G. Larson, J. Quant. Spectrosc. Radiat. Transfer 17 (1977) 613. t171 R.J.S. Crossley, Physica Scripta T8 (1984) 117. WI J.S. Sims and R.C. Whitten, Phys. Rev. A8 (1973) 2220. 1191C.A. Nicolaides, D.R. Beck and 0. Sinanoglu, J. Phys. B 6 (1973) 62. WI K.E. Banyard and G.K. Taylor, Phys. Rev. A10 (1974) 1019. WI A. Hibbert, J. Phys. B 7 (1974) 1417. WI R.F. Stewart, J. Phys. B 8 (1975) 1. ~231 C.M. Moser, R.K. Nesbet and M.N. Gupta, Phys. Rev. Al3 (1976) 17. J. Quant. Spectrosc. I241 D.S. Victorov and U.I. Safronova, Radiat. Transfer 17 (1977) 605. and G.A. Victor, J. Phys. PI C. Laughlin, E.R. Constantinides B 11 (1978) 2243. 1261 P.S Ganas and A.E.s. Green, Phys. Rev. Al9 (1979) 2197. A. Gaupp and L.J. Curtis, J. Phys. B 7 i271 I. Martinson, (1974) L463.

IV. BEAM-FOIL

& SURFACE

INTERACTIONS