Beam-foil studies of energy levels and radiative lifetimes in Be I and Be II

NUCLEAR INSTRUMENTS

AND

METHODS

110

(I973) 51-54; © N O R T H - H O L L A N D P U B L I S H I N G CO.

BEAM-FOIL STUDIES OF ENERGY LEVELS AND RADIATIVE L I F E T I M E S IN Be I AND Be II* S. HONTZEASt, 1. MARTINSON, P. E R M A N and R. BUCHTA

Research Institute for Physics, 10405 Stockholm 50, Sweden We have investigated the beam-foil spectra of beryllium in order to classify new transitions belonging to the doubly-excited nonautoionizing quartet system of Be II or the displaced term system of Be I.

1. Introduction

ratorl). The region 500-1600 A was investigated with a 1-m normal incidence monochromator. Between 2000 and 6000 A a Heath 35 cm monochromator was used. More detailed descriptions of the experimental aspects are found in refs. 1 and 2.

We report a study of the beam-foil spectra of beryllium between 500 and 6000 A. The main emphasis was placed on the doubly-excited, non-autoionizing Be II terms, of the type ls2snl and ls2pn/4L, and the Be I displaced terms, ls22pn/1'3L. In a number of cases the spectral studies were combined with lifetime measurements.

3. Results Figs. 1-3 show examples of our spectra in different wavelength regions. In spite of our low beam energies a number of Be III transitions appeared in the spectra, but their low intensities precluded reliable lifetime measurements.

2. Experiment The measurements were made using 50-166 keV Be + and Be + + ions (1-5/tA) from an isotope sepa* Supported in part by the Swedish Natural Science Research Council (NFR). t Presented the paper. (Present address: University of Saskatchewan, Regina Campus, Regina, Canada.)

3.1. Be II The Be II doubly-excited quartet levels lie energetically above the Be II[ ground state. However, because of the A S = 0 selection rule, they are metastable against autoionization via the electrostatic interaction. So far only the 2s2p 4P-2p2 4 p multiplet, at 2324 A, has been reported in the literature2-4). As fig. 2 shows, this transition was strongly excited in our spectra. Using the calculations of Holeien and Geltman 5) and the available information about the Li I quartet terms 2, 6), we were able to identify several other spec-

"0

CO ,,4. CO

c o o

¢

m

co

.....

~

i

i

600

800

1000

1200 25100

Fig. 1. Beam-foil spectrum of beryllium between 550 and 1250 A. The incoming beam energy was 83 keV. The lines at 714 A, 865 A, 1020 A (see insert) and 1156 A are ascribed to the Be II quartet system.

2Z.O0

2300

2200

Fig. 2. Partial spectrum of beryllium between 2150 and 2500 A. The beam energy was 83 keV. Note the strong 2324 A line which belongs to doubly-excited Be II (2s2p 4p-2p2 4p).

51 I. O P T I C A L SPECTRA

52

s. HONTZEAS et al.

3197

3131

i

3110

3200

L

3180

I

3160

I

31~0

3120

3100

Fig. 3. The vicinity of the Be I1 resonance line, recorded at 83 keV. The lines at 3090 A, 3160 A and 3180 .& have not been reported earlier.

tral lines with the Be II quartet system. From the analogy with Li l, isoelectronic to Be 1I, we expected, e.g., the 2s3s 4S, 2s3d 4D, and 2p3d 4D levels of Be II to be strongly populated in the beam-foil light source. The insert of fig. 1 shows a line at I020 A which we assign to the 2s2p 4p-2s3s 4S combination. The calculated wavelength for this transition is 1023 A (ref. 5). The energies for the Be I| 2snd and 2pnd 4D states have not been calculated, but they can be approximately located because of their relatively small quantum defects. (The latter are typically 0.1 and smaller for Li I.) On the basis of such analyses we assign the lines at 865 and 714 A (fig. 1) to the 2s2p 4P-2s3d 4D and 2s2p 4p_ 2s4d 4D transitions, respectively. We also identified two decay modes of the 2p3d 4D term, to 2p24p (981 A) and to 2s3d 4D (at 3405 A). A few additional lines are also tentatively ascribed to the Be II quartet system. Table 1 summarizes the experimental and theoretical results for the doubly-excited Be II quartet system. Our suggested energy level diagram is displayed in fig. 4. All energies are normalized to Holoien and Geltman's s) calculated 2s2p 4p energy, 115.996 eV above the Be II ground state. The dotted horizontal lines in fig. 4 show the calculated level energies. Except for the odd 4p terms we note quite good agreement between this work and theoryS). Our work seems to indicate that the interaction between the 2snp 4p and 2pns 4p series is somewhat less pronounced than predicted by the calculations. In addition to the transitions given in table 1 we also observed lines at 3510.8+_0.5 A and 4330.2+_0.5 A the excitation functions of which were very similar to those of the classified Be II quartet transitions. Attempts to identify these lines have not yet been successful, however.

TABLE 1 Observed transitions in the doubly-excited quartet system of Be II.

Wavelength

Suggested combination

This work (A) 2324.6±0.3 1020.1 ± 1.0 865.3± 1.0 714.0±2 1155.9±1.0 1909.0± I b 981.4±1.0 3180.7±1.0 2273.5±0.8 2764.24- 1.0 3405.6±0.6

2s2p4P _ 2 p Z 4 p - 2s3s 4S - 2s3d4D - 2s4d 4D 2pZ4p - 2 p 3 s i p - 2s3p 4p -2p3daD 2s3s4S - 2 p 3 s 4 p 2s3p4p - 2 p 3 p a p - 2s4s 4S 2s3d 4D - 2p3d aD

Excitation energy (eV)a

Effective quantum number This work Theory5) n* 121.328 128.150 130.324 133.360 132.054 127.823 133.961 132.047 133.275 132.308 133.963

121.275 128.130

132.516 129.725 132.516 133.400 132.421

1.701 2.509 2.899 3.979 2.596 2.463 2.971 2.595 2.818 3.481 2.972

a All experimental energy values are adjusted to Holoien and Geltman's 5) calculated eigenvalue of the 2s2pip term, - 1.25785 Z 2 Ry, which corresponds to an energy of I 15.996eV above the Be II 2s 2S ground state. b Observed by Andersen et al.4) and Berry et al.2). 3.2. BeI Of the Be I displaced terms (2pnll"3L)which energetically lie between the Be II 2s 2S and 2p Zp levels, those fulfilling the relation l = L are expected to be stable against autoionization (because of parity requirements) whereas the l ~ L terms interact strongly with the continuumT). Most of our present information about the spectrum and energy levels of Be I, including the displaced term system originates from the accurate measurements of Johansson 8) and Holmstr6m and Johanssong). In

ENERGY LEVELS AND RADIATIVE LIFETIMES

53 TABLE 2

eV

2sns

2pns

~S

~po

2snp

2pnp

4pO

2snd

~p

~D

Transitions from the 2pnl configurations o f Be I, observed in this experiment.

2pnd

~D°

I/,(]

Wavelength"

Combination

Term value ( c m - 1)

2s3p 1p _ 2p3p 1p 2s4p ]p _ 2p3p 1p 2s4p 1p _ 2p4p 1p 2s5p 1p _ 2p5p 1p 2p 2 1D - 2p3d 1D 2s3d 1D - 2p3d 1D 2p 2 1D - 2p4d 1D 2s2p 3p _ 2p2 ap 2s3p 3p _ 2p3p 3p 2s4p 3p _ 2p4p 3p 2p2 3p _ 2p3d aD 2s3d 3D - 2p3d aD 2p2 3p - 2p4d 3D 2s4d aD - 2p4d 3D

18 004 18 005 8 933 5 374 13 731 13 731 7 466 47 429 15 108 7 963 12 936 12 936 7 128 7 141 d

(A)

Effective quantum number n*

135 /~

......

-......

3

/~._~.:~/~ ,-7--

3

.......

130

125

12oi

~

%

2

1151 Fig. 4. Suggested term diagram for the doubly-excited quartet levels in Be 11. All indicated transitions appear in beam-foil spectra. The calculated term energies are s h o w n by the dashed lines (see ref. 5). The 2s and 2p limits of Be III lie at 136.797 and 140.126 eV, respectively, above the Be I1 ground state.

ref. 8 an intense Be l line at 3455 ,~ was tentatively classified as the 2s2pXp-2p 2 iS combination. It was pointed out by Weiss1°), however, that the experimentally determined decay time for this line *'11) implies an anomalously low oscillator strength for this particular Be I transition. Later analyses by W e i s s 12'13) suggest a reclassification of the 3455 A singlet, to the 2s3plP-2p3p~P transition. This new interpretation was already confirmed by Berry et al. 2) who also observed the decay of 2p3p~P to 2s2p~P and 2s4plp. We have now obtained a more accurate value for the 2 s 4 p l p - 2 p 3 p l p wavelength, 4526.6_+0.3 •, which yields the same (to within l cm -1) 2p3plp term value as the 3455 A line. Table 2 summarizes our results for the 2p3p~P term and other Be I displaced terms. A previously unidentified Be I line at 3208 ,~ (c.f., e.g., ref. 8) arises from the 2s4p 1p_2p4plp transitions, according to the calculations of Victor14). This assignment is supported by our work, and we also observed the 2 s 5 p l p - 2 p 5 p l p combination, at 3160 A. Fig. 3 and table 2 also show

3455.183 4526.64-0.3 3208.600 3160.6-4-0.3 2738.050 3451.372 b 2337.0+ 1.0 2650.619 e 3019.526 e 3090.3 4-0.4 2898.254 e 3110.986 c 2480.6±0.5 31204- 1

2.4688 2.469 3.5048 4.519 2.8269 2.8269 3.834 1.5210 2.6950 3.712 2.9125 2.9125 3.924 3.920 a

a When error limits are not given, wavelengths have been taken from J o h a n s s o n ' s8) work. b Masked by the 3455/~ line in beam-foil spectra. e Wavelength o f the strongest component of the multiplet, c.f., ref. 8. d We estimate these values to be less accurate than those obtained from the 2480/~ line.

that we observed transitions from the previously unknown 2p4d 1D, 2p4p3p, and 2p4d3D terms. According to the calculations of Victor 14) and Weiss 12,13), the 2p 2 1S term lies above the Be II ground state and it should therefore be strongly autoionizing (14: L). Possible radiative transitions from 2p 2 ~S to 2s2plP lie in the wavelength interval 2793-2808 A (ref. 14). We made very careful spectral scans of this region but found no lines which could be ascribed to radiative decays of the 2p 2 ~S level. This shows that the latter decays mainly by electron emission, as expected from the calculated widthT). 3.3. LIFETIMES Examples of our measured lifetimes are given in table 3. Our remeasured lifetime for the 2p 2 4p term, 3.1 ns, is somewhat lower than the value given in ref. 2, but the agreement is better with the work of Andersen et al. 4) and the theoretical value calculated by Cowan15). Table 3 also includes our experimental results for some additional Be II quartet levels as well as for several Be I displaced terms. For the former no comparisons with theory are possible. The Be I 2p3plP lifetime has previously been deterI. O P T I C A L

SPECTRA

54

S. H O N T Z E A S et al. TABLE 3 Radiative lifetimes in Be II and Be I. Spectrum

a b e d e

Wavelength (A)

This work

II II 1I II II

714 865 981 1156 2324

2s2p 4p 2s2p 4p 2p2 4p 2p2 4p 2s2p 4p

_ 2s4d 4D _ 2s3d 4D _ 2p3d 4D _ 2p3s 4p _ 2pg 4p

I

2738

2p 2 1D - 2p3d 1D

I

2898

2p2 3p _ 2p3d aD

I

3019

2s3p zP - 2p3p ap

I I I I

3110 3160 3208 4526

2s3d 2s5p 2s4p 2s4p

3D1p _ 1p _ 1p _

2p3d 2p5p 2p4p 2p3p

aD 1p 1p 1p

1.3 4-0.2 0.79 4- 0.08 1.0 ± 0 . i 2.4 4-0.3 3.1 4-0.2 0.694-0.09 10.0 :t:2 1.7 4-0.2 2.3 4-0.2 6.0 4-0.7 1.6 4-0.2 5.3 ±0.5 3.9 -4-0.5 5.6 4-0.7

Lifetime of upper level (ns) Other experiments

Theory

3.24-0.2n; 4.14-0.2 b

2.61 °

2.64-0.2 a fl.5 4- 0.2b [6.94- 1.0b 6.8 +0.2 b

7"300

5.0±0.2a'e; 5.44-0.2 b'e

6.83 d

See ref. 4. See ref. 2. SeereL 15. See ref. 12. Measured from the 3455 A. line (2s3p 1P-2p3p Ip).

m i n e d f r o m d e c a y m e a s u r e m e n t s o f t h e 3455 A l i n e 2 ' 4 ' a l ) . O u r studies o f t h e 4526 A line ( 2 s 4 p l P 2 p 3 p ' P ) y i e l d e d essentially t h e s a m e value, w h i c h gives a d d i t i o n a l s u p p o r t to t h e reclassification o f t h e 3455 A line, discussed a b o v e . A n d e r s e n et al. 4) r e p o r t e d m a r k e d l y different d e c a y t i m e s f o r t h e Be I 2898 (2p 2 3 P - 2 p 3 d 3D) a n d 3110 A (2s3d 3 D - 2 p 3 d 3D) lines a n d q u e s t i o n e d t h e v a l i d i t y o f t h e s e s p e c t r a l assignmentsa). W e w e r e n o t a b l e to r e p r o d u c e t h e i r r e s u l t s ; i n s t e a d we o b t a i n e d v e r y s i m i l a r d e c a y c o n s t a n t s f o r b o t h m u l t i p l e t s (table 3). I n b o t h cases o u r d e c a y c u r v e s c o n t a i n e d a c a s c a d e o f 6 + 1 ns w h i c h is fairly close to t h e v a l u e 6.8 ns, previously obtained for the 3110A branch'). Our m e a s u r e d 2 p 3 d 1D lifetime, 0.69 + 0.09 ns, is s u r p r i s i n g l y short, w h e r e a s t h e c a s c a d e lifetime, 10.0_+ 2 ns, seems to give a m o r e r e a s o n a b l e e s t i m a t e o f the l i f e t i m e f o r this h i g h level. S i m i l a r results c a n be f o u n d f o r t h e 2 p 3 p 3p t e r m ; h e r e t h e c a s c a d e d e c a y t i m e 6 . 0 + 0 . 7 ns (table 3) is in g o o d a c c o r d w i t h t h e t h e o r e t i c a l value13). T h e o r i g i n o f t h e fast initial d e c a y s is u n k n o w n , in the absence of calculated spin-orbit autoionization rates. H o w e v e r , it w o u l d be s u r p r i s i n g if t h e l a t t e r w e r e so fast in n e u t r a l a t o m s .

A m o r e d e t a i l e d a c c o u n t o f this w o r k will a p p e a r in P h y s i c a Scripta.

References l) w. s. Bickel, I. Bergstr6m, R. Buchta, L. Lundin and I. Martinson, Phys. Rev. 178 (1969) 118. 2) H. G. Berry, J. Bromander, I. Martinson and R. Buchta, Phys. Scripta 3 (1971) 63. ~) I. Martinson, Nucl. Instr. and Meth. 90 (1970) 81. a) T. Andersen, K. A. Jessen and G. Sorensen, Phys. Rev. 188 (1969) 76. 5) E. Holoien and S. Geltman, Phys. Rev. 153 (1967) 81. 6) H. G. Berry, E. I4. Pinnington and J. L. Subtil, J. Opt. Soc. Am. 62 (1972) 767, and previous references quoted therein. 7) D. L. Moores, Proc. Soc. 91 (1967) 830. s) L. Johansson, Arkiv Fysik 23 (1962) 119. 9) j. E. Holmstr6m and L. Johansson, Arkiv Fysik 40 (1969) 133. lO) A. W. Weiss, Nucl. Instr. and Meth. 90 (1970) 121. 11) I. BergstrOm, J. Bromander, R. Buchta, L. Lundin and I. Martinson, Phys. Letters 28A (1969) 721. ~) A. W. Weiss, private communication (1970). A. W. Weiss, Phys. Rev. A6 (1972) 1261. 14) G. A. Victor, private communication (1972). 15) R. D. Cowan, private communication (1970).