Atoms and Nuclei

Z. Phys. A - A t o m s and Nuclei 299, 1 1 - 1 3 (1981)

Zeitschrift for Physik A

9 Springer-Verlag 1981

Hyperfine Structures and Isotope Shifts of the 5 d 4 D 7 / 2 - ~ 6 p in Xenon Ions

4Pg/2 Transition

G. Borghs, P. De Bisschop*, R.E. Silverans**, M. Van Hove, and J.M. Van den Cruyce Instituut voor Kern- en Stralingsfysika, Katholieke Universiteit Leuven, Belgium Received September 30, 1980; revised version November 27, 1980 Collinear fast beam-laser spectroscopy has been performed on metastable 5d 4D7/2Xenon ions. Hyperfine structure constants for the 6p4Ps~2 level have been derived for 129Xe: A = -1,634.9 _+0.9 MHz and 131Xe: A=485.3_+0.3MHz and B = - l l 6 . 5 _ + 2 . 0 M H z . Changes in mean squared nuclear charge radii are derived from the measured isotope shifts.

1. Introduction Partial information about the hyperfine structures and isotope shifts of the 5d4DT/2-+6p4ps~2 transition in stable Xe ions has been obtained in recent years by ion beam-laser experiments in a crossed beam geometry [-1-3]. For the 5d4DT/2level, the h.f.s. constants were measured with high precision by r.f. magnetic resonance [2], a technique applicable on ground or metastable states only. The accuracy of the reported h.f.s, constants for the 6p4P~2 level was 'limited by the resolution (experimental line widths >130MHz), and isotope shifts of low abundant xenon isotopes are missing. In this paper we report on improved hyperfine structure and isotope shift measurements of this spectral line by collinear laser spectroscopy [4], repeating here the experiment of Meier et al. [5] on xenon ions with improved accuracy and sensitivity.

2. Experiment The experimental arrangement is shown schematically in Fig. 1. Fast beams of all stable xenon ions are produced by the Leuven Isotope Separator equipped with a plasma type ionization source. Stable operating conditions were obtained for ion beam intensities up to a few nA for 124Xe, so about 1 pA of the least * Aspirant N.F.W.O. ** Bevoegdverklaard Navorser N.F.W.O.

........~--~--

J

--

............. _,IN/-. ---~'~1~"2~

--

......... -- --~"-

J

seporo or

window~' ~ lenses*filter dye loser [ ~ photoncounter

[~

Fig. 1. Experimental set-up

abundant xenon isotope was available for the experiment as roughly 0.1% of the ions leave the ion source in the required 5d4D7/2state. The metastable ions were selectively excited to the 6p4ps~2 level by the light of a tunable single frequency dye-laser (605.1nm, A v_~10MHz) and the fluorescent photons of the decay to the 6s4Ps/2 level were counted as a function of laser frequency. The laser scans were frequency calibrated by a 150MHz FSR Fabry-Perot etalon. To eliminate laser stray light, an interference filter (At . . . . . =529nm, A2 = 1 nm) was placed in front of the photomultiplier, so the only background left was due to the dark current of the photomultiplier operating at room temperature (30 c.s-1). Sporadic jumps in the acceleration voltage have been cancelled by repeatedly measuring pairs of Xe isotopes within a short time interval. The obtained spectral linewidth for the 5d 4D7/2~6P 4P~2 transition is 60 MHz, which is less than half the widths obtained in the earlier xenon

0340-2193/81/0299/0011/$01.00

12

G. Borghs et al.: Hyperfine Structures and Isotope Shifts in Xenon Ions

experiments. The difference with the natural linewidth (-~21 MHz) is mainly due to the big velocity spread in the plasma type ion source. This is concluded from a comparison with our previous Ba experiment under identical conditions except the use of a surface ionization source which g a v e : /eexpt - F,, t ~- 15 MHz.

Table 1. Hyperfine structure constantz [MHz] of the 6p 4P5/2 Xe II

3. Results

compared with previous work. The ratio of the magnetic hyperfine constants for 129Xe and 131Xe, compared to the known ratio of the magnetic moments [6], does not reveal a hyperfine anomaly.

3.1. Hyperfine Structure Constants The hyperfine interaction splits the 5d4D7/2 and 6p4Ps~2 levels into doublets in 129Xe(I=1/2) and quartets in 13 ~Xe (I = 3/2). As an example, the experimental hyperfine structure of the 5d4D7/2 ~ 6 P 4P5/2 transition in ~3~Xe is shown in Fig. 2. The transition line for 129Xe is only split into three h.f. components, due to the absence of electric quadrupole interaction. The analysis of these hyperfine structures were made in two steps. First, the h.f.s, constants for both levels have been determined from our experimental data only. The results for the 5d 40,7/2 level were consistent with those of the laser-fluorescence ion-beam magnetic resonance experiment [2]. In a second step, we introduced the very accurate data from the latter in our analysis to extract again the h.f.s, constants for the 6p 4P5~2 level. This procedure led to results, still in agreement with our first analysis but with improved accuracy. In Table 1, our results are presented and 131Xe ]z 5d 4DTt;~ 6p 4Psi2 I605nrn)

l

Intensity [orbitr.]

LL

level

A~29 A~31 B131

W. Ontario [1]

Uppsala [3]

-1,667 -+18.7 487.5-+ 5.2 - 107.2_+52.3

-1,641 _+9 487.3 -+0.5 - 126 -+6

Leuven -1,634.9+0.9 485.3-+0.3 - 116.5-+2.0

3.2. Isotope Shifts In a series of scans, total shifts between all pairs of stable xenon isotopes have been measured, including the low abundant 124'126Xe isotopes. The ion velocities were calculated using the calibration of the acceleration voltage described earlier [7]. Total shifts minus doppler shifts resulted in the isotope shifts, given in the second column of Table 2. A comparison of our results for 12SXe up to a36Xe with earlier experiments shows reasonable agreement with [3] but disagreement with [1]. In the second part of Table 2, these earlier results are presented as quoted in [1] and [3]. F r o m King plots of our isotope shifts with those of several transition lines in the atomic spectrum of Xe [8], the electronic factor as well as the mass shift factor for the 5d4D7/2~6p4Ps~ transition were obtained: (notation according to Heilig and Steudel [9]) Ei=0.068_+0.008

and

M~=-828+17GHz.

The quoted errors do not include uncertainties in the calculation of E and M factors of the XeI transition lines [8]. One of the King plots used is given in Fig. 3. Only pairs of isotopes, quoted in [7], have been used. The corresponding isotope shifts, includ-

-r

-~

m

CO

O4

04

5

150MHz

Fig. 2. Hyperfine structure of the 5d 4D7/z---'6P*P5~2transition of 131XeII

Table 2. Isotope shifts of the

A,A'

5d407/2---~6p4ps/2transition in XeII

I~]A'A' [MHz] A,A' Leuven

124,136 126,136 128,136 129,136 130,136 131,136 132,136 134,136

-476.4+3.4 -392.2_+3.4 -311.3_+3.3 -253.4_+3.3 -231.6_+3.0 -160.5-+3.6 -139.8_+2.8 - 58.4_+2.6

128,132 129,132 130,132 131,132 134,132 136,132

(~vA'A' [MHz] W. Ontario

Uppsala

- 66_+20 - 91_+20

-162_+4 - 99_+4 - 80_+3 - 12_+3 82-+3 145_+4

- 15_+20 +130-t-20 +135_+20

G. Borghs et al.: Hyperfine Structures and Isotope Shifts in Xenon Ions

% -,,so

-I~

5v ~',i, A.~A x I0-/" MHz

-so XeI 979,9 nm

13

ing the error bars, from our data are deduced from Table 2. Changes in mean squared nuclear charge radii, derived from the relation [9]: ~r

._,0"@~

= E i f (Z) b AA'

jr_ Mi(A, _ A)/A A'

are given in Table 3; the errors refer only to the statistical uncertainty on the isotope shifts. A graphical representation (Fig. 4) shows the main features i.e., the slow decrease of (r e) with decreasing neutron nmnber, the odd-even staggering and the closed shell effect. A very similar behaviour has been reported earlier for the isotonic Ba and Cs nuclei [7-13]. Financial support of the I.I.K.W. is acknowledged.

Fig. 3. King plot of the 605.1nm transition in XeII vs. the 979.9 nm transition in Xe I

References 0

I

I

I

[

I /

I

/

/\ /

'~ /'X/

F/9

Y

-15 70

I

72

I

74

i

76

I

I

78

80

82

N

Fig. 4. Changes in mean squared nuclear charge radii of the stable xenon isotopes relative to 136Xe

Table 3. Changes in mean squared nuclear charge radii of the Xe isotopes relative to J36Xe A

cS(rZ>A'136 [10 3fmZ]

124 126 128 129 130 131 132 134

148.1 _+4.5 119.3 --+4.4 90.4--+4.3 106.4-+4.6 64.4 _+3.9 93.6-+4.7 58.2--+3.6 42.2+_3.4

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