Laser spectroscopic investigation of isotope shifts in Nd II lines

Spectrochimica Acta Part B 60 (2005) 447 – 453 www.elsevier.com/locate/sab

Laser spectroscopic investigation of isotope shifts in Nd II lines W. KoczorowskiT, E. Stachowska, B. Furmann, D. Stefan´ska, A. Jarosz, A. Krzykowski, A. Walaszyk, G. Szawiola, A. Buczek Poznan´ University of Technology, Faculty of Technical Physics, Chair of Atomic Physics, Nieszawska 13 B, 60-965 Poznan´, Poland Received 18 November 2004; accepted 11 February 2005 Available online 25 April 2005

Abstract The isotope shift of 11 optical transitions in Nd II in the spectral range 420–450 nm have been recorded (in all cases but one for all pairs of even Nd isotopes). For all observed transitions the values of the field shifts, the specific mass shifts and Djw(0)j2 have been evaluated. Using our new data, combined with data reported in earlier papers, term isotope shifts for all pairs of even Nd isotopes have been determined for 27 energy levels and for the configurations: 4f 46s, 4f 45d, 4f 35d6s and 4f 46p as well. It is shown that configuration assignment of a level on the basis of term shift values only is in some cases not satisfactory (as e.g. of the level at 22,696 cm1) and obviously additional information is required. D 2005 Elsevier B.V. All rights reserved. Keywords: Neodymium ion; Isotope shift; Laser spectroscopy

1. Introduction The investigation of isotope shift (IS) and hyperfine structure (HFS) in atomic spectra is an important tool to improve our precise knowledge of the designation of fine structure levels and the classification of new lines (see e.g. [1,2]), the structure of the nucleus, such as charge radius and distribution and also of the magnetic moment (and its distribution within the nucleus in cases of hyperfine anomalies). The complex structure of the spectral lines of the neodymium ion is caused by four factors: a large number of isotopes, small values of the IS, often very small HFS splittings for the odd isotopes and partially overlap of the HFS with the stronger components of the even isotopes. Investigations of the HFS are only possible with highresolution methods (see e.g. [3]). In Nd II IS of all even isotopes and HFS results for transitions starting from low-lying, thermally populated,

T Corresponding author. Tel.: +48 61 665 32 24; fax: +48 61 665 32 39. E-mail address: [email protected] (W. Koczorowski). 0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2005.02.010

levels can be found in the literature for 34 lines [2–9]. For these investigations well known laser spectroscopic methods at high resolution could be used. But for transitions starting from the higher levels, which are not accessible by usual laser spectroscopy, only one line has been investigated so far at high resolution [2]. Attempts were made using fast beam laser spectroscopy of the neodymium ion to investigate lines starting from metastable levels, but they failed [10]. This paper presents our IS results between all even isotopes for 11 transitions. The HFS of the odd isotopes could not be resolved. The transitions start from levels lying between 513 and 14,090 cm1, their wavelengths lay between 420 and 450 nm (wavelengths in air). The method used was laser induced fluorescence (LIF) in a hollow cathode (HC) discharge. Neodymium samples with natural isotope abundance were used. The presence of the components belonging to all the isotopes in the same spectrum makes determination of the sign of isotope shift unequivocal, since all the observed components can be identified beyond any doubt on the basis of the observed relative intensities, corresponding to their relative abundance in the sample. Information about the structure of spectral lines, consisting of the components corresponding to all naturally occurring

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isotopes, can be very interesting for astronomers, especially in the case of elements abundant in the stars. This is one of the more important markers for the chemically peculiar (CP) stars [11,12]. The IS available in the literature and the IS results presented in this paper have been evaluated for individual energy levels with respect to the 4f 45d 6K 9/2 level at 6005 cm1, which has been chosen as reference level (see e.g. [1]).

2. Theoretical background The theory of IS is well known and was thoroughly described e.g. in the monograph by King [13]. We limit therefore ourselves to the definitions and formulae which are used in this work. Isotope shift is the small difference in the transition energy of electrons for different isotopes. It is the sum of three effects: normal mass shift (NMS), specific mass shift (SMS) and field shift (FS): IS ¼ NMS þ SMS þ FS:

ð1Þ

In a King plot the RIS is used, multiplied by a modifying factor lA1 ;A2 ¼

ðAs1  As2 Þ A1 A2 ðA1  A2 Þ As1 As2

ð5Þ

where A s1 and A s2 are from an arbitrary chosen pair of isotopes, the standard isotope pair (in neodymium the isotopes 146 and 144). In the case that the ybr 2N values are known (from calculations or other experiments) the modified RIS (ARIS) are plotted versus modified ybr 2N for all investigated isotope pairs. Then the FS and the SMS values can easily be determined from a linear regression. The value of the energy term isotope shift for a particular level (A 1A 2DT) can easily be calculated from the IS of the line if the value DT for the other level involved in this transition is known, or when the other level is the reference level. The DT values are e.g. helpful for the determination of the dominant configuration of a particular level, but in the case of levels which show particularly strong configuration mixing they are not sufficient for the determination of the configuration of this level.

The normal mass shift is given by the formula: NMSA1 ; A2 ¼

3. Experiment

1 A1  A2 m; 1822:9 A1 A2

ð2Þ

where A 1 and A 2 denote the mass numbers of the isotopes (A 1NA 2) and m is the transition frequency. The field shift can be written as follows: FSA1 ;A2 ¼  pDjwð0Þj2

a30 f ðZ Þybr2 NA1 ;A2 Z

¼ Fi ybr2 NA1 ;A2 ;

ð3Þ

with i: the transition, a 0: the Bohr radius, ybr 2N: the change of the mean square charge radius between the isotopes and f(Z): the relativistic correction factor (in the case of Nd II f(60)=17.13 GHz/fm2 [14]). Djw(0)j2 is the difference of electron density at the nucleus between the lower and the upper level of the transition [13]. From the FS and Djw(0)j2 values conclusions can be drawn about the electronic states involved in the transition. This approach neglects the contributions of higher nuclear moments. They amount to about 4.4% for neodymium (see Fig. 6.3 in [15]). The SMS effect describes the correlation of motion between the electrons in the electronic shells of the atom. This can not be as easily calculated as the NMS. It can, however, be determined from a King plot [13], when the FS is known. Furthermore a so-called residual isotope shift (RIS) is used: RIS ¼ IS  NMS ¼ Fi ybr2 NA1 ;A2 þ Mi

A1  A2 A1 A2

ð4Þ

In this work our investigations of the IS of the Nd II spectrum using the laser induced fluorescence method in a HC discharge are presented. The scheme of the experimental setup is shown in Fig. 1. The Nd II target ions for LIF are produced in a HC. It consists of a copper cylinder with an axial bore. Neodymium is placed inside the cathode in the form of a rod (a sample with natural abundance of isotopes), with an axial bore of 7 mm diameter. The cathode is then screwed in the centre of a metal tube. On both sides of the HC two ring shaped aluminium anodes are mounted. The anodes are electrically insulated from the metal tube (and cathode) by ceramic pieces. The metal tube is sealed on both sides to glass tubes, which are closed with glass windows. All parts of the source are cooled by liquid nitrogen. The whole device is connected to a simple liquid nitrogen-cooled sorption pump, backed by a rotary vacuum pump. Ultimate pressure of this system is better than 104 mbar. The HC is filled with argon as a buffer gas at a pressure of 0.5 mbar. The discharge current used in the present experiment ranged from 10 to 100 mA. The laser beam for the excitation of the ions was generated by a modified tunable ring dye laser (Coherent CR 699-21), operating with Stilbene 3 and optically pumped by an argon ion laser (Spectra Physics 2085-3.0). The value of the output power of the laser was limited by two factors: the position of the wavelength of the transition on the gain curve of the dye and the degree of degradation of the dye solution. The laser power used ranged from 10 to 100 mW. The laser beam, intensity-modulated at a frequency of ca.

W. Koczorowski et al. / Spectrochimica Acta Part B 60 (2005) 447–453

449

Fig. 1. Experimental setup used for the investigations of isotope shifts in Nd II.

0.7 kHz by a mechanical chopper (in order to allow lock-in detection) and collimated by a telescope system, passed twice through the hollow cathode discharge: directly and after reflection from a mirror placed behind the HC. The reflecting mirror has been used to increase the light intensity in the HC. The laser beam was not focused in the discharge area, but on the reflecting mirror to return the laser light through the hole in the entrance mirror to avoid the laser light to enter the monochromator. Under our excitation conditions and with the laser power used, beam saturation and saturation broadening were not observed. The frequency change of the laser, while scanning across the spectral line, was measured with the help of frequency markers of a confocal Fabry-Perot interferometer (free spectral range 149.98(04) MHz, as measured by Krzykow-

ski et al. [16]). The LIF signal has been recorded by a photomultiplier using a monochromator as a filter. Simultaneously an optogalvanic signal could be detected. Fig. 2 gives a typical example of one run, while typically 15 to 20 runs have been made per line.

4. Results In our investigations we were able to measure the IS of all pairs of even Nd isotopes simultaneously, with the use of samples with the natural isotope abundances: 142:27%, 143:12%, 144:24%, 145:8%, 146:17%, 148:6%, 150:6%. Since intensity and splitting of the HFS-components of both odd isotopes, 143Nd and 145Nd, are small for the lines

Fig. 2. Signal of the line k423.985 nm, recorded from one scan.

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Table 1 Energy levels of the neodymium ion involved in the transitions investigated k (in air) nm

Wavenumber, cm1

Lower level, cm1

J lower

Upper level, cm1

J upper

422.772

23,646.780

15/2

23,616.425

423.985

23,579.258

426.184

23,457.470

428.451

23,333.346

430.443

23,225.325

431.334

23,177.355

431.436

23,171.870

432.793

23,099.230

27448.710 4f 46p 33670.620 4f 35d6p 6L 33670.620 4f 35d6p 6L 35485.930 4f 35d6p 6M 28418.970 4f 46p 6K 35485.930 4f 35d6p 6M 37267.495 4f 35d6p 6L 35503.215 4f 35d6p 6L 27611.725

13/2

423.315

445.639

22,433.390

450.658

22,183.520

3801.917 4f 46s 6I 10054.195 4f 35d6s 6L 10091.360 4f 35d6s 6K 12028.460 4f 35d6s 6L 5085.619 4f 46s 6I 12260.605 4f 35d6s 6L 14090.140 4f 35d6s 4K 12331.345 4f 35d6s 6K 4512.481 4f 46s 6I 5985.580 4f 46s 6I 513.322 4f 46s 6I

11/2 9/2 13/2 17/2 15/2 13/2 11/2 13/2 15/2 9/2

28418.970 4f 46p 6K 22696.885 4f 35d 2

11/2 11/2 15/2 17/2 15/2 15/2 13/2 15/2 17/2 11/2

Energy values and configurations of levels are from [1,17,18].

studied, the HFS of the odd isotopes has not been determined, but their influence on the components of neighbouring even isotopes has been taken into account. The obtained spectra have been analysed by the commercial program package Origin (Microcal). In our case each IS component has been represented by a Voigt profile and the following parameters have been fitted: centre position, half width and intensity. The shape of the line depends on the discharge current. Since the components belonging to various isotopes in most cases strongly overlap, small values of the discharge current (about 10 to 40 mA) were to be preferred, in order to decrease Doppler broadening. However, if a line was very weak, the current had to be increased to relatively large values, thus impairing the resolution. The Doppler broadening, depending on current, ranged in our experiment from 0.6 to 1.4 GHz. No influence of Stark shift has been observed, the positions of particular spectral line components did not change with current, which is typical for the HC discharge. The Doppler effect is dominating the line broadening, which shows from the good fit of the experimental shape of the spectral line components with a Voigt profile. The discharge has not been stabilised, resulting in a significant noise level (a typical example of a recorded line is shown in Fig. 2). For this reason each line has been recorded 15 to 20 times. Each scan has been fitted independently and the values determined from all scans have been compared to check the reproducibility. The values of IS presented are the average values and the errors quoted are statistical ones. In the case of isotope shift between abundant isotopes (142, 144, 146) we obtained a precision of 20 to 80 MHz. Our values,

measured with a natural mixture of isotopes, were at least of the same precision as previous measurements with enriched samples. But the accuracy where less abundant isotopes (148, 150) were involved could be as high as 400 MHz. In Table 1 the transitions and energy levels of Nd II investigated in this work are given. All IS values determined are shown in Table 2. In earlier investigations IS were only observed for low-lying levels, and only for one pair of isotopes, using enriched samples. In this work for the first time excitation from the configuration 4f 35d6s has been investigated for all even isotope pairs of Nd II, using the LIF method. The consistency between our IS values with older results has been checked for two additional lines, 430.357 nm and 431.452 nm, previously measured by Deckwer [5] with saturation spectroscopy of a gas discharge. The results agree very well. The quality of the consistency between the measured IS has been checked by King plots of one line versus all other lines. In order to determine FS and Djw(0)j2 by means of a King plot (an example is shown in Fig. 3), the values of ybr 2N have been taken from [8]. These values were based upon precision optical measurements and are for 144–142:0.269(8) fm2, 146–144:0.259(10) fm2, 148– 146:0.278(7) fm2 and 150–148:0.403(7) fm2. All values of FS, SMS and Djw(0)j2 determined in this work are presented in Table 3. The calculated NMS values range from 33 to 38 MHz for all lines and isotope pairs given in Table 3 (where A 1A 2=2). On the basis of the data in the literature [2,4–9], combined with the new results obtained in this work, isotope shifts, (DT), for individual energy levels could be evaluated; the results obtained for 14 even levels and 13 odd levels are presented in Table 4. The value 150,144DT=0 for the pure configuration 4f 45d has been taken from earlier work [1,17]. The levels of this configuration are expected to

Table 2 Isotope shifts between even isotopes of Nd II in GHz for the spectral lines investigated 144

146

148

150

k (in air), nm

IS

142

144

146

148

422.772 423.315a 423.985 426.184a 428.451 430.443 431.334 431.436 432.793 445.639 450.658

0.68(8) 2.640(40) 2.619(21) 1.03(7)b 1.443(17)b,c 1.957(35) 1.22(8) 1.428(26) 0.82(8) 1.390(40) 0.75(6)b

0.68(10) 2.380(40) 2.352(23) 0.96(8) 1.352(21)c 1.828(45) 1.12(8) 1.374(33) 0.74(7) 1.306(34) 0.75(6)

0.72(18) 2.84(10) 2.732(52) 0.99(12) 1.44(3) 2.05(5) 1.16(12) 1.439(50) 0.80(30) 1.512(52) 0.76(8)

0.85(15) 4.01(15) 3.85(10) 1.20(12) 1.95(7) – 1.86(17) 2.02(8) 0.90(40) 1.619(44) 0.78(10)

Nd– Nd

IS

Nd– Nd

IS

Nd– Nd

IS

Nd– Nd

a Values of IS 150Nd–144Nd observed by Blaise et al. [1] are: 423.315 nm: 9.11(15) GHz, 426.184 nm: 3.57(15) GHz. b Values of IS 144Nd–142Nd observed by Ahmad and Saksena [20] are: 426.184 nm: 0.90(6) GHz, 428.451 nm: 1.32(6) GHz, 450.658 nm: 0.81(6) GHz. c Values of IS in line 428.451 nm: 144Nd–142Nd: 1.412(30) GHz and 146 Nd–144Nd: 1.370 (30) GHz observed by Nfldeke [4].

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451

Fig. 3. King plot (ARIS against modified ybr 2N) for the line k423.985 nm. For the explanation of bmodifiedQ see Eq. (5); the standard isotope pair is 146–144. The uncertainties in the modified ybr 2N are taken from Ma et al. [8] and ARIS errors are related to the values of the IS uncertainties as shown in the Table 2.

have a negligible isotope shift. All DT values listed in Table 4 have been calculated relative to the level 4f 45d 6K 9/2 at 6005 cm1, assuming a value of 150,142DT=0, as in earlier work [1,17]. The estimated errors of the DT values for the levels belonging to the configurations 4f 46s and 4f 35d 6s do in general not exceed 20%. In the case of the remaining configurations the errors may be larger. This earlier work [1,17] gives values of DT, based only upon single isotope pair measurements using enriched samples. Our results concerning the same isotope pairs are consistent with those previously published values [1,17]. From the data concerning pure levels only, isotope shifts for pure configurations have been calculated, again taking the configuration 4f 45d as reference (Table 5). Low-lying levels of the configuration 4f 46s, given in Table 4, have been found to be pure [18]. All have been used in the calculations of the value of DT(4f 46s). For this configuration statistical uncertainties have been evaluated (see

Table 5). For the levels of the configurations 4f 35d6s and 4f 46p strong configuration mixing effects appear. Among the levels under study only one level of each configuration can be assumed to be a pure level: 4f 35d6s 4I 9/2 at 23171 cm1 [1,19] and 4f 46p 6K 19/2 at 30002 cm1 [2,17]. Our results for 150,144DT, using these levels, are in good agreement with those given by Nakhate et al. [17]. Therefore we decided to derive the DT values for the pure configurations for each pair of even isotopes in the same way. These values were never obtained before. They are preliminary ones. The level 4I 11/2 at 22696.885 cm1 was by Blaise [1] attributed to the configuration 4f 35d 2 on the basis of its DT value for one isotope pair: 144–150. In our studies this level has been observed in the transition 6 I 9/2 (513.322 cm1)Y4I 11/2 (22696.885 cm1), the former level being without doubt attributed to the configuration 4f 46s. We have determined DT values for all even isotope pairs; the

Table 3 Values of FS between even isotopes of Nd II for the spectral lines investigated, SMS for the standard isotope pair, 146–144, F i -values, Djw(0)j2 and the correlation coefficient R from the King plot k air, nm 422.772 423.315 423.985 426.184 428.451 430.443 431.334 431.436 432.793 445.639 450.658

FS, GHz

SMS, GHz

144–142

146–144

148–146

150–148

146–144

0.38(7) 3.0(6) 2.8(7) 0.47(6) 1.1(21) 2.6(5) 1.34(18) 1.252(32) 0.3(2) 0.60(20) 0.14(7)

0.36(6) 2.8(6) 2.6(6) 0.44(6) 1.0(2) 2.5(5) 1.26(16) 1.169(30) 0.3(2) 0.58(17) 0.13(6)

0.37(6) 4.0(6) 2.7(6) 0.46(6) 1.1(2) 2.6(5) 1.32(17) 1.225(31) 0.3(2) 0.60(18) 0.14(6)

0.53(9) 4.2(8) 3.8(9) 0.65(9) 1.5(3) – 1.86(24) 1.728(44) 0.4(3) 0.85(25) 0.20(9)

0.35(7) 0.3(6) 0.1(6) 0.58(6) 0.4(2) 0.6(5) 0.09(18) 0.239(32) 0.5(2) 0.81(20) 0.64(7)

F i , GHz/fm2

Djw(0)j2 a 3 0

R

1.38(22) 11.0(2.1) 10.0(2.2) 1.69(22) 4.0(8) 9.5(1.8) 4.85(62) 4.51(12) 1.0(8) 2.2(7) 0.52(22)

1.53(7) 12.1(5) 11.0(7) 1.9(1) 4.4(2) 10.6(3) 5.35(24) 4.98(24) 1.11(4) 2.44(9) 0.567(24)

0.976 0.966 0.956 0.984 0.968 0.983 0.984 0.999 0.689 0.923 0.858

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Table 4 Isotope shifts (DT) for individual levels; the level 4f 45d 6K 9/2 at 6005.270 cm1 has been taken as a reference Level energy, cm1

4

144–142

146–144

148–146

150–148

DT, GHz

Designation [1,17,18] 6

DT, GHz

DT, GHz

DT, GHz

0.000 513.330 1470.105 2585.460 3801.930 5085.640 1650.205 3066.755 4512.495 5985.580 4437.560 5487.655 6005.270 6931.800

4f 6s I 7/2 4f 46s 6I 9/2 4f 46s 6I 11/2 4f 46s 6I 13/2 4f 46s 6I 15/2 4f 46s 6I 17/2 4f 46s 4I 9/2 4f 46s 4I 11/2 4f 46s 4I 13/2 4f 46s 4I 15/2 4f 45d 6L 11/2 4f 45d 6L 13/2 4f 45d 6K 9/2 4f 45d 6K 11/2

2.14(4) 2.12(4) 2.14(6) 2.2(2) 2.2(4) 2.1(6) 2.11(4) 2.1(2) 2.1(4) 2.1(6) 0.05(4) 0.1(2) 0.0 0.1(2)

1.96(4) 1.95(4) 1.97(4) 2.0(2) 1.9(4) 1.9(6) 1.92(5) 1.9(2) 1.9(4) 1.9(6) 0.02(5) 0.0(2) 0.0 0.0(2)

2.23(6) 2.21(6) 2.19(6) 2.3(2) 2.4(4) 2.5(6) 2.15(6) 2.3(3) 2.3(4) 2.6(6) 0.05(6) 0.1(1) 0.0 0.1(2)

3.1(1) 3.1(1) 3.1(2) 3.1(2) 3.1(4) 3.1(6) 3.04(6) 3.0(3) 3.0(5) 2.7(6) 0.1(1) 0.2(1) 0.0 0.1(2)

23,171.110 22,696.885 27,611.725 27,448.710 23,229.980 24,321.245 23,537.380 24,134.080 24,445.380 25,524.470 26,912.765 28,418.970 30,002.310

4f 35d6s 4f 35d 211/2

3.39(4) 1.35(9) 1.3(2) 1.5(3) 1.22(2) 1.5(2) 1.39(3) 1.3(1) 1.65(8) 0.7(2) 0.6(4) 0.7(5) 0.3(3)

3.10(4) 1.20(9) 1.1(3) 1.4(5) 1.09(3) 1.3(2) 1.25(5) 1.1(1) 1.3(1) 0.5(2) 0.5(4) 0.6(5) 0.3(4)

3.57(4) 1.45(15) 1.5(7) 1.6(5) 1.29(5) 1.6(2) 1.51(5) 1.4(1) 1.7(1) 0.8(2) 0.7(4) 1.1(6) 0.6(6)

5.1(1) 2.28(15) 2.2(9) 2.3(5) 1.94(6) 2.3(2) 2.3(1) 2.1(1) 2.5(2) 1.1(2) 1.0(4) 1.1(6) 0.6(6)

15/2

4f 46p 13/2 4f 46p 6K 9/2 9/2

4f 35d 29/2 11/2

4f 46p 6K 11/2 4f 46p 6K 13/2 4f 46p 6K 15/2 4f 46p 6K 17/2 4f 46p 6K 19/2

value observed for the isotope pair 144–150 is in very good agreement with the one given by Blaise [1]. If this level 4I 11/2 (22696.885 cm1) were to be assigned as a pure one, belonging only to the configuration 4f 35d 2, this transition should be a two-electron excitation. The intensity expected for this kind of transition is rather low; moreover, the expected values of Djw(0)j2 for such a transition, involving an s-electron, are rather high, of the order of 10 a 03. The experimentally observed transition intensity resembles on the contrary closely the values observed for transitions of the type 4f 46sY4f 46p. The observed values of Djw(0)j2, determined on the basis of the King plots using the results for all the isotope pairs, are ca. 0.5 a 03, suggesting a transition not involving the change of an s-electron. In conclusion, we suggest that this level contains significant admixtures of the configurations 4f 46p (which might explain the observed high intensity of the transition) and 4f 35d6s (which accounts for the small value of the parameter Table 5 Isotope shifts for pure configurations (only pure levels have been taken into account) Configuration 4f 46s 4f 45d 4f 35d6s 4f 46p

144–142

146–144

148–146

150–148

GHz

GHz

GHz

GHz

2.13(11) 0 3.4 0.3

1.93(11) 0 3.1 0.3

2.3(4) 0 3.6 0.6

3.03(34) 0 5.1 0.6

DT,

DT,

DT,

DT,

Djw(0)j2). The DT values for the former configuration are much larger than experimentally observed for this level, while those for the latter configuration are much smaller, thus the mixture of both might result in the proper value, corresponding to the configuration 4f 35d 2. We do not exclude the possibility, that this configuration is also present in the composition of the level 4I 11/2 (22696.885 cm1). The proportions of individual configurations cannot be easily determined in an unequivocal manner; further investigations might be very helpful. Our remaining results have confirmed the results and classifications of earlier work [1,17,18].

5. Conclusions For 11 lines in the spectral range of 420–450 nm the values of the IS between all even isotopes have been obtained for the first time. From the King plots the values of the FS and the SMS have been determined and from the values of the FS and literature values from [8] the Djw(0)j2 values have been evaluated. Excitation from high lying levels of the configuration 4f 35d6s has been investigated for the first time by the LIF method. Term isotope shifts for individual energy levels (14 even and 13 odd levels) have been obtained on the basis of the available literature data [2,4–9] and our new results. From the data concerning only pure levels isotope shifts for pure configurations have been

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derived. Our results confirm most of the earlier HFS results and level classifications [1,17,18]. In the case of the level at 22,696.885 cm1 we suggest to verify our assignment as a mixture of three configurations using a very high resolution spectroscopic method (e.g. saturation spectroscopy) or by investigating other lines involving this level. We find that the values of isotope shifts for single isotope pairs do not always provide a sufficient basis for the determination of the spectroscopic designation of energy levels, but measurements of the IS for many isotope pairs of an element, as presented in this work, may give further information.

Acknowledgments The authors wish to thank Prof. Jerzy Dembczyn´ski for many fruitful discussions. Special gratitude is due to Prof. Klaus Heilig of the University of Hannover for his valuable hints and remarks concerning the interpretation of the results as well as the editorial side of our manuscript. The present work has been supported by the project of the Poznan´ University of Technology BW 63-025/05.

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