Measurements of the odd-even effect in the mass spectrometric analysis of Ti and

Os isotopes by resonance ionization mass spectrometry have been performed for ΔJ = + 1, 0 and -1 transitions. Under saturating conditions of the ionization and for ΔJ = + 1 transitions odd-even

effects are reduced below the 0.5% level. Depending on the polarization state of the laser large

odd isotope enrichments are observed for ΔJ = 0 and -1 transitions which can be reduced below

the 0.5% level by depolarization of the laser field

Blake, G. A.

Hutcheon, I. D.

Wasserburg, G. J.

Wunderlich, R. K.

English

Caltech Authors - Main

lnst. Phys. Conf Ser. No 128: Section 3 127 
Paper presented at RIS 92, Santa Fe, NM, USA, 24-29 May 1992 
Systematics of the odd-even effect in the resonance ionization of Os and Ti 
R. K. Wunderlich*, G. J. Wasserburg, I. D. Hutcheon and G. A. Blake 
Division of Geological and Planetary Sciences, California Institute of Technology. Pasadena, CA 
91125, *Physikalisches Institut, Universitat Augsburg, W-8900 Augsburg; FRG 
ABSTRACT: Measurements of the odd-even effect in the mass spectrometric analysis ofTi and 
Os isotopes by resonance ionization mass spectrometry have been performed forM = + 1, 0 and 
-1 transitions. Under saturating conditions of the ionization and forM = + 1 transitions odd-even 
effects are reduced below the 0.5% level. Depending on the polarization state of the laser large 
odd isotope enrichments are observed for M = 0 and -1 transitions which can be reduced below 
the 0.5% level by depolarization of the laser field. 
1. INTRODUCTION 
The application of resonance ionization mass spectrometry (RIMS) to accurate isotopic analysis in geo-
and cosmochemistry is complicated by the presence of laser induced shifts in measured isotopic ratios. 
Observed effects range from 3% (Walker and Fassett 1986) to over40% (Fairbank et al1989, Spiegel 
et al. 1992) with a predominant enrichment of the odd mass isotopes. Laser induced shifts can be 
broadly divided into wavelength tuning effects which originate from the finite overlap of the laser 
spectral distribution with the absorption maxima of the different isotopes (Wunderlich et. al. 1992 a and 
b) and the intrinsic odd-even effects (Fairbank et al 1989, Lambropoulos et al. 1989, Payne et al. 
1991, Wunderlich et al. 1992 a and b). In practical applications of RIMS to isotopic analysis intrinsic 
odd-even effects can be compensated for by comparison of measured isotopic ratios with a standard of 
known isotopic composition (Spiegel et al. 1992). However, the size of the effects makes accurate 
determination of isotopic ratios difficult and precludes the ready identification of material of unusual 
isotopic composition. Furthermore, in some systems, like in the Re-Os chronometer, there is no 
standard for the radiogenic isotope available. 
In this study we examined the odd-even effect in the resonance ionization of Ti and Os in order to 
devise methods to avoid or greatly reduce the size of these effects in isotopic analysis. Os and Ti were 
selected because of their importance in geo- and cosmochemistry. 
2: ODD-EVEN EFFECTS AND THE MEASUREMENT OF ISOTOPIC RATIOS 
Intrinsic odd-even effects can be divided into selection rule effects and dynamic effects. Selection rule 
effects can be present when the resonance transition involves angular momentum changes M=O or -1. 
In this case a certain fraction of the ground state MJ and Mp components will not interact with a linearly 
or circularly polarized laser field. Because of the increased number of agular momentum states for 
isotopes with nuclear spin (odd isotopes) this fraction is smaller for the odd isotopes, resulting in odd 
© 1992 lOP Publishing Ltd and individual contributors 
128 Resonance Ionization Spectroscopy 1992 
isotope enrichment. Considering a ~J=O transition between J=1 states, 1/3 of the ground state 
population does not interact with a linearly polarized laser field because of the forbidden MJ = 0 -> M1 
= 0 transition. Upon addition of an half integer nuclear spin this selection rule is no longer in effect 
resulting in an enhancement of the odd isotopes. This situation is met in many resonance transitions in 
Ti and Os. In principle selection rule effects could be corrected by simple level counting. However, in 
many practical applications the laser radiation is not of a pure polarization state causing deviations of 
isotopic ratios obtained with this type of resonance transition from the expected values and a reduction 
of the odd-even effect (Wunderlich et al. 1992 a,b). Measured isotope ratios will then depend on the 
laser intensity, beam parameters and ion optical extraction conditions. The effects will be particularly 
pronounced in multistep ionization schemes with noncolinear laser beams because ionization can be 
produced from regions with widely different saturation of the different MJ ground state components. 
These considerations suggest the use of depolarized light for the excitation of the discrete resonances in 
RIMS. This also provides the advantage of not having to use all M=+ 1 resonance transitions. 
Dynamic effects have been discussed in detail by Lambropoulos and Lyras (1989) and by Payne et al. 
(1991) including the effects of the laser spatial intensity distribution. Dynamic effects pertain to M=+ 1 
transitions where from selection rule arguments no odd isotope enrichment should be present. A 
coherent and incoherent regime can be distinguished. In the first case with interaction times much 
smaller than the HFS coupling time a coherent superposition of HFS levels is excited. With regard to 
population balance ionization proceeds as if the HFS structure was absent and no odd-even effects 
should be observed. For interaction times much larger than the HFS coupling time and large laser 
bandwidhts excitation proceeds via independent HFS states. Due to the higher excited state degeneracy 
for isotopes with nuclear spin there will be on average more atoms in the resonance state for odd 
isotopes as compared to the even isotopes dending on the polarization state of the laser. This can result 
in a large odd isotope enrichment if the ionization is not saturated. Thus, saturation of the ionization is 
essential for obtaining a small odd-even effect. However, considering again the incoherent case 
depolarization of the resonance excitation laser should also result in an equal fraction of excited state 
atoms for the odd and even isotopes resulting in a large reduction of the odd -even effect. 
3. EXPERIMENTS AND RESULTS 
Details of the experimental set up and procedures have been described by Wunderlich et al. (1992 a). A 
frequency doubled pulsed laser system, thermal atomic beam and a modified quadrupole mass 
spectrometer were used for the RIMS experiments. Single photon 1 + 1 schemes with resonances near 
40000. cm-1 were used for the resonance ionization of Ti and Os. The width of the HFS transition 
arrays for the odd Os and Ti isotpes were obtained from intracavity etalon scans. The dye laser 
fundamental could be operated with a bandwidth of 0.4 cm-1 to assure excitation of the whole HFS 
transition arrays. 
The experiments on M=+ 1 transitions in Os and Ti were all performed in the incoherent regime, that is 
the pulse duration of 20 ns was much larger than the inverse HFS splitting. Results from a series of 
Table 1. Odd-even effects in Os and Ti isotope ratios for M=+1 transitions. 
Numbers in paranthesis indicate the 1 o error 
Resonance o47Ti o49n 0189as mode 
State (cm-1) (O/oo) (O/oo) (O/oo) 
39405.9 -3.4 (5) sat 
38544.4 -14 (12) +2.8 (11) sat 
37851.5 +35 (11) +4.6 (11) sat 
38159.5 +60 (15) +144 (23) nsat 
RIS Dynamics 129 
measurements are given in Table 1. The isotopic ratios have been corrected for mass fractionation and 
are given in the per mil deviation from their standard values (o-values) ( Os ratios: Creaser et al. 1991, 
Ti ratios: Niederer et al. 1981). Reference isotopes were 46-n and 190os. Mode indicates saturating 
(sat) or nonsaturating (nsat) conditions of the ionization. 
Under saturating conditions of the ionization no odd-even effect for Os is seen. This result differs from 
the one obtained by Walker and Fassett (1986) where a 3% enrichment in 189Qs was observed. With 
exception of the o47Ti = 35 value, the Ti results obt~ined under saturating conditions are compatible 
with the absence of odd-even effects. The values obtained under nonsaturating conditions show a 
considerable increase in the odd isotope ratios consistent with the arguments given above. The higher 
vavalue of ()49Ti may be caused by the larger number of excited state components in 49Ti compared to 
47Ti (1=5/2 for47Ti and 1=7/2 for49Ti). 
0.0 0.2 
--±-I 
t 
0.4 0.6 0.8 1.0 
Loser Energy lmJ) 
The power dependence of the 
189os;l90os and 189os;l 88os ratios 
Fig. I. Power dependence of 189os;188os. 
measured for the L\1=+ 1 transition is 
shown in Fig.1. The arrow indicates the 
onset of ionization saturation. The ratios 
are constant for laser intensities larger than 
half the ionization saturation intensity and 
agree very well with the standard ratios 
after mass fractionation correction. The 
maximum laser intensity in the experiment 
was 5 x 107 Watt/cm2 so that the 
constancy of the isotopic ratios can not be 
explained by laser bandwidth effects. 
Selection rule and polarization effects were 
studied on ~1 =0 and -1 transitions in Os 
and Ti. Some degree of depolarization was 
generated by passing the expanded laser beam through a quartz window exposed to localized stress 
creating a random amount of birefringence across the laser beam diameter. P ell is the ratio of the 
maximum and minimum intensity measured in two orthogonal directions of a linear polarizer positioned 
behind the experiment chamber. In Table 2. odd-even effects obtained for different values of P ell are 
listed in &-values. With exception of the Os L\1=0 (40361.9 cm-1) result odd-even effects are absent for 
a large degree of depolarization of the laser beam, P ell =27. The Ti result is markedly different from the 
results obtained by Spiegel et al. (1992) in a multistep ionization scheme with a series of ~1=0 
transitions where effects up to o47Ti = 350 were observed. Based on selection rules we calculate a 
maximum odd isotope effect of o4 7Ti = 225 for linearly polarized radiation for this type of transition. 
Table 2. 
Pen 
220 
27 
Polarization dependence of the odd-even effect for M=O and M=-1 transition 
in Os and Ti. Numbers in paranthesis indicate the la error 
+137 (10) 
+2.7 (10) 
o189Qs (AJ=-1) 
+57 (8) 
-1.3(13) 
o189Qs (AJ=O) 
+89 (10) 
+36 (8) 
Thus we suggest that the size of the odd-even effect was most likely increased in the final autoionizing 
step with an additional L\J=-1 transition. The residual odd isotope enrichment of 3.6% for the Os L\1=0 as 
compared to the L\1=-1 resonance transition (39406.cm -1) with P ell =27 appears unusual considering the 
much larger oscillator strength of the L\1=0 transition (i.e. 0.3 compared to 0.003). As such a small 
130 Resonance Ionization Spectroscopy 1992 
admixture of elliptical polarization should saturate the MJ=O to MJ=O transition and no odd isotope 
enrichment should be observed. 
The effect of the spatial intensity distribution in the laser beam on odd-even ratios is shown in Table 3. 
The 189Qsj190Qs ratio was measured for different focussing conditions for the AT=-1 resonance transition 
and with Pen= 27. The confocal parameter in all experiments was larger than the logitudinal extend of the 
ion collection region. It is seen that strong focussing results in a large reduction of the odd isotope 
Table 3. Dependence of ()189os on focussing 
conditions for Pen = 27. 
~J 
-1 
0 
f = 100cm 
74 (11) 
57 (16) 
f=35 
-1.3 (13) 
36 (8) 
4. SUMMARY AND CONCLUSIONS 
enrichment as could be expected from the 
arguments given above. All experiments were 
carried out under saturating conditions of the 
ionization. The ionization rate RI was smaller than 
the HFS splitting so that the ionization was 
generated from an incoherent superposition. In 
case the HFS splitting is much smaller than RI it 
can be expected that odd-even effects will be 
absent for M=O and -1 transitions. 
Odd-even effects in the isotopic analysis of Os and Ti by RIMS were studied. It could be shown that 
under saturating conditions of the ionization odd-even effects were absent for M=+ 1 transitions or 
could be reduced below the 1% level. For ~J=O and -1 transitions it was shown that the odd-even 
effects could be strongly reduced depending on the polarzation state and focussing conditions of the 
laser. In particular for one ~J=O and -1 transition effects were absent for a large degree of 
depolarization and strong focussing conditions. The results given here show that with a careful choice 
of the experimental parameters it is possible to obtain an accuracy better 0.5% for isotopic analysis 
with RIMS. 
ACKNOWLEOOEMENTS 
This work was supported by DOE grant DE FG03-88ER-1351 and NSF grant EAR-8816936 to G. J. 
Wasserburg. The laser system was obtained through support from the Packard and Sloan Foundations 
and NASA grant NAGW -1955 to G. A. Blake. R. K. W. would like to acknowledge the support of 
Prof. H.-J. Fecht for this work. 
REFERENCES 
Creaser R A, Papanastassiou D A and Wasserburg G J 1991 Cosmochirn. Acta 55 597 
Fairbank W M Jr., Spaar M T, Parks J E and Hutchinson J M R 1989 Phys. Rev. A40 2195 
Lambropoulos P and Lyras A 1989 Phys. Rev. A40 2199 
Niederer F R, Papanastassiou D A and Wasserburg G J 1981 Geochirn Acta 45 1017 
Payne M G, Allman S Land Parks J E 1991 Spectrochirn. Acta 46B 1439 
Spiegel DR, Pellin M J, Calaway W F, Burnett J W, CoonS R, Young C E, Gruen D M 
and Clayton R N 1992 Anal. Chern. 64 469 
Walker J R and Fassett J D 1986, Anal. Chern. 58 2393 
Wunderlich R K, Hutcheon I D, Wasserburg G J and Blake G A 1992 a lnt J. Mass 
Spectrorn. and Ion Processes in press 
Wunderlich R K, Hutcheon I D, Wasserburg G J and Blake G A 1992 b submitted Anal. Chern. 


Systematics of the odd-even effect in the resonance ionization of Os and Ti

Phys. Rev. A40 2195 Lambropoulos P and Lyras A

http://authors.library.caltech.edu/34528/1/Wunderlich_InstPhysConfSer1992p127.pdf

Systematics of the odd-even effect in the resonance ionization of Os and Ti

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