Appl. Phys. B 62, 511 513 (1996)

Applied Physics B and " ' ° " OpUcs © Springer-Verlag 1996

Isotope separation by nonlinear resonances in a Paul trap R. Alheit, K. Enders, G. Werth Institut für Physik, Universität Mainz, D-55099 Mainz, Germany

(Fax: +49-6131/39-5169) Received: 8 May 1995/Accepted: 20 December 1995

Abstraet. Deviations from the ideal quadrupole potential in a Paul ion trap create nonlinear resonances at certain operating points inside the stability diagram, where in the absence of potential pertubations storing times are very long. In the presence of those pertubations, however, the ions are lost from the trap. Since these resonances are mass dependent and the mass resolution is of the order of 100 it can be used to separate isotopes of a given element by choosing suitable trap operating conditions. Experiments on a natural mixture of Eu ÷ ions of mass 151 and 153 show that in a simple way, by proper choice of the operating point, the ions can be completely separated and laser-induced optical spectra of a single isotope can be received. This is the first time that mass separation in a Paul trap is performed by nonlinear effects in contrast to the usual way of using the mass dependent boundaries of the stability diagram

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Paul quadrupole ion traps confine charged particles by time-varying electric potentials applied to electrodes which, in the ideal case, are hyperboloids of revolution [1, 2]. The amplitudes and frequencies of the applied fields for which stable confinement can be achieved are given by the solution of the Mathieu equation d2ui d T + (ai - 2 q i c o s ( 2 t ) u i = 0,

values of the stability parameters a, q which are within the boundaries of a regular stability diagram [1 3]. Recently, we have shown by a systematic high-resolution scan across the stability diagram [-4] that for a real trap, the deviation of the potential from the ideal quadrupole shape leads to numerous resonances within the boundaries of the stability diagram. For certain values of the stability parameters, the ions gain energy from the trapping field and are lost from the trap. Those resonances have been observed previously by Paul and von Busch [5], by Dawson and Whetten [6] and other [7] for lower order of the perturbation. Using high resolution in the variation of the trapping parameters and high detection etticiency, we demonstrated, however, that resonances arising fi'om higher-order perturbations are also present and can be detected. Quite generally, these resonances are given by the condition [5, 8]

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(3)

with nr, nz integer; fir= co,./2f2; fi~ = coz/2f2, where co,., co~ are the oscillation frequencies of the ions in the time averaged potential well, which depend on the values of at, qr and az, qz, respectively. The reason for those resonances are deviations from the ideal quadrupol potential, caused by truncation of the trap electrodes, non-hyperbolic shapes or misalignment. In the case of large stored-ion clouds, the space-charge potential also gives rise to different potential shapes. Finally, higher harmonics in the ac trapping field may change the ion behaviour in the trap [5, 8]. If we expand the trap potential in a power series of (r/ro) N, the observed resonances can be assigned to a given order 2N of the perturbing potential by t% + nz = N.

(4)

In a frst experiment, using H2+ ions, we confirmed (3) up to the order 2N = 18. We used a Paul trap of radius ro = 2cm, endcap distance 2z0 = x/2"ro, driven by a 3 MHz oscillator at amplitudes up to 750 V. The superposed dc voltage was varied between - 6 0 and 150 V. The

512

maximal number of stored ions used for our measurements was between 500 and 1000. This corresponds to an ion density of 250 to 500 ions/mm 3. We observed spacecharge shifts of the resonances above 5 x 104 stored ions. In our experiment, we kept the ion number below this limit to avoid perturbation by space charge. Figure 1 shows a high-resolution scan through a part of the stability diagram, taken with H~-. The H2~ ions were created by electrionization from the background gas at 10 9 mbar, stored for 0.5 s and then ejected from the trap and counted in an ion multiplier, placed 8 cm above the upper trap endcap, which was formed by a mesh. Protons which are created simultaneously with the H~- ions, are separated by time-of-ftight between trap and detector. Data were taken for a given value of the dc voltage, while the ac amplitude was changed in increments of 1 V. For H~ ions, this corresponds to a change in the trapping parameter q~ of &q= = 0.00136. Then the dc voltage was changed by 0.7 V, corresponding to &a~ = 0.0019. Ten

cycles are added for each data point to improve the signal-to-noise ratio. In Fig. 1, dark regions correspond to a large number of stored ions, while at the bright lines, the ion number is reduced. All these lines of instabilities can be assigned to a specific higher-order contribution to the trapping potential as shown in [4]. Figure 2 gives details of a particular resonance which we show as an example here. It represents an expansion of the data near the resonance, indicated in Fig. 1 by a black circle. It corresponds to a node of three resonances of different perturbation orders. The ion number decrease at the centre of the resonance and the F W H M is Aq~ = 0.008. Since q is inversely proportional to the ions mass, we have q/Aq = m/Am. Similar results were obtained for other resonances. A typical resolution (FWHM) is of the order of 100-200. This is sufficient to discriminate between different isotopes of one element. In a second experiment, we used the trap to confine a natural mixture of Eu ÷ isotopes containing the isotopes

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Fig. 1. Measured stored ion number in a part of the first stable region of a Paul ion trap. Particular low ion numbers are observed at numerous lines, which correspond to resonances given by higher-order perturbations to the ideal quadrupole potential. Darker regions correspond to higher ion number

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Fig. 3a, b. Laser-induced ftuorescence spectrum from a cloud of a natural mixture of Eu ÷ isotopes, showing the hyperfine structure of the 9S4-9P 5 resonance line at 382 nm: (a) Trap operated outside a resonance. The spectrum shows the superposition of both isotopes 151 and 153 to about equal parts; (b) Trap operated at the resonance for mass 151 shown in Fig. 2. The operating conditions for both Eu + isotopes is indicated. The fluorescence shows the pure spectrum of the isotope 153

151 and 153 to about equal parts for spectroscopic purposes [9]. The ions are produced by surface ionization from a hot Pt-filament, placed near the lower endcap, and slowed down inside the trap by collisions with neural N2 buffer gas at 5 x 10 -~ mbar. Typical storage times are of the order of 10 h. We excite ions on the 9 S 4 - 9 p 5 r e s o n an ce transition (2 = 382 nm) by a frequency-doubled Ti: Sapphire laser. If we store the ions far away from a trap resonance, the laser-induced fluorescence shows the superposition of the two isotopes including their hyperfine components (Fig. 3a). However, if we move the Eu ion of mass 151 to an operating point which corresponds to

a resonance node of dodeca-, deca- and octupole resonances (qz = 0.719, az = - 0.013) (Figs. 1 and 2), we loose this isotope completely and are left with the other (Fig. 3b). The amplitude and storage time of the remaining isotope is virtually unaffected. The same procedure has been successfully applied to the other isotope. Isotope separation by nonlinear resonances in a Paul trap is basically different from the normal use of this device in mass spectrometry. Usually, the mass-dependent motional eigenfrequencies of trapped ions are selectively excited or the mass-dependent boundaries of the stability diagram are used for isotope separation. The use of isotope separation by nonlinear resonances in a Paul trap greatly simplifies the optical spectra and the identification of hyperfine components and avoids the tedious and time-consuming preparation of isotopically pure samples. It may also help to clean the ion trap from unwanted species, which may be produced by the ion source simultaneously with the ion under investigation and creates a space-charge potential which limits the number of storable wanted ions. We also want to point out that the existence of numerous resonances inside the stability diagram, as shown in Fig. 1, may affect the measurement of rate constants for reactions, in which a daughter ion is produced and stored from a confined parent ion. Since, in general, the chargeto-mass ratio of the daughter ion is different from the parent ion, its trapping parameters are different. Care has to be taken to avoid operating points at which the daughter ion hits a nonlinear resonance, since this might greatly change the observed ion number.

Acknowledgemems. We thank S. Kleineidam and G. Marx for help with the measurements. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. References 1. 2. 3. 4. 5. 6. 7. 8. 9.

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