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Selective detection of 13C by laser photodetachment mass spectrometry
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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 3667–3673 www.elsevier.com/locate/nimb
Selective detection of
13
C by laser photodetachment mass spectrometry
P. Andersson a,*, J. Sandstro¨m a, D. Hanstorp a, N.D. Gibson b, K. Wendt c, D.J. Pegg d, R.D. Thomas e a Department of Physics, Go¨teborg University, SE-412 96 Go¨teborg, Sweden Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA c Institut fuer Physik, Johannes Gutenberg-Universita¨t, Mainz, D-55099 Mainz, Germany d Department of Physics, University of Tennessee, Knoxville, TN 37996, USA e Department of Physics, AlbaNova, Stockholm University, SE-106 91 Stockholm, Sweden b
Received 15 June 2007; received in revised form 23 May 2008 Available online 8 June 2008
Abstract In this paper, we demonstrate how laser photodetachment mass spectrometry (LPMS) can be used to selectively detect 13C ions in the presence of 12C ions in a low energy ion beam. An isotopically enriched beam of carbon ions consisting of equal amounts of 13C and 12C ions was extracted from an ion source. The ions interacted with a laser beam in a collinear geometry over a distance of 70 cm. Residual atoms produced in the photodetachment process were detected in a neutral particle detector placed downstream of the collinear interaction region. By making use of the Doppler effect we were able to selectively photodetach 13C ions. The number of detected 13C atoms was 13 times larger than the number of detected 12C atoms. The population of the excited, weakly bound 2D excited state of the C ion was depleted by the use of a second laser. This significantly reduced the background accompanying the signal arising from the photodetachment of the 4S ground state C ion. Different applications of the LPMS method will be discussed in the paper. Ó 2008 Elsevier B.V. All rights reserved. PACS: 32.80.Gc; 07.75.+h Keywords: Negative ion; Photodetachment; Radiocarbon; Isotope selection; Mass spectrometry
1. Introduction Since it was first demonstrated in 1977, accelerator mass spectrometry (AMS) has developed into the most selective, as well as the most frequently used, mass spectrometric technique for ultra-rare elemental species determination with isotopic abundances well below 109 [1]. Today, AMS allows for the routine determination of abundances of rare isotopes like 10Be, 14C, 26Al, 36Cl, 41Ca and 129I with relative levels down to one part in 1015 and detection limits as low as 105 atoms [2–5]. As a result, this highly selective and sensitive method finds applications in numerous applied research fields such as geology and geochronology, *
Corresponding author. Tel.: +46 317723297. E-mail address: [email protected] (P. Andersson).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.126
biomedicine and environmental studies. More traditional techniques, including conventional mass spectrometry or radiometric methods, are typically limited in either sensitivity or selectivity. Most often AMS systems are based on tandem accelerators, which are generally large and expensive to build and maintain. Accordingly, there has been a recent trend towards the development of smaller and less expensive AMS machines, which operate with terminal voltages well below 1 MV [6–8]. As a consequence of this lower terminal voltage, the positive ions produced by the stripping process at the high voltage terminal of the tandem accelerator are formed in lower charge states and the selective power of the succeeding high-energy mass spectrometer is significantly degraded. Consequently, the background associated with neighbouring isotopes and atomic as well as molecular
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isobars becomes enhanced. This potential problem makes it desirable to incorporate other means to improve selectivity. In 1989, Berkovits et al. [9] demonstrated that an isobar of a neighbouring element with significantly lower electron affinity (EA) than the element of interest can be effectively suppressed at the low energy side of the AMS tandem accelerator by overlapping the ion beam with a powerful fixed frequency laser beam. It was shown, that 36S ions could be neutralized by photodetachment, while 36Cl ions in the same beam remained unaffected. In a similar study on 59Ni, an isobaric interference from 59Co was suppressed by a factor of 125 [10]. In a recent experiment [11] we introduced a refined version of the selection method applied by Berkovits et al. This method, called laser photodetachment mass spectrometry (LPMS), is designed to further improve both the isotopic and isobaric selectivity of a mass spectrometric measurement. A feasibility study was carried out at a conventional magnetic sector field mass spectrometer. In order to explain this method we first have to introduce the Wigner threshold law [12]. The energy dependence of the cross section Q at the onset of the production of neutral atoms by photodetachment is described by the expression Q ðE E0 Þ
lþ1=2
;
ciently than the 32S isotope. Sulphur was chosen for this demonstration study since it both produces an s-wave detachment and its electron affinity suitably matched the laser light accessible to us at that time. In the present experiment, we apply the LPMS method to analytically more interesting carbon isotopes. There continues to be a considerable practical interest in this element since the stable trace isotope, 13C, and the unstable but long-lived, ultra trace isotope 14C are the basis of several important analytical processes [5]. The experiment was carried out on 13C and 12C ions. The photodetachment spectra of the 12C, 13C and 14C isotopes will, apart from the Doppler shift and a negligible isotope shift, be identical. Hence, the LPMS method that we have shown to selectively detect 13C can, in principle, be directly applied also to selectively detect 14C. In this paper, we present our results when we apply the LPMS method to investigate carbon isotopes. We will also discuss how this technique can be used to suppress molecular interferences. Finally, we will discuss how the LPMS method could be of great value as a tool in photodetachment studies of fundamental properties of molecular and atomic negative ions.
ð1Þ 2. Experimental technique
where E = hc r represents the photon energy, r is the wavenumber of the transition, E0 the electron affinity of the neutral atom and l is the angular momentum of the outgoing electron. If the emitted electron is an s-wave, Eq. (1) gives a very sharp onset of the cross section, which varies as a square root function just above the threshold. The basic idea in LPMS is to make use of this sharp feature in the photodetachment cross section in order to interact with a selected isotope while leaving others unaffected. The most straight forward method would be to make use of the isotope shift in the electron affinity. Such shifts have been observed for H/D [13], Cl [14] and O [15]. The difference in the threshold energies are, however, too small to be used for an analytical purpose. Instead, we exploited the Doppler shift in the photodetachment threshold that is a result of the fast motion of the ions in an accelerator. Since the ions are monoenergetic, different isotopes with slightly different masses will move at different velocities and experience different Doppler shifts of their photodetachment thresholds. The thresholds of the different isotopes will be sufficiently well separated in energy that it is possible to tune the light from a laser between the thresholds. This allows one to selectively interact with a specific isotope and neutralise the corresponding ions by photodetachment. At the same time the ions of the remaining isotopes are not affected. In this manner we are able to selectively photodetach a selected isotope. This selective detection was probed by detecting the residual atoms produced in the photodetachment process. In our initial proof-of-principle experiment we were able to detect the 34S isotope 50 times more effi-
The apparatus, which is illustrated schematically in Fig. 1, is basically the same as that described in [11]. A compilation of the essential experimental parameters for spectroscopy on negative ions of carbon is given in Table 1. Negative carbon ions were produced in a cesium sputter ion source using a graphite cathode. In the present experiment we used an isotopically enriched carbon sample with an abundance ratio of 13C/12C 1. This sample was well suited to demonstrate enhancement in the abundance ratio and to investigate the sources of background specific to
Fig. 1. Collinear laser-ion beams apparatus that was used in the present experiment. The ion beam is indicated by the narrow solid line, the laser beams r1 and r2 by the thick shaded lines. The laser beam r1 and the ion beam are directed anti-parallel in interaction region 1. The laser beam r2 can be directed either parallel or anti-parallel to the ion beam in interaction region 2.
P. Andersson et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3667–3673 Table 1 Experimental parameter used for isotope depletion of carbon Parameter
Value
Ion beam energy Ion current Photon flux r1 (depletion laser) Photon flux r2 (detection laser) Depletion interaction length 1 Detection interaction length 2 Detector efficiency Vacuum pressure in interaction region 2
5.25 keV 1 nA 1026 photons s1 1024 photons s1 0.5 m 0.7 m 100% 109 mbar
these measurements. The ions were extracted from the ion source, accelerated to a nominal energy of 5.25 keV and focused to form a parallel beam. Mass analysis of the beam ions was carried out using a 90° sector field magnet. The energy spread of the ensemble of ions in the beam is a constant in the conservative force field arising from the static acceleration potential applied in the direction of motion of the ions. It is given by dE ¼ dðmv2=2Þ ¼ mvdv ¼ constant:
ð2Þ
Since the longitudinal velocity spread is inversely proportional to the velocity of the ions, it is compressed when the ions are accelerated. This compression of the velocity distribution upon acceleration and its advantageous consequences were systematically exploited during the development of the technique of high-resolution collinear laser spectroscopy on fast atomic or ionic beams [16,17]. Before describing the experimental apparatus in some more detail, we need to discuss the energy level diagram of the negative carbon ion, as shown in Fig. 2. This ion has a ground state 2p3 4S configuration, and an excited 2p3 2D energy level [18,19]. Since they are of the same parity, single photon electric-dipole induced transitions are
Fig. 2. Energy level diagram of the C ion showing the 4S ground and the 2 D excited state, located just below the photodetachment threshold. The energy r1 of the fixed frequency laser 1 is sufficient to detach ions in the excited state but is insufficient to do the same for ions in the ground state. The higher energy r2 of the tuneable laser 2 is set to investigate the threshold for detachment from the ground state [note: depletion is brought about by the intensity of the laser (photon flux) not the energy of the photons].
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prohibited between them. As a result, the excited 2p3 2D state, situated 1.23 eV above the ground state (33 meV below the detachment threshold), is metastable. As described in the introduction, the main goal in this work is to investigate how a laser with a photon energy that is matched to the energy difference between the ground state of the negative ion and the neutral atom can be used to obtain an isotopically selective detection. This excitation is labelled r2 in Fig. 2. However, for ions in the metastable 2 D state, no isotopic selectivity can be obtained using laser r2. Ions in this state were therefore removed from the beam prior to the interaction with laser r2. As shown in Fig. 2, it is possible to select the photon energy, labelled r1, which is sufficient to neutralise the excited state ions, but it is insufficient to detach electrons from ground-state ions. In the experiment the laser beam of frequency r1 was directed anti-parallel to the ion beam direction in the straight beam line section situated just after the magnet (interaction region 1). Thereafter, the remaining ions in the beam, now predominantly in the ground state, were steered by a 90° electrostatic quadrupole deflector into the interaction region 2, where they where merged collinearly with the second laser beam of frequency r2. The overlap of the ion and laser beams in region 2 was 0.7 m long and was defined by two 3 mm apertures. Downstream of this interaction region, the negative ions remaining in the beam were deflected out of the laser beam and into a Faraday cup by an electric field between two vertically aligned electrodes. Ion currents measured at this point were typically a few nA. Neutral species were generated in the interaction region by neutralisation of the negative ions, either purposely via photodetachment (signal) or via detachment in collisions with the residual gas particles (background). The neutral particles were not affected by the applied electric field and entered a neutral particle detector placed about 20 cm further downstream, where they impinged on a transparent glass plate coated with a thin layer of In2O3:Sn. Secondary electrons produced by the particle impact were finally detected by a channel electron multiplier (CEM). The whole secondary electron detector arrangement was placed inside a Faraday cage biased at 50 V in order to prevent stray electrons from being detected. The transparency of the coated glass plate allowed the laser light to be transmitted, enabling us to use both co- and counter-propagating laser-ion beam geometries. A potential problem with this type of detector is the generation of a considerable number of electrons by a laser-light-induced photoelectric effect on the coated glass plate. Electrons created in this manner, however, were prevented from reaching the CEM by electrically biasing a grid placed in front of the opening of the CEM during the laser pulse. After a pre-set delay time of about 10 ns following the laser pulse, the bias on the grid was turned off to allow the detection of the impinging atoms, which arrive much later from the interaction region. The output signal from the CEM was fed into a gated counter, where a narrow time window was set to take data only in coincidence
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with the arrival of those neutrals at the detector that had been exposed to laser light. As was verified, the signal-tobackground ratio was appreciably enhanced if the gating was properly set. This arrangement allowed efficient discrimination against any continuously distributed electron background arising, for example, from collisional detachment. The gated detection was necessary to achieve meaningful results since we were only able to use a low laser duty cycle of 10 Hz in the experiment. The primary source of background counts, i.e. from collisional detachment, was further minimized by maintaining the apparatus under ultra-high vacuum conditions by use of three differentially pumped sections, each separated by 3 mm diameter apertures. A diffusion pump was used in the ion source region, and ion pumps were used in the magnet and interaction regions. The nominal working pressure in the interaction region was maintained around 109 mbar. Two different lasers were used in the experiments. Metastable state depletion in the straight section of the beam line prior to the electrostatic quadrupole deflector was carried out by using the 1064 nm fundamental output from a Nd:YAG laser (r1) with about 80 mJ pulse energy. The tuneable laser light (r2) used to selectively detach electrons from the ground-state ions of a particular isotope was generated by an optical parametric oscillator (OPO) system that was pumped by the third harmonic of another Nd:YAG laser. Both laser systems provided pulses of 6 ns duration at a rate of 10 Hz and were properly synchronised to interact with the same ion ensemble. The OPO laser system was tuned to a wavelength around 981.2 nm. Taking the Doppler shift into account, this value corresponds to the binding energy of the C ion in its ground state, which is given by the electron affinity of the carbon atom of 10179.67(15) cm1 [20]. Laser pulse characteristics of the OPO system were typically 1 mJ pulse energy and a bandwidth of 6 GHz. The laser intensity was continuously monitored with a power meter situated after the interaction region. The OPO laser beam r2 could be directed either parallel or anti-parallel to the direction of motion of the ion beam. The signals from the neutral atom detector, the laser power meter and the negative ion Faraday cup were all recorded by a PC computer using a LabView-based data acquisition program.
Fig. 3. Ion beam mass spectrum of the carbon mass region showing the isotope abundance ratio, 13C/12C, of approximately 1. The y-axis shows the ion current as measured in the Faraday cup. Gaussian functions are fitted to the mass peaks. The weak peaks on the higher mass side of the main peak are artefacts associated with imperfections in ion source and ion optics.
the ion source and ion optical system. They have no effect on the general outcome of this experiment. The effect of the laser-induced depletion of the excited state population by laser r1 was explored by measuring the change in the production of neutral 12C atoms around the photodetachment threshold when laser r1 is turned on and off. In both cases, laser r2 was scanned from 10190.70 cm1 to 10192.70 cm1 in the ion rest frame (taking into account the Doppler shift of the threshold). The data are plotted in Fig. 4. The circles show the laserinduced production of 12C atoms taken with laser r1 on and the square symbols show the same signal with laser r1 off. The observed reduction in the yield of neutrals below the threshold energy when laser r1 is on amounts to a factor of about 3. This suppression is due to the depletion in the beam of ions in the excited state. The sharpness of the onset of the photon-induced neutral atom signal is limited by the finite laser bandwidth. The isotope-dependent energies of the photodetachment thresholds for the isotopes 12C and 13C were determined by
3. Results and discussion Fig. 3 shows the mass spectrum of the enriched carbon sample, as obtained by scanning the field of the analyzing magnet and recording the negative ion current in the Faraday cup at the end of the interaction region. The mass spectrum shows an approximately equal abundance of the 12C and 13C isotopes in accordance with specifications of the sample placed in the ion source. A weak tailing and a small side peak are observed on the high-mass side of each primary peak. These peaks, which are about 5% of the intensity of the primary peaks, are attributed to experimental artefacts associated with imperfections in
Fig. 4. Threshold region of the cross section for photodetachment of 12C. The signal from the neutral atom detector is recorded during scans of laser r2 across the onset. Circles represent measurements taken with laser r1 turned on in order to suppress excited state population. The squares represent measurements taken with laser r1 turned off.
tuning laser r2, while setting laser r1 for the maximum depletion of the excited ions. These data are shown in Fig. 5. The separation of the photodetachment thresholds for the isotopes 12C and 13C is extracted by fitting the parameters of the Wigner law to the data near threshold (1). The small discrepancy between the experimental data and the Wigner law observed just below the threshold is caused by the finite bandwidth of the laser. This fact must, of course, be taken into consideration in an absolute electron affinity measurement but is of no importance here since we are only interested in the relative threshold energies. A separation of DEexp = 0.45(8) cm1 was determined from the data. The value is in good agreement with the sum of the Doppler shift and the normal mass shift of DEtheo = 0.42 cm1, keeping in mind the uncertainty of a possible specific mass shift contribution, which could easily amount to values of the order of ±0.06 cm1. The rather large experimental uncertainty that is quoted here is due to the finite bandwidth of the laser and deviations from the simple Wigner square root law that were not included in the fitting function. Utilizing this energy separation, we decided to set the photon energy of laser r2 to 10191.60 cm1 (position A) to selectively detach 13C ions while leaving 12C ions unaffected. By simply reversing the direction of laser r2 relative to the ion beam and correspondingly retuning the laser, electrons from 12C instead of 13C could just as easily be selectively detached. A mass spectrum, recorded with the laser r2 set to wavenumber position A in Fig. 5 (10191.60 cm1), is shown in Fig. 6. Here the photon-induced neutral atom signal from the neutral atom detector is shown. The two possible sources of background arise from either collisional detachment or isobaric molecular interferences stemming from neutralization of 12CH ions that are produced in the ion source. Contributions from these sources have been considered in the final data as will be described below. The 12CH ion has an electron affinity of 1.238(8) eV [21], somewhat
Fig. 5. Energy dependence of the photodetachment threshold measured for the carbon isotopes via the neutral atom detector. Circles and squares represent data obtained for the isotopes 13C and 12C, respectively, while the lines are Wigner law fits to the data. The x-axis gives the laboratory frame wave numbers of laser r2. The points A at 10 191.60 cm1 and B at 10190.56 cm1 indicate the positions used in the experiment to accumulate the ‘‘signal” and the background, respectively, as discussed in the text.
Detected neutral particles (a.u.)
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12
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13
Fig. 6. Laser photodetachment mass spectrum of the carbon mass region. The y-axis shows the number of detected neutral atoms produced by the photodetachment process. The inset shows the raw data at laser position A (upper hatched line) and B (lower dotted line), where the direction of the laser light is parallel with the ion beam in both cases. The resulting curve, obtained by subtraction of these two is shown in the main window. Points represent the experimental data and the line represents a fit consisting of the sum of two Gaussian curves.
below the 13C atom electron affinity of 1.262 eV [20], and hence, both the 13C and the 12CH ions are neutralized by laser r2. An ideal solution to eliminate this molecular interference would be to photodetach the molecular ions with laser r1 or another laser, before they reach the primary interaction region. However, at the time of the experiment, no laser capable of generating the required photon energy was accessible. Instead the background contribution from the detachment of ions of molecular species was determined together with the collisional background by recording a separate spectrum with laser r2 tuned to 10190.56 cm1 (position B) in Fig. 5, well below the detachment threshold for both isotopes 12C and 13C. The background data are shown in the inset of Fig. 6, together with the photodetachment measurement that was recorded at energy position A. At position B, only the 12CH ions are neutralised by the laser light. Thus a precise value for the background arising from both isobaric molecular interferences and collisional detachment was determined and could be subtracted from the data obtained at position A. As can be seen in the figure, 13C atoms are detected with much higher efficiency than 12C atoms. Gaussian function fits to the data for both isotopes are represented by the solid line in Fig. 6. From the relative peak areas, the number of detected 13C atoms was found to be 13 times larger than the number of detected 12C atoms. Testing the reproducibility of this measurement gives rise to a final uncertainty of about 20%. Factors affecting the reproducibility are the overall experimental conditions and specifically the operation of the ion source, which determines the production rate of different molecular ions. 4. Outlook and conclusion In the present paper laser photodetachment of negative ions has been used to explore the possibility of selective detection of a specific isotope of carbon. The method
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exploits the mass-dependent Doppler shift experienced by a fast moving ion during the absorption of light. To achieve high isotopic selectivity, we used a narrow bandwidth laser source whose wavelength was tuned to lie between the photodetachment thresholds of the isotopes under study. In this way, ions of one (or more) isotope(s) in the beam can be selectively neutralized by detachment. The number of detected 13C atoms was 13 times higher than the number of detected 12C atoms, although there were equal amounts of 12C and 13C ions in the beam. The problem associated with beam ions in metastable excited states was overcome by applying an additional lower energy laser to deplete their population prior to the primary interaction region. This background was suppressed by a factor of about three. The LPMS method described has two possible future applications. It can be used either for analytical purposes or as a tool in fundamental studies of atomic and molecular negative ions. First we will discuss the possibility of incorporating the experimental scheme described in this paper into a mid-size or small AMS machine used for analytical purposes: By overlapping, in a counter-propagating geometry, the ion beam at the low energy side of the accelerator with one or more laser beams prior to injection into the tandem accelerator one could deplete ions of the lighter, and hence faster moving, isotopes in the beam. For example, 12C and 13C ions could be neutralized leaving 14C ions to pass through the tandem accelerator. In addition, and of equal importance, there would be a discrimination against any interfering beam component with an electron affinity smaller than that of the atom of the element under investigation. For example, in the case of 14C detection, isobaric molecular interferences from 12CH2 and 13CH could be efficiently suppressed by neutralizing the ions of these molecules by photodetachment. Such an experimental scheme, however, would require peak powers that can only be produced with very powerful pulsed lasers. Such lasers, however, typically have a low duty cycle. In the present experiment, the number of ions interacting with the laser light is determined by the time they spend in the interaction region. The time of flight for the carbon ions over the 0.7 m long interaction path is about 3 ls when ions are accelerated to 5 keV. With a 10 Hz laser system, the duty cycle is therefore only 3 105. This value could, of course, be improved substantially by using lasers with high repetition rates of e.g. 10 kHz. This would yield a duty cycle of 3%. The corresponding efficiency losses of ‘‘only” a factor of 30 or less, associated with a comparable gain in isotopic and molecular selectivity, could well be acceptable for a number of experiments. Such a scheme would be a realistic basis for applicability of the LPMS technique. As an alternative, the ion source could be pulsed, for example by switching the Cs source extraction potential. A duty cycle of up to 100% could then, in principle, be achieved with careful synchronization of the lasers and ion beam. In conclusion we state that the capability of the LPMS method to selectively
neutralize specific isotopes was demonstrated and application seems feasible, but it would take an additional development of high power, high repetition rate lasers until the method can be utilized in existing mass spectrometric analytical systems. Recently, Liu et al. [22] installed an ion beam cooler and buncher unit at a low energy mass spectrometer. By using a technique analogous to the LPMS method these authors were able to demonstrate that it is possible to both slow down and energetically compress a negative ion beam in a radio frequency quadrupole trap filled with helium buffer gas. The energy compression had the effect that the geometrical cross section of the ion beam was substantially reduced. This allowed for a better overlap between the ion beam and a well-focussed laser beam. Even more important, the interaction time was substantially increased. As a result of these improvements, the photodetachment process could be saturated even with medium-power continuous wave lasers. A duty cycle of 100% could then be achieved without pulsing the ion source. The reduced velocity of the ions in the buncher would, however, strongly reduce the isotopic selectivity of LPMS, but it could be used, for example, to more efficiently suppress molecular interferences in an AMS system. The original idea behind this work was to develop an experimental scheme that could be used to suppress isotopic and molecular isobaric interferences in analytical mass spectrometry. The first real application of the technique, however, might be in the field of atomic and molecular physics. During the course of the work it has become clear that our present LPMS apparatus is well suited to greatly improve the accuracy of measurements of the structure of atomic and molecular negative ions. Negative ions are of considerable fundamental interest due to the relative importance of the theoretically interesting electron correlation effect. More recent theoretical work of particular relevance for this work is a calculation of the isotope shift in the electron affinity of oxygen [23] and of beryllium [24], and very high precision calculations of the electron affinities of a number of light elements, including carbon, were performed by de Oliveira [25]. On the experimental side, one complication is that traditional optical spectroscopy cannot be applied in order to gain structural information about most negative ions. This is due to the fact that almost all atomic negative ions lack bound excited states with parity opposite to the ground state, the notable exceptions being Os [26] and possibly Ce [27,28]. Instead, the photodetachment process is the only method for spectroscopic investigations that can be used. Since this is a continuum process, all energetically allowed detachment channels will contribute to the signal. These unwanted channels arise from the photodetachment of an electron from an ion in an excited state. An accurate method of determining an electron affinity is to measure the laser photon energy at the onset of detachment of
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electrons from an ion in its ground state. This signal frequently sits on top of a background associated with the photodetachment from ions in excited states, such as the 2D state of C discussed in the present paper. As we have demonstrated, the population in such states can be depleted prior to the interaction region, hence reducing the background arising from the presence of an excited state in the negative ion. With this improvement, laser photodetachment threshold (LPT) spectroscopy [20] should be able to compete with the laser photodetachment microscopy (LPM) method which, in later years, have produced the most accurate electron affinity measurements [29,30]. It should be pointed out that the low duty cycle is not a limitation in experiments where the fundamental properties of negative ions are being investigated. Since the ions are stable there is no limit in the sample volumes available for the experiments. In this type of experiment it would, of course, be of great interest to install an ion beam cooler and buncher in order to facilitate an almost complete depletion of excited state ions. This method could be of even greater interest in studies of molecular negative ions, where the wealth of rotational and vibrational states makes the interpretation of photodetachment spectra very difficult. These studies would require the access to two powerful tuneable laser sources. We expect soon to have access to two such laser systems in our laboratory. Our future plans call for the initial application of such a LPMS system to make a precise determination of the electron affinity of a variety of atomic and molecular systems. Acknowledgements This work was supported by the Swedish Science Foundation. Additional support was provided in part by the National Science Foundation under Grant No. 0456916, DAAD Grant No. 26021 and by STINT. R.D.T. would like to thank the Swedish Science Foundation for funding a Research Fellowship. References [1] C. Tuniz, J.R. Bird, D. Fink, G.F. Herzog, Accelerator Mass Spectroscopy, CRC Press LLC, Boca Raton, 1998. [2] W. Kutschera, Progress in isotope analysis at ultra trace level by AMS, Int. J. Mass Spec. 242 (2005) 145. [3] L.K. Fifield, Accelerator mass spectrometry and its applications, Rep. Prog. Phys. 62 (1999) 1223. [4] S. Bowman, Radiocarbon Dating, University of California Press, Los Angeles, 1990. [5] L.-A. Currie, The remarkable metrological history of radiocarbon dating II, J. Res. NIST 109 (2004) 185. [6] M. Suter, A new generation of small facilities for accelerator mass spectrometry, Nucl. Instr. and Meth. B 139 (1998) 150. [7] H.-A. Synal, S. Jacob, M. Suter, The PSI/ETH small radiocarbon dating system, Nucl. Instr. and Meth. B 172 (2000) 1.
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[8] M. Stocker, R. Bertschinger, M. Dobeli, M. Grajcar, S. Jacob, J. Scheer, M. Suter, H.A. Synal, Status of the PSI/ETH compact AMS facility, Nucl. Instr. and Meth. B 223–224 (2004) 104. [9] D. Berkovits, E. Boaretto, G. Hollos, W. Kutschera, R. Naaman, M. Paul, Z. Vager, Selective suppression of negative ions by lasers, Nucl. Instr. and Meth. A 281 (1989) 663. [10] D. Berkovits, E. Boaretto, G. Hollos, W. Kutschera, R. Naaman, M. Paul, Z. Vager, Study of laser interaction with negative ions, Nucl. Instr. and Meth. B 52 (1990) 378. [11] J. Sandstro¨m, P. Andersson, K. Fritioff, D. Hanstorp, R. Thomas, D.J. Pegg, K. Wendt, Laser photodetachment mass spectrometry, Nucl. Instr. and Meth. B 217 (2004) 513. [12] E.P. Wigner, On the behaviour of cross sections near threshold, Phys. Rev. 73 (1948) 1002. [13] K.R. Lykke, K.K. Murray, W.C. Lineberger, Threshold photodetachment of H, Phys. Rev. A 43 (1991) 6104. [14] U. Berzinsh, M. Gustafsson, D. Hanstorp, A. Klinkmu¨ller, U. Ljungblad, A.-M. Ma˚rtensson-Pendrill, Isotope shift in the electron affinity of chlorine, Phys. Rev. A 51 (1995) 231. [15] C. Blondel, C. Delsart, C. Valli, S. Yiou, M.R. Godefroid, S. Van Eck, Electron affinities of 16O, 17O, 18O, the fine structure of 16O, and the hyperfine structure of 17O, Phys. Rev. A 64 (2001) 052504. [16] S.L. Kaufman, High resolution laser spectroscopy in fast beams, Optics Commun. 17 (1976) 309. [17] R. Neugart, Collinear fast-beam laser spectroscopy, in: H.J. Beyer, H. Kleinpoppen (Eds.), Progr. in Atomic Spectr. Part D, 1987, p. 75. [18] M. Scheer, R.C. Bilodeau, C.A. Brodie, H.K. Haugen, Systematic study of the stable states of C Si Ge and Sn via infrared laser spectroscopy, Phys. Rev. A 58 (1998) 2844. [19] D. Feldmann, Infrared photodetachment measurements near threshold of C, Chem. Phys. Lett. 47 (1977) 338. [20] T. Andersen, H.K. Haugen, H. Hotop, Binding energies of negative ions III, J. Phys. Chem. Ref. Data 28 (1999) 1511. [21] J.C. Rienstra-Kiracofe, G.S. Tschumper, H.F. Schaefer, S. Nandi, G.B. Ellison, Atomic and molecular electron affinities: photoelectron experiments and theoretical computations, Chem. Rev. 102 (2002) 231. [22] Y. Liu, J.R. Beene, C.C. Havener, J.F. Liang, Isobar suppression by photodetachment in a gas-filled RF quadrupole ion guide, Appl. Phys. Lett. 87 (2005) 1, 113504. [23] M.R. Godefroid, C. Froese-Fischer, Isotope shift in the oxygen electron affinity, Phys. Rev. A 60 (1999) R2637. [24] M. Nemouchi, A. Taleb, M. Godefroid, Isotope shift in the electron affinity of beryllium, J. Phys. B 37 (2004) 865. [25] G. de Oliveira, J.M.L. Martin, F. de Proft, P. Geerlings, Electron affinities of the first- and second-row atoms: benchmark ab initio and density-functional calculation, Phys. Rev. A 60 (1999) 1034. [26] R.C. Bilodeau, H.K. Haugen, Experimental studies of Os: observation of a bound–bound electric dipole transition in an atomic negative ion, Phys. Rev. Lett. 85 (2000) 534. [27] Steven M.S.M. O’Malley, Donald R.D.R. Beck, Calculation of Ce binding energies by analysis of photodetachment partial cross sections, Phys. Rev. A 74 (2006) 042509. [28] C.W. Walter, N.D. Gibson, C.M. Janczak, K.A. Starr, A.P. Snedden, R.L. Field III, P. Andersson, Infrared photodetachment of Ce threshold spectroscopy and resonance structure, Phys. Rev. A 76 (2007) 052702. [29] C. Blondel, W. Chaibi, C. Delsart, C. Drag, F. Goldfarb, S. Kroger, The electron affinities of O, Si, and S revisited with the photodetachment microscope, Eur. Phys. J. D 33 (2005) 335. [30] C. Blondel, W. Chaibi, C. Delsart, C. Drag, Images of a photoelectron interfering with itself: the photodetachment microscope, Source: J. Mod. Opt. 53 (2006) 2605.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 3667–3673 www.elsevier.com/locate/nimb
Selective detection of
13
C by laser photodetachment mass spectrometry
P. Andersson a,*, J. Sandstro¨m a, D. Hanstorp a, N.D. Gibson b, K. Wendt c, D.J. Pegg d, R.D. Thomas e a Department of Physics, Go¨teborg University, SE-412 96 Go¨teborg, Sweden Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA c Institut fuer Physik, Johannes Gutenberg-Universita¨t, Mainz, D-55099 Mainz, Germany d Department of Physics, University of Tennessee, Knoxville, TN 37996, USA e Department of Physics, AlbaNova, Stockholm University, SE-106 91 Stockholm, Sweden b
Received 15 June 2007; received in revised form 23 May 2008 Available online 8 June 2008
Abstract In this paper, we demonstrate how laser photodetachment mass spectrometry (LPMS) can be used to selectively detect 13C ions in the presence of 12C ions in a low energy ion beam. An isotopically enriched beam of carbon ions consisting of equal amounts of 13C and 12C ions was extracted from an ion source. The ions interacted with a laser beam in a collinear geometry over a distance of 70 cm. Residual atoms produced in the photodetachment process were detected in a neutral particle detector placed downstream of the collinear interaction region. By making use of the Doppler effect we were able to selectively photodetach 13C ions. The number of detected 13C atoms was 13 times larger than the number of detected 12C atoms. The population of the excited, weakly bound 2D excited state of the C ion was depleted by the use of a second laser. This significantly reduced the background accompanying the signal arising from the photodetachment of the 4S ground state C ion. Different applications of the LPMS method will be discussed in the paper. Ó 2008 Elsevier B.V. All rights reserved. PACS: 32.80.Gc; 07.75.+h Keywords: Negative ion; Photodetachment; Radiocarbon; Isotope selection; Mass spectrometry
1. Introduction Since it was first demonstrated in 1977, accelerator mass spectrometry (AMS) has developed into the most selective, as well as the most frequently used, mass spectrometric technique for ultra-rare elemental species determination with isotopic abundances well below 109 [1]. Today, AMS allows for the routine determination of abundances of rare isotopes like 10Be, 14C, 26Al, 36Cl, 41Ca and 129I with relative levels down to one part in 1015 and detection limits as low as 105 atoms [2–5]. As a result, this highly selective and sensitive method finds applications in numerous applied research fields such as geology and geochronology, *
Corresponding author. Tel.: +46 317723297. E-mail address: [email protected] (P. Andersson).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.126
biomedicine and environmental studies. More traditional techniques, including conventional mass spectrometry or radiometric methods, are typically limited in either sensitivity or selectivity. Most often AMS systems are based on tandem accelerators, which are generally large and expensive to build and maintain. Accordingly, there has been a recent trend towards the development of smaller and less expensive AMS machines, which operate with terminal voltages well below 1 MV [6–8]. As a consequence of this lower terminal voltage, the positive ions produced by the stripping process at the high voltage terminal of the tandem accelerator are formed in lower charge states and the selective power of the succeeding high-energy mass spectrometer is significantly degraded. Consequently, the background associated with neighbouring isotopes and atomic as well as molecular
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isobars becomes enhanced. This potential problem makes it desirable to incorporate other means to improve selectivity. In 1989, Berkovits et al. [9] demonstrated that an isobar of a neighbouring element with significantly lower electron affinity (EA) than the element of interest can be effectively suppressed at the low energy side of the AMS tandem accelerator by overlapping the ion beam with a powerful fixed frequency laser beam. It was shown, that 36S ions could be neutralized by photodetachment, while 36Cl ions in the same beam remained unaffected. In a similar study on 59Ni, an isobaric interference from 59Co was suppressed by a factor of 125 [10]. In a recent experiment [11] we introduced a refined version of the selection method applied by Berkovits et al. This method, called laser photodetachment mass spectrometry (LPMS), is designed to further improve both the isotopic and isobaric selectivity of a mass spectrometric measurement. A feasibility study was carried out at a conventional magnetic sector field mass spectrometer. In order to explain this method we first have to introduce the Wigner threshold law [12]. The energy dependence of the cross section Q at the onset of the production of neutral atoms by photodetachment is described by the expression Q ðE E0 Þ
lþ1=2
;
ciently than the 32S isotope. Sulphur was chosen for this demonstration study since it both produces an s-wave detachment and its electron affinity suitably matched the laser light accessible to us at that time. In the present experiment, we apply the LPMS method to analytically more interesting carbon isotopes. There continues to be a considerable practical interest in this element since the stable trace isotope, 13C, and the unstable but long-lived, ultra trace isotope 14C are the basis of several important analytical processes [5]. The experiment was carried out on 13C and 12C ions. The photodetachment spectra of the 12C, 13C and 14C isotopes will, apart from the Doppler shift and a negligible isotope shift, be identical. Hence, the LPMS method that we have shown to selectively detect 13C can, in principle, be directly applied also to selectively detect 14C. In this paper, we present our results when we apply the LPMS method to investigate carbon isotopes. We will also discuss how this technique can be used to suppress molecular interferences. Finally, we will discuss how the LPMS method could be of great value as a tool in photodetachment studies of fundamental properties of molecular and atomic negative ions.
ð1Þ 2. Experimental technique
where E = hc r represents the photon energy, r is the wavenumber of the transition, E0 the electron affinity of the neutral atom and l is the angular momentum of the outgoing electron. If the emitted electron is an s-wave, Eq. (1) gives a very sharp onset of the cross section, which varies as a square root function just above the threshold. The basic idea in LPMS is to make use of this sharp feature in the photodetachment cross section in order to interact with a selected isotope while leaving others unaffected. The most straight forward method would be to make use of the isotope shift in the electron affinity. Such shifts have been observed for H/D [13], Cl [14] and O [15]. The difference in the threshold energies are, however, too small to be used for an analytical purpose. Instead, we exploited the Doppler shift in the photodetachment threshold that is a result of the fast motion of the ions in an accelerator. Since the ions are monoenergetic, different isotopes with slightly different masses will move at different velocities and experience different Doppler shifts of their photodetachment thresholds. The thresholds of the different isotopes will be sufficiently well separated in energy that it is possible to tune the light from a laser between the thresholds. This allows one to selectively interact with a specific isotope and neutralise the corresponding ions by photodetachment. At the same time the ions of the remaining isotopes are not affected. In this manner we are able to selectively photodetach a selected isotope. This selective detection was probed by detecting the residual atoms produced in the photodetachment process. In our initial proof-of-principle experiment we were able to detect the 34S isotope 50 times more effi-
The apparatus, which is illustrated schematically in Fig. 1, is basically the same as that described in [11]. A compilation of the essential experimental parameters for spectroscopy on negative ions of carbon is given in Table 1. Negative carbon ions were produced in a cesium sputter ion source using a graphite cathode. In the present experiment we used an isotopically enriched carbon sample with an abundance ratio of 13C/12C 1. This sample was well suited to demonstrate enhancement in the abundance ratio and to investigate the sources of background specific to
Fig. 1. Collinear laser-ion beams apparatus that was used in the present experiment. The ion beam is indicated by the narrow solid line, the laser beams r1 and r2 by the thick shaded lines. The laser beam r1 and the ion beam are directed anti-parallel in interaction region 1. The laser beam r2 can be directed either parallel or anti-parallel to the ion beam in interaction region 2.
P. Andersson et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3667–3673 Table 1 Experimental parameter used for isotope depletion of carbon Parameter
Value
Ion beam energy Ion current Photon flux r1 (depletion laser) Photon flux r2 (detection laser) Depletion interaction length 1 Detection interaction length 2 Detector efficiency Vacuum pressure in interaction region 2
5.25 keV 1 nA 1026 photons s1 1024 photons s1 0.5 m 0.7 m 100% 109 mbar
these measurements. The ions were extracted from the ion source, accelerated to a nominal energy of 5.25 keV and focused to form a parallel beam. Mass analysis of the beam ions was carried out using a 90° sector field magnet. The energy spread of the ensemble of ions in the beam is a constant in the conservative force field arising from the static acceleration potential applied in the direction of motion of the ions. It is given by dE ¼ dðmv2=2Þ ¼ mvdv ¼ constant:
ð2Þ
Since the longitudinal velocity spread is inversely proportional to the velocity of the ions, it is compressed when the ions are accelerated. This compression of the velocity distribution upon acceleration and its advantageous consequences were systematically exploited during the development of the technique of high-resolution collinear laser spectroscopy on fast atomic or ionic beams [16,17]. Before describing the experimental apparatus in some more detail, we need to discuss the energy level diagram of the negative carbon ion, as shown in Fig. 2. This ion has a ground state 2p3 4S configuration, and an excited 2p3 2D energy level [18,19]. Since they are of the same parity, single photon electric-dipole induced transitions are
Fig. 2. Energy level diagram of the C ion showing the 4S ground and the 2 D excited state, located just below the photodetachment threshold. The energy r1 of the fixed frequency laser 1 is sufficient to detach ions in the excited state but is insufficient to do the same for ions in the ground state. The higher energy r2 of the tuneable laser 2 is set to investigate the threshold for detachment from the ground state [note: depletion is brought about by the intensity of the laser (photon flux) not the energy of the photons].
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prohibited between them. As a result, the excited 2p3 2D state, situated 1.23 eV above the ground state (33 meV below the detachment threshold), is metastable. As described in the introduction, the main goal in this work is to investigate how a laser with a photon energy that is matched to the energy difference between the ground state of the negative ion and the neutral atom can be used to obtain an isotopically selective detection. This excitation is labelled r2 in Fig. 2. However, for ions in the metastable 2 D state, no isotopic selectivity can be obtained using laser r2. Ions in this state were therefore removed from the beam prior to the interaction with laser r2. As shown in Fig. 2, it is possible to select the photon energy, labelled r1, which is sufficient to neutralise the excited state ions, but it is insufficient to detach electrons from ground-state ions. In the experiment the laser beam of frequency r1 was directed anti-parallel to the ion beam direction in the straight beam line section situated just after the magnet (interaction region 1). Thereafter, the remaining ions in the beam, now predominantly in the ground state, were steered by a 90° electrostatic quadrupole deflector into the interaction region 2, where they where merged collinearly with the second laser beam of frequency r2. The overlap of the ion and laser beams in region 2 was 0.7 m long and was defined by two 3 mm apertures. Downstream of this interaction region, the negative ions remaining in the beam were deflected out of the laser beam and into a Faraday cup by an electric field between two vertically aligned electrodes. Ion currents measured at this point were typically a few nA. Neutral species were generated in the interaction region by neutralisation of the negative ions, either purposely via photodetachment (signal) or via detachment in collisions with the residual gas particles (background). The neutral particles were not affected by the applied electric field and entered a neutral particle detector placed about 20 cm further downstream, where they impinged on a transparent glass plate coated with a thin layer of In2O3:Sn. Secondary electrons produced by the particle impact were finally detected by a channel electron multiplier (CEM). The whole secondary electron detector arrangement was placed inside a Faraday cage biased at 50 V in order to prevent stray electrons from being detected. The transparency of the coated glass plate allowed the laser light to be transmitted, enabling us to use both co- and counter-propagating laser-ion beam geometries. A potential problem with this type of detector is the generation of a considerable number of electrons by a laser-light-induced photoelectric effect on the coated glass plate. Electrons created in this manner, however, were prevented from reaching the CEM by electrically biasing a grid placed in front of the opening of the CEM during the laser pulse. After a pre-set delay time of about 10 ns following the laser pulse, the bias on the grid was turned off to allow the detection of the impinging atoms, which arrive much later from the interaction region. The output signal from the CEM was fed into a gated counter, where a narrow time window was set to take data only in coincidence
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with the arrival of those neutrals at the detector that had been exposed to laser light. As was verified, the signal-tobackground ratio was appreciably enhanced if the gating was properly set. This arrangement allowed efficient discrimination against any continuously distributed electron background arising, for example, from collisional detachment. The gated detection was necessary to achieve meaningful results since we were only able to use a low laser duty cycle of 10 Hz in the experiment. The primary source of background counts, i.e. from collisional detachment, was further minimized by maintaining the apparatus under ultra-high vacuum conditions by use of three differentially pumped sections, each separated by 3 mm diameter apertures. A diffusion pump was used in the ion source region, and ion pumps were used in the magnet and interaction regions. The nominal working pressure in the interaction region was maintained around 109 mbar. Two different lasers were used in the experiments. Metastable state depletion in the straight section of the beam line prior to the electrostatic quadrupole deflector was carried out by using the 1064 nm fundamental output from a Nd:YAG laser (r1) with about 80 mJ pulse energy. The tuneable laser light (r2) used to selectively detach electrons from the ground-state ions of a particular isotope was generated by an optical parametric oscillator (OPO) system that was pumped by the third harmonic of another Nd:YAG laser. Both laser systems provided pulses of 6 ns duration at a rate of 10 Hz and were properly synchronised to interact with the same ion ensemble. The OPO laser system was tuned to a wavelength around 981.2 nm. Taking the Doppler shift into account, this value corresponds to the binding energy of the C ion in its ground state, which is given by the electron affinity of the carbon atom of 10179.67(15) cm1 [20]. Laser pulse characteristics of the OPO system were typically 1 mJ pulse energy and a bandwidth of 6 GHz. The laser intensity was continuously monitored with a power meter situated after the interaction region. The OPO laser beam r2 could be directed either parallel or anti-parallel to the direction of motion of the ion beam. The signals from the neutral atom detector, the laser power meter and the negative ion Faraday cup were all recorded by a PC computer using a LabView-based data acquisition program.
Fig. 3. Ion beam mass spectrum of the carbon mass region showing the isotope abundance ratio, 13C/12C, of approximately 1. The y-axis shows the ion current as measured in the Faraday cup. Gaussian functions are fitted to the mass peaks. The weak peaks on the higher mass side of the main peak are artefacts associated with imperfections in ion source and ion optics.
the ion source and ion optical system. They have no effect on the general outcome of this experiment. The effect of the laser-induced depletion of the excited state population by laser r1 was explored by measuring the change in the production of neutral 12C atoms around the photodetachment threshold when laser r1 is turned on and off. In both cases, laser r2 was scanned from 10190.70 cm1 to 10192.70 cm1 in the ion rest frame (taking into account the Doppler shift of the threshold). The data are plotted in Fig. 4. The circles show the laserinduced production of 12C atoms taken with laser r1 on and the square symbols show the same signal with laser r1 off. The observed reduction in the yield of neutrals below the threshold energy when laser r1 is on amounts to a factor of about 3. This suppression is due to the depletion in the beam of ions in the excited state. The sharpness of the onset of the photon-induced neutral atom signal is limited by the finite laser bandwidth. The isotope-dependent energies of the photodetachment thresholds for the isotopes 12C and 13C were determined by
3. Results and discussion Fig. 3 shows the mass spectrum of the enriched carbon sample, as obtained by scanning the field of the analyzing magnet and recording the negative ion current in the Faraday cup at the end of the interaction region. The mass spectrum shows an approximately equal abundance of the 12C and 13C isotopes in accordance with specifications of the sample placed in the ion source. A weak tailing and a small side peak are observed on the high-mass side of each primary peak. These peaks, which are about 5% of the intensity of the primary peaks, are attributed to experimental artefacts associated with imperfections in
Fig. 4. Threshold region of the cross section for photodetachment of 12C. The signal from the neutral atom detector is recorded during scans of laser r2 across the onset. Circles represent measurements taken with laser r1 turned on in order to suppress excited state population. The squares represent measurements taken with laser r1 turned off.
tuning laser r2, while setting laser r1 for the maximum depletion of the excited ions. These data are shown in Fig. 5. The separation of the photodetachment thresholds for the isotopes 12C and 13C is extracted by fitting the parameters of the Wigner law to the data near threshold (1). The small discrepancy between the experimental data and the Wigner law observed just below the threshold is caused by the finite bandwidth of the laser. This fact must, of course, be taken into consideration in an absolute electron affinity measurement but is of no importance here since we are only interested in the relative threshold energies. A separation of DEexp = 0.45(8) cm1 was determined from the data. The value is in good agreement with the sum of the Doppler shift and the normal mass shift of DEtheo = 0.42 cm1, keeping in mind the uncertainty of a possible specific mass shift contribution, which could easily amount to values of the order of ±0.06 cm1. The rather large experimental uncertainty that is quoted here is due to the finite bandwidth of the laser and deviations from the simple Wigner square root law that were not included in the fitting function. Utilizing this energy separation, we decided to set the photon energy of laser r2 to 10191.60 cm1 (position A) to selectively detach 13C ions while leaving 12C ions unaffected. By simply reversing the direction of laser r2 relative to the ion beam and correspondingly retuning the laser, electrons from 12C instead of 13C could just as easily be selectively detached. A mass spectrum, recorded with the laser r2 set to wavenumber position A in Fig. 5 (10191.60 cm1), is shown in Fig. 6. Here the photon-induced neutral atom signal from the neutral atom detector is shown. The two possible sources of background arise from either collisional detachment or isobaric molecular interferences stemming from neutralization of 12CH ions that are produced in the ion source. Contributions from these sources have been considered in the final data as will be described below. The 12CH ion has an electron affinity of 1.238(8) eV [21], somewhat
Fig. 5. Energy dependence of the photodetachment threshold measured for the carbon isotopes via the neutral atom detector. Circles and squares represent data obtained for the isotopes 13C and 12C, respectively, while the lines are Wigner law fits to the data. The x-axis gives the laboratory frame wave numbers of laser r2. The points A at 10 191.60 cm1 and B at 10190.56 cm1 indicate the positions used in the experiment to accumulate the ‘‘signal” and the background, respectively, as discussed in the text.
Detected neutral particles (a.u.)
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12
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13
Fig. 6. Laser photodetachment mass spectrum of the carbon mass region. The y-axis shows the number of detected neutral atoms produced by the photodetachment process. The inset shows the raw data at laser position A (upper hatched line) and B (lower dotted line), where the direction of the laser light is parallel with the ion beam in both cases. The resulting curve, obtained by subtraction of these two is shown in the main window. Points represent the experimental data and the line represents a fit consisting of the sum of two Gaussian curves.
below the 13C atom electron affinity of 1.262 eV [20], and hence, both the 13C and the 12CH ions are neutralized by laser r2. An ideal solution to eliminate this molecular interference would be to photodetach the molecular ions with laser r1 or another laser, before they reach the primary interaction region. However, at the time of the experiment, no laser capable of generating the required photon energy was accessible. Instead the background contribution from the detachment of ions of molecular species was determined together with the collisional background by recording a separate spectrum with laser r2 tuned to 10190.56 cm1 (position B) in Fig. 5, well below the detachment threshold for both isotopes 12C and 13C. The background data are shown in the inset of Fig. 6, together with the photodetachment measurement that was recorded at energy position A. At position B, only the 12CH ions are neutralised by the laser light. Thus a precise value for the background arising from both isobaric molecular interferences and collisional detachment was determined and could be subtracted from the data obtained at position A. As can be seen in the figure, 13C atoms are detected with much higher efficiency than 12C atoms. Gaussian function fits to the data for both isotopes are represented by the solid line in Fig. 6. From the relative peak areas, the number of detected 13C atoms was found to be 13 times larger than the number of detected 12C atoms. Testing the reproducibility of this measurement gives rise to a final uncertainty of about 20%. Factors affecting the reproducibility are the overall experimental conditions and specifically the operation of the ion source, which determines the production rate of different molecular ions. 4. Outlook and conclusion In the present paper laser photodetachment of negative ions has been used to explore the possibility of selective detection of a specific isotope of carbon. The method
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exploits the mass-dependent Doppler shift experienced by a fast moving ion during the absorption of light. To achieve high isotopic selectivity, we used a narrow bandwidth laser source whose wavelength was tuned to lie between the photodetachment thresholds of the isotopes under study. In this way, ions of one (or more) isotope(s) in the beam can be selectively neutralized by detachment. The number of detected 13C atoms was 13 times higher than the number of detected 12C atoms, although there were equal amounts of 12C and 13C ions in the beam. The problem associated with beam ions in metastable excited states was overcome by applying an additional lower energy laser to deplete their population prior to the primary interaction region. This background was suppressed by a factor of about three. The LPMS method described has two possible future applications. It can be used either for analytical purposes or as a tool in fundamental studies of atomic and molecular negative ions. First we will discuss the possibility of incorporating the experimental scheme described in this paper into a mid-size or small AMS machine used for analytical purposes: By overlapping, in a counter-propagating geometry, the ion beam at the low energy side of the accelerator with one or more laser beams prior to injection into the tandem accelerator one could deplete ions of the lighter, and hence faster moving, isotopes in the beam. For example, 12C and 13C ions could be neutralized leaving 14C ions to pass through the tandem accelerator. In addition, and of equal importance, there would be a discrimination against any interfering beam component with an electron affinity smaller than that of the atom of the element under investigation. For example, in the case of 14C detection, isobaric molecular interferences from 12CH2 and 13CH could be efficiently suppressed by neutralizing the ions of these molecules by photodetachment. Such an experimental scheme, however, would require peak powers that can only be produced with very powerful pulsed lasers. Such lasers, however, typically have a low duty cycle. In the present experiment, the number of ions interacting with the laser light is determined by the time they spend in the interaction region. The time of flight for the carbon ions over the 0.7 m long interaction path is about 3 ls when ions are accelerated to 5 keV. With a 10 Hz laser system, the duty cycle is therefore only 3 105. This value could, of course, be improved substantially by using lasers with high repetition rates of e.g. 10 kHz. This would yield a duty cycle of 3%. The corresponding efficiency losses of ‘‘only” a factor of 30 or less, associated with a comparable gain in isotopic and molecular selectivity, could well be acceptable for a number of experiments. Such a scheme would be a realistic basis for applicability of the LPMS technique. As an alternative, the ion source could be pulsed, for example by switching the Cs source extraction potential. A duty cycle of up to 100% could then, in principle, be achieved with careful synchronization of the lasers and ion beam. In conclusion we state that the capability of the LPMS method to selectively
neutralize specific isotopes was demonstrated and application seems feasible, but it would take an additional development of high power, high repetition rate lasers until the method can be utilized in existing mass spectrometric analytical systems. Recently, Liu et al. [22] installed an ion beam cooler and buncher unit at a low energy mass spectrometer. By using a technique analogous to the LPMS method these authors were able to demonstrate that it is possible to both slow down and energetically compress a negative ion beam in a radio frequency quadrupole trap filled with helium buffer gas. The energy compression had the effect that the geometrical cross section of the ion beam was substantially reduced. This allowed for a better overlap between the ion beam and a well-focussed laser beam. Even more important, the interaction time was substantially increased. As a result of these improvements, the photodetachment process could be saturated even with medium-power continuous wave lasers. A duty cycle of 100% could then be achieved without pulsing the ion source. The reduced velocity of the ions in the buncher would, however, strongly reduce the isotopic selectivity of LPMS, but it could be used, for example, to more efficiently suppress molecular interferences in an AMS system. The original idea behind this work was to develop an experimental scheme that could be used to suppress isotopic and molecular isobaric interferences in analytical mass spectrometry. The first real application of the technique, however, might be in the field of atomic and molecular physics. During the course of the work it has become clear that our present LPMS apparatus is well suited to greatly improve the accuracy of measurements of the structure of atomic and molecular negative ions. Negative ions are of considerable fundamental interest due to the relative importance of the theoretically interesting electron correlation effect. More recent theoretical work of particular relevance for this work is a calculation of the isotope shift in the electron affinity of oxygen [23] and of beryllium [24], and very high precision calculations of the electron affinities of a number of light elements, including carbon, were performed by de Oliveira [25]. On the experimental side, one complication is that traditional optical spectroscopy cannot be applied in order to gain structural information about most negative ions. This is due to the fact that almost all atomic negative ions lack bound excited states with parity opposite to the ground state, the notable exceptions being Os [26] and possibly Ce [27,28]. Instead, the photodetachment process is the only method for spectroscopic investigations that can be used. Since this is a continuum process, all energetically allowed detachment channels will contribute to the signal. These unwanted channels arise from the photodetachment of an electron from an ion in an excited state. An accurate method of determining an electron affinity is to measure the laser photon energy at the onset of detachment of
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electrons from an ion in its ground state. This signal frequently sits on top of a background associated with the photodetachment from ions in excited states, such as the 2D state of C discussed in the present paper. As we have demonstrated, the population in such states can be depleted prior to the interaction region, hence reducing the background arising from the presence of an excited state in the negative ion. With this improvement, laser photodetachment threshold (LPT) spectroscopy [20] should be able to compete with the laser photodetachment microscopy (LPM) method which, in later years, have produced the most accurate electron affinity measurements [29,30]. It should be pointed out that the low duty cycle is not a limitation in experiments where the fundamental properties of negative ions are being investigated. Since the ions are stable there is no limit in the sample volumes available for the experiments. In this type of experiment it would, of course, be of great interest to install an ion beam cooler and buncher in order to facilitate an almost complete depletion of excited state ions. This method could be of even greater interest in studies of molecular negative ions, where the wealth of rotational and vibrational states makes the interpretation of photodetachment spectra very difficult. These studies would require the access to two powerful tuneable laser sources. We expect soon to have access to two such laser systems in our laboratory. Our future plans call for the initial application of such a LPMS system to make a precise determination of the electron affinity of a variety of atomic and molecular systems. Acknowledgements This work was supported by the Swedish Science Foundation. Additional support was provided in part by the National Science Foundation under Grant No. 0456916, DAAD Grant No. 26021 and by STINT. R.D.T. would like to thank the Swedish Science Foundation for funding a Research Fellowship. References [1] C. Tuniz, J.R. Bird, D. Fink, G.F. Herzog, Accelerator Mass Spectroscopy, CRC Press LLC, Boca Raton, 1998. [2] W. Kutschera, Progress in isotope analysis at ultra trace level by AMS, Int. J. Mass Spec. 242 (2005) 145. [3] L.K. Fifield, Accelerator mass spectrometry and its applications, Rep. Prog. Phys. 62 (1999) 1223. [4] S. Bowman, Radiocarbon Dating, University of California Press, Los Angeles, 1990. [5] L.-A. Currie, The remarkable metrological history of radiocarbon dating II, J. Res. NIST 109 (2004) 185. [6] M. Suter, A new generation of small facilities for accelerator mass spectrometry, Nucl. Instr. and Meth. B 139 (1998) 150. [7] H.-A. Synal, S. Jacob, M. Suter, The PSI/ETH small radiocarbon dating system, Nucl. Instr. and Meth. B 172 (2000) 1.
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