Resonance lonization mass spectroscopy with a pulsed thermal atomic beam

Resonance ionization mass spectroscopy (RIMS) and pulsed-laser induced desorption (PLID) have been combined for ultrasensitive detection and spectroscopy of very small samples of refractive elements. The method has been tested and applied to laser spectroscopy of 5×10⁹ atoms (1.5 pg) of¹⁹⁵Au (T _1/2= 183d) implanted at the ISOLDE online mass separator with 60 keV into graphite. A pulsed thermal atomic beam was formed by laser desorption with a 10 ns NdYag laser pulse. Subsequently the atoms were photoionized in a three-colour, three-step resonant excitation to an autoionizing state. The selectivity was enhanced by a time-of-flight measurement of the photo ions. In resonance, one ion was detected per 10⁵ atoms implanted resulting in a gain in detection efficiency by three orders of magnitude in comparison to the use of a continuous atomic beam. In the course of the experiments several unknown autoionizing states were found, and the lifetime of the 6d ² D ^3/2 state of gold was determined to be=10.7(6) ns.     

Resonance Ionization Mass Spectrometry (RIMS) with Pulsed and CW-Lasers on Plutonium

The detection of long-lived plutonium isotopes in ultra-trace amounts by resonance ionization mass spectrometry (RIMS) is a well-established routine method. Detection limits of 10⁶ to 10⁷ atoms and precise measurements of the isotopic composition have been achieved. In this work multi-step resonance ionization of plutonium atoms has been performed with tunable lasers having very different output intensities and spectral properties. In order to compare different ways for the resonance ionization of plutonium broadband pulsed dye and titanium:sapphire lasers as well as narrow-band cw-diode and titanium:sapphire lasers have been applied for a number of efficient excitation schemes. It has been shown, that for identical excitation schemes the optical isotope selectivity can be improved by using cw-lasers (bandwidths < 10 MHz) instead of pulsed lasers (bandwidths > 2 GHz). Pulsed and cw-laser systems have been used simultaneously for resonance ionization enabling direct comparisons of pulsed and continuous ionization processes. So far, a three-step, three-color laser excitation scheme has been proven to be most practical in terms of efficiency, selectivity and laser wavelengths. Alternatively a newly discovered three-step, two-color excitation scheme which includes a strong two-photon transition from an excited state into a high-lying autoionizing state yields similar ionization efficiencies. This two-photon transition was characterized with respect to saturation behavior and line width.     

Efficient three-step,two-color ionization of plutonium using a resonance enhanced 2-photon transition into an autoionizing state

Resonance ionization mass spectrometry (RIMS) has proven to be a powerful method for isotope selective ultra-trace analysis of long-lived radioisotopes. For plutonium detection limits of to atoms have been achieved for various types of samples. So far a three-step, three-color laser excitation scheme was applied for efficient ionization. In this work, a two-photon transition from an excited state into a high-lying autoionizing state, will be presented, yielding a similar overall efficiency as the three-step, three-color ionization scheme. In this way, only two tunable lasers are needed, while the advantages of a three-step, three-color excitation (high selectivity, good efficiency and low non-resonant background) are preserved. The two-photon transition has been characterized with respect to saturation behavior and line width. The three-step, two-color ionization is a possibility for an improved RIMS procedure.Received: 6 January 2004, Published online: 24 February 2004PACS: 82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI) - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz Autoionization     

Laser resonant ionization of plutonium

Resonance ionization mass spectrometry (RIMS) has been tested for the isotope-selective determination of trace amounts of plutonium. An atomic beam is formed by evaporating plutonium atoms from a rhenium-filament heated to 1800 °C. The radiation of a pulsed dye laser excites the atoms in a two-photon process (=595.2 nm) followed by photoionization of the excited atoms. Mass selectivity is obtained by use of a time-of-flight spectrometer. A resonant signal of²³⁹Pu was measured with 10¹³ atoms deposited on the filament.     

237Np and 57Fe Mössbauer study of NpFeGa5

⁵⁷Fe and ²³⁷Np Mössbauer ōmeasurements have been performed for NpFeGa₅, which is one of the so-called neptunium 1-1-5 compounds. The ⁵⁷Fe Mössbauer spectra below T _N = 118 K show the magnetically ordered state. The magnitude of the hyperfine magnetic field at the ⁵⁷Fe nucleus is determined to be 1.98 ± 0.05 T at 10 K. From the ²³⁷Np Mössbauer spectrum at 10 K, the hyperfine magnetic field at the ²³⁷Np nucleus is 203 T and the hyperfine coupling constant is determined to be 237 T/μ_B using the Np atomic magnetic moment of 0.86 μ_B determined by the neutron diffraction study.     

Relationship between 237Np Mössbauer parameters and bond distances in nitrogen coordinated [NpO2(acac)2py]

We now report the ²³⁷Np Mössbauer spectra for [NpO₂(acac)₂py] I. The neptunium(VI) ion in I is coordinated by six oxygen atoms and the one nitrogen atom of pyridine. The isomer shift value obtained for I is ?40 mm/s, which falls in the range for the Np(VI) oxidation state. The relationship between the isomer shift and Np–O bond distance of the neptunyl group for oxygen coordination compounds is in good agreement with the reported structure of I. It is well known that the mean Np–O bond distance of the neptunium compounds is shorter by 0.01 Å than that of analogous uranium compounds. The good agreement in the ²³⁷Np–Mössbauer parameters and the Np–O distance has indicated that the reported analogous [UO₂(acac)₂py] structure may have some inaccuracies. Therefore, we have re-determined the crystal structure of [UO₂(acac)₂py]. The U–O bond distance we have obtained is reasonable.     

Systematic trends in the237neptunium Mössbauer isomer shift: Overlap of IV,V and VI neptunium oxidation states and correlation between isomer shift and crystal structure

The influence of the neptunium ion environment on the²³⁷Np Mössbauer isomer shifts has been studied in various metal coordination complexes: fluorides, oxides, oxide fluorides and polycarboxylates. A linear dependence between the isomer shift and the mean neptunium-ligand distance in a series of hexavalent Np compounds has been evidenced and the feasibility of overlapped isomer shift areas, namely Np(IV), Np(V) and Np(VI) has been established.     

237Np Mössbauer spectroscopy on neptunium doped borosilicate glasses

Mössbauer and magnetization measurements have been carried out on borosilicate glasses containing neptunium. Different melting and annealing times, in various crucibles have been investigated. Using the 59.5 keV Mössbauer resonance in²³⁷Np we have characterized valency states of neptunium ions in glasses and studied hyperfine interactions. Roughly, absorption spectra show two sites easily differenciated above 50 K by two lines. Isomer shifts (relative to NpAl₂) are respectively: – 10.0 (1.0) and + 32.5(.5) mm/s. These resonance lines can be assigned, the first at Np⁴⁺, the second at Np³⁺ species. The intensity ratio Np⁴⁺/Np³⁺ varies with elaboration conditions. At 4.2 K, Np⁴⁺ ions exhibit a hyperfine splitting arising from relaxation phenomenon. Magnetization measurements do not show any magnetic ordering.     

Resonantly enhanced three-photon excitation spectra of lithium

We present the first measurement on the resonantly enhanced three-photon excitation spectra of natural lithium using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. Exploiting the linear and circular polarizations, the np ²P_3/2(8 ? n ? 11) and nf  ²F_7/2 (8 ? n ? 38) series have been observed via three-photon excitation from the ground state. The measured level energies reveal a dynamic shift from calculated values, which increases with an increase of the principal quantum number n. The ac stark shift and line broadening mechanisms are studied as a function of laser intensity. It is noted that the width increases and the line center shifts towards the higher energy side as the laser intensity is increased. The maximum observed shift for the 12f ²F_7/2 line is 0.33 cm⁻¹ corresponding to the laser intensity variation from 1.34 × 10¹² W/m² to 1.03 × 10¹³ W/m², whereas its width increases from 0.36 cm⁻¹ to 0.82 cm⁻¹.     

Systematics of Mössbauer hyperfine parameters in Np intermetallics

Data for intermetallic compounds of neptunium obtained with the 60 keV Mössbauer resonance of²³⁷Np are reviewed. Measurements of temperature, pressure and field dependencies are available. The main questions addressed are: (a) the degree of delocalization of 5f-electrons, (b) the formal charge state of Np, and (c) the influence of the ligand on the neptunium electronic structure. For this purpose, we present an evaluation of systematic behavior concerning mainly the hyperfine field and isomer shift in the cubic Laves phase materials NpX₂, the NaCl-type monochalcogenides and monopnictides, and intermetallics with AuCu₃ and ThCr₂Si₂ structures. Analogies to corresponding rare-earth compounds will be pointed out.     

A high-temperature laser ion source for trace analysis and other applications

We report here on the development of a high-temperature laser ion source useful for trace analysis and other applications. It consists of a high temperature ionization chamber, three tunable dye lasers pumped by copper vapor lasers for stepwise resonant ionization and a Mattauch-Herzog mass spectrometer for the analysis of photo-ions. The principle of the laser ion source and its theoretical efficiency are discussed, where the efficiency of a laser ion source is the ratio of photo-ions extracted out to the number of atoms introduced into the cavity. Experimentally, an efficiency of 2×10^–3 has been achieved for technetium. The scheme of gated detection is described which is used for suppressing isobaric background of molybdenum. The possible improvements are briefly mentioned for achieving isotopic analysis of 10⁸ atoms of ^97,98Tc in the presence of 10¹⁵ atoms of molybdenum, as separated chemically from molybdenum ore. Such an analysis of technetium isotopes produced in molybdenite ore by a (v,e ^–) reaction, is expected to yield information about the solar neutrino flux.Permanent address: Bhabha Atomic Research Centre, Bombay 400085, India     

Radiochemical determination of 237NP in soil samples contaminated with weapon grade plutonium

The Palomares terrestrial ecosystem (Spain) constitutes a natural laboratory to study transuranics. This scenario is partially contaminated with weapon-grade plutonium since the burnout and fragmentation of two thermonuclear bombs accidentally dropped in 1966. While performing radiometric measurements in the field, the possible presence of ²³⁷Np was observed through its 29 keV gamma emission. To accomplish a detailed characterization of the source term in the contaminated area using the isotopic ratios Pu-Am-Np, the radiochemical isolation and quantification by alpha spectrometry of ²³⁷Np was initiated. The selected radiochemical procedure involves separation of Np from Am, U and Pu with ionic resins, given that in soil samples from Palomares ^239+240Pu levels are several orders of magnitude higher than ²³⁷Np. Then neptunium is isolated using TEVA organic resins. After electrodeposition, quantification is performed by alpha spectrometry. Different tests were done with blank solutions spiked with ²³⁶Pu and ²³⁷Np, solutions resulting from the total dissolution of radioactive particles and soil samples. Results indicate that the optimal sequential radionuclide separation order is Pu-Np, with decontamination percentages obtained with the ionic resins ranging from 98% to 100%. Also, the addition of NaNO₂ has proved to be necessary, acting as a stabilizer of Pu-Np valences.     

Laser-induced breakdown of krypton near five-photon ionization threshold

Selective excitation of laser-induced breakdown in krypton in the pressure range 1–4 bar and the spectral range 420–620 nm at a laser intensity of 10¹¹ W/cm² has been studied. It is shown that the breakdown of krypton can be induced selectively due to resonantly enhanced multiphoton ionization via excited atomic states. A number of four- and five-photon atomic resonances have been identified in breakdown excitation spectra. The role of different factors determining the selectivity of the breakdown has been studied. Possible analytical applications of a selective breakdown are discussed.     

Sorption kinetics of uranium-238 and neptunium-237 on a glacial sediment

Studies of uptake of radionuclides by natural materials have shown that sorption may occur via fast equilibrium exchange and/or slow kinetic processes, which can be described using a series of box models. This paper describes the use of such models to evaluate the solid-solution partitioning of ²³⁸U and ²³⁷Np on a clay rich sediment. Experimental data are obtained using the batch sorption technique, which are then analysed using 1, 2, and 3 box kinetic models. Uptake of ²³⁸U is initially rapid, with approximately 90% sorbed within the first 30 minutes. Sorption continues, but at a slower rate. Uptake of ²³⁷Np is initially slower than U, with approximately 30% sorbed within the first 30 minutes. Sorption again continues, at a slower rate. Analysis of the experimental data indicates that sorption can be described using 2 and 3 box kinetic models. The results demonstrate that sorption of uranium and neptunium on clay rich sediments occurs via equilibrium and kinetically controlled pathways, with neptunium being controlled by kinetics to a greater extent than uranium. The 2 or 3 box model can describe sorption of neptunium, uranium requires the 3 box model.     

Line shape analysis of the 60 keV Mössbauer resonance in237Np: Interference effects

The dispersion parameter arising from the interference between photo and conversion electrons has been determined for the 59.6 keV E1-Mössbauer resonance in²³⁷Np to be =–(3.4±0.2)×10^–2. Using this result, simulations of various types of resonance spectra were performed to illustrate the significance of the interference lineshape in the analysis of Mössbauer data of²³⁷Np.     

Quantized infrared-optical triple resonance on a single cold barium ion

A single trapped and cooled Ba⁺ ion is irradiated by resonant visible light (493, 650 nm) alternating with light at 1.76 µm which may excite the ion to its² D _5/2 metastable state. The (absence of) visible resonance scattering probes the excitation, tuning spectra of which show vibrational sidebands that characterize the ion's temperature. Observed values as low as 120 µK, one-eighth the Doppler limit, are ascribed to electronic Raman cooling by the visible light. Tuning spectra of the events of stimulated deexcitation indicate ion heating by the IR interaction. The results demonstrate the feasibility of vibrational spectrometry on a single particle that oscillates in a potential well, forming a quasi-molecule.     

Measurements of photoionization cross sections from the 4p, 5d and 7s excited states of potassium

New measurements of the photoionization cross sections from the 4p ²P_1/2,3/2, 5d ²D_5/2,3/2 and 7s ²S_1/2 excited states of potassium are presented. The cross sections have been measured by two-step excitation and ionization using a Nd:YAG laser in conjunction with a thermionic diode ion detector. By applying the saturation technique, the absolute values of the cross sections from the 4p ²P_3/2 and 4p ²P_1/2 states at 355 nm are determined as 7.2±1.1 and 5.6±0.8 Mb, respectively. The photoionization cross section from the 5d ²D_5/2,3/2 excited state has been measured using two excitation paths, two-step excitation and two-photon excitation from the ground state. The measured values of the cross sections from the 5d ²D_5/2 state by two-photon excitation from the ground state is 28.9±4.3 Mb, whereas in the two-step excitation, the cross section from the 5d ²D_3/2 state via the 4p ²P_1/2 state and from the 5d ²D_5/2,3/2 states via the 4p ²P_3/2 state are determined as 25.1±3.8 and 30.2±4.5 Mb, respectively. Besides, we have measured the photoionization cross sections from the 7s ²S_1/2 excited state using the two-photon excitation from the ground state as 0.61±0.09 Mb.     

Trace detection of plutonium by three-step photoionization with a laser system pumped by a copper vapor laser

Laser photoionization has been used to detect trace amounts of plutonium. A high sensitivity and selectivity has been achieved by applying three-step excitation and ionization of the plutonium atoms with high pulse-repetition rates and additional mass determination by time-of-flight measurements. A laser system was developed which consists of a copper vapor laser pumping three dye lasers simultaneously. Samples containing between 10¹⁰ and 10¹² atoms of²³⁹Pu on Re filaments were measured yielding strong resonance signals with maximum ion count rates of several kHz at a vanishingly low background. A detection efficiency of 10^–7 was determined allowing the detection of about 10⁸ plutonium atoms in a sample.This work comprises part of the doctoral dissertation of W. Ruster, to be submitted to Johannes-Gutenberg-Universität, Mainz     

Effect of residual oxygen on ionization processes of Si and Si sputtered from Si(1 1 1)-7 × 7 surface

The effect of residual oxygen impurity on ionization processes of Si⁺ and Si²⁺ has been studied quantitatively. In this study, ion sputtering experiments were carried out for a Si(1 1 1)-7 × 7 surface, irradiated with 9-11 keV Ar⁰ and Kr⁰ beam. Even if the oxygen concentration is less than the detection limit of Auger electron spectrometry, SiO⁺ and SiO₂⁺ ions have been appreciably observed. Moreover, as the SiO⁺ and SiO₂⁺ yields increases, the Si⁺ yield is slightly enhanced, whereas the Si²⁺ yield is significantly reduced. From the incidence angle dependence of secondary ion yields, it is confirmed that Si⁺* (Si⁺ with a 2p hole) created in the shallow region from the surface exclusively contributes to Si²⁺ formation. By assuming that the SiO⁺ and SiO₂⁺ yields are proportional to the residual oxygen concentration, these observations are reasonably explained: The increase of Si⁺ with the increase of residual oxygen is caused by a similar effect commonly observed for oxidized surfaces. The decrease of Si²⁺ yield can be explained by the inter-atomic Auger transition between the residual oxygen impurity and Si⁺*, which efficiently interferes the Si²⁺ formation process.     

Wavelength dependent Stark compensation in isotope selective Doppler-free two photon ionization

A novel wavelength dependent Stark shift compensation method for isotope selective ionization in Doppler free two-photon spectroscopy is theoretically investigated. In the present paper, certain wavelengths for the ionization step termed as “magic wavelengths” are identified for compensating the Stark shift induced by the excitation laser. The numerical calculations indicate that, ionization efficiencies can be maximized by the judicious choice of the intensities of the excitation and the ionization lasers without much degradation in the optical selectivity. The effects of Stark shift induced asymmetry and its reversal by selecting the appropriate magic wavelength for the ionization step for various excitation and ionization laser intensities have been investigated. The wavelength dependent Stark compensation method can be adapted for the isotope selective Doppler free two-photon excitation followed by non-resonant ionization process for any atomic system. In the present work, the theoretical results pertaining to the specific example of ⁴¹Ca has been presented.