Selective ultratrace analysis of 41Ca by laser resonance ionization

A compact resonance ionization mass spectrometer is presented. It is presently applied for sensitive and highly selective ultratrace determination of the long-lived radioisotope ⁴¹Ca for environmental, biological, and fundamental investigations. The development of coherent multistep resonance ionization enables the realization of experimental detection limits as low as10⁶ atoms per sample and very high isotopic selectivity above 10¹². This revised version was published online in August 2006 with corrections to the Cover Date.     

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     

Radiation detected resonance ionization spectroscopy on208Tl and242fAm

An ultra-sensitive laser spectroscopic method has been developed for the hyperfine spectroscopy of short-lived isotopes far off stability produced by heavy ion induced nuclear reactions at very weak intensity (> 1/s). It is based on resonance ionization spectroscopy in a buffer gas cell with radiation detection of the ionization process (RADRIS). As a first on-line application of RADRIS optical spectroscopy at^242fAm fission isomers is in progress at the low target production rate of 10/s. The resonance ionization has been performed in two steps utilizing an excimer dye laser combination with a repetition rate of 300 Hz. The first resonant step proceeds through terms which correspond to wavelengths of 466.28, 468.17 or 426.56 nm; the second non-resonant step is achieved with the 351 nm radiation of the excimer laser itself, running with XeF. The frequency scans of the tuneable dye laser at 466.28 and 468.17 nm exhibit broad resonance ionization signals, the latter with a large isotope shift between^242fAm and²⁴³Am which is in accordance with the large quadrupole moment of the^242fAm fission isomer.Work supported by the Bundesministerium für Forschung und Technologie under contract 06 MZ 188 I.     

An injection-seeded high-repetition rate Ti:Sapphire laser for high-resolution spectroscopy and trace analysis of rare isotopes

An injection-seeded high-repetition rate (～10 kHz) Ti:sapphire laser with a spectral bandwidth of ～20 MHz and an average output power of above 1.5 W has been developed. We report on its demonstration and characteristics with respect to the spectral, temporal, and spatial properties as well as the output energy. In crossed-beam resonance ionization on a well-collimated thermal atomic beam, the ～200 MHz hyperfine structure of the D₂ transition at 308 nm of ²⁷Al has been well resolved. Applications of the system in the field of insource laser spectroscopy for on-line produced short-lived radioactive isotopes as well as for selective-trace isotope determination are discussed.     

New laser setup at the IRIS facility. Magnetic moments and mean squared charge radii of neutron deficient Tl isotopes

A new laser installation for the resonance ionization spectroscopy in a laser ion source and for rare isotope production has been put into operation at the IRIS mass-separator, working on-line with the 1 GeV proton beam of the Petersburg Nuclear Physics Institute synchrocyclotron. Isotope shift and hyperfine splitting of 276.9 nm atomic transition in the long chain of Tl isotopes and isomers have been measured. New data on the magnetic moments and changes in mean squared charge radius for ^{183,184,185,185m,186m2,195m,197m}Tl have been obtained.     

The Rotational Spectra of H2CCSi and H2C4Si

The microwave rotational spectra of two singlet chains H₂CCSi and H₂C₄Si, have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. Following detection of the singly substituted rare isotopic species and D₂CCSi, an experimental structure (r₀) has been determined to high accuracy for H₂CCSi by isotopic substitution. In addition, the rotational transitions of the ²⁹Si and the two ¹³C isotopic species of H₂CCSi exhibit nuclear spin-rotation hyperfine structure. The component of the spin-rotation tensor along the a-inertial axis is abnormally large for each of these isotopic species, especially for that with ²⁹Si, where the magnitude of C_aa is in excess of 700 kHz.     

Laser Stark Spectroscopy and Isotopic Separation

A potentially useful scheme for photoselective isotope enrichment is presented. The selective laser excitation step is accomplished by utilizing the Stark effect to shift molecular transitions into resonance with laser frequencies. The method is demonstrated for isotope separation of ¹³C, ¹⁵N and ²H. Laser Stark spectroscopy can sensitively detect as few as 10⁹ molecules in the optical path, thus the isotopic separation can be achieved from industrial sample mixtures containing natural abundances. A comparison of the isotopic enrichment efficiency using the present method of “exact” resonance and the previously used method of “near” resonance excitation is also given.     

Photothermal ionization spectroscopy for shallow impurities in ultra-pure silicon

We report the comprehensive results obtained in our group and last few years for the shallow impurities in ultrapure silicon by use of photothermal ionization spectroscopy. The new results reported here include the discovery and investigation of new shallow impurity centers in Si, the detection for the compensation of different types of impurities, the accurate determination for the spin-orbit splitting . of valence band for Si, and the phonon duplicates and Fano resonance for the transitions of shallow impurities in Si. In addition it is also shown experimentally that the sensitivity of the photothermal ionization spectroscopy as used for detecting the concentration of shallow donors in Si can reach as high as 10⁸ cm^–3, much higher than that reported in the literatures up to date, and line width for the sharpest spectral lines in the spectrum is about 0.08 cm^–1, that is, 10 eV.     

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.     

激光共振电离过程中铅同位素歧视效应研究

以速率方程为基础,通过数值模拟方法,对Pb的激光共振电离通道：“6s26p2 3P0—6s26p7s 3P0₁—电离”的激光诱导同位素歧视效应进行了研究,以探讨减小激光诱导歧视效应,准确测定Pb同位素比值的有效途径。通过考虑激光线型、原子吸收谱线线型、同位素位移及同位素超精细结构等因素的影响,对速率方程进行近似。在近似的速率方程基础上,以“1+1”激光共振电离过程为例,进而探讨激光峰值功率密度、带宽和中心波长对Pb的同位素歧视效应的影响。研究结果表明,利用最佳波长法,可基本消除同位素歧视效应的影响;提高激发光光强可使Pb同位素饱和电离,从而消除同位素歧视效应的影响;采用宽带激光激发,可减小同位素歧视效应的影响。     

Laser resonance ionization for ultra-trace analysis on long-lived radioactive isotopes

Benefiting from the continuous laser developments, resonance ionization can be applied for a variety of experiments on radioactive isotopes, e.g. as a laser ion source for producing pure beams of short-lived isotopes at on-line facilities. In this paper the application of a compact set-up for resonance ionization mass spectrometry for ultra-trace analysis of the long-lived isotope Ca-41 is described. With this set-up a purely optical selectivity of 3×10⁹ and an overall detection efficiency of 1.2(4)×10⁻⁵ are demonstrated.     

Detection of very rare isotopes by laser collinear resonant ionization of fast atoms

Results are presented of the work on the development of a method for detecting ultra rare isotopes, based on the collinear isotope-selective laser ionization of atoms in a fast bearn. The rare isotope³He was detected at a selectivity of 10⁹ and isotope-selective ionization of krypton atoms was demonstrated. A scheme is suggested for detecting the rare radioactive isotopes⁸⁵Kr and⁸¹Kr. The technique has been exploited for the measurement of hyperfine structures and isotope shifts of unstable Yb isotopes.     

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.     

Ultratrace Determination of Selenium by Hydride Generation–Inductively Coupled Plasma Mass Spectrometry Operated Under Nonrobust Plasma Conditions

A hydride generation–inductively coupled plasma mass spectrometer (HG‐ICPMS) operated at low RF power (720 W) was demonstrated to be suitable for ultra trace determination of Se, which is an element with high ionization potential (IP). ⁷⁸Se analysis free from ³⁸Ar⁴⁰Ar spectral interference was achieved by operating the plasma under a so‐called warm plasma condition (as opposed to traditionally defined hot and cool plasmas). In this warm plasma, there is a sufficient population of energetic Ar⁺ to ensure ionization of Se, while the number of ³⁸Ar⁴⁰Ar ions is at the minimum, due to the dimer's bimodal profile. A detection limit of 1 ng/L was achieved for ⁷⁸Se. Background spectra of warm and hot plasmas were compared. Two types of HG gas–liquid separators were compared. The technique was applied to measure trace amounts of Se in atmospheric samples; namely, cloud water and aerosol particulate. Good agreement was found between HG warm plasma ICPMS and pneumatic micronebulization hot plasma ICPMS for the cloud water samples. The water extract of aerosol particulate typically contained 50–100 ng/L of Se, equivalent to 0.25–0.5 ng/m³ in the atmosphere. Ours appears to be the first report of the use of nonrobust plasma ICPMS for a high‐IP element; the superb accuracy and sensitivity of this technique rival those of other powerful trace analytical methodologies such as HG‐LIF, HG‐LEI, or DRC‐ICPMS. It is conceivable that warm plasma ICPMS is applicable to other elements with high IP.     

Resonance ionization mass spectroscopy for trace analysis of neptunium

Resonance ionization mass spectroscopic (RIMS) measurements for trace analysis and spectroscopy of ²³⁷Np, the ecologically most important isotope of neptunium, are described. The chemical procedure for the separation of neptunium from aqueous samples as well as the preparation of filaments for RIMS are outlined. Several two- and three-step excitation schemes have been investigated in order to find suitable conditions for the sensitive detection of ²³⁷Np. Using a three-step, three-color excitation and ionization scheme an overall detection efficiency of 3×10^–8 was obtained, resulting in a detection limit of 4×10⁸ atoms (160 fg) of ²³⁷Np. The hyperfine structure splittings of the levels under investigation, which influence the detection limit, were measured. A new method to determine the first ionization potential (IP) was applied to neptunium yielding a value of IP=6.2655(2) eV.     

Vibrational spectroscopy of interstellar molecules

In this paper we describe how infrared vibrational spectroscopy has been applied in our laboratory to three areas of interest to interstellar chemistry: the origin of the unidentified interstellar infrared emission bands, the depletion of the iron in the interstellar medium, and the fractionation of isotope-bearing molecules in space. In the first area, infrared absorption spectra of polycyclic aromatic hydrocarbons and their ions trapped in cryogenic rare gas matrices have been analyzed and shown to coincide well with the frequencies and relative intensities of the unidentified infrared emission bands. These bands have been observed from many sources in interstellar space, including reflection and planetary nebulae and H II regions, in parts of the Milky Way, as well as other galaxies. In the second area, a free electron laser, with tunable output in the infrared, has been utilized to record resonant multiphoton photodissociation vibrational spectra of various polycyclic aromatic hydrocarbons complexed with Fe⁺ in a Fourier transform ion cyclotron resonance mass spectrometer. Stable complexes found for Fe⁺ and a number of PAHs suggest that iron could be sequestered in such species and that they could function as carriers of the unidentified infrared emission bands. In the third area, the photo-induced ¹³C-isotopic scrambling in the C₃ carbon cluster has been investigated by matrix isolation infrared spectroscopy. Fractionation is shown to arise from small zero-point energy differences in the C₃ isotopomers. A proposed mechanism of isotopic scrambling involving the formation of cyclic C₃ intermediates may be applicable to other species for which isotopic fractionation has also been observed in the ISM.     

First observation of a resonance ionization signal on242mAm fission isomers

The feasibility of a hyperfine spectroscopy on^242mAm fission isomers has been demonstrated at the low target production rate of 10/s. The experimental method employed is based on resonance ionization spectroscopy in a buffer gas cell with detection of the ionization process by means of the fission decay of the isomers. The resonance ionization has been performed in two steps, utilizing an excimer dye laser combination with a repetition rate of 300 Hz. The first resonant step proceeds through theJ=7/2 term at 21440.35 cm⁻¹, which has been excited with the tuncable dye laser beam of a wavelength of 466.28 nm, the second non-resonant step is achieved with the 351 nm radiation of the excimer laser itself, running with XeF. The frequency scan of the tuneable dye laser exhibits a broad resonance ionization signal, the width of which is most likely explained by the magnetic hyperfine interaction.     

Unified approach to dispersion forces within macroscopic QED

We describe a laser spectroscopy station that has been proposed to exploit the high yields of radioactive beams at the future FAIR facility at GSI, Darmstadt. Exotic nuclei produced in the Super-Fragment-Recoil-Separator (S-FRS) are stopped in a gas cell, extracted, reaccelerated and transported to the LaSpec setup. Here, collinear spectroscopy on ions and atoms, β-NMR experiments or laser-desorbed resonance ionization can be applied to these rare isotopes.     

Resonance ionization spectroscopy of thorium

Here we describe experiments aimed at developing an element-selective ion source for thorium (Th). The technique applied is resonance ionization spectroscopy (RIS) with a thermal atom beam. Ionization schemes for isotopically nonselective ionization of Th as well as for isotopically selective ionization of ²³⁰Th are proposed. The RIS-scheme used is two-photon two-colour ionization with excitation in the ultraviolet spectral range between 244 nm and 267 nm or in the visible spectral range between 485 nm and 529 nm. Ionization of the excited atoms is performed either by ultraviolet photons or by visible photons, depending on the energy required for this process.     

用于原子能级结构研究的激光共振电离光谱系统

激光共振电离光谱技术是一种利用一路或多路激光将待测原子选择性共振激发与电离,通过测量离子信号来研究原子能级结构的光谱技术。研建了一套激光共振电离光谱装置,用于原子高激发态能级结构参数的测量。分别从该装置的总体结构、关键技术和应用实例等方面进行了详细介绍。该套装置主要包括高调谐精度的染料激光器系统、高效的激光离子源系统和高分辨率的飞行时间质量分析器。染料激光器系统包括3台多纵模可调谐染料激光器和1台单纵模可调谐染料激光器,均为脉冲工作方式,重复频率为10 kHz,泵浦源均为532 nm的Nd∶YAG固体激光器。激光离子源系统包括原子化源、激光与原子相互作用区和离子光学透镜组三部分组成,样品在原子化源中被电加热实现原子化,喷射出的原子被激光选择性激发、电离,产生的离子被离子传输透镜整形成能量分散小、束窄的离子束。飞行时间质量分析器采用了反射式结构设计、脉冲垂直推斥技术和偏转板调节技术。利用此装置,实验测定了U原子的自电离态光谱,获得了U原子一条较佳的三色三光子共振电离路径,对应激光的波长分别为591.7,565.0和632.4 nm。此系统还可用于测量同位素位移和原子超精细结构等参数。另外,由于此系统中联用了质量分析器,因此可用于样品多元素分析、痕量元素分析、同位素丰度分析。