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.     

A New Laser Photochemical Process for Isotope Enrichment

Selective laser excitation of a specific isotopic species to its lowest electronic singlet (S₁) and triplet (T₁) states respectively, then followed by a chemical reaction for isotope separation is examined theoretically. It is found that the direct excitation to T₁ state from ground state (S₀) is an unique excitation channel for obtaining a controllable chemical reaction and for achieving a high isotope separation factor. This is due to the following important facts: 1) Resonant energy transfer from a selectively excited triplet isotopic species to an unexcited species of the other isotope present in the mixture is missing (or is very small); 2) The lifetime of the T₁ state is normally longer than that of the S₁ state; 3) There is no intersystem crossing channel that exists for the deactivation of excited isotopic species from T₁ state.

Numerical calculation is performed for the chlorine isotope separation in a mixture of thiophosgene (0.5 torr) and diethoxyethylene (1.5 torr). The result indicates that the isotopic enrichment factor is at least 2.5 × 10³ times larger for selective T₁ excitation than that for S₁ excitation.     

Lithium isotope separation by laser

A lithium isotope separation was performed using a laser isotope separation method. It was found that the lithium atoms with a natural isotopic abundance enhanced its⁶Li concentration up to over 90% by tuning the laser wavelength to the² P _1/2 of⁶Li. Too high power, however, leads to a loss of enrichment due to the power broadening effect which was analysed by the equation of motion of density matrices.     

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, highly selective laser isotope separation of carbon-13

We recently demonstrated an original approach to highly selective laser isotope separation of carbon-13 that employs vibrational overtone pre-excitation of CF₃H together with infrared multiphoton dissociation [O.V. Boyarkin, M. Kowalczyk, T.R. Rizzo, J. Chem. Phys. 118, 93 (2003)]. The practical implementation of this approach was complicated by the long absorption path length needed for the overtone excitation laser beam. In the present work, we employ a low overtone level for the pre-excitation that shortens this pathway, facilitating engineering of the process. We propose an optimal configuration of the isotope separation scheme and consider a realistic example of a separation unit for isotopic enrichment of carbon-13 to 94–98%. The photon energy expenditure of 97 eV per separated atom is much lower than that of the current commercial laser technology, making this process economically feasible.PACS 82.30.Lp; 89.90.+n     

High-efficiency Zn isotope separation in a photochemical reaction induced by two-photon excitation

High-efficiency laser isotope separation was accomplished for zinc atoms in the situation when isotope shifts are much smaller than the Doppler width. The atoms were excited via two-photon absorption of two counter-propagating waves slightly detuned from the intermediate state, so that all “0requisite” atoms were excited. The isotope separation results from a photochemical reaction between the selectively excited zinc isotope and the CO₂ molecules. The reaction rate is 3–5 orders of magnitude higher than for the unexcited atoms. The mechanisms of two-photon resonance line broadening are considered both theoretically and experimentally, and the possibilities of the method for obtaining isotopes in quantitative yields are estimated.     

Investigations of Yb-doped optical fiber using selective laser excitation

The method of tunable site-selective laser excitation was used for a study of the spectral and fluorescence characteristics of Yb³⁺ optical centers in laser optical fiber. Decay curves and positions of fluorescence spectra maxima at different wavelengths of selective laser excitation and positions of excitation spectra maxima at different fluorescence selective registration provide new information on lifetimes and Stark energy distribution among inhomogeneously broadened Yb³⁺ lines. The obtained Stark splitting energy distribution diagram demonstrates the tuneability of Yb³⁺ laser oscillation wavelength under tunable excitation of a pump laser.     

On-line computer controlled cw dye laser spectrometer for laser isotope separation

An on-line cw dye laser multichannel spectrometer is described, which can be controlled by FORTRAN. Automatic tuning of a dye laser to the wavelength of maximum excitation selectivity for photochemical isotope separation is achieved. Applying the system to the selective photoaddition of iodine chloride ICl to acetylene, an enrichment factor of³⁷Cl in the photoproduct C₂H₂ICl of β=100±5 was obtained. Both stable chlorine isotopes can be enriched to more than 97% purity in a single step, starting from natural samples in both cases.     

Metabolite Pools and Metabolic Branching as Factors of in-vivo Isotope Discriminations by Kinetic Isotope Effects

Abstract

Inter- and intra-molecular non-statistical isotope distributions do not only require the existence of a kinetic isotope effect on a defined enzyme catalyzed reaction, but also the prerequisite that this reaction is located at a metabolic branching point. Furthermore a metabolic and isotopic balance demand that the extent of the isotopic shift is reciprocal to the products' yields. On this base the ¹³C-enrichment of L-ascorbic acid in position C-1 and the depletion of glycerol in C-1 are interpreted. The ¹³C-pattern of natural malic acid is discussed as a consequence of isotope effects on the carboxylation of pyruvate and PEP and on the pyruvate dehydrogenase reaction. The patterns of natural products synthezised by transfer of “active acetaldehyde” is proposed to be due to an isotope effect on the thiamine pyrophosphate containing lyase reaction. An isotope effect on the reduction of “active formaldehyde” to “active methyl” and the existence of corresponding pools is responsible for ¹³C-enrichments and depletions of natural products in positions bearing these intermediates. Finally a model for the main nitrogen pools and for isotope discriminations between α-amino, ω-amino-N and amide pools in plants is proposed.     

Towards 0.99999 28Si

A new approach for producing high-purity silicon with isotopic enrichment of ²⁸Si isotope is reported. The methods of centrifugal enrichment were modified to obtain the initial gaseous silicon tetrafluoride with a record-breaking enrichment of 0.99999664(11) with respect to ²⁸Si. The effective conversion of silicon tetrafluoride into elementary silicon with minimal isotopic dilution was achieved in an electron cyclotron resonance discharge plasma, sustained by gyrotron microwave radiation with a frequency of 24 GHz. We have experimentally demonstrated the deposition of the layers of microcrystalline ²⁸Si with enrichment of 0.999986 ± 0.000003, which is the best result at the present time.     

Rotational-coherence molecular laser isotope separation

We have proposed a laser isotope separation method, utilizing rotational coherence of a simple molecule. In the scheme, photoexcited molecules are isotopically separated by difference of rotational period between them. To illustrate this method, two-pulse photodissociation of mixed ⁷⁹Br₂/⁸¹Br₂ isotopes has been investigated theoretically. The photodissociation probabilities of ⁷⁹Br₂ and ⁸¹Br₂ have been calculated as functions of time delay between the photoexcitation and dissociation laser pulses. We have demonstrated that isotope enrichment factor of ⁷⁹Br relative to ⁸¹Br can be changed from 0.34 to 1.8, by simply changing the time delay only by 0.2 ns. Additionally, we have shown that this method is effective for heavy isotopes, based on mass dependence of the isotope enrichment factor.     

Selective photochemical isotope “burning” upon the interaction of resonant laser radiation with atoms

The method of laser isotope separation based on selective excited-atom “burning” in a flow with buffer and reagent gases was implemented experimentally for zinc and rubidium. Selective excitation of isotopes was accomplished by the one-photon method using weak absorption lines and at the edge of the Doppler contour of atomic absorption with small isotope shift.     

Mass dependence of calcium isotope fractionations in crown-ether resin chromatography

Benzo 18-crown-6-ether resin was synthesised by the phenol condensation polymerisation process in porous silica beads, of which particle diameter was ca 60μ Calcium adsorption chromatography was performed with the synthesised resin packed in a glass column. The effluent was sampled in fractions, and the isotopic abundance ratios of ⁴²Ca, ⁴³Ca, ⁴⁴Ca, and ⁴⁸Ca against ⁴⁰Ca were measured by a thermo-ionisation mass spectrometer. The enrichment of heavier calcium isotopes was observed at the front boundary of calcium adsorption chromatogram. The mass dependence of mutual separation of calcium isotopes was analysed by using the three-isotope-plots method. The slopes of three-isotope-plots indicate the relative values of mutual separation coefficients for concerned isotopic pairs. The results have shown the normal mass dependence; isotope fractionation is proportional to the reduced mass difference, (M – M′)/MM′, where M and M′ are masses of heavy and light isotope, respectively. The mass dependence clarifies that the isotope fractionations are originated from molecular vibration. The observed separation coefficient ? is 3.1×10^?3 for the pair of ⁴⁰Ca and ⁴⁸Ca. Productivity of enriched ⁴⁸Ca by crown-ether-resin was discussed as the function of the separation coefficient and the height equivalent to the theoretical plate.     

Laser isotope separation of carbon by multiple IR photon and subsequent UV excitation of CF3I molecules

A new approach to laser isotope separation is considered. It is based on collisionless multiple photon ir laser excitation and subsequent uv laser dissociation of vibrationally excited molecules. TEA CO₂ and excimer XeF, XeCl lasers are used for ir excitation and uv dissociation, respectively. The products of photolysis (C₂F₆) are enriched with¹²C.     

Boron isotope enrichment in nanosecond pulsed laser-ablation plume

Boron isotopic enrichment is observed in the laser ablation of B₄C target using nanosecond (ns) wide 532 nm laser beam of a Nd-YAG laser. B¹⁰/B¹¹ ratio of 0.9 against the natural abundance of 0.25 is obtained at a laser power density of 8×10⁸ W/cm² (fluence of 6.4 J/cm²). The enrichment as a function of laser power density is demonstrated using a quadrupole mass spectrometer. Apart from higher enrichment factor, only singly charged ions are found in the laser plume from the B₄C target, in contrast to the multiply charged ions from the BN target reported in a recent report using femtosecond (fs) laser pulses. This study indicates the possibility of using less expensive, widely used ns lasers, which can also yield a higher throughput per pulse than a fs laser for isotope enrichment. Received: 28 September 2001 / Accepted: 4 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +91-4114/480-065, E-mail: mj@igcar.ernet.in     

Influence of the Stark effect on multiphoton ionization of atoms when the dynamic polarizability depends strongly on the laser frequency

The influence of the Stark effect on multiphoton ionization of Ba atoms under conditions when the dynamic polarizability depends strongly on the frequency of the laser radiation is investigated. It is found that for some electric field strengths ε of the laser radiation this effect gives rise to resonance peaks in the Ba⁺ ion yield as a function of the laser radiation frequency at frequencies corresponding to single-photon transitions between the excited states. These frequencies can differ substantially from the frequencies corresponding to the conventional multiphoton excitation of these states from the ground states of the atoms. Peaks in the ion yield as a function of ε behave differently from the conventional Stark effect—their position on the frequency scale does not depend strongly on ε. The conditions under which such an induced resonance structure appears are determined. Zh. éksp. Teor. Fiz. 113, 499–512 (February 1998)     

A new laser concept for isotopically selective analysis of noble gases

A new laser approach for the isotopically selective analysis of noble gases is presented. This approach uses noble gas atoms prepared in the 1s ⁵ metastable state. Hyperfine levels in the 1s ⁵ and 2p ⁹ states form two-level systems in Ne, Ar, Kr, and Xe which can be excited by a commercially available single-frequency laser system. Absorption of photons from such a laser, and the resulting momentum transfer, can be used to selectively deflect the desired isotope from a supersonic atomic beam into a detection area. Light from the same laser can then be used to selectively count atoms of the desired isotope using the photon-burst technique. Thus, enrichment and selective detection are accomplished with a single laser in a single pass through the apparatus.The problem of analyzing for⁸⁵Kr in a sample of noble gases extracted from the air is examined in detail. This is a stringent test of the selectivity of this approach because⁸⁵Kr has the same nuclear spin, and thus similar hyperfine splittings, as naturally occurring⁸³Kr. Calculations indicate that isotopic selectivity of the new approach is easily adequate to resolve⁸⁵Kr in a 10¹⁰ excess of⁸³Kr.     

Macroscopic enrichment of12C by a high-power mechanically Q-switched CO2 laser

The isotope-selective multiphoton dissociation of CHClF₂ in a multipass refocusing Herriott cell was used to enrich more than 4 moles of chlorodifluoromethane to 99.99% of¹²C isotopic purity. It is the largest isotope quantity ever separated by a laser process. A cw excited mechanically Q-switched CO₂ laser, which delivers 16 mJ pulses at 5 kHz was used. The enrichment controlled by a mass-spectrometer and guided by a PC was run with a rate of 25 g¹²C per 24 h.     

Ca和Sr原子n′dnl自电离态在低电场中的线性Stark效应

用激光多步激发技术观察了Ca和Sr原子n′dnl双电子激发自电离态的电场效应。Stark光谱显示,在F<1/3n⁵的低电场中,Stark能级分裂与外电场强度线性相关。也观察了近简并l亚能级的Stark混合使自电离共振谱线在外电场中的增宽。这一效应在高温等离子体中对双电子复合速率的增加起重要作用。 关键词：     

Variation in magnesium isotopic composition during zone recrystallization of MgCl<Subscript>2</Subscript>·6H<Subscript>2</Subscript>O

The variation in Mg isotopic composition during MgCl₂·6H₂O zone recrystallization was studied. It was shown that light isotope ²⁴ Mg enrichment occurs at the crystal end to which the recrystallization zone moves. Isotopes ²⁵ Mg and ²⁶ Mg concentrate in the initial crystallization zone. When the molten zone is exposed to a de magnetic field or direct current, the separation factor increases. The data obtained were compared with data on magnesium isotope separation by other physicochemical methods.