Hydrogen Isotopologues Research Articles

Improved reliability and availability of fundamental properties for all hydrogen isotopologues by Gaussian process regression using data from experiments and path-integral simulations

The thermodynamic and transport properties of hydrogen isotopologues in the solid, liquid, and gas phases are crucial for hydrogen energy exploitation, such as the design of the fuel cycle of nuclear fusion reactors. However, experimental data for tritium-containing species are often not available. Alternatively, path integral (PI) simulation can evaluate hydrogen properties taking into account nuclear quantum effects; however, concerns about its reliability are present. This study proposes a method of Gaussian process regression (GPR) using both experimental and PI simulation data to obtain practically plausible estimates for all isotopologues. Using liquid viscosity and diffusivity as test cases, we demonstrate that the present method has high and robust predictive performance. The method has broad applicability and can also be used to design experiments and calculations to efficiently improve the regression model, and thus is expected to contribute to enhancing the availability and reliability of the hydrogen isotopologue property database.

Comparison of Three Compact Raman Systems with Excitation Laser Wavelengths of 405, 532, and 660 nm (µRA-RGB)

Based on good experience with Raman systems in general and the µRA systems in particular, we try to expand the capabilities and possible applications of Raman spectroscopy. A central aspect is the excitation wavelength since signal intensity and fluorescence background depend on that. Besides the common 532-nm laser (green), we used a 660-nm (red) and 405-nm (blue) laser, hence the name µRA-RGB. All three systems share the same basic principle (fiber coupling between laser, Raman head, and spectrometer) and only differ because of necessary adjustments for the excitation wavelength used, like the laser edge filter. As the original µRA system has already proved its capability to simultaneously detect all six hydrogen isotopologues, this first RGB study was limited to H2, D2, and equilibrated mixtures of both. With one of Tritium Laboratory Karlsruhe’s proven LARA systems connected to the same gas mixing loop system, comparing the µRA systems against it was possible. This paper shows the results of the measurement campaign comparing all three µRA systems (405-, 532-, 660-nm excitation wavelengths) and the comparison to the well-established large Raman systems (LARA, 532 nm).

Accurate Reference Gas Mixtures Containing Tritiated Molecules: Their Production and Raman-Based Analysis.

Highly accurate, quantitative analyses of mixtures of hydrogen isotopologues—both the stable species, H2, D2, and HD, and the radioactive species, T2, HT, and DT—are of great importance in fields as diverse as deuterium–tritium fusion, neutrino mass measurements using tritium β-decay, or for photonuclear experiments in which hydrogen–deuterium targets are used. In this publication we describe a production, handling, and analysis facility capable of fabricating well-defined gas samples, which may contain any of the stable and radioactive hydrogen isotopologues, with sub-percent accuracy for the relative species concentrations. The production is based on precise manometric gas mixing of H2, D2, and T2. The heteronuclear isotopologues HD, HT, and DT are generated via controlled, in-line catalytic reaction or by β-induced self-equilibration, respectively. The analysis was carried out using an in-line intensity- and wavelength-calibrated Raman spectroscopy system. This allows for continuous monitoring of the composition of the circulating gas during the self-equilibration or catalytic evolution phases. During all procedures, effects, such as exchange reactions with wall materials, were considered with care. Together with measurement statistics, these and other systematic effects were included in the determination of composition uncertainties of the generated reference gas samples. Measurement and calibration accuracy at the level of 1% was achieved.

Sorption- and diffusion-induced isotopic fractionation in chloroethenes

Isotopic fractionation of groundwater contaminants can occur due to degradation, diffusion and sorption. Of these, only degradation has been extensively explored, yet diffusive isotopic fractionation (DIF) and sorptive isotopic fractionation (SIF) can have significant effects on the isotopic enrichment of groundwater contaminants. Understanding how to mathematically describe and model these processes is vital to the correct interpretation of compound-specific isotope analysis (CSIA) data in the field. Here, models for these physical fractionation processes are developed and described, including the definition of a sorption enrichment factor. These models are then implemented numerically using inverse and finite-element methods to investigate two scenarios, diffusion-sorption and diffusion-sorption-advection, that have been measured in the laboratory. Concentration, δ37Cl, and δ2H data from cis-dichloroethene (cDCE) and trichloroethene (TCE) are used as inputs to the models. Unknown transport parameters including diffusive fractionation exponents are determined from an inverse modelling approach. DIF is shown to have a stronger influence on chlorine isotopologues than on hydrogen isotopologues. For both cDCE and TCE, the sorption enrichment factor of chlorine is found to be negative while that of hydrogen is positive. The presented approach and results provide novel tools and insight into DIF and SIF and underline that these processes should be taken into account when using CSIA to assess contaminant fate.

Kinetic Isotope Effect in Low-Energy Collisions between Hydrogen Isotopologues and Metastable Helium Atoms: Theoretical Calculations Including the Vibrational Excitation of the Molecule.

We present very accurate theoretical results of Penning ionization rate coefficients of the excited metastable helium atoms (4He(23S) and 3He(23S)) colliding with the hydrogen isotopologues (H2, HD, D2) in the ground and first excited rotational and vibrational states at subkelvin regime. The calculations are performed using the current best ab initio interaction energy surface, which takes into account the nonrigidity effects of the molecule. The results confirm a recently observed substantial quantum kinetic isotope effect (Nat. Chem. 2014, 6, 332–335) and reveal that the change of the rotational or vibrational state of the molecule can strongly enhance or suppress the reaction. Moreover, we demonstrate the mechanism of the appearance and disappearance of resonances in Penning ionization. The additional model computations, with the morphed interaction energy surface and mass, give better insight into the behavior of the resonances and thereby the reaction dynamics under study. Our theoretical findings are compared with all available measurements, and comprehensive data for prospective experiments are provided.

Quantitative Long-Term Monitoring of the Circulating Gases in the KATRIN Experiment Using Raman Spectroscopy

The Karlsruhe Tritium Neutrino (KATRIN) experiment aims at measuring the effective electron neutrino mass with a sensitivity of 0.2 eV/c2, i.e., improving on previous measurements by an order of magnitude. Neutrino mass data taking with KATRIN commenced in early 2019, and after only a few weeks of data recording, analysis of these data showed the success of KATRIN, improving on the known neutrino mass limit by a factor of about two. This success very much could be ascribed to the fact that most of the system components met, or even surpassed, the required specifications during long-term operation. Here, we report on the performance of the laser Raman (LARA) monitoring system which provides continuous high-precision information on the gas composition injected into the experiment’s windowless gaseous tritium source (WGTS), specifically on its isotopic purity of tritium—one of the key parameters required in the derivation of the electron neutrino mass. The concentrations cx for all six hydrogen isotopologues were monitored simultaneously, with a measurement precision for individual components of the order 10−3 or better throughout the complete KATRIN data taking campaigns to date. From these, the tritium purity, εT, is derived with precision of <10−3 and trueness of <3 × 10−3, being within and surpassing the actual requirements for KATRIN, respectively.

Design of a Cryostat for Spectroscopic Investigation of All Hydrogen Isotopologues in the Solid, Liquid, and Gaseous Phases

A crucial part of the closed fuel cycle of future fusion power plants will be isotope separation, which takes place in a cryogenic distillation refraction column, where all six hydrogen isotopologues are separated due to their different vapor pressures at a given temperature. For monitoring and process controlling, the Tritium Laboratory Karlsruhe has investigated liquid hydrogen by infrared (IR) absorption spectroscopy and presented the first successful calibration for the inactive isotopologues. Now, the new Tritium Absorption InfraRed Spectroscopy 2 (T2ApIR) experiment, which is fully tritium compatible, is under construction and aims to provide a calibration for concentration measurements of all six hydrogen isotopologues in solid, liquid, and gaseous phases via not only IR absorption but also Raman spectroscopy. One major challenge of the new experiment so far has been the design of the cryostat, which had to fulfill diverse technical and safety requirements regarding tritium compatibility, cryogenics, and overpressure and the combination of optical components for Raman and IR spectroscopy.

Concentrated Nonequilibrium HD for the Cross Calibration of Hydrogen Isotopologue Analytics

For the fuel cycles of fusion power plants, highly specialized in-line analytic systems are crucial for efficient process control, monitoring, and accountancy. One of these systems under development is infrared (IR) absorption spectroscopy of liquid hydrogen isotopologue mixtures that can be used for in-line process control and monitoring of cryogenic distillation. The main challenge of this method is the complex calibration procedure since the integral IR absorption strength is nonlinearly correlated with the isotopologue composition. Typical calibration procedures make use of well-known samples produced by mixing atomic pure samples and referenced by p-V-T-measurement. The samples are catalyzed to produce samples containing heteronuclear molecules. By this procedure, one cannot exceed the chemical equilibrium of high temperatures (mass action coefficient Kc<4). Therefore, it is not possible to produce samples with an HD, HT, or DT concentration above 50% by catalysis or natural equilibration. However, in isotope or isotopologue separation, such as in cryogenic distillation, this equilibrium will be regularly exceeded. In the case of IR absorption spectroscopy on liquid hydrogen isotopologues, additional care needs to be taken for calibration since the calibration functions are highly nonlinear. We tested our calibration in the high-purity HD regime (Kc>4) by producing a sample via cryogenic distillation and performing a cross calibration for three systems: Quadrupole mass spectrometry, Raman spectroscopy, and infrared spectroscopy. Therefore, we can also demonstrate that additional calibration points are indispensable in order to improve the systematic uncertainties below the 5% level, and a simple extrapolation from a calibration of Kc < 4 to Kc > 4 will result in a trueness and accuracy exceeding this 5% level.

Experimental investigation of the ideal selectivity of MFI-ZSM-5 zeolite-type membranes for a first evaluation of the separation of hydrogen isotopologues from helium

In future fusion power plants, the tritium (T) breeding inside the reactor is a key aspect to ensure the so-called tritium self-sufficiency. The continuous availability of the fuel for further fusion reactions requires the continuous separation of small molecules such as hydrogen isotopologues (e.g., HT) from helium. Pressure-driven separation with membranes is a promising method to fulfill these requirements, especially if molecular-sieving membranes are available to separate these gas species. In this work, the single gas permeances of He, H2 and D2 were measured with a MFI-ZSM-5 zeolite-type tubular membrane. These experiments were performed in the membrane’s temperature range: 298–398K. At these temperatures, a transition in the transport regime was observed at 360K, which is attributed to the transition from surface-diffusion dominant to gas-translational diffusion dominant. Using the measured permeances, the permselectivities for H2/He and D2/He were calculated. In the temperature range tested, the permselectivities exceed the Knudsen selectivity, indicating that the membrane is of rather high-quality. Instead, the H2/D2 permeances ratio was around the Knudsen ratio (i.e., ∼1.41). Moreover, the permeances of the binary mixture H2/D2, at the whole concentration range, were also measured at 298K to study the isotopic effects on the transport through the MFI-ZSM-5 membrane. We found that the mixed gas behaves macroscopically as a hydrogen isotopologue molecule, and its permeance falls with Meff-0.5, where Meff is the effective mass of the mixture determined by the concentrations of H2 and D2. Using this result, the Q2 permeances and the Q2/He permselectivities (where Q = H, D, T) for the other hydrogen isotopologues (i.e., HD, HT, DT and T2) were also calculated.

Compact Determination of Hydrogen Isotopes

Scanning calorimetry of a confined, reversible hydrogen sorbent material has been previously proposed as a method to determine compositions of unknown mixtures of diatomic hydrogen isotopologues and helium. Application of this concept could result in greater process knowledge during the handling of these gases. Previously published studies have focused on mixtures that do not include tritium. This paper focuses on modeling to predict the effect of tritium in mixtures of the isotopologues on a calorimetry scan. The model predicts that tritium can be measured with a sensitivity comparable to that observed for hydrogen-deuterium mixtures, and that under some conditions, it may be possible to determine the atomic fractions of all three isotopes in a gas mixture.

Design of a Spectroscopy Experiment for All Hydrogen Isotopologues in the Gaseous, Liquid, and Solid Phase

An integral part of the fuel cycle of future fusion facilities is the isotope separation system (ISS). The Tritium Laboratory Karlsruhe (TLK) is currently developing a system to monitor the concentration of all six hydrogen isotopologues Q2 (H2, HD, D2, HT, DT, T2) in the liquid phase in the cryogenic distillation process of the ISS.Liquid inactive Q2 were already successfully analyzed under cryogenic conditions via infrared (IR) absorption spectroscopy and calibration data for D2 is provided by previous experiments at TLK. The new experiment T2ApIR (Tritium Absorption Infrared Spectroscopy Experiment) is designed to be fully tritium compatible to perform a complete calibration of the IR absorption measurement system with all six hydrogen isotopologues in the liquid phase under conditions similar to the ISS. This provides a unique non-invasive, inline and real-time measurement system for isotopologic concentration determination, ready for implementation in the cryogenic distillation column.

Indirect Spin-Spin Coupling Constants in the Hydrogen Isotopologues.

The results of experimental and theoretical studies of indirect spin-spin coupling constants for hydrogen deuteride (HD), hydrogen tritide (HT), and deuterium tritide (DT) are described. The reduced coupling constants obtained from the gas-phase NMR (nuclear magnetic resonance) experiment conducted at 300 K are 2.338(1), 2.334(3), and 2.316(1) × 10(20) T(2) J(-1), while the ab initio values computed at the full configuration interaction level of theory equal 2.349(3), 2.343(3), and 2.322(3) × 10(20) T(2) J(-1) for HD, HT, and DT, respectively. The agreement of the experimental and theoretical results is improved when proper treatment of the influence of nuclear relaxation on the NMR spectrum is applied. However, there is a minor discrepancy between experiment and theory, exceeding the estimated error bars; potential sources of this discrepancy are discussed.

Raman Spectroscopy at the Tritium Laboratory Karlsruhe

Raman spectroscopy is employed successfully for analysis of hydrogen isotopologues at the Tritium Laboratory Karlsruhe (TLK). In this paper, we summarize the recent achievements in the further development on this technique, and the various applications for which it is used at the TLK. Further, we show that Raman spectroscopy has evolved as a versatile, highly accurate key method for quantitative analysis complementing the portfolio of analytic techniques at the TLK.

How to Make Raman-Inactive Helium Visible in Raman Spectra of Tritium-Helium Gas Mixtures

Raman spectroscopy, a powerful method for the quantitative compositional analysis of molecular gases, e.g. mixtures of hydrogen isotopologues, is not able to detect monoatomic species like helium. This deficit can be overcome by using radioluminescence emission from helium atoms induced by β-electrons from tritium decay. We present theoretical considerations and combined Raman/radioluminescence spectra. Furthermore, we discuss the linearity of the method together with validation measurements for determining the pressure dependence. Finally, we conclude how this technique can be used for samples of helium with traces of tritium, and vice versa.

First Calibration Measurements of an FTIR Absorption Spectroscopy System for Liquid Hydrogen Isotopologues for the Isotope Separation System of Fusion Power Plants

Fusion facilities like ITER and DEMO will circulate huge amounts of deuterium and tritium in their fuel cycle with an estimated throughput of kg per hour. One important capability of these fuel cycles is to separate the hydrogen isotopologues. For this purpose the Isotope Separation System (ISS), using cryogenic distillation, as part of the TRitium Enrichment Test Assembly (TRENTA) is under development at Tritium Laboratory Karlsruhe. Fourier transform infrared absorption spectroscopy (FTIR) has been selected to prove its capability for inline monitoring of the tritium concentration in the liquid phase at the bottom of the distillation column of the ISS. The actual R&D work is focusing on the calibration of such a system. Two major issues are the identification of appropriate absorption lines and their dependence on the isotopic concentrations and composition. For this purpose the Tritium Absorption IR spectroscopy experiment has been set up as an extension of TRENTA. For calibration a Raman spectroscopy system is used. First measurements, with equilibrated mixtures of H2, D2 and HD demonstrate that FTIR can be used for quantitative analysis of liquid hydrogen isotopologues and reveal a nonlinear dependence of the integrated absorbance from the D2 concentration in the 2nd vibrational branch of D2 FTIR spectra.

Electron delocalization in polyenes: a semiexperimental equilibrium structure for (3E)-1,3,5-hexatriene and theoretical structures for (3Z,5Z)-, (3E,5E)-, and (3E,5Z)-1,3,5,7-octatetraene.

Electronic structure theory reveals that π-electron delocalization increases with the chain length in polyenes. To analyze quantitatively this effect a semiexperimental equilibrium structure has been determined for trans-hexatriene by the mixed estimation method. For this fit rotational constants for a number of carbon and hydrogen isotopologues as well as a high-level ab initio structure have been used. The accuracy is 0.001 Å for bond lengths and 0.1° for bond angles. For the three isomers of octatetraene, high-level ab initio calculations have given a comparably accurate structure. These structures have been used in comparison with the structure of s-trans-butadiene to show that "C═C" bonds increase in length and "C-C" bonds decrease in length as the polyene chain lengthens. These structural effects of π-electron delocalization increase toward the center of polyenes. Most likely, π-π conjugation in the molecules studied plays a large part in their planarity that, in turn, forces the hydrogen atoms of cis fragments in bay regions to be in a close contact. Their distance is indeed shorter than the sum of their van der Waals radii, and they seem to participate in a six-membered ring.

Observation of the isotope effect in sub-kelvin reactions

Quantum phenomena in the translational motion of reactants, which are usually negligible at room temperature, can dominate reaction dynamics at low temperatures. In such cold conditions, even the weak centrifugal force is enough to create a potential barrier that keeps reactants separated. However, reactions may still proceed through tunnelling because, at low temperatures, wave-like properties become important. At certain de Broglie wavelengths, the colliding particles can become trapped in long-lived metastable scattering states, leading to sharp increases in the total reaction rate. Here, we show that these metastable states are responsible for a dramatic, order-of-magnitude-strong, quantum kinetic isotope effect by measuring the absolute Penning ionization reaction rates between hydrogen isotopologues and metastable helium down to 0.01 K. We demonstrate that measurements of a single isotope are insufficient to constrain ab initio calculations, making the kinetic isotope effect in the cold regime necessary to remove ambiguity among possible potential energy surfaces.

Accurate depolarization ratio measurements for all diatomic hydrogen isotopologues

The Raman depolarization ratios for individual Q1(J”) branch lines of all diatomic hydrogen isotopologues – H2, HD, D2, HT, DT, and T2 – were measured, for all rotational levels with population larger than 1/100 relative to the Boltzmann maximum at room temperature. For these measurements, the experimental setup normally used for the monitoring of the tritiated hydrogen molecules at KArlsruhe TRItium Neutrino experiment was adapted to optimally control the excitation laser power and polarization, and to precisely define the Raman light collection geometry. The measured Raman depolarization values were compared to theoretical values, which are linked to polarizability tensor quantities. For this, the ‘raw data’ were corrected taking into account distinct aspects affecting Raman depolarization data, including (1) excitation polarization impurities; (2) extended Raman excitation volumes; and (3) Raman light collection over finite solid angles. Our corrected depolarization ratios of the hydrogen isotopologues agree with the theoretical values (based on ab initio quantum calculations by R.J. LeRoy, University of Waterloo, Canada) to better than 5% for nearly all of the measured Q1(J”) lines, with 1σ confidence level. The results demonstrate that reliable, accurate Raman depolarization ratios can be extracted from experimental measurements, which may be substantially distorted by excitation polarization impurities and by geometrical effects. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Path-Integral Calculation of Cross Second Virial Coefficients for Hydrogen Isotopologues

The path-integral Monte Carlo technique and a recent high-accuracy six-dimensional potential are used to compute the cross second virial coefficients for all unlike pairs among the hydrogen isotopologues \(\mathrm{H}_{2}, \mathrm{D}_{2}, \mathrm{T}_{2}\), HD, HT, and DT. Values are calculated from 15 K to 2000 K for these quantities where experimental information is almost completely absent. It is found that the commonly assumed arithmetic mean of the pure-component values does not provide a good approximation for the cross coefficients below approximately 50 K, especially for pairs containing \(\mathrm{H}_{2}\).

Nuclear quantum effects on the He2H+ complex with the nuclear molecular orbital approach

AbstractThe nuclear quantum effects (NQE) on the geometries and energies of the HeH+ and He2H+ complexes and their hydrogen isotopologues are investigated with the recently developed APMO/MP2 code. This code implements the Nuclear Molecular Orbital Approach (NMO) at a MP2 level of theory for electrons and quantum nuclei. In a first investigation of NQE on HeH+, we have observed a reduction in the bond distance as the mass of the hydrogen isotope is increased. Our calculated bond distances are in good agreement with experiment. We have also studied the H/D/T isotope effects on the geometry, total energy and stabilization energy of HeHHe+. We have also determined the relation between nuclear mass and delocalization and found that the lighter the nucleus the more delocalized it is. Our results demonstrate the importance of including nuclear quantum effects in these systems. To our knowledge, these are the first reported results on isotope effects on the HeH+ and He2H+ complexes using a NMO method. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010