Realization of the optical pascal has been limited by systematic errors caused by distortion of the optic. In this work, distortion error is circumvented via synchronous measurement of helium refractivity at two optical frequencies. The resultant pressure realization achieves combined standard uncertainty of 5.7 x 10-6p, chiefly limited by ab initio knowledge of helium dispersion. Two-color measurements are also presented for neon, argon, and nitrogen, which enable semiprimary realization of the pascal in a more practical embodiment. For argon dispersion, measurement and ab initio calculation barely agree within mutual expanded uncertainty; experiment is about 16 times more accurate than theory.

Optical refractometry techniques enable realization of both pressure and temperature directly from properties of the gas. The NIST refractometer, a fixed length optical cavity (FLOC) has previously been evaluated for operation as pressure standard, and now in this paper, is evaluated for the feasibility of operation as a primary temperature standard as well. The challenge is that during operation, one cavity is filled with gas. Gas dynamics predicts that this will result in heating which in turn will affect the cavity temperature uniformity, impeding the ability to measure the gas temperature with sufficient accuracy to make the standard useful as a primary standard for temperature or pressure. Temperature uniformity across the refractometer must be less than 0.5 mK for measurements of the refractivity to be sufficiently accurate for the FLOC. This paper compares computer modeling to laboratory measurements, enabling us to validate the model to predict thermal behavior and to accurately determine the measurement uncertainty of the technique. The results presented in this paper show that temperature of the glass elements of the refractometer and ‘thermal-shell’ copper chamber are equivalent to within 0.5 mK after an equilibration time of 3000 s (when going from 1 kPa to 100 kPa). This finding enables measurements of the copper chamber to determine the gas temperature to within an uncertainty (k = 1) of 0.5 mK. Additionally, the NIST refractometer is evaluated for feasibility of operation as temperature standard.

Laser refractometers are approaching accuracy levels where gas pressures in the range 1 Pa < p < 1 MPa inferred by measurements of gas refractivity at a known temperature will be competitive with the best existing pressure standards and sensors. Here, the authors develop the relationship between pressure and refractivity p = c 1 ⋅ ( n - 1 ) + c 2 ⋅ ( n - 1 ) 2 + c 3 ⋅ ( n - 1 ) 3 + ⋯ , via measurement at T = 293.1529(13) K and λ = 632.9908(2) nm for p ≤ 500 kPa. The authors give values of the coefficients c 1, c 2, c 3 for six gases: Ne, Ar, Xe, N2, CO2, and N2O. For each gas, the resulting molar polarizability A R ≡ 2 R T 3 c 1 has a standard uncertainty within 16 × 10-6·A R . In these experiments, pressure was realized via measurements of helium refractivity at a known temperature: for He, the relationship between pressure and refractivity is known through calculation much more accurately than it can presently be measured. This feature allowed them to calibrate a pressure transducer in situ with helium and subsequently use the transducer to accurately gage the relationship between pressure and refractivity on an isotherm for other gases of interest.

The measurement science in realizing and disseminating the unit for pressure in the International System of Units, the pascal (Pa), has been the subject of much interest at the National Institute of Standards and Technology (NIST). Modern optical-based techniques for pascal metrology have been investigated, including multiphoton ionization and cavity ringdown spectroscopy. Work is ongoing to recast the pascal in terms of quantum properties and fundamental constants and in doing so make vacuum metrology consistent with the global trend toward quantum-based metrology. NIST has ongoing projects that interrogate the index of refraction of a gas using an optical cavity for low vacuum, and count background particles in high vacuum to extreme high vacuum using trapped laser-cooled atoms.

In cell-based laser refractometers, interferometer pathlength uncertainty introduced by deformation and stress in the windows through which the beams pass can be the chief factor limiting measurement accuracy. The fractional contribution of pathlength uncertainty to our recent determination of the Boltzmann constant was 9.8 × 10 -6 , and more than two times larger than the next largest uncertainty component. We briefly describe the error and propose a design in which cell window effects contribute less than 3 × 10 -6 fractional error to the measurement of helium refractivity; performance that would be competitive with state-of-the-art primary thermometry and barometry.

Nanometer-sized structures, surfaces and sub-surface phenomena have played an enormous role in science and technological applications and represent a driving-force of current interdisciplinary science. Recent developments include the atomic-scale characterization of nanoparticles, molecular reactions at surfaces, magnetism at the atomic scale, photoelectric characterization of nanostructures as well as two-dimensional solids. Research and development of smart nanostructured materials governed by their surface properties is a rapidly growing field. The main challenge is to develop an accurate and robust electronic structure description. The density of surface-related trap states is analyzed by transient UV photoconductivity and temperature-dependent admittance spectroscopy. An advanced application of thin films on shaped substrates is the deposition of catalytic layers on hollow glass microspheres for hydrogen storage controlled exothermal hydrolytic release. Surface properties of thin films including dissolution and corrosion, fouling resistance, and hydrophilicity/hydrophobicity are explored to improve materials response in biological environments and medicine. Trends in surface bio-functionalization routes based on vacuum techniques, together with advances in surface analysis of biomaterials, are discussed. Pioneering advances in the application of X-ray nanodiffraction of thin film cross-sections for characterizing nanostructure and local strain including in-situ experiments during nanoindentation are described. Precise measurements and control of plasma properties are important for fundamental investigations and the development of next generation plasma-based technologies. Critical control parameters are the flux and energy distribution of incident ions at reactive surfaces; it is also crucial to control the dynamics of electrons initiating non-equilibrium chemical reactions. The most promising approach involves the exploitation of complementary advantages in direct measurements combined with specifically designed numerical simulations. Exciting new developments in vacuum science and technology have focused on forward-looking and next generation standards and sensors that take advantage of photonics based measurements. These measurements are inherently fast, frequency based, easily transferrable to sensors based on photonics and hold promise of being disruptive and transformative. Realization of Pascal, the SI unit for pressure, a cold-atom trap based ultra-high and extreme high vacuum (UHV and XHV) standard, dynamic pressure measurements and a photonic based thermometer are three key examples that are presented.

New techniques using refractometry have enabled gas pressure to be measured using laser interferometry. Two key techniques have been studied at NIST which include the Fixed Length Optical Cavity (FLOC) and the Variable Length Optical Cavity (VLOC). The measurement techniques are described and the traceability of these measurements through quantum mechanics that enables them to be primary standards. This technology is critical for gas pressure metrology to move away from artifact based standards (and especially mercury based) and move to quantum based methods for realization of the pascal.

The future of pressure, vacuum and even temperature measurement will employ lasers, Fabry-Perot optical cavities, cold atom traps and lots of quantum physics. For pressure measurement of a gas, photons interact at the quantum level such that light travels at a slower speed in gas than it does in vacuum. For extreme vacuum measurements, cold atom traps will be used to detect single collisions between gas trapped cold atoms enabling the number density of the gas to be measured. For temperature measurement is performed using silicon photonics to detect the small changes in refractive index in micro machined siliconphontoic cavities coupled to optical fibers. For dynamic pressure, NIST is developing a method where the unique quantum mechanical characteristics of the molecules are themselves the standard for pressure, making it consistent with the quantum-SI. Our approach is to use independent molecular spectroscopy as a dynamic measurement of pressure, where the pressure and temperature is ascertained by measuring time-resolved pressure-broadened spectra of CO molecules. This paper briefly reviews the status of these projects currently underway at the NIST Thermodynamic Metrology Group.

The measurement science in realizing and disseminating the unit for pressure in the International System of Units (SI), the pascal (Pa), has been the subject of much interest at the National Institute of Standards and Technology (NIST). Modern optical-based techniques for pascal metrology have been investigated, including multi-photon ionization and cavity ringdown spectroscopy. Work is ongoing to recast the pascal in terms of quantum properties and fundamental constants and in so doing, make vacuum metrology consistent with the global trend toward quantum-based metrology. NIST has ongoing projects that interrogate the index of refraction of a gas using an optical cavity for low vacuum, and count background particles in high vacuum to extreme high vacuum using trapped laser-cooled atoms.

Since the beginning of measurement of pressure in the 17th century, the unit of pressure has been defined by the relationship of force per unit area. The present state of optical technology now offers the possibility of using a thermodynamic definition-specifically the ideal gas law-for the realization of the pressure unit, in the vacuum regime and slightly above, with an accuracy comparable to or better than the traditional methods of force per area. The changes planned for the SI in 2018 support the application of this thermodynamic definition that is based on the ideal gas law with the necessary corrections for real-gas effects. The paper reviews the theoretical and experimental foundations of those optical methods that are considered to be most promising to realize the unit of pressure at the highest level of metrology.

We describe a method for determining the density of helium via measurements of optical refractivity. In combination with the equation of state, this allows realization of the pascal. Our apparatus is based on the integration of a gas triple-cell into a quasi-monolithic heterodyne interferometer: the stability of the interferometer is ±50  pm over 10 h. We claim the contribution of cell window thinning to pathlength uncertainty can be canceled within an uncertainty of 0.37 fm/Pa per window pass, of which for our 25 cm cell length corresponds to a fractional error of 9.3×10-6 in the measure of helium refractivity. We report the ratio (n-1)N2 /(n-1)He=8.570354(13) at p=367.420(4)  kPa, T=293.1529(13)  K and λ=632.9908(6)  nm, which can be used to calibrate less-accurate refractometers. By measuring helium refractivity at known temperature and pressure, we determined the Boltzmann constant with standard uncertainty kB=1.380652(17)×10-23  JK-1.

We have developed a new low pressure sensor which is based on the measurement of (nitrogen) gas refractivity inside a Fabry-Perot (FP) cavity. We compare pressure determinations via this laser refractometer to that of well-established ultrasonic manometers throughout the range 100Pa to 100000Pa. The refractometer demonstrates 10 -6 precision for p > 50 kPa; - as good or better than the manometer - we argue that a laser refractometer represents a state-of-the-art transfer standard of the pascal. We also claim the refractometer has an accuracy of U(p FP ) = [(16mPa) 2 + (11.9 × 10 -6 · p) 2 ] 1/2 , as realized through the properties of nitrogen gas.

We have developed a new low-pressure sensor which is based on the measurement of (nitrogen) gas refractivity inside a Fabry-Perot cavity. We compare pressure determinations via this laser refractometer to that of well-established ultrasonic manometers throughout the range 100 Pa to 180 000 Pa. The refractometer demonstrates 10(-6) ⋅ p reproducibility for p > 100 Pa, and this precision outperforms a manometer. We also claim the refractometer has an expanded uncertainty of U(pFP) = [(2.0 mPa)(2) + (8.8 × 10(-6) ⋅ p)(2)](1/2), as realized through the properties of nitrogen gas; we argue that a transfer of the pascal to p < 1 kPa using a laser refractometer is more accurate than the current primary realization.

Lasers from four national metrological institutes (NMIs) were compared in 2012 as part of the CCL-K11 ongoing key comparison, initiated by the 13th meeting of the Comite Consultative des Longuers (CCL) in 2007. The absolute frequency of R(127) 11-5 transitions of molecular iodine was measured for these lasers following the technical protocol for CCL-K11. The results of these measurements are compiled in this report. The comparison reports, as communicated by each participant, are included as appendices.This document constitutes the fourth final report for the ongoing key comparison CCL-K11.

We have built and characterized a refractometer that utilizes two Fabry-Perot cavities formed on a dimensionally stable spacer. In the typical mode of operation, one cavity is held at vacuum, and the other cavity is filled with nitrogen gas. The differential change in length between the cavities is measured as the difference in frequency between two helium-neon lasers, one locked to the resonance of each cavity. This differential change in optical length is a measure of the gas refractivity. Using the known values for the molar refractivity and virial coefficients of nitrogen, and accounting for cavity length distortions, the device can be used as a high-resolution, multi-decade pressure sensor. We define a reference value for nitrogen refractivity as n-1=(26485.28±0.3)×10(-8) at p=100.0000  kPa, T=302.9190  K, and λ(vac)=632.9908  nm. We compare pressure determinations via the refractometer and the reference value to a mercury manometer.

In 2000, a key comparison, CCL-K3 (optical polygon and angle blocks) was started, piloted by NMISA. Based on it, in 2007, the SIM metrological region started a SIM.L-K3 key comparison piloted by INMETRO. The results of this regional comparison (RMO key comparison) contribute to the Mutual Recognition Arrangement (MRA) between the national metrology institutes of the Metre Convention. It is linked with the CCL-K3 key comparison via laboratories that participated in both the CIPM and the RMO comparisons. This common participation establishes the link between the comparisons and ensures equivalence of national metrology institutes, according to the MRA between NMIs. The SIM NMIs that took part in the CCL-K3 were NIST, NRC and CENAM. However, NRC withdrew from it. GUM from Poland (EURAMET) and NPLI from India (APMP) were invited to participate in the SIM.L-K3 key comparison. The circulation of artefacts (a 12 faces polygon and 4 angle blocks) started in 2008 and was completed in 2009.

An apparatus capable of comparing displacements with picometer accuracy is currently being designed at NIST. In principle, we wish to compare one displacement in vacuum to a second, equal displacement in gas, in order to determine gas refractive index. If the gas is helium, the refractive index is expected to be amenable to high-accuracy ab initio calculations relating refractive index to gas density or to the ratio of pressure and temperature ( P / T ); the measured refractive index can then be used to infer ( P / T ) with an accuracy goal of about 1×10 -6 (relative standard uncertainty). If either the pressure or temperature is known, the refractive index measurement will allow us to determine the second quantity. Our goal is to achieve an uncertainty limited primarily by the uncertainty of the Boltzmann constant (before redefinition of SI units, which will give the Boltzmann constant a defined value). The technique is an optical analog of dielectric constant gas thermometry and can be used in a similar manner. The dimensional metrology is uniquely challenging, requiring picometer-level uncertainty in the comparison of the displacements.

For several years we have been studying the use of Fabry-Perot interferometers for precise measurement of the refractive index of gasses, where the primary motivation has been to improve interferometer-based length measurement. Because the refractive index of a gas depends on its pressure and temperature, we can also use refractive index to monitor either of these quantities if the second is known. Recently we have embarked on a project that will utilize refractive index to infer pressure with a smaller measurement uncertainty than is currently possible, hoping to reach a relative standard uncertainty of 1.4™~10-6. The projected uncertainty budget is currently dominated by uncertainty of the Boltzmann constant, but following the coming redefinition of SI units, the Boltzmann uncertainty will be replaced by uncertainty in the temperature scale, at which point refractive index measurements can be expected to play a central role in precise realization of thermodynamic temperature. Dimensional metrology with picometer uncertainties is the core of our technique and is the subject of this paper. Refractive index will be measured by comparing two precisely equal displacements ( .150 mm), where one displacement is in vacuum and the second is in helium and will appear to be slightly longer due to the refractive index. The two displacements must be compared with < 3 pm uncertainty. The intrinsic precision achievable with Fabry-Perot cavities far exceeds our needed accuracy, but two or more independent interferometers have never been compared to such high accuracy when undergoing macroscopic displacements. The major challenges include many of the typical sources of error in dimensional measurement, such as Abbe errors, alignment errors, material dimensional stability, etc. Careful consideration must be given to second-order effects that are not normally large enough to merit mention. Our proposed experimental design will minimize such errors and provide additional metrology (including angle measurements with nanoradian precision) needed to correct the residual errors.