Gregory E. Scace

Frequency-Stabilized Single-Mode Cavity Ring-Down Apparatus for High-Resolution Absorption Spectroscopy

We present a cavity ring-down spectroscopy apparatus suitable for high-resolution absorption spectroscopy. The central feature of the spectrometer is a ring-down cavity whose comb of eigenfrequencies is actively stabilized with respect to a tuneable, frequency-stabilized reference laser. By using dichroic ring-down cavity mirrors that are designed to have relatively high losses and low losses at the respective wavelengths of the reference laser and probe laser, the cavity stabilization dynamics are decoupled from frequency jitter of the probe laser. We use the cavity eigenfrequencies as markers in spectral scans and achieve a frequency resolution of ≈1 MHz. Five rovibrational transitions in the (2,0,1) vibrational band of water vapor near 0.935 μm are probed with a continuous-wave external-cavity diode laser, and their line strengths are determined and compared to literature values. Collisional narrowing effects and pressure shifting are observed, illustrating the applicability of the method for quantitat...

Wavelength-modulation laser hygrometer for ultrasensitive detection of water vapor in semiconductor gases.

Water vapor is measured by use of a near-infrared diode laser and wavelength-modulation absorption spectroscopy. Humidity levels as low as 5 nmol/mol [1 nmol/mol = 1 ppb (1 ppb equals 1 part in 10(9))] of water vapor in air are measured with a sensitivity of better than 0.2 nmol/mol (3varsigma). The sensitivity, linearity, and stability of the technique are determined in experiments conducted at the National Institute of Standards and Technology, Gaithersburg, Maryland, by use of the low frost-point humidity generator over the range from 5 nmol/mol to 2.5 mumol/mol of water vapor in air. The pressure-broadening coefficients for water broadened by helium [0.0199(6) cm(-1) atm(-1) HWHM] and by hydrogen chloride [0.268(6) cm(-1) atm(-1) HWHM] are reported for the water line at 1392.5 nm.

Performance of a dual Fabry–Perot cavity refractometer

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.

The second-generation NIST standard hygrometer

A second-generation standard hygrometer has been completed at the National Institute of Standards and Technology (NIST). This hygrometer measures humidity using a gravimetric method: it separates the water from the carrier gas and afterwards measures the water mass and carrier gas mass. These two measurements determine the mass ratio r (the ratio of the measured water mass to the measured dry-gas mass). The new design allows automated continuous gas collection at up to 3?L?min?1. This enables the hygrometer to collect larger amounts of gas and thereby measure humidity values lower than that measured by the previous NIST standard hygrometer. When operated in an optimal thermal environment (minimal thermal loads in the laboratory), the total expanded relative uncertainty (k = 2) of the gravimetric hygrometer is approximately 0.1% for atmospheric-pressure frost points higher than ?35??C (r = 250??g?g?1). Below this frost point the total expanded relative uncertainty gradually increases to approximately 1% at ?55??C (r = 13??g?g?1). The hygrometer has measured the humidity of gas samples produced by the NIST Hybrid Generator and the NIST Low Frost-Point Generator with dew/frost points from ?35??C to 71??C. For both generators the differences between the humidity generated and the humidity measured by the gravimetric hygrometer are less than the combined uncertainties of the generator and the hygrometer.

Spectroscopic Measurement of the Vapour Pressure of Ice

We present a laser absorption technique to measure the saturation vapour pressure of hexagonal ice. This method is referenced to the triple-point state of water and uses frequency-stabilized cavity ring-down spectroscopy to probe four rotation–vibration transitions of at wavenumbers near 7180 cm−1. Laser measurements are made at the output of a temperature-regulated standard humidity generator, which contains ice. The dynamic range of the technique is extended by measuring the relative intensities of three weak/strong transition pairs at fixed ice temperature and humidity concentration. Our results agree with a widely used thermodynamically derived ice vapour pressure correlation over the temperature range 0°C to −70°C to within 0.35 per cent.

Final report on CCT-K6: Comparison of local realisations of dew-point temperature scales in the range ?50 ?C to +20 ?C

A key comparison in dew-point temperature was carried out among the national standards held by NPL (pilot), NMIJ, INTA, VSL, INRIM, MIKES, NIST, NIM, VNIIFTRI-ESB and NMC. A pair of condensation-principle dew-point hygrometers was circulated and used to compare the local realisations of dew point for participant humidity generators in the range ?50 ?C to +20 ?C. The duration of the comparison was prolonged by numerous problems with the hygrometers, requiring some repairs, and several additional check measurements by the pilot. Despite the problems and the extended timescale, the comparison was effective in providing evidence of equivalence. Agreement with the key comparison reference value was achieved in the majority of cases, and bilateral degrees of equivalence are also reported. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Metrology for comparison of displacements at the picometer level

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.

Application of precise line shape measurements to determine the vapor pressure of ice in the temperature range from 0 to −70° C

We performed spectroscopic measurement of the vapor pressure of ice. We describe an experimental method that enables such a measurement over a wide temperature range that covers the change of the vapor pressure of ice by a few orders of magnitude. Our preliminary results have relative uncertainties at the sub‐percent level and are compared to correlations given by Wexler [A. Wexler, J. Res. NBS 81A, 5 (1977)] and Marti and Mauersberger [J. Marti and K. Mauersberger, Geophys. Res. Lett. 20,363 (1993)].

Picometer metrology for precise measurement of refractive index, pressure, and temperature

Abstract: Fabry-Perot interferometers can be used for very precise measurement of the refractive index of gasses. This can enable increased accuracy of interferometer-based length measurement. In addition, because the refractive index of a gas depends on its pressure and temperature, measurements of refractive index can be used to monitor either one of these quantities if the second is known. Recently we have embarked on a project with a goal of measuring pressure with a relative standard uncertainty below 1.4 × 10−6. Dimensional metrology with picometer uncertainties is the core of this 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. 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. The proposed experimental design will minimize the major sources of error while providing additional metrology (including angle measurements with nanoradian precision) to correct residual errors.

Trace-Humidity Calibration and Testing Services at the National Institute of Standards and Technology

Abstract: The National Institute of Standards and Technology (NIST) provides low-humidity calibration and testing services for hygrometers and humidity generators over a humidity range between 3 nanomole per mole (nmol/mol, parts per billion, or ppb) of water vapor in nitrogen and 3 000 micromole per mole (μmol/mol, parts per million, or ppm). Hygrometer calibrations are typically performed by subjecting the hygrometer to various H2O/N2 mixtures produced by the NIST Low Frost-Point Generator (LFPG), and comparing the water vapor concentration reported by the device under test to that of the LFPG. Humidity generator calibrations are typically performed by adjusting both the LFPG and the humidity generator under test to produce nominally the same concentration of H2O in N2, then measuring the difference in water vapor concentration through sequential measurements taken with a sensitive hygrometer. In the case of permeation-tube humidity generators, NIST offers either complete generator calibration, or calibration of water permeation-tubes. In addition to calibration services, NIST provides individually-customized testing services in which the LFPG routinely provides low-uncertainty H2O/N2 mixtures for prototype testing, development of new measurement technologies, and for scientific experiments. In this paper we discuss the trace-humidity calibration and testing services available at NIST. The LFPG, and its performance are presented. Examples are provided of how the calibration and testing of hygrometers and humidity generators are performed. We supply details of how to obtain NIST calibration and testing services.