Jonathan E. Hardis

National Institute of Standards and Technology High-Accuracy Cryogenic Radiometer

A high-accuracy cryogenic radiometer has been developed at the National Institute of Standards and Technology to serve as a primary standard for optical power measurements. This instrument is an electrical-substitution radiometer that can be operated at cryogenic temperatures to achieve a relative standard uncertainty of 0.021% at an optical power level of 0.8 mW. The construction and operation of the high-accuracy cryogenic radiometer and the uncertainties in optical power measurements are detailed.

Fourteen-decade photocurrent measurements with large-area silicon photodiodes at room temperature.

Recent improvements in commercial silicon photodiodes and operational amplifiers permit electrical noise to be reduced to an equivalent of 0.1 fA of photocurrent when a measurement time of 400 s is used. This is equivalent to a photocurrent resulting from fewer than 800 photons/s, and it implies a dynamic range of 14 orders of magnitude for a detector circuit. We explain the circuit theory, paying particular attention to the measurement bandwidth, the causes of noise and drift, and the proper selection of circuit components. These optical radiation detectors complement the primary radiometric standards. These detectors may replace photomultiplier tubes that have been used traditionally and or that were too costly to be used.

Vibrationally resolved photoelectron studies of the 7σ−1 channel in N2O

We present vibrationally resolved photoelectron studies of the 000, 100, 200, and 001 modes of the A state (7σ−1) of N2 O+ in the 17.4–26 eV photon‐energy range. The vibrational branching ratios σ(100)/σ(000) and σ(001)/σ(000) agree very well with fluorescence measurements by Kelly et al. and qualitatively with recent theoretical predictions of Braunstein and McKoy. The large non‐Franck–Condon variations in the σ(100)/σ(000) and σ(200)/σ(000) branching ratios are associated with a predicted 7σ→eσ shape resonance near 20 eV. Overall, the vibrational branching ratios imply lower resonant energies for the stretching modes (100 and 200) and a similar resonant energy for the asymmetric stretch (001), compared with the 000 mode. The vibrational asymmetry parameters (β) display a strong variation with energy which is qualitatively reproduced by theory; however, the experimental values for β(100) and β(001) exhibit additional structure around 20 eV. When combined with theory and recent fluorescence data, these r...

THE NIST DETECTOR-BASED LUMINOUS INTENSITY SCALE

The Système International des Unités (SI) base unit for photometry, the candela, has been realized by using absolute detectors rather than absolute sources. This change in method permits luminous intensity calibrations of standard lamps to be carried out with a relative expanded uncertainty (coverage factor k = 2, and thus a 2 standard deviation estimate) of 0.46 %, almost a factor-of-two improvement. A group of eight reference photometers has been constructed with silicon photodiodes, matched with filters to mimic the spectral luminous efficiency function for photopic vision. The wide dynamic range of the photometers aid in their calibration. The components of the photometers were carefully measured and selected to reduce the sources of error and to provide baseline data for aging studies. Periodic remeasurement of the photometers indicate that a yearly recalibration is required. The design, characterization, calibration, evaluation, and application of the photometers are discussed.

Calibration of a monochromator/spectrometer system for the measurement of photoelectron angular distributions and branching ratios

Abstract We describe the techniques used in calibrating a monochromator/spectrometer system for gas-phase photoelectron angular distribution and branching ratio measurements. We report a self-consistent set of values for the Ne 2p, Ar 3p, Kr 4 p 3 2 and 4 p 1 2 , and Xe 5 p 3 2 and 5 p 1 2 photoelectron asymmetry parameters and for the Kr 4 p 3 2 : 4 p 1 2 and Xe 5 p 3 2 : 5 p 1 2 branching ratios for the kinetic energy regions from threshold to approximately 15 eV.

Resonance structure in the vibrationally resolved photoelectron branching ratios and angular distributions of the 2π-1 channel of NO

We report on vibrationally resolved measurements of photoelectron angular distributions and branching ratios for NO+(2π−1)X 1Σ+ using synchrotron radiation over hν=11.5–26 eV. Normally weak vibrational levels are strongly enhanced below 18 eV, and the photoelectron asymmetry parameters and branching ratios display a vibrationally dependent, broad spectral structure over hν≂11–18 eV. These observations may reflect the presence of the expected σ shape resonance, however, various interchannel coupling mechanisms may also be involved. Resonance structure in the photoelectron asymmetry parameters is also observed in the hν≂19–22 eV region. This structure is likely associated with Rydberg excitations from the 4σ orbital.

Vibronic coupling and other many‐body effects in the 4σ−1g photoionization channel of CO2

Vibrational branching ratios and photoelectron angular distributions were measured for 4σ−1g photoionization of CO2 in the energy range 20–28 eV. Of particular interest are three vibrational components of the resulting CO+2 C 2Σ+g state—the allowed (000) and (100) bands and the forbidden (101) band. The wavelength dependence of the beta parameter for the forbidden band deviated significantly from that of the two allowed bands, showing instead a strong resemblance to that of the B 2Σ+u state. This behavior suggests that vibronic coupling to the B 2Σ+u state is responsible for the appearance of the forbidden (101) band in the C 2Σ+g state photoelectron spectrum. We also observe evidence for other many‐body effects—shape‐resonance‐induced continuum–continuum coupling and doubly excited autoionizing resonances—in the present data.

The Detector-Based Candela Scale and Related Photometric Calibration Procedures at NIST

The candela, one of the SI base units, has been realized by using absolutely calibrated detectors rather than sources. A group of eight photometers was constructed using silicon photodiodes, precision apertures, and glass filters for V (λ) match. Their absolute spectral responsivities were calibrated against the NIST absolute spectral responsivity scale. The measurement chain has been significantly shortened compared with the old scale based on a blackbody. This resulted in improving the calibration uncertainty to 0.46% (2σ), a factor-of-2 improvement. This revision has made various photometric calibrations at NIST more versatile and flexible. Luminous intensities of light sources ranging from 10 -3 to 10 4 candelas are directly calibrated with the standard photometers, which have a linear response over that range. Illuminance meters are calibrated directly against the standard photometers. A luminance scale has also been realized on the detector base using an integrating sphere source. Total flux ranging from 10 -2 to 10 5 lumens can be measured in a 2 m integrating sphere using a photometer with a wide dynamic range. The revisions of the calibration procedures significantly improved the calibration uncertainty.

Comparison of the NIST high Accuracy Cryogenic Radiometer and the NIST scale of Detector Spectral Response

Two independent methods of measurement were used to determine the absolute spectral responsivity and external quantum efficiency of light-trapping silicon photodiode packages. These trap packages were calibrated first by the NIST High Accuracy Cryogenic Radiometer at laser wavelengths of 633 nm and 442 nm. They were also measured in the NIST Spectral Comparator Facility with working standards traceable to a 100% quantum efficient radiometer (QED-200). The two sets of measurements agree to better than 0,1% at 633 nm and 0,25% at 442 nm.

Use of heterodyne detection to measure optical transmittance over a wide range

We are developing a heterodyne detection technique to measure optical transmittance with high accuracy over an unprecedented dynamic range. We have measured filters spanning a wide range of transmittances (12 orders of magnitude) and have evaluated the absolute uncertainties and discuss the ultimate accuracies that may be achieved. Our setup uses a two-beam Mach-Zehnder interferometer with acoustooptic frequency shifting to produce a frequency difference between the two light beams. We determine the optical transmittance of a filter by inserting it into one of the interferometer arms and measuring the change in amplitude of the signal at the difference frequency on the interferometer output beam. This method allows direct comparisons between optical and rf attenuators, ultimately tying optical transmittance measurements to rf attenuation standards in an absolute way.