Observed Wavelength Range Research Articles

A Comparative Analysis of Gravity Waves in He and Ar Densities in the Martian Thermosphere

AbstractIn this study, we conducted a comparative analysis of gravity waves (GWs) in the Martian upper thermosphere, utilizing He and Ar densities measured by the Neutral Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile EvolutioN spacecraft. Our investigation revealed that GWs are a persistent phenomenon in Martian thermospheric He densities, akin to their presence in heavier species like Ar and CO2. Intriguingly, we observed that local time and seasonal variations in He GW amplitudes exhibit an almost opposite pattern compared to those in Ar. A significant breakthrough was the identification of a temperature‐dependent transition wavelength distinctly demarcating short and long wavelengths. This transition wavelength, which increases with increasing background temperatures, profoundly influences the dominance of GW amplitudes in He and Ar. In the short‐wavelength limit, GW amplitudes in He prevail, while GW amplitudes in Ar take precedence in the long‐wavelength limit. During periods of lower temperatures during midnight and the dawn terminator, transition wavelengths fall within the observed wavelength range enabling the study of dominance of GWs in He and Ar. Conversely, higher temperatures (>180 K) during daytime and the dusk terminator eliminate transition wavelengths within our observational range. Consequently, the amplitudes of GWs in He dominate over those in Ar. Furthermore, under nominal dust conditions, GWs in both He and Ar exhibit an inverse relationship with their respective background densities. During global dust storm, this inverse relationship is maintained for GWs in He, while GWs in Ar display a slight positive correlation with background Ar densities.

Circular Dichroism Spectral Similarity Plots to Extend Validation and Correction to All Measured Wavelengths.

Interlaboratory comparisons of circular dichroism (CD) spectra are useful for developing confidence in the measurements associated with optically active molecules. These measurements also help define the higher-order (secondary and tertiary) structure of biopolymers. Unfortunately, the extent of the validity of these measurements has been unclear. In this work, a method is described to extend CD validation over the entire observed wavelength range using what will be called spectral similarity plots. The method involves plotting, wavelength by wavelength, all measured spectral intensities of a sample at one concentration against the intensity values of the same material at a different concentration or pathlength. These spectral similarity plots validate the instrument in terms of spectral shape and whether the shape is shifted in intensity and/or in wavelength. This comparison tests the linearity of instrument's signal, the balance of its left and right polarizations, its wavelengths, and its spectral intensity scales. When the process is applied to materials with accepted and archived intensity values, the method can be linked to older single-wavelength and double-wavelength calibration techniques. Further, spectral similarity testing of CD spectra from samples with different concentrations run in different labs suggests that improved interlaboratory validation of CD data is possible. Since a database of archival CD measurements is available online, spectral similarity comparisons could possibly provide the ability to compare linearity, polarization balance, wavelength, and spectral intensity between all current CD instruments. If the preliminary results published here prove robust and transferable, then comparisons of full-wavelength range spectra to archived data using spectral similarity plots should become part of the standard process to validate and calibrate the performance of CD instruments.

Revealing a High Water Abundance in the Upper Mesosphere of Mars With ACS Onboard TGO

AbstractWe present the first water vapor profiles encompassing the upper mesosphere of Mars, 100–120 km, far exceeding the maximum altitudes where remote sensing has been able to observe water to date. Our results are based on solar occultation measurements by Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). The observed wavelength range around 2.7 μm possesses strong CO2 and H2O absorption lines allowing sensitive temperature and density retrievals. We report a maximum H2O mixing ratio varying from 10 to 50 ppmv at 100–120 km during the global dust storm (GDS) of Martian Year (MY) 34 and around southern summer solstice of MY 34 and 35. During other seasons water remains persistently below ∼2 ppmv. We claim that contributions of the MY34 GDS and perihelion periods into the projected hydrogen escape from Mars are nearly equivalent.

Asteroid spectral taxonomy using neural networks

Aims. We explore the performance of neural networks in automatically classifying asteroids into their taxonomic spectral classes. We particularly focus on what the methodology could offer the ESA Gaia mission. Methods. We constructed an asteroid dataset that can be limited to simulating Gaia samples. The samples were fed into a custom-designed neural network that learns how to predict the samples’ spectral classes and produces the success rate of the predictions. The performance of the neural network is also evaluated using three real preliminary Gaia asteroid spectra. Results. The overall results show that the neural network can identify taxonomic classes of asteroids in a robust manner. The success in classification is evaluated for spectra from the nominal 0.45–2.45 μm wavelength range used in the Bus-DeMeo taxonomy, and from a limited range of 0.45–1.05 μm following the joint wavelength range of Gaia observations and the Bus-DeMeo taxonomic system. Conclusions. The obtained results indicate that using neural networks to execute automated classification is an appealing solution for maintaining asteroid taxonomies, especially as the size of the available datasets grows larger with missions like Gaia.

Machine-learning Classifiers for Intermediate Redshift Emission-line Galaxies

Abstract Classification of intermediate redshift (z = 0.3–0.8) emission line galaxies as star-forming galaxies, composite galaxies, active galactic nuclei (AGNs), or low-ionization nuclear emission regions (LINERs) using optical spectra alone was impossible because the lines used for standard optical diagnostic diagrams: [N ii], Hα, and [S ii] are redshifted out of the observed wavelength range. In this work, we address this problem using four supervised machine-learning classification algorithms: k-nearest neighbors (KNN), support vector classifier (SVC), random forest (RF), and a multilayer perceptron (MLP) neural network. For input features, we use properties that can be measured from optical galaxy spectra out to z < 0.8—[O iii]/Hβ, [O ii]/Hβ, [O iii] line width, and stellar velocity dispersion—and four colors (u − g, g − r, r − i, and i − z) corrected to z = 0.1. The labels for the low redshift emission line galaxy training set are determined using standard optical diagnostic diagrams. RF has the best area under curve score for classifying all four galaxy types, meaning the highest distinguishing power. Both the AUC scores and accuracies of the other algorithms are ordered as MLP > SVC > KNN. The classification accuracies with all eight features (and the four spectroscopically determined features only) are 93.4% (92.3%) for star-forming galaxies, 69.4% (63.7%) for composite galaxies, 71.8% (67.3%) for AGNs, and 65.7% (60.8%) for LINERs. The stacked spectrum of galaxies of the same type as determined by optical diagnostic diagrams at low redshift and RF at intermediate redshift are broadly consistent. Our publicly available code (https://github.com/zkdtc/MLC_ELGs) and trained models will be instrumental for classifying emission line galaxies in upcoming wide-field spectroscopic surveys.

Atomic Transition Probabilities for UV and Blue Lines of Fe ii and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937

Abstract We report new branching fractions (BFs) for 121 UV lines from the low-lying odd-parity levels of Fe ii belonging to the z6Do, z6Fo, z6Po, z4Fo, z4Do, and z4Po terms of the 3d6(5D)4p configuration. These lines range in wavelength from 2250 to 3280 Å and originate in levels ranging in energy from 38,459 to 47,626 cm−1. In addition, we report BFs for 10 weak blue lines connecting to the z4Do term that range in wavelength from 4173 to 4584 Å. The BFs are combined with radiative lifetimes from the literature to determine transition probabilities and log(gf) values. Comparison is made to selected experimental and theoretical data from the literature. Our new data are applied to iron abundance determinations in the Sun and in metal-poor star HD 84937. For the Sun, eight blue lines yield log ε(Fe) = 7.46 ± 0.03, in agreement with standard solar abundance estimates. For HD 84937 the observable wavelength range extends to the vacuum UV (λ ≥ 2327 Å), and from 75 lines we derive log ε(Fe) = 5.26 ± 0.01 (σ = 0.07), near to the metallicity estimates of past HD 84937 studies.

Measurement of Circular Dichroism Spectra without Control of a Phase Modulator using Retardation Domain Analysis

This study investigated the measurement of circular dichroism (CD) spectra without controlling a phase modulator. In a conventional CD system, the peak retardation of the phase modulator must remain constant over the observed wavelength range. Thus, the phase modulator must be controlled to maintain an appropriate modulation degree at an observed wavelength. In contrast, CD obtained using retardation domain analysis is not affected by peak retardation. Consequently, CD spectra can be measured without control of the phase modulator, which was experimentally demonstrated in this study. Additionally, linear dichroism spectra were obtained using retardation domain analysis.

Higher order fano resonances in plasmonic nanostructures with two disks outside one ring

A kind of plasmonic nanostructure with two disks placed outside a ring is introduced. When incident light is perpendicular to the surface of the structure, the extinction spectra in the observed wavelength range show that octupolar modes are reduced as the radii of two-disks are different. In the case of two disks with the same size symmetrically around the ring (the SDDR structure), octupolar modes are suppressed absolutely. For SDDR structure, the larger the size ratios of the ring to the disks, the easier higher order dark modes in the ring are excited; as higher order resonances appear, lower order resonances are reduced. The decapolar mode of the SDDR structure with the ring radius of 250nm and two-disk radius of 60nm is in the near-infrared wavelength region, its contrast ratio (CR) and figure of merit (FOM) can reach 87% and 22.5, respectively, which demonstrates application potentials for high-performance biochemical sensing.

Study on spectral and refractive index sensing characteristics of etched excessively tilted fiber gratings.

We investigated the spectral and refractive index (RI) sensing characteristics of the excessively tilted fiber grating (Ex-TFG) with different cladding diameters. The Ex-TFG is inscribed in standard single-mode fiber, and the cladding reduces from 125 μm to around 15 μm by the chemical etching method. Experimental results show that the number of cladding modes decreases, and the spacing of adjacent resonance peaks becomes larger and larger with the reduction of the cladding diameter in the observed wavelength range of 1250-1650 nm. The average RI sensitivity in the index region of 1.33-1.38, the one near 1.33, and the one at around 1.38 of the etched Ex-TFG with a diameter of 15 μm is ∼6.3, ∼5.3, and ∼6.67 fold compared to those of the no-etched Ex-TFG, respectively. Also, the RI sensing performances of the etched Ex-TFG with a diameter smaller than 30 μm are better than those of the Ex-TFG inscribed in SM1500 (4.2 μm/80 μm) fiber in the index region of 1.33. The proposed micronano Ex-TFG has higher RI sensitivity and a more compact structure in biosensing applications, compared to the standard Ex-TFGs and Ex-TFGs inscribed in SM1500 fiber.

Single Aperture Spectral+ToF Compressive Camera: Toward Hyperspectral+Depth Imagery

Spectral imaging involves the sensing of a large amount of spatial information across a multitude of wavelengths. Conventional approaches rely on scanning techniques to construct a spectral data cube. Recently, compressive spectral imaging (CSI) has allowed to estimate spectral images with as few as a single coded snapshot. On a different front, 3-D ranging imaging often involves scanning along one of the spatial dimensions to estimate the depth of an scene using structured light, or the use of two cameras as required by stereo-imaging techniques. Recently, Time-of-Flight (ToF) snapshot imaging has gained considerable attention, due to its accuracy and speed. To date, however, these imaging modalities (CSI and ToF) have been realized and implemented by separate independent imaging sensors. This paper presents the development of a single aperture compressive spectral + depth imaging camera that employs a commodity 3-D range ToF sensor as the sensing device of a coded-aperture-based compressive spectral imager. The proposed system uses a single aperture/single sensor; thus, representing a significant improvement over existing RGB+D cameras that integrate two separate image sensors, one for RGB and another for depth. In addition, the observable wavelength range of the CSI device is expanded from the visible to the near-infrared, resolving up to as many as 16 independent channels. The proposed system allows the addition of side-information by placing a grayscale or RGB camera in the same path of the single-aperture system, such that the quality of the spectral estimation is improved, while maintaining high-frame rates. We demonstrate the proposed ideas through real experimentation conducted on an assembled CSI+ToF testbed camera system.

IR heterodyne spectrometer MILAHI for continuous monitoring observatory of Martian and Venusian atmospheres at Mt. Haleakalā, Hawaii

A new Mid-Infrared Laser Heterodyne Instrument (MILAHI) with >106 resolving power at 7–12μm was developed for continuous monitoring of planetary atmospheres by using dedicated ground-based telescopes for planetary science at Mt. Haleakalā, Hawaii. Room-temperature-type quantum cascade lasers (QCLs) that cover wavelength ranges of 7.69–7.73, 9.54–9.59, and 10.28–10.33μm have been newly installed as local oscillators to allow observation of CO2, CH4, H2O2, H2O, and HDO. Modeling and predictions by radiative transfer code gave the following scientific capabilities and measurement sensitivities of the MILAHI. (1) Temperature profiles are achieved at altitudes of 65–90km on Venus, and the ground surface to 30km on Mars. (2) New wind profiles are provided at altitudes of 75–90km on Venus, and 5–25km on Mars. (3) Direct measurements of the mesospheric wind and temperature are obtained from the Doppler-shifted emission line at altitudes of 110km on Venus and 75km on Mars. (4) Detections of trace gases and isotopic ratios are performed without any ambiguity of the reproducing the terrestrial atmospheric absorptions in the observed wavelength range. A HDO measurement of twice the Vienna Standard Mean Ocean Water (VSMOW) can be obtained by 15-min integration, while H2O of 75ppm is provided by 3.62-h integration. The detectability of the 100ppb-CH4 on Mars corresponds to an integration time of 32h.

Stellar science from a blue wavelength range

AbstractFrom stellar spectra, a variety of physical properties of stars can be derived. In particular, the chemical composition of stellar atmospheres can be inferred from absorption line analyses. These provide key information on large scales, such as the formation of our Galaxy, down to the small‐scale nucleosynthesis processes that take place in stars and supernovae. By extending the observed wavelength range toward bluer wavelengths, we optimize such studies to also include critical absorption lines in metal‐poor stars, and allow for studies of heavy elements (Z ≥ 38) whose formation processes remain poorly constrained. In this context, spectrographs optimized for observing blue wavelength ranges are essential, since many absorption lines at redder wavelengths are too weak to be detected in metal‐poor stars. This means that some elements cannot be studied in the visual‐redder regions, and important scientific tracers and science cases are lost. The present era of large public surveys will target millions of stars. It is therefore important that the next generation of spectrographs are designed such that they cover a wide wavelength range and can observe a large number of stars simultaneously. Only then, we can gain the full information from stellar spectra, from both metal‐poor to metal‐rich ones, that will allow us to understand the aforementioned formation scenarios in greater detail. Here we describe the requirements driving the design of the forthcoming survey instrument 4MOST, a multi‐object spectrograph commissioned for the ESO VISTA 4 m‐telescope. While 4MOST is also intended for studies of active galactic nuclei, baryonic acoustic oscillations, weak lensing, cosmological constants, supernovae and other transients, we focus here on high‐density, wide‐area survey of stars and the science that can be achieved with high‐resolution stellar spectroscopy. Scientific and technical requirements that governed the design are described along with a thorough line blending analysis. For the high‐resolution spectrograph, we find that a sampling of ≥2.5 (pixels per resolving element), spectral resolution of 18000 or higher, and a wavelength range covering 393–436 nm, is the most well‐balanced solution for the instrument. A spectrograph with these characteristics will enable accurate abundance analysis (±0.1 dex) in the blue and allow us to confront the outlined scientific questions. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

UVUDF: ULTRAVIOLET THROUGH NEAR-INFRARED CATALOG AND PHOTOMETRIC REDSHIFTS OF GALAXIES IN THE HUBBLE ULTRA DEEP FIELD

We present photometry and derived redshifts from up to eleven bandpasses for 9927 galaxies in the Hubble Ultra Deep field (UDF), covering an observed wavelength range from the near-ultraviolet (NUV) to the near-infrared (NIR) with Hubble Space Telescope observations. Our Wide Field Camera 3 (WFC3)/UV F225W, F275W, and F336W image mosaics from the ultra-violet UDF (UVUDF) imaging campaign are newly calibrated to correct for charge transfer inefficiency, and use new dark calibrations to minimize background gradients and pattern noise. Our NIR WFC3/IR image mosaics combine the imaging from the UDF09 and UDF12 campaigns with CANDELS data to provide NIR coverage for the entire UDF field of view. We use aperture-matched point-spread function corrected photometry to measure photometric redshifts in the UDF, sampling both the Lyman break and Balmer break of galaxies at z~0.8-3.4, and one of the breaks over the rest of the redshift range. Our comparison of these results with a compilation of robust spectroscopic redshifts shows an improvement in the galaxy photometric redshifts by a factor of two in scatter and a factor three in outlier fraction over previous UDF catalogs. The inclusion of the new NUV data is responsible for a factor of two decrease in the outlier fraction compared to redshifts determined from only the optical and NIR data, and improves the scatter at z<0.5 and at z>2. The panchromatic coverage of the UDF from the NUV through the NIR yields robust photometric redshifts of the UDF, with the lowest outlier fraction available.

Active galactic nuclei at z ∼ 1.5 – I. Spectral energy distribution and accretion discs

The physics of active super massive black holes (BHs) is governed by their mass (MBH), spin (a*), and accretion rate (⁠|$\dot{M}$|⁠). This work is the first in a series of papers with the aim of testing how these parameters determine the observable attributes of active galactic nuclei (AGN). We have selected a sample in a narrow redshift range, centred on z ∼ 1.55, that covers a wide range in MBH and |$\dot{M}$|⁠, and are observing them with X-shooter, covering rest wavelengths ∼1200–9800 Å. The current work covers 30 such objects and focuses on the origin of the AGN spectral energy distribution (SED). After estimating MBH and |$\dot{M}$| based on each observed SED, we use thin accretion disc (AD) models and a Bayesian analysis to fit the observed SEDs in our sample. We are able to fit 22/30 of the SEDs. Out of the remaining eight SEDs, three can be fit by the thin AD model by correcting the observed SED for reddening within the host galaxy and four can be fit by adding a disc wind to the model. In four of these eight sources, Milky Way-type extinction, with the strong 2175 Å feature, provides the best reddening correction. The distribution in spin parameter covers the entire range, from −1 to 0.998, and the most massive BHs have spin parameters greater than 0.7. This is consistent with the ‘spin-up’ model of BH evolution. Altogether, these results indicate that thin ADs are indeed the main power houses of AGN, and earlier claims to the contrary are likely affected by variability and a limited observed wavelength range.

OPTICAL AND NEAR-INFRARED POLARIMETRY OF HIGHLY REDDENED Type Ia SUPERNOVA 2014J: PECULIAR PROPERTIES OF DUST IN M82

We presented optical and near-infrared multi-band linear polarimetry of the highly reddened Type Ia SN~2014J appeared in M82. SN~2014J exhibits large polarization at shorter wavelengths, e.g., $4.8$\% in $B$ band, and the polarization decreases rapidly at longer wavelengths, with the position angle of the polarization remaining at approximately $40^{\circ}$ over the observed wavelength range. These polarimetric properties suggest that the observed polarization is likely to be caused predominantly by the interstellar dust within M82. Further analysis shows that the polarization peaks at a wavelengths much shorter than those obtained for the Galactic dust. The wavelength dependence of the polarization can be better described by an inverse power law rather than by Serkowski law for Galactic interstellar polarization. These suggests that the nature of the dust in M82 may be different from that in our Galaxy, with polarizing dust grains having a mean radius of $<0.1\ \mu$m.

DETECTION OF OCEAN GLINT AND OZONE ABSORPTION USINGLCROSSEARTH OBSERVATIONS

The Lunar CRater Observation and Sensing Satellite (LCROSS) observed the distant Earth on three occasions in 2009. These data span a range of phase angles, including a rare crescent phase view. For each epoch, the satellite acquired near-infrared and mid-infrared full-disk images, and partial-disk spectra at 0.26-0.65 microns (R~500) and 1.17-2.48 microns (R~50). Spectra show strong absorption features due to water vapor and ozone, which is a biosignature gas. We perform a significant recalibration of the UV-visible spectra and provide the first comparison of high-resolution visible Earth spectra to the NASA Astrobiology Institute's Virtual Planetary Laboratory three-dimensional spectral Earth model. We find good agreement with the observations, reproducing the absolute brightness and dynamic range at all wavelengths for all observation epochs, thus validating the model to within the ~10% data calibration uncertainty. Data-model comparisons reveal a strong ocean glint signature in the crescent phase dataset, which is well matched by our model predictions throughout the observed wavelength range. This provides the first observational test of a technique that could be used to determine exoplanet habitability from disk-integrated observations at visible and near-infrared wavelengths, where the glint signal is strongest. We examine the detection of the ozone 255 nm Hartley and 400-700 nm Chappuis bands. While the Hartley band is the strongest ozone feature in Earth's spectrum, false positives for its detection could exist. Finally, we discuss the implications of these findings for future exoplanet characterization missions.

Galaxy And Mass Assembly (GAMA): spectroscopic analysis

The Galaxy And Mass Assembly (GAMA) survey is a multiwavelength photometric and spectroscopic survey, using the AAOmega spectrograph on the Anglo-Australian Telescope to obtain spectra for up to ~300000 galaxies over 280 square degrees, to a limiting magnitude of r_pet < 19.8 mag. The target galaxies are distributed over 0<z<0.5 with a median redshift of z~0.2, although the redshift distribution includes a small number of systems, primarily quasars, at higher redshifts, up to and beyond z=1. The redshift accuracy ranges from sigma_v~50km/s to sigma_v~100km/s depending on the signal-to-noise of the spectrum. Here we describe the GAMA spectroscopic reduction and analysis pipeline. We present the steps involved in taking the raw two-dimensional spectroscopic images through to flux-calibrated one-dimensional spectra. The resulting GAMA spectra cover an observed wavelength range of 3750<lambda<8850 A at a resolution of R~1300. The final flux calibration is typically accurate to 10-20%, although the reliability is worse at the extreme wavelength ends, and poorer in the blue than the red. We present details of the measurement of emission and absorption features in the GAMA spectra. These measurements are characterised through a variety of quality control analyses detailing the robustness and reliability of the measurements. We illustrate the quality of the measurements with a brief exploration of elementary emission line properties of the galaxies in the GAMA sample. We demonstrate the luminosity dependence of the Balmer decrement, consistent with previously published results, and explore further how Balmer decrement varies with galaxy mass and redshift. We also investigate the mass and redshift dependencies of the [NII]/Halpha vs [OIII]/Hbeta spectral diagnostic diagram, commonly used to discriminate between star forming and nuclear activity in galaxies.

Approach for measuring complex refractive index of molten Sb2Te3 by spectroscopic ellipsometry

The complex refractive index of liquid and solid Sb2Te3 around its melting point has been measured by spectroscopic ellipsometry. The real and imaginary parts of the complex refractive index of liquid Sb2Te3 are totally different from those of solid Sb2Te3. The imaginary part of the complex refractive index for the liquid is significantly lower than that of the solid over the observed wavelength range. Our results are consistent with the idea that a molten area created in the center of a focused laser spot can function as a small aperture in the super-resolution readout effect in optical disks.

WARM H2O AND OH DISK EMISSION IN V1331 Cyg

We present high resolution (R=24,000) L-band spectra of the young intermediate mass star V1331 Cyg obtained with NIRSPEC on the Keck II telescope. The spectra show strong, rich emission from water and OH that likely arises from the warm surface region of the circumstellar disk. We explore the use of the new BT2 (Barber et al. 2006) water line list in fitting the spectra, and we find that it does a much better job than the well-known HITRAN (Rothman et al. 1998) water line list in the observed wavelength range and for the warm temperatures probed by our data. By comparing the observed spectra with synthetic disk emission models, we find that the water and OH emission lines have similar widths (FWHM ~ 18 km s-1). If the line widths are set by disk rotation, the OH and water emission lines probe a similar range of disk radii in this source. The water and OH emission are consistent with thermal emission for both components at a temperature ~ 1500 K. The column densities of the emitting water and OH are large, ~ 10^{21} cm-2 and ~ 10^{20} cm-2, respectively. Such a high column density of water is more than adequate to shield the disk midplane from external UV irradiation in the event of complete dust settling out of the disk atmosphere, enabling chemical synthesis to continue in the midplane despite a harsh external UV environment. The large OH-to-water ratio is similar to expectations for UV irradiated disks (e.g., Bethell and Bergin 2009), although the large OH column density is less easily accounted for.

Refractive index measurements of solid parahydrogen

Solid para-H2 is a promising gain medium for stimulated Raman scattering, due to its high number density and narrow Raman linewidth. In preparation for the design of a cw solid hydrogen Raman laser, we have made the first measurements, to our knowledge, of the index of refraction of a solid para-H2 crystal, in the wavelength range of 430-1100 nm. For a crystal stabilized at 4.4 K, this refractive index is measured to be n(p-H2)=1.130±0.001 at 514 nm. A slight, but significant, dependence on the final crystal-growth temperature is observed, with higher n(p-H2) at higher crystal-growth temperatures. Once a crystal is grown, it can be heated up to 10 K with no change in n(p-H2). The refractive index varies only slightly over the observed wavelength range, and no significant birefringence was observed.