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Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors

R. Stewart McWilliamsa,b,c,d,e,1, D. Allen Daltona, Zuzana Konôpkováf, Mohammad F. Mahmooda,e, and Alexander F. Goncharova,g

aGeophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015; bSchool of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom; cCentre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom; dDepartamento de Geociencias, Universidad de Los Andes, Bogotá DC, Colombia; eDepartment of Mathematics, Howard University, Washington, DC 20059; fDeutsches Elektronen-Synchrotron Photon Science, 22607 Hamburg, Germany; and gKey Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China

Edited by Vladimir E. Fortov, Russian Academy of Sciences, Moscow, Russia, and approved May 15, 2015 (received for review November 13, 2014) The noble gases are elements of broad importance across science Here we report experiments in the laser-heated diamond anvil and technology and are primary constituents of planetary and cell (15, 16, 26–29) on high-density and high-temperature states stellar atmospheres, where they segregate into droplets or layers of the noble gases Xe, Ar, Ne, and He (Fig. 1). Rapid heating that affect the thermal, chemical, and structural evolution of their and cooling of compressed samples using pulsed laser heating host body. We have measured the optical properties of noble (26, 27) is coupled with time domain spectroscopy of thermal gases at relevant high pressures and temperatures in the laser- emission (26) to determine sample temperature and transient heated diamond anvil cell, observing insulator-to-conductor trans- absorption to establish corresponding sample optical properties formations in dense helium, neon, argon, and xenon at 4,000– (Figs. S1 and S2). A sequence of heat cycles to increasing tem- 15,000 K and pressures of 15–52 GPa. The thermal activation and perature documents optical changes in these initially transparent frequency dependence of conduction reveal an optical character insulators. dominated by electrons of low mobility, as in an amorphous semi- conductor or poor metal, rather than free electrons as is often Results assumed for such wide band gap insulators at high temperatures. Time domain spectroscopic data on pulsed heated samples are White dwarf stars having helium outer atmospheres cool slower presented in Figs. 2 and 3. Absorption coefficients are de- and may have different color than if atmospheric opacity were termined as controlled by free electrons. Helium rain in Jupiter and Saturn becomes conducting at conditions well correlated with its in- α = −lnðIH =ICÞ=d, [1] creased in metallic hydrogen, whereas a deep layer of insulating neon may inhibit core erosion in Saturn. where IC and IH are the transmitted probe intensity through cold (transparent) and hot samples, respectively, and d is the time- rare gases | extreme conditions | warm dense matter | giant planet | dependent thickness of the absorbing region, typically 4 ± 2 μm white dwarf (Fig. 1C and Fig. S3). The corresponding conductivity is given as

oble gases play important roles in the evolution and dy- σ = nαc«0 [2] Nnamics of planets and stars, especially where they appear in a condensed, purified state. In gas giant planets, helium and for high frequencies, where n is the index of refraction (of order « neon can precipitate as rain in metallic hydrogen envelopes, 1), c is the speed of light, and 0 is the permittivity of free space. leading to planetary warming and specifically the anomalously Conductivity at low frequency was determined from shock slow cooling of Saturn (1–8). In white dwarf stars cooling can be especially fast due to the predicted low opacity of dense helium Significance atmospheres, affecting the calibration of these objects as cosmo- logical timekeepers (9–12). In these systems, the transformation of Planets and stars contain matter at extreme pressures and tem- dense noble gases (particularly He) from optically transparent peratures hidden deep beneath their opaque surfaces. Unable to insulators to opaque electrical conductors is of special importance see these states of matter directly, we instead produce them in

(2, 9, 11, 12). laboratory experiments. Here a novel method of studying ex- PHYSICS Dense noble gases are expected to show systematic similarities treme states in a tabletop experiment is described and applied in their properties at extreme conditions (13–17); however, a to common planet- and star-forming materials, the noble gases. general understanding of their insulator–conductor transformation Helium, neon, argon, and xenon transform in the experiments from remains to be established. Xe is observed to metallize near room transparent electrical insulators to opaque electrical conductors. In temperature under pressures similar to those at Earth’s core– Saturn, rain composed of becomes conductive as it falls mantle boundary (18, 19). Ar and He are observed to conduct only and can form a protective layer around the planetary core that at combined high pressure and temperature (12, 13, 17). Ne is prevents the core from dissolving into surrounding metallic hy- predicted to have the highest metallization pressure of all known drogen. White dwarf stars have unexpectedly opaque helium materials—103 times that of Xe and 10 times that of He (14, 18, atmospheres, causing them to age slower than anticipated. 20, 21)—and has never been documented outside of its insulating Author contributions: R.S.M., D.A.D., M.F.M., and A.F.G. designed research; R.S.M., D.A.D., state. Experimental probes of extreme densities and temperatures Z.K., M.F.M., and A.F.G. performed research; R.S.M. and A.F.G. analyzed data; and R.S.M., in noble gases have previously relied on dynamic compression by D.A.D., Z.K., M.F.M., and A.F.G. wrote the paper. shock waves (12, 13, 17, 22–24). However, in such adiabatic ex- The authors declare no conflict of interest. periments, light and compressible noble gases heat up significantly This article is a PNAS Direct Submission. and can ultimately reach density maxima (12, 13, 17, 21, 24, 25), so 1To whom correspondence should be addressed. Email: [email protected]. that conditions created often lie far from those deep within This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. planets (7, 8) and stars (9). 1073/pnas.1421801112/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1421801112 PNAS | June 30, 2015 | vol. 112 | no. 26 | 7925–7930 Downloaded by guest on September 29, 2021 ≈ [3] A diamond- kTC 0.078 Eg, heating probe anvil cell laser spectro- streak where k is Boltzmann’s constant, defining a global transforma- graph camera tion boundary (Fig. 5). This dependence is similar to that pre- dicted by free electron theory (kTC ∼ 0.071 Eg + 0.33 eV; Fig. 5) but is offset to lower temperature by several thousand degrees K. Direct measurements of absorption spectra (Fig. 2) provide a B C clear insight into the nature of the opaque high-temperature foil state. Rather than showing absorption (Fig. 2E) and conductivity anvil anvil T (kK) melt sample ( Position Radial 10 -10 (Fig. 2F) decreasing with photon energy, as expected for free hole electron optical character (9, 11, 18, 32, 33), these quantities 8 opaque increase. Optical conductivity is high compared with the DC probe 6 0 conductivity but of similar magnitude. 4 The data thus evince an enhancement of conduction with

2 m) temperature; however, free electrons do not describe its spectral heating solid 10 character or thermal activation behavior. laser ruby 0 Discussion -20 -10 0 10 As interband excitations do not account for the optical changes Axial Position ( m) observed here, other explanations must be examined. Absorption D may be dominated by localized states within the band gaps, a thermal common trait of disordered semiconductors associated with a emission characteristic Urbach absorption behavior (33, 34), which well describes our data (Fig. 2E and SI Text). Orbital-hybridized or charge-sharing clusters in the fluid (11, 15, 35) could provide a amplitude (pulsed) cw probe bb probe localized basis of absorption (34). First-principles electronic (continuous) structure models for warm dense noble gases should thus not neglect localization effects a priori (2). Data are also well de- scribed by a common conductivity model for a poor metal with time 1 s low electron mobility (32) (Fig. 2F), which reproduces the weakly increasing conductivity from DC to optical frequencies and Fig. 1. Creating and probing extreme states of noble gases. (A) Configu- predicts a maximum in conductivity at finite frequency. Such ration of laser heating and transient absorption probing of the diamond features also characterize other warm dense matter near metal- anvil cell, with probe beams transmitted through the cell into the detection system. (B) Microscopic view of the diamond cell cavity, which contains a lization transitions, including hydrogen (36), aluminum (37), and noble gas sample and a metal foil (Ir) which converts laser radiation to heat mercury (32). Indirect-gap (18) and Tauc (33) absorption and has small hole at the heated region through which probe beams are behaviors, known to occur in rare gas solid and disordered transmitted to test optical character of samples. (C) Finite element model semiconductors, respectively, could not describe these data self- (26) (Fig. S3) of the temperature distribution in heated Ar at 51 GPa (Fig. 2), consistently (SI Text). − with solid–melt (16) and insulator–conductor (α = 0.1 μm 1) boundaries in Ar and Xe show significant reductions in transformation the sample marked dashed and dotted, respectively. (D) Schematic of time temperature with increasing pressure (Fig. 4), consistent with the domain probing during transient heating. Temperature is determined from high sensitivity of the band gaps to compression (18, 38–41) thermal emission (red) and absorption from transmitted probe beams: a con- tinuous laser (cw; green) and pulsed supercontinuum broadband (bb; blue). (Figs. S4 and S5). At higher pressures, fluid Ar and Xe should transform to conductors upon melting: from Eq. 3, fluid Xe is conducting along the melting curve (15, 16) above ∼60 GPa wave direct current (DC) conductivity data (where available; and fluid Ar (16) above ∼160 GPa (Fig. 4). This confirms that SI Text). anomalous melting behavior in Xe under pressure (15, 16) can The onset of absorption occurs at temperatures well above be attributed to electronic transformation in the fluid (15) and noble gas melting points (15, 16) and so in fluid states (Fig. 4). suggests anomalous thermal emission reported from laser-heated ∼ Electronic band gaps (Eg) at these conditions are larger than the Ar near melting, at 90 GPa and 4,500 K (29), could be due to energies of light used, ruling out direct interband (bound-free) opacity of hot Ar. He and Ne require the highest temperatures to absorption. Optical changes due to thermally excited free elec- exhibit optical darkening, of 10,000–12,000 K and 15,000–18,000 K, trons, often believed to dominate in dense insulators at high respectively, confirming the especially insulating character pre- temperatures (2, 9, 11, 12, 30), yield a simple criterion for the dicted for Ne (14, 20, 21). Due to the relative insensitivity of He temperature of optical transformation and absorption onset, and Ne band gaps to changes in density (2, 11, 14, 20, 25, 30) (Fig. based on Drude free electron behavior in a semiconductor (12, S6), these threshold temperatures should be similar over a wide 30) (SI Text). However, strong absorption appears at lower domain of pressures in planetary (6–8) and stellar (9, 10) interiors temperature than expected from this model, at conditions where (Fig. 4 and SI Text). free electron absorption should be small to negligible. For ex- White dwarf stars possess dense atmospheres of He at con- ample, in Ar at 51 GPa (Fig. 2), the band gap is 7.4 eV, whereas ditions directly probed in our experiments (9–12) (Figs. 4 and 5). our spectral range is below 2.7 eV, ruling out interband ab- Our measurements affirm that these atmospheres are highly − . High absorption (α > 0.1 μm 1) is observed above 6,700 K transparent at visible and near-IR wavelengths, a feature believed (0.6 eV). At this temperature the DC conductivity is ∼1 S/cm to produce particularly rapid stellar cooling due to efficient trans- (12, 13, 17, 30, 31) (SI Text), but the corresponding free electron port of thermal radiation (9, 10, 12). However, atmospheric − absorption (∼0.01 μm 1) is an order of magnitude lower than opacity will be higher than if caused by free electrons (Fig. 5), the observed absorption. implying relatively slower cooling (9, 10). An atmospheric Across all noble gases studied, the temperature TC for the onset opacity increasing toward the blue (e.g., Fig. 2) rather than the of high absorption increases systematically with the band gap Eg as red as predicted by free electron models (9, 11) could lead to

7926 | www.pnas.org/cgi/doi/10.1073/pnas.1421801112 McWilliams et al. Downloaded by guest on September 29, 2021 A Time ( s) B Time ( s) C Time ( s) 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 450 1.5 Intensity T history T 4.4 kK Emission (counts) 10 K) 3 500 8 1.0 Probe (cw) 0 1000 6 Luminance (a.u.) 550 4 Laser 0.5 Probe (bb) Probes: power Transmittance T 8.7 kK Wavelength (nm) Wavelength 2 (a.u.) cw Temperature (10 Temperature bb 600 0 0.0 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 D E F 540 40 ) 0.84 Probe -1

m data Reference Urbach fit 30 590 0.80

Intensity 20 data (counts) 0.76 DC conductivity 640 Drude prediction 10 Smith-Drude fit Wavelength (nm) Wavelength Conductivity (S/cm) 0 100 200 0.72 Absorption coef. ( 690 0 4 5 6 7 1.7 1.8 1.9 2.0 2.1 2.2 2.3 0 1 2 3 4 Time (ns) Energy (eV) Energy (eV)

Fig. 2. Absorption and conduction in argon at 51 GPa. (A) Spectrogram of emission and transient absorption using continuous monochromatic (cw) and pulsed broadband (bb) (48) probes during a heat cycle (method of Fig. 1). (B) Temperature history with uncertainty, thermal luminance, and laser power for A. (C) Sample transmittance (532–615 nm) for A and at a lower temperature (bb error bars are uncertainty due to broadband source power). (D) Spectrogram of transient absorption (bb) during a heating event similar to A but taken on a faster timescale to remove thermal background and including one probe pulse and its reference. (E) Absorption spectrum derived from D assuming a 4-μm-thick absorbing sample and fit to a semiconductor Urbach (33) model (error bars are standard deviation from three spectra). (F) Conductivity spectrum derived from E, the optical index of refraction (50, 53), and the DC conductivity (12, 13, 17, 30), with metallic Drude (12, 30) and poor-metal Smith–Drude (32) models.

deficiencies in blue color (9), which have been documented in a thermal production of singly ionized He, a chemical species iso- subset of white dwarfs (9). electronic with atomic hydrogen that is a likely source of observed Significant theoretical and observational evidence for phase optical and electronic changes (9, 11, 25), is one possible cause of separation of helium and neon in the hydrogen-rich outer en- increased miscibility. The data also highlight the importance of velopes of giant planets has been reported (1–8), yet questions localized rather than extended electronic changes in the warm over the nature and even existence of this phase separation re- dense noble gases, which may play an important role in chemical main (2, 42, 43). This is in part due to limited direct experimental behavior. He electronic change has sometimes been expected to data on the hydrogen–helium–neon system at the conditions of be not relevant to the miscibility question, on the argument that phase separation: although hydrogen is observed in a metallic He remains insulating near demixing conditions (3, 43); the data state at the interior conditions of giant planets (44), measure- confirm this not to be the case. If He electronic changes con- ments on the electronic character of helium and neon at relevant trolled miscibility, then the insensitivity of He electronic prop- densities and temperatures have been limited (12, 13, 17, 45). As erties to pressure (2, 11, 14, 25, 30) (Fig. 4 and Fig. S6) would phase separation may be controlled by the electronic properties imply pressure independence in the maximum temperature of of the constituents (1, 2, 4), the present measurements have a immiscibility, as seen in recent theory (3, 4) at high pressures PHYSICS direct bearing on this problem. (SI Text). Our measurements show that in Saturn above ∼0.25 of the Based on these considerations, phase separated He should planet’s radius (RS) and in Jupiter above ∼0.70 of its radius (RJ), tend to dissolve just above Saturn’s core (Fig. 5). However, un- precipitated He would be insulating (Fig. 5). He droplets would like He, Ne remains an insulator at core conditions in Saturn be weakly conducting below these depths and to the core– (T < 11,000 K). If Ne exhibited a correlation between its electronic envelope boundary of Saturn, lying just beyond the transformation character and mixing behavior similar to that indicated for He, at ∼0.15 RS. In Jupiter, phase-separated He would transform then Ne sequestered and concentrated by He rain in the deep further at depth, developing metallic properties (12, 30) below interior (1, 5) may remain exsolved even if He is dissolved. Thus, ∼0.25 RJ andabovethecoresurfaceat∼0.10 RJ. noble gas precipitate could accumulate on the core surface (7, 8), However, the onset of conducting states observed here closely even in the event of He dissolution. This noble gas layer, corresponds with the conditions where helium is predicted (3, 4) chemically unreactive with metallic hydrogen, would inhibit to become fully soluble in metallic hydrogen (Fig. 5 and SI Text), erosion (6, 8, 46) of the underlying core, even if very thin (SI consistent with expectations that electronic transformations and Text). Although plausible for Saturn, such a layer is unlikely for miscibility are related in hydrogen–helium mixtures (1, 2, 4). In Jupiter, given the insulator-to-conductor transformations of both Saturn, for example, the maximum temperature of immiscibility is He and Ne at the temperature of Jupiter’s core. This could predicted at 8,000 ± 800 K (3, 4), whereas electronic trans- contribute to differential core erosion in these planets and so formation has progressed significantly by ∼9,800 K (Fig. 5). The to the differences in their core size, envelope heavy element

McWilliams et al. PNAS | June 30, 2015 | vol. 112 | no. 26 | 7927 Downloaded by guest on September 29, 2021 diamond- Fig. 3. Absorption onset in helium at 22 GPa. (A)Con- A heating B foil probe anvil cell anvil anvil figuration of laser heating and transient absorp- laser sample tion with probe beams reflected in the diamond cell. (B) Microscopic view of the cavity, where probe probe beams are reflected from the foil surface in a laser heated area. (C and D) Spectrograms for transparent and absorbing samples, respectively, with broadband probing. (E and F) Sample temperature (black points, left axes) and transmittance (red points, right axes) spectro- streak heating ruby shown with transmission levels (horizontal lines) for graph camera laser − α = 0andα = 0.1 μm 1 (red error bars and bands are uncertainty due to broadband source power). From C and E to D and F, absorption increases concomi- 560 tantly with emission intensity, duration, and temper- C D ature. A 4-μm-thick absorbing sample and two-pass absorption is used to compute α. 600

0 300 600

Wavelength (nm) Wavelength 0300600 640 Intensity (counts) Intensity (counts)

) 1.5

K

Transmittance 3 13 E -1 F -1 α = 0 µm α = 0 µm 1.0

α -1 α -1 0.5 11 = 0.1 µm = 0.1 µm

Temperature ( 10 Temperature 0.0 2 6 10 14 2 6 10 14 Time (µs) Temperature Time (µs) Transmittance Laser Power (a.u.)

enrichment, and luminosity (3, 4, 6–8). Saturn and Jupiter may producing spectrograms of emission and transient absorption (Figs. 2 and 3). thus lie at a boundary between internally condensed and in- Thermal emission is detected as broadband and temporally continuous sig- ternally mixed gas giants controlled by formation of a protective nal (Fig. 1D) and is fit to a Planck function to determine temperature (Fig. S2). noble gas envelope around the cores of cooler planets. Probe beams for transient absorption passing through the heated sample are detected simultaneously with emission but are discrete in either wavelength Materials and Methods (green continuous laser) or time (pulsed supercontinuum broadband) (48) Diamond anvils with 300-μm-diameter flat culets were used, with Re gaskets, and are in this way discriminable from emission background (Fig. 1D). Laser pressure-loaded ultrahigh purity noble gases, and Ir laser-coupling foils; heating pulses of several microseconds duration were used (26, 27). sample pressure was determined via ruby fluorescence (47). Confocal optical Sample optical transformation was manifested by several concomitant spectroscopy (Fig. S1) is used to record visible spectra from the IR (1,075 nm) changes in the spectrograms upon increasing laser power (SI Text): (i)asudden laser-heated sample (27). In place of a CCD (27), a streak camera is mounted enhancement of transient absorption and (ii) a shift to higher temperature, to the spectrometer to record spectra continuously during heating cycles, emission intensity, and heat cycle duration. These phenomena are attributed

20 20 A conducting B fluid Ne conducting 15 15 K) K) fluid 3 3 He insulating 10 fluid 10

8 kK 6.5 kK insulating fluid Jupiter Fig. 4. Phase diagrams of Ne (A), He (B), Ar (C), and Xe (D) from experimental measurements. Present 5 5 Temperature (10 Temperature Temperature (10 Temperature measurements of optical absorption are horizontal Saturn − − solid white dwarf 4.5 kK dashes (red, α < 0.1 μm 1; black, α > 0.1 μm 1); solid 0 0 vertical bars represent the range of temperatures 0 50 100 150 0 50 100 150 observed in each material state, plus uncertainty 20 Pressure (GPa) 20 Pressure (GPa) (Fig. S7). Black lines are phase boundaries for C D fluid–fluid insulator–conductor transformation (Eq. 3), Ar Xe melting (15, 16) and solid–solid insulator–conductor

15 K) 15 K) transformation in Xe (18, 19). Prior data are squares 3 3 (conductivity) (13, 17, 22, 54) or circles (absorptivity) − conducting (12, 18) with red indicating σ < 1S/cm,α < 0.1 μm 1, 10 conducting −1 10 fluid and black indicating σ > 1S/cm,α > 0.1 μm ;black fluid triangle is region of anomalous emission in Ar (29); red (black) crosses indicate insulating (metallic) 5 5 Temperature (10 Temperature i. fluid (10 Temperature i. fluid electrical behavior in Xe (19); and black diamond solid solid c. solid indicates dense plasma conditions in He (24). Also 0 0 shown are interior conditions of Jupiter (brown) (6), 0 50 100 150 0 50 100 150 Saturn (blue) (7), and white dwarfs (green) for ef- Pressure (GPa) Pressure (GPa) fective temperatures of 8.0, 6.5, and 4.5 × 103 K (9).

7928 | www.pnas.org/cgi/doi/10.1073/pnas.1421801112 McWilliams et al. Downloaded by guest on September 29, 2021 Broadband transient absorption probe data include uncertainty due to random pulse spectral intensity variation (48) (Figs. 2C and 3 E and F), or 25 Reflecting reflective these variations were cancelled out by pulse referencing (Fig. 2E). The broad- He (12) band probe was often edge-filtered (Figs. 2A and 3 C and D) to reserve a portion of the spectrogram for measurement of emission alone. Probing was made by 20 reflecting beams from the metallic (reflective) foil in a laser-heated area and K) 3 through overlying hot sample (Fig. 3) or by transmitting beams through a small − hole in the foil at the laser heated area (Figs. 1 and 2). The critical α of 0.1 μm 1 15 (σ ∼ 1 S/cm; Eq. 2) is roughly the detection limit for absorbing conditions, above (obs.) Absorbing c which sample transformation was clearly evident (Fig. 3). > Band gaps for noble gases Xe, Ar, and He were estimated at the volume 10 Transparent (31, 45, 49, 50) and temperature of optical transformation using experi- (free-free) white mental (12, 13, 17, 18, 31, 38, 51, 52) and theoretical (2, 11, 30, 39–41) results > c Temperature (10 Temperature dwarfs (Figs. S4–S7 and SI Text). The Ar gap was modeled as (Fig. S4) 5 Jupiter E = 18.5 − 75.6 V−0.926 − 0.000527 T 0.902, [4] (18) g Saturn Xe Ar He Ne and the Xe gap was modeled as (Fig. S5) 0 0 5 10 15 20 −1.74 Eg = 10.2 − 601 V , [5] Gap Energy (eV) 3 with Eg in eV, V in cm /mol, and T in K. The gap variation in He was estimated Fig. 5. Optical properties of noble gases at 450–1,075 nm as a function from a prior model (30). The gap of Ne is less well known and was estimated of band gap. Absorption coefficients α below (red) or above (black) the from its initial value (20) and assuming a temperature sensitivity similar to He α = μ −1 critical value ( c 0.1 m ) are from this study (horizontal dashes) on (Figs. S6 and S7 and SI Text), giving Eg ≈ 16.5 eV. Uncertainties in band gap fluid Ne (15 GPa), He (22, 52 GPa), Ar (22, 51 GPa), and Xe (22, 44 GPa) estimates (Fig. S7) are small compared with the variation across the noble and prior work (circles) on solid Xe (120 GPa) (18). Corresponding band gases and are accounted for in our conclusions. For example, the increase in gaps are estimated at the conditions of optical transformation. The data temperature of absorption onset with band gap is well constrained. define a boundary of linear character (black solid line; Eq. 3) where Detailed methods can be found in SI Text. conductivities are ∼1 S/cm. The free electron model (12, 30) location of this boundary is the dashed black line (SI Text and Fig. S7). High reflectivity ACKNOWLEDGMENTS. We thank C. T. Seagle and R. Boehler for helpful (>1%) and conductivity (>100 S/cm) occur above the dotted black line (12, 23, discussions; M. Ahart, M. Somayazulu, and T. Strobel for technical assistance; 30). Colored lines are conditions of He atmospheres of white dwarfs (9, 30) and N. Gómez-Pérez, V. V. Struzhkin, D. G. Hicks, R. Boehler, and two anon- (green; for effective temperatures 8.0 and 4.5 kK) and He rain in planetary ymous reviewers for reviews of this work. This work was supported by the gaseous envelopes (blue and brown) (6, 7, 25, 30) with thicker areas in- National Science Foundation (NSF) Major Research Instrumentation program, dicating predicted upper limit of He–H immiscibility (3, 4). NSF EAR-1015239 and NSF EAR/IF-1128867; the Army Research Office (56122- CH-H); the Carnegie Institution of Washington; the Deep Observatory Instrumentation grant; the British Council Researcher Links programme; the US Department of Energy (DOE) National Nuclear Security Administration Carne- to broadband (visible to IR) enhancement in sample optical absorptivity and gie/DOE Alliance Center (DE-FC52-08NA28554); the DOE Energy Frontier Re- emissivity such that the efficiency of incident heating laser absorption in- search Center for Energy Frontier Research in Extreme Environments; and creases (28) together with probe absorption. National Science Foundation of China (21473211).

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