Optical constants of ammonium hydrosulfide ice and ammonia ice

Optical constants of ammonium hydrosulfide iceand ammonia ice

Carly J. A. Howett

Atmospheric, Oceanic and Planetary Physics, University of Oxford, Clarendon Laboratory, Parks Road,Oxford OX1 3PU, UK

Robert W. Carlson

Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA

Patrick G. J. Irwin


Simon B. Calcutt


Received May 12, 2006; revised September 1, 2006; accepted September 6, 2006;posted September 19, 2006 (Doc. ID 70889); published December 20, 2006

Thin-film transmission spectra of ammonium hydrosulfide �NH4SH� ice and ammonia �NH3� ice between 1300and 12,000 cm−1 were used to determine the ice’s optical constants. The films were grown on a sapphire sub-strate, and a Fourier-transform spectrometer and a grating spectrometer were used together to record thespectra. Lambert’s law was used to directly determine the imaginary component of the complex refractive in-dices; from this, the real component was derived using the Kramers–Kronig algorithm. It is shown that, con-trary to what is expected, the optical constants determined for NH3 ice at 80 K are in good agreement withthose in the cubic phase, rather than the metastable one. The phase of the NH4SH ice was observed to changefrom amorphous to polycrystalline as the film was annealed to 160 K. © 2006 Optical Society of America

OCIS codes: 310.6860, 300.6340, 010.1280.

1. INTRODUCTIONThe optical constants of ammonium hydrosulfide�NH4SH� ice and ammonia �NH3� ice are crucial in the ac-curate modeling of cloud decks in many of the gas giantplanets of our solar system. However, no previous studyhas published the optical constants of NH4SH ice in atabulated form. Therefore, the aim of this study was todeduce actual values for the optical constants of NH4SHice. The techniques used were verified by comparing theoptical constants of NH3 ice determined in this study tothose widely available in the literature.

Previous studies and reviews of the optical constants ofNH3 ice are shown in Table 1. Two reviews are included inthe table: Taylor2 and Martonchik et al.9 Using these pre-viously published results, it has been shown that the dif-ference between the optical properties of NH3 ice in itsvarious phases is significant.12 The absorption featuresgrow more narrow and strong as the phase changes frommetastable to cubic, and the 9.46 �m �1057 cm−1� absorp-tion feature observed in the metastable phase splits intothree features of roughly equal intensity at 9.42, 9.37, and9.09 �m (1062, 1067, and 1100 cm−1, respectively) in thecubic phase.

Previous investigations into NH4SH ice are far fewer;

those that have been conducted are summarized in Table2. The most recent two studies on NH4SH ice are themost comprehensive, covering both a larger wavelengthand temperature range.

Bragin et al.15 showed the lattice structure of NH4SHice at low temperatures, after annealing, were compa-rable to the unannealed at higher temperatures. The lat-tice of NH4SH is made from NH4

+SH− ions, with NH4+

forming a tetrahedral structure. The NH4+ has four inter-

nal modes observed within the fundamentals of NH4SH:the NH degenerate stretching mode at 3000 cm−1, the NHsymmetric stretching mode at 2925 cm−1, and two HNHbending modes with fundamentals at 1437 and1418 cm−1. The only external vibration of NH4

+ observedin NH4SH is the Eu mode at 220 cm−1, and it is veryweak. A comparison of the wavenumbers these fundamen-tals were observed at by Bragin et al.15 and Ferraro etal.12 is given in Table 3. Ferraro et al. did not observe theexternal vibrations of Eu and A2u in amorphous NH4SH,since they occur at very low wavenumbers, outside thoseinvestigated either by that study or by this one.

The studies conclude that no phase change occurs be-tween 80 and 390 K other than the change from the amor-phous to the crystalline lattice structure at 160 K.12,15 An-

126 J. Opt. Soc. Am. B/Vol. 24, No. 1 /January 2007 Howett et al.

0740-3224/06/010126-11/$15.00 © 2006 Optical Society of America

nealing of the crystalline lattice results in the absorptionfeatures becoming sharper and increasing in magnitude,with very little change in their wavelengths ��0.01 �m�.

Out of the four previous studies, the only one that couldhave produced optical constants was the work of Ferraroet al., since no attempt was made to measure the thick-ness of the films in any of the studies. However, the re-sults from the work of Ferraro et al. were never publishedin a tabulated form, nor was any further work publishedon the optical constants of NH4SH ice by the authors.

Despite the fact that no tabulated results have beenpublished, two data of optical constants for NH4SH icecirculate within the planetary community. Pearl16 assertsthat both data sets are from the experimental work con-ducted by Ferraro et al. The data sets are hereafter re-

ferred to as sets A and B, both are shown in Fig. 1. Theimaginary component is believed to have been obtaineddirectly from transmission experiments, and a Kramers–Kronig analysis was performed to determine the real com-ponent. These data sets were then extrapolated from2.6 to 0.5 �m.17

Upon comparison of these results with graphical repre-sentations given by Ferraro, it is concluded that the broadweaker absorption features present in set A are morecomparable to those expected in amorphous NH4SH,while the narrower features of set B appear to be moresimilar to those of the polycrystalline NH4SH ice. The off-set in the real components value of the complex refractiveindex between sets A and B, as shown in Fig. 1, is prob-ably due to differences in the way the Kramers–Kroniganalysis was performed.

The Kramers–Kronig algorithm requires a single valueof the real component of the complex refractive index at

Table 1. Previous Investigations intoOptical Constants of NH3 Icea

ReferencesTemperature

(K)Wavelength

��m�QuantityMeasured Method

1 77 0.14–0.22 �b Transmission2 77 2–125 n and kc Transmission3 16–92 2.6–25 n and kb Transmission4 193 1.9–10.4 n and kc Transmission5 77 0.19–0.32 n and kc Reflection6 80 1.5–20 n and kc Transmission7 88 1–200 n and kc Transmission8 20 and 80 2.7–20 n and kb Transmission9 77–80 0.14–200 n and kc Transmission10 23 2.5–15.4 n and kc Transmission

an and k represent the real and imaginary components of the complex refractiveindex, respectively. The absorption coefficient is represented by �. Tabulated resultsare presented in Ref. 11.

bResults are only published using graphical methods.cResults are presented in a tabular form.

Table 2. Investigation into the Absorption Spectraof NH4SH Ice

ReferencesWavelength Range

��m�Temperature

(K) Method

13 1–4 77 Reflection14 0.3–1.03 77 Reflection15 2.5–50 83–390 Raman12 1–167 88–180 Transmission

Table 3. Comparison of the Results ofInvestigations into the Assignment of

NH4SH Ice’s Absorption Bands

Fundamental Transition

WavelengthAssignment

of Bragin et al.��m�

WavelengthAssignment

of Ferraro et al.��m�

�4 �A2u� 7.09 7.05�4 �Eu� 6.99 6.96

HS stretch 3.90 3.89�3 �A2u� 3.43 3.42�3 �Eu� 3.33 3.33

Fig. 1. Real and imaginary components of the complex refrac-tive indices of NH4SH ice. Sets A and B, as described in the text,are given by the solid and dashed curves, respectively.

Howett et al. Vol. 24, No. 1 /January 2007 /J. Opt. Soc. Am. B 127

any wavelength within the wavelength range of theimaginary components to be converted. A difference inthis real component’s value used to convert the imaginarycomponent of sets A and B would explain the observed off-set. Since the geometry of the equipment did not allow forreflection measurements to be taken, the real compo-nent’s value could not be determined experimentally bythis study. Therefore, it was not possible to establishwhich of these two data sets is more accurate.

2. INSTRUMENTATIONThe vacuum chamber used to determine the optical con-stants presented in this paper was composed of three sec-tions, each holding eight ports. The stainless-steel cham-ber was purposely built using segments produced byKimbell Physics. Transmission measurements using thegrating spectrometer were made through magnesiumfluoride windows, while the Fourier-transform spectrom-eter (FTS) made its measurements through zinc selenideones. Quartz windows were used for measurements takenwith the helium–neon (He–Ne) laser and the remainingwindows were sapphire. A vacuum of 10−6 Pa could beroutinely obtained at room temperature using both aturbo and a roughing pump. A schematic of each sectionincluding the instrumentation connected to each port isgiven in Fig. 2.

Two spectrometers were used to cover the infrared re-gion: an Oriel Mir-8000 modular infrared FTS with anOriel potassium bromide (KBr) beam splitter covering awavenumber region of 1300–6000 cm−1, and an Acton Re-search Company 505F Czerny–Turner grating spectrom-eter covering a wavenumber region of 4200–12,000 cm−1.

An LN2-cooled mercury–cadmium–telluride (MCT) de-tector with a D* value of 5�1010 cm Hz1/2 W−1 was usedwith a silicon carbide �SiC2� infrared source for the FTS.The output of the source was a collimated beam, neglect-ing the need for further optics. A quartz–tungsten–halogen (QTH) lamp was used as an infrared source forthe grating spectrometer, with a series of calcium fluoride�CaF2� lenses to collimate the beam. An enclosed opticalchopper and a lock-in amplifier were used together to in-crease the signal-to-noise ratio of the grating spectrom-eter. A thermoelectrically cooled lead-sulfide (PbS) detec-tor was used at a typical operating temperature of 253 K,which gave a D* value of 2.5�1011 cm Hz1/2 W−1.

A Janis Research ST-100 continuous flow cryostat wasmounted inside the vacuum chamber, in which thewedged sapphire substrate of 1 mm thickness was held.Sapphire has a high transmittance, at 293 K it’s trans-mittance is �80% from 0.2 to 4 �m decreasing to �60%at 5 �m and �10% at 7 �m at 293 K. These values fortransmittance increase with decreasing temperatures.The wedged shape of the substrate allowed for the filmthickness to be determined on both sides of the substratesimultaneously. No film was ever detected on the back ofthe substrate.

The background signal for both the FTS and the grat-ing spectrometer were measured at each temperature.The background signal for the grating spectrometer wasnegligible. A low background signal, �40% transmittance,was observed between 1400 and 3000 cm−1 by the FTS.

The drift in the background signal was found to be negli-gible by recording several zero-sample spectra. The re-moval of the background signal from the transmissionmeasurements taken is discussed further in the followingsection.

The cryostat was cooled using liquid nitrogen and con-trolled using a Lakeshore 330 autotuning temperaturecontroller mounted directly onto the sample mount. Thetemperature could be held stable to ±0.2 K at 70 K, with amonitoring accuracy of ±80 mK.

3. PROCEDUREWhile the cryostat was cooling, the substrate was heated,thus any impurities solidified on the cool radiation shieldrather than the relatively warmer windows or substrate.Typical pressures inside the vacuum chamber at 70 Kwere 10−7 Pa.

Fig. 2. Schematics taken from various views of the infrared op-tical transmission chamber, the FTS, and the grating spectrom-eter. The vacuum chamber is composed of three stainless-steelsections, each holding eight ports. Each of the three sections areshown separately. The path taken by the radiation from source todetector are shown in gray and are a solid line for the gratingspectrometer, dashed-dotted line for the FTS, and dotted line forthe laser paths. All paths travel up from the bottom, through thesubstrate in the center of the middle section and out of the topsection, except one laser path, which reflects off the substrateback up to the top section. 1, He–Ne laser. 2, Chopper. 3, 5, and21, CaF2 lens. 4, Plane mirror. 6, QTH light source for the grat-ing spectrometer. 7, Iris. 8, SiC2 light source for the FTS. 9 and20, PbS detector for the laser. 10, Ion gauge. 11, Ports. 12,Vacuum chamber. 13, Plane mirror, tilted at 45° to the base of thevacuum chamber. 14, Mass spectrometer. 15, Cryostat (dottedline shows position inside the vacuum chamber). 16, Gas-handling system. 17, Turbo pump. 18, Sapphire substrate. 19,FTS. 22, Grating spectrometer. (a) Bottom section, (b) middle sec-tion, (c) top section.


The thin films were deposited by vapor deposition. Gaswas allowed to enter the chamber through four jets aimedat the substrate. The jets effectively sprayed the vaporonto the cold substrate. The NH3 gas used was of “Mathe-son purity” 99.999% pure, while the SH gas used was “CPgrade” 99.5% pure. No data have been published by themanufacturer outlining possible species of the trace con-taminants, although an independent study found no dis-cernible trace contaminants in the gas.18 In particular, noevidence was found for water vapor contamination, eitherfrom the gases used in the vapor deposition process orfrom atmospheric contamination. Water vapor is a strongabsorber in the infrared, with significant lines at3228 cm−1 �3.1 �m�.19 Such lines are not observed in anyof the transmission spectrum. The pressure and purity ofthe gas were also continually monitored by an ion gaugeand a residual gas analyzer inside the vacuum chamber,while the pressure of the gas entering the jets was mea-sured using two Baraton pressure transducers.

Depositing the NH3 ice film onto the substrate was at-tempted at various temperatures. Temperatures above140 K were initially attempted, so that the phase of theice would be cubic12 and thus most comparable to the NH3ice expected to form clouds in the Jovian atmosphere.20

Nondiffuse, uncracked films were only observed to be pro-duced at temperatures equal to or below 70 K for NH3 iceand 80 K for NH4SH ice. Hence, all depositions weremade at these temperatures.

The thickness of the thin films deposited were mea-sured by shining a He–Ne laser through the thin film andrecording the interference patterns it produced as the filmgrew in thickness. By counting the number of interferencefringes �m� and knowing the wavelength of the laser ��and the real component of the complex refractive index atthis wavelength �n�, the thickness of the film �d� could be

determined using the relationship m�=2nd. An exampleof the typical interference pattern produced is shown inFig. 3. The real component of the complex refractive indexfor NH3 at the wavelength of the He–Ne laser �632.8 nm�was taken to be 1.423 from Martonchik et al.,9 while thevalue for NH4SH ice was taken to be 1.750 from set A ofFerraro et al.,12 for reasons outlined in the following sec-tion. The laser’s wavelength was outside those detectedby either the FTS or the grating spectrometer.

The geometry of the system was such that a dual-anglelaser beam technique, which negates the need for previ-ous knowledge of the real component of the complex re-fractive index at the laser’s wavelength, could not be uti-lized. This technique works by reflecting two laser beamsoff the substrate at different angles. As the gas condensesonto the substrate, two interference patterns are detectedwith different periods. From the ratio of these periods, therefractive index of the film can be determined, from whichthe thickness of the film can be deduced.21

A deposition rate of 15 nm/min was found to give theleast diffuse NH3 ice films. This is significantly slowerthan the 0.3 �m/min deposition rate previously indicatedas satisfactory in the literature.7 In this experimentalsetup, the vapor was deposited using four jets positionedclose to the substrate, in contrast to the single flarednozzle used in previous experiments. For a film of uniformthickness to be laid, each jet had to deposit an equalamount of vapor onto the substrate. It was found that thiscould only be achieved by using a slower deposition rate.

NH4SH ice films were created using a one-to-one ratioof NH3 gas and SH gas. The combined spectra appear con-siderably different from the spectra of both constituentgases and do not contain any of the same fundamentalfrequencies, indicating that a new species was created. Itwas found that an increase in either of the gases led to

Fig. 3. Variation in film thickness across the substrate when backlit by a Na lamp. (a) shows a photograph of a NH4SH ice film takenby a Canon Digital IXUS 500 camera. From the rings of uniform thickness shown in (a) and knowledge of the absolute film thicknesswhere the laser beam passes through the film, it is possible to determine the absolute film thickness of the rings, as shown by the blackcrosses in (b). A surface, such as the one also shown in (b), can then be fitted to these regions to model the change in film thickness acrossthe substrate.


features of that species appearing in the transmissionspectra, rather than other species being created.

Figure 4(a) shows the variation in appearance acrossthe substrate of a typical NH4SH ice film when illumi-nated by a sodium (Na) lamp. The magnitude of thesevariations was considered to be too high to approximatethe film as uniform. Accurate determination of the filmthickness is crucial to obtaining accurate optical con-stants, since any error in the absolute film thickness is di-rectly introduced into the film’s optical constants.

The variations in film thickness across the substratewere modeled to produce a surface map of the film, asshown by Fig. 4(b). The surface map was produced by us-ing the absolute film thickness, as determined by the in-terference pattern of the He–Ne laser at a known locationand the relative change in film thickness across the sub-strate, as determined by the brightness variations acrossthe film in the images.

4. RESULTSEach transmission spectrum taken by the FTS had aspectral resolution of 1 cm−1 and was produced by coad-ding 256 interferograms. Three order-sorting filters wereused in the grating spectrometer over the4200–12,000 cm−1 region investigated. An average slitwidth of 50 �m and a grating with a groove density of300 g/mm gave the spectrometer an average spectralresolution of 0.7 cm−1. Slight alterations were made to theslit width of the spectrometer in later runs to allow forweaker absorption features to be investigated. A greaterslit width increases the throughput of the spectrometer,causing the strong features to saturate out but allowingfor the weaker features to increase in strength to a detect-able level.

Transmission spectra, with respect to wavenumber �,were recorded at each temperature before and after a filmwas deposited [hereafter denoted as I0�� and I��, respec-tively]. An infrared background signal at each tempera-ture was also recorded and removed from the transmis-sion spectra (this correction is henceforth considered tohave been performed to all transmission spectra). Thebaseline of each spectrum was also corrected, since varia-tions in it can arise due to detector drift and thermal re-sponses of the optics. The wavelength of the spectrumtaken by the grating spectrometer was also corrected for,using an offset determined by previously observing Helines. It was found that the FTS had no such offset, andtherefore this correction was not required.

The transmission, �, is defined as the ratio of the spec-tra taken after the film is deposited to the spectra takenbefore �I /I0�. The absorption coefficient, ��, is related tothe transmission by Lambert’s law, defined as I�� /I0��=exp�−��d�. However, because the film was found not tobe of uniform thickness across the substrate, the trans-mission had to be calculated for each position on the sub-strate. At a given location �x ,y� and thickness �z�, the ab-sorption coefficient will be related to the transmissionaccording to

I�� =�x1

x2�y1

y2

I0��e−��z�x,y�dydx, �1�

hence

�� =I��

I0��=�

x1

x2�y1

y2

e−��z�x,y�dydx. �2�

The absorption coefficient was calculated from Eq. (2) byfitting the known ratio of the transmission spectra tovarious values of the absorption coefficient, using the pre-determined film thickness, until a match was found. Oncethe absorption coefficient is known, the imaginary compo-nent of the complex refractive index, k��, can then befound directly9 according to k��=�� /4��.

As previously discussed, the real component of the com-plex refractive index could not be directly determined ex-perimentally in this study as the reflection measurementsrequired to calculate it were unable to be taken due to therig’s current geometry. So, the real component of the com-plex refractive index was determined from the imaginary

Fig. 4. Example of laser interference fringes, corresponding to afilm of 1.22 �m being laid. (a) Photograph of NH4SH ice film. (b)Modeled surface (in micrometers) of an NH4SH ice film.


component using the Kramers–Kronig algorithm.22,23 If itcan be assumed that there is a negligible contribution tothe real component’s value outside the regions of strongabsorption, then a more simple approximation can beused, known as the subtractive Kramers–Kronig algo-rithm.

The subtractive Kramers–Kronig algorithm relates thereal and imaginary components value at frequency ��,n��, and k��, to the real and imaginary components ata reference frequency �m, n��m�, and k��m�, by

n�� = n��m� +2

��

0

��k�� − ��k��

�2 − ��2

−�k�� − �mk��m�

�2 − �m2 �d�. �3�

A real component value of 2.194 at 3774 cm−1 was used,from set A, in the Kramers–Kronig analysis for both theannealed 160 K and the unannealed 80 K results, since

Table 4. Optical Constants of NH3 Ice at 80 K and NH4SH Ice after Deposition at 80 Kand after Annealing at 160 K from 1300 to 3250 cm−1

�

NH4SH160 K

k

NH4SH160 K

n

NH4SH160 K

kError

NH4SH160 K

nError

NH4SH80 K

k

NH4SH80 K

n

NH4SH80 K

kError

NH4SH80 K

nError

NH380 K

n

NH380 K

k

NH380 K

nError

NH380 K

kError

1300 2.91E−03 2.83 5.39E−04 0.53 7.83E−04 2.72 1.05E−04 0.36 8.73E−04 1.48 1.43E−04 0.011350 1.74E−02 2.87 3.40E−03 0.56 4.78E−02 2.78 6.33E−03 0.37 4.21E−04 1.48 9.56E−06 0.011400 1.63E−01 3.26 4.49E−02 0.90 2.54E−01 2.77 3.47E−02 0.38 3.32E−03 1.48 4.09E−04 0.011450 1.96E−02 2.26 4.59E−04 0.05 3.29E−01 2.23 1.02E−01 0.69 2.50E−03 1.48 8.92E−04 0.011500 6.84E−03 2.31 2.87E−04 0.10 2.73E−02 2.32 3.19E−03 0.27 6.82E−03 1.48 1.10E−03 0.011550 1.84E−03 2.35 9.89E−05 0.13 2.77E−03 2.39 3.50E−04 0.30 5.13E−03 1.49 8.44E−04 0.011600 1.69E−03 2.37 1.04E−04 0.15 4.16E−03 2.44 5.75E−04 0.34 3.42E−02 1.47 5.58E−03 0.011650 3.22E−05 2.39 2.17E−06 0.16 7.43E−02 2.45 8.86E−03 0.26 2.96E−02 1.38 1.19E−02 0.011700 1.43E−03 2.41 1.03E−04 0.17 5.29E−03 2.46 7.16E−04 0.33 4.69E−03 1.42 1.09E−03 0.011750 1.17E−02 2.42 9.06E−04 0.19 1.26E−02 2.49 1.72E−03 0.34 1.65E−03 1.43 3.29E−05 0.011800 7.66E−02 2.46 7.06E−03 0.23 4.45E−02 2.49 6.13E−03 0.34 1.99E−04 1.44 1.03E−04 0.011850 5.93E−02 2.37 3.65E−03 0.15 5.46E−02 2.46 7.72E−03 0.35 5.11E−03 1.45 1.99E−05 0.011900 1.23E−02 2.37 7.71E−04 0.15 2.39E−02 2.44 3.43E−03 0.35 1.13E−02 1.44 1.91E−03 0.011950 1.30E−03 2.39 8.86E−05 0.16 2.93E−03 2.46 4.20E−04 0.35 1.55E−03 1.44 2.86E−04 0.012000 9.22E−04 2.40 6.53E−05 0.17 9.44E−04 2.48 1.36E−04 0.36 2.92E−04 1.45 3.55E−05 0.012050 5.66E−03 2.42 4.36E−04 0.19 2.46E−03 2.49 3.56E−04 0.36 8.21E−04 1.45 2.21E−04 0.012100 2.52E−02 2.41 1.91E−03 0.18 1.04E−02 2.49 1.51E−03 0.36 7.35E−05 1.45 3.83E−05 0.012150 3.42E−04 2.42 2.71E−05 0.19 8.13E−04 2.49 1.19E−04 0.37 3.50E−04 1.45 1.77E−05 0.012200 7.05E−04 2.44 5.98E−05 0.21 2.46E−04 2.50 3.61E−05 0.37 1.47E−03 1.45 6.95E−05 0.012250 9.49E−04 2.45 8.20E−05 0.21 9.75E−05 2.51 1.45E−05 0.37 1.65E−03 1.45 1.06E−04 0.012300 2.09E−03 2.45 1.85E−04 0.22 6.84E−04 2.51 1.02E−04 0.37 1.24E−04 1.45 4.89E−05 0.012350 4.05E−04 2.46 3.64E−05 0.22 6.08E−03 2.54 9.45E−04 0.40 2.71E−04 1.45 4.76E−05 0.012400 4.83E−04 2.46 4.45E−05 0.23 3.15E−02 2.56 3.75E−03 0.45 4.01E−04 1.45 1.35E−04 0.012450 1.66E−03 2.47 1.57E−04 0.23 3.65E−02 2.56 4.35E−03 0.43 7.93E−04 1.45 1.00E−04 0.012500 1.79E−03 2.48 1.73E−04 0.24 3.31E−02 2.58 3.95E−03 0.45 2.50E−03 1.46 3.69E−05 0.012550 7.89E−03 2.49 7.90E−04 0.25 4.05E−02 2.62 4.83E−03 0.47 2.09E−04 1.46 2.10E−05 0.012600 1.25E−02 2.50 1.29E−03 0.26 5.51E−02 2.69 6.59E−03 0.59 3.67E−04 1.46 2.83E−05 0.012650 3.23E−02 2.53 3.58E−03 0.28 1.34E−01 2.77 1.62E−02 0.66 1.78E−03 1.46 2.83E−04 0.012700 7.44E−02 2.59 9.53E−03 0.33 2.97E−01 2.82 3.66E−02 0.71 1.51E−03 1.46 2.40E−04 0.012750 1.62E−01 2.66 2.39E−02 0.39 5.00E−01 2.72 6.36E−02 0.60 4.55E−03 1.46 7.22E−04 0.012800 6.97E−01 2.56 8.47E−02 0.31 5.75E−01 2.50 7.44E−02 0.35 4.60E−03 1.46 7.31E−04 0.012850 2.28E−01 2.40 1.63E−02 0.17 4.97E−01 2.41 6.39E−02 0.24 1.99E−03 1.46 3.16E−04 0.012900 6.27E−01 2.53 7.09E−02 0.29 5.38E−01 2.36 6.96E−02 0.19 6.51E−03 1.46 1.03E−03 0.012950 3.93E−01 2.09 1.67E−02 0.09 5.37E−01 2.19 6.98E−02 0.01 6.86E−03 1.47 1.09E−03 0.013000 2.92E−01 2.09 1.20E−02 0.09 3.97E−01 2.03 5.09E−02 0.18 6.54E−03 1.46 1.04E−03 0.013050 1.11E−01 2.10 4.33E−03 0.08 2.33E−01 2.07 2.89E−02 0.14 8.25E−03 1.46 5.34E−04 0.013100 2.08E−01 2.17 2.45E−03 0.03 2.12E−01 2.10 2.62E−02 0.10 1.65E−03 1.47 2.61E−04 0.013150 8.97E−02 2.11 2.91E−03 0.07 1.46E−01 2.08 1.78E−02 0.13 5.23E−03 1.48 8.29E−04 0.013200 6.53E−02 2.15 1.22E−03 0.04 9.44E−02 2.11 1.15E−02 0.10 3.41E−02 1.51 5.42E−03 0.023210 6.42E−02 2.15 1.10E−03 0.04 8.68E−02 2.11 1.05E−02 0.09 3.69E−02 1.50 5.86E−03 0.013220 6.29E−02 2.16 9.66E−04 0.03 7.91E−02 2.12 9.57E−03 0.09 3.08E−02 1.50 4.88E−03 0.013230 6.09E−02 2.16 8.36E−04 0.03 7.29E−02 2.12 8.81E−03 0.08 2.68E−02 1.50 4.26E−03 0.023240 5.36E−02 2.16 8.14E−04 0.03 6.68E−02 2.13 8.06E−03 0.07 2.84E−02 1.51 4.51E−03 0.023250 5.11E−02 2.16 7.31E−04 0.03 6.06E−02 2.13 7.31E−03 0.07 3.42E−02 1.52 5.43E−03 0.02


the magnitude of the imaginary component of set A wasmore comparable to both sets of results than that of set B.The same relative variations in the real components valuefor both results were observed when a value of 1.2083 at3344 cm−1 was used, from set B, but with an average off-set of 0.99.

There are many values for the real component of thecomplex refractive index of NH3 ice in the literature. Themore comprehensive review of the optical constants ofNH3 ice published to date is Martonchik et al.9 Therefore,it is from this work that a real component value wastaken, 1.406 at 6000 cm−1, for use in the Kramers–Kronigalgorithm.

The optical constants for both NH3 ice and NH4SH iceafter it was deposited at 80 K and after annealing at160 K between 1300 and 4000 cm−1 are given in Tables 4and 5. Higher spectral resolution tables, including uncer-tainty, from 1300 to 12,000 cm−1 are available on-line atwww.atm.ox.ac.uk/user/howett/opticalconstants.

A. Ammonia Ice SpectraTwenty separate transmission spectra were taken of NH3ice films. Films between 0.22 and 3.11 �m thick were laidand investigated. In every run, the transmission spectraat 70 and 80 K were recorded, and in eight of the 20 runs,the transmission spectra of NH3 ice was investigated upto temperatures of 130 K.

Once the transmission spectra had been corrected, theoptical constants were determined using the techniquesoutlined in the previous section for each temperature ineach run. The results from each run were combined to ob-tain the results presented in Fig. 5. The one- variation ofthese results is given in Tables 4 and 5.

It was found that heating the NH3 ice from 70 to 80 Kresulted in no change of film thickness or appearance, nopressure increase in the chamber, no increase in theabundance of any species, and no change in the transmis-sion spectrum. The implications of these findings are dis-cussed further in Section 5.

B. Ammonium Hydrosulfide IceEight separate measurements of the transmission spectraof NH4SH ice were made. Ice films of thicknesses between0.29 and 1.15 �m were laid. A phase change of theNH4SH ice film from an amorphous to a polycrystallinelattice structure at 160 K is well documented in theliterature.15 Hence, after deposition at 80 K, all NH4SHice films were annealed, by heating the substrate to160 K, recooling it to 80 K, and reheating it to 160 K. Theoptical constants were deduced from each run at both80 K and for the annealed film at 160 K. They are com-bined to obtain the results shown in Figs. 6 and 7, respec-tively. The one- error of these results is given in Tables 4and 5.

Table 5. Optical Constants of NH3 Ice at 80 K and NH4SH Ice after Deposition at 80 Kand after Annealing at 160 K from 3260 to 4000 cm−1

�

NH4SH160 K

k

NH4SH160 K

n

NH4SH160 K

kError

NH4SH160 K

nError

NH4SH80 K

k

NH4SH80 K

n

NH4SH80 K

kError

NH4SH80 K

nError

NH380 K

n

NH380 K

k

NH380 K

nError

NH380 K

kError

3260 4.91E−02 2.17 5.77E−04 0.03 5.46E−02 2.14 6.58E−03 0.06 4.60E−02 1.53 7.30E−03 0.023270 4.35E−02 2.17 5.08E−04 0.03 4.74E−02 2.15 5.71E−03 0.05 6.30E−02 1.53 9.99E−03 0.023280 3.67E−02 2.17 4.06E−04 0.02 4.19E−02 2.16 5.03E−03 0.04 7.58E−02 1.53 1.20E−02 0.023290 3.89E−02 2.17 3.64E−04 0.02 4.32E−02 2.16 5.19E−03 0.03 8.21E−02 1.52 1.30E−02 0.023300 3.35E−02 2.17 2.77E−04 0.02 4.31E−02 2.17 5.18E−03 0.02 7.63E−02 1.52 1.21E−02 0.023310 2.81E−02 2.18 1.74E−04 0.01 4.51E−02 2.17 5.43E−03 0.02 7.12E−02 1.53 1.13E−02 0.023320 3.00E−02 2.18 1.94E−04 0.01 4.81E−02 2.18 5.79E−03 0.02 7.47E−02 1.55 1.19E−02 0.023330 2.60E−02 2.18 1.47E−04 0.01 4.75E−02 2.17 5.72E−03 0.02 8.44E−02 1.57 1.34E−02 0.033340 2.53E−02 2.18 1.10E−04 0.01 4.71E−02 2.17 5.68E−03 0.02 1.09E−01 1.62 1.73E−02 0.033350 2.27E−02 2.19 8.38E−05 0.01 4.34E−02 2.17 5.23E−03 0.03 1.77E−01 1.68 2.81E−02 0.043360 2.20E−02 2.19 5.23E−05 0.01 3.61E−02 2.17 4.34E−03 0.03 3.63E−01 1.80 5.76E−02 0.063370 1.86E−02 2.19 4.41E−05 0.01 2.92E−02 2.16 3.50E−03 0.04 7.71E−01 1.68 1.23E−01 0.043380 1.51E−02 2.19 3.71E−05 0.01 1.24E−02 2.17 1.48E−03 0.03 4.48E−01 0.71 7.12E−02 0.113390 1.67E−02 2.19 1.89E−05 0.01 7.99E−03 2.18 9.52E−04 0.02 6.98E−02 1.02 1.11E−02 0.063400 1.34E−02 2.18 7.28E−05 0.01 5.50E−03 2.18 6.55E−04 0.01 1.50E−02 1.16 2.38E−03 0.043450 7.13E−03 2.18 3.88E−05 0.01 3.04E−03 2.18 3.29E−04 0.01 7.21E−03 1.33 1.12E−03 0.013500 8.60E−04 2.18 4.82E−06 0.01 5.70E−04 2.18 2.98E−06 0.01 6.21E−03 1.34 9.63E−04 0.013550 2.39E−03 2.19 6.05E−06 0.01 4.14E−04 2.19 1.84E−06 0.01 1.98E−04 1.34 3.07E−05 0.013600 3.92E−03 2.20 7.29E−06 0.01 2.57E−04 2.19 7.10E−07 0.01 4.15E−04 1.36 3.50E−07 0.013650 4.62E−03 2.20 8.02E−06 0.01 3.23E−04 2.19 7.30E−07 0.01 1.45E−04 1.36 5.28E−06 0.013700 5.32E−03 2.19 8.75E−06 0.01 3.89E−04 2.19 7.40E−07 0.01 1.09E−03 1.37 9.10E−07 0.013750 3.88E−03 2.19 3.38E−06 0.01 1.85E−04 2.19 4.20E−07 0.01 9.74E−04 1.37 1.54E−04 0.013800 3.74E−03 2.19 2.60E−07 0.01 1.59E−04 2.19 1.20E−07 0.01 1.77E−03 1.37 2.80E−04 0.013850 2.85E−03 2.20 1.23E−06 0.01 3.03E−04 2.20 2.50E−07 0.01 1.44E−03 1.38 2.28E−04 0.013900 1.95E−03 2.20 2.20E−06 0.01 4.47E−04 2.20 3.90E−07 0.01 2.82E−04 1.38 4.47E−05 0.013950 1.06E−03 2.20 1.19E−06 0.01 3.88E−04 2.20 4.50E−07 0.01 5.08E−04 1.38 8.06E−05 0.014000 1.61E−04 2.20 1.80E−07 0.01 3.29E−04 2.20 5.10E−07 0.01 4.03E−03 1.39 3.77E−04 0.01


As the film was heated above the evaporation tempera-ture of NH3 ice and SH ice, a slight pressure increase wasobserved, and their abundances were increased on themass spectrometer. However, the pressure quickly de-creased, and it was found that the film’s thickness and ap-pearance did not change during this process. Since thespectral signature of these ices remains constant, it is be-lieved they had not evaporated from the film but from theradiation shield.

5. DISCUSSIONFigure 8 compares the imaginary component of the com-plex refractive index determined in this study for NH3 iceand those determined by Sill et al.7 for both cubic andmetastable NH3 ice over the limited wavenumber rangegiven by Sill et al. for the optical constants of metastable

NH3 ice. Only the imaginary components can be com-pared, since the real component’s value of the complex re-fractive index was not published by Sill et al. Figure 8also shows the optical constants determined in this studyare significantly more comparable with those of the cubicphase, as opposed to the metastable phase, over both the1057 and the 3374 cm−1 (9.46 and 2.96 �m) absorptionfeatures. This indicates that the phase of the NH3 ice ini-tially deposited in this study was, in fact, cubic, callinginto question the initial prediction based on the results ofFerraro et al.12 that it should be metastable.

Much debate existed in the literature prior to the workof Ferraro et al. as to the types of phases exhibited byNH3 ice, and under what conditions they are formed. Fer-raro et al. brought the previous studies together, reinter-preting the results of many of them. Hence, the literaturedoes not provide a single consensus of the phases of NH3

Fig. 5. (a) Imaginary and (b) real components of the complex re-fractive index of NH3 ice over wavenumbers of 1300–12,000 cm−1

at 80 K.

Fig. 6. (a) Imaginary and (b) real components of the complex re-fractive index of NH4SH ice, over wavenumbers of1300–12,000 cm−1, at 80 K after deposition.


ice, but rather those presented by Ferraro et al. representthe most reliable review to date. The results given hereand those determined by Ferraro et al. were prepared us-ing comparable methods and temperatures. To unambigu-ously resolve this issue further, results are required overan increased range of temperatures.

For films not to be subject to significant evaporationlosses, a problem at higher temperatures, thicker filmsare required. However, transmission measurements ofthick films saturate at the main absorption features, andso reflection measurements are required instead. Unfor-tunately, the current geometry of the rig does not allowfor the reflection measurements to be made, and so thereal component of the complex refractive index was un-able to be directly determined.

The absorption features in the complex refractive indexfor NH4SH ice when deposited at 80 K and after anneal-

ing at 160 K are focused upon in Figs. 9 and 10. The fea-tures of the 80 K NH4SH film, shown in Fig. 9, are farmore comparable to the shape of set B than set A of thepreviously circulated data sets, despite its increased mag-nitude. Bragin et al.15 showed such features are presentin unannealed or amorphous NH4SH. The presence ofsimilar trends at this temperature in set B data impliesthat this data set is indeed for NH4SH but in its amor-phous phase.

Comparison of Figs. 6 and 7 show that as the ice washeated to 160 K, the absorption features became sharperand more absorbing, particularly at 1418, 2810, 2925,3000, and 3110 cm−1. Ferraro et al. assigned these fea-tures to be due to the �4, 2�4, �3 (vibration), �3 (rota-tional), and �2+�4 fundamentals. The fundamentals ob-served here occur at comparable wavenumbers to thosedetermined by Ferraro et al. Such changes in the absorp-

Fig. 7. (a) Imaginary and (b) real components of the complex re-fractive index of NH4SH ice, over wavenumbers of1300–12,000 cm−1, after the final heating to 160 K.

Fig. 8. Imaginary component of the optical constants of NH3 icedetermined by this study (solid curve) compared to those deter-mined by Ref. 7 for a metastable phase (dotted curve) and cubicphase (dashed curve) of NH3 ice over the 1057 and 3374 cm−1 ab-sorption features. (a) Absorption features of 1000–1150 cm−1, (b)3340–3400 cm−1.


tion features are expected to occur as the film changesfrom an amorphous to a polycrystalline state.12

Further cooling anneals the film, so that the absorptionfeatures reach a maximum intensity at the recooled tem-perature of 80 K and settle at a value comparable with setA when reheated to 160 K. The change in the absorptionfeature’s intensity is characteristic of the phase changeprocess.12 It is expected that further annealing effortswould have a minimal effect on changing the magnitudeof the optical constants, since set A was subject to signifi-cantly higher temperatures throughout the annealingprocess and gave very similar results. The similarity be-tween these results and those of set A, as shown in Fig.10, supports the claim that this data set represents theoptical constants for NH4SH in a polycrystalline state. No

evidence was found for a second crystalline phase ofNH4SH in this work, in agreement with previousresults.12,15

6. CONCLUSIONS AND FUTURE WORKTransmission measurements of ammonium hydrosulfide�NH4SH� ice and ammonia �NH3� ice thin films weremade, using both a grating and a Fourier transform spec-trometer. A number of corrections were applied to thedata, and the imaginary component of the complex refrac-tive index was directly determined. The subtractiveKramers–Kronig relation was used to derive the real com-ponent from the imaginary one. Once both components of

Fig. 9. (a) Imaginary and (b) real components of the complex re-fractive index of NH4SH ice between 1000 and 4000 cm−1, a spec-tral region that includes a number of absorption features. The re-fractive indices of the previously circulated data sets A (dottedcurve), B (dashed curve), and the results from this study unan-nealed at 80 K (solid curve) are compared.

Fig. 10. (a) Imaginary and (b) real components of the complexrefractive index of NH4SH ice between 1000 and 4000 cm−1, aspectral region, which includes a number of absorption features.The refractive indices of the previously circulated data sets A(dotted curve), B (dashed curve), and the results from this studyafter annealing at 160 K (solid curve) are compared.


both ices were determined, the data sets were validatedby comparing these results to optical constant data avail-able in the literature.

The NH3 ice results are in good agreement with previ-ously published results for NH3 ice in a cubic phase for alldeposition temperatures, calling into question previousconclusions made on the formation temperature of phasesof the ice. Further investigation using thicker films andreflection measurements could help to resolve this issue,since the optical constants of NH3 ice could then be deter-mined at higher temperatures, as evaporation losses be-come less significant.

The NH4SH optical constants presented here for allphases of the ice are the first to be published in a tabu-lated form. The results agree well with the absorbancespectra previously published for both the amorphous andthe polycrystalline phases of the ice. A previous study ofthe ice has shown that its optical constants and colorchange with exposure to ultraviolet.14 Further investiga-tion using such radiation would be advantageous for un-derstanding whether clouds of NH4SH have any role toplay in the unexplained colors of the Jovian clouds.

It is hoped that the facility will be modified to includereflection measurements. If this were the case, the experi-ments could be repeated determining both components ofthe complex refractive index simultaneously.

ACKNOWLEDGMENTSThis work was supported by the Particle Physics and As-tronomy Research Council and the Jet Propulsion Labo-ratory.

C. J. A. Howett’s e-mail address [email protected].

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methane, ammonia, and ice in the vacuum ultraviolet,” J.Chem. Phys. 33, 270–274 (1960).

2. F. W. Taylor, “Preliminary data on the optical properties ofsolid ammonia and scattering parameters for ammoniacloud particles,” J. Atmos. Sci. 30, 667–683(1973).

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4. C. W. Robertson and H. D. Downing, “Optical constants ofsolid ammonia in the infrared,” J. Opt. Soc. Am. 65,432–435 (1975).

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constants of water and ammonia ice,” J. Opt. Soc. Am. 65,919–926 (1975).

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8. B. E. Wood and J. A. Roux, “Infrared optical properties ofthin H2O, NH3, and CO2 cyrofilms,” J. Opt. Soc. Am. 72,720–728 (1982).

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13. H. Kieffer and W. D. Smythe, “Frost spectra: comparisonwith Jupiter’s satellites,” Icarus 21, 506–512 (1974).

14. L. A. Lebofsky and M. B. Fegley, “Laboratory reflectionspectra for the determination of chemical composition of icybodies,” Icarus 28, 379–387 (1976).

15. J. Bragin, M. Diem, D. Guthals, and S. Chang, “Thevibrational spectrum and lattice dynamics ofpolycrystalline ammonium hydrosulphide,” J. Chem. Phys.67, 1247–1256 (1977).

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19. O. B. Toon, M. A. Tolbert, B. G. Koehler, A. M.Middlebrook, and J. Jordan, “Infrared optical constants ofH2O ice, amorphous nitric acid solutions, and nitric acidhydrates,” J. Geophys. Res. 99, 25631–25654 (1994).

20. F. Bagenal, T. E. Dowling, W. B. McKinnon, D. Jewitt, C.Murray, J. Bell, R. Lorenz, and F. Nimmo, Jupiter: ThePlanet, Satellites and Magnetosphere (Cambridge U. Press,2004).

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Optical constants of ammonium hydrosulfide ice and ammonia ice