The experimental procedures and precautions required to measure liquid-alloy reflectivities with a cw far infrared (FIR) laser are described.  The output of a carefully stabilized optically pumped FIR laser was channeled to a melted sample contained in a silica holder under a He atmosphere. By maintaining specular reflection and alloy homogeneity, reflectivities reproducible to ± 7% were routinely obtained

Clayman, B.P.

Johnston, S.F.

English

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       Johnston, S.F.and Clayman, B.P. (1980) Reflectivity measurements on hot reactive liquids using an FIR laser. Applied Optics, 19 (18). pp. 3118-3120. ISSN 1559-128X  http://eprints.gla.ac.uk/56822  Deposited on: 17 November 2011   Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Reflectivity measurements on hot reactive liquidsusing a FIR laserS. F. Johnston and B. P. ClaymanThe experimental procedures and precautions required to measure liquid-alloy reflectivities with a cw farinfrared (FIR) laser are described. The output of a carefully stabilized optically pumped FIR laser waschanneled to a melted sample contained in a silica holder under a He atmosphere. By maintaining specularreflection and alloy homogeneity, reflectivities reproducible to 47% were routinely obtained.1. IntroductionFar infrared (FIR) lasers are well suited to measuringtemperature-dependent optical constants. They pro-vide relatively intense monochromatic radiation at anyof several hundred frequencies. The conventional FIRtool, Fourier transform spectroscopy, which is moreappropriate for broadband constant-temperaturemeasurements, is hampered by low power levels.Previous FIR laser work' on liquids has been confinedto conditions near STP. This paper describes the ap-plication of a FIR laser to measurements of the reflec-tivity of volatile and reactive liquid metal alloys at ele-vated temperatures. The composition dependence ofthe reflectivity of Ga-Te liquid alloys is presented.II. Experimental ArrangementFigure 1 is a block diagram of the complete experi-ment. The laser system is quite similar to that of Beanand Perkowitz.2 The CO2 laser is a 25-W CoherentRadiation model 42 with an integral diffraction gratingwhose output mirror is vibrated at frequency 2 by apiezoelectric transducer (PZT). The grating is used toselect the correct line from the CO2 manifold, and thePZT modulation is used for cavity length stabiliza-tion.The CO2 laser output optically pumps methanolvapor (or other gases) flowing through a gold surfacecylindrical waveguide (1-m X 2.5-cm diam), whichcomprises the FIR laser cavity. The input window isvibrated at frequency v3, and the laser output is sent ontwo alternate paths by a mechanical chopper at fre-quency v1.The authors are with Simon Fraser University, Physics Depart-ment, Burnaby, B.C., V5A 1S6.Received 12 May 1980.0003-6935/80/183118-03$00.50/0.© 1980 Optical Society of America.One path leads via brass light pipes through aFabry-Perot interferometer (used to check outputwavelength) to a pyroelectric detector (REF. DET.).Its output is demodulated at frequencies 2 and V3 toadjust the laser cavities for maximum output eithermanually or under servo-loop control and at vl to pro-vide a reference signal for the reflectivity measure-ments.The second path leads via brass light pipes to thesample furnace (described below) and then to anotherpyroelectric detector (SAMP. DET.) whose output isdemodulated at frequency v. 3 This signal and thereference signal are fed to a ratiometer (Ithaco model3512) whose output is recorded on chart recorders; theratiometer corrects for fluctuations in FIR laseroutput.The optical furnace is an evacuable stainless steelcylinder containing a resistance-heated sample holder,thermocouples, and stirring rod. The walls and the lid,which contains light pipe and vacuum connections, arewater-cooled. Details of the sample area are shown inFig. 2. Silica was used to construct crucibles and stirrerto avoid contamination of the typically reactive liquidalloys. After passing through a 1.5-mm thick poly-ethylene window, the FIR radiation is channeled byreplaceable stainless steel light pipes at 160 incidenceto the sample surface. To ensure specular reflectionfrom the sample surface, the height of which could varywith thermal expansion, evaporation, or meniscuschanges, the height of the crucible assembly can beadjusted by an external micrometer. A window was notused to flatten the liquid surface and protect it fromevaporation or reaction, because its absorption wouldrestrict measurements to low frequencies, and the facesof the window could introduce spurious reflections orinterference effects. The close proximity of light pipesto the relatively large-area liquid surface prevents thecurved edges of the sample from reflecting radiation tothe sample detector. Meniscus effects are limited to3118 APPLIED OPTICS / Vol. 19, No. 18 / 15 September 1980Fig. 1. Block diagram of the experiment.Fig. 2. Furnace sample-area detail: (1) stainless steel light pipe;(2) silica stirring rod; (3) silica crucible; (4) boron-nitride cylinder,supporting resistive heater wire; (5) adjustable-height sample support;(6) thermocouple; (7) sample ingot.within 2 mm of the walls of the 25-mm diam crucible,and the beam diameter that could be collected by theexit light pipe was <15 mm.Three methanol-vapor frequencies, 8.22, 17.5, and84.1 cm-1, spanning the FIR, were used in the experi-ments. The experimental procedure consisted first ofmeasuring the compensated signal when a gold-coatedglass disk was placed in the sample crucible at roomtemperature4 and subsequently replacing it with analloy sample, evacuating the furnace, and introducinga dry He atmosphere to reduce liquid evaporation. Thesample was then heated through its melting point, andits height was optimized for maximum signal. To en-sure alloy homogeneity, the stirring rod was lowered intothe melt and rotated. Stirring, height optimization, andthe reference measurement were repeated at least onceper run. The true reflectivity was taken to be the ratioof the mean sample and reference measurements.Ill. Results and DiscussionInitial test results using a solder metal sample, shownin Fig. 3, illustrate the problems that were encountered.In this case, the optimum sample height had been de-termined after initial melting. The apparently differ-ent melting and freezing temperatures are in fact dueto hysteresis, resulting from too-rapid cooling. By re-stricting temperature changes to < 120 C/min, liquidustemperatures in accord with published figures5 wereobtained. The differing reflectivities of samples 1 and15 September 1980 / Vol. 19, No. 18 / APPLIED OPTICS 3119100180*- 60' 40a!201100 200 300Temperature (C ) 400I I I I I I I I I1008C_ 604020Ga0.2 0.4x _0.6 0.8 TFig. 3. Solder-metal reflectivity at 84.2 cm- 1. Discontinuities Fig. 4. Liquid GalxTe. alloy reflectivity at 84.2 cm-1 and T = 1200represent sample freezing/melting. K (9270 C). Series I and II represent two sets of alloy samples.2 are attributed to slight oxidation of the surface ofsolder sample 1. Subsequent repair of a furnace leakpermitted evacuation to <10-4 Torr and backfilling toa 2-atm pressure of dry He; this improved reproduc-ibility. The solid-sample reflectivities still agreed lesswell, however, because of nonideal surface shapes(warping by thermal expansion upon freezing) and, forthe alloys described below, formation of crystallites,which scattered radiation. For this reason, solid-sample reflectivities were not recorded in these exper-iments.A final problem was interference effects at 8.22 cm-1and occasionally at 17.5 cm-': by varying sampleheight, signal modulations of up to 20% were producedwith maxima at X/2 multiples in the micrometer posi-tion. This is ascribed to backreflection of radiationfrom the sample surface to the furnace entrance windowand subsequent interference with an incoming ray.Backreflection was only possible for nonparaxial rays;long wavelength radiation was highly diverged by dif-fraction at the 2-mm diam hole of the laser exist mirror.Fixes, not yet carried out, could include replacement ofthe exit-hole configuration by a partially reflecting endmirror (at the expense of coupling-out higher-ordermodes) or enlargement of the exit hole (to the detrimentof FIR output power).Typical results after correction of the other problemsare shown in Fig. 4. Liquid Ga-Te alloys at 927 0C(1200 K) were individually and repeatedly measured.Reproducibility is -7%; about half of this randomerror is caused by shifts of the apparatus (and hencechanges in light pipe channeling) upon interchange ofthe sample and reference crucibles in the furnace.Comparison to broadband reflectivity spectra for twoliquid alloys, obtained by conventional Fourier trans-form spectroscopy, indicates that no systematic erroris present. The Ga-Te data will be described in greaterdetail elsewhere.6References1. 0. A. Simpson, B. L. Bean, and S. Perkowitz, J. Opt. Soc. Am. 69,1723 (1979).2. B. L. Bean and S. Perkowitz, J. Opt. Soc. Am. 67, 911 (1977).3. Both detectors are Molectron model P4-43.4. Initial tests indicated that nonlinearity or offsetting of the detectorsignal was not introduced by the blackbody radiation of the fur-nace.5. R. C. Weast, Ed., CRC Handbook of Chemistry and Physics (CRCPress, Cleveland, Ohio, 1973).6. S. F. Johnston, E. D. Crozier, and B. P. Clayman (to be pub-lished).We would like to acknowledge the helpful advice ofB. Simson and the financial support of the NaturalSciences and Engineering Research Council ofCanada.3120 APPLIED OPTICS / Vol. 19, No. 18 / 15 September 1980I I I I SnPbII-n heatingo cooling* sample 2 o initial series IA repeated series I0 series IIl I I I I I I v: = - ! - - - 11,... -L - l I-* I

Clayman (to be published). We would like to acknowledge the helpful advice of B. Simson and the financial support of the Natural Sciences and Engineering Research Council of Canada.

Reflectivity measurements on hot reactive liquids using an FIR laser

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