Photoluminescence and Optical Transmission ofDiffusion-Pump OilsRoger L. Kroes

The photoluminescence and optical transmission of the four widely used diffusion-pump oils, DC-705,DC-704, Convalex-10, and Convoil-20, were measured. Each of the oils was found to be transparentthroughout the visible region, showed some absorption in the near uv region, and became very opaquebelow approximately 300 nm. Both Convalex-10 and Convoil-20 turned yellow after exposure to uv light.No such change was noted in DC-705 or DC-704. Photoluminescence was produced in each of the fouroils when irradiated with uv light. Both DC-705 and DC-704 had a nearly identical luminescence spectrathat peaked at 350 nm. The spectrapeaks in the visible region.

1. Introduction

Much interest has been shown recently in the opti-cal properties of diffusion-pump oils. This interesthas arisen mainly because of problems associatedwith the backstreaming of pump oil vapor, resultingin a film of oil condensing on the interior compo-nents of the system being evacuated. Various tech-niques, such as the use of cryogenically cooled baf-fles, dramatically reduce this backstreaming but donot completely eliminate it. A particularly seriousproblem arises in vacuum chambers that contain op-tical components such as mirrors, gratings, and lens-es. A thin contaminant coating of oil can greatly re-duce the reflectivity or transmittance of such compo-nents and can also cause interference effects.' Inthe case of gratings, a reduction in efficiency, in-creased scatter, and a shift in the Wood's anomaliescan result.2 '3 These effects are particularly pro-nounced at the short uv wavelengths, where theseoils become very opaque.

A specific example of this type of problem arises inthe prelaunch environmental testing of space vehi-cles containing exposed optical surfaces. These ve-hicles are tested in a large vacuum chamber that uti-lizes oil diffusion pumps. Backstreaming of oil fromthese pumps can condense on exposed optical surfac-es and seriously compromise their usefulness.

A knowledge of the optical properties of these oilscan be utilized in several ways. If the thickness andabsorption coefficient of an oil film on an optical ele-ment are known, the change in reflectance or trans-

The author is with the Space Sciences Laboratory, NASA Mar-shall Space Flight Center, Alabama 35812.

Received 16 August 1972.

of Convalex-10 and Convoil-20 were fairly complex, with several

mittance produced by the film can be calculated.Another useful approach is to construct an opticalmonitor using the known change in transmittance orreflectance of a surface produced by the buildup of acondensed oil film to measure the degree of contami-nation taking place in any given system.

II. Experiment

A. Diffusion-Pump Oils StudiedIn the present investigation four widely used diffu-

sion-pump oils-DC-704, DC-705, Convalex-10, andConvoil-20-were studied. Their transmittance,photoluminescence, and the effect of prolonged uv ir-radiation on their optical properties were measured.DC-704 and DC-705 are silicone oils produced by theDow Corning Corporation. DC-704 is tetramethyl-tetraphenyltrisiloxane and has the following struc-ture:

CH3 CH3 CH3

CH3 [

DC-705 is trimethylpentaphenyltrisiloxane and hasthe following structure:

CH3 CH3 CH3

O-Si- 0 Si- -i -

Convalex-10 and Convoil-20 are produced by theConsolidated Vacuum Company. Convoil-20 is a

September 1973 / Vol. 12, No. 9 / APPLIED OPTICS 2075

hydrocarbon oil, whereas Convalex-10 is a polyphen-ylester and has the general structure formula:

( )0

B. Optical AbsorptionThe optical absorption spectrum of each of the

pump oils was measured using a Heath model EU-721 ratio recording spectrophotometer. This is asingle-beam spectrophotometer in which the sampleis alternately placed in the light beam, then re-moved, and the ratio of the light intensities record-ed. The spectral region over which these measure-ments were made was from 600 nm to approximately240 nm, the short wavelength end being limited bythe ability to measure light through very opaquesamples. The absorption coefficient (cm-1) =(lnIo/I)/[x(cm)], where Io is the incident light inten-sity, I is the transmitted light intensity, and x is thesample thickness in centimeters, was calculated foreach oil using the recorded values of the ratio bo/Iover the measured wavelength range.

Correction was made for light loss caused by backreflections from the sample cell. When oil diluted inethyl alcohol was used in the standard 1-cm samplecell, a similar cell containing only ethyl alcohol wasplaced in the light beam during the Io reference mea-surements. This corrected Io for reflection lossesand also cancelled the absorption caused by the al-cohol. The thin sample cell consisted of two flatquartz plates separated by a metal disk spacer. Thethickness of the oil sample was determined by thethickness of the spacer used. When making mea-surements with the thin cell, only undiluted oil wasused. No second cell was placed in the referencebeam. Io was corrected electronically by setting theoutput signal at 100% for the visible part of the spec-trum where these thin samples were completelytransparent.

Figure 1 shows the absorption spectrum of Con-voil-20. This oil was not as opaque as the others;therefore, it was possible to measure directly itsspectrum using an undiluted sample in the 0.01-cmthick sample cell. Because all the oils were verytransparent ( 1) in the visible spectral region,none of the absorption curves are extended beyond360 nm.

Difficulties arose with DC-704, DC-705, and Con-valex-10 because of their extreme opacity in the uv.The thinnest sample cell was 0.003 cm thick.When this thin cell was used, the transmitted lightintensity was reduced by more than 4 orders of mag-nitude from the incident beam for most measure-ments below 280 nm. With the monochromator setat some fixed wavelength, there will always be somescattered light from the monochromator of otherthan the fixed wavelength incident on the sample.This scattered light may be as small as a fraction of1% of the total light incident on the sample. How-

ever, if it contains longer wavelengths at which theoil is relatively transparent and one makes measure-ments below 280 nm, a situation will arise wheremuch of the measured transmitted light is fromthese scattered wavelengths. Thus, for these veryopaque samples, large measurement errors can arise.

To alleviate these problems, a second series ofmeasurements was made utilizing a somewhat differ-ent approach. The sample oils were highly dilutedin ethyl alcohol. These dilute samples were placedin the spectrophotometer and their absorptancemeasured against a reference sample containing pureethyl alcohol. The equivalent thicknesses of the di-luted oils were calculated from the known dilutionsand the absorption coefficient calculated. Severaldifferent dilutions were used with each oil, and sev-eral measurements were made with different prepa-rations of the same dilution to check reproducibilityand accuracy of dilution. By using a dilution 1-1500of oil to ethyl alcohol for DC-704 and DC-705 in a1-cm cell, absorption measurements were easilymade down to 230 nm with good reproducibility.Figure 1 shows the absorption curves thus obtainedfor DC-704 and DC-705.

As a check on the validity of this dilution method,very thin films of undiluted DC-704 and DC-705were made by placing the oil between two flat quartzplates and pressing them together tightly. We werenot able to measure directly the film thicknesses, butthey were thin enough so that absorption measure-ments could be made down of 235 nm. These ab-sorption curves had exactly the same shape as thosein Fig. 1 for the dilute samples, and they could beexactly fitted to those curves by the proper choice ofa film thickness. The film thicknesses thus chosenwere in the 0.0003-0.0005-cm range, depending onthe sample chosen.

The most difficult absorption spectrum to measurewas that of Convalex-10. Its opacity was muchgreater than the other oils. It was not possible to

7000 v

2000 / VJDD-7 7

.2 1000 X b0 I ,CA

0240 260 28 300 320 340 360

WAVELENGTH (NANOMETERSFig. 1. Absorption coefficients of Convoil-20, DC-704, and DC-

705 in the 234-360-nm wavelength region.

2076 APPLIED OPTICS / Vol. 12, No. 9 / September 1973

measurement are presented in arbitrary units so thata direct quantitative comparison cannot be made

40 000 with present work. However, a qualitative compari-E son shows the general features to be the same.

Supple and Gloria7 have reported the absorption30, 000 | / 0spectrum of DC-705 in the range from 240 nm to 320

nm. Their curve shows DC-705 to have relativelylittle absorptance down to 280 nm, large absorptance

CONVALEX -10 between 280 nm and 255 nm, and zero absorptanceC-,Z 20,000 below 250 nm. They used a very thin sample of

unspecified thickness.

co lC. Photoluminescence< 10,000 Various wavelengths of monochromatic uv light in

the range from 250 nm to 350 nm were used for exci-tation. The luminescent light was analyzed and de-

0 260 280 300 320 340 360 tected by a monochromator and photomultiplier240 2WAVELENGTH (NANOMETERS) tube positioned at a right angle to the direction of

Fig. 2. Absorption coefficient of Convalex-10 in the 249-360-nm the incident exciting light. Figure 3 shows the pho-Fig.length region. toluminescence spectra of DC-704 and DC-705.wavelength region. They have almost identical spectra-a single broad-

band that peaks at 350 nm and trails off into the

10

9 10

ZA 8 ~~~~~~~~~~~~~~~~~~~~~~~~~~98 3 / \..CONVOIL-20

O~~~ ~ ~ ,-// i\ \

ig. 3. LmnsnesDC-7C pu3 6 0 480 C5NVA5E02 - 3.. /

320 360 400 440 40 520 560 600 -'

WAVELE (NANOMETERS)

Fig. 3. Luminescence spectra of DC-704 and DC-705 produced WAVELENGTH0 44NANOMETERS0 60by excitation With1 300-nm light.WAENGHNNOTRS

Fig. 4. Luminescence spectra of Convalex-10 excited by 375-nmlight and Convoil-20 excited by 325-nm light.

get a measurable amount of light through our thin-nest sample (0.001 cm) below 290 nm. Thus, theabsorption spectrum in Fig. 2 was obtained by dilut-ing one part of Convalex-10 in 9000 parts (by vol-ume) of ethyl alcohol and making the measurement CONVOIL-20 f\ / \ CONVALEX-10as previously described for DC-704 and DC-705. / \ / \

A number of other researchers have previously re- < / I ported measurements on some of these oils. Gillis4 |has measured the absorption coefficient of DC-704 /and DC-705 in the range from 300 nm to 400 nm and 0 2also in the near ir. His values and those presentedin this work are in good agreement.

Using a somewhat different approach, a group atthe Oak Ridge National Laboratory has determined /the optical constants of DC-704 and DC-705 in the 320 360 400 440 480 520 560 60VUV region.5 ' 6 These researchers have also re- WAVELENGTH (NANOMETERS)

ported transmission measurements in the 620-238- Fig. 5. Absorption bands in Convalex-10 and Convoil-20 pro-nm range for DC-704. Their data for this latter duced by prolonged exposure to uv light.

September 1973 / Vol. 12, No. 9 / APPLIED OPTICS 2077

visible, giving them a light blue appearance. Figure4 shows the photoluminescence spectra from Conva-lex-10 and Convoil-20. These oils luminesce in thevisible region; however, their spectra are not identi-cal. Convalex-10 has two pronounced peaks at 426nm and 460 nm. Convoil-20 has three peaks at 430nm, 438 nm, and 470 nm.

D. Ultraviolet Irradiation-Induced Absorption Bands

An interesting phenomenon observed during thisstudy was the appearance in two of the oils of ab-sorption bands produced by prolonged exposure touv radiation. Convalex-10 and Convoil-20, whichare normally clear, colorless oils, turned yellow afterseveral hours of exposure to intense uv radiationfrom a deuterium lamp. It was noted that for thesame amount of irradiation, a much denser absorp-tion was produced in Convalex-10 than in Convoil-20. The spectra of these absorption bands are inFig. 5. In this figure both oils show the same maxi-mum opacity. This is the result of the Convalex-10sample having received less irradiation than theConvoil-20 sample. Both oils absorb in the blue re-gion, giving rise to the yellow appearance; however,they are not identical. The absorption band in Con-valex-10 peaks at 440 nm, whereas the bands in theConvoil-20 peak at 376 nm and 407 nm. The way in

which the yellowing occurred shows that these ab-sorption bands were produced by the uv portion ofthe spectrum and not the visible portion. It wasmost pronounced at the surface and trailed off intothe bulk in the same manner as the uv light was ab-sorbed. This was also checked by irradiating firstwith visible and then with uv light. Only the uvlight produced absorption bands.

Both DC-704 and DC-705 were found to be stablewith respect to uv irradiation; they formed no ab-sorption bands when irradiated for several hourswith a deuterium lamp under exactly the same con-dition that produced the absorption bands in Conva-lex-10 and Convoil-20.References

1. R. C. Linton, Measurement and Application of ContaminantOptical Constants, AIAA Paper 71-460, American Institute ofAeronautics and Astronautics 6th Thermophysics Conference,Tullahoma, Tennessee (April 1971).

2. J. J. Cowan and E. T. Arakawa, Z. Phys. 235, 97 (1970).3. J. J. Cowan and E. T. Arakawa, Phys. Stat. Sol. (a) , 695

(1970).4. H. L. Gillis, J. Vac. Sci. Technol. 8, 610 (1971).5. B. L. Sowers, M. W. Williams, R. N. Hamm, and E. T. Araka-

wa, J. Appl. Phys. 42, 4252 (1971).6. G. D. Kerr, M. W. Williams, R. D. Birkhoff, and L. R. Paint-

er, J. Appl. Phys. 42, 4258 (1971).7. R. W. Supple and H. R. Gloria, J. Vac. Sci. Technol. 4, 276

(1967).

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2078 APPLIED OPTICS / Vol. 12, No. 9 / September 1973

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