Reflectivity Enhancement of Vacuum Monochromator Optics by the Action of Oxygen Atoms Philip M. Jolinson

Chemistry Department, State University of New York, Stony Brook, New York 11790. Received 12 December 1972. In spite of their inherent optical advantages, multiple

reflecting monochromators have been found wanting for use in the vacuum ultraviolet because of low optical throughput. For use below 110 nm the fault lies in the reflectivity of the metal coatings; and little can be done about this. Above 110 nm the reflectivity of a fresh MgF2-coated aluminum surface is quite good, and multi­ple reflecting monochromator mounts should perform ade­quately. It has been found, however, that under irradia­tion by strong light sources the optical transmission of a vacuum instrument is gradually decreased. The first ef­fects of this are a movement of the low wavelength cutoff of the instrument to longer wavelength and an apparent absorption band around 190 nm. The rate of optical deg­radation is greatly enhanced by poor vacuum conditions in the monochromator.

940 APPLIED OPTICS / Vol. 12, No. 5 / May 1973

Fig. 1. The spectrum of a high pres­sure argon lamp taken through a Czerny-Turner monochromator before (lower trace) and after (upper trace) cleaning of the optics by oxygen

atoms.

The coating, which forms on the mirrors and grating of the instrument, is not removed by any known solvent. It is believed to be a polymer caused by irradiation of resid­ual oils in the vacuum, and as such is probably similar to films formed consciously by irradiation of monomers.1

When working with high powered lamps it has been nec­essary to have the optics recoated occasionally, an opera­tion that is both costly and inconvenient, due in part to the necessity to realign the monochromator. Since this inconvenience. hampered use of both high powered lamps and multiple reflecting monochromators, we sought a method of cleaning mirrors and gratings of the deposited film without removing them from their mounts.

Gillette and Kenyon made an extensive study of con­taminant films produced by proton bombardment.2 They devised a technique for cleaning these films from optical surfaces by bombarding them with oxygen atoms pro­duced in a radio frequency discharge. Since the films produced in a vacuum system by proton bombardment are probably very similar to those produced by high ener­gy photons, an in situ cleaning method using oxygen atoms seemed promising.

The cleaning of monochromator optics by oxygen atoms was tested on two different monochromators. They were both McPherson 218 instruments that employed a crossed 0.3 m Czerny-Turner mount. Each had a grating blazed at 1500 A, and all the optical surfaces were aluminum overcoated with magnesium fluoride. These monochro-mators had been exposed to hundreds of hours of exposure to irradiation from a high pressure argon arc3 that had a magnesium fluoride window. The irradiation had been both in air and in vacuum. The vacuum had been main­tained at about 5 X 10-7 Torr with a turbomolecular pump in one case and a ion-titanium sublimation pump in the other. The contamination levels were low relative to those experienced previously when a diffusion pump system had been used.3

A new lid was made for the monochroπiators that had an 0-ring feed-through equidistant from the two mirrors and the grating. Through this 0-ring fitting a 12-mm glass tube extended vertically down into the monochroma-tor and made a right angle turn so it could be swivelled to point at each optical element in turn. The glass tubing ended about 6 cm from the 5 cm X 5 cm elements. Oxy-

gen atoms were produced by flowing molecular oxygen (commercial grade) through a microwave discharge just outside the vacuum chamber. This discharge was run at about 45 W of power in an Evanson type cavity. The same microwave equipment is used to run microwave dis­charge lamps used as vacuum ultraviolet sources and is thus already at hand in many laboratories. The mono-chromator was continuously pumped with the turbomole-cular pump, and the flow rate of the oxygen adjusted to control the pressure in the system. The pressure in the monochromator was then about 1-2 Torr.

A window was provided above each surface to observe the atom flow onto the optical surface. Although oxygen itself does not have an afterglow, and the stream cannot be seen, proper operating orientation and pressure can be established by running air or nitrogen through the dis­charge. • The afterglow shows the dimensions of the atom flow. It was found that the air afterglow filled the whole monochromator at low flow rate (and thus low pressure), but at higher flow rate a well defined stream was formed that could be directed at will.

The results of the oxygen atom treatment were quite dramatic in both monochromators. The first instrument had not been as extensively used as the second, and its optics were in fairly good condition. Between 150 nm and 200 nm there was very little change in the instrumental transmission. However at 230 nm the throughput in­creased by a factor of 1.5. Below 130 nm the improve­ment was even more pronounced with a factor of 2 im­provement at 126 nm, a factor of 2.7 at 120 nm, 5.3 at 118 nm, etc. until the MgF2 cutoff at 115 nm.

In the accompanying figure we show the spectrum of a high pressure argon lamp3 taken by the second monochro-mator before and after ten minutes of oxygen atom clean­ing on each optical surface. This lamp was fairly new; and various impurity lines are evident, particularly Lyman a and some carbon lines. The detection system consisted of an EMI 6256 photomultiplier preceded by a sodium salicylate film on a quartz window. Scattered visible light was subtracted from the spectra. The im­provement in monochromator transmission is pronounced at both short and long wavelengths, but is little changed around 160 nm. This change agrees with the reflectivity change seen by Gillette and Kenyon but is a more dra­matic effect due in part to the multiple reflections.

Due to the limited opportunity for experimentation with naturally contaminated mirrors it was not possible to establish the effects of repeated treatment. No damage to the optics was apparent to the eye. If it is established that several cleaning cycles can be tolerated before recoat-ing is necessary, it would be convenient to have the neces­sary tubing installed in vacuum monochromators by the manufacturer.

Although our study on the monochromators did not ex­tend that far, the data of Gillette and Kenyon show that the effect of the polymer formation extends out into the visible region. We have experimented with films formed on quartz discs irradiated while the side away from the lamp was in a 10-6 Torr vacuum. After many hours a fairly dark orange-brown spot had appeared where the light emerged from the quartz. Oxygen atom treatment in a bell jar easily removed these spots. Thus the con­taminant film can be formed on any UV irradiated optical component that has a surface in a vacuum, and the ab-sorbance of this film extends through the visible. The oxygen atom method is therefore applicable to cleaning any optical element that cannot be subjected to abrasives, such as coated optics; and improvement of optical perfor­mance will not be restricted to the ultraviolet region. Of

course any aluminum surface would have to be protected by a coating such as MgF2 to avoid oxidation of the alu­minum.

We acknowledge support of this work by the National Science Foundation and the Petroleum Research Fund of the American Chemical Society. Some of the preliminary experiments were performed by Sandra Winegard.

References 1. A. M. Mearns, Thin Solid Films 3, 201 (1969). 2. R. B. Gillette and B. A. Kenyon, Appl. Opt. 10, 545 (1971). 3. P. M. Johnson, J. Opt. Soc. Am. 60, 1669 (1970).

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