FEBRUARY, 1937

The Effect of Irradiation on the Transmission of Lithium Fluoride

EDWIN G. SCHNEIDER

Harvard University, Cambridge, Massachusetts

(Received November 20, 1936)

The transmission of lithium fluoride in the visible and ultraviolet may be considerably

reduced by exposure to very intense ultraviolet radiation, electron bombardment or contact

with a low pressure electrical discharge. Strong absorption bands with maxima at 5200, 3100,

and 2500A and a gradually increasing absorption below 1800A appear after a few minutes

exposure to electron bombardment or contact with the low pressure discharge. Since 75 hours

exposure to the light of a hydrogen discharge tube caused only about a maximum decreaseof 5 percent in transmission, lithium fluoride may be considered as satisfactory as fluorite for

use in ultraviolet optical instruments. Since all of the absorption bands build up at about

the same rate, the visible discoloration, faint yellow to deep red, may be used as an indication of

the change in the Lltraviolet. Lithium fluoride colored by irradiation may be useful as a filterin ultraviolet studies.

A FAINT visible discoloration throughout thebody of a lithium fluoride window was

observed after a prolonged exposure to extremeultraviolet radiation. Since a similar increase inthe absorption in the ultraviolet might put aserious limitation on the use of this substance inmany optical instruments, an investigation ofthe effects of irradiation seemed advisable.

A plate of lithium fluoride placed about two

centimeters from the end of the capillary of a lowpressure hydrogen discharge tube carrying about50 m.a. at 25,000 volts showed a barely per-ceptible darkening after 35 hours exposure andwas about the color of a Wratten K filter at theend of 75 hours. The yellow color was most in-tense on the side of the crystal which had beennearest the light source, the other side beingalmost clear. A test of the transmission of thiscolored specimen showed a decrease in trans-

parency of less than five percent in the regionbelow 1200A and broad absorption bands whichdecreased the transmission by about five percentat 2500, 3100 and 5200A. The two near ultra-violet bands have been observed in specimens oflithium fluoride exposed to x-rays;l the band at5200A did not appear.

A crystal exposed to an electron beam pro-duced in a cold cathode discharge tube by aninduction coil giving approximately 100,000 voltsdeveloped all of these absorption bands. Since

the region of discoloration was confined to a

1 Ottmer, Zeits. f. Physik 46, 798 (1928).

surface layer so thin that a few strokes on thepolishing lap were sufficient to remove all tracesof color, the depth of this layer was probably nogreater than the range of the electrons in thecrystal. The action of the electron beam wasmuch more rapid than that of the ultravioletlight, a noticeable darkening appearing within afew seconds. The specimen became a deep redcolor by transmitted light and a bright blue byreflected light after a few minutes exposure.

An effect similar in all respects to that pro-

duced by electron bombardment occurred whencrystals were placed in a discharge tube operated

with hydrogen or air at pressures somewhatbelow 0.001 mm of mercury. Under these condi-

tions the whole tube became filled with a blue

glow. The coloring of the lithium fluoride by thedischarge was probably due to the presence of a

large number of relatively high speed electrons.In order to eliminate the possibility that the

color was due to material deposited by the dis-charge, a piece of glass was substituted for thelithium fluoride. Exposure for five hours pro-duced no visible change.

Curves A and A' show the transmission of twopieces of lithium fluoride after treatment with theglow discharge. The more transparent specimenwas bombarded for five minutes and became alight orange. The other crystal, exposed for tenminutes, became about the color of ruby glassbut appeared slightly turbid. The red crystal wasso opaque to visible radiation below 5500A that

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VOLUM E 2 7J. . S. A.

TRANSMISSION OF LITHIUM FLUORIDE

no light could be seen through it with a handspectroscope pointed directly at the sun. CurvesB and B' were taken after the crystals had beenallowed to stand on a shelf exposed to air and thelight of the room for two months. Curves C andC' were taken a little over a year later.

A weakening of the absorption bands similar tothat observed in the irradiated crystals whichwere kept for over a year can be produced rapidlyby heating the crystal. Prolonged heating at atemperature just below red heat removed all ofthe discoloration produced by the ultravioletlight and restored the ultraviolet transmission ofthe specimen to its original value. In the case ofthe crystals treated by electron bombardment orglow discharge, the absorption bands could becompletely removed by heating only if the visiblecolor was a light yellow or fainter. The verydeeply colored specimens showed a tenfold in-crease in transmission in the near ultraviolet andconsiderably less change in the visible and farultraviolet when heated to a dull red for half anhour. Heating for longer periods at this temper-ature increased the transmission only slightly.Raising the temperature to the fusing pointfailed to remove all of the visible color.

A series of plates taken during the irradiationof the lithium fluoride by ultraviolet light and byglow discharge show that all of the absorptionbands build up at about the same rate at first,the extension of the extreme ultraviolet cutoffincreasing a little more slowly than the otherbands. Prolonged irradiation by glow dischargeincreases the absorption in the visible to a greaterextent than in the ultraviolet. Presumably ultra-violet irradiation would show the same effect ifcarried to the point where the crystal ap-peared red.

Before continuing the discussion, it might beadvisable to state briefly a few of the observa-tions of Gudden, Hilsch, Pohl and many others2

on the behavior of the alkali halides. Irradiationof an alkali halide crystal with high energy elec-trons, x-rays or ultraviolet light of sufficientlyshort wave-length results in the formation of

2 Since this material is widely scattered throughout theliterature and many of the publications are not readilyavailable, the reader is referred to a recent summary ofthis subject by A. L. Hughes, Rev. Mod. Phys. 8, 294(1936).

absorbing centers known as F centers. Only alimited concentration of F centers can be pro-duced in a crystal. These absorbing centers maybe bleached by the absorption of light or byheating, the bleaching process resulting in threechanges of interest in the present discussion;(a) the reversion of the lattice to its original form,(b) the formation of colloidal particles of thealkali metal, and (c) the formation of U centers.The U centers have an absorption band lying justabove the short wave-length cutoff of the noi malcrystal. Absorption of light by the U centers orheating a crystal containing U centers results inthe formation of F centers. Heating a crystalcontaining colloidal metal may also result in theformation of F centers.

The F centers of lithium fluoride have an ab-sorption maximum at 2500A.' Since the irradia-tion of the crystal was carried out with the fullmolecular spectrum of hydrogen, conditions werefavorable for the repeated formation and bleach-ing by light of the F centers. Bleaching during thetreatment with glow discharge or electron bom-bardment may have occurred through local heat-ing by impact of the electrons. These processesshould have resulted in the formation of colloidallithium and U centers in the crystals.

The turbid appearance and the stability of thevisible absorption band is consistent with thehypothesis that this band is due to colloidal-lithium. Further evidence for this view is fur-nished by the fact that the visible absorbingcenters may be produced in concentrations farbeyond those of the F centers. The failure of thex-ray treatment to produce the visible absorptionband may be attributed to the lack of radiationactive in bleaching the F centers.

The absorption below 1800A produced byirradiation may be partially due to the formationof U centers. The gradual extension of the foot ofthe short wave-length limit rather than thedevelopment of a discrete band suggests that thegreater part of this absorption is due to latticedistortion as the result of the formation of col-loidal particles rather than to U centers. This is areasonable assumption because lithium fluoridecrystals containing small amounts of impuritiesshow a similar extension of the ultraviolet ab-

73

EDWIN G. SCHNEIDER

z;JI I oI r

ZP , a "

0

Lo ~~~ORANGE SPECIMEN

z 150v)

B

A5

1000 2000 3000 4000 5000 6000WAVE-LENGTH- A

FIG. 1. Transmission of lithium fluoride exposed to glowdischarge, A, B and Cfor 5 min., A', B' and C' for 10 min.Curves A taken immediately after treatment, curves Bafter 2 months, curves C a year later.

sorption band. Since the shape of the extension isonly slightly influenced by the nature of the

impurity, the greater part of the effect may beascribed to lattice distortion.

The cause of the band at 3100A is unknown.In conclusion, it might be well to emphasize a

few practical aspects of these experiments.1. Since all of the absorption bands build up at

about the same rate when a crystal of lithiumfluoride is exposed to the general radiation froma discharge, the visible coloring may be usedas an indication of the change in ultraviolettransmission.

2. Lithium fluoride windows should be placedso that they do not come in direct contact withthe discharge.

3. The change in transmission of lithiumfluoride on irradiation with ultraviolet light isnegligible under ordinary conditions of use. Sincefluorite turns purple at about the same rate thatlithium fluoride becomes yellow, the latter maybe used to replace fluorite in optical instruments.

4. Lithium fluoride colored by irradiation maybe useful as a filter transparent in the ultravioletbut opaque in the visible. The low transmissionin the ultraviolet will, however, limit the use ofthe colored salt for this purpose.

Corrections to "Changes in Color Temperature of Tungsten-Filament Lamps atConstant Voltage"(J. 0. S. A. 26, 409 (1936))

DEANE B. JUDD,

National Bureau of Standards, Washington D. C.

IN this paper which appeared in the November (1936) issue, the followingcorrections should be made:

Eq. (7) should read:

R- (L/d02)[p'(1 -f)+p"(f/c)].

Eq. (11) should read:

C3.1= p/p/.99 p'(l-f)].

(7)

(11)

In the text between these two equations wherever the terms I, 0, and p ap-pear, the terms I", ", and p", respectively, should be substituted for them.

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