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COMMUNICATIONS TO THE EDITOR
CONCERNING THE EFFECT OF ILLUMINATION UPON MEASUREMENTS O F GAS ADSORPTION Sir: I n the course of our studies on gas adsorption we have observed that normal illumination of an adsorbent can introduce serious errors in low pressure adsorption measurements. These observations, which should be of immediate interest to workers in the field, are as follows. A sample of germanium powder bearing 0.4 monolayer of physically adsorbed krypton a t 77.8” K. was illuminated by immersing an 0.5-watt incandescent lamp in the flask of liquid nitrogen employed as a thermostat. The adsorbate pressure rose rapidly from 1.1 to 6.0 p ; it remained a t this value until the illumination was stopped, whereupon it decayed exponentially to its original value. When the lamp intensity was varied (by introducing a resistance in series) the pressure rise increased proportionally, but, for a given intensity, it was not very sensitive to changes in the position of the lamp relative to the sample chamber. The effect of illumination was determined for several equilibrium krypton pressures, with the results summarized in Table I.
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mirrored flask-which was accordingly heated to a temperature above its surroundings. The temperature change (AT,) calculated from the pressure rise and the differential heat of krypton adsorption ( A E ) is consistent with the input of radiant energy, the heat capacity of the sample, and the rate of heat dissipation. An approximate fit of the data is given by Equation (l),which was derived on the assumption of a constant rate of absorption of radiant energy, (dE/dt),, opposed by a dissipation of heat proportional to the displacement of the temperature (AT) above equilibrium. (kl
- kz A T ) / k l
=
(1)
where Icl = (l/C) (dE/dt), (C = specific heat of sample) and kz is a first order rate constant dependent on the geometry of the sample chamber and the thermal conductivity of the medium. The values of kl calculated from Equation (1) show fair agreement (Table I) giving (dE/dt)a M 0.003 watt, or 0.6% of the total power emitted by the lamp. The same fractional absorption of radiant flux was calculated from the pressure rise observed when a 100-watt lamp was supported two feet above the flask. I n this case the flux of visible light across the flask, measured with a light meter, was 60 meter-candles. When a fluorescent lamp yielding the same visible flux was substituted for the incandescent source, the pressure rise was TABLEI reduced 10-fold, a result to be expected inasmuch as 1.13 g. germanium, 2600 cm.2; thermostat temperature, the emission of a fluorescent lamp is almost ex77.8’K.; illumination by 0.5-watt incandescent lamp. clusively confined to the visible. Equil. Pressure The above interpretation is further supported by krypton illuminasimilar observations with different adsorbates on pressure, tion, Ai?,. ATm, kn kl. (dE/dt),p microns microns aal./mole OK. min:-1 O/min. watts the same adsorbent (krypton and oxygen on oxi0.00 0 .oo . . . .. . . . . .,. . . . dized germanium), and with the same adsorbate 1.14 6.00 3200 6 . 6 0.09 0.60 0.0033 on different adsorbents (krypton on bare germa11.70 26.0 2700 3 . 6 .18 .65 .0035 nium, on oxidized germanium, on graphite, and on 70.0 100.0 2500 1 . 8 .23 .42 .0023 TiOz). I n the latter case (dE/dt). was found to 1063. 1218. 2380 0 . 7 .55 .38 .0021 vary from 0.2% (on TiOz) t o 1.6% (on graphite) A. J. Rosenberg and C. S.Martel, Jr., to be published. of the radiant flux, differences which are attributable to the different reflectances. The rise in pressure was contrary to the expectaThus, even while a glass sample chamber may be tion that illumination, which increases the charge immersed in liquid nitrogen, rather limited illumidensity of a semiconductor surface, would enhance nation can raise the sample temperature to a prothe polarizability, hence, the adsorption potential. hibitive degree for accurate adsorption studies. Furthermore, it is highly improbable that the For instance, a 50-watt incandescent lamp supobserved effects are attributable to specific desorp- ported two feet above a Dewar flask of three-inch tion of krypton atoms. Although a chemisorbed diameter transmits enough radiant energy to the atom may be dissociated from a surface by the sample chamber to raise the temperature of 0.17 g. absorption of a sufficiently energetic quantum, a of graphite, under a pressure of three microns of corresponding effect is unlikely in physical adsorp- krypton, from 77.8 to 79.0”K. tion where the binding energy is not concentrated These results point out the need for caution in in single bonds,” and the adsorbate-adsorbate excluding light from the sample chamber during interaction energy is comparable to that between measurements of gas adsorption, particularly a t low adsorbate and surface. Indeed, on experimental pressures. grounds alone, the magnitude of the pressure rise is Acknowledgment.-The research reported in difficult to understand unless the entire sample, this document was supported jointly by the Army, and not merely the peripheral portion directly Navy, and Air Force under contract with the illuminated, was involved. Massachusetts Institute of Technology. For these reasons it seemed likely that the LINCOLN LABORATORY ARTHUR J. ROSENBERG observed effects were thermal, i.e., a fraction of the MASSACHUSETTS OF TECHNOLOGY INSTITUTE radiant energy emitted by the lamp was absorbed LEXINQTON, MASSACHUSETTS CHARLES S. MARTEL, JR. by the sample-the only “grey” object in the RECEIVED JANUARY 24, 1957
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