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MEASUREMENT OF ULTRA-LOW PRESSURES WITH AN ADSORPTION MANOMETER
As is known, ordinary ionization manometers are unsuitable for measuring pressures of the order of \(10^{-8}\) mm Hg and below. For this purpose, special-design ionization manometers may be used.
The authors of the paper under review\(^2\) have proposed, for measuring ultra-low pressures, an instrument which they call an adsorption manometer, whose operation is based on a completely different principle.
The number of molecules striking a certain surface per unit time is proportional to the pressure
\[ \frac{\Delta \nu}{\Delta t}=\text{const}\,\frac{p}{\sqrt{MT}}, \]
where \(\frac{\Delta \nu}{\Delta t}\) is the number of molecules striking the surface per unit time, \(p\) is the gas pressure, \(M\) is the molecular weight of the gas, and \(T\) is the absolute temperature.
These molecules are partially sorbed by the surface, and the rate of adsorption is also proportional to the magnitude of the pressure. To detect adsorption, one may make use of the fact that the slightest contamination of a surface, say of tungsten, noticeably changes the work function of electrons from this surface. If it is assumed that the rate of adsorption of residual-gas molecules is constant and that the effect of each adsorbed gas molecule on the increase in the work function is the same, then \(\frac{d\varphi}{dt}\) is proportional to \(\frac{\Delta \nu}{\Delta t}\), and one may write that
\[ \left(\frac{d\varphi}{dt}\right)_{p=\text{const}}=C, \]
where \(\varphi\) is the work function of an electron from the surface of tungsten.
Thus, one may expect that the work function will depend linearly on time and that the tangent of the angle of inclination of the straight line will be proportional to the pressure.
Fig. 1.
Fig. 2.
Figure 1 schematically shows the design of the adsorption manometer used by the authors in their work. A strip of tungsten filament \(K\) is placed opposite a quartz window \(Q\), sealed into the glass with the aid of graded seals. Through this window the tungsten strip is illuminated by the light of a mercury lamp, passed through a double monochro-
The work function of electrons from the surface of tungsten is measured photoelectrically, by Fowler’s method, with the aid of an auxiliary cylindrical electrode \(A\). The constancy of the light intensity during the measurement is monitored with the aid of a carefully and repeatedly calibrated thermopile.
Figure 2 shows the experimentally obtained dependence of the work function on time at a pressure of \(10^{-8}\) mm Hg (a) and lower (b). The experimental points lie well on straight lines up to the upper limit \(\varphi = 4.59\) eV. The extrapolated value of the work function for a clean tungsten surface \((t = 0)\), \(\varphi = 4.50\) eV, agrees with the literature data. For values \(\varphi > 4.59\) eV the linear dependence \(\varphi = f(t)\) is violated.
By heating the cathode \(K\), the tungsten strip was cleaned, and the measurements were repeated, with quite satisfactory reproducibility of the results.
Although the described method for measuring ultra-low pressures does not make it possible to measure the absolute values of the pressure with high accuracy, it may prove useful for detecting pressure changes in the pressure range below \(10^{-7}\) mm Hg, when ordinary methods can no longer be applied.
L. Kh.
CITED LITERATURE
- UFZh, XLVI, issue 3, 425 (1952).
- M. Seddig und G. Haase, Zeits. angew. Phys., 4, No. 3, 105 (1952).