Gaseous and surface reactions involving helium metastable atoms and resonance photons By R. F. Stebbings Department of Physics, University College London ( Communicated by H. S. W. Massey, —Received 5 March 1957) The absolute electron yield (yM) for He (23S) metastable atoms incident on a gold surface has been measured. The method requires passage of a metastable atom flux through a collision chamber containing argon and thence to the gold surface. From observations on the current of argon ions arising from collisions of the type He (23£) + A-> He + A+ + e, together with measurements of the electron emission from the gold surface, yM may be determined.
Helium metastable atoms and resonance photons 271 tion of atoms in a given metastable state by measuring the selective absorption, in the discharge tube, of a line characteristic of the transition between the metastable state and some higher state.
R. F. Stebbings 272 The detector current is now reduced to /2 where h = 7m e. (2) Then ^ — /2 — 7m(^Yi — -^2) where — iV2) is the number of metastable atoms de-excited per second in collisions of the type shown. Now, since each argon ion is apparent as the consequence of the de-excitation of a metastable atom, the argon ion current (/3) in the collision chamber is an absolute measure of the reduction in the metastable atom flux incident on the detector.
Helium metastable atoms and resonance photons 273 From these measurements the secondary emission from A and B may be found. It is not sufficient, however, to determine the corrected /2 by the addition of this current to the detector current since this would imply the equality of the electron yields from the two systems. It is first necessary, by means of a subsidiary measure ment, to calibrate the collision chamber electrode surfaces in terms of the detector surface. Accordingly, helium is admitted to the collision chamber and now sub stantially no metastable atoms are destroyed in gaseous collisions, the only possible collisions being of an elastic or excitation exchange nature. For an increment of pressure cLP in the collision chamber the reduction in the detector current (d/) and the increase in the collision chamber current (d/') are measured. Since these two current increments arise essentially from the transfer of a number (d of metastable atoms from one detector to the other their magnitudes are in direct proportion to the electron yields of their surfaces. Thus dI = yMdN, dF = y'MdN, + + Figure 3. (1) A+ ions + secondary electrons from (2) A+ ions; (3) A+ ions + secondary electrons from B.
274 R. F. Stebbings to which potentials were applied eliminating charged particles from the beam. The remanent neutral beam then traversed the collision chamber C and entered the wedge detector W. The collision chamber electrodes and the wedge detector assembly were constructed of brass and subsequently gold-plated. A further meta stable atom detector G which intercepted a fraction of the metastable flux was included to provide a control current, thereby eliminating errors arising from fluctuations in beam intensity. It was soon found, however, that the stability of the beam was such as to render this refinement unnecessary.
Helium metastable atoms and resonance photons 275 Photon flux It was evident that using this arrangement photons from the source could also pass through the collision chamber and strike the detector surfaces. Investigations by Found (1931) and Kenty (1933) left considerable doubt as to the relative fluxes of He* and resonance radiation likely to reach the collision chamber, although the more recent work of Reynolds (1952) suggested that the photon flux would be negligible in comparison with the metastable atom flux. Provision was nevertheless made for the introduction of a collodion film,f transparent to photons, into the path of the beam, in order that any photo currents could be identified.
R. F. Stebbings 276 It is interesting to note that the total cross-sections in the non-resonance cases increased linearly with increasing atomic number of the target atom (figure 7). The author is not aware of further experimental data on these collisions but the sym- helium pressure (10-3 mm Hg) Figure 5. Typical plot showing the absorption of the beam in helium. M, metastable atoms; P, photons.
Helium metastable atoms and resonance photons 277 metrical case of He* in He has been treated theoretically by Buckingham & Dalgarno (1952) who have determined the total elastic scattering cross-section as a function of incident energy. They have expressed their total cross-section, in units of na\, bv the formula Qua =148+ 128/# —61/X2, where K2 = T/28-6, T being the equivalent temperature of the colliding particles. At a temperature of 300° K, which is considered appropriate to the metastable atoms in the present work, the theoretical value for the cross-section is i807ra|.
R. F. Stebbings 278 Initially the ionization increases sharply and arises mainly from metastable atoms. As the pressure is raised the probability of an ionizing encounter increases but the number of metastable atoms arriving at the collision chamber diminishes, because of elastic scattering in transit from the source. As the pressure is increased still further this loss of metastable atoms becomes the dominant factor and the ionization from this source drops until at 20 /x it has become very small. The photo-ionization, 10~13 A 10-14 A argon pressure (10~3 mm Hg) (a) (b) Figure 8. Ionization in the collision chamber when it arises (a) from incident metastable atoms and photons, ( ) from photons only.
Helium metastable atoms and resonance photons 279 however, increases substantially linearly with pressure up to the highest pressures employed. The ionization may, therefore, be resolved into its two components shown in figure 9. The exact shape of the metastable curve is not known at the higher pressures but it would appear to have a long tail. It is seen, then, that in the collision chamber also, the contributions to the various currents arising from metastable atoms and from photons may be separately determined. It is also evident that owing argon pressure (] 0 3 mm Hg) Figure 11. The currents at the various electrodes as a function of pressure.