The adsorption mechanisms proposed here have involved
interaction of gas and sorbate phase molecules or ions with three
different kinds of active sites - electron donors, 'shallow' traps
and deeper-lying trapping centres associated with molecular water.
The shallow traps have been tentatively identified with oxygen -
vacancies, and the donors and deep traps with hydroxyl-vacancies.
The band-structure of the solid-surface may be modified to include
the deep traps, or sinks, which may lie just above or below
the donor levels, depending on the exo- or endo-thermal nature
of the reaction
Interpretation of the photo-adsorption data has here been
in terms of an overall scheme which is basically a compound of
previously suggested ideas. For example, the 'A' and 'B'-site concept of Kennedy (2) has been largely retained, including
the migration-controlled O₂ uptake mechanism, though the proposed
nature of the donors and "oxygen sites" is more akin to
the ideas of McLean (3). Similarities also exist to the other
reaction schemes of McLintock (4) and Bews (17): but it is felt
that the present scheme is more comprehensive than formerly,
being capable, for example, of embracing many diverse features
of the interrelated oxygen, hydrocarbon and nitric oxide uptakes.
No claim is made, however, to have provided a unique explanation
of the intricacies involved in the photosorptions. It is
merely suggested that the treatment described in the previous
pages serves as a reasonable account of the electronic processes
that may occur within the surface layers under illumination.
Proof that these processes do occur is not possible at the present
level of experimental investigation, but some degree of confirmation
may have been achieved from the quantitative approach
towards predicted C₂H₄ uptake values, and free donor concentrations.
Future investigation might be directed at elucidating the
nature of the finally sorbed species. For example, infra-red
A.T,R. techniques might be developed in order to examine the
hydrocarbon-surface bond, and the possible presence of CH₃
groups on ethylene-covered surfaces. Magnetic susceptibility
measurements could well provide valuable information on the
concentration of paramagnetic centres (e.g. O.) in the dark, under
illumination, and in the presence of various adsorbates, and hence
might settle the nature of surface oxygen. Extension of photoconductivity
measurements of the type made by McLintock (4) to
include a study of the NO sorption would also be of interest, and
the origin of photodesorbed H₂O might profitably be examined,
using labelling techniques.