A system of physical chemistry - Index of

THE PHOTO-ELECTRIC EFFECT 129

thus calculated bear no obvious relation to those obtained by Hughes

and others.^

Let us return for a moment to the expression = /zvq V^. To test

this expression we may take the case **of** oxygen. Hughes has found

that the ionisation **of** air sets in at a wave-length i35)u,/x. This is like-

wise the position **of** the absorption band **of** oxygen in the ultra-violet.

We may conclude, therefore, that the effect in air is due to the ionisation

**of** oxygen, and hence we should be able to calculate the ionisation

potential **of** oxygen from the expression V = hv^e. The value **of** V thus

obtained is 9"2 volts which is identical with the observed value.

In dealing with collisions between electrons and molecules or atoms,

it is necessary to distinguish between two kinds, namely, elastic collisions

and inelastic collisions. In the first, the time **of** contact or encounter

is very short compared with the second. No "chemical" effects are

produced as a result **of** such collisions, the particles separating in the

same chemical state and with the same momentum (though not, **of**

course, with the same direction) as they possessed prior to the encounter.

These are the kind **of** collisions postulated in kinetic molecular theory

to account for the gas laws, for example. In the second kind **of** colli-

sion the time **of** encounter is longer, the total momentum after collision

is less than it was before, arid there is opportunity for transfer **of** energy

between the two particles, which may result in observable chemical or

**physical** change. This is consequently the more important kind from

the standpoint **of** atomic interaction, ionisation, and resonance, etc.

Tate and Foote {Proc. Nat. Acad. Sci., 4, 9 (1918)) point out that "no

considerable transfer **of** energy from the light electron to the relatively

heavy gas can take place except when the time **of** encounter between

electron and atom bears some simple relation to the characteristic period

**of** one **of** the vibrational degrees **of** freedom in the atom ".

"Two types **of** /^elastic encounter between electrons and gas atoms

have been observed. One **of** these results in the emission **of** a radiation

**of** a single frequency, without ionisation **of** the gas, whilst the other ionises

the gas and causes it to emit a composite spectrum **of** radiations.

The potential giving the first type **of** encounter may be termed a resonance

potential, that giving the second typ^ an ionisation potential. The

line which defines the shortest wave limit or " convergence frequency "

in the spectrum which is produced when the ionisation is potential applied,

occurs much further in the ultra-violet than does the single line

produced by the resonance potential. Naturally the single line characteristic

**of** resonance also appears when the applied voltage causes

ionisation. The following table is that given by Tate and Foote, and

contains the values **of** resonance and ionisation potentials respectively.

The calculated values **of** V in each case are obtained by employing the

quantum expression eY = hv. Thus the conditions are chosen experimentally

under which the gas emits only the single line. Its frequency

^ These limiting wave-lengths appear, however, to lie fairly close to the d spersional

wave-lengths. Accurate values for the dispersional wave-lengths have been

obtained in a number **of** cases by Baly, Phil. Ma^., 27, 632, 1914.

VOL. III. 9