Sensitization by monatomic gases

Reactions sensitized by monatomic gases have been very extensively studied. The majority of the cases involve mercury vapor. Direct reaction of excited mercury atoms with other gases has also received considerable attention of late. 

A monatomic gas at low pressures shows line absorption at wavelengths longer than those required to produce ions. Transitions from atoms in low energy states to higher states will be observed. If levels and their assignments are well known, one may use the Boltzmann equation 

The quantity pt is the number of states into which this level will split in the presence of a foreign magnetic field. For cases which show the usual coupling between L and S one may write 

Since hyperfine structure may sometimes be important we must consider in more detail absorption by atoms with nuclear moment i. If atoms with nuclear moments i have total electronic angular momenta J, the i and the J combine to give a total angular momentum which may have the values 

The selection rules state that the total angular momentum quantum number may change by 1 or 0. Thus an element with several isotopes each with its own nuclear spin will present a line spectrum with a very complex and, under most experimental conditions, unresolved hyperfine structure. Nevertheless, as we shall see later, the overlap between the hyperfine components of a spectrum line is sufficiently incomplete to permit preferential excitation of one isotope in a mixture of isotopes by radiation from a lamp containing that same isotope. 

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