A study of metastable atomic and molecular levels

In contrast to the large number of measurements which have been made of the transition probabilities of electric dipole allowed spectral lines, there have been very few experimental determinations of the absolute transition probabilities of electric dipole "forbidden" lines. The radiative lifetimes of the metastable states from which forbidden radiation is emitted may be as long as one second or more and in most laboratory spectral lamps such species are destroyed by collision processes before forbidden radiation can be emitted. However, metastable atoms and molecules play important roles in determining the energy balance of the upper atmosphere and many of the prominent spectral features of the aurorae and the airglow are forbidden by the selection rules for electric dipole radiation.
An experimental determination of the radiative lifetime of the 1S0 metastable state of atomic oxygen is presented in this thesis. The principal emission from this state is the electric quadrupole 21S0 - 21D2 line at 5577Å, which is one of the strongest features of the auroral spectrum. The metastable states of oxygen were populated in a pulsed low current discharge through moderate pressures of either neon or argon which were mixed with a trace concentration (typically 10-4 torr) of oxygen. In such a discharge, oxygen molecules are dissociated by collision with metastable inert gas atoms and the oxygen atoms so produced are excited to the metastable levels by electron exchange collisions. Atomic recombination is slow so that the oxygen is present mainly in the atomic form.
The intensity of the 5577Å forbidden line was recorded as a function of time during the afterglow period which followed a discharge pulse. The light was detected by a photomultiplier tube and the individual photoelectric events were recorded by the multi-channel scaling technique. The channel address cycle was initiated at the end of each discharge pulse and the photopulses were then counted as the channels of the multiscaler were successively addressed during the afterglow period. In this way, the intensity detected from the afterglow was obtained as a function of channel number or time. The statistical scatter on the recorded signal was reduced by adding the intensities detected from many afterglow cycles.
During the late afterglow, the decay of the metastable state population was exponential with decay constant
Γ = A′⁄p + Γ0 + α′p + β′p + α′opo
where the terms represent respectively diffusion to the walls of the vessel, the reciprocal Γ0 of the 0(1S0) radiative lifetime and deactivation of metastable atoms during two and three body collisions with both inert gas atoms and oxygen atoms. Here p and po are the partial pressures of the inert gas and of atomic oxygen respectively, and the coefficients α′, β′ and α′o are the rate constants for the different collisional deactivation processes. A large volume discharge vessel and inert gas pressures of typically 10 torr were used in order to reduce the loss of metastable population by diffusion. The spectroscopically pure gases used in the experiments were mixed in a bakeable gas handling system.
The oxygen partial pressure could not be measured directly because of adsorption of atomic oxygen on the walls of the vessel. Instead, the effect of the 0(1S0) deactivation by atomic oxygen was estimated by correlating the measured decay constants with the observed intensity of the afterglow signal. This intensity decreased as the oxygen was adsorbed. A "reduced" decay constant was determined at each inert gas pressure investigated as the value which would be expected at zero signal intensity or zero oxygen pressure.
Afterglow curves were recorded using both neon and argon at pressures ranging between 1 torr and 40 torr and the radiative lifetime of the 0(1S0) metastable state was evaluated by correlating the values of the reduced decay constants obtained at different inert gas pressures. The result
°Cexpt [0(1S0)] = 0.76 ± 0.03 seconds
is considerably more accurate than the previous experimental determinations and is in good agreement with the best theoretical estimate
°Ctheory [0(1S0)] = 0.80 seconds
Furthermore, the result is the first experimental determination of a metastable state radiative lifetime which is sufficiently reliable to be compared to theoretical estimates.
Measurements were also made of the coefficients for diffusion of 0(1S0) metastable atoms through neon and argon, and the rate constants for deactivating collisions with neon and argon. The products of the inert gas pressure and the diffusion coefficients were determined as
416 ± 12 torr cm2 s-1 for neon at 298°K, and
268 ± 8 torr cm2 s-1 for argon at 298°K
in good agreement with previous measurements. These results are consistent with a Lennard-Jones collision diameter of 2.85Å and a force constant ε/k = 21.7°K for elastic collisions between two 0(1S0) atoms. Upper limits on the rate constants for two body deactivating collisions between 0(1S0) atoms and inert gas atoms were determined as
4 × 10-19 cm3 s-1 for neon, and
5.5 × 10-18 cm3 s-1 for argon.
Deactivation during three body collisions was found to be insignificant compared to the other loss processes.