STUDY OF EFFECTS OF RADIATION TRAPPING ON DOPPLER SHIFTS OF EMISSIONS
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Date
1995
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Ohio State University
Abstract
It is well known that radiation trapping may occur if an emitting upper state is optically connected to a highly populated lower state. We have calculated that when an emitting upper state is radiation trapped, the Doppler shift of the emission from this state will be reduced by a factor $\delta$. For a given geometry of an emission source, $\delta$ depends only on the absorption coefficient. For the Doppler shifts of two different emissions to have an equal reduction factor $\delta$, the corresponding absorption coefficients must therefore be equal, which leads a simple relation between the densities of the two absorbing lower states that are involved in radiation trapping. This relation allows one to determine the relative density of the trapping states. Experimentally we have measured the Doppler shifts for neutral He emissions from both trapped states and untrapped states and determined the $\delta$ values for the emissions, $2^{1}S - 3^{1}P$ and $2^{3}S-2^{3}P$. Since the $3^{1}P$ state is optically connected and trapped by the ground state while the $2^{3}P$ state is trapped by the metastable $2^{3}S$ state, using the measured $\delta$ values, the absolute density of metastable $He(2^{3}S)$ can be determined from the known ground state atomic density.
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Author Institution: The Ohio State University, Columbus, OH 43210