Spectral mode changes in an alkali rf discharge

As a result of observations made by Shaw (M.S. thesis, Cornell University, 1964) in the mid-1960s, alkali rf discharges are known to operate in two spectral modes, the so-called ring mode and the red mode. Experience has shown that the ring mode is best for discharge lamps used in quantum-electronic devices such as atomic clocks and optically pumped magnetometers and that the performance of these devices seriously degrades when the lamp operates in the red mode. Understanding the origin of these modes therefore has application to understanding and improving various quantum-electronic devices. Here we show that Shaw’s model for these modes is inconsistent with observation, and we propose an alternate model based on the role of radiation trapping in multistep ionization.