Difficulties associated with stimulated emission in a bismuth - Neon discharge

The population of the metastable
6p^{3}
²
D_{3/2}^{0}
bismuth level has been determined by time-resolved absorption spectroscopy at times in the range
1-10 \mu
s after the start of the discharge in a bismuth vaporneon mixture, for a neon pressure of 13 mbar and a specific discharge energy of 1 mJ . cm
^-3
. An exponential decay of the population is observed and lifetimes of 3.0, 4.5, and 5.3 μs are obtained at temperatures of 830, 790, and 755°C, respectively. Measurements of the nine strongest fluorescent transitions to the metastable level confirm that a population of the order of 10
¹³
cm
^-3
is established in the early stage of the discharge and exceeds by three orders of magnitude the peak population of the bismuth resonance level
6p^{2} 7s^{4}P_{1/2}
. It is concluded that the absence of laser emission at 472.2 nm in the system studied is due to excessive population of the metastable level
6p^{3}
²
D_{3/2}^{0}
during the early stage of the discharge. It is proposed that this high population arises from the dissociation of Bi
2
dimers by electron impact and that the loss of electron energy to dissociation and to excitation of the vibrational and rotational levels of the dimers is responsible for the very low population achieved in the
6p^{2} 7s
⁴
p_{1/2}
level.