Laser-induced Zeeman coherences and non-linear processes in the neon 2p 5 (J = 1) to 1s 5 (J = 2) transition have been studied experimentally and theoretically. By varying the observation geometry and choice of polarisation, the fluorescence observed in the 2p 5 to 1s 3 (J = 0) transition becomes sensitive to different processes within the atom. A detailed theoretical treatment enables profiles to be matched to the experiment using an appropriate choice of parameters. Moreover, analysis of the equations of motion for the system allows the origins of the various features of the fluorescence profiles to be precisely identified in terms of the generation of higher-order Zeeman coherences in the 1s 5 level and coherent trapping of part of the atomic population there. The population trapping can be interpreted and analysed, for a particular choice of geometry, in terms of simpler subsystems of the full transition.
The interaction of an intense optical-maser field with excited atoms in a gas discharge leads to a considerable change in the population of those levels which are resonant at the maser frequency. This will cause a change in the population of other levels that are connected to the maser levels through collision-induced nonradiative as well as radiative transitions. Using the 1.15-\ensuremath{\mu} optical maser, these effects have been studied in detail for the
2s levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the
2s2 and
2s3 levels. In pure neon this has been found to be ${\ensuremath{\sigma}}_{23}=(2.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$
cm2. For a helium-neon mixture the measured cross section is ${\ensuremath{\sigma}}_{23}=(1.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$
cm2. The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2{s}_{2}\ensuremath{\rightarrow}2{p}_{10}$ and $2{s}_{3}\ensuremath{\rightarrow}2{p}_{10}$. The measured ratio is $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.
The fluorescence observed when a single-mode laser drives the 2p 3 (J=0) to 1s 4 (J=1) neon transition displays a narrow minimum as an applied magnetic field is varied. A detailed theoretical analysis of the system confirms that this resonance is due to the formation of a Zeeman coherence in the lower level which traps the population there. The dominant contribution to the population trapping comes from the atomic velocity classes for which the atom-laser detuning is close to zero, and for the case of exact resonance analytic expressions are obtained for dip width and depth. A broad minimum which also appears in the profile is shown to originate from incoherent processes in the atomic ensemble.