The effect of amplified spontaneous emission on the measurement of the multiplet state distribution of ground state oxygen atoms

Amplified spontaneous emission (ASE) is a process which competes with laser-induced fluorescence (LIF) in three level systems. This was demonstrated in the case of oxygen atoms, for which both ASE and LIF were observed at 845 nm. Ground state oxygen atoms were generated by photodissociating 02, SOa, and NO2 in a pump and probe experiment. Both nascent and thermalized O(3P,+p) atoms were measured using two-photon LIF at 226 nm. The fine structure populations deduced from the 845 nm ( 3p3Pj’ + 325’) emission were considerably hotter than the distributions obtained from the 130 nm ( 3s3S+2p3P) transition. A kinetic model is developed showing that the distributions observed at 845 are distorted by ASE, which selectively depletes the populations of atoms originating from more highly populated multiplet levels.