Influence of polarization effects and magnetic fields on time-resolved laser-induced fluorescence measurements

The theory of time-resolved laser-induced fluorescence (LIF) in a static magnetic field is reformulated by means of the graphical method of angular momentum theory. Invariant general geometrical relations of the time-resolved fluoresence signal are obtained in an explicit form for arbitrary polarization and propagation directions of the primary and secondary radiation and for any direction of the magnetic field. On this basis an extensive experimental study was carried out to find general conditions for lifetime experiments using laser-induced fluorescence, under which the direction of polarization in the excitation and the observation channel and the influence of external magnetic fields does not change the time evolution of the fluorescence signal. It is shown that besides the well-known ‘‘magic-angle’’ excitation also measurements at very low pressures (p≊${10}^{\mathrm{-}4}$ mbar) provide conditions where laser-induced alignment is negligible and does not disturb the time evolution of the decay signal. Furthermore the experiments confirm that the magnitude of laser-induced alignment depends very sensitively on the chosen transition scheme, on the value of the cross section for alignment destroying collisions, and on the geometry of the experiment. Interferences on the time-resolved fluorescence signal by external magnetic fields are also discussed. It is shown that the presence of a strong magnetic field can lead to lifetime results systematically too short under conditions typical for LIF lifetime experiments.