Aspects of laser induced fluorescence employing pulsed and modulated excitation

This work reports on various aspects of employing strong, pulsed or modulated excitation in laser induced fluorescence (LIF) experiments. Part I is motivated by two-photon LIF measurements in nitric oxide, used for the determination of temperatures and concentrations in cold flows. It is found that dynamical Stark effects modify the lineshapes of individual rotational transitions significantly, thereby invalidating conventional methods of data analysis. It is shown that, to maintain calibration, one must extract the true intensity dependence of the underlying nonlinear interactions from data that are spatially and temporally unresolved. This inversion problem is developed and solved for nonsaturated as well as saturated excitation, namely by the use of Abel inversion, moment expansion, and collocation in terms of kernel or spline basis functions. Further applications of these techniques to problems in laser diagnostics are discussed.

Part II is concerned with fundamental aspects of a proposed laser fluorescence velocimeter. In this instrument, flow velocities are derived from the depth-of-modulation or the phase-shift of the fluorescence signal with respect to the bichromatic (fully amplitude modulated) input field. By solving the pertinent Bloch equations it is found that, unlike for weak excitation, the inhomogeneously broadened response of such a system deviates considerably from that of a similar system driven by a broadband field. Thus, conventional results from phase-sensitive fluorometry are inapplicable. Instead, much insight can be gained from the concepts of multiple quantum resonances and Bloch-Siegert shifts, taken from magnetic resonance. However, this work moves beyond merely transferring these concepts to optical resonance, in particular by studying the behavior of harmonic interaction components. Also, it is discussed how similar insights can be applied to other optical interactions, for example those involving phase-modulation.