Femtosecond real-time probing of reactions XXII. Kinetic description of probe absorption, fluorescence, depletion and mass spectrometry

A theoretical description of femtosecond transition-state dynamics is developed and compared with experimental results, with numerous examples. We focus on the population flow, employing a kinetic model, with the objective of studying the general trends and signal control parameters; we do not include coherence for the sake of acquiring a simplified picture with operational parameters. Different methods of detection are addressed, such as laser-induced fluorescence, depletion, absorption, and mass spectrometry. The response of the molecular system is derived for various types of dynamics with one or more rate determining processes. The significance of the relative time scales associated with the rates is explored and the effect on transients of the ratio of probing cross-sections is examined. Furthermore, the influence on the observed signal of the finite temporal width and shape of the laser pulses is quantified. Effects are discussed both in the unsaturated and saturated regimes of transition probabilities, demonstrating characteristic features of both regimes