  Level-crossing spectroscopy involves lifting the degeneracy of an excited
state and using the interference of two nearly degenerate levels to measure the
excited state lifetime. Here we use the idea of interference between different
pathways to study the momentum-dependent wave packet lifetime due an excited
state level-crossing (conical intersection) in a molecule. Changes in
population from the wave packet propagation are reflected in the detected
fluorescence. We use a chirped pulse to control the wave packet momentum.
Changing the chirp rate affects the transition to the lower state through the
conical intersection. It also affects the interference of different pathways in
the upper electronic state, due to the geometric phase acquired. Increasing the
chirp rate decreases the coherence of the wave packet in the upper electronic
state. This suggests that there is a finite momentum dependent lifetime of the
wave packet through the level-crossing as function of chirp. We dub this
lifetime the wave packet momentum lifetime.
