A Comparison of Strong and Weak Field Ionization as a Probe of Excited State Molecular Dynamics*

Ionization can serve as a universal probe of excited state molecular dynamics, such as internal conversion, dissociation, and isomerization. We conduct time-resolved photoelectron and photoion spectroscopy measurements of excited state state dynamics in both weak field (UV-pump/VUV-probe) and strong field (UV-pump/IR-probe) ionization (WFI and SFI). We investigate the relative merits of WFI versus SFI as probes in two different classes of molecules - halogenated methanes (CH2I2) and cyclic organic molecules (uracil). Experimentally, both WFI and SFI approaches show similar dynamics - CH2I2 undergoes rapid internal conversion followed by dissociation and uracil has substantial population trapping in the excited state in addition to rapid internal conversion back to the ground state. Theoretically, we compare the experimental results with electronic structure and dynamics calculations. We find that while SFI and WFI provide qualitatively similar information about the excited state dynamics, only WFI results can be compared quantitatively with calculations.