Strong-field- versus weak-field-ionization pump-probe spectroscopy

Abstract

Ionization can serve as a universal probe of excited-state dynamics in molecules, such as internal conversion, dissociation, and isomerization. These processes are of fundamental importance to a wide array of dynamics in biology, chemistry, and physics. In recent years, there has been significant debate about the relative merits of strong-field ionization (SFI), which involves multiphoton absorption, versus weak-field ionization (WFI), where a single photon is absorbed, as probes of these dynamics. SFI is advantageous because it uses wavelengths that are relatively easy to generate, and one can always ionize the molecule with sufficient intensity. However, for SFI it is difficult to calculate observables, such as the time-dependent ion yield, since the calculation of the ionization dynamics including multiphoton processes is computationally expensive and difficult to carry out for many molecular geometries. WFI has the advantage that calculations of observables are tractable. However, the generation and implementation of the appropriate wavelengths (photon energies) can be challenging, and the fixed energy of the probe can lead to technical complications in following the dynamics from excited states back down to the ground state. Here we present a quantitative comparison of the two approaches for following the excited-state dynamics of two molecules, diiodomethane and uracil. The combination of internal conversion and dissociation in these molecules provides an ideal comparison of WFI and SFI as a probe. We compare the measurements with calculations of the dynamics. Our work indicates that while SFI and WFI provide qualitatively similar information about the excited-state dynamics, only WFI results can be compared quantitatively with present-day calculations.