Abstract
This paper employs Bayesian probability theory for analyzing data generated in femtosecond pump-probe photoelectron-photoion coincidence (PEPICO) experiments. These experiments allow investigating ultrafast dynamical processes in photoexcited molecules. Bayesian probability theory is consistently applied to data analysis problems occurring in these types of experiments such as background subtraction and false coincidences. We previously demonstrated that the Bayesian formalism has many advantages, amongst which are compensation of false coincidences, no overestimation of pump-only contributions, significantly increased signal-to-noise ratio, and applicability to any experimental situation and noise statistics. Most importantly, by accounting for false coincidences, our approach allows running experiments at higher ionization rates, resulting in an appreciable reduction of data acquisition times. In addition to our previous paper, we include fluctuating laser intensities, of which the straightforward implementation highlights yet another advantage of the Bayesian formalism. Our method is thoroughly scrutinized by challenging mock data, where we find a minor impact of laser fluctuations on false coincidences, yet a noteworthy influence on background subtraction. We apply our algorithm to data obtained in experiments and discuss the impact of laser fluctuations on the data analysis.
Highlights
Coincidence measurements are a widely-used and powerful experimental technique in physics and chemistry
We recently presented a Bayesian approach to photoelectron-photoion coincidence (PEPICO), which treats both coincidences and background subtraction on the same footing [22,29]
We used Bayesian probability theory to analyze data obtained from pump-probe photoionization experiments with photoelectron-photoion coincidence detection
Summary
Coincidence measurements are a widely-used and powerful experimental technique in physics and chemistry. The correct pairwise assignment (true coincidence) may be affected by certain experimental conditions: If a laser pulse triggers a number of simultaneous ionization events arising from different neutral molecules, the assignment of correlated electron-ion pairs is impaired and causes so-called false coincidences [11]. (2) One electron and one ion are detected, which can originate from the same molecule (true coincidence) or (3) an electron and ion can originate from different molecules (false coincidence) Let this be illustrated using the example of exactly two ionization events. It is feasible to distinguish between true and false coincidences by pure experimental finesse, such as in cold target recoil ion momentum spectroscopy (COLTRIMS) [23,24], it demands quite a technical and financial effort and is entirely impossible for time-of-flight detection, as used in the presented experiment.
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