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Register a new userBayesian Analysis of Femtosecond Pump-Probe Photoelectron-Photoion Coincidence Spectra with Fluctuating Laser Intensities
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Michael Rumetshofer
Publication date
2019
fields
Physics
and research's language is
English
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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.
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Ultrafast dynamical processes in photoexcited molecules can be observed with pump-probe measurements, in which information about the dynamics is obtained from the transient signal associated with the excited state. Background signals provoked by pump and/or probe pulses alone often obscure these excited state signals. Simple subtraction of pump-only and/or probe-only measurements from the pump-probe measurement, as commonly applied, results in a degradation of the signal-to-noise ratio and, in the case of coincidence detection, the danger of overrated background subtraction. Coincidence measurements additionally suffer from false coincidences. Here we present a probabilistic approach based on Bayesian probability theory that overcomes these problems. For a pump-probe experiment with photoelectron-photoion coincidence detection we reconstruct the interesting excited-state spectrum from pump-probe and pump-only measurements. This approach allows to treat background and false coincidences consistently and on the same footing. We demonstrate that the Bayesian formalism has the following advantages over simple signal subtraction: (i) the signal-to-noise ratio is significantly increased, (ii) the pump-only contribution is not overestimated, (iii) false coincidences are excluded, (iv) prior knowledge, such as positivity, is consistently incorporated, (v) confidence intervals are provided for the reconstructed spectrum, and (vi) it is applicable to any experimental situation and noise statistics. Most importantly, by accounting for false coincidences, the Bayesian approach allows to run experiments at higher ionization rates, resulting in a significant reduction of data acquisition times. The application to pump-probe coincidence measurements on acetone molecules enables novel quantitative interpretations about the molecular decay dynamics and fragmentation behavior.
We describe a setup to study ultrafast dynamics in gas-phase molecules using time-resolved photoelectron and photoion spectroscopy. The vacuum ultraviolet (VUV) probe pulses are generated via strong field high-order harmonic generation from infrared femtosecond laser pulses. The band pass characteristic in transmission of thin indium (In) metal foil is exploited to isolate the $9^{text{th}}$ harmonic of the 800 nm fundamental (H9, 14 eV, 89 nm) from all other high harmonics. The $9^{text{th}}$ harmonic is obtained with high conversion efficiencies and has sufficient photon energy to access the complete set of valence electron levels in most molecules. The setup also allows for direct comparison of VUV single-photon probe with 800 nm multi-photon probe without influencing the delay of excitation and probe pulse or the beam geometry. We use a magnetic bottle spectrometer with high collection efficiency for electrons, serving at the same time as a time of flight spectrometer for ions. Characterization measurements on Xe reveal the spectral width of H9 to be $190pm60$ meV and a photon flux of $sim1cdot10^{7}$ photons/pulse after spectral filtering. As a first application, we investigate the S$_1$ excitation of perylene using time-resolved ion spectra obtained with multi-photon probing and time-resolved electron spectra from VUV single-photon probing. The time resolution extracted from cross-correlation measurements is $65pm10$ fs for both probing schemes and the pulse duration of H9 is found to be $35pm8$ fs.
Recently, two novel techniques for the extraction of the phase-shift map (Tomassini {it et.~al.}, Applied Optics {bf 40} 35 (2001)) and the electronic density map estimation (Tomassini P. and Giulietti A., Optics Communication {bf 199}, pp 143-148 (2001)) have been proposed. In this paper we apply both methods to a sample laser-plasma interferogram obtained with femtoseconds probe pulse, in an experimental setup devoted to laser particle acceleration studies.
Studies of ultrafast dynamics along with femtosecond-pulse metrology rely on non-linear processes, induced solely by the exciting/probing pulses or the pulses to be characterized. Extension of these approaches to the extreme-ultraviolet (XUV) spectral region opens up a new, direct route to attosecond scale dynamics. Limitations in available intensities of coherent XUV continua kept this prospect barren. The present work overcomes this barrier. Reaching condition at which simultaneous ejection of two bound electrons by two-XUV-photon absorption becomes more efficient than their one-by-one removal it is succeeded to probe atomic coherences, evolving at the 1fs scale, and determine the XUV-pulse duration. The investigated rich and dense in structure autoionizing manifold ascertains applicability of the approach to complex systems. This initiates the era of XUV-pump-XUV-probe experiments with attosecond resolution.
The Linac Coherent Light Source changes configurations multiple times per day, necessitating fast tuning strategies to reduce setup time for successive experiments. To this end, we employ a Bayesian approach to transport optics tuning to optimize groups of quadrupole magnets. We use a Gaussian process to provide a probabilistic model of the machine response with respect to control parameters from a modest number of samples. Subsequent samples are selected during optimization using a statistical test combining the model prediction and uncertainty. The model parameters are fit from archived scans, and correlations between devices are added from a simple beam transport model. The result is a sample-efficient optimization routine, which we show significantly outperforms existing optimizers.
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