Uncovering Highly-Excited State Mixing in Acetone Using Ultrafast VUV Pulses and Coincidence Imaging Techniques

Couch, David E.; Kapteyn, Henry C.; Murnane, Margaret M.; Peters, William K.

doi:10.1021/acs.jpca.7b01112

Cited by 11 publications

(24 citation statements)

References 30 publications

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“…Introduced in the 1960s [6], coincidence methods have rapidly developed and are nowadays also applied in time-resolved investigations of ultrafast dynamics in molecules or clusters. In these dynamical studies PEPICO detection has proven to be essential to learn about the underlying processes if competing intramolecular relaxation pathways are active [3,[7][8][9], or if different species are present [10].…”

Section: Introductionmentioning

confidence: 99%

Analysis of femtosecond pump-probe photoelectron-photoion coincidence measurements applying Bayesian probability theory

Rumetshofer¹,

Heim²,

Thaler³

et al. 2018

Phys. Rev. A

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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, requiring long data-acquisition times to keep erroneous signals at an acceptable level. Here we present a probabilistic approach based on Bayesian probability theory that overcomes these problems. For a pumpprobe experiment with photoelectron-photoion coincidence detection, we reconstruct the interesting excited-state spectrum from pump-probe and pump-only measurements. This approach allows us 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 us to run experiments at higher ionization rates, resulting in a significant reduction of data acquisition times. The probabilistic approach is thoroughly scrutinized by challenging mock data. The application to pump-probe coincidence measurements on acetone molecules enables quantitative interpretations about the molecular decay dynamics and fragmentation behavior. All results underline the superiority of a consistent probabilistic approach over ad-hoc estimations. The software implementation of the Bayesian formalism presented in this paper is provided at https://github.com/fslab-tugraz/PEPICOBayes/. * m.rumetshofer@tugraz.at † M.R. and P.H. contributed equally to this work. arXiv:1709.04456v3 [physics.atom-ph]

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Section: Introductionmentioning

confidence: 99%

Analysis of femtosecond pump-probe photoelectron-photoion coincidence measurements applying Bayesian probability theory

Rumetshofer¹,

Heim²,

Thaler³

et al. 2018

Phys. Rev. A

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“…Previously observed population decay from the 3d yz state is somewhat more contradicting: Farmanara el al. 14 report a 330 fs decay, while Hüter et al., 13 Couch et al, 12 and Solling et al 5 obtain much shorter time constants of (50 ± 5) to (90 ± 5) fs, <50 fs, and <60 fs, respectively. Our observed decay time constant lies with τ 3dyz = (96 ± 17) fs within the same range.…”

Section: Resultsmentioning

confidence: 96%

“…Dynamics initiated by photoexcitation of the 3p and 3d Rydberg states, by contrast, show that nonradiative deactivation and corresponding fragmentation pathways are governed by Rydberg–valence coupling. 12 − 14 , 5 The interplay of Rydberg and valence states becomes even more important for excitation to higher Rydberg states above 3d, for which fewer experimental results are available. 13 , 15 , 16 …”

Section: Introductionmentioning

confidence: 99%

“…Other experiments targeting these higher Rydberg states also observe population decay through IC, but the reported time scales vary from 50 to 400 fs, resulting in different interpretations concerning the relaxation pathways. 12 − 14 , 5 Since these experiments differed in terms of their excitation conditions, the inconsistent results suggest that the relaxation dynamics depend sensitively on excitation parameters such as the exact photon energy or one- vs two-photon excitation. Therefore, a conclusive picture describing the exact relaxation pathways and the corresponding temporal behavior could not be obtained for excitation to states above 3p so far.…”

Section: Introductionmentioning

confidence: 99%

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The Role of Rydberg–Valence Coupling in the Ultrafast Relaxation Dynamics of Acetone

et al. 2017

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The electronic structure of excited states of acetone is represented by a Rydberg manifold that is coupled to valence states which provide very fast and efficient relaxation pathways. We observe and characterize the transfer of population from photoexcited Rydberg states (6p, 6d, 7s) to a whole series of lower Rydberg states (3p to 4d) and a simultaneous decay of population from these states. We obtain these results with time-resolved photoelectron–photoion coincidence (PEPICO) detection in combination with the application of Bayesian statistics for data analysis. Despite the expectedly complex relaxation behavior, we find that a simple sequential decay model is able to describe the observed PEPICO transients satisfactorily. We obtain a slower decay (∼320 fs) from photoexcited states compared to a faster decay (∼100 fs) of states that are populated by internal conversion, demonstrating that different relaxation dynamics are active. Within the series of Rydberg states populated by internal conversion, the decay dynamics seem to be similar, and a trend of slower decay from lower states indicates an increasingly higher energy barrier along the decay pathway for lower states. The presented results agree all in all with previous relaxation studies within the Rydberg manifold. The state-resolved observation of transient population ranging from 3p to 4d can serve as reference for time-dependent simulations.

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“…Photoelectron-photoion coincidence (PEPICO) spectroscopy utilizes not only information obtained from the detection of electrons and ions, but also the fact that they stem from the very same ionization event [1][2][3][4][5][6]. Frequently used in photoionization studies of gas phase molecules or clusters, this technique allows for conclusions about the ionization process such as the disentanglement of competing intramolecular relaxation channels [4,[7][8][9] or multiple species [10], a depth of insight that cannot be achieved without assigning electrons to the ions from which they originate. Thus, the success of PEPICO is based on these recordings of pairs being unambiguous, energy-resolved for electrons, and mass-resolved for the cations.…”

Section: Introductionmentioning

confidence: 99%

Bayesian Analysis of Femtosecond Pump-Probe Photoelectron-Photoion Coincidence Spectra with Fluctuating Laser Intensities

Heim

Rumetshofer

Ranftl

et al. 2019

Entropy

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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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Uncovering Highly-Excited State Mixing in Acetone Using Ultrafast VUV Pulses and Coincidence Imaging Techniques

Cited by 11 publications

References 30 publications

Analysis of femtosecond pump-probe photoelectron-photoion coincidence measurements applying Bayesian probability theory

Analysis of femtosecond pump-probe photoelectron-photoion coincidence measurements applying Bayesian probability theory

The Role of Rydberg–Valence Coupling in the Ultrafast Relaxation Dynamics of Acetone

Bayesian Analysis of Femtosecond Pump-Probe Photoelectron-Photoion Coincidence Spectra with Fluctuating Laser Intensities

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