Direct probe of the bent and linear geometries of the core-excited Renner-Teller pair states by means of the triple-ion-coincidence momentum imaging technique

The doubly degenerate core-excited Pi state of CO2 splits into two due to static Renner-Teller effect. Using the triple-ion-coincidence momentum imaging technique and focusing on the dependence of the measured quantities on the polarization of the incident light, we have probed, directly and separately, the linear and bent geometries for the B1 and A1 Renner-Teller pair states, as a direct proof of the static Renner-Teller effect.     

Rescattering of ultralow-energy electrons for single ionization of Ne in the tunneling regime

Electron emission for single ionization of Ne by 25 fs, 1.0 PW/cm(2) laser pulses at 800 nm has been investigated in a kinematically complete experiment using a "reaction microscope." Mapping the complete final state momentum space with high resolution, a distinct local minimum is observed at P(e parallel )=0, where P(e parallel ) is the electron momentum parallel to the laser polarization. Whereas tunneling theory predicts a maximum at zero momentum, our findings are in good agreement with recent semiclassical predictions which were interpreted to be due to "recollision."     

Electron correlations observed through intensity interferometry

Intensity interferometry was applied to study electron correlations in doubly ionizing ion-atom collisions. In this method, the probability to find two electrons emitted in the same double ionization event with a certain momentum difference is compared to the corresponding probability for two uncorrelated electrons from two independent events. The ratio of both probabilities, the so-called correlation function, is found to sensitively reveal electron correlation effects, but it is rather insensitive to the collision dynamics.     

The impact of the third O2 addition reaction network on ignition delay times of neo-pentane

We studied the oxidation of neo-pentane by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O₂ addition reaction network on ignition delay time predictions. The experiments are based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHP intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O₂ addition reaction network. A satisfactory agreement has been observed between the experimental data points and the simulation results, thus adding confidence to the model's overall performance. Finally, this mechanism was used to predict ignition delay times reported previously in shock tube and rapid compression machine experiments (J. Bugler et al., Combust. Flame 163 (2016) 138–156). While the model accurately reproduces the experimental data, simulations with and without the 3rd O₂ addition reaction network included reveal only a negligible effect on the predicted ignition delay times at 10 and 20 atm. According to model calculations, low temperatures and high pressures promote the importance of the 3rd O₂ addition reactions.     

Entanglement of n-heptane and iso-butanol chemistries in flames fueled by their mixtures

This paper concerns itself with the entanglement of the high-temperature oxidation chemistry of n-heptane and iso-butanol in flames fueled by their mixtures. While in many cases the chemistries of the individual fuel components do not interact in mixture flames, in this work, we revealed interactions between the individual species pools originating from n-heptane and iso-butanol oxidation. In a coordinated experimental and modeling effort, chemical structures of three low-pressure premixed flames fueled by different blends of n-heptane and iso-butanol were determined using flame-sampling molecular-beam mass spectrometry with synchrotron-based single-photon ionization and chemical kinetic modeling. The chemical kinetic model, which is based on the reaction set that was used previously [Braun-Unkhoff et al., Proc. Combust. Inst., 2017, 36, 1311–1319], was now extended by an n-heptane sub-mechanism. The overall good performance of the model allows for an extraction of chemically relevant information that highlights the entanglement between the individual fuel-specific species pools. For example, it was shown that methyl radicals, in part from iso-butanol oxidation (i.e., from the decomposition of α-iso-butanol radicals) can participate in n-heptane consumption processes through H-abstraction reactions. Further interactions are related to the formation of the methylallyl radical and aromatics formation. The relevance of such interactions is also discussed regarding the formation of oxygenated byproducts.     

Manipulating atomic fragmentation processes by controlling the projectile coherence

We have measured the scattering angle dependence of cross sections for ionization in p+H2 collisions for a fixed projectile energy loss. Depending on the projectile coherence, interference due to indistinguishable diffraction of the projectile from the two atomic centers was either present or absent in the data. This shows that, due to the fundamentals of quantum mechanics, the preparation of the beam must be included in theoretical calculations. The results have far-reaching implications on formal atomic scattering theory because this critical aspect has been overlooked for several decades.     

Wavelength dependence of the suppressed ionization of molecules in strong laser fields

We study ionization of molecules by an intense laser field over a broad wavelength regime, ranging from 0.8 to 1.5 μm experimentally and from 0.6 to 10 μm theoretically. A reaction microscope is combined with an optical parametric amplifier to achieve ionization yields in the near-infrared wavelength regime. Calculations are done using the strong-field S-matrix theory and agreement is found between experiment and theory, showing that ionization of many molecules is suppressed compared to the ionization of atoms with identical ionization potentials at near-infrared wavelengths at around 0.8 μm, but not at longest wavelengths (10 μm). This is due to interference effects in the electron emission that are effective at low photoelectron energies but tend to average out at higher energies. We observe the transition between suppression and nonsuppression of molecular ionization in the near-infrared wavelength regime (1-5 μm).