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Angle-dependent strong-field ionization and fragmentation of carbon dioxide measured using rotational wave packets
In this work, we experimentally study the angle-dependent single ionization of carbon dioxide (CO 2 ) by linearly and circularly polarized pulses. The angle dependence of the ionization probability by linearly polarized pulses extracted from time-domain measurements on an impulsively excited rotational wave packet is compared with data obtained from a direct angle-scan measurement. The results from the measurement with linear and circular polarization are consistent with the adiabatic ionization approximation. Further, we extend the time-domain method to extract the dependence of the asymptotic momentum distribution of fragment ions on the orientation of the molecular axis, and apply it to investigate dissociative double ionization of CO 2 . We show that such measurements can directly test the validity of the axial recoil approximation.
Guiding of high-intensity laser pulses in 100-mm-long hydrodynamic optical-field-ionized plasma channels
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Disentangling the subcycle electron momentum spectrum in strong-field ionization
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Isotope-selective strong-field ionization of semiheavy water
Semiheavy water (HOD) is one of the simplest molecules in which the bonds are labeled by isotope. We demonstrate that a pair of intense few-femtosecond infrared laser pulses can be used to selectively tunnel ionize along one of the two bonds. The first pulse doubly ionizes HOD, inducing rapid bond stretching and unbending. Femtoseconds later, the second pulse arrives and further ionization is selectively enhanced along the OH bond. These conclusions arise from 3D time-resolved measurements of H + , D + , and O + momenta following triple ionization. Published by the American Physical Society 2025
The use of a double focusing field ionization mass spectrometer for the study of gas phase ion decomposition kinetics at 10 picosec to 10 microsec
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A field-ionization neutral-particle mass spectrometer
Description of laboratory model of a neutral-particle mass spectrometer that could make localized direct measurements of the neutral densities at 1 AU. Using a magnet that has a circular pole face 2.54 cm in diameter and a gap flux density of 8.0 kG, the following sensitivities were obtained for the various species: 10 to the 10th power counts per second per Torr for H2, 3 times 10 to the eighth power counts per second per Torr for He, and 2 times 10 to the 9th power counts per second per Torr for N2. Overall sensitivity may be increased through the use of electrostatic focusing elements between the ion source and the magnetic analyzer.
Apparent charge localization effects in the McLafferty rearrangement of acetate esters - Field ionization kinetic measurements
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Kinetics and mechanisms of unimolecular gas-phase reactions of radical cations at times of 0.00001 to 10 microsec following field ionization
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A convenient time scale calculation for field ionization kinetic experiments with a double focusing mass analyzer
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Skeletal rearrangement of the methylcyclopentane ion. Field-ionization kinetic measurements
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Coherent electronic-vibrational dynamics in deuterium bromide probed via attosecond transient-absorption spectroscopy
Ultrafast laser excitation can trigger multiplex coherent dynamics in molecules. Here, we report attosecond transient-absorption experiments addressing simultaneous probing of electronic and vibrational dynamics in a prototype molecule, deuterium bromide (DBr), following its strong-field ionization. Electronic and vibrational coherences in the ionic X$^2\Pi_{3/2}$ and X$^2\Pi_{1/2}$ states are characterized in the Br-$3d$ core-level absorption spectra via quantum beats with 12.6-fs and 19.9-fs periodicities, respectively. Polarization scans reveal that the phase of the electronic quantum beats depends on the probe direction, experimentally showing that the coherent electronic motion corresponds to the oscillation of the hole density along the ionization-field direction. The vibrational quantum beats are found to maintain a relatively constant amplitude, whereas the electronic quantum beats exhibit a partial decrease in time. Quantum wave-packet simulations show that the decoherence effect from the vibrational motion is insignificant because of the parallel relation between the X$^2\Pi_{3/2}$ and X$^2\Pi_{1/2}$ potentials. Finally, a comparison between the DBr and HBr results suggests that rotation motion is responsible for the decoherence since it leads to initial alignment prepared by the strong-field ionization.
IONIZED GASES AT LOW DENSITY IN AXIALLY SYMMETRIC MAGNETIC FIELDS
Ionized gases at low density in axisymmetric magnetic fields
Ionization of HCCI Neutral and Cations by Strong Laser Fields Simulated With Time Dependent Configuration Interaction
Strong field ionization of neutral iodoacetylene (HCCI) can produce a coherent superposition of the X and A cations. This superposition results in charge migration between the CC π orbital and the iodine π-type lone pair which can be monitored by strong field ionization with short, intense probe pulses. Strong field ionization of the X and A states of HCCI cation was simulated with time-dependent configuration interaction using singly ionized configurations and singly excited, singly ionized configurations (TD-CISD-IP) and an absorbing boundary. Studies with static fields were used to obtain the 3-dimensional angular dependence of instantaneous ionization rates by strong fields and the orbitals involved in producing the cations and dications. The frequency of charge oscillation is determined by the energy separation of the X and A states; this separation can change depending on the direction and strength of the field. Furthermore, fields along the molecular axis can cause extensive mixing between the field-free X and A configurations. For coherent superpositions of the X and A states, the charge oscillations are characterized by two frequencies–the driving frequency of the laser field of the probe pulse and the intrinsic frequency due to the energy separation between the X and A states. For linear and circularly polarized pulses, the ionization rates show marked differences that depend on the polarization direction of the pulse, the carrier envelope phase and initial phase of the superposition. Varying the initial phase of the superposition at the beginning of the probe pulse is analogous to changing the delay between the pump and probe pulses. The charge oscillation in the coherent superposition of the X and A states results in maxima and minima in the ionization yield as a function of the superposition phase
The selection of transistors for use in ionizing radiation fields.
Transistor selection for use in ionizing radiation fields considering forward current gain degradation and collector-base leakage current increase
Femtosecond pulse parameter estimation from photoelectron momenta using machine learning
Abstract Deep learning models have provided huge interpretation power for image-like data. Specifically, convolutional neural networks (CNNs) have demonstrated incredible acuity for tasks such as feature extraction or parameter estimation. Here we test CNNs on strong-field ionization photoelectron spectra, training on theoretical data sets to ‘invert’ experimental data. Pulse characterization is used as a ‘testing ground’, specifically we retrieve the laser intensity, where ‘traditional’ measurements typically lead to 20% uncertainty. We report on crucial data augmentation techniques required to successfully train on theoretical data and return consistent results from experiments, including accounting for detector saturation. The same procedure can be repeated to apply CNNs in a range of scenarios for strong-field ionization. Using a predictive uncertainty estimation, reliable laser intensity uncertainties of a few percent can be extracted, which are consistently lower than those given by traditional techniques. Using interpretability methods can reveal parts of the distribution that are most sensitive to laser intensity, which can be directly associated with holographic interferences. The CNNs employed provide an accurate and convenient ways to extract parameters, and represent a novel interpretational tool for strong-field ionization spectra.
Light control of intramolecular nuclear dynamics by vortex electron localization
In strong-field ionization of molecules, intense light pulses are thought to have a negligible direct influence on atomic nuclei. Molecular dissociation is thus expected to be determined by the geometrical configuration of the molecular ion at the ionization instant. Contrary to this picture, we observe a counterintuitive electron-proton angular correlation and the formation of proton vortices following strong-field ionization of H 2 molecules by bicircularly polarized two-color laser fields. We explain this phenomenon by the pathway interference and localization of the residual H 2 + electron in different angular-momentum states formed in the tail of the driving laser pulse. We validate this interpretation by combining a quantum-mechanical numerical simulation of the field-driven coupled electronic-nuclear dynamics and a semiclassical-trajectory model for the phase accumulation of the laser-driven electronic-nuclear wave packet. Our joint experimental and theoretical study reveals a general picture of vortex electron localization which can be used for controlling molecular-bond breaking with circularly polarized laser fields.