arXiv subjects

Andrew J. Howard

Publications and source records attributed to Andrew J. Howard.

6 recordsLinked to original sources

arXiv2026

Electron spectra from strong-field enhanced ionization in heavy water

Strong-field enhanced ionization (EI) is a phenomenon in which stretching of interatomic bonds into a distorted molecular geometry leads to an increase in the tunneling ionization rate driven by a strong field. While the nuclear dynamics leading to EI are well established, the electron dynamics of the ionization step remain largely unexplored. Isolating the momentum distribution of the electrons involved in EI is critical to fully characterizing the phenomenon. We have measured the electron momentum distribution associated with EI in triple ionization of D$_2$O using 6-fs pulse pairs together with full fragment momentum imaging and electron-ion correlation methods. We find that the EI electron momentum distribution differs substantially from that of standard strong-field tunneling from molecules, exhibiting an increased yield of electrons with large momentum in the direction of the laser polarization, and a change from the expected Gaussian distribution. These observations indicate that the instantaneous EI tunneling rate is maximized at a critical value of the laser electric field, rather than at the peak of an optical cycle. This finding distinguishes EI from Keldysh tunneling rate predictions, where tunneling rate increases monotonically with field strength. These pronounced differences between EI and non-EI electron spectra are critical tests of models of enhanced ionization and suggest a route towards control of the sub-cycle timing of electron emission.

physics.atom-ph↗

arXiv2026

Tracking molecular hydrogen formation from ionized water in real time

Removing an electron from a water molecule can drive its two hydrogen atoms to pair up and depart as molecular hydrogen. However, even for this elementary reaction, the route from start to finish has remained hidden because measurements have yet to follow the electronic and nuclear motion simultaneously. Combining correlated photoelectron and ion imaging, few-femtosecond pump--probe measurements, and nonadiabatic simulations, we track the complete pathway in isolated heavy water (D$_2$O) molecules. The reaction takes an indirect route and dissociates along three distinct pathways (direct, roaming, and delayed) with formation times of about 34 and 72 femtoseconds for the direct and delayed branches. Yet bond formation requires the molecule to first break its own symmetry. Only random asymmetric motion enables the electronic-state switch at a conical intersection, joining the two hydrogen atoms before the oxygen--hydrogen bond breaks. These results establish a time-resolved picture of molecular hydrogen formation from water and provide a general strategy for linking electronic excitation to chemical outcomes in settings from radiation damage to hydrogen production.

physics.chem-ph↗

arXiv2025

Optical Interference Effect in Strong-field Electronic Coherence Spectroscopy

We have investigated strong-field-induced electronic coherences in argon and molecular nitrogen ions created by high-intensity, few-cycle infrared laser pulses. This is a step toward the long-sought goal of strong-field coherent control in molecular chemistry. We employed high-intensity, few-cycle infrared laser pulses in a pump-probe setup to investigate a recent prediction that electronic coherences in nitrogen molecules change the ion yields vs. pump-probe delay. [Yuen and Lin, Phys. Rev. A 109, L011101 (2024)]. The predicted coherence signals in molecular nitrogen could not be resolved above the optical interference of the pump and probe pulses; a simultaneous measurement clearly resolved the induced cation fine-structure coherence in strong-field-ionized argon. The results of our comparison with simulations suggest that optical interference effects manifest differently in each ionic species and must be carefully accounted for when interpreting experimental data. We found that nonsequential double ionization in the low-intensity region of the focal volume can reduce the visibility of coherence generated by two-pulse sequential ionization, and we quantify the importance of pulse shape and spectral characteristics for isolating the desired coherence signals.

physics.atom-ph↗

arXiv2024

Isotope-Selective Strong Field Ionization of Semi-Heavy Water

Semi-heavy water (HOD) is one of the simplest molecules in which the bonds are labelled 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.

physics.chem-ph↗

arXiv2023

Femtosecond electronic and hydrogen structural dynamics in ammonia imaged with ultrafast electron diffraction

Directly imaging structural dynamics involving hydrogen atoms by ultrafast diffraction methods is complicated by their low scattering cross-sections. Here we demonstrate that megaelectronvolt ultrafast electron diffraction is sufficiently sensitive to follow hydrogen dynamics in isolated molecules. In a study of the photodissociation of gas phase ammonia, we simultaneously observe signatures of the nuclear and corresponding electronic structure changes resulting from the dissociation dynamics in the time-dependent diffraction. Both assignments are confirmed by ab initio simulations of the photochemical dynamics and the resulting diffraction observable. While the temporal resolution of the experiment is insufficient to resolve the dissociation in time, our results represent an important step towards the observation of proton dynamics in real space and time.

physics.chem-ph↗

arXiv2021

Strong Field Ionization of Water II: Electronic and Nuclear Dynamics En Route to Double Ionization

We investigate the role of nuclear motion and strong-field-induced electronic couplings during the double ionization of deuterated water using momentum-resolved coincidence spectroscopy. By examining the three-body dicationic dissociation channel, D$^{+}$/D$^{+}$/O, for both few- and multi-cycle laser pulses, strong evidence for intra-pulse dynamics is observed. The extracted angle- and energy-resolved double ionization yields are compared to classical trajectory simulations of the dissociation dynamics occurring from different electronic states of the dication. In contrast with measurements of single photon double ionization, pronounced departure from the expectations for vertical ionization is observed, even for pulses as short as 10~fs in duration. We outline numerous mechanisms by which the strong laser field can modify the nuclear wavefunction en-route to final states of the dication where molecular fragmentation occurs. Specifically, we consider the possibility of a coordinate-dependence to the strong-field ionization rate, intermediate nuclear motion in monocation states prior to double ionization, and near-resonant laser-induced dipole couplings in the ion. These results highlight the fact that, for small and light molecules such as D$_2$O, a vertical-transition treatment of the ionization dynamics is not sufficient to reproduce the features seen experimentally in the strong field coincidence double-ionization data.

physics.atom-ph↗