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We report the generation and shaping of ∼ 20 femtosecond pulses in the deep ultraviolet (DUV). Our approach combines spectral broadening in a stretched hollow-core fiber with compression and shaping via an acousto-optic modulator-based pulse shaper. The pulse shaper allows for automatic compression, useful for in-air delivery of pulses to two-dimensional (2D) spectroscopy and other time-resolved experiments. Unlike previous DUV pulse generation schemes that lack phase stability, our method enables programmable generation of phase-locked pulse-pairs with attosecond-level stability. We demonstrate pulse characterization via self-diffraction frequency-resolved optical gating and dispersion scan methods in the DUV and validate performance through a pump-probe experiment resolving electronic dynamics on attosecond timescales. 

The excited-state dynamics of o-nitrophenol have been explored using trajectory surface hopping nonadiabatic dynamics combined with floating occupation molecular orbital complete active space configuration interaction. We focus on the effect of excitation energy on the subsequent dynamics. The absorption spectrum of o-nitrophenol has two peaks, centered at 3.9 eV (∼320 nm) and 5.1 eV (∼240 nm), and we performed dynamics starting from each of these peaks. The results show that even though the relaxation time constants are similar for the two excitation windows, the underlying dynamics are different. When exciting to the low energy peak, the dynamics are dominated by intramolecular proton transfer followed by internal conversion to the ground state, while exciting to the high-energy peak leads to fast internal conversion to the first excited state and slower decay to the ground state. In this case, intramolecular proton transfer does not occur as frequently, and many trajectories decay to the ground state through conical intersections without proton transfer. By calculating spin–orbit coupling values along the trajectories, we also show that intersystem crossing is possible. Based on the Landau–Zener probability formula, we estimate that there is about a 30%–40% probability that intersystem crossing will occur within 1 ps. 

We present momentum resolved covariance measurements of entangled electronic-nuclear wave packets created and probed with octave spanning phaselocked ultrafast pulses. We launch vibrational wave packets on multiple electronic states via multiphoton absorption, and probe these wave packets via strong field double ionization using a second phaselocked pulse. Momentum resolved covariance mapping of the fragment ions highlights the nuclear motion, while measurements of the yield as a function of the relative phase between pump and probe pulses highlight the electronic coherence. The combined measurements allow us to directly visualize the entanglement between the electronic and nuclear degrees of freedom and follow the evolution of the complete wavefunction. Published by the American Physical Society2024 

Time-resolved photoionization measurements were performed on o-nitrophenol pumped with UV laser pulses at a central wavelength of 255 nm (4.9 eV) and probed with vacuum ultraviolet (VUV) pulses at 153 nm (8.1 eV). The photoelectron spectrum and time of flight mass spectrum for ions were recorded at each pump–probe delay. The measurements are interpreted with the aid of electronic structure calculations for both the neutral and ionic states. Evidence is found for the formation of a bicyclic intermediate followed by NO dissociation through a process of internal conversion and intersystem crossing. The combination of photoelectron and photoion spectroscopy, together with computational results, provides strong evidence of intersystem crossing that is difficult to establish with only a single technique. 

Abstract Filming atomic motion within molecules is an active pursuit of molecular physics and quantum chemistry. A promising method is laser-induced Coulomb Explosion Imaging (CEI) where a laser pulse rapidly ionizes many electrons from a molecule, causing the remaining ions to undergo Coulomb repulsion. The ion momenta are used to reconstruct the molecular geometry which is tracked over time (i.e., filmed) by ionizing at an adjustable delay with respect to the start of interatomic motion. Results are distorted, however, by ultrafast motion during the ionizing pulse. We studied this effect in water and filmed the rapid “slingshot” motion that enhances ionization and distorts CEI results. Our investigation uncovered both the geometry and mechanism of the enhancement which may inform CEI experiments in many other polyatomic molecules.