Observing coupled electronic and nuclear dynamics, and the flow of energy between electronic and vibrational degrees of freedom during photochemical reactions, remains a central challenge in ultrafast chemistry. Here, we combine time-resolved Coulomb explosion imaging with ab initio quantum wavepacket calculations to resolve the nonadiabatic dynamics of UV-excited NO 2 as it evolves toward and relaxes through a conical intersection. Excitation at 400 nm, just below the dissociation threshold, launches large-amplitude vibrational motion that explores extended regions of the ground-state potential energy surface following electronic relaxation. Coincident fragment-ion momenta reveal a highly delocalized nuclear wavepacket with correlated bending and asymmetric-stretch motion, providing direct insight into ultrafast vibrational energy redistribution predicted by theory.

We investigate the wave packet that remains bound in the ground and excited cationic states of oxygen after strong-field ionization by an intense 800-nm pulse. Much weaker probe pulses (800 or 264 nm) are used to dissociate these still-bound cations. The momentum distribution of O^+ is measured as a function of pump-probe delay and Fourier-transformed to obtain kinetic-energy-dependent and rotational-state-resolved quantum beat spectra. The sub-cm^-1 resolution of the Fourier transform allows unambiguous identification of the electronic, vibrational, and rotational states populated by the pump and then dissociated by the probe. Although strong-field ionization is expected to populate the lower-lying X^2Π_g and a^4Π_u states more effectively than the b^4Σ^-_g state, a wave packet in the X^2Π_g state is seen only with the 264-nm probe and only weak signatures of the a^4Π_u states are found with either probe. The experiment confirms the role of the resonant coupling between the b^4Σ^-_g and a^4Π_u states by the 800 nm pulses [Xue et al., Phys. Rev. A 97, 043409 (2018)] and reveals the importance of rovibrational excitation in determining the momentum distribution of the O^+ fragments. The strong X^2Π_g state contribution observed with the 264-nm probe also shows the importance of resonant coupling in the probe pulse. The sub-cm^-1 resolution also resolves spin-orbit splitting in both the X^2Π_g and a^4Π_u state wave packets.

Light-induced molecular dynamics often involve the excitation of several electronic, vibrational, and rotational states. Since the ensuing electronic and nuclear motion determines the pathways and outcomes of photoinduced reactions, our ability to monitor and understand these dynamics is crucial for molecular physics, physical chemistry, and photobiology. However, characterizing this complex motion represents a significant challenge when different degrees of freedom are strongly coupled. In this Letter, we demonstrate how the interplay between vibrational, rotational, and electronic degrees of freedom governs the evolution of molecular wave packets in the low-lying states of strong-field-ionized sulfur dioxide. Using time-resolved Coulomb explosion imaging (CEI) and quantum mechanical wave packet simulations, we directly map the bending vibrations of the molecule, show how the vibrational wave packet is influenced by molecular alignment, and elucidate the consequences of nuclear motion for the coupling between the two lowest electronic states of the cation. Our results demonstrate that multi-coincident CEI can be an efficient experimental tool for characterizing coupled electronic and nuclear motion in polyatomic molecules.

Vibrational Cooper minima due to weak dipole coupling were previously observed in strong-field dissociation (2009)]. We demonstrate in two independent experiments that these vibrational Cooper minima occur more generally and can be used as "fingerprints" to identify the final electronic state in the dissociation pathway. In the first experiment, we show the wavelength dependence of the vibrational Cooper minima observed in the KER spectrum of O2+ dissociation induced by 396- and 264-nm photons. These minima appear at the locations predicted by our first-order perturbation-theory calculations. In the second experiment, a delay-dependent KER spectrum is obtained from O2 using an ionizing pump and a dissociating probe, then Fourier-transformed to generate a spin-rotation quantum beat spectrum of O2+. This spectrum shows vibrational Cooper minima at the expected locations, confirming their presence in another observable. In this case, these minima are used to identify the dissociation pathway.

Coulomb explosion imaging (CEI) with x-ray free electron lasers has recently been shown to be a powerful method for obtaining detailed structural information of gas-phase planar ring molecules [R. Boll et al. Nat. Phys. 18, 423-428 (2022)]. In this Letter, we investigate the potential of CEI driven by a tabletop laser and extend this approach to differentiating three-dimensional (3D) structures. We study the static CEI patterns of planar and nonplanar organic molecules that resemble the structures of typical products formed in ring-opening reactions. Our results reveal that each molecule exhibits a well-localized and distinctive pattern in 3D fragment-ion momentum space. We find that these patterns yield direct information about the molecular structures and can be qualitatively reproduced using a classical Coulomb explosion simulation. Our findings suggest that laser-induced CEI can serve as a robust method for differentiating molecular structures of organic ring and chain molecules. As such, it holds great promise as a method for following ultrafast structural changes, e.g., during ring-opening reactions, by tracking the motion of individual atoms in pump-probe experiments.

The 53rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics will take place from May 30 – June 3, 2022 in Orlando, Fl, USA. Virtual Presenter Help Desk

We probe the time-dependent ionization dynamics of impulsively excited rotational wave packets of N2, CO2, and C2H4 using broadband ultraviolet pulses centered at 262 nm. Photoelectron momentum distributions recorded by velocity-map imaging show a strong dependence on alignment, on multiphoton order, and on the electronic and vibrational states of the cation. We show that substantial information about the molecular-frame photoelectron angular distribution can be obtained from the high-order laboratory-frame asymmetry parameters without any prior knowledge of the photoionization process. We also compare few-photon ionization with one-photon ionization and strong-field ionization.

We present the results of an experiment investigating the generation of high-order harmonics by a femtosecond near-infrared (NIR) laser pulse in the presence of an extreme ultraviolet (XUV) field provided by a free-electron laser (FEL), a process referred to as XUV-assisted high-order harmonic generation (HHG). Our experimental findings show that the XUV field can lead to a small enhancement in the harmonic yield when the XUV and NIR pulses overlap in time, while a strong decrease of the HHG yield and a red shift of the HHG spectrum is observed when the XUV precedes the NIR pulse. The latter observations are in qualitative agreement with model calculations that consider the effect of a decreased number of neutral emitters but are at odds with the predicted effect of the correspondingly increased ionization fraction on the phase matching. Our study demonstrates the technical feasibility of XUV-assisted HHG experiments at FELs, which may provide new avenues to investigate correlation-driven electron dynamics as well as novel ways to study and control propagation effects and phase matching in HHG.

Using strong-field ionization as a probe, we observe highly nonperiodic evolution of the spin-rotation wave packet launched by a nonionizing femtosecond pulse in oxygen. The nonperiodicity is readily apparent only in rotationally cold molecules that are pumped with a weak alignment pulse. We show that this behavior is a consequence of the spin-rotation and the spin-spin couplings in the triplet ground state of the neutral molecule. A model that includes these couplings in the field-free Hamiltonian but in neither the alignment nor the ionization step explains most of the observed dynamics, suggesting that neither process depends explicitly on the electronic spin. We also show that the angle dependence of strong-field ionization can be retrieved from the delay-dependent signal even when coupling to spin complicates the rotational dynamics.

Using the CD3OH isotopologue of methanol, the ratio of D2H+ to D3+ formation is manipulated by changing the characteristics of the intense femtosecond laser pulse. Detection of D2H+ indicates a formation process involving two hydrogen atoms from the methyl side of the molecule and a proton from the hydroxyl side, while detection of D3+ indicates local formation involving only the methyl group. Both mechanisms are thought to involve a neutral D2 moiety. An adaptive control strategy that employs image-based feedback to guide the learning algorithm results in an enhancement of the D2H+/D3+ ratio by a factor of approximately two. The optimized pulses have secondary structures 110-210 fs after the main pulse and result in photofragments that have different kinetic energy release distributions than those produced from near transform limited pulses. Systematic changes to the linear chirp and higher order dispersion terms of the laser pulse are compared to the results obtained with the optimized pulse shapes.

In this work, we experimentally study the angle-dependent single ionization of carbon dioxide (CO2) 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. 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 CO2. We show that such measurements can directly test the validity of the axial recoil approximation.

The XUV field emitted by impulsively aligned ethylene molecules during high-order harmonic generation is retrieved as a function of molecular orientation. The results can be ascribed to multielectron contributions to the harmonic emission.

Coincidence momentum imaging is a powerful method to study molecular structure and dynamics. Here, we used the Coulomb explosion imaging method together with synchrotron radiation and ultrafast laser pulses in order to distinguish between geometric isomers of C2H2Br2, C2H2Cl2, and difluoroiodobenzene and to investigate their fragmentation dynamics following both strong-field and inner-shell photoionization.

Carrier-envelope phases (CEPs) from a kHz repetition rate, non-CEP stabilized laser system are measured and tagged with two different methods: an f-2f interferometer and a stereo-above-threshold-ionization carrier-envelope-phase-meter. Both methods utilize the octave spanning spectrum generated in the hollow-core fiber (HCF) that broadens the laser spectrum to produce few-cycle pulses. Phases from both methods are carefully synchronized and compared on a single shot level. The results show that the CEPs measured by both methods are in good agreement and demonstrate that a HCF based f-2f interferometer is well suited for CEP tagged experiments.

We measure multi-orbital contributions to high harmonic generation from aligned nitrogen. We show that the change in revival structure in the cutoff harmonics has a counterpart in the angular distribution when a lower-lying orbital contributes to the harmonic yield. This angular distribution is directly observed in the laboratory without any further deconvolution. Because of the high degree of alignment we are able to distinguish angular contributions of the highest occupied molecular orbital 1 (HOMO-1) orbital from angle-dependent spectroscopic features of the HOMO. In particular, we are able to make a direct comparison with the cross section of the HOMO-1 orbital in the extreme ultraviolet region.

N2 HOMO-1 orbital cross section revealed through high-order-harmonic generation Jan Troß,1,2 Xiaoming Ren,1,* Varun Makhija,1,† Sudipta Mondal,1,‡ Vinod Kumarappan,1 and Carlos A. Trallero-Herrero1,§ 1James R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, USA 2Institut für Kernphysik, Johann Wolfgang Goethe Universität Frankfurt, Max-von-Laue-Str. 1, 60438 Frankfurt, Germany (Received 13 January 2017; published 20 March 2017)