Stephen R. Leone

We report on the observation of coherent molecular vibrations in the methyl radical, which is produced by 267 nm photodissociation of methyl iodide. The radical is excited to high levels in its ν 2 “umbrella” vibrational mode by the dissociation, and the ensuing coherent vibrational dynamics are observed by measuring ultrafast time-dependent changes in the x-ray transition energy from the C 1 s to the singly occupied valence orbital. Due to symmetry, the real-space vibrational motion appears predominantly in the x-ray energy shift at the difference frequencies of the ν 2 progression, although the fundamental frequencies of the ν 2 mode are also observed. By constructing a quantum-mechanical model of the dynamics, the coherent superposition is characterized and the real-space motion of the radicals is reconstructed. The retrieved trajectories are dominated by pronounced quantum beating governed by the high degree of coherent excitation and the strong negative anharmonicity of the ν 2 mode.

All-optical logic-gate-based switching is a prerequisite for photonic computing. This article introduces a logic-gate protocol for noncollinear four-wave mixing (FWM) of one attosecond extreme ultraviolet (XUV) with two few-femtosecond near infrared (NIR) pulses. Simulations show that the NIR carrier-envelope phases (CEPs) alter the spatial distribution of the XUV FWM emission, using doubly-excited states of gas-phase helium as an example. A complete set of logic gates—X(N)OR, (N)AND, and (N)OR—is realized for the 2s3p FWM signal at 63.66 eV with switching contrasts of 3.6 to 10.4. This theoretical study extends all-optical logic switching to the XUV and x-ray regimes and opens a new pathway for ultrafast photonic logic.

Accessing coherences is key to fully understand and control ultrafast dynamics of complex quantum systems like molecules. Most photochemical processes are mediated by conical intersections, which generate coherences between electronic states in molecules. We show with accurate calculations performed on gas-phase methyl iodide that electronic coherences of spin-orbit-split states persist in atomic iodine after dissociation. Our simulation predicts a maximum magnitude of vibronic coherence in the molecular regime of 0.75% of the initially photoexcited state population. Upon dissociation, one-third of this coherence magnitude is transferred to a long-lived atomic coherence where vibrational decoherence can no longer occur. To trace these dynamics, we propose a tabletop experimental approach—heterodyned attosecond four-wave-mixing spectroscopy. This technique can temporally resolve small electronic coherence magnitudes and reconstruct the full complex coherence function via phase cycling. Hence, heterodyned attosecond four-wave-mixing spectroscopy leads the way to a complete understanding and optimal control of spin-orbit-coupled electronic states in photochemistry.

The reported "dissociation times" for the Br₂ C (¹Π_u 1_u) state by various measurement methods differ widely across the literature (30 to 340 fs). We consider this issue by leveraging attosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the Br M_4,5 3d_3/2,5/2 edges (66 to 80 eV), tracking core-to-valence (3d → 4p) and core-to-Rydberg (3d → ns, np, n ≥ 5) transitions from the molecular to the atomic limit. The progress of dissociation can be ascertained by the buildup of the atomic absorption in time. Notably, the measured rise times of the 3d_5/2,3/2 → 4p transitions depend on the probed core level final state, 38 ± 1 and 20 ± 5 fs for ²D_5/2 and ²D_3/2 at 64.31 and 65.34 eV, respectively. Simulations by the nuclear time-dependent Schrödinger equation reproduce the rise-time difference of the 3d → 4p transitions, and the theory suggests several important factors. One is the transition dipole moments of each probe transition have different molecular and atomic values for ²D_5/2 versus ²D_3/2 that depend on the bond length. The other is the merger of multiple molecular absorptions into the same atomic absorption, creating multiple time scales even for a single probe transition. On the other hand, the core-to-Rydberg absorptions did not allow accurate atomic Br buildup times to be extracted due to spectral overlaps with ground state bleaching, otherwise an even more comprehensive picture of the role of the probe state transition would be possible. This work shows that the measured XUV probe signals accurately contain the dissociative wavepacket dynamics but also reveal how the specific core-to-valence transition affects the apparent progress toward dissociation with bond length. These results highlight the potential probe-transition-dependent effects that need to be considered when interpreting measured signals and their time scales.

The decay of core-excited states is a sensitive probe of autoionization dynamics and correlation effects in many-electron systems, occurring on ultrafast timescales. Xenon, with its dense manifold of autoionizing resonances that can be coupled with near-infrared light, provides a platform to investigate these processes. In this work, the autoionization decay lifetimes of 4d−16ℓ (ℓ = s, p, d, …) core-excited states in xenon atoms are probed with extreme ultraviolet (XUV) attosecond noncollinear four-wave-mixing (FWM) spectroscopy. The 4d{5/2,3/2}−16p XUV-bright states (optically dipole-allowed) exhibit decay lifetimes of ∼6 fs, which is consistent with spectator-type decay. In contrast, the 4d{5/2,3/2}−16s and 4d{5/2,3/2}−16d XUV-dark states (optically dipole forbidden) show longer decay lifetimes of ∼20 fs. Photoionization calculations confirm that all core-hole states with 4d character should decay via spectator channels in ≤6 fs, suggesting that the apparently longer dark-state decay times arise from an alternative mechanism. A few-level simulation of the FWM process shows that the inclusion of a nearby, longer-lived dark state can mimic the experimental FWM signal, suggesting population cycling with a second electronic state with non-4d character. Ab initio calculations support the presence of such multielectron excited states in the 60–70 eV range. These results demonstrate that FWM signals can encode coupled-state dynamics when probing complex systems, highlighting the importance of combining theoretical and experimental approaches to disentangle accurate core-level decay pathways and lifetimes.

Circularly polarized light offers opportunities to probe symmetry-dependent properties of matter such as chirality and spin. Circular-dichroic measurements typically require further intrinsic or extrinsic breaking of symmetry by, e.g., enantiomeric excess, orientation, magnetic fields, or direction-sensitive detectors. Here we introduce circular-dichroic attosecond transient absorption spectroscopy by leveraging the angular momentum of two circularly polarized pulses, both pump and probe, in an isotropic medium, optically orienting the angular momentum of excited states on an attosecond timescale. We investigate a circular-dichroic measurement of the attosecond transient absorption of He Rydberg states. By limiting the allowed pathways via dipole selection rules for co- and counterrotating circularly polarized near-infrared and extreme ultraviolet (XUV) pulses, different spectral reshapings of the XUV transient absorption due to the AC Stark effect are observed. Paired with time-dependent Schrödinger equation calculations, the results show the role of selection and propensity rules and open up new opportunities to study coupling pathways of excited states as well as spin-dependent dynamics in atoms and beyond via attosecond optical orientation.