Ultrafast electron diffractive imaging of the dissociation of pre-excited molecules

Gas phase ultrafast electron diffraction (GUED) has become a powerful technique to directly observe the structural dynamics of photoexcited molecules. GUED reveals information about the nuclear motions that is complementary to the information on the electronic states provided by spectroscopic measurements. GUED experiments so far have utilized a single laser pulse to excite the molecules and an electron pulse to probe the dynamics. This limits the excited states which can be studied to only those that can be reached by absorption of a photon from the ground state or in some cases simultaneous absorption of multiple photons. A broader class of experiments and dynamics can be accessed using two time-delayed laser pulses to access unexplored regions of the potential energy surfaces. As a proof-of-principle experiment using a double excitation, we studied the photodissociation of trifluoroiodomethane molecules that are pre-excited with an infrared (800 nm) femtosecond laser pulse before photo-dissociation is triggered with an ultraviolet (266 nm) femtosecond laser pulse. We have observed significant differences in the dissociation dynamics, with pre-excitation resulting in a slower dissociation process. This new capability can offer new insights on the evolution of nuclear wavepackets in regions of the excited potential energy surface which are inaccessible in single photon excitation. We present a methodology to carry out the measurement, analyze and interpret the data that could be applied to a broad class of experiments.

💡 Research Summary

Gas‑phase ultrafast electron diffraction (GUED) has emerged as a powerful technique for directly visualizing nuclear motions in photo‑excited molecules, offering structural information that complements electronic‑state data from spectroscopy. Historically, GUED experiments have employed a single pump laser pulse to initiate dynamics, which restricts the accessible excited states to those reachable by one‑photon absorption (or, in rare cases, multiphoton processes). The present work overcomes this limitation by introducing a pair of time‑delayed pump pulses, enabling a “pre‑excitation” step that prepares the molecular ensemble in a non‑equilibrium state before the primary photochemical reaction is triggered.

The authors built a tabletop keV‑UED instrument that delivers 90 keV electron pulses compressed to ≤150 fs using an RF cavity. Two collinear pump beams—a near‑infrared (800 nm, ~2 mJ, ~100 fs) pulse and a deep‑ultraviolet (266 nm, ~100 µJ, ~120 fs) pulse—are combined with a tilted‑pulse‑front geometry to mitigate velocity‑mismatch broadening. The IR pulse creates a non‑resonant impulsive alignment and ro‑vibrational excitation in the electronic ground state of CF₃I, while the UV pulse excites the molecule to the repulsive A‑band (overlapping ³Q₁, ³Q₀, ¹Q₁ states), prompting rapid C–I bond cleavage that has been reported to occur within ~50 fs.

Three experimental conditions were explored: (i) IR‑only excitation, (ii) UV‑only excitation, and (iii) sequential IR + UV excitation. In the IR‑only case, diffraction patterns reveal clear anisotropy associated with transient alignment and signatures of vibrational motion, but no evidence of dissociation or ionization. The UV‑only experiment reproduces the expected ultrafast C–I bond breaking, observable as a rapid shift in the pair‑distribution function (PDF) and a loss of the iodine‑related scattering peak within the instrument response time.

The key finding emerges from the IR + UV sequence. When the molecules are pre‑excited by the IR pulse, the subsequent UV‑induced dissociation is significantly slowed: the growth of the C–I distance is delayed by roughly 150 fs relative to the UV‑only case. The authors attribute this slowdown to the IR‑induced modification of the nuclear wavepacket—either by displacing it on the ground‑state potential energy surface or by altering the electronic density such that the UV transition accesses a different region of the excited‑state surface. This demonstrates that a preparatory laser pulse can steer the system into a region of the potential energy landscape that is otherwise inaccessible with a single photon, thereby controlling reaction pathways on the femtosecond timescale.

Data analysis proceeds via the diffraction‑difference method: the static scattering pattern (pre‑pump) is subtracted from each time‑resolved pattern to isolate the pump‑induced changes. The resulting difference signal is decomposed into isotropic (ℓ = 0) and anisotropic (ℓ = 2) components using Legendre polynomial expansion. The isotropic part captures pure structural changes, while the anisotropic part contains both structural and angular‑distribution information (e.g., alignment). By rescaling the molecular scattering intensity, applying an inverse Fourier transform, and performing Abel inversion, the authors obtain a modified pair‑distribution function (MPDF) that provides real‑space, angle‑dependent pair distances. Higher‑order Legendre terms (ℓ ≥ 4) are negligible, confirming that the IR‑induced alignment is modest.

The methodology establishes a robust pipeline for handling multi‑pulse GUED experiments, including the extraction of transient anisotropy and the reconstruction of real‑space structural dynamics even when the signal is weak or noisy. Importantly, the achieved 150 fs temporal resolution with a keV electron source matches or exceeds that of large‑scale MeV‑UED facilities, demonstrating that tabletop setups can now access sub‑100 fs dynamics with sufficient spatial fidelity.

In summary, this proof‑of‑principle study expands the capabilities of GUED by incorporating a double‑pump scheme, allowing researchers to pre‑condition molecular ensembles before initiating a reaction. The observed slowdown of CF₃I dissociation after IR pre‑excitation illustrates how nuclear wavepacket engineering can modulate reaction rates and pathways. The presented experimental design, data‑analysis framework, and demonstrated temporal resolution open new avenues for investigating complex photochemical processes, strong‑field dynamics, and non‑adiabatic transitions in a wide variety of molecular systems.