On the Wavelength-Dependent Photochemistry of the Atmospheric Molecule CF3COCl

The wavelength control of photochemistry usually results from ultrafast dynamics following the excitation of different electronic states. Here, we investigate the CF3COCl molecule, exhibiting wavelength-dependent photochemistry both via (i) depositing increasing internal energy into a single state and (ii) populating different electronic states. We reveal the mechanism behind the photon-energy dependence by combining nonadiabatic ab initio molecular dynamics techniques with the velocity map imaging experiment. We describe a consecutive mechanism of photodissociation where an immediate release of Cl taking place in an excited electronic state is followed by a slower ground-state dissociation of the CO fragment. The CO release is subject to an activation barrier and is controlled by excess internal energy via the excitation wavelength. Therefore, a selective release of CO along with Cl can be achieved. The mechanism is fully supported by both the measured kinetic energy distributions and anisotropies of the angular distributions. Interestingly, the kinetic energy of the released Cl atom is sensitively modified by accounting for spin-orbit coupling. Given the atmospheric importance of CF3COCl, we discuss the consequences of our findings for atmospheric photochemistry.

Controlling the Spatial and Momentum Distributions of Plasmonic Carriers: Volume vs Surface Effects

Spatial and momentum distributions of excited charge carriers in nanoplasmonic systems depend sensitively on optical excitation parameters and nanoscale geometry, which therefore control the efficiency and functionality of plasmon-enhanced catalysts, photovoltaics, and nanocathodes. Growing appreciation over the past decade for the different roles of volume- vs surface-mediated excitation in such systems has underscored the need for explicit separation and quantification of these pathways. Toward these ends, we utilize angle-resolved photoelectron velocity map imaging to distinguish these processes in gold nanorods of different aspect ratios down to the spherical limit. Despite coupling to the longitudinal surface plasmon, we find that resonantly excited nanorods always exhibit transverse (sideways) multiphoton photoemission distributions due to photoexcitation within volume field enhancement regions rather than at the tip hot spots. This behavior is accurately reproduced via ballistic Monte Carlo modeling, establishing that volume-excited electrons primarily escape through the nanorod sides. Furthermore, we demonstrate optical control over the photoelectron angular distributions via a screening-induced transition from volume (transverse/side) to surface (longitudinal/tip) photoemission with red detuning of the excitation laser. Frequency-dependent cross sections are separately quantified for these mechanisms by comparison with theoretical calculations, combining volume and surface velocity-resolved photoemission modeling. Based on these results, we identify nanomaterial-specific contributions to the photoemission cross sections and offer general nanoplasmonic design principles for controlling photoexcitation/emission distributions via geometry- and frequency-dependent tuning of the volume vs surface fields.

Time-resolved ion imaging at free-electron lasers using TimepixCam

The application of a novel fast optical-imaging camera, TimepixCam, to molecular photoionization experiments using the velocity-map imaging technique at a free-electron laser is described. TimepixCam is a 256 × 256 pixel CMOS camera that is able to detect and time-stamp ion hits with 20 ns timing resolution, thus making it possible to record ion momentum images for all fragment ions simultaneously and avoiding the need to gate the detector on a single fragment. This allows the recording of significantly more data within a given amount of beam time and is particularly useful for pump-probe experiments, where drifts, for example, in the timing and pulse energy of the free-electron laser, severely limit the comparability of pump-probe scans for different fragments taken consecutively. In principle, this also allows ion-ion covariance or coincidence techniques to be applied to determine angular correlations between fragments.

Velocity of a Molecule Evaporated from a Water Nanodroplet: Maxwell-Boltzmann Statistics versus Non-Ergodic Events

The velocity of a molecule evaporated from a mass-selected protonated water nanodroplet is measured by velocity map imaging in combination with a recently developed mass spectrometry technique. The measured velocity distributions allow probing statistical energy redistribution in ultimately small water nanodroplets after ultrafast electronic excitation. As the droplet size increases, the velocity distribution rapidly approaches the behavior expected for macroscopic droplets. However, a distinct high-velocity contribution provides evidence of molecular evaporation before complete energy redistribution, corresponding to non-ergodic events.

Does Infrared Multiphoton Dissociation of Vinyl Chloride Yield Cold Vinylidene?

Velocity map imaging of the infrared multiphoton dissociation of vinyl chloride shows the formation of HCl in rotational levels below J = 10 that are associated with the three-center elimination pathway. The total translational energy release is observed to peak at 3-5 kcal/mol, which is consistent with the low reverse barrier predicted for the formation of HCl with vinylidene coproducts. Direct dynamics trajectory studies from the three-center transition state reproduce the observed distributions and show that the associated vinylidene is formed with only modest rotational excitation, precluding Coriolis-induced mixing among the excited vibrational levels of acetylene that would lead to distribution of vinylidene character into many vibrationally mixed acetylene vibrational levels. The results suggest that infrared multiphoton dissociation of vinyl chloride is an efficient route to synthesis of stable, cold vinylidene.

Femtosecond Photoelectron Imaging of Aligned Polyanions: Probing Molecular Dynamics through the Electron-Anion Coulomb Repulsion

The first time-resolved photoelectron imaging study of a polyanion is presented. Using the alignment induced through resonance excitation, the photoelectron angular distributions can be qualitatively understood in terms of the position of localized excess charges on the molecular skeleton, which influence the photoemission dynamics. Pump-probe experiments are used to demonstrate that the photoelectron angular distribution is also sensitive to molecular dynamics. This is shown here for the rotational dynamics of a polyanion, in which the photoelectron anisotropy tracks the rotational coherence as it dephases. The methodology can in principle be applied to general molecular dynamics in large polyanions, providing a new route to studying ultrafast structural dynamics in complex gas-phase systems.