Timescales for adiabatic photodissociation dynamics from the à state of ammonia

Femtosecond Electronic and Hydrogen Structural Dynamics in Ammonia Imaged with Ultrafast Electron Diffraction

Directly imaging structural dynamics involving hydrogen atoms by ultrafast diffraction methods is complicated by their low scattering cross sections. Here we demonstrate that megaelectronvolt ultrafast electron diffraction is sufficiently sensitive to follow hydrogen dynamics in isolated molecules. In a study of the photodissociation of gas phase ammonia, we simultaneously observe signatures of the nuclear and corresponding electronic structure changes resulting from the dissociation dynamics in the time-dependent diffraction. Both assignments are confirmed by ab initio simulations of the photochemical dynamics and the resulting diffraction observable. While the temporal resolution of the experiment is insufficient to resolve the dissociation in time, our results represent an important step towards the observation of proton dynamics in real space and time.

Photochemical carbon-sulfur bond cleavage in thioethers mediated via excited state Rydberg-to-valence evolution

Time-resolved photoelectron imaging and supporting ab initio quantum chemistry calculations were used to investigate non-adiabatic excess energy redistribution dynamics operating in the saturated thioethers diethylsulfide, tetrahydrothiophene and thietane. In all cases, 200 nm excitation leads to molecular fragmentation on an ultrafast (<100 fs) timescale, driven by the evolution of Rydberg-to-valence orbital character along the S-C stretching coordinate. The C-S-C bending angle was also found to be a key coordinate driving initial internal conversion through the excited state Rydberg manifold, although only small angular displacements away from the ground state equilibrium geometry are required. Conformational constraints imposed by the cyclic ring structures of tetrahydrothiophene and thietane do not therefore influence dynamical timescales to any significant extent. Through use of a high-intensity 267 nm probe, we were also able to detect the presence of some transient (bi)radical species. These are extremely short lived, but they appear to confirm the presence of two competing excited state fragmentation channels - one proceeding directly from the initially prepared 4p manifold, and one involving non-adiabatic population of the 4s state. This is in addition to a decay pathway leading back to the S₀ electronic ground state, which shows an enhanced propensity in the 5-membered ring system tetrahydrothiophene over the other two species investigated.

The ultrafast vibronic dynamics of ammonia's D̃ state

Using vacuum-ultraviolet time-resolved velocity map imaging of photoelectrons, we study ultrafast coupled electronic and nuclear dynamics in low-lying Rydberg states of ammonia. Vibrationally-resolved internal vibrational relaxation (IVR) is observed in a progression of the e' bending modes. This vibrational progression is only observed in the D̃ state, and is lost upon ultrafast internal conversion to the C̃ and B̃ electronic states. Due to the ultrashort time scale of the internal conversion (ca. 64 fs), and the vibronic resolution, the non-adiabatic coupling vectors are identified and verified with ab initio calculations. The time-scale of this IVR process is highly surprising and significant because IVR is usually treated as an incoherent process that proceeds statistically, according to a "Fermi's Golden Rule"-like model, where the process scales with the available degrees of freedom. Here, we show that it can be highly non-statistical, restricted to only a very small subset of vibrational motions.

Mode-specific excited-state dynamics of N-methylpyrrole

State-selective deactivation rates of N-methylpyrrole in the S₁ state have been measured by using the picosecond pump-probe method. The S₁ decay time leading to the N-CH₃ bond dissociation is found to be strongly mode-dependent as manifested in both S₁ decay and methyl-fragment growth dynamics. Time-resolved velocity-map ion images of the ˙CH₃ fragment, as far as the fragment of the Gaussian-shaped high kinetic energy distribution is concerned, suggest that the N-CH₃ cleavage reaction might occur through an intermediate. Sudden decrease of the S₁ lifetime at ∼700 cm^-1 above the S₁ origin is accompanied by the fragmentation of the Boltzmann-type low kinetic energy distribution. The appearance rate of this low-kinetic energy fragment turns out to be quite slow to give τ∼ 5 ns compared to the S₁ lifetime of ∼174 ps at the +806 cm^-1 band, for instance, confirming previous findings that the S₁ decay process starts to be overwhelmed by a new fast nonradiative transition in the corresponding excitation energy region. The lifetime at the S₁ origin accessed by the two-photon absorption is firstly measured to give τ∼ 8 ns. Using one and two photon absoption processes, a number of S₁ vibronic bands are identified to give mode-dependent lifetimes spanning an enormously wide temporal range of 8 ns-5 ps in the quite narrow excitation energy region of 0-1800 cm^-1 above the S₁ origin. Understanding of the N-methylpyrrole dynamics on multidimensional excited-state potential energy surfaces governing energy dissipating processes will get much benefit from our detailed mode-specific lifetime measurements.

Multidimensional H Atom Tunneling Dynamics of Phenol: Interplay between Vibrations and Tunneling

Multidimensional facets of the hydrogen tunneling dynamics of phenol excited in S₁ (ππ*) have been unraveled to give particular S₁ vibronic states strongly coupled or actively decoupled to the O-H tunneling coordinate. Strong mode-dependent variation of the tunneling rate measured with picosecond lasers indicates that tunneling probability is extremely sensitive to low-frequency vibrational modes seemingly orthogonal to the O-H elongation coordinate unless the rate of energy randomization exceeds that of tunneling. The multidimensional nature of tunneling has also been manifested in efficient internal-to-translational energy transfers observed at S₁ vibronic modes strongly coupled to the tunneling coordinate, giving insights into otherwise the formidable multidimensional map of tunneling process. The nonadiabatic bifurcation dynamics in the later stage of the chemical reaction has been disentangled by analyzing picosecond time-resolved product state distributions, resolving a long controversial issue regarding the origin of high or low kinetic energy component of the product translational energy distributions.

From fundamental science to product: a bottom-up approach to sunscreen development

Despite the pivotal role of ultraviolet (UV) radiation in sustaining life on Earth, overexposure to this type of radiation can have catastrophic effects, such as skin cancer. Sunscreens, the most common form of artificial protection against such harmful effects, absorb UV radiation before it reaches vulnerable skin cells. Absorption of UV radiation prompts ultrafast molecular events in sunscreen molecules which, ideally, would allow for fast and safe dissipation of the excess energy. However, our knowledge of these mechanisms remains limited. In this article, we will review recent advances in the field of ultrafast photodynamics (light induced molecular processes occurring within femtoseconds, fs, 10-15 s to picoseconds, ps, 10-12 s) of sunscreens. We follow a bottom-up approach to common sunscreen active ingredients, analysing any emerging trends from the current literature on the subject. Moreover, we will identify the main questions that remain unanswered, pinpoint some of the main challenges and finally comment on the outlook of this exciting field of research.

Hydrogen Bonds in Excited State Proton Transfer

Hydrogen bonding interactions between biological chromophores and their surrounding protein and solvent environment significantly affect the photochemical pathways of the chromophore and its biological function. A common first step in the dynamics of these systems is excited state proton transfer between the noncovalently bound molecules, which stabilizes the system against dissociation and principally alters relaxation pathways. Despite such fundamental importance, studying excited state proton transfer across a hydrogen bond has proven difficult, leaving uncertainties about the mechanism. Through time-resolved photoelectron imaging measurements, we demonstrate how the addition of a single hydrogen bond and the opening of an excited state proton transfer channel dramatically changes the outcome of a photochemical reaction, from rapid dissociation in the isolated chromophore to efficient stabilization and ground state recovery in the hydrogen bonded case, and uncover the mechanism of excited state proton transfer at a hydrogen bond, which follows sequential hydrogen and charge transfer processes.

Resonant multiphoton ionisation probe of the photodissociation dynamics of ammonia

The dissociation dynamics of the Ã-state of ammonia have been studied using a resonant multiphoton ionisation probe in a photoelectron spectroscopy experiment. The use of a resonant intermediate in the multiphoton ionisation process changes the ionisation propensity, allowing access to different ion states when compared with equivalent single photon ionisation experiments. Ionisation through the E' ¹A₁' Rydberg intermediate means we maintain overlap with the ion state for an extended period, allowing us to monitor the excited state population for several hundred femtoseconds. The vibrational states in the photoelectron spectrum show two distinct timescales, 200 fs and 320 fs, that we assign to the non-adiabatic and adiabatic dissociation processes respectively. The different timescales derive from differences in the wavepacket trajectories for the two dissociation pathways that resonantly excite different vibrational states in the intermediate Rydberg state. The timescales are similar to those obtained from time resolved ion kinetic energy release measurements, suggesting we can measure the different trajectories taken out to the region of conical intersection.

Ultrafast relaxation dynamics of electronically excited piperidine: ionization signatures of Rydberg/valence evolution

Time-resolved photoelectron spectroscopy reveals distinct ionization signatures of Rydberg-to-valence state evolution in the secondary aliphatic amine piperidine.

Accurate nonadiabatic dynamics

This Perspective addresses the use of coupled diabatic potential energy surfaces (PESs) together with rigorous quantum dynamics in full or reduced dimensional coordinate spaces to obtain accurate solutions to problems in nonadiabatic dynamics.

Time-resolved photoionization spectroscopy of mixed Rydberg-valence states: indole case study

Time-resolved photoelectron imaging reveals subtle new mechanistic insight into the ultraviolet relaxation dynamics of gas-phase indole.

Ultrafast dynamics of electrons in ammonia

Solvated electrons were first discovered in solutions of metals in liquid ammonia. The physical and chemical properties of these species have been studied extensively for many decades using an arsenal of electrochemical, spectroscopic, and theoretical techniques. Yet, in contrast to their hydrated counterpart, the ultrafast dynamics of ammoniated electrons remained completely unexplored until quite recently. Femtosecond pump-probe spectroscopy on metal-ammonia solutions and femtosecond multiphoton ionization spectroscopy on the neat ammonia solvent have provided new insights into the optical properties and the reactivities of this fascinating species. This article reviews the nature of the optical transition, which gives the metal-ammonia solutions their characteristic blue appearance, in terms of ultrafast relaxation processes involving bound and continuum excited states. The recombination processes following the injection of an electron via photoionization of the solvent are discussed in the context of the electronic structure of the liquid and the anionic defect associated with the solvated electron.