Probing photochemical dynamics using electronic vs vibrational sum-frequency spectroscopy: The case of the hydrated electron at the water/air interface.

Photo-dynamics can proceed differently at the water/air interface compared to in the respective bulk phases. Second-order non-linear spectroscopy is capable of selectively probing the dynamics of species in such an environment. However, certain conclusions drawn from vibrational and electronic sum-frequency generation spectroscopies do not agree as is the case for the formation and structure of hydrated electrons at the interface.

The valence electron affinity of uracil determined by anion cluster photoelectron spectroscopy.

The unoccupied π* orbitals of the nucleobases are considered to play important roles in low-energy electron attachment to DNA, inducing damage. While the lowest anionic valence state is vertically unbound in all neutral nucleobases, it remains unclear even for the simplest nucleobase, uracil (U), whether its valence anion (U) is adiabatically bound, which has important implications on the efficacy of damage processes. Using anion photoelectron spectroscopy, we demonstrate that the valence electron affinity (EA) of U can be accurately measured within weakly solvating clusters, U(Ar) and U(N).

Effect of N Atom Substitution on Electronic Resonances: A 2D Photoelectron Spectroscopic and Computational Study of Anthracene, Acridine, and Phenazine Anions.

The accommodation of an excess electron by polycyclic aromatic hydrocarbons (PAHs) has important chemical and technological implications ranging from molecular electronics to charge balance in interstellar molecular clouds. Here, we use two-dimensional photoelectron spectroscopy and equation-of-motion coupled-cluster calculations of the radical anions of acridine (CHN) and phenazine (CHN) and compare our results for these species to those for the anthracene anion (CH). The calculations predict the observed resonances and additionally find low-energy two-particle-one-hole states, which are not immediately apparent in the spectra, and offer a slightly revised interpretation of the resonances in anthracene.

Photoelectron spectroscopy of the deprotonated tryptophan anion: the contribution of deprotomers to its photodetachment channels.

Photoelectron spectroscopy and electronic structure calculations are used to investigate the electronic structure of the deprotonated anionic form of the aromatic amino acid tryptophan, and its chromophore, indole. The photoelectron spectra of tryptophan, recorded at different wavelengths across the UV, consist of two direct detachment channels and thermionic emission, whereas the = 4.66 eV spectrum of indole consists of two direct detachment features.

Dynamics of Anions: From Bound to Unbound States and Everything In Between.

Gas-phase anions present an ideal playground for the exploration of excited-state dynamics. They offer control in terms of the mass, extent of solvation, internal temperature, and conformation. The application of a range of ion sources has opened the field to a vast array of anionic systems whose dynamics are important in areas ranging from biology to star formation.

Spectroscopy and dynamics of the hydrated electron at the water/air interface.

The hydrated electron, e, has attracted much attention as a central species in radiation chemistry. However, much less is known about e at the water/air surface, despite its fundamental role in electron transfer processes at interfaces. Using time-resolved electronic sum-frequency generation spectroscopy, the electronic spectrum of e at the water/air interface and its dynamics are measured here, following photo-oxidation of the phenoxide anion.

Photodissociation of permanganate (MnO) produces the manganese dioxide anion (MnO) in an excited triplet state.

Photoelectron imaging, electron action spectroscopy and electronic structure calculations are used to probe the structure and dynamics of MnO. Following excitation to the first bright absorption band of MnO (1T), photodetachment, ground state electron loss, and photodissociation, to produce MnO, are both observed to occur simultaneously. MnO is produced in an excited electronic state, identified as a triplet state, which indicates that the dissociation proceeds on singlet potential energy surfaces spin conservation.

Alkylated green fluorescent protein chromophores: dynamics in the gas phase and in aqueous solution.

Fluorescent labelling of macromolecular samples, including using the green fluorescent protein (GFP), has revolutionised the field of bioimaging. The ongoing development of fluorescent proteins require a detailed understanding of the photophysics of the biochromophore, and how chemical derivatisation influences the excited state dynamics. Here, we investigate the photophysical properties associated with the S state of three alkylated derivatives of the chromophore in GFP, in the gas phase using time-resolved photoelectron imaging, and in water using femtosecond fluorescence upconversion.

Unraveling the decarboxylation dynamics of the fluorescein dianion with fragment action spectroscopy.

The decarboxylation dynamics of the doubly deprotonated fluorescein dianion, Fl2-, are investigated by recording fragment action spectra for the anion, Fl-, and its decarboxylated analog, Fl-CO2-, using a new reflectron secondary mass spectrometer. The formation of the anion, Fl-, is directly investigated by photoelectron imaging. The Fl- and Fl-CO2- action spectra indicate that, for λ < 400 nm, one-photon dissociative photodetachment, i.

Allophycocyanin A is a carbon dioxide receptor in the cyanobacterial phycobilisome.

Light harvesting is fundamental for production of ATP and reducing equivalents for CO fixation during photosynthesis. However, electronic energy transfer (EET) through a photosystem can harm the photosynthetic apparatus when not balanced with CO. Here, we show that CO binding to the light-harvesting complex modulates EET in photosynthetic cyanobacteria.

Kasha's Rule and Koopmans' Correlations for Electron Tunnelling through Repulsive Coulomb Barriers in a Polyanion.

The long-range electronic structure of polyanions is defined by the repulsive Coulomb barrier (RCB). Excited states can decay by resonant electron tunnelling through RCBs, but such decay has not been observed for electronically excited states other than the first excited state, suggesting a Kasha-type rule for resonant electron tunnelling. Using action spectroscopy, photoelectron imaging, and computational chemistry, we show that the fluorescein dianion, Fl, partially decays through electron tunnelling from the S excited state, thus demonstrating anti-Kasha behavior, and that resonant electron tunnelling adheres to Koopmans' correlations, thus disentangling different channels.

Resonances in nitrobenzene probed by the electron attachment to neutral and by the photodetachment from anion.

We probe resonances (transient anions) in nitrobenzene with the focus on the electron emission from these. Experimentally, we populate resonances in two ways: either by the impact of free electrons on the neutral molecule or by the photoexcitation of the bound molecular anion. These two excitation means lead to transient anions in different initial geometries.

Photooxidation of the Phenolate Anion is Accelerated at the Water/Air Interface.

Molecular photodynamics can be dramatically affected at the water/air interface. Probing such dynamics is challenging, with product formation often probed indirectly through its interaction with interfacial water molecules using time-resolved and phase-sensitive vibrational sum-frequency generation (SFG). Here, the photoproduct formation of the phenolate anion at the water/air interface is probed directly using time-resolved electronic SFG and compared to transient absorption spectra in bulk water.

Photoelectron Imaging Study of the Diplatinum Iodide Dianions [PtI] and [PtI].

Photoelectron spectroscopy has been used to study the electronic structure, photodetachment, and photodissociation of the stable diplatinum iodide dianions [PtI] and [PtI]. Photoelectron spectra over a range of photon energies show the characteristic absence of low kinetic energy photoelectrons expected for dianions as a result of the repulsive Coulomb barrier (RCB). Vertical detachment energies of ∼1.

Effect of Microhydration on the Temporary Anion States of Pyrene.

The influence of incremental hydration (≤4) on the electronic resonances of the pyrene anion is studied using two-dimensional photoelectron spectroscopy. The photoexcitation energies of the resonances do not change; therefore, from the anion's perspective, the resonances remain the same, but from the neutral's perspective of the electron-molecule reaction, the resonances decrease in energy by the binding energy of the water molecules. The autodetachment of the resonances shows that hydration has very little effect, showing that even the dynamics of most of the resonances are not impacted by hydration.

Photoelectron imaging of PtI and its PtI photodissociation product.

The photoelectron imaging of PtI is presented over photon energies ranging from hν = 3.2 to 4.5 eV.

Photochemistry of the pyruvate anion produces CO, CO, CH, CH, and a low energy electron.

The photochemistry of pyruvic acid has attracted much scientific interest because it is believed to play critical roles in atmospheric chemistry. However, under most atmospherically relevant conditions, pyruvic acid deprotonates to form its conjugate base, the photochemistry of which is essentially unknown. Here, we present a detailed study of the photochemistry of the isolated pyruvate anion and uncover that it is extremely rich.

A Hückel Model for the Excited-State Dynamics of a Protein Chromophore Developed Using Photoelectron Imaging.

Chemistry can be described as the movement of nuclei within molecules and the concomitant instantaneous change in electronic structure. This idea underpins the central chemical concepts of potential energy surfaces and reaction coordinates. To experimentally capture such chemical change therefore requires methods that can probe both the nuclear electronic structure simultaneously and on the time scale of atomic motion.

Photo-isomerization of the isolated photoactive yellow protein chromophore: what comes before the primary step?

Photoactive proteins typically rely on structural changes in a small chromophore to initiate a biological response. While these changes often involve isomerization as the "primary step", preceding this is an ultrafast relaxation of the molecular framework caused by the sudden change in electronic structure upon photoexcitation. Here, we capture this motion for an isolated model chromophore of the photoactive yellow protein using time-resolved photoelectron imaging.

Nonadiabatic Dynamics between Valence, Nonvalence, and Continuum Electronic States in a Heteropolycyclic Aromatic Hydrocarbon.

Internal conversion between valence-localized and dipole-bound states is thought to be a ubiquitous process in polar molecular anions, yet there is limited direct evidence. Here, photodetachment action spectroscopy and time-resolved photoelectron imaging with a heteropolycyclic aromatic hydrocarbon (hetero-PAH) anion, deprotonated 1-pyrenol, is used to demonstrate a subpicosecond (τ = 160 ± 20 fs) valence to dipole-bound state internal conversion following excitation of the origin transition of the first valence-localized excited state. The internal conversion dynamics are evident in the photoelectron spectra and in the photoelectron angular distributions (β values) as the electronic character of the excited state population changes from valence to nonvalence.

Time-resolved electronic sum-frequency generation spectroscopy with fluorescence suppression using optical Kerr gating.

Excited state dynamics of molecules at interfaces can be studied using second-order non-linear spectroscopic methods such as time-resolved electronic sum-frequency generation (SFG). However, as such measurements inherently generate very small signals, they are often overwhelmed by signals originating from fluorescence. Here, this limitation is overcome by optical Kerr gating of the SFG signal to discriminate against fluorescence.

Photoelectron spectroscopy of the protoporphyrin IX dianion.

Two-dimensional photoelectron spectroscopy using nanosecond and femtosecond lasers has been used to study the protopophyrin IX dianion at photon energies between 1.8-4.1 eV.