Photoelectric emission from the alkali metal doped vacuum-ice interface

The photoelectron photoemission spectra and thresholds for low coverages of Li and K adsorbed on water-ice have been measured, compared with photoionization spectra of the gas-phase atoms, and modeled by quantum chemical calculations. For both alkali metals the threshold for photoemission is dramatically decreased and the cross section increased on adsorption to the water-ice surface. Quantum chemical calculations suggest that the initial state is formed by the metal atoms adsorbed into the water-ice surface, forming a state with a delocalized electron distribution. This state is metastable and decays on the hundreds of seconds time scale at 92 K. The decay is markedly faster for Li than for K, probably due to diffusion into the ice film

Time resolved structural dynamics of butadiyne-linked porphyrin dimers

In this work the timescales and mechanisms associated with the structural dynamics of butadiyne-linked porphyrin dimers are investigated through time resolved narrowband pump / broadband probe transient absorption spectroscopy. Our results confirm previous findings that the broadening is partly due to a distribution of structures with different (dihedral) angular conformations. Comparison of measurements with excitations on the red and blue sides of the Q-band unravel the ground and excited state conformational re-equilibration timescales. Further comparison to a planarized dimer, through addition of a ligand, provide conclusive evidence for the twisting motion performed by the porphyrin dimer in solution

Ultrafast Structure and Dynamics in the Thermally Activated Delayed Fluorescence of a Carbene-Metal-Amide

Thermally activated delayed fluorescence has enormous potential for the development of efficient light emitting diodes. A recently discovered class of molecules (the carbene – metal – amides, CMAs) are exceptionally promising as they combine the small singlet - triplet energy gap required for thermal activation with a large transition moment for emission. Calculations suggest that excited state structural dynamics modulate the critical coupling between singlet and triplet states, but do not agree on the nature of those dynamics. Here we report ultrafast time resolved transient absorption and Raman studies of CMA photodynamics. The measurements reveal complex structural evolution following intersystem crossing on the tens to hundreds of picoseconds timescale, and a change in the low frequency vibrational spectrum between singlet and triplet states. The latter is assigned to a change in frequency or amplitude associated with a Raman active mode localized on the metal centre

Mapping the Excited‐State Potential Energy Surface of a Photomolecular Motor

A detailed understanding of the operation and efficiency of unidirectional photomolecular rotary motors is essential for their effective exploitation in molecular nanomachines. Unidirectional motion relies on light‐driven conversion from a stable (1 a) to a metastable (1 b) conformation, which then relaxes through a thermally driven helix inversion in the ground state. The excited‐state surface has thus far only been experimentally characterised for 1 a. Here we probe the metastable, 1 b, excited state, utilising ultrafast transient absorption and femtosecond stimulated Raman spectroscopy. These reveal that the “dark” excited‐state intermediate between 1 a and 1 b has a different lifetime and structure depending on the initial ground‐state conformation excited. This suggests that the reaction coordinate connecting 1 a to 1 b differs to that for the reverse photochemical process. The result is contrasted with earlier calculations

Vibrational coherences in broadband 2D electronic spectroscopy: spectral filtering vs. excited state displacement

Coherences in ultrafast 2D electronic spectroscopy (2DES) reveal superpositions of quantum states corresponding to the motion of wavepackets within the potential energy surface of molecular systems. Whilst electronic coherences imply the transfer of energy between coupled chromophores, vibrational coherences track the motion of nuclear wavepackets, with their intensities governed by the displacement of the electronic excited states with respect to the ground state equilibrium geometry. Analysis of vibrational coherences thus provides valuable information on the ground and excited state structure of molecules, with ground state bleach (GSB) and stimulated emission (SE) pathways reporting on the S0 – S1 displacement and excited state absorption (ESA) pathways also involving an S1 – Sn displacement. Recent development of broadband 2DES experiments have enabled access to a greater range of coherences involving higher energy electronic states. However, a complete analysis must consider involvement of multiple vibrational modes, and any filtering of Liouville pathways due to the finite width of the excitation spectrum. Here, combining the equation of motion-phase matching approach for finite laser spectra with the hierarchical equation of motion to correctly account for dephasing and dissipation, we model half-broadband and broadband 2DES of cresyl violet to demonstrate the impact of spectral filtering vs. the relative displacement of two excited states (S1 and Sn) on the intensity distribution of peaks in the beating maps for two vibrational modes with frequencies 350 cm-1 and 585 cm-1. This study is motivated by recent experimental results from our group which interestingly show the greatest intensity of the beating maps for the 350 cm 1 mode localised in the excited state absorption region