We used two-dimensional infrared spectroscopy to disentangle the broad infrared band in the amide II vibrational regions of Bombyx mori native silk films, identifying the single amide II modes and correlating them to specific secondary structure. Amide I and amide II modes have a strong vibrational coupling, which manifests as cross-peaks in 2D infrared spectra with frequencies determined by both the amide I and amide II frequencies of the same secondary structure. By cross referencing with well-known amide I assignments, we determined that the amide II (N-H) absorbs at around 1552 and at 1530 cm^–1 for helical and β-sheet structures, respectively. We also observed a peak at 1517 cm⁻¹ that could not be easily assigned to an amide II mode, and instead we tentatively assigned it to a Tyrosine sidechain. These results stand in contrast with previous findings from linear infrared spectroscopy, highlighting the ability of multidimensional spectroscopy for untangling convoluted spectra, and suggesting the need for caution when assigning silk amide II spectra.

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250–200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1¹πσ* (¹A₂) state; and (ii) initial photoexcitation to the higher energy ¹ππ* (¹B₂) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1¹πσ* (¹A₂) at 250 nm, we observe a single time-constant of 126 ± 28 fs for N–H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1¹πσ* (¹A₂) surface in the vertical Franck–Condon region, followed by non-adiabatic coupling through a 1¹πσ*/S₀ conical intersection to yield pyrrolyl radicals in their electronic ground state (C₄H₄N()) together with H-atoms. At 238 nm, direct excitation to, and N–H dissociation along, the 1¹πσ* (¹A₂) surface is observed to occur with a time-constant of 46 ± 22 fs. Upon initial population of the ¹ππ* (¹B₂) state at 200 nm, a rapid ¹ππ* (¹B₂) → 1¹πσ* (¹A₂) → N–H fission process takes place within 52 ± 12 fs. In addition to ultrafast N–H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S₀ ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 ± 0.4 ns. Analogous measurements on pyrrole-d₁ reveal that these statistical H-atoms are released only through C–H bond cleavage.

The green fluorescent protein (GFP) is employed extensively as a marker in biology and the life sciences as a result of its spectacular fluorescence properties. Here, we employ femtosecond time-resolved photoelectron spectroscopy to investigate the ultrafast excited state dynamics of the isolated GFP chromophore anion. Excited state population is found to decay bi-exponentially, with characteristic lifetimes of 330 fs and 1.4 ps. Distinct photoelectron spectra can be assigned to each of these timescales and point to the presence of a transient intermediate along the decay coordinate. Guided by ab initio calculations, we assign these observations to twisting about the C–C–C bridge followed by internal conversion to the anion ground state. The dynamics in vacuo are very similar to those observed in solution, despite the difference in absorption spectra between the two media. This is consistent with the protein environment restricting rotation about the C–C–C bond in order to prevent ultrafast internal conversion and preserve the fluorescence.

We discuss how Coulomb explosion imaging (CEI), triggered by intense femtosecond laser pulses and combined with laser-induced alignment and covariance analysis of the angular distributions of the recoiling fragment ions, provides new opportunities for imaging the structures of molecules and molecular complexes. First, focusing on gas phase molecules, we show how the periodic torsional motion of halogenated biphenyl molecules can be measured in real time by timed CEI, and how CEI of one-dimensionally aligned difluoroiodobenzene molecules can uniquely identify four structural isomers. Next, focusing on molecular complexes formed inside He nano-droplets, we show that the conformations of noncovalently bound dimers or trimers, aligned in one or three dimensions, can be determined by CEI. Results presented for homodimers of CS₂, OCS, and bromobenzene pave the way for femtosecond time-resolved structure imaging of molecules undergoing bimolecular interactions and ultimately chemical reactions.

Ultrafast time-resolved velocity map ion imaging (TR-VMI) and time-resolved ion-yield (TR-IY) methods are utilised to reveal a comprehensive picture of the electronic state relaxation dynamics in photoexcited catechol (1,2-dihydroxybenzene). After excitation to the S₁ (¹ππ*) state between 280.5 (the S₁ origin band, S₁(v = 0)) to 243 nm, the population in this state is observed to decay through coupling onto the S₂ (¹πσ*) state, which is dissociative with respect to the non-hydrogen bonded ‘free’ O–H bond (labelled O¹–H). This process occurs via tunnelling under an S₁/S₂ conical intersection (CI) on a timeframe of 5–11 ps, resulting in O¹–H bond fission along S₂. Concomitant formation of ground state catechoxyl radicals (C₆H₅O₂(X)), in coincidence with translationally excited H-atoms, occurs over the same timescale as the S₁ state population decays. Between 254–237 nm, direct excitation to the S₂ state is also observed, manifesting in the ultrafast (∼100 fs) formation of H-atoms with high kinetic energy release. From these measurements we determine that the S₁/S₂ CI lies ∼3700–5500 cm⁻¹ above the S₁(v = 0) level, indicating that the barrier height to tunnelling from S₁(v = 0) → S₂ is comparable to that observed in the related ‘benchmark’ species phenol (hydroxybenzene). We discuss how a highly ‘vibrationally-enhanced’ tunnelling mechanism is responsible for the two orders of magnitude enhancement to the tunnelling rate in catechol, relative to that previously determined in phenol (>1.2 ns), despite similar barrier heights. This phenomenon is a direct consequence of the non-planar S₁ excited state minimum structure (C₁ symmetry) in catechol, which in turn yields relaxed symmetry constraints for vibronic coupling from S₁(v = 0) → S₂ – a scenario which does not exist for phenol. These findings offer an elegant example of how even simple chemical modifications (ortho-hydroxy substitution) to a fundamental, biologically relevant, UV chromophore, such as phenol, can have profound effects on the ensuing excited state dynamics.