Adam S. Chatterley

Direct Observation of Hydrogen Tunneling Dynamics in Photoexcited Phenol

The excited-state dynamics of phenol following ultraviolet (UV) irradiation have received considerable interest in recent years, most notably because they can provide a model for understanding the UV-induced dynamics of the aromatic amino acid tyrosine. Despite this, there has been some debate as to whether hydrogen tunneling dynamics play a significant role in phenols excited-state O-H bond fission when UV excitation occurs below the (1)ππ*/(1)πσ* conical intersection (CI). In this Letter, we present direct evidence that (1)πσ*-mediated O-H bond fission below the (1)ππ*/(1)πσ* CI proceeds exclusively through hydrogen tunneling dynamics. The observation of hydrogen tunneling may have some parallels with proton tunneling dynamics from tyrosine residues (along the O-H bond of the phenol moiety) in a wide range of natural enzymes, potentially adding further justification for utilizing phenols as model systems for investigating tyrosine-based dynamics.

Taking the green fluorescence out of the protein: dynamics of the isolated GFP chromophore anion

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.

Probing ultrafast dynamics in photoexcited pyrrole: timescales for 1πσ* mediated H-atom elimination

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.

Time-resolved photoelectron imaging of the isolated deprotonated nucleotides

Using time-resolved photoelectron spectroscopy, the excited state dynamics of gas-phase mass-selected nucleotide anions have been monitored following UV excitation at 4.66 eV. The spectra reveal that the dynamics of the 2′-deoxyguanosine 5′-monophosphate anion (dGMP−) are very similar to those of the adenosine nucleotide (dAMP−) and are insensitive to solvent. Comparison of our results with other literature suggests that nucleotides of the two purine bases share a common relaxation pathway, whereby the initially populated 1ππ* states relax to the ground electronic state without involvement of any other intermediary electronic states. In the analogous pyrimidine nucleotides of thymine and cytosine, dTMP− and dCMP−, no such unified mechanism is observed. Photoexcited dTMP− behaves much like the isolated nucleobase thymine, exhibiting rapid relaxation to the ground electronic state, although with a minor long-lived channel. On the other hand, isolated dCMP− is longer lived than its cytosine nucleobase, and hence it appears that the presence of the sugar and phosphate in the nucleotide arrangement leads to a modification of the available relaxation pathways. Nucleotides are the basic monomer building blocks of DNA and our results present important new benchmark data to develop an understanding of the molecular mechanism by which photodamage can be mediated when DNA is exposed to UV light.

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.

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier.

Many properties of isolated multiply-charged anions (MCAs) are dictated by the strong intra-molecular Coulomb interactions that are present. The most striking property of MCAs is a long-range repulsive Coulomb barrier (RCB) that arises from the repulsive interaction between an electron and an anion which must be overcome to form a MCA. Excited states provide a route to probing this RCB and the focus of this Perspective is on recent photoelectron experiments, including angularly and temporally resolved, that have provided detailed physical insight into the RCB surfaces, their anisotropy, and their use to monitor molecular dynamics in real-time. An outlook provides some future prospects that studies on MCAs provide in terms of monitoring structural, charge-migration, and solvation dynamics.

Base-specific ionization of deprotonated nucleotides by resonance enhanced two-photon detachment

The intrinsic ionization energy of a base in DNA plays a critical role in determining the energies at which damage mechanisms may emerge. Here, a two-photon resonance-enhanced ionization scheme is presented that utilizes the (1)ππ* transition, localized on the DNA base, to elucidate the base-specific ionization in a deprotonated nucleotide. In contrast to previous reports, the scheme is insensitive to competing ionization channels arising from the sugar-phosphate backbone. Using this approach, we demonstrate that for all bases except guanine, the lowest electron detachment energy corresponds to detachment from the sugar-phosphate backbone and allows us to determine the lowest adiabatic ionization energy for the other three bases for the first time in an isolated nucleotide.

Strongly aligned molecules inside helium droplets in the near-adiabatic regime

Iodine (I2) molecules embedded in He nanodroplets are aligned by a 160 ps long laser pulse. The highest degree of alignment, occurring at the peak of the pulse and quantified by ⟨cos2𝜃2D⟩, is measured as a function of the laser intensity. The results are well described by ⟨cos2𝜃2D⟩ calculated for a gas of isolated molecules each with an effective rotational constant of 0.6 times the gas-phase value and at a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to describe rotating molecules in superfluid helium rationalizes why the alignment mechanism is similar to that of isolated molecules with an effective rotational constant. A major advantage of molecules in He droplets is that their 0.4 K temperature leads to stronger alignment than what can generally be achieved for gas phase molecules-here demonstrated by a direct comparison of the droplet results to measurements on a ∼1 K supersonic beam of isolated molecules. This point is further illustrated for a more complex system by measurements on 1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species studied, the highest values of ⟨cos2𝜃2D⟩ achieved in He droplets exceed 0.96.

Relaxation dynamics of photoexcited resorcinol: internal conversion versus H atom tunnelling

The excited state dynamics of resorcinol (1,3-dihydroxybenzene) following UV excitation at a range of pump wavelengths, 278 ≥ λ ≥ 255 nm, have been investigated using a combination of time-resolved velocity map ion imaging and ultrafast time-resolved ion yield measurements coupled with complementary ab initio calculations. After excitation to the 1(1)ππ* state we extract a timescale, τ1, for excited state relaxation that decreases as a function of excitation energy from 2.70 ns to ~120 ps. This is assigned to competing relaxation mechanisms. Tunnelling beneath the 1(1)ππ*/(1)πσ* conical intersection, followed by coupling onto the dissociative (1)πσ* state, yields H atoms born with high kinetic energy (~5000 cm(-1)). This mechanism is in competition with an internal conversion process that is able to transfer population from the photoexcited 1(1)ππ* state back to a vibrationally excited ground state, S0*. When exciting between 264-260 nm a second decay component, τ2, is observed and we put forth several possible explanations as to the origins of τ2, including conformer specific dynamics. Excitation with 237 nm light (above the 1(1)ππ*/(1)πσ* conical intersection) yields high kinetic energy H atoms (~11,000 cm(-1)) produced in ~260 fs, in line with a mechanism involving ultrafast coupling between the 1(1)ππ* (or 2(1)ππ*) and (1)πσ* state followed by dissociation. The results presented highlight the profound effect the presence of additional functional groups, and more specifically the precise location of the functional groups, can have on the excited state dynamics of model heteroaromatic systems following UV excitation.

Direct observation of ring-opening dynamics in strong-field ionized selenophene using femtosecond inner-shell absorption spectroscopy

Femtosecond extreme ultraviolet transient absorption spectroscopy is used to explore strong-field ionization induced dynamics in selenophene (C4H4Se). The dynamics are monitored in real-time from the viewpoint of the Se atom by recording the temporal evolution of element-specific spectral features near the Se 3d inner-shell absorption edge (∼58 eV). The interpretation of the experimental results is supported by first-principles time-dependent density functional theory calculations. The experiments simultaneously capture the instantaneous population of stable molecular ions, the emergence and decay of excited cation states, and the appearance of atomic fragments. The experiments reveal, in particular, insight into the strong-field induced ring-opening dynamics in the selenophene cation, which are traced by the emergence of non-cyclic molecules as well as the liberation of Se+ ions within an overall time scale of approximately 170 fs. We propose that both products may be associated with dynamics on the same electronic surfaces but with different degrees of vibrational excitation. The time-dependent inner-shell absorption features provide direct evidence for a complex relaxation mechanism that may be approximated by a two-step model, whereby the initially prepared, excited cyclic cation decays within τ1 = 80 ± 30 fs into a transient molecular species, which then gives rise to the emergence of bare Se+ and ring-open cations within an additional τ2 = 80 ± 30 fs. The combined experimental and theoretical results suggest a close relationship between σ* excited cation states and the observed ring-opening reactions. The findings demonstrate that the combination of femtosecond time-resolved core-level spectroscopy with ab initio estimates of spectroscopic signatures provide new insights into complex, ultrafast photochemical reactions such as ring-opening dynamics in organic molecules in real-time and with simultaneous sensitivity for electronic and structural rearrangements.