Dave Townsend

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright (1)L(a) and (1)L(b) electronic states of (1)ππ∗ character and spectroscopically dark and dissociative (1)πσ∗ states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited (1)L(a) state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived (1)L(b) state or the (1)πσ∗ state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the (1)πσ∗ state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.

Caveats in the interpretation of time-resolved photoionization measurements: A photoelectron imaging study of pyrrole

We report time-resolved photoelectron imaging studies of gas-phase pyrrole over the 267-240 nm excitation region, recorded in conjunction with a 300 nm probe. Of specific interest is the lowest-lying (3s/πσ*) state, which exhibits very weak oscillator strength but is thought to be excited directly at wavelengths ≤254 nm. We conclude, however, that the only significant contribution to our photoelectron data at all wavelengths investigated is from non-resonant ionization. Our findings do not rule out (3s/πσ*) state excitation (as appears to be confirmed by supporting time-resolved ion-yield measurements) but do potentially highlight important caveats regarding the use and interpretation of photoreactant ionization measurements to interrogate dynamical processes in systems exhibiting significant topological differences between the potential energy surfaces of the neutral and cation states.

Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source

Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond ultraviolet laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C3H4N2O+ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theoretically predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold molecular beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths <250 nm, revealing extremely rapid (<200 fs) relaxation of the bright S3(1ππ*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility.

Structural dynamics: general discussion

Piero Decleva, Andrew J. Orr-Ewing, Markus Kowalewski, Oleg Kornilov, Jon P. Marangos, Hans Jakob Wörner, Allan S. Johnson, Ruaridh Forbes, Daniel Rolles, Dave Townsend, Oliver Schalk, Sebastian Mai, Tom J. Penfold, R. J. Dwayne Miller, Martin Centurion, Kiyoshi Ueda, Wolfgang Domcke, Peter M. Weber, Kyoung Koo Baeck, Oksana Travnikova, Chelsea Liekhus-Schmaltz, João Pedro Figueira Nunes, Daniel M. Neumark, Oliver Gessner, Albert Stolow, Artem Rudenko, Pankaj Kumar Mishra, Adam Kirrander, Danielle Dowek, Fernando Mart́ın, Ágnes Vibók, Michael P. Minitti, Brian Stankus and Christian Burger

Electronic and non-adiabatic dynamics

Andrew J. Orr-Ewing, Jan R. R. Verlet, Tom J. Penfold, Russell S. Minns, Michael P. Minitti, Theis I. Sølling, Oliver Schalk, Markus Kowalewski, Jon P. Marangos, Michael A. Robb, Allan S. Johnson, Hans Jakob Worner, Dmitrii V. Shalashilin, R. J. Dwayne Miller, ̈ Wolfgang Domcke, Kiyoshi Ueda, Peter M. Weber, Raluca Cireasa, Morgane Vacher, Gareth M. Roberts, Piero Decleva, Filippo Bencivenga, Daniel M. Neumark, Oliver Gessner, Albert Stolow, Pankaj Kumar Mishra, Iakov Polyak, Kyoung Koo Baeck, Adam Kirrander, Danielle Dowek, Alvaro ́ Jimenez-Galan, Fernando Martın, Shaul Mukamel, ́ ́ ́ Taro Sekikawa, Maxim F. Gelin, Dave Townsend, Dmitry V. Makhov and Simon P. Neville