17 research outputs found
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 3(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
Ultrafast Non-Radiative Decay of Gas-Phase Nucleosides
De-excitation of DNA nucleosides on picosecond timescales was measured and found to be twice as fast as the equivalent nucleobases.</p
The influence of substituent position on the excited state dynamics operating in 4-, 5- and 6-hydroxyindole
The importance of molecular axis alignment and symmetry-breaking in photoelectron elliptical dichroism
Photoelectron angular distributions (PADs) produced from the photoionization of chiral molecules using elliptically polarized light exhibit a forward/backward asymmetry with respect to the optical propagation direction. By recording these distributions using the velocity-map imaging (VMI) technique, the resulting photoelectron elliptical dichroism (PEELD) has previously been demonstrated as a promising spectroscopic tool for studying chiral molecules in the gas phase. The use of elliptically polarized laser pulses, however, produces PADs (and consequently, PEELD distributions) that do not exhibit cylindrical symmetry about the propagation axis. This leads to significant limitations and challenges when employing conventional VMI acquisition and data processing strategies. Using novel photoelectron image analysis methods based around Hankel transform reconstruction tomography and machine learning, however, we have quantified—for the first time—significant symmetry-breaking contributions to PEELD signals that are of a comparable magnitude to the symmetric terms in the multiphoton ionization of (1R,4R)-(+)- and (1S,4S)-(−)-camphor. This contradicts any assumptions that symmetry-breaking can be ignored when reconstructing VMI data. Furthermore, these same symmetry-breaking terms are expected to appear in any experiment where circular and linear laser fields are used together. This ionization scheme is particularly relevant for investigating dynamics in chiral molecules, but it is not limited to them. Developing a full understanding of these terms and the role they play in the photoionization of chiral molecules is of clear importance if the potential of PEELD and related effects for future practical applications is to be fully realized.</p
Velocity-map imaging of photoelectron circular dichroism in non-volatile molecules using a laser-based desorption source
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Compact intense extreme-ultraviolet source
High-intensity laser pulses covering the ultraviolet to terahertz spectral
regions are nowadays routinely generated in a large number of laboratories. In
contrast, intense extreme-ultraviolet (XUV) pulses have only been demonstrated
using a small number of sources including free-electron laser facilities [1-3]
and long high-harmonic generation (HHG) beamlines [4-9]. Here we demonstrate a
concept for a compact intense XUV source based on HHG that is focused to an
intensity of W/cm, with a potential increase up to
W/cm in the future. Our approach uses tight focusing of the
near-infrared (NIR) driving laser and minimizes the XUV virtual source size by
generating harmonics several Rayleigh lengths away from the NIR focus.
Accordingly, the XUV pulses can be refocused to a small beam waist radius of
600 nm, enabling the absorption of up to four XUV photons by a single Ar atom
in a setup that fits on a modest (2 m) laser table. Our concept represents a
straightforward approach for the generation of intense XUV pulses in many
laboratories, providing novel opportunities for XUV strong-field and nonlinear
optics experiments, for XUV-pump XUV-probe spectroscopy and for the coherent
diffractive imaging of nanoscale structures