Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution

Abstract

We describe a transient absorption (TA) spectroscopy system in the ultraviolet (UV) spectral range, for the study of the ultrafast optical response of biomolecules. After reviewing the techniques for the generation and characterization of ultrashort UV pulses, we describe the experimental setup of our ultrabroadband UV TA spectrometer. The setup combines sub-20-fs UV pump pulses tunable between 3.35 and 4.7 eV, with broadband white-light-continuum probe pulses in the 1.7-4.6 eV range. Thanks to the broad tunability of the pump pulses in the UV spectral range, the extremely high temporal resolution and the broad spectral coverage of the probe, this TA system is a powerful and versatile tool for the study of many biomolecules. As an example of its potential, we apply the TA spectrometer to track ultrafast internal conversion processes in pyrene after excitation in the UV, and to resolve an impulsively excited molecular vibration with 85-fs period.

Strongly Coupled Coherent Phonons in Single-Layer MoS2

“…Tunable pump pulses with photon energy ranging from 2 to 4.3eV are generated by a Non-collinear Optical Parametric Amplifier (NOPA) [72]. The pump pulses are temporally compressed by 2 chirped mirrors in the visible and by a MgF 2 prism compressor in the UV [73]. The pulse characterization is performed by frequency-resolved optical gating in the visible, and by a two-dimensional spectral-shearing interferometry method (2DSI) in the UV [74].…”

Two-dimensional UV spectroscopy: a new insight into the structure and dynamics of biomolecules

“…The generation, characterization and manipulation of ultrashort light pulses in the UV region are difficult, due to the lack of suitable broadband optical gain media and the strong material dispersion. 21 Moreover, 2D spectroscopy requires interferometric stability between pulse pairs, which amounts to controlling their path-length difference to be within a small fraction of the wavelength. This requirement is harder to satisfy in the UV region due to the short wavelengths.…”

“…Pyrene (see the inset of Fig. 6(a) for the molecular structure) is a polycyclic aromatic hydrocarbon with intriguing features which make it an excellent candidate for assessing novel spectroscopic techniques: 21,71,89,106,111 (i) well-resolved vibronic progressions in the UV region (29 000-33 000 cm À1 and 36 000-40 000 cm À1 ) associated with two bright states (labelled S 2 and S 4 , see the inset in Fig. 6(b)); (ii) ultrafast (i.e., in the sub-100 fs regime) photophysics involving non-adiabatic population decay mediated through CIs (mechanisms shown in the inset of Fig.…”

“…Upon photoexcitation, the bright states S 2 and S 4 are depopulated on a sub-100 fs time scale, as inferred from the disappearance of their state-specic PA signals. 21,71 Eventually, the population relaxes to the lowest (dark) electronic state (labelled S 1 ) where it remains trapped on a ns-timescale exhibiting long-lived uorescence. The S 1 state is characterized by PA signals in the visible (PA 1 band peaking at 19 500 cm À1 ) and the UV region (PA 2 band peaking at 27 500 cm À1 and PA 3 band peaking at 39 000 cm À1 ).…”

“…7(d) shows the details of the molecular structure of horse myoglobin, highlighting the heme moiety as well as two tryptophan (Trp) residues, Trp 7 and Trp 14 , located in the a helix A. Previous studies have shown that the uorescence of Trp 7 (Trp 14 ) is quenched on a timescale of 110-140 (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) ps, and attributed this process to uorescence resonance energy transfer (FRET) to the heme. 107,108 2DUV over the 325-280 nm excitation range reveals a different scenario.…”