Femtosecond fluorescence upconversion experiments were combined with CASPT2 and time dependent DFT calculations to characterize the excited state dynamics of the mutagenic etheno adduct 1,N2-etheno-2'-deoxyguanosine (ϵdG). This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The ϵdG fluorescence is strongly modified with respect to that of the canonical nucleoside dG, notably by an about 6-fold increase in fluorescence lifetime and quantum yield at neutral pH. In addition, femtosecond fluorescence upconversion experiments reveal the presence of two emission bands with maxima at 335 nm for the shorter-lived and 425 nm for the longer-lived. Quantum mechanical calculations rationalize these findings and provide absorption and fluorescence spectral shapes similar to the experimental ones. Two different bright minima are located on the potential energy surface of the lowest energy singlet excited state. One planar minimum, slightly more stable, is associated with the emission at 335 nm, whereas the other one, with a bent etheno ring, is associated with the red-shifted emission.

Let's shine! The 2'-deoxyguanosine etheno adduct exhibits enhanced emission features with respect to the canonical nucleobase. The peculiar fluorescence arising from two independent species was unveiled by upconversion fluorescence and rationalized by quantum mechanical calculations. More information can be found in the Research Article by R. Improta, V. Lhiaubet-Vallet and co-workers (DOI: 10.1002/chem.202401835). Artwork designed by Cora Pardo Asuncion.

Future high-luminosity experiments require timing precision of the order of tens of picoseconds for pileup mitigation while aiming at instrumenting large active areas. PICOSEC Micromegas detectors combine a Cherenkov radiator with a photocathode and a two-stage gaseous amplification structure to achieve timing precision of $\lt25 \mathrm{ps}$ for minimum ionising particles [1] while allowing for economic area coverage. We present the development of tileable detector modules achieving high timing precision and optimisations in detector construction and signal routing which improve the achievable resolution to better than 20 ps. In parallel, new developments towards maintaining performance in challenging environments are pursued including alternative photoconversion layers which tolerate higher levels of ion back flow and resistive amplification structures. Carbon-based photocathodes including diamond-like carbon and $B_{4} C$ are evaluated and timing resolution values in the range of $30-40$ ps were obtained. Resistive detector technologies including resistive Micromegas and $\mu$ RWELL detectors were implemented to withstand high-rate environments. To profit from efficient protection against discharges and minimise the influence of delayed signal components on timing performance, high resistivity electrodes are used and comparable time resolution to non-resistive prototypes is demonstrated. The performance of different scalable readout approaches is compared to demonstrate the availability of suitable electronic readout schemes for multi-module detector systems. Dedicated preamplifiers coupled with waveform-digitising TDCs are used for constantfraction discrimination while a threshold-based timing ASIC is shown to allow for timewalk correction with complementary energy information. The presented developments illustrate the versatility of PICOSEC Micromegas precise timing detectors and their compatibility with challenging environments for high-pileup environments as well as photo detection.

The paper deals with the fluorescence of guanine quadruplexes (G4) formed by association of two DNA strands d(GGGGTTTTGGGG) in the presence of K+ cations, noted as OXY/K+ in reference to the protozoon Oxytricha nova, whose telomere contains TTTTGGGG repeats. They were studied by steady-state and time-resolved techniques, time-correlated single photon counting, and fluorescence upconversion. The maximum of the OXY/K+ fluorescence spectrum is located at 334 nm, and the quantum yield is 5.8 × 10-4. About 75% of the photons are emitted before 100 ps and stem from ππ* states, possibly with a small contribution of charge transfer. Time-resolved fluorescence anisotropy measurements indicate that ultrafast (<330 fs) excitation transfer, due to internal conversion among exciton states, is more efficient in OXY/K+ compared to previously studied G4 structures. This is attributed to the arrangement of the peripheral thymines in two diagonal loops with restricted mobility, facilitating the interaction among them and with guanines. Thymines should also be responsible for a weak intensity excimer/exciplex emission band, peaking at 445 nm. Finally, the longest living fluorescence component (∼2.1 ns) is observed at the blue side of the spectrum. So far, high-energy long-lived emitting states had been reported only for double-stranded structures but not for G4.

The PICOSEC Micromegas (MM) detector is a precise timing gaseous detector consisting of a Cherenkov radiator combined with a photocathode and a MM amplifying structure. A 100-channel PICOSEC MM prototype with 10 × 10 cm 2 active area equipped with a Cesium Iodide (CsI) photocathode demonstrated a time resolution below σ = 18 ps. The objective of this work is to improve the PICOSEC MM detector robustness aspects, i.e. integration of resistive MM and carbon-based photocathodes, while maintaining good time resolution. The PICOSEC MM prototypes have been tested in laboratory conditions and successfully characterised with 150 GeV/c muon beams at the CERN SPS H4 beam line. The excellent timing performance below σ = 20 ps for an individual pad obtained with the 10 × 10 cm 2 area resistive PICOSEC MM of 20 MΩ/□ showed no significant time resolution degradation as a result of adding a resistive layer. A single-pad prototype equipped with a 12 nm thick Boron Carbide (B 4 C) photocathode presented a time resolution below σ = 35 ps, opening up new possibilities for detectors with robust photocathodes. The results made the concept more suitable for the experiments in need of robust detectors with good time resolution.

The PICOSEC Micromegas detector is a precise timing gaseous detector based on a Cherenkov radiator coupled to a semi-transparent photocathode and a Micromegas amplifying structure. First single-pad prototypes demonstrated a time resolution below sigma = 25 ps, however, to make the concept appropriate to physics applications, several developments are required. The objective of this work was to achieve an equivalent time resolution for a 10 x 10 cm(2) area PICOSEC Micromegas detector. The prototype was designed, produced and tested in the laboratory and successfully operated with a 80 GeV/c muon beam. Preliminary results for this device equipped with a CsI photocathode demonstrated a time resolution below sigma = 25 ps for all measured pads. The time resolution was reduced to be below sigma = 18 ps by decreasing the drift gap to 180 mu m and using dedicated RF amplifier cards as new electronics. The excellent timing performance of the single-channel proof of concept was not only transferred to the 100-channel prototype, but even improved, making the PICOSEC Micromegas detector more suitable for large-area experiments in need of detectors with high time resolutions.

During the past few years, several studies reported that a significant part of the intrinsic fluorescence of DNA duplexes decays with surprisingly long lifetimes (1-3 ns) at wavelengths shorter than the ππ* emission of their monomeric constituents. This high-energy nanosecond emission (HENE), hardly discernible in the steady-state fluorescence spectra of most duplexes, was investigated by time-correlated single-photon counting. The ubiquity of HENE contrasts with the paradigm that the longest-lived excited states correspond to low-energy excimers/exciplexes. Interestingly, the latter were found to decay faster than the HENE. So far, the excited states responsible for HENE remain elusive. In order to foster future studies for their characterization, this Perspective presents a critical summary of the experimental observations and the first theoretical approaches. Moreover, some new directions for further work are outlined. Finally, the obvious need for computations of the fluorescence anisotropy considering the dynamic conformational landscape of duplexes is stressed.

The PICOSEC Micromegas precise timing detector is based on a Cherenkov radiator coupled to a photocathode operating in a semi-transparent mode and a Micromegas amplification structure. The first proof of concept single-channel prototype was able to achieve a time resolution below 25 ps. One of the crucial aspects in the development of precise timing gaseous detectors applicable in high-energy physics experiments is a modular design that enables large area coverage. The first 19-channel multi-pad prototype with an active area of approximately 10 cm(2) suffered from degraded timing resolution due to the non-uniformity of the preamplification gap thickness. A new 100 cm(2) detector module with 100 channels based on a rigid hybrid ceramic/FR4 Micromegas board for improved drift gap uniformity was developed. Initial measurements with 80 GeV/c muons showed improvements in timing response over the measured pads and a time resolution below 25 ps. More recent measurements with a thinner drift gap detector module and newly developed RF pulse amplifiers show that the pad centre resolution can be enhanced to the level of 17 ps. This work will present the development of the detector from structural simulations, design, and beam test commissioning with a focus on the timing performance of a thinner drift gap detector module in combination with new electronics using an automated timing scan method.

This paper will review the origins, development, and examples of new versions of Micro-Pattern Gas Detectors. The goal for MPGD development was the creation of detectors that could cost-effectively cover large areas while offering excellent position and timing resolution, and the ability to operate at high incident particle rates. The early MPGD developments culminated in the formation of the RD51 collaboration which has become the critical organization for the promotion of MPGDs and all aspects of their production, characterization, simulation, and uses in an expanding array of experimental configurations. For the Snowmass 2021 study, a number of Letters of Interest were received that illustrate ongoing developments and expansion of the use of MPGDs. In this paper, we highlight high precision timing, high rate application, trigger capability expansion of the SRS readout system, and a structure designed for low ion backflow.

Using femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics in strongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads to discrete excitonic transitions and strong Stark features in the TA spectra. This prevents the use of conventional relaxation analysis methods extracting the carrier temperature or measuring the buildup of the band-edge bleaching. Instead, we show that the TA spectral line shape near the band-edge reflects the state of the system, which can be used to probe the exciton cooling dynamics. The ultrafast hot exciton relaxation in one- to three- monolayer-thick NPLs confirms the absence of intrinsic phonon bottleneck. However, excitation fluence-dependent measurements reveal a hot phonon bottleneck effect, which is found to be independent of the nature of the internal cations but strongly affected by the ligands and/or sample surface state. Together, these results suggest a role of the surface ligands in the cooling process.

The publication deals with polymeric pA●pT and oligomeric A20●T20 DNA duplexes whose fluorescence is studied by time-correlated single photon counting. It is shown that their emission on the nanosecond timescale is largely dominated by high-energy components peaking at a wavelength shorter than 305 nm. Because of their anisotropy (0.02) and their sensitivity to base stacking, modulated by the duplex size and the ionic strength of the solution, these components are attributed to mixed ππ*/charge transfer excitons. As high-energy long-lived excited states may be responsible for photochemical reactions, their identification via theoretical studies is an important challenge.

Timing information in current and future accelerator facilities is important for resolving objects (particle tracks, showers, etc.) in extreme large particles multiplicities on the detection systems. The PICOSEC Micromegas detector has demonstrated the ability to time 150 GeV muons with a sub-25 ps precision. Driven by detailed simulation studies and a phenomenological model which describes stochastically the dynamics of the signal formation, new PICOSEC designs were developed that significantly improve the timing performance of the detector. PICOSEC prototypes with reduced drift gap size (∼119 µm) achieved a resolution of 45 ps in timing single photons in laser beam tests (in comparison to 76 ps of the standard PICOSEC detector). Towards large area detectors, multi-pad PICOSEC prototypes with segmented anodes has been developed and studied. Extensive tests in particle beams revealed that the multi-pad PICOSEC technology provides also very precise timing, even when the induced signal is shared among several neighbouring pads. Furthermore, new signal processing algorithms have been developed, which can be applied during data acquisition and provide real time, precise timing.

Improving the understanding of multiple exciton interactions and dynamics in semiconductor nanostructures is mandatory for their successful use as photoactive materials in light convertors such as electroluminescent diodes, lasers, or single-photon sources. Here high-fluence and high-energy excitation effects are investigated in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs) using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Nonradiative Auger recombination (AR) is the dominant pathway for multiexciton recombination. Its dynamics are found to be subquadratic with the exciton density. Indeed, because of the limited exciton wave-function delocalization length, AR is limited by exciton diffusion in the 2D plane at moderate excitation fluence and takes place in several hundreds of picoseconds, with typical recombination rates on the order of 10(-2) cm(2)/s. At high excitation fluence leading to an average interexciton distance comparable with the exciton delocalization length, the measured "intrinsic" AR time is faster than 10 ps and independent of the NPL composition. The strong dependence of the AR rate on the interexciton distance allows us to identify the recombination resulting from multiple exciton generation, involving the reaction of "geminate biexcitons", upon excitation at low fluence with high-energy photons.

In this article, the solvation in ionic liquid/molecular solvent (IL/MS) mixtures is investigated. The steady-state and time-resolved measurements of the solvation of coumarin 153 (C153) in BmimBF4/propylene carbonate are presented. A slight bathochromic shift was observed in the steady-state spectra upon increasing the IL molar fraction xIL. The time correlated photon counting TCSPC and fluorescence upconversion techniques were used to obtain the experimental solvation response functions (SRFs). The SRFs were found to be multi-exponential and the average solvation times increased with xIL. The MD simulations were performed to help the interpretation of the experimental data. An unprecedentedly good agreement between the measured and the calculated SRFs was achieved. The partition of the coulombic energy of interaction between the atoms of C153 and cations, anions and PC molecules provides a deep understanding of the time signatures of the mixture components participation in the solvation response. Indeed, the slow portion of the SRFs was found to be dominated by the IL. These time signatures change in function of xIL and represent the previously established regimes of IL/MS mixtures: electrolyte solution (xIL < 0.2) and lubricated IL (xIL > 0.2). These results will be of crucial interest for the understanding of the molecular rearrangements occurring in IL/MS electrolyte models for dye sensitized solar cells (DSSCs) after photo-excitation.

The intrinsic fluorescence of nucleic acids is extremely weak compared to that of the fluorescent labels used to probe their structural and functional behavior. Thus, for technical reasons, the investigation of the intrinsic DNA fluorescence was limited for a long time. But with the improvement in spectroscopic techniques, the situation started to change around the turn of the century. During the past two decades, various factors modulating the static and dynamic properties of the DNA fluorescence have been determined; it was shown that, under certain conditions, quantum yields may be up 100 times higher than what was known so far. The ensemble of these studies opened up new paths for the development of label-free DNA fluorescence for biochemical applications. In parallel, these studies have shed new light on the primary processes leading to photoreactions that damage DNA when it absorbs UV radiation.We have been studying a variety of DNA systems, ranging from the monomeric nucleobases to double-stranded and four-stranded structures using fluorescence spectroscopy. The specificity of our work resides in the quantitative association of the steady-state fluorescence spectra with time-resolved data recorded from the femtosecond to the nanosecond timescales, made possible by the development of specific methodologies.Among others, our fluorescence studies provide information on the energy and the polarization of electronic transitions. These are valuable indicators for the evolution of electronic excitations in complex systems, where the electronic coupling between chromophores plays a key role. Highlighting collective effects that originate from electronic interactions in DNA multimers is the objective of the present Account.In contrast to the monomeric chromophores, whose fluorescence decays within a few picoseconds, that of DNA multimers persists on the nanosecond timescale. Even if long-lived states represent only a small fraction of electronic excitations, they may be crucial to the DNA photoreactivity because the probability to reach reactive conformations increases over time, owing to the incessant structural dynamics of nucleic acids.Our femtosecond studies have revealed that an ultrafast excitation energy transfer takes place among the nucleobases within duplexes and G-quadruplexes. Such an ultrafast process is possible when collective states are populated directly upon photon absorption. At much longer times, we discovered an unexpected long-lived high-energy emission stemming from what was coined "HELM excitons". These collective states, whose emission increases with the duplex size, could be responsible for the delayed fluorescence of ππ* states observed for genomic DNA.Most studies dealing with excited-state relaxation in DNA were carried out with excitation in the absorption band peaking at around 260 nm. We went beyond this and also performed the first time-resolved study with excitation in the UVA spectral range, where a very weak absorption tail is present. The resulting fluorescence decays are much slower and the fluorescence quantum yields are much higher than for UVC excitation. We showed that the base pairing of DNA strands enhances the UVA fluorescence and, in parallel, increases the photoreactivity because it modifies the nature of the involved collective excited states.

The PICOSEC Micromegas detector can time the arrival of Minimum Ionizing Particles with a sub-25 ps precision. A very good timing resolution in detecting single photons is also demonstrated in laser beams. The PICOSEC timing resolution is determined mainly by the drift field. The arrival time of the signal and the timing resolution vary with the size of the pulse amplitude. Detailed simulations based on GARFIELD++ reproduce the experimental PICOSEC timing characteristics. This agreement is exploited to identify the microscopic physical variables, which determine the observed timing properties. In these studies, several counter-intuitive observations are made for the behavior of such microscopic variables. In order to gain insight on the main physical mechanisms causing the observed behavior, a phenomenological model is constructed and presented. The model is based on a simple mechanism of "time-gain per interaction" and it employs a statistical description of the avalanche evolution. It describes quantitatively the dynamical and statistical properties of the microscopic quantities, which determine the PICOSEC timing characteristics, in excellent agreement with the simulations. In parallel, it offers phenomenological explanations for the behavior of these microscopic variables. The formulae expressing this model can be used as a tool for fast and reliable predictions, provided that the input parameter values (e.g. drift velocities) are known for the considered operating conditions.

We present a study of the excited state relaxation dynamics of the photosensitizer P1 used in p-type dye-sensitized solar cells. Comparative femtosecond fluorescence upconversion measurements in solution and in films show that the dye undergoes a picosecond electronic relaxation from the bright Franck–Condon (FC) state to a low-emitting charge-transfer (CT) state in polar environment. The fluorescence is moderately quenched in solution and on the mesoporous Al2O3 isolator but dramatically more on NiO semiconductor. We assign this sub-picosecond process to the hole injection thus confirming that the electron transfer is from the FC state directly into the NiO valence band.

Joint femtosecond fluorescence upconversion experiments and theoretical calculations provide a hitherto unattained degree of characterization and understanding of the mutagenic etheno adduct 3,N4-etheno-2'-deoxycytidine (εdC) excited state relaxation. This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The εdC fluorescence is modified with respect to that of the canonical base dC, with a 3-fold increased lifetime and quantum yield at neutral pH. This behavior is amplified upon protonation of the etheno ring (εdCH+). Quantum mechanical calculations show that the lowest energy state ππ*1 is responsible for the fluorescence and that the main nonradiative decay pathway to the ground state goes through an ethene-like conical intersection, involving the out-of-plane motion of the C5 and C6 substituents. This conical intersection is lower in energy than the ππ* state (ππ*1) minimum, but a sizable energy barrier explains the increase of εdC and εdCH+ fluorescence lifetimes with respect to that of dC.

The multi-pad PICOSEC-Micromegas is an improved detector prototype with a segmented anode, consisting of 19 hexagonal pads. Detailed studies are performed with data collected in a muon beam over four representative pads. We demonstrate that such a device, scalable to a larger area, provides excellent time resolution and detection efficiency. As expected from earlier single-cell device studies, we measure a time resolution of approximately 25 picoseconds for charged particles hitting near the anode pad centres, and up to 30 picoseconds at the pad edges. Here, we study in detail the effect of drift gap thickness non-uniformity on the timing performance and evaluate impact position based corrections to obtain a uniform timing response over the full detector coverage.

In order to introduce students to the fascinating field of femtochemistry, we propose here a practical laboratory training course conceived for second-year master's students in chemistry. We describe the use of a broadband femtosecond transient absorption (pump-probe) experiment for monitoring a fast light-triggered chemical reaction in solution. The experiments are performed on the pyranine photoacid, which upon photoexcitation at 390 nm undergoes a proton transfer to the solvent in about 90 ps. While this practical course involves advanced equipment and techniques, the measured transient absorption data allow easy analysis and interpretation. The transient absorption spectra at a few selected delay times can be analyzed qualitatively in terms of bleach, induced absorption, and stimulated emission. Likewise, the transient absorption signals at a few chosen wavelengths can be quantitatively analyzed and explained with simple kinetic models to determine the time constant of the proton-transfer reaction. This training aims at giving the students the opportunity to face some of the current challenges in contemporary chemistry by learning the basics of ultrafast spectroscopy.