Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil

One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl (●OH) and azide (●N3) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of ●OH with 2-SeU are dominated by an absorption band with an λmax = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2●). The dimeric radical (2●) is formed in the reaction of a selenyl-type radical (6●) with 2-SeU, and both radicals are in equilibrium with Keq = 1.3 × 104 M−1 at pH 4 (below the pKa of 2-SeU). Similar equilibrium with Keq = 4.4 × 103 M−1 was determined for pH 10 (above the pKa of 2-SeU), which admittedly involves the same radical (6●) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2●−). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λmax = 390 nm, which was assigned to the ●OH adduct to the double bond at C5 carbon atom (3●) based on DFT calculations. Similar spectral and kinetic features were also observed during the ●N3-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU.

Clocking the buildup dynamics of light-induced states through attosecond transient absorption spectrum

The buildup processes of the light-induced states (LISs) in attosecond transient absorption spectroscopy are studied by solving the time-dependent Schrödinger equation and compared with the quasistatic Floquet theory, revealing a time lag of the maximal shift and strongest absorbance of the LIS with respect to the zero delay that is referred to as the buildup time. We analytically derive a scaling law for the buildup time that confirms the numerical results over a wide range of detunings. Our theory verifies the commonly accepted scenario of nearly instantaneous response of matter to light if the pump field is blue-detuned, but some differences are found in the near-resonant and red-detuning cases. Implications of the buildup time in petahertz optoelectronics are discussed.

Singlet and Triplet Exciton Dynamics of Violanthrone

<div>The exciton dynamics of violanthrone-79 are investigated in solution and in the solid</div><div>state. In solution, the photo-prepared singlet is found to exhibit a strong ground-state bleach</div><div>and stimulated emission feature, but when sensitized in its triplet state, exhibits only a narrow</div><div>and weak ground-state bleach. As supported by density functional theory calculations,</div><div>this is explained by the triplet state having absorptions in the same region, with a similar</div><div>oscillator strength, as the ground state molecule. In solid films, the excited singlet is</div><div>found to survive only 100 ps, giving way to a long-lived transient absorption spectrum with</div><div>characteristics reminiscent of the triplet in solution. This is interpreted in terms of singlet</div><div>fission in the solid film.</div>

Singlet and Triplet Exciton Dynamics of Violanthrone

<div>The exciton dynamics of violanthrone-79 are investigated in solution and in the solid</div><div>state. In solution, the photo-prepared singlet is found to exhibit a strong ground-state bleach</div><div>and stimulated emission feature, but when sensitized in its triplet state, exhibits only a narrow</div><div>and weak ground-state bleach. As supported by density functional theory calculations,</div><div>this is explained by the triplet state having absorptions in the same region, with a similar</div><div>oscillator strength, as the ground state molecule. In solid films, the excited singlet is</div><div>found to survive only 100 ps, giving way to a long-lived transient absorption spectrum with</div><div>characteristics reminiscent of the triplet in solution. This is interpreted in terms of singlet</div><div>fission in the solid film.</div>

Bound-State Electron Dynamics Driven by Near-Resonantly Detuned Intense and Ultrashort Pulsed XUV Fields

We report on numerical results revealing line-shape asymmetry changes of electronic transitions in atoms near-resonantly driven by intense extreme-ultraviolet (XUV) electric fields by monitoring their transient absorption spectrum after transmission through a moderately dense atomic medium. Our numerical model utilizes ultrashort broadband XUV laser pulses varied in their intensity (1014–1015 W/cm2) and detuning nearly out of resonance for a quantitative evaluation of the absorption line-shape asymmetry. It will be shown how transient energy shifts of the bound electronic states can be linked to these asymmetry changes in the case of an ultrashort XUV driving pulse temporally shorter than the lifetime of the resonant excitation, and how the asymmetry can be controlled by the near-resonant detuning of the XUV pulse. In the case of a two-level system, the numerical model is compared to an analytical calculation, which helps to uncover the underlying mechanism for the detuning- and intensity-induced line-shape modification and links it to the generalized Rabi frequency. To further apply the numerical model to recent experimental results of the near-resonant dressing of the 2s2p doubly excited state in helium by an ultrashort XUV free-electron laser pulse we extend the two-level model with an ionization continuum, thereby enabling the description of transmission-type (Fraunhofer-like) transient absorption of a strongly laser-coupled autoionizing state.

Tracking Intrinsic Properties of CH3NH3PbI3 Perovskite Thin Films Grown by Spin Coating Technique at Ambient Temperature

Methyl ammonium lead iodide has become a burgeoning class of hybrid halide perovskites of solution-processed semiconductors. Advancements in its processing and characterization underscore structural, optical, and electronic properties. They have led to the development of perovskite solar cells, photo detectors, lasers, and photo diodes with power conversion efficiencies mature to be classified as first and second-generation technologies. Characterizing forms an integral understanding the operating principles and fundamental limitations for optoelectronics applications. Studies outlined in this paper covers CH3NH3PbI3 using time-resolved pump-probe spectroscopy, X-ray diffractometry, spectrophotometry and other measurements. Thus this investigatiosn may serve as principle tool in analyzing excited state decay kinetics and optical nonlinearities in CH3NH3PbI3 thin films. It is demonstrated herein that non-resonant photoexcitation yields a large fraction of free carriers on a sub-picosecond time scale. If applied in practical optoelectronic applications then any photogenerated carriers may travel long carrier lengths before they are extracted to realize large external quantum efficiencies and efficient charge extraction. The optical constants of CH3NH3PbI3 are interpreted using ab initio calculations through models. Findings show good agreement between the optical constants derived from QSGW and those from related literature. Transition from the highest valence band (VB) to the lowest conduction band (CB) was found to be responsible for almost all the optical responses between 1.2 and 5.5 eV. It was concluded that optical constants and energy band diagrams of CH3NH3PbI3 can be used to simulate the contributions from different optical transitions to a typical transient absorption spectrum for many optoelectronic applications.