Search results for "Internal conversion"
showing 10 items of 58 documents
Results from the Project 8 phase-1 cyclotron radiation emission spectroscopy detector
2017
The Project 8 collaboration seeks to measure the absolute neutrino mass scale by means of precision spectroscopy of the beta decay of tritium. Our technique, cyclotron radiation emission spectroscopy, measures the frequency of the radiation emitted by electrons produced by decays in an ambient magnetic field. Because the cyclotron frequency is inversely proportional to the electron's Lorentz factor, this is also a measurement of the electron's energy. In order to demonstrate the viability of this technique, we have assembled and successfully operated a prototype system, which uses a rectangular waveguide to collect the cyclotron radiation from internal conversion electrons emitted from a ga…
ChemInform Abstract: Excitation of Nucleobases from a Computational Perspective I: Reaction Paths
2016
The main intrinsic photochemical events in nucleobases can be described on theoretical grounds within the realm of non-adiabatic computational photochemistry. From a static standpoint, the photochemical reaction path approach (PRPA), through the computation of the respective minimum energy path (MEP), can be regarded as the most suitable strategy in order to explore the electronically excited isolated nucleobases. Unfortunately, the PRPA does not appear widely in the studies reported in the last decade. The main ultrafast decay observed experimentally for the gas-phase excited nucleobases is related to the computed barrierless MEPs from the bright excited state connecting the initial Franck…
Intersystem Crossing and Light-Induced Bistability in Iron(?) Spin-Crossover Compounds
1996
Abstract The dynamics of the high-spin→low-spin intersystem crossing process in iron(?) spin-crossover compounds are strongly influenced by cooperative effects of elastic origin which are due to the large difference in volume between high-spin and low-spin complexes. The deviation from first order kinetics is attributed to a build-up of an internal pressure as the relaxation proceeds, leading to a characteristic self-acceleration. The elastic interactions may lead to a light-induced bistability for systems which otherwise remain in the high-spin state down to cryogenic temperatures.
Harvesting Fluorescence from Efficient Tk -> Sj (j, k > 1) Reverse Intersystem Crossing for ??* Emissive Transition-Metal Complexes
2013
Using a bimetallic Au(I) complex bearing alkynyl-(phenylene)3-diphosphine ligand (A-3), we demonstrate that the fluorescence can be exquisitely harvested upon T1 → Tk (k > 1) excitation followed by Tk → Sj (j, k > 1) intersystem crossing (ISC) back to the S1 state. Upon S0 → S1 355 nm excitation, the S1 → T1 intersystem crossing rate has been determined to be 8.9 × 108 s–1. Subsequently, in a two-step laser pump–probe experiment, following a 355 nm laser excitation, the 532 nm T1 → Tk probing gives the prominent blue 375 nm fluorescence, and this time-dependent pump–probe signal correlates well with the lifetime of the T1 state. Careful examination reveals the efficiency of Tk → Sj (j, k > …
Photophysical properties of the triangular [Au(HNCOH)]3 complex and its dimer
2020
Rate constants for radiative and non-radiative transitions of the [Au(HNCOH)]3 complex and its dimer were calculated within the Herzberg–Teller approximation based on quantum mechanical principles. A high triplet quantum yield was estimated for the monomer. Internal conversion (IC) was found to be the major competing process to the intersystem crossing (ISC) from the lowest excited singlet state (S1) to the lowest triplet state (T1). ISC and IC from the spin-mixed 1 state also dominate the triplet relaxation process resulting in a negligible phosphorescence quantum yield for the monomer. The IC and ISC rate constants of the dimer are considerably smaller due to much lower Franck–Condon fact…
Isomeric state of 80Y and its role in the rp-process
2003
The HIGISOL facility has been used to investigate carefully the isomeric transition 228.5 keV in 80Y. We have measured the electron internal conversion coefficient for this transition αK = 0.50 ± 0.07 which gives the value for half-life of “bare” isomeric state T 1/2 = 6.8 ± 0.5 s. The isomeric state should play an important role in the rp-process calculations.
The Role of Adenine Excimers in the Photophysics of Oligonucleotides
2009
Energies and structures of different arrangements of the stacked adenine homodimer have been computed at the ab initio CASPT2 level of theory in isolation and in an aqueous environment. Adenine dimers are shown to form excimer singlet states with different degrees of stacking and interaction. A model for a 2-fold decay dynamics of adenine oligomers can be supported in which, after initial excitation in the middle UV range, unstacked or slightly stacked pairs of nucleobases will relax by an ultrafast internal conversion to the ground state, localizing the excitation in the monomer and through the corresponding conical intersection with the ground state. On the other hand, long-lifetime intra…
Singlet Excited-State Interactions in Naphthalene-Thymine Dyads
2004
Two thymidine-derived nucleosides 1 and 2 were prepared by attaching a chiral naphthalene to the positions 5′ and 3′ of the sugar. The resulting dyads, which contain key substructures present in drugs and nucleic acids, exhibit different spatial arrangements (transoid or cisoid) of the fluorophore relative to the thymine unit. Emission measurements on these compounds in the presence of ROH molecules revealed a remarkable intramolecular fluorescence quenching for dyad 1. The obtained results are consistent with quenching of the singlet excited state of 1 by hydrogen-bond donor solvents. Thus, a physical deactivation process (vibronically induced internal conversion) would be the pathway resp…
Theoretical insight into the intrinsic ultrafast formation of cyclobutane pyrimidine dimers in UV-irradiated DNA: thymine versus cytosine.
2008
The higher formation yields measured in the ultrafast photoinduced formation of cyclobutane thymine dimers (T T) with respect to those of cytosine (C C) are explained, on the basis of ab initio CASPT2 results, by the existence in thymine of more reactive orientations and a less efficient photoreversibility, whereas in cytosine the funnel toward the photolesion becomes competitive with that mediating the internal conversion of the excited-cytosine monomer.
Determination of absolute internal conversion coefficients using the SAGE spectrometer
2016
Abstract A non-reference based method to determine internal conversion coefficients using the SAGE spectrometer is carried out for transitions in the nuclei of 154 Sm, 152 Sm and 166 Yb. The Normalised-Peak-to-Gamma method is in general an efficient tool to extract internal conversion coefficients. However, in many cases the required well-known reference transitions are not available. The data analysis steps required to determine absolute internal conversion coefficients with the SAGE spectrometer are presented. In addition, several background suppression methods are introduced and an example of how ancillary detectors can be used to select specific reaction products is given. The results o…