Isomeric Yield Research Articles

Energy- and conformer-dependent excited-state relaxation of an E/Z photoswitchable thienyl-ethene.

Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E/Z photoisomerization following absorption of visible light. cis-to-trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565-420 nm) transient absorption (TA) studies to probe the photophysics of cis-to-trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (PA and PB) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the PB conformer is found to contribute to triplet population and the switching of cis-4TCE: high-energy excitation (<470 nm) of PB involves direct excitation to S2, relaxation from which prepares an ISC-active S1 geometry (ISC QY 0.4-0.67, kISC∼ 1.6-2.6 × 10-9 s-1) that is the gateway to triplet population and isomerization. We ascribe low cis-to-trans isomerization yield to excitation of the nonreactive PA conformer (75-85% loss) as well as loses along the PB S2→ S1→ T1 cascade (10-20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive "antiparallel" ring conformation.

β -decay half-lives of Sb134,134m and their isomeric yield ratio produced by the spontaneous fission of Cf252

A number of fission products possess isomeric states which have a nuclear spin significantly different from that of the ground state. The yield ratio of these states following fission is influenced by the angular momentum present in the fissioning system. The $^{134m,134}\mathrm{Sb}$ yield ratio had not been previously measured in the spontaneous fission of $^{252}\mathrm{Cf}$; however, it had previously been observed to favor the (${7}^{\ensuremath{-}}$) isomer over the (${0}^{\ensuremath{-}}$) ground state in $^{235}\mathrm{U}({n}_{th},f$) and $^{232}\mathrm{Th}$(25 MeV $p,f$). Using a mass-separated beam of low-energy $^{134,134m}\mathrm{Sb}$ ions produced by $^{252}\mathrm{Cf}$ spontaneous fission at the CARIBU facility, $\ensuremath{\beta}$ particles and $\ensuremath{\gamma}$ rays were detected using the SATURN/X-Array decay station to determine the fission-yield ratio and $\ensuremath{\beta}$-decay half-lives. The $^{134m}\mathrm{Sb}$ to $^{134}\mathrm{Sb}$ fission yield was determined to be $2.03\ifmmode\pm\else\textpm\fi{}0.05$ and the half-lives of $^{134m}\mathrm{Sb}$ and $^{134}\mathrm{Sb}$ were found to be $9.87\ifmmode\pm\else\textpm\fi{}0.08$ s and $0.674\ifmmode\pm\else\textpm\fi{}0.004$ s, respectively. These results represent the first isomeric yield ratio measurement for this nucleus, and improved measurements of the $^{134}\mathrm{Sb}$ ground state and the $^{134m}\mathrm{Sb}$ isomer half-lives.

First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments

We report the first experimental determination of independent isomeric yield ratios using direct ion counting with a Penning trap, which offered such a high resolution in mass that isomeric states ...

Isomeric yield ratio of 152m1Eu(8−)–152m2Eu(0−) produced in (γ, n) photonuclear reaction in the giant dipole resonance region

In this work, the isomeric ratio of 152m1Eu(8−)–152m2Eu(0−) produced from the 153Eu(γ, n)152Eu reaction have been measured in the whole giant dipole resonance region by the activation method. In order to improve the accuracy of the experimental results, the necessary corrections were made in the gamma activity measurements and data analysis. The results were discussed and compared with the similar data from literature to examine the excitation energy, the spin difference and the nuclear reaction channel effects and can be used for the interpretation on the structure of the nucleus and the mechanisms of nuclear reactions. The data for bremsstrahlung end-point energies of 19–23 MeV are first-time measurements.

Intersystem Crossing Drives Photoisomerization in o-Nitrotoluene, a Model for Photolabile Caged Compounds.

o-Nitrobenzyl (oNB) derivatives are widely used photolabile caged compounds in chemical and biological applications. The primary step in the photoinduced deprotection is an excited state intramolecular hydrogen transfer (ESIHT) leading to tautomerization of the oNB compound and subsequent release of the protecting group. The prototype molecule for studying such ESIHT is o-nitrotoluene (oNT), where hydrogen transfers from the methyl to the nitro group. Using the complete active space self-consistent field (CASSCF) method with second-order perturbative energy corrections (CASPT2), we have comprehensively investigated the photoisomerization and photo decay mechanisms in oNT. We have obtained the minimum energy crossing points (MECPs) between relevant electronic states and identified the singlet and triplet pathways. There is a barrierless path for oNT to relax to the lowest triplet state. In this T1 state, the ESIHT products are more stable than T1 oNT. Hydrogen-transfer occurs on the T1 state followed by relaxation to the ground state to give the isomerized product. A biradical intermediate proposed by previous studies is characterized to be the hydrogen-transferred T1 product. On the singlet pathway, in contrast to the triplet, the ground state tautomer is formed from the S1 oNT through a geometrically distant and energetically higher S1/S0 conical intersection. Although nonadiabatic dynamical studies are essential for determining branching ratios, our study, which considers the accessibility of different MECPs based on geometry and energy, and the magnitude of spin-orbit coupling at singlet-triplet MECPs, suggests that a significant fraction of the isomerization yield is due to the triplet channel.

Measurement of cross sections and isomeric yield ratios for 110m, gIn, 111m, gIn and 112m, gIn from natIn( $\gamma$ γ , xn) reactions in the bremsstrahlung end-point energies of 65-75 MeV

The flux weighted average cross sections and the isomeric yield ratios of 110m, gIn, 111m, gIn and 112m, gIn in the natIn( $\gamma$ ,xn) 112-110In reactions with the bremsstrahlung end-point energies of 65, 70 and 75 MeV have been determined by activation and off-line $\gamma$ -ray spectrometric technique using the 100 MeV electron linac at Pohang accelerator laboratory, Korea. It was found that the natIn( $\gamma$ ,xn) 112-110In reaction cross sections increase very fast from the threshold energy to a certain value of the bremsstrahlung energy, where the other reaction channel opens. After a certain energy, it remains constant because the cross section of the other reaction channel starts to increase. This indicates the partition of the excitation energy in different reaction channles. The isomeric yield ratios of 110m, gIn, 111m, gIn and 112m, gIn in the natIn( $\gamma$ ,xn) reactions decrease very fast with the bemsstrahlung end-point energy from the threshold value to 15-30 MeV due to the giant dipole resonance (GDR) effect. Above that, the isomeric yield ratios increase slowly with the bremsstrahlung energy due to the increase of excitation energy.

Measurement of isomeric yield ratios of 86m,gY and 87m,gY in the quasi-monoenergetic neutron induced reactions of 89Y

The isomeric yield ratios (IR) of 86m,gY and 87m,gY were measured in the 23 to 37 MeV quasi-monoenergetic neutron induced reaction of 89Y by using an off-line γ-ray spectrometric technique. The IR values in the 89Y(n,xn) reactions were also calculated using the TALYS 1.8 code and found to be in good agreement for 87m,gY but slightly less for 86m,gY. The IR values of 86m,gY and 87m,gY from the present work in the 89Y(n,xn) reactions were compared with the literature data in the 89Y(γ,xn), 89Y(p,x) and 89Y(α,x) reactions. It was found that the IR values of 86m,gY and 87m,gY in all these reactions increase with excitation energy. At the same excitation energy, the IR values are higher in the alpha induced reaction than in the proton, neutron and photon induced reactions due to higher input angular momentum in the former than in the later. These two observations indicate the role of entrance parameters. However, it is surprising to see that at the same excitation energy, the IR values of 86m,gY and 87m,gY in the 89Y(n,xn) reaction are nearly constant and significantly higher than in the 89Y(γ,xn) reaction.

New measurements on isobaric fission product yields and mean kinetic energy for 241Pu thermal neutron-induced fission

Nuclear fission yields data measurements for thermal neutron induced fission of 241Pu have been carried out at the Institut Laue Langevin (ILL) in Grenoble, using the Lohengrin mass spectrometer. Mass, isotopic and isomeric yields have been extracted for the last measurements. A focus is given in this document to the mass yield results which are obtained for almost the entire heavy peak and most of the light high yields masses, along with the covariance matrix. The mean kinetic energy as a function of the fission product mass has also been extracted from the measurements. The total mean kinetic energy pre and post neutron emission have been assessed and compared to other works showing a rather good agreement.

From fission yield measurements to evaluation: status on statistical methodology for the covariance question

Studies on fission yields have a major impact on the characterization and the understanding of the fission process and are mandatory for reactor applications. Fission yield evaluation represents the synthesis of experimental and theoretical knowledge to perform the best estimation of mass, isotopic and isomeric yields. Today, the output of fission yield evaluation is available as a function of isotopic yields. Without the explicitness of evaluation covariance data, mass yield uncertainties are greater than those of isotopic yields. This is in contradiction with experimental knowledge where the abundance of mass yield measurements is dominant. These last years, different covariance matrices have been suggested but the experimental part of those are neglected. The collaboration between the LPSC Grenoble and the CEA Cadarache starts a new program in the field of the evaluation of fission products in addition to the current experimental program at Institut Laue-Langevin. The goal is to define a new methodology of evaluation based on statistical tests to define the different experimental sets in agreement, giving different solutions for different analysis choices. This study deals with the thermal neutron induced fission of 235U. The mix of data is non-unique and this topic will be discussed using the Shannon entropy criterion in the framework of the statistical methodology proposed.

Measurement of fission yields and isomeric yield ratios at IGISOL

Data on fission yields and isomeric yield ratios (IYR) are tools to study the fission process, in particular the generation of angular momentum. We use the IGISOL facility with the Penning trap JYFLTRAP in Jyväskylä, Finland, for such measurements on 232Th and natU targets. Previously published fission yield data from IGISOL concern the 232Th(p,f) and 238U(p,f) reactions at 25 and 50 MeV. Recently, a neutron source, using the Be(p,n) reaction, has been developed, installed and tested. We summarize the results for (p,f) focusing on the first measurement of IYR by direct ion counting. We also present first results for IYR and relative yields for Sn and Sb isotopes in the 128-133 mass range from natU(n,f) based on γ-spectrometry. We find a staggering behaviour in the cumulative yields for Sn and a shift in the independent fission yields for Sb as compared to current evaluations. Plans for the future experimental program on fission yields and IYR measurements are discussed.

Photoionization and trans-to-cis isomerization of β-cyclodextrin-encapsulated azobenzene induced by two-color two-laser-pulse excitation

Azobenzene (1) and the complex resulting from the incorporation of 1 with cyclodextrin (1/CD) are attractive for light-driven applications such as micromachining and chemical biology tools. The highly sensitive photoresponse of 1 is crucial for light-driven applications containing both 1 and 1/CD to reach their full potential. In this study, we investigated the photoionization and trans-to-cis isomerization of 1/CD induced by one- and two-color two-laser pulse excitation. Photoionization of 1/CD, which was induced by stepwise two-photon absorption, was observed using laser pulse excitation at 266nm. Additionally, simultaneous irradiation with 266 and 532nm laser pulses increased the trans-to-cis isomerization yield (Υt→c) by 27%. It was concluded that the increase in Υt→c was caused by the occurrence of trans-to-cis isomerization in the higher-energy singlet state (Sn), which was reached by S1→Sn transition induced by laser pulse excitation at 532nm. The results of this study are potentially applicable in light-driven applications such as micromachining and chemical biology tools.

Measurement of isomeric yield ratios for the heavy cerium isotopes in the (γ,n) reactions

Measurement of isomeric yield ratios for the heavy cerium isotopes in the (γ,n) reactions

Measurements and analysis of [formula omitted]Hf production cross sections under [formula omitted]Ta and [formula omitted]W irradiation by protons with 0.04–2.6 GeV energies

This paper presents the results of experimental determination and computational simulation of the independent cross sections of 178m2Hf production in natTa and natW thin targets irradiated by mono-energetic proton beams in the energy range from ∼0.04 up to 2.6 GeV. The cross sections were determined using the γ-spectrometry technique without chemical separation of irradiated samples. The obtained experimental results were compared with the results of simulations based on the modified CEM03.03-ext intranuclear cascade model, which allows to analyze both the cross sections of residual nuclei production and the probabilities of long-lived isomeric states formation. Verification of the isomeric yield calculation model was demonstrated based on experimental data on high-spin isomer production cross sections in 208Pb target irradiated by protons.

Hemiindigo: Highly Bistable Photoswitching at the Biooptical Window.

Hemiindigo is a long known chromophore that absorbs in the blue part of the spectrum but has almost completely been ignored as potential photoswitch. Herein we show how the absorption of hemiindigo is shifted to the red part of the visible spectrum and how nearly perfect photoswitching can be achieved using blue or green and red light. Five derivatives were investigated giving very high isomeric yields in both switching directions, i.e. >90% E isomer after irradiation with 470 to 530 nm light and 99% Z isomer with 590 up to 680 nm light. At the same time the thermal bistability is extraordinarily high leading to half-lives of the pure isomeric states of up to 83 years at 25 °C. The herein developed photoswitches show photochromism in the visible enabling the two isomeric states to be distinguished by the naked eye. Substituted hemiindigos therefore constitute extremely promising new photoswitches with excellent properties for applications in biology, chemistry, or material sciences.

On the photochemical properties of the organometallic trans cyrhetrenyl chalcone (E). Deceleration of the photoisomerization and quenching of the luminescence effected by the photogenerated cis isomer (Z)

The photoprocesses of two organometallic chalcone complexes: one trans (E) [(η5-C5H4C(O)CH=CH-Ph)Re(CO)3], (1a), and the other functionalized with a macrocyclic trans (E) [(η5-C5H4C(O)CH=CH-4-benzo-15-crown-5)Re(CO)3], (1b), were investigated in fluid solution. Isomerization from the trans (E isomer) to the cis (Z isomer) and a weak luminescence from (1b) were observed when deaerated solutions of (1a) and (1b) were respectively irradiated at 300nm and 350nm. A marked non-linear dependence of the product Z concentration on the irradiation time was observed. Similarly the intensity of the (1b) luminescence decreased non-linearly with the irradiation time. These departures form linearity have been rationalized on the basis of a mechanism incorporating a weak inner filter effect and an energy transfer reaction from an excited state of E to Z. Other effects on the photolysis regime were of a structural origin. The smaller isomerization quantum yield of (1b) relative to (1a) (ϕisom=0.59 for (1b) vs. ϕisom=0.69 for (1a)) was attributed, to a large extent, to steric hindrance resulting from the appended macrocycle. Complexation of Ca2+ by the macrocycle in (1b) rises the quantum yield of isomerization to ϕisom=0.62, an effect associated with an inductive displacement of π electronic density that weakens the chalcone’s double bond and defeats the macrocycle’s steric hindrance.

Measurement of isomeric yield ratios of 99m,g;101m,g;102m,gRh in the natPd(γ,pxn) reactions with the bremsstrahlung end-point energies of 50–70 MeV

The isomeric yield ratios of 99m,gRh, 101m,ggRh, and 102m,gRh isomeric pairs produced from the natPd(γ,pxn) reactions were measured with the bremsstrahlung end-point energies of 50-, 55-, 60-, 65-, and 70-MeV at the 100MeV electron linac of the Pohang Accelerator Laboratory, Korea. The measurements were carried out with the activation method in combination with off-line γ-ray spectrometry. In order to improve the accuracy of the activity measurements, the separation of the overlapping γ-rays and the necessary corrections for the counting losses were made. The new experimental results are compared with the theoretical values of the TALYS-1.6 code.

Izomerni vidnoshennya ta seredni kutovi momenty pervynnykh frahmentiv 97Nb pry fotopodili 235U ta 238U

The isomeric yield ratios are measured for 97Nb fragments obtained at the bremsstrahlung photofission of 235U and 238U nuclei with gamma-quantum end-point energies of 10.5, 12.0, and 18.0 MeV. The mean angular momenta of primary fission fragments are determined, by using the generalized Huizenga–Vandenbosh statistical model and with the help of TALYS 1.6 and EMPIRE 3.2 codes.

Measurement of photo-neutron cross sections and isomeric yield ratios in the 89Y(γ,xn)89−x Y reactions at the bremsstrahlung end-point energies of 65, 70 and 75 MeV

Abstract The flux-weighted average cross sections of the 89Y(γ,xn; x=1–4)89−x Y reactions and the isomeric yield ratios of the 87m,gY, 86m,gY, and 85m,gY radionuclides produced in these reactions with the bremsstrahlung end-point energies of 65, 70 and 75 MeV have been determined by an activation and off-line γ-ray spectrometric technique using the 100 MeV electron linac in Pohang Accelerator Laboratory, Korea. The theoretical 89Y(γ,xn; x=1–4)89−x Y reactioncross sections for mono-energetic photonshave been calculated using the computer code TALYS 1.6. Then the flux-weighted theoretical values were obtaind to compare with the present data. The flux-weighted experimental and theoretical 89Y(γ,xn; x=1–4)89−x Y reaction cross sections increase very fast from the threshold values to a certain bremsstrahlung energy, wherethe other reaction channels open up. Thereafter it remains constant a while and then slowly decreases with the increase of cross sections for other reactions. Similarly, the isomeric yield ratios of 87m,gY,86m,gY and 85m,gY in the 89Y(γ,xn; x=2–4)89−x Y reactions from the present work and literature data show an increasing trend from their respective threshold values to a certain bremsstrahlung energy. After a certain point ofenergy, the isomeric yield ratios increase slowly with the bremsstrahlung energy. These observations indicate the role of excitation energy and its partitioning in different reaction channels.

Application of isochronous mass spectrometry for the study of angular momentum population in projectile fragmentation reactions

Isochronous mass spectrometry was applied to measure isomeric yield ratios of fragmentation reaction products. This approach is complementary to conventional gamma-ray spectroscopy in particular for measuring yield ratios for long-lived isomeric states. Isomeric yield ratios for the high-spin I = 19/2 states in the mirror nuclei 53Fe and 53Co are measured to study angular momentum population following the projectile fragmentation of 78Kr at energies of 480A MeV on a beryllium target. The 19/2 state isomeric ratios of 53Fe produced from different projectiles in the literature have also been extracted as a function of mass number difference between projectile and fragment (mass loss). The results are compared to ABRABLA07 model calculations. The isomeric ratios of 53Fe produced using different projectiles suggest that the theory underestimates not only the previously reported dependence on the spin but also the dependence on the mass loss.

Measurements of isomeric yield ratios of fission products from proton-induced fission onnatU and232Th via direct ion counting

Independent isomeric yield ratios (IYR) of 81Ge, 96Y, 97Y, 97Nb, 128Sn and 130Sn have been determined in the 25 MeV proton-induced fission of natU and 232Th. The measurements were performed at the ...