The photoisomerization of azobenzene in solution was studied experimentally and by calculations. trans-to-cis and cis-to-trans dynamics are described through broadband transient absorption, ...fluorescence, and stimulated Raman spectroscopy. Transient absorption was extended to cover not only the nπ* band but also the ππ* band in the ultraviolet. Isomerization yields are used for a quantitative comparison of trans and cis transient spectra under different excitation. For the trans-to-cis path upon nπ*(S 1) excitation, the evolution develops with 0.3, 3, and 16 ps. The first two times reflect population relaxation to a local minimum S 1t L and subsequent transition to a dark intermediate S 1t D over an 8 kJ/mol barrier. The existence of stationary points S 1t L and S 1t D is confirmed by quantum-chemical calculations. The third time corresponds to S 1t D → S 0 relaxation to the ground state via an S 1 /S 0 conical intersection over a 12 kJ/mol barrier. Thus, the 16 ps time constant is attributed to the isomerization process and not to vibrational cooling, contrary to the current view and in line with the previous interpretation by Lednev et al. (J. Phys. Chem. 1996, 100, 13338). The decay of the long-lived intermediate S 1t D is consistent with the hula twist rather than with the inversion mechanism. For the cis-to-trans reaction following nπ* excitation, signal decay is strongly nonexponential, with 0.1 and 1 ps. The latter (1 ps) is much shorter than the 16 ps decay of the trans isomer, implying different S 1/S 0 conical intersections and relaxation paths for the cis-to-trans and trans-to-cis reaction. New results are also obtained with ππ*(S n ) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S 1 region, which is not accessible under nπ* excitation. For cis-azobenzene, up to 30% of the excited species isomerize to trans via an S n /S 1 intersection, resulting in a mixed cis/trans S 1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.
There is a common belief that the main uncertainties in the theoretical analysis of neutrinoless double beta (0νββ) decay originate from the nuclear matrix elements. Here, we uncover another ...previously overlooked source of potentially large uncertainties stemming from nonperturbative QCD effects. Recently perturbative QCD corrections have been calculated for all dimension 6 and 9 effective operators describing 0νββ-decay and their importance for a reliable treatment of 0νββ-decay has been demonstrated. However, these perturbative results are valid at energy scales above ∼1 GeV, while the typical 0νββ scale is about ∼100 MeV. In view of this fact we examine the possibility of extrapolating the perturbative results towards sub-GeV nonperturbative scales on the basis of the QCD coupling constant “freezing” behavior using background perturbation theory. Our analysis suggests that such an infrared extrapolation does modify the perturbative results for both short-range and long-range mechanisms of 0νββ-decay in general only moderately. We also discuss that the tensor⊗tensor effective operator cannot appear alone in the low energy limit of any renormalizable high-scale model and then demonstrate that all five linearly independent combinations of the scalar and tensor operators, which can appear in renormalizable models, are infrared stable.
The rotamerism and photoisomerization of trans- and cis-1,2-di-(2-naphthyl)ethylene (tN and cN) are studied with stationary and transient absorption spectroscopies assisted by quantum chemical ...calculations. Absorption and emission spectra of rotamers (rotational isomers) tN-S (C 2h -symmetric), tN-A (C 1), and tN-S′ (C 2) are derived with a 53:47 ratio of tN-S to tN-A. Upon photoexcitation, the equilibration of the rotamers in S 0 (rotamerization) is observed in the bleach region with characteristic time τrotamer ≈ 0.5 ns. With excitation at 364 nm, the S 0 equilibrium shifts because, mainly, tN-A is bleached and the rotamerization becomes traceable, whereas with excitation at 345 nm, the equilibrium is preserved and the bleach spectrum remains unchanged. It is just long-lived (∼2 ns) S 1 that allows for monitoring the rotamer dynamics in S 0. Replacement of the stilbene phenyl rings with larger naphthyls increases the S 1 → P torsional barrier E 1act toward perpendicular configuration P both from cis and trans configurations. In tN, the radiative relaxation with τR ≈ 3.7 ns becomes the main deactivation channel, and accordingly, the measured decays show nearly no dependence on the solvent viscosity. The cis-to-trans photoisomerization occurs via two paths: adiabatic c S 1 → P → t S 1 (20%) and more common nonadiabatic c S 1 → P → S 0 (80%). The barrier c S 1 → P in the cis-isomer is reduced in polar solvents because of a zwitterionic character of P. The P-state is directly detected with the cN isomer in acetonitrile by an excited-state absorption band at 400 nm developing with 0.7 ps and decaying with 1.6 ps. Two dihydrophenanthrene (DHP)-like products result from photoexcited cN. The metastable DHP-A builds up transiently from cN-A, and its spectrum at about 550 nm matches the published DHP absorption. The stable DHP-S′ accumulates under stationary illumination and is formed either from excited cN-S′ or metastable DHP-A.
Photoinduced isomerization of 1,1′-bis-indanyliden (stiff-stilbene) in solution was studied with broadband transient absorption and femtosecond Raman spectroscopies, and by quantum-chemical ...calculations. Trans-to-cis S 1 isomerization proceeds over a 600 and 400 cm–1 barrier in n-hexane and acetonitrile, respectively. The reaction develops on multiple time scales with fast (0.3–0.4 ps) viscosity-independent and slower (2–26 ps) viscosity-dependent components. In the course of intramolecular torsion (which should be the main reaction coordinate) some excited molecules pass through the perpendicular conformation P and reach the cis geometry, to be temporarily trapped there. Subsequently they relax back to P and further to the ground state S 0. The cis-to-trans isomerization reveals ultrafast (0.06 ps) oscillatory relaxation followed by 13 ps decay in n-hexane and 2 ps decay in acetonitrile, corresponding to barriers of 800 and 400 cm–1, respectively. Raman S 0 and S 1 spectra are reported and discussed. The perpendicular conformation P was not detected, possibly due to its low oscillator strength and short lifetime, or because of strong overlap with hot product spectra. XMCQDPT2 calculations locate a stationary S 1 point on the cis side and two perpendicular-pyramidalized stationary points, to be reached from the former over 300 and 680 cm–1 barrier. Implications for parent stilbene are discussed; in this case we also see evidence for the trans-to-cis adiabatic path, as in stiff-stilbene. Very similar viscosity dependence for the two compounds supports the common isomerization pathway: torsion about the central double bond.
The decay rate of neutrinoless double beta (0nu beta beta ) decay contains terms from heavy particle exchange, which lead to dimension-9 (d=9) six fermion operators at low energies. Limits on the ...coefficients of these operators have been derived previously neglecting the running of the operators between the high scale, where they are generated, and the energy scale of 0nu beta beta decay, where they are measured. Here we calculate the leading-order QCD corrections to all possible d=9 operators contributing to the 0nu beta beta amplitude and use renormalization group running to calculate 1-loop improved limits. Numerically, QCD running dramatically changes some limits by factors of the order of or larger than typical uncertainties in nuclear matrix element calculations. For some specific cases, operator mixing in the running changes limits even by up to 3 orders of magnitude. Our results can be straightforwardly combined with new experimental limits or improved nuclear matrix element calculations to rederive updated limits on all short-range contributions to 0nu beta beta decay.
How proteins with a specific amino acid sequence are synthesized in living organisms is no longer a mystery after the deciphering of the genetic code six decades ago. However, the origin of the ...system of genetically coded protein synthesis is still the subject of hypotheses that are only partially experimentally justified. Based on the review of works mainly of the last decade, the author formulated a hypothesis that largely concretizes certain stages of the formation of the translation system and the genetic code. The hypothesis is based on the concept of evolutionary reduction of the ambiguity of the primordial code. The distinctive positions of the hypothesis are as follows: (i) the emergence of aptamers, enriching the protocell by the rare catalytically active amino acids, and their evolutionary transformation into aaRS-like ribozymes which resulted in the operational code, (ii) simultaneous participation of a large number of amino acids (including noncanonical) in the formation of operational and genetic codes, (iii) the recognition of the whole codon of mRNA by the anticodon loop of proto-tRNA from the very beginning of coded protein synthesis, (iv) the coevolution of the operational and genetic codes that eliminated their initial ambiguity.
We analyze the impact of QCD corrections on limits derived from neutrinoless double beta decay (0νββ). As demonstrated previously, the effect of the color mismatch arising from loops with gluons ...linking the quarks from different color-singlet currents participating in the effective operators has a dramatic impact on the predictions for some particular Wilson coefficients. Here, we consider all possible contributions from heavy particle exchange, i.e. the so-called short-range mechanism of 0νββ decay. All high-scale models (HSM) in this class match at some scale around a ∼ few TeV with the corresponding effective theory, containing a certain set of effective dimension-9 operators. Many of these HSM receive contributions from more than one of the basic operators and we calculate limits on these models using the latest experimental data. We also show with one nontrivial example, how to derive limits on more complicated models, in which many different Feynman diagrams contribute to 0νββ decay, using our general method.
The Amur River basin is widely known as one of the regions where the earliest pottery first appeared, however few are aware that it happened simultaneously in two separate areas associated with two ...different archaeological complexes -the Gromatukha and Osipovka. This duality is confirmed here through three lines of inquiry. The first section reviews the most recent general data on both complexes including radiocarbon ages and stone tool assemblages, the second gives detailed descriptions and analysis of both cultures’ pottery including composition, forming, and decorating techniques, and the third provides data on their changes over time. The primary distinctions between the two complexes are stressed including the distribution area, pottery and stone tool production, chronological discrepancies, complex origins, and affiliations. Based on these observations we conclude that the Osipovka and Gromatuka pottery are likely produced by different cultural groups in distinct regions, with little cultural interactions. These observations provide a more complex view of the invention and development of pottery within a smaller regional setting, facilitating future comparisons of the general context of pottery invention and opening new pathways for the reconstruction of human behavioral reactions to cultural, climatic, and environmental challenges in the past.
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