Contribution of incoherent effects to the orientation dependence of bremsstrahlung from rapid electrons in crystal

The bremsstrahlung cross section for relativistic electrons in a crystal is split into the sum of coherent and incoherent parts (the last is due to a thermal motion of atoms in the crystal). Although the spectrum of incoherent radiation in crystal is similar to one in amorphous medium, the incoherent radiation intensity could demonstrate substantial dependence on the crystal orientation due to the electrons' flux redistribution in the crystal. In the present paper we apply our method of the incoherent bremsstrahlung simulation developed earlier to interpretation of some recent experimental results obtained at the Mainz Microtron MAMI.Comment: VIII International Symposium "Radiation from Relativistic Electrons in Periodic Structures" (RREPS-09) Zvenigorod, Russia, September 7-11, 200

Watching Ultrafast Barrierless Excited-state Isomerization Of Pseudocyanine In Real Time

The photoinduced excited-state processes in 1,1\u27-diethyl-2,2\u27-cyanine iodine are investigated using femtosecond time-resolved pump-probe spectroscopy. Using a broad range of probe wavelengths, the relaxation of the initially prepared excited-state wavepacket can be followed down to the sink region. The data directly visualize the directed downhill motion along the torsional reaction coordinate and suggest a barrierless excited-state isomerization in the short chain cyanine dye. Additionally, ultrafast ground-state hole and excited-state hole replica broadening is observed. While the narrow excited-state wavepacket broadens during pump-probe overlap, the ground-state hole burning dynamics takes place on a significantly longer time-scale. The experiment reported can be considered as a direct monitoring of the shape and the position of the photoprepared wavepacket on the excited-state potential energy surface

Wavepacket Motion Via A Conical Intersection In The Photochemistry Of Aqueous Transition-metal Dianions

The photochemical reaction paths in aqueous PtBr(6)(2-) and OsBr(6)(2-) have been studied by femtosecond broad-band pump probe spectroscopy supported by CASPT2 and DFT/TDDFT calculations. These paths lead to the separation of negative charges and propagate through distortions of nascent, penta-coordinated metal fragments caused by Jahn-Teller C(4v) and D(3h) conical intersections (CIs), respectively. Within 150 fs following 420 nm excitation of PtBr(6)(2-), the molecule undergoes internal conversion and intersystem crossing into the dissociative lowest triplet excited (3)T(1g) state, loses a ligand, and relaxes via the C(4v) CI to the nearly trigonal bipyramid (3)PtBr(5)(-) product in the triplet state. Direct 530 nm excitation of PtBr(6)(2-) to (3)T(1g) yields. the same product. Oscillations observed in the bending and umbrella a(1) modes of (3)PtBr(5)(-) arise from impulsive excitation of, respectively, one of the reaction coordinate modes, which is parallel to the gradient difference vector of the C(4v) CI, and the spectator mode that preserves the electronic degeneracy

TIME-RESOLVED RELAXATION DYNAMICS OF NEAR-INFRARED EXCITED ELECTRONIC STATES IN TRANSITION METAL COMPLEXES.

Sub-100 fs time-resolved, broadband transient absorption spectroscopy was employed to investigate ultrafast radiationless relaxation dynamics of near-infrared, metal-centered (MC), electronic excited states of several d$^{5}$ and d$^{9}$ transition metal complexes (e.g., \chem{CuCl_4}$^{2-}$, \chem{CuBr_4}$^{2-}$, \chem{IrBr_6}$^{2-}$, \chem{IrCl_6}$^{2-}$, etc.) in acetonitrile solution. The results yield insights into the topology of the involved potential energy surfaces, Jann-Teller distortions, and the dynamics through conical intersections connecting the first excited and ground electronic states (energy gap, less than 8000 \wn). Furthermore, it was found that the addition of water to the solutions efficiently quenches the MC excited states via energy transfer

ULTRAFAST DYNAMICS OF DIBROMOCYCLOALKANES

Carbocyclic geminal dibromides are important intermediates in synthesis of complex heterocyclic molecules and natural products.\footnote{Ganesh, N. V.; Jayaraman, N. J. Org. Chem. 2007, 72, 5500-5504.} Ultrafast time-resolved techniques can help to obtain information about the electronic structure of the intermediates which appears in the chemistry and photochemistry of these molecules. Also, relaxation dynamics and reactivity of these intermediates in different solvents can be characterized. This information can be used for the design of different compounds with desired properties. In the current work, 1,1-dibromocycloalkanes with 3-, 4-, and 5-member rings were synthesized using previously described procedures.\footnote{Napolitano, E.; Fiachi, R.; Mastrorilli, E. Synt. Comm. 1986, 122-125.} \footnote{ingh, R. K.; Danishefsky, S. J.Org.Chem. 1975, 40, 2969-2970.} \footnote{Blankenship, C.; Paquette, L. A. Synt. Comm. 1984, 14, 983-987.} For all three samples, ultrafast time-resolved absorption experiments with UV-excitation were performed in acetonitrile and methylcyclohexane solvents. It was shown that excitation of dibromocycloalkane solutions with 250 nm short (40 fs) pulses forms excited state absorption (ESA) of parent molecules in the spectral range from 360 to 760 nm. Within 800 fs, ESA decays with beginning of formation product bands. Next within 50 ps, these broad bands reach their maximum intensity and form well-defined broad peaks centered at 550, 600 and 400 nm for 3-, 4-, and 5-member rings respectively. These product species are long-lived and begin to decay at 1 ns. Obtained results suggest the formation of isomer products \chem{(CH_2)\chem{n}-C-Br-Br} (n=1-3), similar to the isomeric species (HBrCBr-Br) observed in isomerization of bromoform.\footnote{Pal, S. K.; Mereshchenko, A. S.; Butaeva, E. V.; El-Khoury, P. Z.; Tarnovsky, A. N. J. Chem. Phys. 2013, 138, 124501.} \footnote{Mereshchenko, A. S.; Butaeva, E. V.; Borin, V. A.; Eyzips, A.; Tarnovsky, A. N. Nat. Chem. 2015, 7, 562-568.

Dynamical Chaos and Level Splitting under the Channeling of the High Energy Positrons in [100] Direction of the Silicon Crystal

The motion of charged particles in a crystal in the axial channeling regime can be both regular and chaotic. The chaos in quantum case manifests itself in the statistical properties of the energy levels set. These properties have been studied previously for the electrons channeling along [110] direction of the silicon crystal, in the case when the classical motion was completely chaotic, as well as for the ones channeling along [100] direction, when the classical motion can be both regular and chaotic for the same energy depending on the initial conditions. Here we study the positrons channeling in [100] direction. This case is of special interest due to the substantial tunneling probability between dynamically isolated regular motion domains in the phase space. The interaction of the energy levels via tunneling distinctly changes the level spacing statistics. All transverse motion energy levels as well as corresponding stationary wave functions are computed numerically for the 30 GeV positrons channeling in [100] direction of the silicon crystal. The values of the matrix elements for the tunnel transitions are extractad from these data. These results confirm the chaos assistance for the tunneling and the level splitting. These values will be used in the further researches of the quantum chaos manifestations in the channeling phenomenon.Comment: Presented on the XIV International Symposium "Radiation from Relativistic Electrons in Periodic Structures", September 18-22, 2023, Tsaghkadzor, Armeni

NA49/NA61: results and plans on beam energy and system size scan at the CERN SPS

This paper presents results and plans of the NA49 and NA61/SHINE experiments at the CERN Super Proton Synchrotron concerning the study of relativistic nucleus-nucleus interactions. First, the NA49 evidence for the energy threshold of creating quark-gluon plasma, the onset of deconfinement, in central lead-lead collisions around 30A GeV is reviewed. Then the status of the NA61/SHINE systematic study of properties of the onset of deconfinement is presented. Second, the search for the critical point of strongly interacting matter undertaken by both experiments is discussed. NA49 measured large fluctuations at the top SPS energy, 158A GeV, in collisions of light and medium size nuclei. They seem to indicate that the critical point exists and is located close to baryonic chemical potential of about 250 MeV. The NA61/SHINE beam energy and system size scan started in 2009 will provide evidence for the existence of the critical point or refute the interpretation of the NA49 fluctuation data in terms of the critical point.Comment: 11 pages, invited talk at Quark Matter 201