Medium effects in photopion reactions (original) (raw)

Study of the reactions and near the coulomb barrier

Nuclear Physics A, 1973

The vector analyzing power and differential cross section have been measured at a deuteron energy of 12 3 MeV for 20Spb(~ ' p) transitions to seven states of2°9pb (Ex = 0 0, 0 79, 1 42, 1 56, 2 01, and 2 47 +2 51 MeV), for 2 o Spb(~ ' t) transitions to three states of 2°7pb (Ex = 0 0, 0 57, and 0 89 MeV), and for 2 o 8pb (a, d) Two deuteron optical model potentials were obtained from analysis of the elastic scattermg measurements, and were used in a DWBA analysis of the present transfer-reaction results and of previous results for 2°Tpb (d, po)2°sPb The sensmvlty of the DWBA analysis to the nuclear interior is investigated The relative importance of Coulomb and of nuclear distortions is discussed The sign of the vector analyzing power for given l. depends on j. and on Q-value for both the (d, p) and the (d,t) reactions The Q-dependence invalidates the simple rule proposed by Grlffith and Roman to relate jn to the predominant sign of iT11 Comparison of the measurements and DWBA predictions for lT1~ allows unambiguous determination ofjo for all transmons except 2°7pb(~, Po) All spins determined in the present work confirm previous assignments The vector analyzing powers for the 2°sPb(d, to)2°Vpb and 2°Tpb(~, Po) 2°8pb transitions are observed to be similar in angular dependence but opposite in sign at all angles The sign reversal reflects differences in the distortions of the proton and triton waves NUCLEAR REACTIONS 2°8Pb(d, d), Ea = 12 3 MeV, measured vector analyzing power tTll(O), cross section cr(O) Deduced optical model parameters 2°8pb(~,p), 2°SPb(d, t), Ea = 12 3 MeV, measured tTll(Q, 0), 6(Q, O) 2°7pb, 2°9pb deduced j~r, spectroscopic factors Enriched target

In-Medium Modifications of the ππ Interaction in Photon-Induced Reactions

Physical Review Letters, 2002

Differential cross sections of the reactions (γ, π • π • ) and (γ, π • π +/− ) have been measured for several nuclei ( 1 H, 12 C, and nat Pb) at an incident-photon energy of Eγ =400-460 MeV at the taggedphoton facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the ππ invariant-mass distribution is found in the π • π • channel. This dependence is not observed in the π • π +/− channel and is consistent with an in-medium modification of the ππ interaction in the I=J=0 channel. The data are compared to π-induced measurements and to calculations within a chiral-unitary approach.

Ultra-fast photochemistry in the strong light-matter coupling regime

Strong coupling between molecules and confined light modes of optical cavities to form polaritons can alter photochemistry, but the origin of this effect remains largely unknown. While theoretical models suggest a suppression of photochemistry due to the formation of new polaritonic potential energy surfaces, many of these models do not account for the energetic disorder among the molecules, which is unavoidable at ambient conditions. Here, we combine experiments and simulations to show that for an ultra-fast photochemical reaction such thermal disorder prevents the modification of the potential energy surface and that suppression is due to radiative decay of the lossy cavity modes. We demonstrate that by increasing the coupling strength we can reduce such losses and enhance reactivity of the strongly coupled system, in contrast to the theoretical paradigm, which would predict stronger suppression. We also show that the excitation spectrum under strong coupling is a product of the e...

Quantum dynamics studies of the photodissociation of molecular systems

2013

Bibliography 218 Contents v List of Tables xxiii Glossary xxv the absorption of light. Within the framework of non-relativistic quantum mechanics, such a process may be cast in the following form. An initial state |ψ i is taken to propagate on the ground state potential until time t = 0. Taking |ψ i to be an eigenstate of the ground state potential, the initial wavefunction changes only by a phase factor. At time t = 0, the absorption of a single photon results in the promotion of the state |ψ i to an electronically excited state, which, assuming vertical excitation, is given in Photodissociation processes are typically challenging to model in an accurate manner for two primary reasons. Firstly, unlike the preponderance of ground state processes, the excitation of a molecular system to an electronically excited state denies us, in general, the possibility of treating in a satisfactory manner the electronic and nuclear degrees of freedom as being decoupled. Thus, the evolution of the wavepacket |φ i has be considered to occur over a manifold of vibronically coupled electronic states. Secondly, the πσ * , valence-type characters as the the X-H dissociation coordinate is traversed. Explanation of this change in character is typically made by taking the adiabatic potential in question to correspond to an avoided crossing between two diabatic states: a lower, bound 3s Rydberg state that is vibronically coupled to a higher-lying, purely dissociative πσ * state by the X-H

Orthogonality, antisymmetry and c.m. effects in direct photoreactions

Nuclear Physics A, 1984

The relevance of different approximations usually adopted in the description of direct knockout of nucleons in photoreactions at medium energies is investigated. Orthogonality between initial and final states in DWIA and antisymmetrization of the outgoing particle are studied in the presence of recoil terms arising from a correct treatment of the c.m. motion. The different effects are discussed in the specific example of the '*C target nucleus for different combinations of bound and scattering states. Whereas orthogonality and antisymmetrization appear to play a rather relevant role only at backward angles, the recoil term is particularly important for the (y. n) reaction and cannot be simulated by the use of multipole energy-independent effective charges.

Medium effects in coherent pion photo- and electroproduction on He and C

Nuclear Physics A, 1999

Coherent π 0 photo-and electroproduction on 4 He and 12 C nuclei is investigated in the framework of a distorted wave impulse approximation in momentum space. The elementary process is described by the recently developed unitary isobar model. Medium effects are considered by introducing a phenomenological ∆ self-energy. The recent experimental data for 4 He and 12 C can be well described over a wide range of energies and emission angles by the assumption that the ∆-nuclear interaction saturates.