Nuclear and atomic quantum dynamics

The borderline between atomic and nuclear physics offers a special playground for quantum dynamics effects. As a primary incentive, quantum control techniques known from atomic systems can be transferred to nuclear systems. This is encouraged by the advent of coherent x-ray light sources such as the x-ray free electron laser (XFEL) that opens the possibility for quantum optics schemes with nuclei. The search for versatile tools for enhancing control in nuclear physics and for coherent manipulation of nuclear states has a two-fold motivation. On the one hand, the lack of coherent gamma-ray sources argue in favor of trying to control nuclear gamma-ray transitions and transform nuclei in such sources. On the other hand, nuclei present long-lived excited states called isomers that can store large amounts of energy over long periods of time. Controlled release of the stored energy may thus lead to nuclear batteries which operate without fusion or fission.

Closer to atomic physics, one may explore the coupling between the atomic and nuclear degrees of freedom in nuclear processes that involve atomic electrons. Nuclear excitation by electron capture (NEEC) and nuclear excitation by electron transition (NEET) are such examples. Processes coupling nuclei to the atomic shell have rather small cross sections compared to purely atomic ones. However, NEEC and NEET can be the most efficient nuclear excitation mechanisms for small transition energies and are expected to play an important role in dense plasmas. The importance of the coupling of nuclei to the atomic shell for isomer depletion and for nucleosynthesis and the element abundance in the universe is to be investigated from the theoretical side.

Interaction of coherent x-ray light with nuclei

Figure 1: (a) A nuclear three-level system for STIRAP. The initial nuclear population is concentrated in state 1, which decays slowly and thus stores energy. The pump laser P drives the transition 1 → 3 and the Stokes laser S the transition 2 → 3. The population is coherently transferred from state 1 to state 2, which decays rapidly and releases the stored energy. (b) Two partially overlapping and copropagating x-ray laser pulses P and S interact with relativistically accelerated nuclei. The Doppler effect ensures that both nuclear transitions are in one-photon resonance with the laser pulses.

While optical lasers have proved to be the basis for many successful methods to control and explore the dynamics of atomic electron shells, the forthcoming coherent XFEL opens the possibility to explore also nuclei with similar methods. We have shown that the XFEL makes possible the direct resonant interaction of nuclei with light and the excitation of low-lying nuclear levels [1]. In order to exploit also the coherence of the novel x-ray light sources, we study the nuclear coherent population transfer occurring when two x-ray laser beams drive a nuclear three-level system in a stimulated Raman adiabatic passage setup (STIRAP) [2] reminiscent of atomic quantum optics. Moderate acceleration of the target nuclei can be used to tune the x-ray photon energy in resonance with the nuclear transition energies when necessary. The coherent population transfer occurring between an isomer and a short-lived nuclear state can be used for the release the energy stored in the former.

Nuclear excitation mechanisms in plasmas

Figure 2: If the atomic and nuclear transition energies match, an electron can recombine into an ion (left panel) with the simultaneous excitation of the nucleus (right panel) from the ground state G to the excited state E [1].

Ultra-strong laser fields can produce nowadays plasmas which approach solid-state values for the electron density and simultaneously have high photon densities. Such conditions simulate dense astrophysical plasmas, considered to be the sites of nucleosynthesis processes. In such environments, both nuclear excitation by photoabsorption as well as via coupling to the atomic shells is possible, populating higher nuclear states and potentially influencing the formation paths for heavy elements. The two nuclear excitation mechanisms involving atomic electrons are NEEC - the recombination of a free electron into a highly charged ion with the excitation of the nucleus [1], and NEET - the simultaneous excitation of the nucleus during an atomic decay transition, provided the energies of the two transitions match [2]. Our results show that for small transition energies NEEC is more efficient than photoabsorption for excitation of nuclei [3], a fact that speaks for the importance of the coupling of nuclei to atomic shells. Planned experiments at ultra-strong laser facilities in Germany and the US are aiming at the identification and quantification of the nuclear excitation mechanisms in plasmas. Such a task is only possible with substantial theoretical support.

Collective effects in the interaction of nuclei with X-ray light

Figure 3: Coherently scattered light (red line) compared to incoherent natural decay (black line) and superradiant decay immediately after the light pulse (green line).

With the commissioning of the new XFEL facilities, it becomes possible to investigate the direct excitation of low-lying nuclear states with x-rays. Photoexcitation experiments with synchrotron radiation have already shown that collective effects may occur in the interaction between nuclei in solid-state targets and light [1,2]. These can be explained by the creation of an exciton, i.e., a single delocalized excitation spread over all nuclei in the sample. The collective effects lead to the appearance of a coherent decay channel that accelerates the nuclear decay.

Due to the high intensity and coherence of the XFEL light the situation changes dramatically compared to excitation by synchrotron radiation. On the one hand, a XFEL pulse may produce more than a single nuclear excitation, such that several excitons may come to life and interact with each other. This can play an important role for the broadening of the nuclear widths and opens the possibility to control them [3]. On the other hand, the high intensity of the XFEL laser pulse leads to the fast explosion or melting of the nuclear target shortly after the pulse, which in turn affects the time evolution of the collective effects. The decay of the exciton may offer thus important information about the time dependence of the energy exchange in the interaction between the XFEL and matter.

Laser-assisted tunneling of quasistationary states

Figure 3: Test case 1-D short-range potential (a rectangular barrier) and the qualitative photoelectron spectra for strong-field ionization from a true bound state (B) and a quasi-stationary state (QS).

Starting from the so-called Strong-Field Approximation (SFA) and its formulation in terms of trajectories in imaginary time, we have developed a new method to describe both qualitatively and quantitatively the tunneling of quasistationary states in laser fields in the semiclassical parameter regime [1]. For a start, this method was applied for the test case of a short-range potential. As a next step, our results can be generalized to describe tunneling through Coulomb barriers. This is particularly interesting due to a number of applications in atomic physics where laser-assisted tunneling of auto-ionizing states could be investigated. The extension of this method would however by no means be restricted to atomic physics. Important processes in nuclear physics, such as alpha decay, or in solid-state physics in the field of photon-assisted transport in superconductor junctions are based on the same underlying phenomena and can be treated as laser-assisted tunneling of quasistationary states. Due to the general statement of the problem and to the moderate numerical effort required, our method is offering an advantageous alternative to other approaches such as the Floquet method for tunneling in the multi-photon regime.

Theory Division