Keldysh parameter for laser-nucleus interactions

We show that the Keldysh parameter, which plays a central role in laser-driven ionization in atoms, molecules, and solid-state physics, can be meaningfully extended to strong-field laser-nucleus interaction settings, providing a key benchmark for laser-assisted nuclear dynamics, including the dynamics that drives nuclear fusion. We demonstrate that, similar to its counterpart in atomic, molecular, and solid-state physics, the nuclear Keldysh parameter, ${\ensuremath{\gamma}}_{\mathrm{n}}$, can be defined via the square root of the ratio of the height of the potential barrier separating the reactant and product states of a quantum system to the kinetic energy that a reduced-mass particle representing the respective two-body interaction acquires from the laser field. Adding to a striking parallelism with atomic, molecular, and solid-state physics, the ${\ensuremath{\gamma}}_{\mathrm{n}}\ensuremath{\approx}1$ borderline is shown to separate two distinctly different regimes of quantum dynamics in laser-nucleus interactions. While in the ${\ensuremath{\gamma}}_{\mathrm{n}}\ensuremath{\gg}1$ regime, such interactions are adequately described in terms of the standard Gamow tunneling exponential; in the ${\ensuremath{\gamma}}_{\mathrm{n}}\ensuremath{\ll}1$ regime, the Gamow-exponential approximation fails, as the tunneling of nuclear wave packets becomes strongly dispersive, connecting to the zeptosecond dynamics of laser-field-dressed nuclear wave functions.