Rate for Laser-Induced Nuclear Dipole Absorption

Using the Brink-Axel hypothesis we derive the rate $R$ for nuclear dipole excitation by a laser pulse carrying $N\gg 1$ photons with average energy $\mathrm{\hslash }{\omega }_0\approx 5$ MeV. As expected $R\propto (\mathrm{\hslash }{\omega }_0{)}^3$. The rate is also proportional to the aperure $\alpha$ of the laser pulse. Perhaps less expected is the fact that $R\propto N$, irrespective of the degree of coherence of the laser pulse. The expression for $R$, derived for a nearly stationary laser pulse, is valid also for short times and can, thus, be used in simulations via rate equations of multiple nuclear dipole excitations by a single pulse. The explicit dependence of $R$ on the parameters of the laser pulse and on nuclear parameters given in the paper should help to optimize experiments on laser-nucleus reactions.