Rate for laser-induced nuclear dipole absorption

Abstract

Using the Brink-Axel hypothesis we derive the rate R for nuclear dipole excitation by a laser pulse carrying N 1 photons with average energyhω 0 ≈ 5 MeV. As expected R ∝ (hω 0) 3. The rate is also proportional to the aperture α of the laser pulse. Perhaps less expected is the fact that R ∝ 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.

Transport equations for driven many-body systems

“…Every such photon absorption process is followed by partial or complete equilibration, depending on the ratio of the rate for dipole absorption (derived in Ref. [7]) and the rate Γ ↓ / for equilibration. For the theoretical description of the sequence of alternating absorption and equilibration processes, the Schrödinger equation is useless (nuclear level densities attain enormous values already several 10 MeV above the ground state, so that a numerical treatment is out of the question), and transport equations are called for.…”

“…These are given in Ref. [7]. We also suppress the (small) fluctuations of the actual photon frequencies in the laser pulse around the mean value ω 0 .…”

“…[2,3]. The rate for dipole absorption used in these papers was later shown [7] to follow from the the Brink-Axel hypothesis [5,6]. If, on the other hand, the rates for dipole absorption and for internal relaxation are approximately equal, both processes occur simultameously.…”

“…For a laser pulse carrying N photons with mean energy ω 0 , the rate R for photon absorption from an arbitrary initial nuclear state |iJ i with energy E i and spin J i has been calculated perturbatively in Ref. [7] with the help of the Brink-Axel hypothesis. As stated in Section II A, the hypothesis says that dipole absorption by the state |iJ i populates the normalized dipole modes |d(i)J f pertaining to that state.…”

“…Combined with plausible estimates for R (that have later been confirmed in Ref. [7]), the resulting rate equations have been used in Refs. [2,3] to calculate laser-induced multi-photon absorption in nuclei in the quasiadiabatic regime.…”