A validation of a simple model for the calculation of the ionization energies in X-ray laser-cluster interactions

Abstract

The outer-ionization of an electron from a cluster is an unambiguous quantity, while the inner-ionization threshold is not, resulting in different microscopic quantum-classical hybrid models used in laser-cluster interactions. A simple local ionization threshold model for the change in the ionization energy is proposed and examined, for atoms and ions, at distances in between the initial configuration of the cluster to well into the cluster's disintegration. This model is compared with a full Hartree-Fock energy calculation which accounts for the electron correlation effects using the coupled cluster method with single and double excitations with perturbative triples (CCSD(T)). Good agreement is found between the two lending a strong theoretical support to works which rely on such models for the final and transient properties of the laser-cluster interaction.

Quantum Monte Carlo calculations of energy gaps from first principles

“…51(1) eV and 21.96(2) eV respectively [with MAE values of 0.47(6)% and 1.62(8)%], and here we see the most improvement from reoptimization. The MAE of the SJB-DMC result is 0.52(5)%, meaning that the results from SJB-VMC and SJB-DMC with reoptimized backflow functions are effectively as good as each other-although SJB-VMC underestimates and SJB-DMC overestimates the ionization potential.A recent coupled cluster [CCSD(T)] calculation determined the first and second ionization potentials of Ne as 21.564 eV and 44.3 eV, respectively [absolute errors of 0.04930 eV (0.23%) and 3.30890 eV (8.1%) with respect to the "exact" nonrelativistic results that we have compared against] 117. A less recent configuration interaction calculation determined the eighth ionization potential of Ne as 238.78440 eV [absolute error of 0.00509 eV (0.0021%)] 118.…”

Induced transparency in the XUV: a pump-probe test of laser-cluster interactions

“…The ionization rates are determined from a mix of experimental (when available) and theoretical cross-sections in the gas phase [20]. During ionization, the perturbation on the ions due to the cluster environment (the nanoplasma) has been shown to be well represented by our local ionization threshold model [28], which maintains the use of atomic ionization potentials. Using the local ionization threshold model, we include single-and multi-photon ionization, collisional ionization, augmented collisional ionization [20], and many-body recombination [30].…”

“…These plasma mechanisms attempt to distort the atomic potentials and allow ionizations to occur which would otherwise be energetically impossible for photoionization. Work which only included augmented collisional ionization using the local ionization threshold model [20,28], which treats the cluster potential as a constant potential perturbation, has successfully reproduced the results of multiple experiments [29], including experiments where Auger ionization is dominant [30]. Thus, the question arises: are the plasma effects on ionization, such as ionization potential lowering, negligible in small clusters?…”

Intense VUV–xenon-cluster interaction revisited

“…The excitation and ionization cross sections are taken from gas experimental results when available and theoretical calculations otherwise [23]. The local ionization threshold model is used to extend the gas ionization model into the cluster environment [10].…”

“…This reduced IP was restricted to photoionization and gave the system access to higher charge states by direct photoionization. A version of this model has been applied beyond the VUV into the extreme ultraviolet (XUV) [8,9], although subsequent direct quantum calculations suggest problems with the model [10]. Recent proposals have sought to test the existence of enhanced ionization mechanisms directly [11,12] since it was apparently absent in experiments where an aluminum foil was irradiated in the hard-x-ray regime [13,14].…”