<title>Charged-particle emission from dielectric materials initiated by a tunable picosecond mid-infrared laser</title>

Abstract

In the ultraviolet, visible and near-infrared, single and multiphoton electronic transitions can explain the production and emission of charged atoms, molecules and photoelectrons during laser ablation and desorption. However, the process of charge transfer and ionization during ablation of dielectrics in the mid-infrared is not well understood. Even though significant electronic excitation is unlikely, copious emission of charged particles, e.g. atoms, molecules and electrons, is observed. No evidence of laser plume interactions is observed and inverse Breinsstrahlung (IB) is ruled out as a primary ionization/charge transfer mechanism. By irradiating with an ultrashort pulse-width mid-infrared laser tuned to a vibrational resonance it is possible to generate a high vibrational excitation density in dielectric materials. This high excitation density creates a non-equilibrium state of matter that exists until the deposited energy fully thermalizes. In this paper we report measurements of the kinetic energy of ions and electrons from CaCO3, NaNO3 and dihydroxybenzoic (DHB) acid that are highly non-thermal. This non-thermal energy distribution is evidence that the primary production of charged species occurs while the material is in a non-equilibrium state. The fact that it occurs in quite different materials, and without some of the characteristic signatures of electronically induced desorption and ionization, points toward a new mechanism.