Electron thermalization in ammonia clusters induced by femtosecond laser fields

Abstract

The ionization of large-sized ammonia clusters in 800 nm femtosecond laser fields is investigated by using the velocity map imaging method. The photoelectron spectra obtained at laser intensities of $\sim$10¹³ W/cm² exhibit two distinct regions with structureless, exponentially decaying distributions. By utilizing electron temperature, these distributions are characterized and the different mechanisms are revealed. In the low-energy region ($ϵ$ $\le$ 0.4 eV), the electron temperature increases with rising laser intensity, primarily due to the enhanced probability of frustrated recombination, where quasi-free electrons are temporarily trapped before eventual ionization. In contrast, in the high-energy region (0.4 eV $\le$ $ϵ$ $\le$ 20 eV) the electron temperature decreases with increasing laser intensity, highlighting the crucial role of cluster expansion during electron thermalization. The photoelectron angular distributions reveal the isotropic nature of thermalized electrons, while field-induced electron scattering significantly influences even high-energy electrons ($ϵ$ $\ge$ 2U${}_p$) beyond thermalization. Our results provide new insights into the underlying mechanisms governing electron thermalization in ammonia clusters under intense laser fields.