Resonance-like enhancement in high-order above-threshold ionization of formic acid

Abstract

We have investigated the high-order above-threshold ionization (HATI) spectrum of formic acid by means of high-resolution electron spectroscopy using 800-nm femtosecond laser light. The resonance-like enhancement structures in the HATI spectrum are shown to be experimentally reproducible and three enhancement-suppression patterns are observed in different energy regimes with increasing laser intensity. We simulated these patterns successfully by computing the exact solution of time-dependent Schr\"odinger equation based on the simple hydrogen-like atom, subject to a time-dependent high-intensity laser pulse. The agreement between experiment and theory shows that ``channel closing'' effects play an important role in the observed resonance-like enhancements (RLEs). The existence of the RLEs for formic acid, which lacks any symmetry, shows that molecular symmetry does not affect the RLEs.