Investigations of Laser Evaporation in Ambient Pressure Helium with Ultrafast Hard X-ray Pulses

Abstract

High-spatial and -temporal resolution direct imaging of the plumes created in laser evaporation in ambient pressure helium was achieved for the first time with an ultrafast hard X-ray source. Picosecond and nanosecond laser pulses were used to evaporate a thin metal film in ∼1 atm of helium. The plumes, which contained atoms, ions, and possibly nanoparticles, were imaged with hard X-ray pulses emitted from a tungsten rod ultrafast hard X-ray apparatus. No evaporation was observed in the first nanosecond or so after irradiation with either picosecond or nanosecond ablation pulses. Approximately 10 jam thick plumes were clearly observed at ∼100 nanoseconds after laser irradiation. The corresponding slow speed of the plumes was attributed mainly to the high-pressure background gas. Attempts were made to identify the composition of these plumes with a dual laser pulse evaporation-ultrafast X-ray pulse imaging scheme.