Compared to the expansion into a vacuum, the interaction of the plasma plume with a reactive gas is a far more complex gas dynamics process due to the rise of many physical phenomena involved including deceleration, recombination, formation of shock waves and clustering. At intermediate ambient gas pressure, the ablated material and the ambient gas are compressed, and at a certain time the plume pressure equals the background gas pressure. As a result the plume pulsates backward [11]. The plasma reflection was already reported by Geohegan [12] using fast ICCD photography of graphite laser ablation into high-pressure argon. Also an oscillatory behavior of plasma induced by laser ablation of YBa2Cu3O7−x in oxygen and noble gases was revealed by Bulgakov and Bulgakova [13]. Their study was theoretically and experimentally made using a two-fluid gas dynamics model and time-of-flight mass spectrometry, respectively.
Our work is related to the characterization of graphite laser ablation in a nitrogen environment by emission spectroscopy at relatively high laser fluences. In the current paper we report the results obtained by analyzing the temporal profile of the radical CN as a function of distance from the target surface for two N2 pressures, 0.5 and 1 mbar. Our experimental finding shows a double structure of the CN optical time of flight (TOF) spectra. The two components (slow and fast) evolve differently in time and in space. The study of the spatio-temporal evolution of the two components shows a moving back of the CN slow species, which were submitted to a reflection by the shock wave at the plasma front–gas interface.