  We show that multiple-photon absorption of radiation from a 10.56 {\mu}m cw
CO2 laser by intermediates (ethylene, vinylidene) generated in pure acetylene
flow makes them decompose to carbon dimers and excited hydrogen. The latter
associates with downstream acetylene to feedback those laser absorbing
intermediates thus making the reactions self-sustained in the absence of
oxygen. This process is different from acetylene self-decomposition that may
occur at higher temperature and pressure. The results of our work may be useful
for understanding the generation of various carbon allotropes and interstellar
dust from acetylene.
