Analysis of influence of the Maxwell distribution of the transverse thermal velocities and of the flight-time-determinedcharacteristic relaxation rates (i.e. the inverse relaxation times T1,2) of the acetylene (C2H2) molecules in the hollow-corephotonic crystal fiber on nonlinear optical effects are presented. The theoretical predictions are compared with theexperimental data obtained in the ～0.4Torr acetylene-filled fiber cell at the wavelength 1530.37nm of the most effectiveP9 vibrational-rotational transition of ~（12）C_2H_2. At room temperature and the fiber mode field diameter of 7.5μm, the averagetransverse thermal velocity of ～390m/s ensured relaxation times T1,2 ～8-10ns. These are in good agreement with thecorresponding values experimentally measured using delayed optical nutation and two-photon echo techniques. Theexperimentally observed nonlinear effect of the polarization ellipse self-rotation proves to be at least two orders ofmagnitude less efficient comparing with that reported earlier for the alkali metals vapors.
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