  The spectral dynamics of a mid-infrared multimode Cr^2+:ZnSe laser located in
a vacuum sealed chamber containing acetylene at low pressure is analyzed by a
stepping-mode high-resolution time-resolved Fourier transform interferometer.
Doppler-limited absorption spectra of C_2H_2 in natural isotopic abundance are
recorded around 4000 cm^-1 with kilometric absorption path lengths and
sensitivities better than 3 10^-8 cm-1. Two cold bands are newly identified and
assigned to the n_1+n_4^1 and n_3+n_5^1 transitions of ^12C^13CH_2. The
n_1+n_5^1 band of ^12C_2HD and fourteen ^12C_2H_2 bands are observed, among
which for the first time n_2+2n_4^2+n_5^-1.
