  We report an approach for high-resolution spectroscopy using a widely tunable
laser emitting in the molecular fingerprint region. The laser is based on
difference-frequency generation (DFG) in a nonlinear orientation-patterned GaAs
crystal. The signal laser, a CO2 gas laser, is operated in a kHz-pulsed mode
while the pump laser, an external-cavity quantum cascade laser, is finely
mode-hop-free tuned. The idler radiation covers a spectral range of 11.6 - 15
{\mu}m with a laser linewidth of 2.3 MHz. We showcase the versatility and the
potential for molecular fingerprinting of the developed DFG laser source by
resolving the absorption features of a mixture of several species in the
long-wavelength mid-infrared. Furthermore, exploiting the wide tunability and
resolution of the spectrometer, we resolve the broadband absorption spectrum of
ethylene (C2H4) over 13 - 14.2 {\mu}m and quantify the self-broadening
coefficients of some selected spectral lines.
