  Non-thermal molecular plasmas play a crucial role in numerous industrial
processes and hold significant potential for driving essential chemical
transformations. Accurate information about the molecular composition of the
plasmas and the distribution of populations among quantum states is essential
for understanding and optimizing plasma processes. Here, we apply a
mid-infrared frequency comb-based Fourier transform spectrometer to measure
high-resolution spectra of plasmas containing hydrogen, nitrogen, and a carbon
source in the 2800 - 3400 cm$^{-1}$ range. The spectrally broadband and
high-resolution capabilities of this technique enable quantum-state-resolved
spectroscopy of multiple plasma-generated species simultaneously, including
CH$_4$, C$_2$H$_2$, C$_2$H$_6$, NH$_3$, and HCN, providing detailed information
beyond the limitations of current methods. Using a line-by-line fitting
approach, we analyzed 548 resolved transitions across five vibrational bands of
plasma-generated HCN. The results indicate a significant non-thermal
distribution of the populations among the quantum states, with distinct
temperatures observed for lower and higher rotational quantum numbers, with a
temperature difference of about 62 K. Broadband state-resolved-spectroscopy via
comb-based methods provides unprecedented fundamental insights into the
non-thermal nature of molecular plasmas - a detailed picture that has never
been accomplished before for such complex non-thermal environment.
