  Optical spectroscopy with broadband femtosecond laser pulses often involves
simultaneous excitation of multiple molecular species with close resonance
frequencies. Interpreting the collective optical response from molecular
mixtures typically requires Fourier analysis of the detected time-resolved
signal. We propose an alternative method of separating coherent optical
responses from two molecular species with neighboring excitation resonances
(here, vibrational modes of oxygen and carbon dioxide). We utilize
ro-vibrational coupling as a mechanism of suppressing the strong vibrational
response from the dominating molecular species (O$_{2}$). Coherent
ro-vibrational dynamics lead to long "silence windows" of zero signal from
oxygen molecules. In these silence windows, the detected signal stems solely
from the minority species (CO$_{2}$) enabling background-free detection and
characterization of the O$_2$/CO$_2$ mixing ratio. In comparison to a Fourier
analysis, our technique does not require femtosecond time resolution or
time-delay scanning.
