The capability to produce femtosecond laser pulses with wavelengths in the atmospheric absorption windowrequires a new understanding of pulse propagation effects. In this work, we characterize the changes in temporalpropagation of middle infrared femtosecond laser pulses by cross-correlation frequency resolved optical gating(XFROG). The temporally distorted infrared pulses are cross-correlated with 800 nm pulses by a four-wavemixing process in air. For the first time, we investigate these propagation effects through gas molecules that arenot present in the atmosphere. Each molecule is shown to have a unique effect on the temporal propagation ofthe pulse that is wavelength dependent. We verify our experimental data with simulations based on a Kramers-Kronig transformation of spectral data from the HITRAN database. The propagation effects are similar to opticalfree induction decay. Multiple vibrational and rovibrational absorption lines are excited by the middle infraredpulse and constructive interference occurs at various delay times relative to the initial pulse. The constructiveinterference impresses a unique fingerprint onto the pulse because the spectral lines of each molecule are unique.The fingerprint behaves as a nonlinear function related to the molecular concentration. To account for this, aregression model is developed to predict the concentration of unknown gas species. The middle infrared beam isthe only laser beam sensitive to the analytes. Thus, standoff detection is a possibility since the XFROG can beperformed locally.
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