Knowledge Bank

We report direct measurement of S-branch Raman coherence lifetimes of CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ due to CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$-CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ collisions by employing picosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy. A custom-built, high-peak-power, nearly transform-limited ps laser system offers an ideal combination of frequency and temporal resolution for such measurements. The rotational S-branch transitions of CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ ground state [0,0$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$,0] with rotational quantum number J$=0-52$ were simultaneously excited by using a broadband ($\sim$3-nm) laser pulse with a full-width-half-maximum (FWHM) of $\sim$100ps.Thecoherencelifetimesofself-broadenedCO${2}$ for a pressure range of 0.05-1.5 bar were directly measured by probing the rotational coherence with a nearly transform-limited, 80-ps-long laser pulse. The measured linewidth of J=6 and J=50 transitions are found to be $\sim$0.106$\pm$0.0002 and $\sim$0.070$\pm$0.0002,respectively.Asexpected,theenergy-transferfromhighJlevelshasasignificantlylongercoherencelifetimebecauseoftheinertiaassociatedwithhigherangularmomentum.ThesemeasurementsareverysignificantforperformingaccuratethermometryorCO${2}$ concentration measurements in gas-phase reacting flows.