Velocity dependence of the sensitized fluorescence of nitrogen by metastable argon

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Abstract
The velocity dependence of the excitation of nitrogen to N2(C 3Πu) by metastable argon has been redetermined using a time‐of‐flight velocity selected argon beam. The cross section velocity dependence is in good agreement with the previous determination using electronic pulsing of the metastable Ar(3P2,0). With mechanical pulsing, which eliminates the dc ground state Ar beam, the cross section function is different, rising more slowly to a maximum at a collision energy of about 0.3 eV and gradually decreasing up to 0.8 eV. All three TOF determinations extrapolate to zero threshold energy. Mechanical pulsing is believed to give the best cross section function, and the differences between this function, the electronically pulsed function, and a function determined using a rotary velocity selector are attributed to exchange collisions in the argon beams. The functions are normalized to the thermal rate constant to give the results in terms of absolute cross section vs collision energy.