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The acetylenic CH stretch mode (v1) of propargyl (H2CCCH) radical has been studied at sub-Doppler resolution (~60 MHz) via infrared laser absorption spectroscopy in a supersonic slit-jet discharge expansion, where low rotational temperatures (Trot = 13.5(4) K) and lack of spectral congestion permit improved determination of band origin and rotational constants for the excited state. For the lowest J states primarily populated in the slit jet cooled expansion, fine structure due to the unpaired electron spin is resolved completely, which permits accurate analysis of electron spin-rotation interactions in the vibrationally excited states (εaa = -518.1(1.8), εbb = -13.0(3), εcc = -1.8(3) MHz). In addition, hyperfine broadening in substantial excess of the sub-Doppler experimental linewidths is observed due to nuclear spin-electron spin contributions at the methylenic (--CH2) and acetylenic (--CH) positions, which permits detailed modeling of the fine/hyperfine structure line contours. The results are consistent with a delocalized radical spin density extending over both methylenic and acetylenic C atoms, in excellent agreement with simple resonance structures as well as ab initio theoretical calculations. [ABSTRACT FROM AUTHOR]