The reaction of ground-state carbon, C(P-3(j)), with the propargyl radical, C3H3((XB2)-B-2), is investigated at an average collision energy of 42.0 kJmol(-1) employing the crossed molecular beams technique and a universal mass spectrometric detector. The laboratory angular distribution and time-of-flight spectra of the C4H2 product are recorded at m/e = 50. Forward-convolution fitting of our data reveals the formation of diacetylene, HCCCCH, in its X-1 Sigma(g)(+) electronic ground state. The reaction dynamics are governed by an initial attack of C(P-3(j)) to the pi-electron density at the acetylenic carbon atom of the propargyl radical, followed by a [1,2]-hydrogen migration to the n-C4H3 isomer. A final carbon-hydrogen bond rupture yields atomic hydrogen and diacetylene through a tight exit transition state located 30-60 kJmol(-1) above the products. This first successful crossed molecular beams study of a reaction between an atom and a free radical marks the beginning of the next generation of crossed beams experiments elucidating the formation of molecular species in combustion processes, chemical vapor deposition, in the interstellar medium, outflows of carbon stars, and hydrocarbon-rich planetary atmospheres via radical-radical reactions. (C) 1997 American Institute of Physics.