Total cross section measurements for 1-400 eV positrons and electrons in C2H2

Total cross section (TCS) for 0.7-400 eV positrons and 1-400 eV electrons colliding with acetylene (C 2 H 2 ) molecules are measured using a retarding potential time-of-flight method. The results are compared with those in ethylene (C 2 H 4 ), ethane (C 2 H 6 ) and methane (CH 4 ) molecules. The effect of the triple bond in C 2 H 2 and the double bond in C 2 H 4 on the TCS curves is explained by the polarisation interaction. The positronium (Ps) formation cross section is also estimated just above threshold.