Discrimination of product isomers in the photodissociation of propyne and allene at 193 nm

The photodissociation dynamics of propyne and allene are investigated in tw o molecular beam/ photodissociation instruments, one using electron impact ionization and the other using tunable vacuum ultraviolet (VUV) light to ph otoionize the photoproducts. The primary dissociation channels for both rea ctants are C3H3 + H and C3H2 + H-2. Measurement of the photoionization effi ciency curves on the VUV instrument shows that the C3H3 product from propyn e is the propynyl (CH3CC) radical, whereas the C3H3 product from allene is the propargyl (CH2CCH) radical. The dominant C3H2 product from both reactan ts is the propadienylidene (H2CCC) radical. We also observe a small amount of secondary C3H2 product from photodissociation of the C3H3 radicals in bo th cases. (C) 1999 American Institute of Physics. [S0021-9606(99)02008-5].