Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet detection of the products

Vacuum ultraviolet ~VUV! laser photoionization is combined with time-of-flight ~TOF! mass spectrometry to determine the photofragments produced from the laser photodissociation of allene and propyne in a molecular beam. Detection of C3H3 1 confirms that atomic hydrogen elimination is the primary process for both of these molecules. A hydrogen molecule elimination channel and a low mass carbon fragmentation channel of allene to produce C3H21H2 and CH21C2H2 , respectively, have also been identified. Different ratios of various dissociation channels from these two molecules suggest that the dissociation mechanisms of these two isomers are different. Dissociation must occur before complete isomerization. These results are discussed in terms of recent theoretical calculations on the ground and excited states of these molecules. Secondary photodissociation of the products has been observed, even though the laser energies that have been used are less than 8 mJ/cm and the photolysis laser is not focused. Therefore, the present results show how important it is to determine product distributions as a function of the laser energy. © 1999 American Institute of Physics. @S0021-9606~99!00807-7#