Photodissociation dynamics of gas phase polyatomic molecules studied using laser multiphoton ionization mass spectrometr

Molecular dissociations can generally be classified as statistical or non-statistical. In statistical dissociations, rapid electronic to vibrational energy redistribution occurs prior to dissociation, forming fragments which can have substantial internal energy. In non-statistical dissociations excitation to purely repulsive potential energy surfaces results in the dissociation being faster than energy redistribution, forming fragments with high kinetic energy but low internal energy.;The picosecond dynamics of energy redistribution in 1,4-dichlorobenzene and other polyatomics is examined by a two-color optical delay multiphoton absorption mass spectrometry experiment. The UV mulitphoton absorption mass spectrum of 2,4-hexadiyne is examined as a function of laser wavelength. The ionic fragment intensity ratio C$sb4$H$sb{4sp+}$/C$sb4$H$sb{3sp+}$ is in good agreement with the previously published one VUV photon ratio suggesting that statistical dissociation of the parent ion is occurring. The wavelength dependence of the observed kinetic energies of some other ionic fragments of 2,4-hexadiyne are presented. When 2,4-hexadiyne absorbs multiple photons of UV light both statistical and non-statistical dissociations occur.;The fragment I$sp+$ observed in the methyl iodide mass spectrum is formed by both statistical (from CH$sb3$I$sp+$ dissociation) and non-statistical (from the dissociation of neutral CH$sb3$I followed by ionization of I) dissociation channels, depending on the 266 nm laser pulselength. Similar results are observed in the multiphoton absorption of iodobenzene. The mass peak lineshape of the I$sp+$ signal from CH$sb3$I is examined as a function of laser polarization in a time-of-flight mass spectrometer at three laser wavelengths. The transitions at 243.68, 266 and 304.02 nm are predominantly parallel, in contradiction to previous conclusions from magnetic circular dichroism studies. The observed kinetic energy of I$sp+$ suggests that the dissociation occurs slightly slower at 300 nm than at 266 nm. Results from a newly developed time-of-flight technique, which measures the relative translational energy of fragments formed from non-statistical photodissociation, are presented. This is applied to a series of iodo-butanes. It is found that the value and the distribution of the alkyl fragment internal energy following dissociation is sensitive to the iodine atom location, with more electron-vibrational coupling (leading to a larger distribution of internal energy) observed if the iodine is attached to a middle carbon. (Abstract shortened with permission of author.).