Abstract
We report data from a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at several wavelengths in the range 236-266 nm, within their respective A-bands. The use of non-resonant single-photon ionization at 118.2 nm allows detection and velocity-map imaging of all fragments, regardless of their vibrotational or electronic state. The resulting photofragment kinetic energy and angular distributions and the quantum yields of ground-state and spin-orbit excited iodine fragments are in good agreement with previous studies employing state-selective detection via REMPI. The data are readily rationalised in terms of three competing dissociation mechanisms. The dominant excitation at all wavelengths studied is via a parallel transition to the (3)Q0 state, which either dissociates directly to give an alkyl radical partnered by spin-orbit excited iodine, or undergoes radiationless transfer to the (1)Q1 potential surface, where it dissociates to an alkyl radical partnered by iodine in its electronic ground state. Ground state iodine atoms can also be formed by direct dissociation from the (1)Q1 or (3)Q1 excited states following perpendicular excitation at the shorter and longer wavelength region, respectively, in the current range of interest. The extent of internal excitation of the alkyl fragment varies with dissociation mechanism, and is considerably higher for ethyl fragments from ethyl iodide photolysis than for methyl fragments from methyl iodide photolysis. We discuss the relative advantages and disadvantages of single-photon vacuum-ultraviolet ionization relative to the more widely used REMPI detection schemes, and conclude, in agreement with others, that single-photon ionization is a viable detection method for photofragment imaging studies, particularly when studying large molecules possessing multiple fragmentation channels.
Highlights
In recent years, there has been a move away from highly detailed studies of diatomic and triatomic molecules towards investigations into the photofragmentation dynamics of larger molecules of perhaps more general chemical interest
We have carried out a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at a range of wavelengths within their respective A-band absorptions. with non-resonant single-photon ionization at 118 nm allowing detection and velocity-map imaging of all photofragments
The measured distributions are readily rationalised in terms of three competing dissociation mechanisms, with the relative contributions from each mechanism varying as a function of photolysis wavelength
Summary
There has been a move away from highly detailed studies of diatomic and triatomic molecules towards investigations into the photofragmentation dynamics of larger molecules of perhaps more general chemical interest. One solution is to employ a ‘universal’ ionization scheme that allows all fragments with sufficiently low ionization potentials to be ionized and detected.[5,6,7,8,9,10,11,12,13] We have recently developed a velocity-map imaging instrument that achieves ionization of all photolysis products with ionization potentials less than 10.49 eV via single-photon vacuum ultraviolet (VUV) ionization at 118.2 nm This allows images to be obtained relatively quickly for each fragment, and though in general this comes at the expense of detailed state-to-state information, in favourable cases this information is still revealed in the images. This study represents an extension of the single-photolysis wavelength studies of Pratt and coworkers.[5,8] We begin with a brief overview of previous studies
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