Dynamics of the A-band ultraviolet photodissociation of methyl iodide and ethyl iodide via velocity-map imaging with ‘universal’ detection

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.

UV photodissociation dynamics of CHI2Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

“…6 The gas-phase photodissociation dynamics of organoiodine compounds has a long history. CH3I in particular is well studied, and a good summary can be found in a recent article by Gardiner et al 7 The A band of CH3I peaks near 257 nm and results from σ*←n excitation. Absorption at wavelengths near the band maximum occurs primarily via a parallel transition to the repulsive 3 Q1 state which correlates with spin-orbit excited I*( 2 P1/2) atoms.…”

Near-UV photodissociation dynamics of CH2I2

“…At 118.2 nm, an accidental resonance with an autoionizing Rydberg state results in 19.2 times greater sensitivity for I over I*. 49,50 The I* quantum yield increases monotonically from zero at 355 nm to a maximum of around 0.46 at 248 nm. Consequently, I* atoms account for at most B2% of the total ion counts in the images acquired using the VUV probe.…”

“…The photochemistry of CH 2 I 2 can be compared to that of other iodine-containing molecules. CH 3 I has been extensively studied and its photochemistry has been recently been summarized by Gardiner et al 50 The first absorption band results from excitation to states, labeled 1 Q 1 , 3 Q 0 and 3 Q 1 , that are dissociative along the C-I coordinate. Parallel transition to the 3 Q 0 state is predominantly responsible for the absorption spectrum and correlates with CH 3 + I*, although the photochemistry is complicated by the existence of a conical intersection with the 1 Q 1 state.…”

Transition state region in the A-Band photodissociation of allyl iodide—A femtosecond extreme ultraviolet transient absorption study

“…This assumption is justified by similar I/I* branching ratios and translational energy distributions observed in the I/I* channels (unimodal and bimodal for I* and I, respectively) for both molecules. 56,58,69 In addition, the shape of the potential energy curves and the location of the 3 Q 0 / 1 Q 1 conical intersection are found to be invariant across a series of linear/branched alkyl iodide molecules. 62 In this context, given the equiergic valence excitation, 4d static XUV absorption, and product I atom resonances in both molecules, transients A and B in allyl iodide are assigned to (4d 5/2 ) −1 σ * and (4d 3/2 ) −1 σ * core-excited states, respectively, accessed via 4d → n(I) transitions from the nσ * states (i.e., similar to CH 3 I).…”