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The site-specific hydrogen-atom elimination mechanism previously reported for photoexcited ethyl radicals (CH 3 CH 2 ) [D. V. Chicharro et al. , Chem. Sci. , 2019, 10 , 6494] is interrogated in the photodissociation of the ethyl isotopologues CD 3 CD 2 , CH 3 CD 2 and CD 3 CH 2 through the velocity map imaging (VMI) detection of the produced hydrogen- and deuterium-atoms. The radicals, generated in situ from photolysis of a precursor using the same laser pulse employed in their excitation to Rydberg states, decompose along the C α -H/D and C β -H/D reaction coordinates through coexisting statistical and site-specific mechanisms. The experiments are carried out at two excitation wavelengths, 201 and 193 nm. The comparison between both sets of results provides accurate information regarding the primary role in the site-specific mechanism of the radical internal reservoir. Importantly, at 193 nm excitation, higher energy dissociation channels (not observed at 201 nm) producing low-recoil H/D-atoms become accessible. High-level ab initio calculations of potential energy curves and the corresponding non-adiabatic interactions allow us to rationalize the experimental results in terms of competitive non-adiabatic decomposition paths. Finally, the adiabatic behavior of the conical intersections in the face of several vibrational modes - the so-called vibrational promoting modes - is discussed.

A prompt site-specific hydrogen-atom elimination from the α-carbon atom (Cα) has been recently reported to occur in the photodissociation of ethyl radicals following excitation at 201 nm [Chicharro et al., Chem. Sci., 2019, 10, 6494]. Such pathway was accessed by means of an initial ro-vibrational energy characterizing the radicals produced by in situ photolysis of a precursor. Here, we present experimental evidence of a similar dynamics in a series of alkyl radicals (C2H5, n-C3H7, n-C4H9, and i-C3H7) containing the same reaction coordinate, but different extended structures. The main requirements for the site-specific mechanism in the studied radicals, namely a rather high content of internal energy prior to dissociation and the participation of vibrational promoting modes, is discussed in terms of the chemical structure of the radicals. The methyl deformation mode in all alkyl radicals along with the CH bending motion in i-C3H7 appear to promote this fast H-atom elimination channel. The photodissociation dynamics of the simplest unsaturated alkyl radical, the vinyl radical (C2H3), is also discussed, showing no signal of site-specific fast H-atom elimination. The results are complemented with high-level ab initio electronic structure calculations of potential energy curves of the vinyl radical, which are compared with those previously reported for the ethyl radical.

The photodissociation dynamics of the ethyl radical following excitation into the 3s and 3p Rydberg states are revisited in a joint experimental and theoretical study. Two different methods to produce the ethyl radical, pyrolysis and in situ photolysis, are employed in order to modify the initial ro-vibrational energy distribution characterizing the ethyl radical beam. H-atom velocity map images following excitation of the radical at 243 nm and at 201 nm are presented and discussed along with ab initio potential energy curves focussing on the bridged C2v geometry. The reported results show that the dynamics following excitation to the 3s Rydberg state is insensitive to the initial internal energy of the parent radical, in contrast to the dynamics on the 3p Rydberg state, which is strongly modified. The role of the bridged C2v geometry on both photodynamics is highlighted and discussed.

The photodissociation dynamics and stereodynamics of ethyl iodide from the origin of the second absorption B-band have been investigated combining pulsed slicFe imaging with resonance enhanced multiphoton ionization (REMPI) detection of all fragments, I( 2 P 3/2 ), I*( 2 P 1/2 ) and C 2 H 5 . The I*( 2 P 1/2 ) atom action spectrum recorded as a function of the excitation wavelength permits one to identify and select the 0 origin of this band at 201.19 nm (49 704 cm -1 ). Translational energy distributions and angular distributions for all fragments and semiclassical Dixon's bipolar moments for the C 2 H 5 fragment are presented and discussed along with high-level ab initio calculations of potential energy curves as a function of the C-I distance. A predissociative mechanism governs the dynamics where in a first step a bound Rydberg state corresponding to the 5pπ I → 6s I transition is populated by the 201.19 nm-photon absorption. A curve crossing with a repulsive state located within the Franck-Condon geometry leads to direct dissociation into the major channel C 2 H 5 + I*( 2 P 1/2 ). A small amount of I( 2 P 3/2 ) atoms is nevertheless observed and presumably attributed to a second curve crossing with a repulsive state from the A-band.

We present the photoelectron spectroscopy of the simplest Criegee intermediate, CH 2 OO, close to the first ionization energy. Comparison with existing theoretical data yields the experimental adiabatic ionization energy and provides a benchmark for theoretical studies on larger Criegee intermediates, which play an important role in the ozonolysis of alkenes.

The photodissociation dynamics of bromoiodomethane (CH 2 BrI) have been investigated at the maximum of the first A and second A' absorption bands, at 266 and 210 nm excitation wavelengths, respectively, using velocity map and slice imaging techniques in combination with a probe detection of both iodine and bromine fragments, I( 2 P 3/2 ), I*( 2 P 1/2 ), Br( 2 P 3/2 ) and Br*( 2 P 1/2 ) via (2 + 1) resonance enhanced multiphoton ionization. Experimental results, i.e. translational energy and angular distributions, are reported and discussed in conjunction with high level ab initio calculations of potential energy curves and absorption spectra. The results indicate that in the A-band, direct dissociation through the 5A' excited state leads to the I( 2 P 3/2 ) channel while I*( 2 P 1/2 ) atoms are produced via the 5A' → 4A'/4A'' nonadiabatic crossing. The presence of Br and Br* fragments upon excitation to the A-band is attributed to indirect dissociation via a curve crossing between the 5A' with upper excited states such as the 9A'. The A'-band is characterized by a strong photoselectivity leading exclusively to the Br( 2 P 3/2 ) and Br*( 2 P 1/2 ) channels, which are likely produced by dissociation through the 9A' excited state. Avoided crossings between several excited states from both the A and A' bands entangle however the possible reaction pathways.

The photodissociation dynamics of the methyl radical from the 3s and 3pz Rydberg states have been studied using the velocity map and slice ion imaging in combination with pump-probe nanosecond laser pulses. The reported translational energy and angular distributions of the H((2)S) photofragment detected by (2+1) REMPI highlight different dissociation mechanisms for the 3s and 3pz Rydberg states. A narrow peak in the translational energy distribution and an anisotropic angular distribution characterize the fast 3s photodissociation, while for the 3pz state Boltzmann-type translational energy and isotropic angular distributions are found. High level ab initio calculations have been performed in order to elucidate the photodissociation mechanisms from the two Rydberg states and to rationalize the experimental results. The calculated potential energy curves highlight a typical predissociation mechanism for the 3s state, characterized by the coupling between the 3s Rydberg state and a valence repulsive state. On the other hand, the photodissociation on the 3pz state is initiated by a predissociation process due to the coupling between the 3pz Rydberg state and a valence repulsive state and constrained, later on, by two conical intersections that allow the system to relax to lower electronic states. Such a mechanism opens up different reaction pathways leading to CH2 photofragments in different electronic states and inducing a transfer of energy between translational and internal modes.