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The soft X-ray absorption spectrum of the allyl free radical

HR XAS of the allyl free radical (CH2CHCH2)) was measured using a3D-ion-coincidence TOF mass spectrometer. The intense features in the spectrum could be assigned with the aid of ab initio simulation at the MCSCF level, required because of the multi-reference nature of the core-excited state wavefunctions of the open shell molecule. M. Alagia et al PCCP 15 (2013) 1310


The allyl radical is one of the most studied polyatomic hydrocarbon radicals owing to its role as a model system for radical chemical dynamics. It is also an important intermediate species in high energy environments such as hydrocarbon-rich flames and has been suggested to play an important role in interstellar chemistry. The interaction of radicals with high energy radiation is of considerable interest for the production of ions whose structure, energetics and reactivity are relevant in the ion–molecule chemistry. Furthermore, radicals are formed as intermediate products in high energy radiation damage of organic and inorganic condensed materials. High resolution inner-shell spectroscopies, such as XAS, NEXAFS and XPS, can thus play a role of paramount importance to characterize free radicals, and allow their identification as intermediate species in chemical reactions occurring on surfaces and in solid thin films. A large number of investigations were carried out in the low energy regime, but no inner-shell experimental spectroscopic studies of the allyl radical have hitherto been published. This work is aiming at investigating the XAS of this radical and providing a detailed spectral assignment by means of a joint experimental and theoretical investigation.
The allyl free radical (CH2CHCH2) was generated by flash pyrolysis of two precursors, allyl iodide and 1,5-hexadiene. Heavy dilution in He was required to prevent radical reactions and minimize its destruction by collisions with inner surfaces of the nozzle. In the experiment a ~96% pyrolytic efficiency was achieved, permitting the acquisition of spectra of the radical using a 3D-ion-momentum imaging mass spectrometer. The experimental XAS of the allyl free radical was recorded as total-ion-yield in the 280–290 eV photon energy range. It is characterized by four  main features. The lowest lying band displays a rich vibrational structure with sharp and partially resolved components. A group of vibrational modes is excited in this core electron transition, suggesting that the equilibrium molecular geometry has significantly changed upon core

 

 excitation. The investigation of the nuclear dissociation dynamics of allyl core excited states can provide information on the nature of the observed resonant structures. In particular, since the multiple ion momentum imaging spectrometer allows us to measure the momentum vectors of all the fragment ions emitted in a single event after X-ray absorption, the ion angular distribution of ion–ion coincidence emission channels, as well as their relative intensities, have been measured at the most relevant photon energies in the C1s excitation region.The open-shell nature of the allyl radical implies that some of the core excitations will inevitably lead to low-spin states with three unpaired electrons. This makes the use of a multi-determinant theoretical approach necessary.

The number of transitions and their excitation energies and oscillator strengths are properly described by ab initio calculations performed at the MCSCF level of theory. The failure of the one electron description of the XAS of the open shell radical and the required MCSCF approach emphasized the importance of the multi-reference nature of the corehole states of the allyl molecule. Investigation of the nuclear dissociation dynamics of allyl dications formed at the resonant photon energies of the XAS bands, provided insight into the nature of the core electron transitions, and corroborated the spectral assignment, guided by the theoretical predictions.


 

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The soft X-ray absorption spectrum of the allyl free radical

M Alagia,E Bodo, S FalcinelliA Ponzi, R Richter, S Stranges
Phys. Chem. Chem. Phys. 15 (2013) 1310

 

Last Updated on Thursday, 02 February 2017 21:58