RADICALCHEM · Study of the energetics and dynamics of UV photodissociation of hydrocarbon radicals and the development of a new molecular beam instrument for the investigation of gas-surface interactions

Final Activity Report Summary - RADICALCHEM (Study of the energetics and dynamics of UV photodissociation of hydrocarbon radicals and the development of a new molecular beam instrument ...)

The dissociation of hydrocarbon radicals is of great interest, not only for the fundamental understanding of radical reaction mechanisms, but also due to the importance of their reactions in hydrocarbon cracking, hydrocarbon flames and modelling of the atmospheres of the large gaseous planets such as Saturn and Jupiter. This project proposed the study of the photodissociation (at 193 and 248 nm) of a series of hydrocarbon radicals using the photofragment translation spectroscopy method in combination with a pyrolysis technique developed to generate high density supersonic beams of radicals from precursor molecules and a universal detector comprising of electron bombardment ionisation and quadrupole mass spectrometry. The advantage of this technique is that all dissociation channels can be detected and their relative importances characterised. The experimental results obtained during this project on the photodissociation of the allyl radical at 193 and 248 nm revealed the operation of no less than 4 channels: direct H atom loss, a 1,3 H-atom shift followed by C-C bond breaking to CH3 + C2H2, two 1,2 H atom shifts followed by C-C bond cleavage to CH3+C2H2 and, finally, direct cleavage of the C-C bond to give CH2 +C2H3 (at 193 nm only). The analysis of these results together with the high level quantum chemistry calculations performed by the fellow resulted in a more in depth analysis of the dissociation the allyl system than ever previously achieved. This work has lead to 3 publications of which one submitted to the Journal of Chemical Physics and two in preparation. Studies on the organic azide decomposition (both thermal and photoinduced) were performed using the same apparatus. A new technique was developed to study the pyrolysis of these azides in which the short residence times of the molecules in the pyrolytic source with respect to conventional oven methods (30microsec vs. 3 millisec) together with stabilisation of the resulting products by supersonic expansion allowed the identification of intermediates of the thermal decomposition process not previously observed. The application of this technique to the study of acetone azide CH3COCH2N3 (together with extensive quantum chemistry calculations performed by the fellow) allowed a detailed description of the thermal decomposition mechanisms. This work resulted in a publication in the Journal of Physical Chemistry A (accepted for publication). It is hoped that in the future this new technique will lead to greater understanding of the decomposition of this important class of molecules (their energy storage properties have lead to many applications including airbags, chemical vapour deposition and organic synthesis). Work on the photodissociation of the above azide was also studied and a publication is in preparation on this topic. Another highlight of the fellowship resulted from the unexpected results on the photodissociation of I2. In this work the spectroscopy of exotic excited molecular states known as superexcited states were investigated. These states are highly excited neutral states which exist above the ionisation threshold of the molecule due to the promotion of an electron from the valence orbitals into very large orbitals called Rydberg orbitals. The interesting aspect of these states is that even though the molecule has absorbed sufficient energy to eject an electron, the molecule remains neutral due to the fact that the electron in the Rydberg orbital has difficulty in exchanging energy with the ionic core. Indeed, these states can undergo neutral dissociation and it was the characterisation of this process in molecular iodine which lead to a publication on this subject (see J. Chem. Phys. 127, (2007) 144309). This work may form the basis of a new spectroscopic technique capable of characterising dissociative ionic states which are difficult to investigate using existing spectroscopic methods.

Data: CORDIS, © European Union