Isolated aromatics ions photofragmentation analysis by mass spectrometry. Applications to astrophysics and analytic chemistry.


This thesis is devoted to the study of the photofragmentation of polycyclic aromatic hydrocarbons (PAHs) cations, suspected to be abundant in the interstellar medium, under continuous irradiation with visible photons. The results, presented here, are at the interface between molecular physics and astrophysics.
Owing to the absence of collisions, radiative energy exchanges can only occur. Under these conditions, fragmentation of anthracene cations (C14H10+) has been obtained with photons of energy lower than 2 eV. This can be explained as the result of a gradual increase of the internal energy due to multiple photon absorption, and can be treated by the microcanonical statistics.
The introduction about the interstellar medium physics is followed by a description of the experimental set-up. The device combines an ion cyclotron resonance (ICR) cell with analysis by Fourier transform mass spectrometry (FT/MS). As a result of high isolation conditions which can be achieved, one can obtained trapping times as big as one hour.
A numerical simulation was developed in the second part. One built a model which can be extended to the whole PAH "family". The data used in the simulation are discussed in next part.
Chapter third presents the quantitative study of photofragmentation of isolated ions. Experimental evidence points out the fact that, besides infrared cooling, the contribution of electronic cooling has to be accounted for in the evaluation of the cooling rate. The formation and the dissociation rates of organometallic complexes, such as the evolution under solar irradiation of a neutral PAH population, were validated the hypothesis of our model.
In the last chapter, the production and characterisation of many new stable species, which might be more relevant as astrophysics than ordinary PAHs, were investigated. Particularly, different steps of hydrogen loss have been characterised and highly dehydrogenated PAH ions (like C16H5+ or C24H6+) have been obtained.



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