SUMMARY OF MY THESIS
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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