Biomedical Engineering Reference
In-Depth Information
collision energy). The negative ions extraction pulse is properly delayed in respect
to the K
C
=
K
ı
beam, allowing the latter to travel to the interaction region prior to the
anions extraction. The base pressure in the collision chamber is
2 10
7
mbar and
the working pressure does not rise upon heating the solid samples. Nevertheless, this
pressure is more than enough to guarantee a binary collision process. The recorded
anionic yields are obtained by subtracting the background signal from the sample
signal. Mass spectra are calibrated through the well known anionic species formed
in collisions of potassium atoms with nitromethane molecules [
13
].
The negative ion mass spectra presented here were obtained with a 1.4 m linear
TOF mass spectrometer with an estimated resolution m
.After
extraction, ions are accelerated and guided into the TOF and detected with a channel
electron multiplier from Dr. Sjuts Optotechnik GmbH operated in a pulse counting
mode. Mass spectra were taken at different collision energies and for all anions,
identified as products of dissociative electron attachment.
The liquid sample of nitromethane was purchased from Sigma-Aldrich with a
minimum purity of
=
m
D150
%. All samples were degassed by a repeated freeze-pump-
thaw cycle before admission to the collision chamber.
The solid samples of thymine and uracil were obtained from Sigma-Aldrich with
a minimum stated purity of
96
% and 99%, respectively, and used as delivered.
Samples were heated typically at 473 K and temperature controlled through a
PID (a Proportional-Integral-Derivative) unit. In order to identify possible oven
temperature effects on the present data, anion mass spectra were recorded for 100 eV
potassium impact upon gas phase uracil at 473K and 458 K. No differences were
observed in the relative peak intensities for the two measurements. The vacuum
chamber as well as the extraction region and TOF electrodes, were kept heated in
order to avoid any condensation of the solid samples.
96
3.4
Results
The topic of this chapter is a part of a broader experimental program concerning
the analysis of the time-of-flight (TOF) mass experiments on electron transfer in
collisions of nitromethane and pyrimidine molecules (thymine and uracil) with fast
potassium atoms. These results have been published elsewhere [
10
,
13
]. As typical
experimental results, Fig.
3.1
shows a section of the TOF mass spectrum of the anion
fragments formed in the collision system potassium-CH
3
NO
2
, whereas Figs.
3.2
and
3.3
are related to thymine and uracil, respectively. A background spectrum has
been collected for each collision energy studied and subtracted to the molecule's
TOF mass spectrum.
The three sets of molecules revisited here, nitromethane, thymine and uracil,
have very large dipole moments
, and therefore can support a stable dipole-
bound state (DBS) and so, under appropriate conditions, a weakly bound electron
can be transferred to form a dipole-bound anion which may serve as a “doorway” to
valence states [
20
].
.>3:4
D
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