Etude par Sonde Atomique Tomographique de la formation de nano ...

tel-00751814, version 1 - 14 Nov 2012
Chapter 2. Materials, experimental and simulation techniques
Figure 2.3. Potential energy diagrams for atom (Ua) and ion (Ui) on the surface of a
sample under an electric field [16].
After the atom is ionized, it flies towards a detector. The chemical nature of the
evaporated ions is then determined by time of flight mass spectrometry. The time of flight is
the time between the electrical (or Laser) pulse that initiate the ionisation and the impact of
the ion into the detector (micro channel plates and aDLD).
The mass-over-charge ratio, m/n, of ions is then deduced from the applied voltage,
Vtotal=Vo+Vp (with Vo and Vp standing and pulse voltages) and the time of flight, t, by using
the energy balance relationship:
m
n
�
2
� t �
2e( V0
�V
) ��
�
� L �
p (2.3.)
where m is the ion mass, n is the ion charge state (1 + , 2 + ...), e is the electron elementary
charge and t and L the flight time and length.
The data collected during an experiment are represented in the form of a mass spectrum,
showing the number of detected ions as a function of their mass-over-charge ratio (given in
atomic mass units, amu). Each peak could be identified and associated to the corresponding
element or its isotopes or combination (molecular ions). The concentration of an element i, in
the analyzed volume is given in atomic percent by:
N
N
i Ci � (2.4.)
t
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