Physical Principles of Electron Microscopy: An Introduction to TEM ...

Analytical Electron Microscopy 159
letter; thus K implies that nl = 1, L implies nl = 1, M implies nl = 1 and so on.
This Roman symbol is followed by a Greek letter that represents the change
in quantum number: � denotes (nu � nl) = 1, � denotes (nu � nl) = 2, and �
denotes (nu � nl) = 3. Sometimes a numerical subscript is added to allow for
the fact that some energy levels are split into components of slightly
different energy, due to quantum-mechanical effects.
The transition sequence represented in Fig. 6-1b would therefore result in
a K� x-ray being emitted, and in the case of carbon there is no other
possibility. With an atom of higher atomic number, containing electrons in
its M-shell, an M- to K-shell transition would result in a K� x-ray (of greater
energy) being emitted. Similarly, a vacancy created (by inelastic scattering
of a primary electron) in the L-shell might result in emission of an L�
photon. These possibilities are illustrated in Fig. 6-2, which shows the x-ray
emission spectrum recorded from a TEM specimen consisting of a thin film
of nickel oxide (NiO) deposited onto a thin carbon film and supported on a
molybdenum (Mo) TEM grid.
Figure 6-2. X-ray emission spectrum (number of x-ray photons as a function of photon
energy) recorded from a TEM specimen (NiO thin film on a Mo grid), showing characteristic
peaks due to the elements C, O, Ni, Mo, and Fe. For Ni and Mo, both K- and L-peaks are
visible. Mo peaks arise from the grid material; the weak Fe peak is from the TEM polepieces.