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

Recent Developments 189
holder containing an environmental cell. To allow adequate gas pressure
inside, the latter employs small differential-pumping apertures or very thin
carbon-film “windows” above and below the specimen.
Recent efforts have shortened the time resolution to the sub-picosecond
region. One example (Lobastov et al., 2005) is a 120-keV TEM fitted with a
lanthanum hexaboride cathode from which electrons are released, not by
thermionic emission but via the photoelectric effect. The LaB6 is illuminated
by the focused beam from an ultrafast laser that produces short pulses of UV
light, repeated at intervals of 13 ns and each less than 100 femtoseconds long
(1 fs = 10 -15 s). The result is a pulsed electron beam, similar to that used for
stroboscopic imaging in the SEM (see page 141) but with much shorter
pulses. As the electron is a charged particle, incorporating many of them into
the same pulse would result in significant Coulomb repulsion between the
electrons so that they no longer travel independently, as needed to focus
them with electromagnetic lenses. This problem is circumvented by reducing
the laser-light intensity, so that each pulse generates (on average) only one
electron. Although the laser pulses themselves can be used for ultrafast
imaging, the image resolution is then limited by the photon wavelength.
One goal of such ultrafast microscopy is to study the atomic-scale
dynamics of chemical reactions; it is known that the motion of individual
atoms in a molecular structure occurs on a femtosecond time scale. Another
aim is to obtain structural information from beam-sensitive (e.g., biological)
specimens before they become damaged by the electron beam (Neutze et al.,
2000).