  Electron energy-loss spectroscopy (EELS) can measure similar information to
X-ray, UV-Vis, and IR spectroscopies but with atomic resolution and increased
scattering cross sections. Recent advances in electron monochromators have
expanded EELS capabilities from chemical identification to the realms of
synchrotron-level core-loss measurements and to low-loss, 10-100 meV
excitations such as phonons, excitons, and valence structure. EELS measurements
are easily correlated with electron diffraction and atomic-scale real-space
imaging in a transmission electron microscope (TEM) to provide detailed local
pictures of quasiparticle and bonding states. This perspective provides an
overview of existing high-resolution EELS (HR-EELS) capabilities while also
motivating the powerful next step in the field - ultrafast EELS in a TEM.
Ultrafast EELS aims to combine atomic level, element specific, and correlated
temporal measurements to better understand spatially specific excited state
phenomena. Ultrafast EELS measurements also add to the abilities of
steady-state HR-EELS by being able to image the electromagnetic field and use
electrons to excite photon-forbidden and momentum-specific transitions. We
discuss the technical challenges ultrafast HR-EELS currently faces, as well as
how integration with in situ and cryo measurements could expand the technique
to new systems of interest, especially molecular and biological samples.
