On the Absence of Knock-On Damage in Lithium Metal at Cryogenic Temperatures

Knock-on damage and sputtering (both electron-nuclear scattering) along with radiolysis (electron-electron scattering), are the three main types of damage that occur when the electron beam interacts with the sample in a transmission electron microscope (TEM). The kinetic threshold of knock-on damage gives an electron beam kinetic energy below which crystals such as graphite (120 keV) and silicon (200 keV) are effectively stable under this β radiation exposure, independent of temperature. These thresholds are well within the typical kinetic energies used in modern TEMs, whose kinetic energies are between 30 keV and 300 keV. The atomic nuclei, once ejected from a crystal, have their own kinetic energy that is the related to the incident electron beam’s kinetic energy and the threshold energy in the crystal. In lithium, imaged under a 300 keV beam, this produces at most a lithium nuclei with a kinetic energy of about 110 eV (given a 9 eV potential well or activation energy), which is much greater than the thermal energy at room temperature. Cryogenic temperatures should offer no protection in lithium metal bombarded by 300 keV electrons, where the threshold kinetic energy is 30 keV. Yet, despite these basic kinetics, lithium can remain undamaged at liquid nitrogen temperatures for fluences over 1000 electrons/Ų, and is damaged easily at room temperature. We will examine this mystery in more detail, with both experimental evidence and possible theoretical explanations.