That free electrons do not spontaneously leave metals during ordinary mechanical motion due to surface potential barriers and lattice binding is a matter of elementary physical principles, requiring no specialized citation.
The claim states that free electrons do not fall from metals when shaken due to the work function and crystal lattice forces. This is standard physics and basic everyday observation regarding electron containment in conductors; none of the retrieved papers directly study the mechanical shaking of bulk metals to test electron emission, but the underlying principles are textbook physics (common knowledge). Therefore, the verdict is common knowledge and no citations are needed.