  We study the problem of injecting single electrons into interacting
one-dimensional quantum systems, a fundamental building block for electron
quantum optics. It is well known that such injection leads to charge and energy
fractionalization. We elucidate this concept by calculating the nonequilibrium
electron distribution function in the momentum and energy domains after the
injection of an energy-resolved electron. Our results shed light on how
fractionalization occurs via the creation of particle-hole pairs by the
injected electron. In particular, we focus on systems with a pair of
counterpropagating channels, and we fully analyze the properties of each chiral
fractional excitation which is created by the injection. We suggest possible
routes to access their energy and momentum distribution functions in
topological quantum Hall or quantum spin-Hall edge states.
