1. Prove that two colliding particles cannot transform into a single photon.$\mathrm{}$

$\mathrm{}\mathrm{}2.$ Explain why a photon that strikes a free electron cannot be absorbed: $\mathrm{????}+\mathrm{????}\nrightarrow \mathrm{????}.$ Such a reaction does take place if the electron is bound in an atom, and causes atomic excitation or even ejection of the electron from the atom. Why can absorption occur in this case?