Radiation pressure is the result of electromagnetic energy (focused sunlight or photons of any energy) transferring momentum to surfaces either by absorption or reflection. The surface is a usually a solid object. The radiation-pressure momentum-transfer process can be demonstrated by a Crookes radiometer. See
http://en.wikipedia.org/wiki/Crookes_radiometer
For a beam of charged particles, like electrons, the incident sunlight can interact only by Thomson scattering (low energy Compton scattering). In this case, the photon energy cannot be fully absorbed due to conservation of both energy and momentum requirements, so the photon scatters off the electron and transfers some (but not all) energy and momentum to it. Sunlight can also scatter off of a beam of protons, but the probability is much lower.
For a beam of neutral atoms, like hydrogen atoms, the sunlight will not interact until the photons reach an energy of about 10.2 eV (~1215 Angstroms), which is short-wavelength UV light. For the hydrogen atom to absorb the photons, the photon energy has to match the transition energy between bound states, like the 2p-1s transition (above). The absorbed photon is re-radiated in a few nanoseconds.
Sunlight can also "reflect" off of neutral atoms, like oxygen and nitrogen atoms in the atmosphere. In this case, the radiation excites an electric dipole moment in the entire atomic electron cloud, which re-radiates in a classic radiation pattern. This was first explained by Lord Rayleigh. Like the other cases above, this photon scattering transfers momentum.
Bob S