Comments below this answer suggests that a laser ablation-based propulsion system, where the laser is separate from the object to be accelerated, could accelerate an object to about 1/1000 of the speed of light, and that the electric field would accelerate ablated atoms to nearly the speed of light and therefore generate substantial momentum transfer.
While it's a hypothetical situation, I'm just curious about this acceleration. I normally think of laser ablation as a thermal process; absorbed light dramatically increases the temperature locally in a very thin layer of strongly absorbing material which then expands due to a large increase in temperature. So it would be somewhat analogous to a rocket in that it's thermal expansion that creates the velocity and therefore momentum, though it's a lot hotter than what you can get from combustion of the reaction mass alone.
But is there really a mechanism where the electric field of the laser beam itself can generate enough acceleration to bring the plasma particles to a velocity remotely close to that of light? The light is "AC" at several times 10^14 Hz, how could it provide such a net "DC" acceleration?
Or could it be some kind of self-bias of the plasma due to the short pulse length? Highly mobile electrons escape first, leaving a lot of self-repulsive positive charge that accelerates itself?
Are there sources about the physics behind this kind of proposed propulsion where I can read further?