Photoelectric effect and conservation of momentum

[BearOfNH]

Imagine a photon zipping along in space until CRASH it runs into an atom and is absorbed by an electron, sending the electron to a higher orbital.

What happened to the photon's momentum? If momentum is conserved, something else must be going on that I don't see. Does the entire atom inherit the momentum and start moving, ever so slowly, in the photon's direction?

Similarly when the electron falls back to a lower level, it's going to emit a photon that has a certain momentum. But you can't just materialize momentum out of thin air empty space -- that would violate conservation. Again, something else must happen ... but what?

This was nearly touched on in a previous article titled
"Where are the photons ?" but I think a new topic is the best way to go. I don't care about photons, I care about momentum.

[swansont]

Imagine a photon zipping along in space until CRASH it runs into an atom and is absorbed by an electron, sending the electron to a higher orbital.

 

What happened to the photon's momentum? If momentum is conserved, something else must be going on that I don't see. Does the entire atom inherit the momentum and start moving, ever so slowly, in the photon's direction?

 

Similarly when the electron falls back to a lower level, it's going to emit a photon that has a certain momentum. But you can't just materialize momentum out of thin air empty space -- that would violate conservation. Again, something else must happen ... but what?

 

This was nearly touched on in a previous article titled

"Where are the photons ?" but I think a new topic is the best way to go. I don't care about photons, I care about momentum.

The momentum is transferred to/from the atom. This is the principle behind laser cooling and trapping (Nobel Prize 1997) [namedrop]I've met two of the three recipients[/namedrop]

 

The momentum transfer from a single photon is small (e.g. for Rb each absorption or emission changes the speed by about 6 mm/sec) but you can scatter millions of photons each second, if you are on resonance.

[John Cuthber]

Sometimes, if the circumstances are right, you can impart the momentum,not to just one atom, but to the entire crystal

http://en.wikipedia.org/wiki/M%C3%B6ssbauer_spectroscopy

[Enthalpy]

The photons' momentum is very real and tranferred to the absorbing medium, or more to a reflecting medium.

 

This "radiation pressure" is perfectly observed on spacecraft - it's most often the strongest parasitic force and torque that act on them. Many telecom satellites have special surfaces at the ends of they Solar arrays to stabilize the craft's orientation and save attitude-keeping fuel instead of wasting fuel from an uncontrolled radiation pressure.

 

Near Earth the incoming pressure is 4.5µPa. Several spacecraft have used it for their propulsion, not just attitude control - but as a demonstration up to now. For these Solar sails to become good, they must weigh like 10g/m²; that's easy for the plastic film, not obvious for the booms that hold it - and damned difficult for the payload that demands huge areas, like 1 hectare for just 100kg. That's the challenge. Build huge, launch in a rocket fairing, reliably deploy in space, which I interpret as "test on the ground" hence in Earth's gravity.

 

Solar sails can go in any direction but work much better nearer to the Sun. Very high energy missions like a polar Solar orbit (Ulysses spacecraft), a Mercury orbit, or joining the heliopause would be better with Solar sails

http://en.wikipedia.org/wiki/Ulysses_(spacecraft)

provided we invent the technology. It doesn't look very difficult, but space agencies (except Jaxa) do a very modest effort; several clubs promote them and bring solid inventions.