Comments

You must log in or register to comment.

RobusEtCeleritas t1_jc64itk wrote

>A single photon strikes an atom, raising its energy level. How many photons are then re-emitted, isotropically?

There's not enough information to determine that; it depends on the level scheme of the atom. If the atom is excited to its first excited state, there can only be one photon emitted. But if it's in a higher excited state, it's possible that multiple photons will be emitted in a cascade.

>To me, wave energy propagation in all directions would no longer have a discrete direction, so how can I conceptualize the "number" of photons re-emitted?

I'm not sure what you mean here, or why you're relating the angular distribution of emitted radiation to the number of photons.

2

subjectivity_one OP t1_jc7349y wrote

Thanks for the response, I really appreciate it.

Maybe my assumption is wrong. Can you tell me if it is true or not that an atom acts as an "isotropic radiator" upon "spontaneous emission" of a previously absorbed photon?

https://en.m.wikipedia.org/wiki/Isotropic_radiator#:~:text=An%20isotropic%20radiator%20is%20a,sphere%20centred%20on%20the%20source.

Here is where I ran across this:

See at 6:24 to 6:40 of this clip... https://youtu.be/SDqCx4FiJSo

2

RobusEtCeleritas t1_jc9okqm wrote

If the atom is not initially polarized before it emits the photon, then the angular distribution of the emitted photon is isotropic in space.

If the atom is initially polarized, then there will be some non-uniform angular distribution based on the angular momentum change of the transition.

2

subjectivity_one OP t1_jcac8ut wrote

I see, so in nature, an unpolarized atom in an atomic gas may be absorbing and emitting resonant frequencies, emitting each individual photon "in one direction" but in a random direction. Given enough time, and enough absorption/emission cycles, the emission pattern would appear as a random, or "all- directional" distribution (isotropic distribution).

As a thought experiment, if the atom was contained in a chamber with 4 walls, each wall being a photon detector capable of detecting the x, y coordinate of any photon that impacts the wall, and a beam of photons resonant with the atom was directed at the atom, the walls would eventually look like equally distributed swiss cheese, and eventually being 100% covered, but only after a sufficient period of time. One wall would detect the first emission at a specific x,y coordinate on the wall, and then the next emission would be detected sequentially, randomly by either that or another wall, and the process would continue until the photon beam stops.

So the atom is not in fact acting as a point-source "isotropic radiator" during re-emission (as I understood it being described in the linked video), whereby if you impact the atom with a single photon, then all 4 walls of our detection chamber would simultaneously detect the re-emission.of energy.

2

RobusEtCeleritas t1_jcd2xwg wrote

The single photon is emitted in a superposition of all directions, with the angular distribution defined by the angular momentum it carries. However when you detect the photon, you entangle the state of your detector with the state of the photon such that the state of the photon decoheres to a single direction. So it looks like the photon traveled in a single, random direction, as opposed to a superposition of all directions.

2

subjectivity_one OP t1_jcigo1g wrote

Thanks for taking the time to explain this. I don't pretend to fully understand it yet, but it leads me to many more questions, which I deeply enjoy.

2