What is the direction of radiation emitted from an atom ?
Your question has always been an area of research whenever new processes are found and is a very valid one. The answer depends on the specific process in question. For one thing, a coordinate system must be chosen. If the atom is moving, then you, might use the momentum vector of the atom as the direction of reference. Usually with atoms though, the problem is solved in the rest frame. If no external field such as laboratory magnetic field is present, then there is no preferred coodrinate frame so just pick one.
The next thing is that the radiation is emitted randomly. This means you cannot predict the angular direction of any specific photon emitted. However, there is an overall probability distribution that predicts where most of the photons will be emitted over time. By the way, a photon is a quantum of radiation, a piece of radiation which is emitted by the atom in a bunch which acts in some ways like a particle. The energy of the photon is equal to the difference in energy of the energy levels involved in the transition giving rise to the radiation. So when I say a specific process, I usually mean a specific set of energy levels. Obviously, if you know the direction of emission, on average, and the energy, then in majority of the cases, you can build a detector to measure the radiation you want to observe.
As far as places to look. I would start with a book or article on atomic physics such as one you could buy in Borders or Waldenbooks. Fermilab's technical documents involve high energy particle angular distributions and the are considerably more complicated. One can search through recent articles (preprints) on the HEP SLAC server also.
Thank you for your reply to my question about the direction of visable radiation emitted from an excited atom changing to a lower energy state.
You indicated that this radiation is random. Does this mean that there is no specific direction and that the radiation is symmetrical? If it is symmetrical, then that is the answer to my question. However, I am interested in the direction of a spedific event, not a statistical average. Is this an unreasonable inquiry? If all of these events result in an average that is symmetrical, what then is the nature of a single event? By "the nature", I mean what is the mechanism.
You indicate that there is an overall probability distribution that predicts where most of the photons will be emitted over time.
That's great! How do I find out about this? It would be very helpful.
I have gone thru most of the library and book store texts you suggested, and find no answers. I searched the HEP SLAC server with negative results.
Thank you again for your help. However, I remain unsatisfied with an answer to my original questions. If you do not want to follow-up on these complicated questions, I'll understand.
Thank you again,
Breifly, it depends on the states involved in the transition. If you have taken a class in chemistry, you know that the atom has major levels defined by the principal quantum number, n, which are separated in energy by a fair amount (Energy is proportional to -1/n^2). Each of these levels is really broken down to sublevels, some which are slightly different in energy from one another. For example, the n = 2 level has the 2S sublevel as well as the 2P level. The S and P designations refer to the amount of orbital angular momentum the electron(s) has. An S state has 0 ang. mom. and the distribution of where it is in space as it orbits the nucleus is spherical. The P state has 1 unit of ang. mom. Its distribution in space is not spherical. So when a transition takes place between levels, one must take into account the where the electron could have been in the initial state and where it can possibly go in the final state. In other words, what the starting and ending distributions are like.
If you really want the specific distributions of photons in terms of a probability distribution over the emission angles, the best thing to do is to define what the quantum numbers of the initial and final states are and then look up in an atomic physics textbook. The majority of transitions that one would be concerned with are electric dipole transitions because they have the largest rates for happening. These occur between the S and P states. For example, in hydrogen, the 2P -> 1S transition would have an angular distribution P(w) = Constant * (sin(w))^2 where w is the angle between the direction you are looking from and the electric dipole moment direction. This creates a radiation pattern. If you take a cross section using a plane which includes the dipole vector and an axis perpendicular to it, the distribution has two lobes (searchlight type lobes) on either side of the dipole vector (perpendicular to it). The dipole vector is basically centered on the atom. Of course, unless your atoms are all moving in a certain direction such as in a beam or are polarized, their dipole vectors will have random orientations. The type of state will determine the orientation of the dipole vector if it has one but with respect to some axis in the atom. If that axis is randomly distributed because you have no control over the atoms direction (such as in a gas), then your radiation pattern is likewise random (ie. spherical).
The reason a laser works like it does is that even though the atoms inside radiate spherically, mirrors on both ends of the laser cavity keep only the photons moving in a particular direction bouncing back and forth. The rest are lost.
Hope this helps.
Thank you again for your replies. I am beginning to get an appreciation for the complexity of atomic visible light radiation direction. Your patience and toleration to my questions are greatly appreciated.
From what you have described, it is necessary to define the transition states involved in light radiation from atoms. To be specific, I would select the transition of an electron of a hydrogen atom from the 1px state to the 1s state. where the position of the electron is or could have been at each end of the transition is, to me, unimportant. Select any appropriate or useful point for each, and proceed to calculate or describe the direction of resulting visible radiation. Is it spherically symmetrical from the atom? If not, what is it's distribution?
Thanks again for your help.
Thank you again for your reply to my questions about the direction of visible radiation from an atom having undergone an energy transition. I think I am beginning to get the idea; but still can use some help in getting over some of the confusing bumps directly in front of me.
I don't have a supply of Physics Texts for reference; and those I have looked at in our local library have not been all that useful to me. If I did have a supply, I'm not sure I could use them effectively anyway.
Consider a H atom transition: (A) 2P(x) --> 1S and (B) 2P(y) --> !S Questions:
I will try to answer your latest questions as I believe you meant them.
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