Re: "A single atom can undergo only one transition at a time"


I can't answer everything, but I'll take a stab at some of your
questions.

> "N.B: A single atom can undergo only one transition at a time."
>
> Three ignorant questions are:
> (i) Where can I find more about this rule?

My guess is that the author was trying to get across the idea that
different atoms undergo different transitions. That is, there isn't
just one atom sitting there doing all the spectroscopy. The same atom
isn't simultaneously doing everything. In other words, you cannot
simultaneously excite the same ground state hydrogen atom from 1s to
both 2p and 4p (I will use this notation for one-electron states, for
simplicity). The electron can only stop in one state. The two
absorption lines corresponding to 1s->2p and 1s->4p are probably due to
different atoms absorbing the light. In principle, the same atom could
absorb many photons over time - after the 1s->2p transition, the atom
could re-emit the photon, relax back to the ground state, then absorb a
1s->4p photon. However, it could not absorb a 1s->4p photon while in
the 2p state. I think this is what the author was trying to get
across. Clear as mud?

That said, I understand that multi-photon processes are indeed
possible. For example, I believe two 600nm photons can excite a 300nm
transition. I think that the laser jocks told me that something like
this has an absorption that increases as laser power squared. That is,
like a termolecular reaction, you have a molecule that needs to
simultaneously find TWO partners to interact with, with the encumbrance
on rate that this implies. In other words, it doesn't happen very
often, if you aren't shining a high power laser at it.

> (ii)This is interesting and we can consider that at a "single instant"
> only "one photon" will be absorbed or emitted. My question is that if
> we have a SINGLE multielectron atom say Na atom and assume that
> sodium atom can absorb light of wavelengths of 300, 400 and 500 nm
> corresponding to all allowed transitions. If we were to shine light on
> that "SINGLE ATOM" which contains all these three wavelengths
> simultaneously, would we observe a SINGLE absorption (due to 300 or 400
> or 500 nm) line or three lines at that very instant? If we were to
> observe three lines which wavelength would have caused transition
> first, since we are assuming that one transition is possible at a time
> (or are they equally probable)?

A single atom would undergo one of those transitions. In general, the
absorption spectrum of an atom (or a molecule, for that matter) is not
the same in the ground state and any given excited state. That is,
after absorbing the 400nm photon, it may not be allowed to absorb a
300nm photon anymore. In the case of sodium, if these are analogous to
transtions like 3s->4p and 3s->5p, the reason why should be clear (no
more 3s electron after the transition!). In a more general case, once
you change the electron distribution, all of the remaining electrons
will probably change in binding energy in response to altered
shielding, repulsions, etc., so the 300nm transition may now have an
analog at 298nm in the excited state.

The absorption transitions are generally not all equally probable,
given an equal flux of appropriate photons. It has to do with how well
the initial and final wavefunctions overlap, mainly. This is why the
peaks in absorption spectra are not all the same size. :)

> (iii) Another related question is that for a multielectron atom, is it
> possible that more than one electron can be excited simultaneously or
> there is quantum restriction that only one electron will be excited at
> one instant.

I'm not sure about this part.

- Craig

.

Relevant Pages

  • Re: Will Radio Engineering be QMs worst nightmare?
    ... >>> This article suggests that for stimulated emission they can ... >> The time during which the transition can happen is not necessarily the ... Well, they're talking about an atom driven by an external field, not ... absorb a photon increases the line width. ...
    (sci.physics)
  • Re: Will Radio Engineering be QMs worst nightmare?
    ... >>> time taken by the transition. ... > time from when the atom is known to be in the ... It seems to me that spontaneous emission could ... A thermal 'photon' comes along and bumps the electron ...
    (sci.physics)
  • Re: Photons-atoms interactions
    ... any photon can be absorbed & emitted. ... Any atom will interact with any photon, ... Suppose you have an atom in which the first excited state has energy E ... here we say that the atom didn't absorb any ...
    (sci.physics.research)
  • Re: Will Radio Engineering be QMs worst nightmare?
    ... >> strongly driven transition (or rather, ... >> occurred or not) to see if an atom was in a particular state. ... >>, the photon is ... It's not at all unusual for a level lifetime ...
    (sci.physics)
  • Re: Ye Old Fantasies; Explained!
    ... you can absorb it, and you can emit it and ... No single photon ever makes it to the surface. ... in the sun or any dense mass a photon will jump from atom to atom being absorbed and released. ...
    (talk.origins)