Difference b/w excited state and metastable excited sate?

  1. difference b/w excited satate and metastable excited sate?and why the time period for m.s.excited state is greater than the common excited state?and how a photon deexite the electron when it is interacted with electron in m.s.excited state?



    m.s.excite=metstable excited state
     
  2. jcsd
  3. It is just a definition of a metastable state to be long-living.

    Normally there are dipole transitions from excited states. For that the excited and the ground state should be different in dipole moment (different l, for example). But if the excited state does not have a dipole moment (l=0), the transition happens via quadrupole or two-photon mechanism which takes longer time.

    Even a one-photon, dipole transition is not so easy, otherwise there will not be "excited" discrete states. A two-photon transition is even harder to carry out.

    A one-photon transition is a transition between two different electron configurations, like from this | to this _ Obviously the electromagnetic filed should be emitted because the initial and final states create different electric configurations. But if the excited state is also like this _ , then a transition does not change much the electric filed, so there is practically nothing to radiate. This explains the "difficulty" in such transitions.
     
    Last edited: Dec 5, 2009
  4. It usually means that the transition to a lower energy state is forbidden by quantum numbers, and can can only proceed via tunneling to another excited state which can decay to the lower energy state by an allowed transition. I think the 2S atomic state in hydrogen is an example.
    Bob S
     
  5. sorry:
    I m a student of F.Sc and in our course their is no datails about atomic structure,excitation,dexictitation only very minor details are given so plz sir.BOB S discuss it simply and in detaile.
     
  6. An "allowed" atomic excited state has a lifetime, meaning the expected time before the excited state returns to the ground state by photon emission. Allowed state lifetimes depend on a complex formula for energy difference, wave function overlap, and quantum state parameters. For example, the calculated hydrogen 2p -> 1s allowed transition lifetime is about 1.6 nanoseconds. The hydrogen 2s-> 1s transition on the other hand, is not an allowed transition (called "forbidden"), and takes much longer to return to the 1s state. The quantum numbers are n, l, s, and m. Transition lifetimes from higher level hydrogen states are longer: 5p->1s is about 24 nanoseconds. These are all calculated and tabulated in Bethe and Salpeter, "Quantum Mechanics for One and two Electron Atoms", page 266. For higher Z hydrogen-like atoms, the lifetimes are shorter. Often, transitions with lifetimes over some arbitrary value, like 100 nanoseconds, are called metastable, even thought they may be "allowed"..
    Sometimes, an atomic or molecular state is conditionally stable, but will decay to the ground state if it collides with another atom that stimulates the transition (like in a gas).
    Bob S
     
  7. Hi
    im sorry ... I completed my B.Tech 3 years back ... i forgot this topic........

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