Re: Photon hits an atom

From: Andr? Michaud (srp_at_microtec.net)
Date: 11/14/04 

Date: 14 Nov 2004 08:43:19 -0800

billybilly54321@hotmail-dot-com.no-spam.invalid (Doodedski) wrote in message news:<41956386$1_1@Usenet.com>...
> I've been reading my notes on a course on relativity I did and several
> examples talk about a photon hitting an atom. The atom is said to
> then have a kinetic energy equal to the energy of the photon. I just
> don't see how this happens. My friend says that when a photon has an
> energy less than the gap between two energy levels of an electron,
> this electron can absorb the photon and transfer the energy
> (instantly) to the atom. I just don't see this being possible (an
> electron will only absorb energy of a photon if it matches a gap).
> Who's right, and what is going on in these examples?
> Thanks,
> Dood

Hi Dood (Dude?)

Presently, they are both right.

When hit by a photon of energy that exactly matches a gap, the
electron will absorb the energy and jump the gap (move to the
matching location further away from the nucleus), with no kinetic
energy transfer to the atom (the atom may remain stationary if it
was before absorbtion).

When a photon of energy less than the smallest gap allowed hits
the electron, the electron does not jump, but simply oscillates
more strongly on its orbital. That kinetic energy may be transfered
to the atom. That's how radiant heat (infrared) is absorbed.

There also are cases when only part of the energy of an incoming
photon is absorbed. Regarding this, read on Compton scattering
and Raman scattering.

André Michaud

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