Homework Help: Absorption spectra

1. Nov 30, 2007

Oerg

erm a simple question. If all light comes from the electrom-photon interference in an atom, that means that we can't obtain a continuous spectrum theoratically. How then is an absorption spectrum possible??

2. Nov 30, 2007

Astronuc

Staff Emeritus
Light arises from electrons changing (decreasing) energy levels in an atom.

If one can have emission, one can have absorption, whereby a photon is absorbed and the electron increases in energy within the atom.

Absorption and emission occur at discrete energies with respect to the atom. On the other hand, atoms are vibrating (in a solid, liquid and polyatomic molecules) or in the case of a gas, the atoms/molecules have some velocity, therefore the Doppler effect affects the frequency of the light. In addition, there is scattering of photons by electrons, and the scattered photon has a reduced frequency (Compton effect) or increased frequency (inverse Compton effect).

3. Nov 30, 2007

Oerg

actually what I am asking is, i always see continuous spectrums from my textbooks. And the absorption spectrum are those missing lines from a continous spectrum. But how do we obtain a continuous spectrum when light is emmited at various specific frequencies depending on the atom?

4. Nov 30, 2007

Astronuc

Staff Emeritus
That is what I explained. The atoms absorb at discrete frequencies, so light intensity at those frequencies is reduced in comparison to the emitted light.

Light is emitted at discrete frequencies, but the Doppler effect and scattering change the frequency with respect to the characterisitic frequency both increasing and decreasing the frequency over a range.

5. Nov 30, 2007

Oerg

Then again, for stars that are moving away from us, wouldnt we be unable to observe a normal spectrum since the spectrum would have undergone the Doppler effect? Is it still possible to detect the composition of the star then?

6. Nov 30, 2007

Astronuc

Staff Emeritus
Well stars and galaxies (large collections of stars) have red shifts if they move away from us. Also if one looks across galaxies with the galactic plane parallel with the line of sight, one observes a variation in red shift because one side is moving toward us (relatively speaking) and the other side away. Actually one has to look at the combined effect of translational velocity and rotational velocity.

In the red shift - the entire spectrum is shifted.

This might be helpful - http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redshf.html

and

http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html#c1

7. Dec 27, 2007

Gokul43201

Staff Emeritus
I know this is really late, but there's a good question here that has not been adequately answered.

Oerg: We measure a continuous absorption spectrum for materials (in a spectrometer, for instance) because the light is NOT being absorbed by atomic electrons. If it were, then we would indeed see essentially discrete spectra (as is observed in the absorption spectra of dilute gases) as you've guessed. What the light is absorbed by is usually (particularly among insulators) the vibrational state of the lattice of atoms (i.e., phonons) that make up the sample. Phonon states are essentially continuous (they are actually discrete but the discreteness is so fine, it is immeasurable for any mesoscopic/macroscopic sample). Take a look at the post in the Physics FAQ sticky thread in the General Physics sub-forum. In metallic samples, light is also absorbed by collective vibrational modes of the conduction electron gas (i.e., plasmons). Like the phonon dispersion, the plasmon dispersion is also essentially continuous.

The statement about light interacting essentially with electrons is intended for "isolated" (non-interacting) atoms, as are found in a dilute system. In dilute solutions, for instance, it is possible to also see absorption from individual electronic levels (in the UV range) or individual vibrational/rotational levels (in the IR/microwave range), making IR, Raman and UV-vis spectroscopy of organic compounds possible. When you deal with solids, however, the atoms/molecules lose their individuality. Light then interacts with the "collective excitations" in the solid.

Last edited: Dec 27, 2007