1. Apr 2, 2005

### smokie

Can the electromagnetic radiation produced by a hot gas of hydrogen atoms be exactly of one frequency? Assume that all of the atoms undergo a transition from the state n=2 to n=1. Note that the atoms in the gas are moving.

2. Apr 2, 2005

### SpaceTiger

Staff Emeritus
Check out this thread. The dominant broadening mechanism in your case will be thermal; that is, due to the doppler effect in the moving atoms.

Last edited: Apr 2, 2005
3. Apr 6, 2005

### smokie

that link confused me even more...

my understanding is they can all be at the same frequency, that is assuming that they all begin and drop to the same energy level, since the frequency and energy are related by E2 - E1 = h x frequency.
If they are dropping from the same energy state, it means same wavelengths, therefore same frequency.

4. Apr 6, 2005

### SpaceTiger

Staff Emeritus
Even from the quantum mechanics point of view, that's not exactly right, but the details of that are a bit hard to grasp, so let's stick with the Doppler broadening. Do you know how the Doppler effect works? The basic idea behind it in this context is that an atom moving either toward or away from us will emit light that is slightly shifted in wavelength. In other words, the energy of the photon will be close to E2-E1, but not exactly because the atom is moving relative to us.

Anyway, if a gas has a temperature greater than absolute zero, then that means the atoms will be moving around randomly. The magnitude of this random motion is determined by the temperature and mass of the gas particles. When you look at an emission line like the one you're describing, it will be smeared by the combined Doppler effect of the many different atoms, effectively "broadening" the line.

5. Apr 6, 2005

### smokie

So getting an exact frequency is only posible if the gas is at a temperature of absolute zero. Other than that, it wouldn't be realistic because the Doppler effect of the atoms moving relative to us would cause 'broadening' of the lines... right?

6. Apr 6, 2005

### SpaceTiger

Staff Emeritus
It's never really possible to get an infinitely narrow line, but you're mostly right. There are other mechanisms that can broaden the line, including an intrinsic uncertainty in the energy of the states.