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- Is there any online site that gives the radiation lines for the elements? Like–the spectra they radiate at when excited… (I know the term for this but can’t remember it… ). The main area that I see this discussed is in radio astronomy, but their scope is limited to common compounds encountered in space; I was more concerned with terrestrial physics.
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- Also (just for fun) is it possible to mathematically detirmine the radiation frequencies of a molecule, if you know what frequencies the component atoms radiate at?
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- I think the “term” would be simply "radiation spectrum for a given element. Is the java applet toward this bottom of this page what you’re looking for?
- Yes, I believe it is. In fact, I believe that’s how scientist determine the composition of distant stars and planets (accounting for Doppler shift, of course). Is it something you can do easily? I really don’t know. In every science class, I’ve ever taken, the concept has been covered but the details have not.
You’re probably looking for the term emission spectrum. Google for that to find some sites and databases.
It’s not clear what wavelengths you’re interested in; the answer to the rest of your question depends on this. Shorter wavelengths probe higher-energy transitions. At extremely short wavelengths (X and gamma), you’d be seeing nuclear transitions, which are pretty insensitive to anything but the particular atomic nucleus. At longer wavelengths (UV) you’d be seeing photoemission by electrons dropping back into inner shells; again, this is relatively insensitive to the chemical neighborhood. At still longer wavelengths (visible and near IR) you’re probably seeing excitations of valence electrons, which are of course delocalized among several nuclei in a molecule and so are strongly dependent on whether you’re seeing an atomic or molecular form. Going to still longer wavelengths (far IR and microwave) you start seeing vibrational and rotational excitations of the molecule.
So, basically, at short wavelengths the emission spectrum of the molecule probably looks a lot like the spectra of its component atoms, but at long wavelengths it looks quite different and could not easily be deduced from the individual atomic spectra.
It depends on what you mean by “mathematically determine”, and also on how much you know about the molecule. The only spectrum which can be exactly calculated is two point charges (among atoms, this is restricted to hydrogen and almost-completely-ionized ions). More complicated spectra can be calculated using perturbation theory or other approximation methods, to as much precision as is desired, but not exactly.
For a molecule, you also need to know the structure of the molecule. It’s possible for two compounds to contain exactly the same atoms, but arranged differently. In such a case, there will not be any particular similarity between the spectra of the two compounds.
If you know the structure and composition of a molecule, it is possible to (again, approximately) calculate its spectrum, but it would be exceedingly hairy. It is probably even possible, given a precisely-known spectrum, to back-calculate what the composition and structure must be of the molecule which generated it. It’s much easier, though, to just get a sample of the chemical in the lab, and look at its spectrum.
Molecular emission spectra are, indeed, very hairy to calculate. They don’t produce nice sharp spectral lines like individual atoms do – they produce wide bands of emission corresponding to whole narrow ranges of energy.
Astrophysicists analyze molecular band-spectra when looking at cool red stars or (recently) at brown dwarfs. Molecules tend not to stay intact at temperatures like you’d find in the photosphere of the sun. Of course, astrophysicists are usually looking at absorption spectra rather than emission spectra, but the wavelengths and widths of the bands are the same.
Well, the OP did say “elements” not “molecules”…
Still, it’s not simple, and I think you’ll find different lists and databases depending on application. In my line of work (solar physics) we need to calculate emission from high-temperature plasma, and I usually use the CHIANTI package. It’s a software package that calculates emission from an optically thin plasma. If you just want the wavelengths, the web site has a list of lines in the database.