Frequencies of atoms/molecules

[jgiannis]

It has been a long time since I have studied science. I have forgotten some of the basics. And in some cases, it seems that I have forgotten to ask my professors some questions. So I hope this community can help. I have a series of questions, all relating to "frequency" of atoms/molecules.

1. When discussing electromagnetic radiation (especially the visible spectrum), we often refer to the wavelength (in units of nm). Since the speed of light is equal to the wavelength times the frequency, would we be wrong--or at least introduce the possibility of error--if we referred to the radiation by its corresponding frequency (in Hz) instead of wavelength?

2. Correct me if I a wrong: When we look at emission/absorption spectra, we are seeing the wavelength (or frequency, as per the above question) of electromagnetic waves which the atom(s) emit/absorb when its electron(s) change states. If this is true, then I have some related questions:
(A) Since atoms can have a complex array of electrons (i.e., multiple electrons, and in multiple orbitals, with multiple possibilities for excited states), then some atoms will display multiple bands in the absorption/emission spectra. Moreover, the spectra of such an atom will not always be the same. For instance, if an atom has 2 electrons in the ground state, and one gets excited to the first-excited states, whereas the other gets excited to the second-excited state, then you can presume two bands will appear on the spectra. However, what if the second time you try this experiment, both electrons get excited to the first-state, or both to the second-state. Wouldn't you only see one band in each of these cases?
(B) Can two elements absorb/emit the exact same wavelength, or is each wavelength reserved for a specific element? For instance, if an element absorbs/emits exactly 600 nm, does that mean that no other element can absorb/emit 600 nm?

3. Do emission/absorption spectra only occur for elements, or can they occur for compounds? For example, if the element nitrogen (a single atom) absorbs/emits 650 nm, will notrogen-gas (2 nitrogen atoms covalently bonded) have a unique absorption/emission spectra (i.e., something other than 650 nm)? I think that the answer here is that any compound will be broken down into its basic element before any absorption/emission occurs, and therefore only elements can absorb/emit EMR.

4. When it comes to the "frequencies" of molecules, my understanding is that any given molecule can vibrate at a variety of frequencies. This would be due to vibrational modes, such as symmetric, asymmetric, wagging, etc. These vibrations are measured with infrared spectroscopy. The units of measure are the inverse-centimeter. I have two questions about this:
(A) Would it make sense to convert the inverse-centimeter into units of Hertz, or would that be nonsensical? For instance, can't I convert the inverse-cm into cm, then into nm, and then into Hz?
(B) Do molecules have a fundamental frequency (that can be converted into Hz) at which they vibrate, or is there no such thing? I am referring to an overall vibrational frequency, rather than individual modes of vibration.

Thanks!

 

[Borek]

In general - nothing wrong with the idea of using frequency instead of wavelength, you are right these things are interchangeable. However, we use wavelength by convention, so it is easier to stick to that convention as it makes discussion with others easier. Try to discuss your car fuel consumption using mpg with someone using l/100 km and you will see what I mean

We (almost) never observe single atoms, and we never record spectra of single atoms. What we see is a spectra of many atoms/molecules, so to some extent they are averaged over possible initial states. You are right that single emission events don't have to be identical - that's why we observe several series for hydrogen atom (Lyman, Balmer and so on)

I don't think there exist any theoretical reason why two different molecules/atoms could not emit at exactly the same frequency, although I have never heard about such a case (and I mean EXACTLY the same frequency). Quite often you will have problems when you are looking for a signal masked by another signal which accidentally is at very similar wavelength.

There are spectra for atoms, elements (like diatomics) and molecules. Note you have listed vibrating molecules in the next paragraph - how is the vibrational spectrum different?

No, there is no such thing as and "overall" molecule frequency. To some extent every vibration we observe is that of a whole molecule, even if it is typically dominated by the single bond or a structure (in other words, while carbonyl always has a very similar vibrational frequency, the exact value is a little bit different for every different molecule).