Absorbance vs wavelength graphs

[lzxnl]

Sometimes in spectroscopy, we are given absorbance vs wavelength graphs to try and work out which wavelength of light would be most suitable for the analysis. As we all know, light energy is absorbed by molecules when the light energy is exactly equal to the energy difference between two energy levels, usually electron energy levels if we are talking about UV-visible spectroscopy. My question is, why on earth do we have a smooth curve? As energy levels are quantised and can only have specific energies, the energy differences should be countable and we should only have peaks at the exact wavelength of light corresponding to an energy level difference. This means that the graph should consist of merely peaks as opposed to a smooth curve, implying that all wavelengths of light are absorbed to some degree. WHY are all wavelengths of light absorbed?

My teacher mentioned a model of molecular bonds as springs. However, surely the allowed energies for atoms in molecules is also quantised and cannot take a continuum of values. Please correct me if my understanding on any of this is incorrect.

[EspoirTron]

Okay I am going to answer this based off my knowledge of Infrared Spectroscopy (seeing as the absorbance vs wavelength graphs are formed from this form of analysis). Okay so we know that infrared energy is not enough to promote electrons to a higher energy state; however, the infrared energy has enough energy to change the nature of bonds, i.e. through stretching, bending, etc. (some forms such as scissoring is beyond the scope of VCE Chemistry, I believe). The reason why the curves are smooth is because the trough in the curve represents the sample absorbing the infrared energy to go to higher vibrational energy states. If you recall if UV-vis-spec there is not just a specific wavelength in which the sample will absorb light, there is a range; however, there is one specific wavelength where it absorbs most strongly. Similarly, in infrared spectroscopy the 'turning point' or maximum absorption represents the maximal absorbance that is required to promote the bonds to its highest vibrational energy state. The smoothed curves represent a spectrum of wavelengths that can promote the molecule to higher vibrational states, however, the trough represents the maximal absorbance.

There is a complex matrix that constitutes our sample and thus there are millions-billions of molecules; therefore, some molecules will absorb at ranges that seem different to what the majority are absorbing at.
I'm not sure you mean by 'quantatised' but if you mean 'capped at a limit' they can absorb lower than that value, it just won't be enough to promote the molecule to its highest vibrational energy state.

I hope that helped! 

[jgoudie]

Great question, I think what nilu1995 is talking about is the difference between the set 'quantised' wavelength is AAS compared with the broad absorption in UV-vis.

AAS deals with one single atom, mostly (if not always) metals.  In a single atom you have the set energy levels, the difference in energy between electron shells.  By looking at how uv-vis is absorbed, the energy is absorbed by electrons within molecules.  Molecules are much more complex than the single atoms in AAS.  The various interaction in the bonding of the molecules creates numerous other 'molecular energy states' electrons can occupy.

This is all beyond the need to know of VCE.  Really you just need to know that the energy is absorbed by the electrons with in the molecules.

Hope this helps a little.

[09Ti08]

I think this might be useful for you: http://www.800mainstreet.com/elsp/Elsp.html

[lzxnl]

If you recall if UV-vis-spec there is not just a specific wavelength in which the sample will absorb light, there is a range; however, there is one specific wavelength where it absorbs most strongly.
There is a complex matrix that constitutes our sample and thus there are millions-billions of molecules; therefore, some molecules will absorb at ranges that seem different to what the majority are absorbing at.
I'm not sure you mean by 'quantatised' but if you mean 'capped at a limit' they can absorb lower than that value, it just won't be enough to promote the molecule to its highest vibrational energy state.

I hope that helped! 

By quantised, I mean that energies within the molecule can only take certain values due to the wave nature of matter. It's not capped at a limit as such. What I'm disputing is precisely the fact that molecules absorb a range of frequencies. The quantised nature of molecular energy does not allow for every wavelength of light to be absorbed. But I know what you're trying to get at.

Great question, I think what nilu1995 is talking about is the difference between the set 'quantised' wavelength is AAS compared with the broad absorption in UV-vis.

AAS deals with one single atom, mostly (if not always) metals.  In a single atom you have the set energy levels, the difference in energy between electron shells.  By looking at how uv-vis is absorbed, the energy is absorbed by electrons within molecules.  Molecules are much more complex than the single atoms in AAS.  The various interaction in the bonding of the molecules creates numerous other 'molecular energy states' electrons can occupy.

This is all beyond the need to know of VCE.  Really you just need to know that the energy is absorbed by the electrons with in the molecules.

Hope this helps a little.

Yes, I'm aware this is beyond the reach of VCE, but I don't like having questions unresolved. Also, as 09Ti08 mentions, the electronic transitions in molecules are identical to those of atoms. In atoms, we have electrons jumping between atomic orbitals. In molecules, the transitions take place between molecular orbitals. It's the same concept; why the different absorbing properties?

[09Ti08]

I'm not sure whether you have read this or not:"Because of a molecule's greater complexity, we can often construct a molecule that will give a particular spectrum, rather than having to just accept the spectra available as we do with atoms. This possibility arises because of the interdependence of molecular orbital energy level values for the molecule, molecular shape, bonding, and distribution of electron density within the molecule."

Also, I think it might have something to do with the recorder as well. You know, engineers might program the recorder in a way such that the data is convenient to use/read. Think about the trend the smooth curve reveals for further research (even though we have to be careful since there are invalid points on the graph, but what we need is the trend) since we do not fully understand quantum physics. Anyway, I did not have a chance to work on the aas machine though, so these are just my assumptions.

[lzxnl]

I'm not sure whether you have read this or not:"Because of a molecule's greater complexity, we can often construct a molecule that will give a particular spectrum, rather than having to just accept the spectra available as we do with atoms. This possibility arises because of the interdependence of molecular orbital energy level values for the molecule, molecular shape, bonding, and distribution of electron density within the molecule."

I read that, but even in a molecule, due to the wave-like nature of electrons and molecules in general, their energies are still quantised. Their energies can still only take certain value, regardless of complexity. I agree, MO energy level values will vary with all of the factors that you mention, but it only varies between molecules. They won't vary in time. They should still only be able to absorb particular wavelengths of light.

My teacher suggested something on those lines, that maybe it's the fault of the recording equipment to produce smooth curves. However, I'm still confused.

[09Ti08]

I did not say anything about time. In fact, we are just considering "absorbance" and "wavelength", aren't we? Well, you can try and imagine a graph which consists of peaks only and see how it might be more confusing.

[lzxnl]

Yes we are, and I know it would confusing. However, what I'm looking for is where the molecule actually absorbs light, and the graph's response to that question is unfortunately even more confusing for me. If we had just peaks, the chemistry would make sense to me. It would look ugly, but I'd be fine with it. Emission spectra consist of just lines.

[09Ti08]

Well, if you still find it confusing, then you might want to review my idea about the recorder. You are at a new position now and you don't want to miss any possibility of finding out something new about  a particular substance.

Oh, and one more thing. I think it may have something to do with other subatomic particles inside the nucleus since they might have energy levels, too. Anyhow, I'm unable to comment further on this.

[mark_alec]

The reasons an absorption/emission line is not only one exact wavelength are complex and I am almost sure are beyond the course: http://en.wikipedia.org/wiki/Spectral_line#Spectral_line_broadening_and_shift