Spectroscopy

Introduction to the Electromagnetic Spectrum and Spectroscopy

To understand what the electromagnetic (EM) spectrum is, you first have to understand the concept of radiation. You've probably heard the word radiation before. It could be from somebody describing their cancer treatment, or for the spectroscopy of chemicals in the lab. As it turns out, radiation is all around you all the time. All radiation is just a type of energy that moves and spreads as it travels.

Did you know that all visible light is radiation? What about when you're in the car and turn on the radio? That's radiation too! These are both forms of electromagnetic radiation. Different types of EM radiation are characterized and organized in the electromagnetic spectrum based on their wavelength.

Spectroscopy definition

Now that we've been given a brief intro, let's look at the definition of spectroscopy.

Electromagnetic Spectrum Diagram

So what is the electromagnetic spectrum?

Figure 1: A basic diagram of the electromagnetic spectrum.

You've probably seen an electromagnetic spectrum graph like the one shown above. As we move from the right to the left, wavelengths get shorter, and in turn, frequency increases. As it turns out, this simple rule can be used to help scientists deduce what molecules they're working with using a few principles.

• Molecules, depending on their structure, absorb a certain amount of energy that corresponds to a specific frequency.
• If we can figure out this frequency- we can figure out the energy it is proportional to and then figure out the actual molecular structure.

This rule is fundamental to almost all types of spectroscopy. While the specifics won't be tested on the AP exam, knowing when to use and what kind of spectroscopy to use for a given situation will be. To know which spectroscopy to use, a rudimentary understanding of what each type of experiment detects.

As you most likely have guessed by now, IR spectroscopy has to do with substances that absorb radiation within the IR electromagnetic range. UV-Vis deals with absorption within the UV and visible range, and so on. However, each type of spectroscopy that we'll cover has different primary usages and reveals different characteristics about atoms and molecules- so types of spectroscopy are NOT interchangeable. (You can't use IR spectroscopy instead of UV-Vis spectroscopy and expect to get the same information or results, and so on.)

Photoelectron Spectroscopy

Photoelectron spectroscopy (PES) manipulates the fact that electrons within atoms and molecules will have differing amounts of relative energies. We know that atoms have a specific number of electrons. We also know that bonds involve sharing electrons or donating them. Therefore, because we know that the energy for these electrons is going to be specific for atoms present in the sample and for the bonding energies associated with them, we can determine both of these with PES.

Focusing on the chemical elements, we can use PES to essentially "zoom in" on atoms in a sample to see energy levels. This is done by 'plucking off' electrons from the sample using high-energy EM radiation like X-Rays or Ultra Violet (UV). By doing this, we can measure what the ionization energy is for each of these "plucked" electrons ending up with a graph that looks something like the following. (Ionization energy is also known as binding energy.)

Above, there is a PES spectrum of a pure, idealized sample of nitrogen. We know nitrogen's energy levels and that they have an electron configuration of 1s²2s²2p. Analyzing the PES of our nitrogen gas sample, we can see that we have three peaks that correspond with these three discrete levels of energy. We can also see that the height of the peaks is relative to how many electrons are in each subshell. For example, the first two peaks are equal in height because 1s² and 2s² both have 2 electrons. The third peak, which represents 2p is half in height because there is only one electron in its subshell. This technique can be applied to the PES spectrum to determine what element is being analyzed.

Now that we understand the basics of how it works, what does PES usage look like?

IR Spectroscopy Chart

Now, let's move a little further outwards. Rather than analyzing a sample atom-by-atom, what if we want to figure out what molecule we're working with? Infrared spectroscopy helps us to do this by experimentally determining how our substance interacts with IR light.

We can do this in a few different ways. Sometimes chemists will look at absorption spectra to find in what unique manner the sample will absorb IR radiation. Emission spectra also tell the opposite: what pattern does our sample make as it emits IR radiation? In rare cases, IR reflection spectra are used to see how the sample reflects IR radiation.

IR spectra can quickly become complicated, but for the sake of the AP Chemistry exam, you'll need to know that IR spectra look something like the following:

Figure 3: Drawn example of an Infrared Spectroscopy graph.

IR spectra are usually measured in wavenumbers, cm^-1, which is just the reciprocal of the wavelength. The peaks on this graph represent different groups of characteristic atoms, otherwise known as functional groups. These peaks appear on different portions of the graph due to the different bond vibrations in reaction to IR radiation across the molecule.

Now, what does IR spectroscopy look like in an experimental design?

UV-Vis Spectroscopy

Ultra Violet-Visible (UV-Vis) spectroscopy is similar in principle to IR spectroscopy. However, what is detected in UV-Vis spectroscopy is different from IR spectroscopy, the produced spectra look different, and there are different implications for the meaning behind what is detected. UV-Vis spectroscopy focuses on the ultraviolet and visible range of the EM radiation spectrum. As a result, UV-Vis spectroscopy primarily detects molecules that are rich in double bonds and metal cations.

A resulting UV-Vis spectrum might look something like the following:

Figure 4: Drawn example of a UV-Vis Spectroscopy graph.

Notice how our graph compares absorbance with wavelength. This spectrum shows the relative molecular absorbance at each specific wavelength and produces a curve that reveals the wavelength of maximum absorbance. This information might be important if you wanted to use this experimental data to find concentration, which could be done with Beer's Law.

Mass Spectroscopy

The last type of spectroscopy that could be mentioned is mass spectroscopy. Mass spectroscopy is quite different from the other types of spectroscopy mentioned. Mass spectrometry uses electrons with a high amount of energy to 'attack' a sample, which forces the ionization of the sample. This causes the sample to be 'split' into multiple ions.

For the sake of AP Chemistry, however, we care much more about what happens if we stick a sample of a pure element into a mass spectrometer. If we do this, it turns out that the sample splits into atoms with different masses. These are different isotopes of an element! (Remember, isotopes are atoms with a differing amount of neutrons.) This means that mass spectroscopy can be used to prove the existence of the neutron.

A mass spectroscopy spectrum might look something like this:

Figure 5: Drawn example of copper under mass spectroscopy.

This graph means that ⁶³Cu is 69% abundant, while ⁶⁵Cu is 31% abundant. Be sure to note the mass-to-charge ratio as well, which can also be written as m/z.