What do I do? 2: Spectroscopy

When people find out that I am doing a PhD in Chemistry, they often ask, somewhat foolishly (by those who aren't really that interested), what it is I do.  I then find myself in a position of trying perform a juggling act to find the balance between a number of things.  How in detail do they want me to go?  Where's the right place between talking over their head, and sounding like a condescending jerk.  Are they actually interested, or are they just being polite.  Based on their background, what parts will they find most interesting?  On top of that, I need to plot a coherent course through my thought process so that if they are interested, they don't get lost in the maze inside my head.

So, I've decided to write it down, and see if I can make a logical series of articles about what I do, starting with the background information that's needed, eventually ending up on what I do day to day. Today I'm going to talk about the basics of spectroscopy.

Spectroscopy an important tool used in interrogating molecules (my favorite way of describing it.  Conjures an image of a Hollywood crime drama, with a molecule cuffed to a table,a detective throwing a chair at the window trying to get answers).  It's used in the food industry as a quality assurance tool, by pharma companies to analyze composition of drugs, in the medical field for many tests looking for symptoms of disease, in forensic science for investigating crimes, and by scientists in many fields to study whatever it is they are interested in.

So what is spectroscopy?  Last week I wrote about the different ways that molecules store energy, and the ladder of states that result from that.  I've updated that post with a visual of what those states might look like.  The goal of spectroscopy is to use light to measure the distances between the rungs on the molecular energy ladder.

So how is light related to energy?  Trick question.  It's not related, it is a form of energy.  Light is more than just the familiar brightness that allows you to see..  Visible light makes up a small portion of the electromagnetic spectrum, only the wavelengths between 400 and 800 nanometers. (if you imagine an infinite field of equally spaced waves, the wavelength is the distance between two consecutive peaks.  1 nanometer = 0.000000001 meters, or 1/1 billion meters).  Different wavelengths correspond to different colors that we see.  I found this cool tool that lets you move a slider around on to find your favorite color's approximate wavelength (I like 469).  Visible light is not the only portion of the spectrum that you have had interactions with.  If you've ever had a sunburn, it's from the interaction of ultraviolet light with the DNA in your skin.  The warmth that you feel from the sun is mostly infrared light.  Doctors use x-ray light to examine bones.  Your car radio receives information through radiowaves.  Your cell phone does the same, but in the microwave region.  Your microwave oven?  Yep.  That's light too.  These are all words used to describe light at different wavelengths.

There is an equation called Planck's equation that allows us to convert the wavelength of light into the energy corresponding to that wavelength.  As the wavelength decreases, energy goes up, so shorter wavelengths, like those of UV and x-ray, have more energy that longer, such as radio or infrared.  And here's why we care, in the context of spectroscopy.  Different wavelengths of light roughly map onto the spacing of the different types of rungs on the molecular energy ladders.  I've included a table showing those relationships.


Ok.  So know we've talked about light, how it's energy, and how the energy of different regions of the spectrum can map on to the spacings between the molecular energy ladders.  Now we just need to put these concepts together to understand spectroscopy.  Lets say we have a molecule that is in the X¹∑⁺ v=0  state in the diagram on the right (that corresponds to the lowest electronic rung (or state), and the lowest vibrational state inside that electronic state. X¹∑⁺ is what's called a term symbol.  They contain a lot of information about a molecules electronic state, but I'm not going to go into that now). If we shine a light on this molecule, with an energy that is equal to the difference in energy between X¹∑⁺ v=0 and V¹∑⁺ v=3 state, the energy from that light can be absorbed by the molecule (for a visual see the red arrow).  This condition of the energy of the light matching the energy difference of the states is called the resonance condition, and is the basis for spectroscopy.  It can also go the other way.  If a molecule is in an excited state, it can drop down to a lower energy state, and emit a flash of light, as shown in the diagram as the green arrow.


So there are two ways we can use this simplified picture of spectroscopy to measure properties about the molecule.  We can hit the molecule with many different wavelengths of light, and see which ones it absorbs, or we can give the molecule an excess of energy, and look at the light that it emits.   Both methods give us the basic information on relative spacing of states in the molecule.  Because the molecular energy ladder spacings are unique to each molecule, we can identify which molecule we're looking at, based on the spectroscopy we perform.  That's how it works in theory.

In practice, we have to build an instrument that allows us to select which light the molecule absorbs/emits.  The basic schematic for an absorption spectrometer can be seen below.  I'll outline some of the critical components. The light source, which is represented by a lightbulb, is the light that is used for measurement.  This light source could be as simple as an actual light bulb, or it could be something like a laser, microwave generator, or particle accelerator.  The type of source used is dependent on what is being studied, and the specifics of the instrument.  It is also necessary to have a way of distinguishing the wavelength of the light, as represented by the vertical black screen with a slit below.  This doesn't have to be before the sample, it could be after, or it could be in the processing of the data later.  How well we can distinguish between wavelengths gives the resolution of the technique.  The box below represents the sample that is being measured.  Light passes through it, and some is absorbed.  What is left passes into the detector, represented by the eye.  By comparing this to the light that would be detected with nothing in the sample cell, the wavelengths the molecule absorbs can be identified, and used to characterize the sample.

Next week, I'm going to talk a little more about the emission types of spectroscopy (not because they reaaally apply to what I do, but because I think they're awesome, and you've seen their effects in real life).  So stay tuned.