What do I do? 3: Emission 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 emission.

Last week you might remember that I wrote about spectroscopy, which uses light to investigate the energy levels of molecules.  After talking about it generally, I talked a little more about absorption spectroscopy, which involves the study of the light that absorbed by the molecule.  That's actually the type of spectroscopy that I have been using during my time in France, and I'll go into more details about that in a later post.  But today I want to talk about another major type of spectroscopy, emission spectroscopy, which can use either fluorescence or phosphorescence.

The main difference between fluorescence and phosphorescence stems from what is "allowed."  While I didn't talk about it last week, there are rules to what transitions you can have between energy states (rungs).  So certain transitions can be "allowed" or they might be "forbidden."  A lot of times, you'll hear the phrase "classically forbidden," because those transitions can still happen, as the rules are only there in the simplifications that physicists made in trying to solve molecular systems.  However, forbidden transitions are usually orders of magnitude slower than the allowed transitions (and oftentimes much weaker).  That's going to come back and be important later.

You might remember a diagram like the one to the right from the previous posts.  I added an extra
electronic energy rung on this one, because we're going to need it for phosphorescence.  For now, focus on the rungs on the left.  You'll see our normal absorption arrow, just like last week, as well as an emission down arrow for the type of emission known as fluorescence.  It's called fluorescence for historical reasons, as one of the early molecules studied with this phenomenon was fluorite, but a molecule does not require fluorine in it to fluoresce.  For something to fluoresce, it must first absorb a photon of light and transition from v=0 in the lowest ladder to v=3 in the upper left ladder. (for now, think of a photon as 1 unit of light).  Then through processes that don't emit energy as light, the molecule relaxes to a lower energy state, before dropping down and emitting a photon of light at a wavelength corresponding to the difference in the higher and lower energy levels.  This is an allowed transition, so the process is very fast (much less than a second).  So if you turn off your pump source (the light that is exciting the molecules), the molecule stops fluorescing very quickly.  If you've ever  been to a roller skating rink, or a laser tag arena, you've seen this phenomenon.  It's what causes the white dye in your clothes to glow under a black (UV) light.  It's actually pretty clever of clothing manufacturers.  The dye's they use for white contain a molecule that absorbs light very efficiently in the UV region of the spectrum, which then emit fluorescence in a broad region of the visible spectrum.  The result is white light emitted, which make whites look whiter in the sunlight.  They also make you a highly visible target in a lasertag deathmatch.

Now look at the right side rung.  This represents an electronic energy state with which a transition with the ground state is "forbidden."  But when you read "forbidden" you should really just read it as "slow".  For phosphorescence to occur, the molecule must first be excited to the upper state through absorption.  Then, following this "allowed" transition, if the molecule is in an energy state on the left ladder that is very close in energy to a forbidden state in the right ladder, it can cross over into that electronic energy state.  From this excited electronic state, light can return to the ground state through an emission event called phosphorescence, releasing photons of light again at a different wavelength than was absorbed.  For a sample of many molecules, this process is slow, and will continue to take place for seconds after the pump light source is taken away.  If you've ever had the pleasure of falling asleep beneath a field of "glow-in-the-dark" stars, you've experienced phosphorescence.  Mark Rober has an awesome video, where he builds a glow-in-the-dark shadow wall and talks even more about this process, that I recommend checking out.

I've never done any phosphorescence spectroscopy (although, apparently it has been done), but fluorescence is one of the detection techniques that we're using in my lab in California.  Fluorescence spectroscopy setups are similar to absorption, but the light you're interested in looking at is not the light that you're pumping the molecule with.  Several methods are used to ignore the light from the source, and only look at photons originating from the sample.  The first is to detect perpendicular (90 degrees off) to the light source.  This minimizes the number of photons that are coming from the light source to the detector (90 degrees gives you the least amount of scattered light).  We also use extra filters typically, only looking at light that is longer in wavelength than the excitation source.  So you'll often see wavelength filters both before the sample, and after the sample.  The equations that ultimately get used to quantify the sample are also different from absorption spectrometers, but that is more complicated than I want to get into here.

You now have enough background, that I could write about the basics of what I have been doing with my time in France.  Next week I'll start writing about what it is I've been doing here, and why it matters, before moving on towards what my main project has been during my PhD work at UC Davis.