What do I do? 1: Using the word Quantum correctly.

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 quantum mechanics.

In Ant-Man and the Wasp, there is a moment where Scott is sitting confused in a room with a bunch of "physicists" who are talking about the quantum realm.  Eventually, he asks them , "Do you guys just put the word quantum in front of everything?"  I wanted to cheer.  Thanks to Hollywood, some people might not realize that quantum is a word that actually means something; it's not just a way of adding the sound of legitimacy to the science that you're talking about.

When I talk about something being "quantum," what I mean is that it can be described by discrete values (whole numbers, none of that fraction or decimal crap).  To illustrate the difference, start with the speed of a car measured by classical physics.  When we talk about the speed, it's a continuum of speeds.  You can be going 90 mph, or 90.1 mph, or 90.000000000000009 mph, etc.  The jumps in speed (or energy) of large objects are infinitesimally small.  The playground analogy to this difference is a slide.  Smooth transitions between each measurement, or smooth enough that it doesn't make a difference at the level we can measure it. In quantum mechanics (physics) the transitions between energy levels are not so smooth.  This happens as the object you're looking at gets smaller and smaller. (This can be explained with the particle-in-a-box model, which I may write a post about at some point in the future).  When you enter this realm the steps between different energies become less smooth, and you end up with discrete energy levels that have gaps between them.  The playground analogy to this type of energy spacing is a ladder, with the rungs on the ladder representing discrete energy levels.  A cool point here is to remember that these two theories aren't actually separate.  As something gets bigger, the spacing between the energy levels gets smaller and smaller, and eventually becomes close enough to a continuum that it doesn't make a difference.

So now that we've talked about what the energy levels look like, we need to talk about how molecules store their energy.  There are 4 main ways that molecules store energy (ignoring a bunch of the other ways they can store energy).  The first 3 are related to the classical physics description of kinetic energy.  Imagine a baseball screaming it's way into the stands after a homerun.  Its energy is manifest through the speed it is moving.  This is the first way a molecule can store energy, through translational movement.  Next, imagine a tennis serve with a lot of topspin.  Some of the power from the serve goes into the translational movement of the ball, but some of it goes into the rotational spin of the ball.  The second way molecules store energy is through rotation (there are several types of "spin" a molecule can have, but for my work, only the overall molecular spin matters for this explanation).  Next, imagine some curly hair, that you stretch downwards, then let go of, and it springs back and forth (maybe a spring is a better analogy, but I like the curly hair one, so I'm sticking with it).  You've put energy into it when you pull it down, then the springing motion is how the hair stores that energy. If you've got two atoms in a molecule, bonded together, the bond acts like a spring, it can store energy in these vibrations.  The last way a molecule stores energy is related to potential energy in classical physics. Potential energy can be thought of a child sitting at the top of the slide.  Before they go down the slide, they have a lot of potential energy.  When they're going down the slide, some of that potential energy is transformed into kinetic energy.  At the bottom of the slide, they have less potential energy than before (although, if they're a typical toddler, not that much less energy (too be clear, that's a joke, unrelated to the analogy...)).  The last way that molecules can store energy is as potential energy, with the energy stored in the orbitals of the electrons.  These are called electronic energy levels.

The spacing between these energy levels varies, but in general, it can be thought that electronic

This is a visual diagram showing relative energies between states. 

Here, we're zoomed in on 2 of the electronic energy rungs, and we show

a few of the vibrational energy rungs  It is not to scale, but the values show

what the spacing is for these states.  EE stansd for Electronic Energy which

is the amount of energy each state has, relative to the lowest energy state. 

ve gives the distance between the vibrational "rungs" of the energy ladder.

energy spacing is bigger than vibrational energy spacing which is bigger than rotational energy spacing, which is bigger than translational energy spacing. 
So imagine the electronic energies as the rungs on that playground ladder.  If you zoom into a close up of 1 rung, you'll be able to see that rung is actually made up of a ladder with many vibrational energy rungs.  Then if you zoom in on one of those rungs, you'll be able to see each of those are made of an even smaller ladder of rotational energy rungs.  And if you do it again, you will be able to see the translational energy rungs. 

Now, this is all a simplification.  The truth is always more complicated.  But this is a basic idea of what "quantum" actually means, and it gives us a framework to understand what my next post will be, which is spectroscopy.