Why some electronic transitions are forbidden and why that actually means jackshit
You'd be forgiven to ask, "Lena, what the fuck are you talking about" right away. "Forbidden transitions" sound odd if you don't know much about spectroscopy - but they're a very important concept. Still, electrons - the stars of electronic spectroscopy - are quantum bastards, and you can tell them something is "important" until your lungs give out, but they'll always do whatever the hell they want. And so is the case here.
Rules are meant to be broken.
---every electron ever, probably
In electronic spectroscopy we shine some light on matter and see what what happens to the electrons inside it. You may remember from your chemistry classes that atoms are built sort of like very complex onions: there are shells and subshells where all the electrons sit. Shining light on an atom may cause it to absorb some energy and go from its regular ground state to an excited state.
You may also remember your science teacher telling you that light absorption causes an electron inside an atom to "jump up to a higher energy level". It's bullshit, because electrons aren't fucking kangaroos and they can't jump. It'd be more precise to say that light absorption changes the quantum state of an electron, but a. hopping electrons are easier to imagine, and b. that just sounds like I'm evading giving you the full picture (which I am. Do you seriously want me to start yapping about atomic terms and angular momentum?).
But I digress. When an atom goes from its ground state to an excited state due to its electrons doing whatever the hell it is that they do when they absorb energy, that's an electronic transition. But just like traffic rules make some theoretically doable actions illegal, there are the so-called selection rules which allow for some electronic transitions and prohibit others.
The most important of them all states simply that energy can only be absorbed in discrete packets, and isn't very interesting. Another one says that the total spin of a system mustn't change during an electronic transition. Personally, I really like the Laporte selection rule (aka the parity rule). It forbids transitions between states of the same parity, like those between d orbitals.
But like I said, electrons are bastards. They break rules all the time. If they could drive a car, they'd go the wrong way up a one-way street and run over pedestrians with a gleeful screech. You can tell them what to do, but they'll just flip you off and fuck your mom. And you know what? It's fucking awesome that they're like that!
Screwing the Laporte rule gives us many pretty colors
A white powder always makes a chemist happy: you never know if it's sugar or amphetamine.
---a tour guide in my uni city's museum of pharmacy
But frankly, colorful crystals and solutions make us even happier. How boring chemistry - and the world, really - would be if electrons never violated the Laporte rule! I mean, just look at these:
[top left / top right / bottom pics are mine]
None of these compounds would be colored if it wasn't for electrons giving the finger to the Laporte rule. And the really cool thing is if you want to explain how this rule gets lifted, you have to turn to symmetry (when you really get to the bottom of things, there's always either QM or math there). The two major mechanisms that cause violations of the Laporte rule are orbital mixing and vibronic coupling (which is what happens when molecular vibrations team up with electronic transitions), both of which are very neatly dealt with by taking into account symmetry considerations. It's all very pleasing. Can you really judge me for having a favorite selection rule? Look how cool all this stuff is!
Screwing the spin selection rule gives us glowing things
I don't like luminescence thermometers. I miss being able to just buy a mercury thermometer wherever.
---my fundamentals of chemistry lecturer, who researches luminescence thermometry
Like I've said many times before, an electron is not a marble: it isn't actually spinning or orbiting the nucleus. Still, there are two important vectors associated with it, representing the orbital angular momentum and spin angular momentum (yet another proof that electrons are bastards). When these two interact, they can give rise to the so-called spin-orbit coupling which, in turn, is able to change the total spin of the system.
But violating the spin selection rule is sort of harder for electrons to do, so it takes them a long time to emit all the extra energy and return to the ground state. We call this relatively slow process phosphorescence. Your childhood glow-in-the-dark star stickers? Some exit signs in public spaces? That's spin-orbit coupling and electrons being rebellious baby!
What's not to love. Spectroscopy is so fucking awesome <3
You can read the rest of my sci comm-ish posts here!
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