My Favorite Reaction’s Not A Reaction

As 7-Up was once called the “Un-Cola,” I am going to call my favorite reaction for CENtral Science’s Chemistry Blog Carnival the “Un-reaction.” When I was a graduate student and then postdoc, I wasn’t a synthesizer of things—I was a studier of molecular interactions: drugs sticking to DNA, proteins sticking to surfaces, lipids assembling into cell membrane mimics.

Behold, the power of the HeNe. Credit: Flickr user fcomartinezc


Some in the field might think this makes me an “un-chemist,” but I was indeed trained as a legitimate one. I was a physical chemist, which means that I still have books on my shelf with fun titles such as “Introduction to Electrodynamics,” “Symmetry and Spectroscopy,” and “Lasers in Chemistry.”

It also means that I spent a lot of time in the dark and used physics-based tools in my work. Lasers, for instance. Spectroscopists in particular couldn’t survive without ‘em. My favorite laser would have to be the “HeNe” (pronounced hee-nee).

That’s “helium neon” to all you synthesizers of compounds out there. In the lab, HeNe lasers are indispensible for their ability to help spectroscopists align optical tables. For instance, when you’re working with a high-repetition Nd:YAG laser, which puts out not-visible-to-the-naked-eye near-infrared light that can burn things (hair, especially, if you don’t tie it up in a ponytail while leaning over the optics), you want a visible, non-dangerous stand-in to put all the mirrors and lenses you’re using in place. Only then can you safely send the high-powered beam bouncing along down the optical pathway toward your sample.

HeNe lasers are great stand-ins. And they work by electronic excitation and collisional energy transfer. That might not be a reaction in the purest sense of making and breaking bonds, but you’ll have to forgive this un-chemist for bending the rules of the Carnival a little bit.

HeNe laser diagram. Credit: Wikipedia/Creative Commons

The first HeNe ever constructed was also the first available commercially—in 1962. It was then, and is still, composed of a small glass tube filled with, you guessed it, helium and neon atoms. An electrical discharge excites the electrons of the low-pressure helium atoms, which then collide with the neon atoms and their electrons, passing along the “excitement.” This energy transfer is possible because helium’s excited state is close in energy to neon’s.

Excited-state neon atoms accumulate, creating what’s called a population inversion, and eventually, with nowhere left to go but down, they emit light and drop to an intermediate energy level before bumping up against the tube walls, releasing further energy in collision and reaching ground state.

The burst of light the neon atoms emit is a red shade—632.8 nm—based on the difference in energy levels of their excited and intermediate states. Other weak energy transitions that cause lasing are possible—one of them makes green light at 543.5 nm—but red is dominant.

Aside from being great alignment tools, HeNe lasers’ claim to fame is that they were the original laser pointers before being replaced with diode-based varieties. But they’re still used to pop balloons in children’s science demonstrations and in Ig Nobel Prize promos. And they’ve also been known to scan a bar-code in a supermarket or two and make music by reading CDs in optical players.

Author: Lauren Wolf

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