I suppose I deserve this one. Some years ago on the blog, I wrote about my days in grad school having to learn about symmetries and vibrational spectroscopy. Sparingly has that knowledge come in handy since then, but the course is still a vivid memory for me, since that’s the clearest example I had yet faced in a classroom of having hit the limits of what I knew. I’d never been exposed to symmetry operations before, so the lecture that morning might as well have been in Basque for all I could make out of it, and that had never quite happened to me up until that point.
That first link also reproduces a Lewis Carroll parody I wrote at the time (which makes that a metaparody of Wordsworth), which came to mind the other day when I read a new paper in Nature. First, the poem, since the last time I quoted it was thirteen years ago now, which should be a decent interval:
He waved his hands and asked me why
Some peak would polarize
But I was thinking of my lunch
And looked up in surprise.
He then showed me a diagram
And I found to my shame
I didn’t know what good it was,
And couldn’t say its name.
And if now I chance to put
My tongue in super-glue
Or madly cram my chiral foot
In its enantiomeric shoe,
Well, this new paper, amazingly enough, details a technique that lets one observe just those normal vibrational modes on a single molecule, in bizarrely high resolution. And I saw “bizarrely” because it actually works much better than it should (and I’m sure far better than the authors were expecting!), providing both interesting data and an interesting theoretical problem on top of the observations.
This is an application of Raman spectroscopy, specifically surface-enhanced Raman spectroscopy (SERS). Typical Raman work involves shining a laser beam through (or off) a sample and looking for small shifts in the original light’s wavelength due to inelastic scattering – some of the photons scattered have slightly lower energies, having lost some by exciting vibrational modes in the sample molecules. This effect can be hugely enhanced (by factors of billions) when the molecule(s) involves are sitting on a metal surface, and that’s how SERS got started. I am no expert in the field, but it’s my impression that the exact theory behind the effect is still being debated – what’s for sure, though, is that it has to do with the interaction between visible light and metallic nanostructures (typically gold or silver), and that the effect is at its peak at certain “hot spots” on the metal surface. You can bring a metal tip down to a single surface-adsorbed molecule and get the effect that way, which is TERS, tip-enhanced Raman spectroscopy. (Here’s a Nature News and View piece on the new paper that goes into some background on all this as well).
This latest work (from UC Irvine) shows a cobalt-tetraphenylporphyrin molecule on a copper surface, cooled to cryogenic temperatures and scanned by a silver probe tip under laser illumination. The setup is crucial – for example, if the molecule is sitting on a gold surface, it literally slips away as the silver metal tip gets close enough, apparently because of a different charge balance on its cobalt atom. On copper, for reasons that also need to be worked out more, it’s much more tied down. As for the readout from the experiment, things change completely as you get very close indeed to the surface – the data at a three-angstrom distance are totally different from those recorded at a two-angstrom separation. Tunneling kicks in, and the resulting data show extremely high spatial resolution, to the point that you can actually reconstruct the normal vibrational modes of the single molecule. Shown are the experimental data (top row), simulation (middle) and the assigned vibrational modes themselves at bottom.
This is pretty startling – there they are, the stretching, wagging, bending, and waving motions that you learn about when you study vibrational spectroscopy, actual chemical bonds behaving like little springs, exactly as the textbook says. But exactly how this resolution is achieved will be something to work out, because the usual theoretical underpinnings of SERS break down on this small a scale. This technique has gone Full Quantum Mechanics, down there in the “atomistic near field”, and the authors go as far as they can before remarking that “In this atomistic limit, the rich physics of plasmonic junctions is more naturally explained in terms of cavity-confined charge densities and currents, which must be included for a more complete treatment in the tunnelling limit.” But that more complete treatment isn’t here yet, and working it out will keep some folks occupied for a bit, you’d think. “Rich physics” is sort of the tipoff phrase, a synonym for “#@$%! complicated”. But somewhere in there are enhancements of 1012 over what any less comprehensive theory would tell you could be seen. And that’s what’s letting us see this. . .