Vibrational Spectroscopy of a Single Polyatomic Molecule

Polyatomic molecular ions are of fundamental importance to fields ranging from biochemistry to atmospheric chemistry. Action spectroscopy methods which couple spectroscopy with mass spectrometry have enabled high resolution structural studies of otherwise inaccessible species, but all reported action techniques destroy the molecules being studied. We have developed a novel action technique in which cold molecular ions are co-trapped with laser-cooled atomic ions and complexed with weakly bound, neutral "messenger" atoms/molecules. We remove these messengers by driving infrared vibrational transitions in the molecule, and we use co-trapped atomic ions as a non-destructive probe to monitor this process. By observing the frequency dependence of this messenger ejection rate we are able to directly determine the vibrational spectrum of the molecule while preserving it in our ion trap. Here we provide an overview of the instrument we have built for this new method, and present our first spectrum of a single tropylium cation. To our knowledge this is the first reported spectrum of a single gas-phase polyatomic molecule. Our method is general to a broad class of molecules, and will open up new pathways for non-destructive, high-resolution spectroscopy and structural analysis of molecular ions.