UC Santa Barbara

Spectroscopy is a key analytical tool that provides valuable insight into molecular structure and is widely used to identify chemical samples. Action spectroscopy combines spectroscopic methods with mass spectrometry, enabling the analysis of low-density gas phase molecular ions. So far, commonly reported action spectroscopy techniques destructively record spectra of ensembles of molecular ions, where the presence of multiple chemical and isomeric species greatly complicates the interpretation of the spectra. Here, we extend the action spectroscopy to the purest possible sample: a single gas phase molecule. The resulting single molecule vibrational spectroscopy is initially demonstrated by the first recorded spectrum of a single gas phase polyatomic molecule to our knowledge. This thesis presents the experimental setup, method, and results of the achieved single-molecule vibrational spectroscopy. The apparatus centers around a cryogenic ion trap, where a single molecular ion is confined after being mass-selected by a quadrupole mass filter. A laser-cooled 88Sr+ ion is co-trapped with the molecular ion. The two ions are sympathetically cooled to a Coulomb crystal at milliKelvin temperatures. We developed nondestructive mass spectrometry based on the crystal's secular motion frequencies. This technique enables ``tagging'' spectroscopy with a single molecular ion, where vibrational transitions are monitored by a nitrogen molecule tag. The single molecule vibrational spectra are deduced based on the vibration-induced de-tagging rate. We showed this single molecule vibrational spectroscopy is generalizable to a broad class of molecular ions and is a powerful analytical tool for the nondestructive identification of single molecules.