Few things affect your quality of life more than the air you breathe and the temperature of your immediate environment. Since more than 80% of the energy used in the industrialized world today is still derived from fossil fuels, these two quantities are not unrelated. Most organic molecules injected into the troposphere are degraded via oxidative processes involving free radical intermediates, and many of these intermediates are the same as the ones involved in the combustion of fossil fuels. Key oxidizing intermediates are hydroxyl, OH (day), and nitrate, NO₃ (night), and early intermediates of oxidized organic compounds include the alkoxy (RO) and peroxy (RO₂) families of radicals. Recently we have explored the spectroscopy of RO, RO₂, and NO₃ radicals both for diagnostic purposes and to characterize their molecular properties and benchmark quantum chemistry calculations. We have utilized moderate resolution cavity ringdown spectroscopy (CRDS) to study ambient temperature radicals and high resolution CRDS and laser induced fluorescence (LIF) to study jet-cooled radicals. Peroxy radicals and NO₃ have weak ~A−~X electronic transitions in the near infrared which we have studied with CRDS. Comparable LIF measurements have been made for the alkoxy species in the UV. Both vibrational and rotational resolution of the electronic spectra is observed. Data obtained from the spectral observations provide information about both the geometric and electronic structure of these radicals as well as their dynamics and also provide the capability for unambiguous diagnostics of their concentrations and reactions.

Rydberg-Rydberg electronic transitions provide information about the electronic structure of the ion-core and each of the fundamental mechanisms by which a light electron exchanges energy and angular momentum with heavy nuclei. Normally, Rydberg electronic states have been indirectly observed via a sequence of laser-excitation steps, for which detection of transitions is accomplished by either fluorescence- or ionization-based schemes. Electronic transitions of |∆n^*| < 1 between Rydberg states (n* is the effective principal quantum number) have kilo-Debye electric dipole transition moments when n^* > 30. Such enormous transition moments render Rydberg-Rydberg electronic transitions directly observable. A chirped millimeter wave pulse can simultaneously polarize a 23 GHz chunk of two-level systems. In our spectra of Ca atoms (10⁴ Rydberg atoms/cm³ in a volume of 100 cm³), the resultant Free Induction Decay (FID) from each of these two level systems is down-converted and heterodyne detected at < 500 kHz resolution (at 3:1 S:N in a single chirp). Willis Flygare and Brooks Pate are to be thanked! But there is more, especially for molecules! Recently, the Doyle and DeMille research groups have developed a cryogenic buffer gas cooled ablation source, our version of which produces beams of alkaline earth monohalide molecules that are > 100x brighter and 10x slower than those produced by our Smalley type supersonic jet ablation source. Our 20 K Neon buffer gas cooled ablation source, in combination with redesign of the resonance region (300 cm³, mm-wave radiation on-axis with the molecular beam) of our CPmmW spectrometer, has resulted in a 1000x increase in brightness of a BaF molecular beam (10⁸ Rydberg molecules/cm³ in a single quantum state) and a 10x improvement in resolution (50 kHz @ 100 GHz). When buffer gas cooled ablation sources are combined with direct detection of FID, a new domain of high resolution molecular spectroscopy begs for exploration!

Nanoparticles are important catalysts. Understanding their structure-activity correlation is paramount for developing better catalysts, but hampered by their inherent inhomogeneity: individual nanoparticles differ from one to another, and for every nanoparticle, it can change from time to time, especially during catalysis. Furthermore, each nanoparticle presents on its surface various types of sites, which are often unequal in catalytic activity. I will present our work of using single-molecule fluorescence microscopy to overcome these challenges and study single-nanoparticle catalysis at the single-turnover resolution and nanometer precision. I will present how we interrogate the catalytic activity and dynamics of individual metal nanoparticles, map the reactivity of different surface sites, and uncover surprising spatial reactivity patterns within single facets at the nanoscale. This spatiotemporally resolved catalysis mapping also enables us to probe the communication between catalytic reactions at different locations on a single nanocatalyst, in much relation to allosteric effects in enzymes.