The absorption of light can initiate chemical change and understanding the outcome is both a fundamental challenge and of practical importance to the atmosphere we live in!
Molecules display a wealth of photochemistry. Our emphasis is on understanding what governs dissociation and reactivity, by determining the energetics and rate of this initial step, and the rates of subsequent reactions. Determination of quantum yields and the kinetics that control branching into different products tie laboratory studies to atmospheric implications. We specialize in applying laser-based spectroscopic techniques to explore these questions.
1. Velocity-map ion imaging studies of unconventional photochemistry
Can we gain insight into the mechanism governing a photodissociation? In these experiments, a molecular beam is produced in high vacuum to prepare cold molecules (less than 30 K). We use pump and probe laser pulses to prepare the initial state and study the outcome. We measure the velocity of nascent photofragments, which are first ionized by either REMPI or VUV and then extracted using velocity ‘mapping’ electric fields to a position sensitive detector. The resulting ion images reveal their speed and angular distributions.
We have recently looked at the photodissociation dynamics of cold acetaldehyde cations over a broad wavelength range. Example Ion images of the four major ionic photoproducts resulting from photolysis of acetaldehyde cations at a photolysis wavelength of 228 nm are shown above.
2. Spectroscopy and kinetics of reactive intermediates
Alkene ozonolysis is an important tropospheric reaction that produces a highly internally excited primary ozonide which then decays into a stable carbonyl and an unstable carbonyl oxide, R1R2COO, known as a Criegee intermediate. Subsequent reactions with trace atmospheric gases leads to formation of acids, peroxides and new particles, affecting air quality. We use broadband transient absorption spectroscopy, along with cavity ringdown spectroscopy, to study these intermediates.
Our recent work has studied the kinetics of the reactions of the simplest Criegee intermediate, CH2OO, with inorganic acids and alcohols. The findings indicate that reactions with HNO3 and HCl are very fast, and that HNO3 is a significant sink for CH2OO in polluted urban environments under dry conditions.
3. Pre-reactivity in van der Waals complexes
In addition to the transition state, the long-range region of intermolecular potential energy surfaces can have a profound influence on the dynamics of bimolecular reactions. By preparing van der Waals complexes between reactive species, it will be possible to optically prepare quasibound states that will predissociate or prereact. We use a combination of molecular beam, laser spectroscopy and ion imaging techniques to explore these transient species and the ensuing dynamics in great detail.
4. Time-resolved laser induced fluorescence
5. Particle trapping using femtosecond laser pulses