Hendrik Nahler

Photodissociation dynamics at surfaces

With a new spectroscopic technique (CELIF), co-developed in our group, we aim to advance the understanding of photochemical reactions at ice surfaces relevant in the earth's atmosphere. The results will reveal the differences between gas-phase and surface photodissociation for the system nitric acid on ice. The CELIF technique is open to the investigation and control of catalytic reactions and reaction dynamics on thin films, topical areas we aim to extend our research to.

1.  Photochemistry at the air-ice interface

In our new surface apparatus we study the photodissociation dynamics of atmospherically relevant molecules such as nitric acid on ice surfaces, shedding light into its importance in explaining pollutant levels in the polar atmospheric boundary layer. In order to detect photodissociation products desorbed from surfaces under near-ambient conditions (210 – 273 C), we utilise spectroscopic methods such as cavity ring-down spectroscopy (CRDS) and laser-induced fluorescence (LIF).

2.  Cavity-enhanced laser-induced fluorescence

We developed a novel spectroscopic technique dubbed cavity-enhanced laser-induced fluorescence (CELIF) where we combine CRDS and LIF in such a way that we maintain the high sensitivity of LIF and at the same time achieve absolute calibration in terms of absorption coefficients through the simultaneous cavity ring-down measurement. We successfully demonstrated the technique in the detection of SD radicals in molecular beams down to 215 molecules in the probe volume. CELIF is particularly powerful in the measurement of absolute absorption coefficients in localised detection volumes.

3. Charge and energy transfer at surfaces

Break-down of the Born-Oppenheimer approximation is shown upon scattering of vibrationally excited NO from a Cs/Au surface. The observed electron emission scales inversely with the velocity of the NO molecules attributed to a mechanism we call vibrational auto-detachment. The large amplitude motion (NO, v=18) close to the metal surface leads to a surface-to-molecule electron transfer and subsequent emission. The probability of this process increases with interaction time (low velocity).