A simple high-intensity UV-photon source for photochemical studies in UHV: Application to the photoconversion of norbornadiene to quadricyclane

Photochemical in situ studies in a well-controlled surface science environment can help to understand photochemical reactions in organic thin films in more detail. To perform such studies without external focusing or light guiding systems, we designed a high-intensity UV-photon source, which is compatible with an ultra-high vacuum (UHV) environment. The UV source is based on a high power light-emitting diode (LED), soldered onto a copper heat reservoir to avoid overheating. The LED can be placed in close vicinity in front of a single crystal, providing flux densities of 2 × 1018 photons s−1 cm−2 at a wavelength of 365 nm. Thus, the device provides light intensities one order of magnitude higher as compared to conventional continuous wave arc lamps, at only a small variation of the flux of less than ±20% over a sample surface of 10 × 8 mm2. The UV source is mounted in a UHV infrared reflection absorption spectroscopy system and triggered by using the IR spectrometer. This allows fully automatized in situ I...