As part of the spectroscopic work in the Laser Group, a pulsed UV/Visible optical parametric oscillator (OPO) system has been developed that provides a much lower bandwidth than conventional dye laser systems. The improved resolution this makes possible is illustrated in the figure below.
The PF spectrum shown allows an extra level of detail beyond rotational structure to be seen in the electronic spectrum, in this case hyperfine structure. This provides a particularly direct probe of the wavefunction of the excited electronic state, allowing such things as % of s-orbital character to be determined.
The diagram above shows an outline of the system; it is an optical parametric oscillator consisting of a ring cavity containing two BBO crystals pumped by an injection seeded 355 nm Nd:YAG laser. The narrow bandwidth is made possible by seeding and locking the cavity to a solid state diode laser in the 770-1050 nm range. By combining this setup with various frequency doubling and mixing schemes most of the 210-1050 nm wavelength range can be covered.
Sub-Doppler spectroscopy is achieved by selectively imaging the fluorescence from the centre of the intersection of the laser beam with a molecular beam. The picture below, taken with an image intensified camera, shows the system in action. The object at the top is the nozzle with an electric discharge inside. The molecular beam has molecules radiating from the bright spot at the bottom of the nozzle (actually the nozzle aperture) and the laser beam crosses the molecular beam about half way down the picture. For this picture, the laser was positioned between two closely spaced peaks, so only molecules travelling somewhat towards or away from the laser are excited and can fluoresce. Those travelling straight down are perpendicular to the laser beam and are therefore not excited.
The interpretation of molecular magnetic hyperfine interactions. J.A.J. Fitzpatrick, F.R. Manby and C.M. Western, J. Chem. Phys. 122, 084312 (2005).
Sub-Doppler Spectroscopy of the Ã 2Σ+ – X 2Π and B 2 Π – X 2Π Transitions of NCO, N.L. Elliott, J.A. J. Fitzpatrick and C.M. Western, J. Chem. Phys. 129, 164301 (2008).