The application of double resonance spectroscopy can be very useful for identifying transitions in complex spectra. Rotationally resolved ultraviolet (electronic) spectroscopy of aromatic molecules and clusters often results in very dense spectra. The first application of microwave-ultraviolet double resonance spectroscopy on such systems has been demonstrated on 1-cyanonnaphthalene and is described on this web-page.
Rotationally resolved fluorescence excitation spectra are obtained using a narrow bandwidth UV laser system and a molecular beam apparatus. The sample is heated in a quartz oven to bring it into the gas phase, seeded in 0.2-1.0 bar argon, and expanded through a nozzle with a diameter of 0.15 mm. The nozzle is kept at a slightly higher temperature to prevent condensation of the sample in the orifice. The molecular beam is skimmed twice in a differential pumping system and is crossed perpendicularly with a UV laser beam at about 30 cm from the nozzle.
UV radiation with a bandwidth of 3 MHz is generated by intracavity frequency doubling in a single frequency ring dye laser. Typically 0.1-5 mW of tunable radiation can be obtained. For relative frequency calibration a temperature stabilized Fabry-Perot interferometer is used with a free spectral range of 75 MHz. For absolute frequency calibration, the iodine absorption spectrum is recorded simultaneously. The total undispersed fluorescence is imaged on a photomultiplier connected to a photon counting system interfaced with a computer.
To perform microwave-ultraviolet (MW-UV) double resonance experiments, microwave radiation (2-10 GHz) was fed into the region where the UV laser interacts with the molecular beam. The antenna was an open loop with a diameter of about 1.5 cm and was positioned in such a way that the laser beam can pass through it. Amplitude modulation of the MW radiation was used to enhance the signal to noise ratio.