A High Power Amplifier for Advanced Chirped Pulse Microwave Spectroscopy

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Our research groups are interested in the study of weak intermolecular interactions, in particular with regard to their role in chirality recognition, solvation, and atmospheric chemistry processes. For example, hydrogen-bonded solute-solvent clusters in solution are now recognized to play highly significant roles in determining bulk properties (“clusters-in-a-liquid model”). Gas phase studies of isolated clusters are particularly well suited to determine the structures, internal motions, and relative energies of different solute-solvent conformers, in part because the solvation process can be studied by adding one solvent molecule at a time. In atmospheric chemistry, small clusters involving organic acids, for example, play a crucial role as nucleation precursors in the formation of secondary organic aerosol. Such complexes can also promote or suppress atmospheric photo-chemical reactions, depending on the particular conformer. High-resolution microwave spectroscopy is perhaps the only experimental method which can routinely provide the required detailed information about structures and relative energies of different conformations of solute-solvent clusters. The introduction of the chirped pulse Fourier transform technique in 2008 was a revolution in microwave spectroscopy, made possible by the tremendous advances in high-speed electronics. We were an early adopter of this technique and have since published a number of high quality studies, several of them in Angew. Chem. (including a VIP with inside cover). We are now pushing the limits of this technique to tackle systems of sizes and (conformational) complexity (and even greater relevance), which were thought impossible to study before. The associated dramatic increase in spectral density (hundreds of lines in a 2 GHz interval), however, makes assignments of quantum numbers and molecular carriers virtually impossible. This impasse can be overcome by the application of double resonance techniques, which allow one to identify transitions connected by a common energy level and belonging to the same conformer. We have significant double resonance experience, but encountered an obstacle in our double resonance experiments. This obstacle is our microwave amplifier which has a low output power of 20 W. In this weak field regime, a chirped pulse excitation affects only very small changes in energy level populations, resulting in weak double resonance effects. This application is for the purchase of a high power travelling wave tube microwave amplifier (300 W) to push our experiments into the strong field regime, where population inversion leads to easily detectable double resonance effects. This amplifier is crucial for us to continue our successful research programs in molecular recognition and solvation. Without the amplifier, we will lose our position at the forefront of this exciting and competitive field of research.