Microwave Spectroscopy in Helium Nanodroplets.

We have implemented a microwave resonator, i.e. a Fabry-Perot cavity, into a helium nanodroplet instrument. The cavity consists of two spherical aluminum mirrors with radius of curvature of 13 cm and diameter of 10 cm. The cavity is mounted in a coaxial fashion into the instrument to maximize the interaction length between radiation and doped helium droplets. The helium droplet beam enters and exits the cavity through a hole in each of the mirrors. One of the mirrors can be adjusted to tune the cavity into resonance. The output of a cw microwave synthesizer can be amplified by a traveling wave tube amplifier to powers of about 25 Watt and is coupled into the cavity through a simple wire hook antenna. Detection is accomplished using the depletion technique. We have measured the spectrum of the J=2-1 transition of carbonylsulfide demonstrating the sensitivity of this method. Power saturation was observed and will be analyzed as will be the observed line width of the transition.