Diagnostics of a Supersonic Beam Using a Microwave Cavity Fourier Transform Spectrometer.

Available from UMI in association with The British Library. Requires signed TDF. The use of a pulsed Fourier transform microwave cavity spectrometer combined with a synchronous pulsed supersonic nozzle beam is described for spectroscopy and beam diagnostics. The use of an appropriate cylindrical cavity mode was shown to give signals without the Doppler splitting characteristics of similar spectrometers employing Fabry-perot cavities. The high sensitivity of the spectrometer and measured linewidths as low as 20 KHz make it ideally suited to the observation of hyperfine components in multi-quadrupole molecules. The tri-quadrupolar structure of the 3 --> 2 transition of PCl_3 was investigated and a measured Cl-P-Cl bond angle of 104.5(4)^circ, differing significantly from 100.27(9)^circ from electron diffraction measurements led to the conclusion of either some form of axial asymmetry in the bond or "bent bonds". Experiments to determine the vibrational and rotational temperature of spectrally active molecules in the beam are described. The rotational beam temperature for the 3_{03} --> 2_{12} OA transition of ethanal was found to reach less than 2 K for dilute mixes in helium. In marked contrast the vibrational temperature of the CS stretching mode of OCS was found to exceed 230 K, showing a small collision cross section for transfer of vibrational energy to the atoms of the carrier gas. The properties of the beam in the centre and at the leading and trailing edges were investigated and found to differ, with the trailing edge showing collisional decay with the buildup of static gas and a decay time of around 80 mus. The use of cavity modes showing a Doppler splitting produced an additional centre peak not theoretically predicted and showing evidence of fringing fields.