15min:
DESIGN AND PERFORMANCE OF A DIRECT DIGITAL CHIRPED-PULSE FOURIER TRANSFORM MICROWAVE (CP-FTMW) SPECTROMETER OPERATING IN THE 2 - 8 GHZ FREQUENCY RANGE.

STEVEN T. SHIPMAN, LEONARDO ALVAREZ-VALTIERRA, JUSTIN L. NEILL AND BROOKS H. PATE, Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904; ALBERTO LESSARI, Universidad de Valladolid, Departmento Química Física y Química Inorgánica, Facultad de Ciencias, Prado de la Magdalena, s/n, 47005 Valladolid, Spain; Z. KISIEL, Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland.

We have constructed a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer operating in the 2 - 8 GHz frequency range. The chirped excitation pulse executes a linear frequency sweep covering 2 - 8 GHz in pulses with duration of 9 µs or less. This chirped pulse is created using the 8-bit D/A converter with 20 Gs/s sample rate in a high-speed arbitrary waveform generator (AWG). The output of the AWG is sent to a high-power pulsed microwave amplifier. We have used both a 300W traveling wave tube amplifier and a 4W solid state amplifier in the spectrometer. The amplified chirped pulse is broadcast onto the molecular beam sample using a WRD250 double-ridge standard gain horn with 15 dBi gain. A second WRD250 gain horn is used to collect the broadband rotational free induction decay. The molecular emission is amplified by a broadband, low-noise amplifier and converted back to a digital signal using the 8-bit A/D converter with 20 Gs/s sample rate in a digital oscilloscope. The frequency domain spectrum is obtained through fast Fourier transform of the time-domain averaged FID with 40 µs duration. The performance of the spectrometer is demonstrated by measurment of the pure rotational spectrum of iodobenzene, its 13C isotopomers, and a weakly bound complex with the neon carrier gas in the molecular beam expansion. A frequency accuracy of 4 kHz for the spectrometer is demonstrated by comparison to previous measurements of the iodobenzene spectrum using several spectrometers.\footnoteDorosh, O. et al. , J. Mol. Spec. \textbf246 (2007), 228-232. Accurate transition intensities are obtained for both number density (13C isotopomer signals) and transition moment (nuclear quadrupole hyperfine patterns). The open interaction region between the horn antennas makes it easy to incorporate a ``Stark cage'' for dipole moment determination.\footnoteEmilsson, T. et al. , J. Chem. Phys. \textbf112 (2000), 1287-1294. Performance of the broadband dipole moment determination of iodobenzene is tested by comparison to previous work. The implementation of broadband microave-microwave double-resonance spectroscopy is also presented for the iodobenzene spectrum.