Chirped-pulse Fourier Transform Microwave (CP-FTMW) Spectroscopy

Rotational spectroscopy is a highly structure-specific technique characterized by its superb frequency resolution (10 kHz linewidth for transitions in the 2-18 GHz frequency range). It utilizes the sensitive relationship between a molecule’s structure and its rotational frequencies, where the 106 unique resolution elements (kHz to GHz) enable the study of complex mixtures. Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy uses fast digital electronics to produce broadband excitation and detection pulses that cover large spectral ranges in a single measurement cycle that occurs on the microsecond time scale. To record a broadband spectrum, a frequency chirp (e.g., a linear sweep over a 1 GHz range) is produced using an arbitrary waveform generator (AWG). The strong electric field polarizes the rotational transitions in the targeted bandwidth. The resulting molecular free induction decay is recorded with a fast digitizer and a fast Fourier transform (FFT) is used to obtain a spectrum in the frequency domain. The frequency patterns can then be fit to theoretical predictions to provide sets of accurate rotational parameters for each species present that has a permanent dipole moment.

Graphic of a microwave circuit.
Microwave circuit. The AWG produces a linear sweep. The resulting frequencies are up converted to 6-18 GHz using a synthesizer and a mixer, then it is amplified by a solid-state amplifier. The microwave radiation is transmitted into the vacuum chamber using a broadcasting horn antenna. The sample is introduced into the chamber using a pulse valve that creates a supersonic expansion. The polarized molecules will emit an FID that is collected using a receiving horn. The molecular signal is amplified by a LNA and down converted. The FID is coherently averaged by an oscilloscope.