96
4 Rotation of the Polyatomic Molecule
a pulsed nozzle source of molecules synchronized with the microwave pulse. Thanks
to the jet the molecules are cooled considerably simplifying the spectra and is well
adapted to the study of weakly bound complexes. The Fabry–Perot cavity brings a
significant enhancement in sensitivity and resolution. The weak point is the bandwidth of individual measurements that is severely restricted (about 1 MHz). However,
the scanning is usually automated, so that the spectrum over a frequency range of
several GHz can be obtained without user intervention. However, this scanning is
still time consuming. Figure 4.4 shows the block diagram of such a spectrometer
with laser ablation.
This last difficulty was solved by Pate et al. (Brown et al. 2008; Shipman and
Pate 2011) by using a high-speed (>4 G samples/s) arbitrary waveform generator to
generate a “chirped” microwave pulse that sweeps up to 12 GHz in less than one μs
combined with the detection of the full frequency band in a single experimental event.
In this way, broadband operation (typically 10 GHz) can be achieved routinely. This
spectrometer has a sensitivity per pulse smaller than the Balle–Flygare spectrometer,
but the large reduction in measurement time allows for increased sensitivity over
long scans. In the last years, the use of chirped-pulse techniques has become standard
and will probably replace other MW techniques. Chirped-pulse techniques can be
combined with double-triple resonance experiments and extended to the mmw-range.
Very recently (Patterson et al. 2013; Lobsiger et al. 2015), chirped-pulse methods
Fig. 4.4 Block diagram of a Fourier transform microwave (FTMW) spectrometer with a supersonic
jet and laser ablation
4 Rotation of the Polyatomic Molecule
a pulsed nozzle source of molecules synchronized with the microwave pulse. Thanks
to the jet the molecules are cooled considerably simplifying the spectra and is well
adapted to the study of weakly bound complexes. The Fabry–Perot cavity brings a
significant enhancement in sensitivity and resolution. The weak point is the bandwidth of individual measurements that is severely restricted (about 1 MHz). However,
the scanning is usually automated, so that the spectrum over a frequency range of
several GHz can be obtained without user intervention. However, this scanning is
still time consuming. Figure 4.4 shows the block diagram of such a spectrometer
with laser ablation.
This last difficulty was solved by Pate et al. (Brown et al. 2008; Shipman and
Pate 2011) by using a high-speed (>4 G samples/s) arbitrary waveform generator to
generate a “chirped” microwave pulse that sweeps up to 12 GHz in less than one μs
combined with the detection of the full frequency band in a single experimental event.
In this way, broadband operation (typically 10 GHz) can be achieved routinely. This
spectrometer has a sensitivity per pulse smaller than the Balle–Flygare spectrometer,
but the large reduction in measurement time allows for increased sensitivity over
long scans. In the last years, the use of chirped-pulse techniques has become standard
and will probably replace other MW techniques. Chirped-pulse techniques can be
combined with double-triple resonance experiments and extended to the mmw-range.
Very recently (Patterson et al. 2013; Lobsiger et al. 2015), chirped-pulse methods
Fig. 4.4 Block diagram of a Fourier transform microwave (FTMW) spectrometer with a supersonic
jet and laser ablation