The CP-FTS (Chirped Pulse Fourier Transform Spectrometer)

The CAS laboratories operate a broadband Chirped-Pulse Fourier Transform Spectrometer (CP-FTS) covering the frequency regions 6–26 GHz and 75–110 GHz. In this type of spectrometers, a molecular sample is polarised by a chirped pulse (~1 µs) and the subsequent free induction decay (FID) is recorded with an instantaneous spectral bandwidth of several GHz. The spectrum is obtained by Fourier Transformation of the FID. The capability to record broad scans with high frequency resolution and high dynamic range on microsecond timescales makes this instrument very suitable for studying the spectra of astrophysically relevant molecules, in particular if they obey dense spectra (e.g. complex organic molecules, COMs) or if frequency predictions are very uncertain. The electronics are situated in a movable rack, so that it can be most easily coupled to a variety of chambers/cells, depending on the desired sample to study. Besides a 1.5 m long waveguide sample cell, the spectrometer can be coupled with the supersonic jet apparatus for studying unstable and short-lived molecules, or in future to a buffer-gas cooling cell.

Setup

The chirped pulse is generated by a 5 GHz arbitrary waveform generator (Keysight, M8190A). The signal is upconverted by an IQ modulator and subsequently amplified by a solid state amplifier (6-18 GHz with 20 W, and 18-26 GHz with 4W) and fed into the sample chamber (e.g. waveguide). The molecular signal (FID) is later on first amplified (low noise amplifier) and downconverted in frequency (IQ mixer), before it is recorded either with a 25 GHz digital scope or 2.5 GHz digitizer card (Acqiris U5310A). A signal generator (Keysight E8257D) acts as local oscillator. Its signal is split by a power divider and fed into IQ modulator and the IQ mixer in the detection unit.

A frequency extension to 75-110 GHz is available. It consists of two active sextuplers. A medium power amplifier allows to generate chirped pulses of about 250 mW and a low noise amplifier attached to the harmonic mixer (75-110 GHz) is applied for downconversion.

The CP-FTS coupled to the 1.5m-long waveguide cell.

Spectra

The recording of a 4 GHz scan of the inversion spectrum of an ammonia sample. This spectrum was obtained after an integration of 250,000 pulses over ca. 20 minutes.

12 – GHz broad spectrum of HC3N at 3mm.
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