Instruments

CPFT-Instruments


Chirped Pulse Fourier Transform Spectroscopy (CP-FTS) is carried out in a newly built Chirped-Pulse Supersonic Jet experiment. The spectrometer is based in parts on the design of the Impact Chirped-Pulse spectrometer at the University of Hannover.  The Figure below hows a schematic diagram of the experimental setup. Central part of the instrument is a free molecular jet setup which has been used previously in the OROTRON spectrometer. The jet expansion is probed perpendicularly by the chirped pulse microwave configuration.

Schematic of the CP-FTS jet spectrometer


Schematic of the CP-FTS jet spectrometer


The Figure displays primarily its design and individual electronic parts. Elements for the molecular excitation process are shown on the left side of the Figure while those used for recording the molecular signal are displayed on the right. A 500 ns long chirped-pulse in the range of 0-500 MHz is created by a Keysight M8190a 12 GS/s arbitrary waveform generator (AWG). The two channels of the AWG are used to drive an IQ-Modulator in order to run a 0 - 1 GHz chirp. When mixed with the local oscillator (LO) frequency in the range 12-26.5 GHz a respective LO+/-500 MHz excitation sweep is generated. This up-converted signal is amplified either by a 20 W (12-18 GHz) or a 4W (18-26.5 GHz) solid state power amplifier and coupled via a directional high-gain horn antenna (25 dBI) into the vacuum chamber where it interacts with the pulsed free jet. Since the antennae are comparable in size with the dimensions of the vacuum chamber (360x360x200 mm), they are located outside the vacuum and the microwave excitation pulses as well as the free induction decay (FID) signals are transmitted via Teflon windows.

The molecular FID is then collected by a second, equally sized horn antenna (right part of the Figure). The received signal is passed through a limiter, a protection pin-diode switch and a bandpass filter. Following the low noise amplifier (LNA) the FID signal is further amplified to set the dynamic range of the FID to the maximum vertical resolution of a 25 GHz analog bandwidth oscilloscope (Tektronix DPO72504D). Employing the same LO used for upconverting the chirp excitation pulse the FID signal is downconverted using an IQ demodulator to generate the in-phase and quadrature FID signal. Recording both channels of this FID signal the upper and lower side band emission spectrum is determined by a complex fast Fourier transformation. The result of such a broad band excitation and FID emission signal will be shown in the result section for selected examples.