## 2 Experiment  
### Experimental Instrument and Methods  
The microwave spectrum of benzophenone in the 2-8 GHz range was collected by a recently developed broadband chirped-pulse Fourier transform microwave (cp-FTMW) spectrometer with a heating nozzle at Nanjing University of Science and Technology (NJUST) as shown in Figure 1, which has been described in detail previously [10, 11]. Basically, a vacuum sample chamber (1.0 m in length and 0.5 m in diameter) housing a reflection spherical aluminum mirror (350 mm in diameter and 800 mm in radius of curvature), a sample pulsed solenoid nozzle ((Parker Series 9, 1.0 mm in diameter)), and a feedhorn antenna makes the main mechanical part of the spectrometer. The nozzle, penetrating the center of the mirror, is in a coaxial arrangement with the mirror, the antenna, and the chamber. The vacuum sample chamber was pumped by a molecular pump to maintain a background pressure about \(1\times 10^{-5}\) Pa. The spectrometer is based on the linear frequency sweeps (chirped pulses) generated by an arbitrary wave generator (AWG), as described by [PERSON] and others [12, 13], but equipped with an economic quadrature detection plan. In this work, a homodyne circuit design was applied to the cp-FTMW spectrometer system: the continuous microwave generated by the microwave source was split in two through a power distributor. Half of the microwave up-converted the broadband source generated by arbitrary waveform generator to excite the sample, and the other half down-converted the molecular excited signal to the oscilloscope acceptable bandwidth range. In this work, a 5 us, 0-500 MHz chirped pulse from AWG (Siglent SDG6052x-e, 1.25 GS/s) was mixed in mixer 1 (Marki M1-0220 LA Mixer) with the LO output from a microwave synthesizer (Anapico Apsyn420, 0.01-20 GHz) to generate an exciting pulse with 1 GHz chirp bandwidth, that was then amplified using a solid-state amplifier (BONN, 5 W) before being coupled into a custom-built vacuum sample chamber containing one high gain horn antenna and a reflection spherical aluminum mirror for broadcasting and receiving the molecular emission signals. The molecular signal, i.e., the free induction decay (FID), was first amplified by a low-noise amplifier (Miteq AFS44 LNA, 1-18 GHz) before being split by a homemade quadrature detection structure to achieve image rejection. The image rejected FID signals were further amplified by two identical low-noise amplifiers ((Mini-Circuits ZFL 500 LN+, 0.1-500 MHz), which were subsequently sampled by the two channels of a digitizer (Spectrum M  
Figure 1: The schematic diagram of the cp-FTMW spectrometer, which is divided into three parts: (1) microwave signal source; (2) sample vacuum chamber; (3) signal detection system. 1.5 GHz bandwidth). Fast Fourier transformation of the resulting free induction decay leads to the broadband spectrum over the full bandwidth of the chirp. The whole sequence was repeated to produce multiple FIDs so that the resulting spectrum gave an improved signal-to-noise ratio. Furthermore, the DF4351 frequency synthesizer, the SDG6052x-e AWG and the M4i.2223 digitizer were referenced to a 10 MHz rubidium standard oscillator (SRS, FS725) for external stability. In order to avoid damage to the downstream amplifier and oscilloscope caused by high-power microwave pulses, a single-pole single-throw electronic switch (SPST, F9114A, 1-18 GHz) was set behind mixer 2 (Marki M1-0220 LA Mixer), which could be closed in time to protect the downstream electronic equipment. Benzophenone (99.9% purity) was purchased from Aldrich without further purification. A sample gas mixture was obtained by flowing high purity argon (99.9% purity) through benzophenone right before the Parker nozzle, which was coupled with a band heater to obtain sufficient vapor pressure for solid chemicals.