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Pure Rotational Spectrum of Benzophenone Detected by Broadband Microwave Spectrometer in the 2–8 Ghz Range

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Pure Rotational Spectrum of Benzophenone Detected by Broadband Microwave Spectrometer in the 2–8 Ghz Range applied sciences Article Pure Rotational Spectrum of Benzophenone Detected by Broadband Microwave Spectrometer in the 2–8 GHz Range Haoyang Tan, Miaoling Yang, Chenbo Huang, Shengwen Duan, Ming Sun *, Qian Chen *, Chao Jiao * and Yi Wu School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; [email protected] (H.T.); [email protected] (M.Y.); [email protected] (C.H.); [email protected] (S.D.); [email protected] (Y.W.) * Correspondence: [email protected] (M.S.); [email protected] (Q.C.); [email protected] (C.J.) Received: 2 November 2020; Accepted: 26 November 2020; Published: 27 November 2020 Abstract: The investigation on microwave spectrum of benzophenone was conducted with a recently constructed broadband chirped-pulse Fourier transform microwave spectrometer with a heating nozzle in the 2–8 GHz range. In this work, 138 b-type pure rotational transitions were assigned to bridge the measuring gap in the microwave region. The rotational constants for benzophenone were accurately determined by a combined microwave data fitting with frequency coverage between 2–14 GHz and have the following values: A = 1692.8892190(119) MHz, B = 412.6446602(43) MHz and C = 353.8745644(43) MHz. Keywords: benzophenone; rotational spectroscopy; microwave; cp-FTMW 1. Introduction Benzophenone is an aryl ketone that could be prepared by a Friedel–Crafts acylation of benzoyl chloride with benzene [1] or by the oxidation of diphenylmethane [2]. Benzophenone is of great use in industrial applications to manufacture agricultural chemicals such as insecticides, and pharmaceuticals such as hypnotics [3]. Due to its photochemical reactivity, benzophenone can work as a photoinitiator and an ultraviolet curing agent [3]. Benzophenone might exist in galaxies similar to polycyclic aromatic hydrocarbons (PAHs), chemicals believed to be widespread through circumstellar envelopes and interstellar medium (ISM) of carbon rich stars [4,5]. Different from many PAHs with tiny dipole moments, benzophenone with a dipole moment about of 3.0 Debye has decent signals of rotational transitions for molecular hunting in deep space by radio telescopes. Benzophenone could also occur in the environment, such as ambient atmosphere, to bring health risks [6,7]. Under these conditions, laboratory rotational spectroscopic data on benzophenone can be a useful guide for radioastronomical and atmospheric searches for this molecule. Currently, benzophenone has been investigated by microwave/millimeter-wave spectroscopy in the 8–14 GHz and 60–73 GHz region [8,9]. The purpose of this work is to extend the laboratory measurements of the rotational spectrum of benzophenone at lower frequency to further support the astrophysical observation and atmospheric remote sensing of benzophenone molecule. The measurement and analysis of the detected spectra of benzophenone in the vibrational ground state between 2 and 8 GHz are reported. Appl. Sci. 2020, 10, 8471; doi:10.3390/app10238471 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 8471 2 of 8 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 8 2. Experiment Experiment 2.1. Experimental Experimental Instrument Instrument and and Methods The microwave spectrum of benzophenone in in th thee 2–8 GHz range was collected by a recently developed broadband broadband chirped-pulse chirped-pulse Fourier Fourier transf transformorm microwave microwave (cp-FTMW) (cp-FTMW) spectrometer spectrometer with with a heatinga heating nozzle nozzle at Nanjing at Nanjing University University of Science of Science and Technology and Technology (NJUST) (NJUST) as shown as shown in Figure in Figure1, which1, haswhich been has described been described in detail in previously detail previously [10,11]. [ 10Basi,11cally,]. Basically, a vacuum a vacuum sample sample chamber chamber (1.0 m in (1.0 length m in andlength 0.5 andm in 0.5 diameter) m in diameter) housing housing a reflection a reflection spherical spherical aluminum aluminum mirror (350 mirror mm (350 in diameter mm in diameter and 800 mmand in 800 radius mm inof radiuscurvature), of curvature), a sample pulsed a sample solenoid pulsed nozzle solenoid ((Parker nozzle Series ((Parker 9, 1.0 Series mm in 9, diameter)), 1.0 mm in anddiameter)), a feedhorn and antenna a feedhorn makes antenna the main makes mechanical the main pa mechanicalrt of the spectrometer. part of the spectrometer. The nozzle, penetrating The nozzle, thepenetrating center of the the center mirror, of is the in mirror, a coaxial is inarrangem a coaxialent arrangement with the mirror, with thethe mirror, antenna, the and antenna, the chamber. and the Thechamber. vacuum The sample vacuum chamber sample was chamber pumped was by pumped a molecu bylar a molecular pump to maintain pump to maintaina background a background pressure 5 pressure about−5 1 10 Pa. about 1 × 10 Pa. × − Figure 1.1. The schematicschematic diagram diagram of of the the cp-FTMW cp-FTMW spectrometer, spectrometer, which which is divided is divided into threeinto three parts: parts:(1) microwave (1) microwave signal source; signal (2) source; sample (2) vacuum sample chamber; vacuum (3) chamber; signal detection (3) signal system. detection system. The spectrometer is based on the linear frequencyfrequency sweeps (chirped pulses) generated by an arbitrary wave generator (AWG), (AWG), as described by Pate and others [12,13] [12,13],, but equipped with an economic quadrature quadrature detection detection plan. plan. In In this this work, work, a homodyne a homodyne circuit circuit design design was was applied applied to the to cp- the FTMWcp-FTMW spectrometer spectrometer system: system: the continuous the continuous microw microwaveave generated generated by the by microwave the microwave source source was split was insplit two in twothrough through a power a power distributor. distributor. Half Half of ofthe the microwave microwave up-converted up-converted the the broadband broadband source generated by arbitrary waveform generator to exci excitete the sample, and the other half down-converted the molecular molecular excited excited signal signal to to the the oscilloscope oscilloscope acceptable acceptable bandwidth bandwidth range. range. In Inthis this work, work, a 5 aµs, 5 µ0–s, 5000–500 MHz MHz chirped chirped pulse pulse from from AWG AWG (Siglent (Siglent SDG6052x-e, SDG6052x-e, 1.25 1.25 GS/s) GS was/s) wasmixed mixed in mixer in mixer 1 (Marki 1 (Marki M1- 0220LAM1-0220LA Mixer) Mixer) with with the theLO LOoutput output from from a microw a microwaveave synthesizer synthesizer (Anapico (Anapico Apsyn420, Apsyn420, 0.01–20 0.01–20 GHz) GHz) to generate anan excitingexciting pulse pulse with with 1 1 GHz GHz chirp chirp bandwidth, bandwidth, that that was was then then amplified amplified using using a solid-state a solid- stateamplifier amplifier (BONN, (BONN, 5 W) before 5 W) beingbefore coupled being couple into a custom-builtd into a custom-built vacuum sample vacuum chamber sample containing chamber containingone high gain one horn high antenna gain horn and antenna a reflection and sphericala reflection aluminum spherical mirror aluminum for broadcasting mirror for andbroadcasting receiving andthe molecularreceiving emissionthe molecular signals. emission The molecular signals. The signal, molecular i.e., the signal, free induction i.e., the decayfree induction (FID), was decay first (FID),amplified wasby first a low-noiseamplified amplifierby a low-noise (Miteq amplifier AFS44 LNA, (Miteq 1–18 AFS44 GHz) LNA, before 1–18 being GHz) split before by a homemadebeing split byquadrature a homemade detection quadrature structure detection to achieve structure image to rejection.achieve image The rejection. image rejected The image FID signalsrejected were FID signalsfurther were amplified further by twoamplified identical by low-noisetwo identical amplifiers low-noise ((Mini-Circuits amplifiers ((Mini-Circuits ZFL 500LN+, 0.1–500ZFL 500LN+, MHz), 0.1–500which were MHz), subsequently which were sampled subsequently by the twosampled channels by the of atwo digitizer channels (Spectrum of a digitizer M4i.2223, (Spectrum 2.5 GS/s, M4i.2223, 2.5 GS/s, 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 Appl. Sci. 2020, 10, 8471 3 of 8 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 8 1.5was GHz repeated bandwidth). to produce Fast multiple Fourier transformationFIDs so that the of resulting the resulting spectrum free inductiongave an improved decay leads signal-to- to the broadbandnoise ratio. spectrum Furthermore, over thethe fullDF4351 bandwidth frequency of the sy chirp.nthesizer, The wholethe SDG6052x-e sequence was AWG repeated and the to M4i.2223 produce multipledigitizer FIDswere soreferenced that the to resulting a 10 MHz spectrum rubidium gave standa an improvedrd oscillator signal-to-noise (SRS, FS725) for ratio. external Furthermore, stability. theIn order DF4351 to frequency avoid damage synthesizer, to the the downstream SDG6052x-e am AWGplifier and and the M4i.2223oscilloscope digitizer caused were by referenced high-power to amicrowave 10 MHz rubidium pulses, standarda single-pole oscillator single-throw (SRS, FS725) electronic for external switch stability. (SPST, InF9114A, order to1–18 avoid GHz) damage was set to thebehind downstream mixer 2 (Marki amplifier M1-0220LA and oscilloscope Mixer), which caused coul byd high-power be closed in microwave
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