Polycyclic aromatic hydrocarbons (PAHs), possibly the key link to the chemical evolution and the origin of life in the Universe, have been strongly believed to exist across all galaxies, including interstellar medium (ISM), circumstellar outflows, planetary nebulae, photo-dissociating and star-formation regions^{[1, 2]}. For a long while, the reasonable, but ambiguous spectroscopic results of deep-space hunting for PAHs have been demonstrated by worldwide astronomical sources ranging from ultraviolet to Infrared bands^{[3, 4, 5]}. Despite the strong pieces of evidence of unidentified infrared emission band (UIR)for the presence of PAHs in the envelopes of evolved carbon rich stars, concrete identification of individual PAH has not been achieved by any ultraviolet/optical/infraredtelescopes^{[3, 4, 5]}. However, in recent years, the pure rotational spectra of benzonitrile and two cyanonaphthalenes were detected in the interstellar medium by radio telescopes^{[1, 2]}, lighting the way forward for more PAH related species to be detected soon.

We previously proposed a radio telescope survey of a potential astrophysical molecule, Dibenzofuran that is probablyrelated to the formation process of PAHs, and carried out the first laboratory spectroscopic research in the microwave band^[6]. In this work, we proposed an astrophysical PAH molecule, 9-fluorenone (C₁₃H₈O), and conducted a pure rotational spectroscopic investigation, mostly at the microwave C-band (4~8 GHz) via a cp-FTMW spectrometer.

9-fluororenoneis a typical oxygenated PAH, the derivatives of PAHs containing carbonyl functional groups. Oxygenated PAHs can be formed by direct photolysis or oxidation of parent PAHs^[7]. As an inexpensive and nontoxic photocatalyst, 9-fluororenone with such an aromatic carbonyl groupcan act as a prototype molecule for the study of photochemical process, mostly via the time-resolved infrared and resonance Raman spectroscopy^[8]. 9-fluorenone could be a significant target of radio astronomy, because such oxygenated PAHs with decent dipole moments can provide a new and effective way to confirm the existence of PAHs in deep space and reveal the connotation of interstellar medium chemistry^[9]. The pure rotational spectra of 9-fluorenone have been gradually captured in the millimeterwave and microwave range^{[10, 11]}. The present work aims to expand the scope of laboratory observation of 9-fluorenone in the microwave C-band (4~8 GHz), further supporting the astrophysical research of this molecule and other PAHs. Specifically, the pure rotational measurement (mostly between 4.5 and 6.6 GHz)of 9-fluorenone in the vibrational ground state is reported to promote more extensive exploration of PAHs in the fields of astrophysics. In addition, a global fitting analysis is carried out by combining with the existing rotational data in the microwave region, conducive to the accurate characterization of the ground state structure of this molecule.

During this work, Watson S-reduced effective Hamiltonian^[19] was applied to the analysis of observed pure rotational spectra in the microwave region. Furthermore, the whole measured rotational transitions were fitted and assigned using the I^r representation in Pickett's SPFIT/SPCAT (spectral fitting program)^[20].The latest fitting results were reformatted to convert SPFIT errors to standard errors using PIFORM^[21]. The calculations and experiments have proven that 9-fluorenone is of C_2v symmetry in the ground state. The optimized structure is of C_2v symmetry as well as planarity. The corresponding C_2v rotational axis coincides with the carbonyl bond and b axis of 9-fluorenone, making its total dipole moment equal to the dipole moment of the b axis, that is, μ _tot=μ _b. This completely agrees with experimental phenomena in which only b-type rotational transitions were detected. Besides the calculations, the rotational constants obtained from previous work in microwave and mm-wave band also contributed to assigning new rotational transition lines. In the range of 3~7 GHz, 27 new b-type rotational transitions were assigned in total, with J_max=16 and K_amax=5, which are provided in Supplementary materials. The newly assigned rotational transitions were fitted together with microwave data measured by van Wijngaarden^[11] at 8~13 GHz, and the correlative spectroscopic constants are shown in Table 2. The root-mean-square (RMS) deviation σ of this combined fit is 8 kHz, and the typical linewidth is about 250 kHz.

Table 2

Table 2

Table 2 Fitting spectroscopic parameters for 9-fluorenone This work Previous worka, b 3.0~13.0 GHz 8.0~13.0 GHz 52.0~74.4 GHz A/MHz 1 445.884 739 29(10) 1 445.884 50(26) 1 445.890 0(21) B/MHz 584.871 673 6(71) 584.871 762(99) 584.885 9(72) C/MHz 416.550 607 8(81) 416.550 51(11) 416.534 6(39) DJ/kHz 0.002 953(38) 0.003 23(17) - DJK/kHz 0.007 16(14) 0.008 52(67) 0.024 5(22) DK/kHz 0.036 59(43) 0.035 2(45) 0.026 4(41) d1/kHz -0.000 548 9(94) -0.001 199(63) - d2/kHz -0.000 264(71) -0.000 290(26) - σ /kHz 8 10 54 Nc 204 178 68 Note: a Constants are from Ref.[10]; b Constants are from Ref.[11]; c Number of assigned transitions

Table 2 Fitting spectroscopic parameters for 9-fluorenone

Pure rotational transitions of the ground state of 9-fluorenone were precisely measured by the spectrometer at C-band (4~8 GHz) mainly for the astrophysical purpose, providing new spectroscopic data for radio telescope observation. The new assignment also made up for the measuring blank of 9-fluorenone in the lower microwave range. As shown in Table 2, a combined fit of this work covered 3~13 GHz, allowing more accurate determination of the rotational constants.

Numerous computational studies have been carried out on the 9-fluorenone^{[10, 11]}, including this work. To sum up, ab initio and Density Functional Theory methods were mainly applied to optimize its structure, which undoubtedly provided the global minimum of planar and C_2v geometry with a single component of dipole moment along the b-axis. As shown in Table 1, a predicted dipole moment of 3.2~3.9 Debye could make 9-fluorenone an ideal PAH for radio telescope survey in deep space. Moreover, the level of asymmetry of 9-fluorenone, described by kappa, is only -0.67, which also helps to simultaneously determine the entire three rotational constants only from the b-type rotational transitions. Furthermore, compared with the spectroscopic data of higher microwave frequency, the accuracy of rotational constants of our combined fitting is markedly improved, as shown in Table 2. The RMS deviations of 8 kHz from this fitting are no more than 5 percent of the ~250 kHz experimental linewidth, indicating that the effective Hamiltonian has accurately described the observed spectral features in the microwave range. However, for certain centrifugal distortion constants, such as D_J, due to the available high K_a and K_c transitions, a global fitting from higher frequency data can certainly derive more exact results.

The pure rotational investigation on 9-fluorenone at C-band (4~8 GHz) region was presented in this work. Accurate observation of its new rotational transitions provided more spectroscopic data for the PAH molecule hunting in deep space. This work also demonstrated the ability of our cp-FTMW spectrometer to detect non-volatile molecules in supersonic jet expansionsby using the heating nozzle. Precise rotational constants obtained by the combined fitting are not only conducive to the accurate characterization of the ground state structure, but also provide the basis for the computational study of such molecules.