We report the first detection of radio recombination lines (RRLs) of ions heavier than helium. In a highly sensitive multi-band (12–50 GHz) line survey toward Orion KL with the TianMa 65-m Radio Telescope (TMRT), we successfully detected more than fifteen unblended α lines of RRLs of singly ionized species (XII) recombined from XIII. The Ka-band (26–35 GHz) spectrum also shows tentative signals of β lines of ions. The detected lines can be successfully crossmatched with the rest frequencies of RRLs of C II and/or O II. This finding greatly expands the scope of our understanding of ion RRLs, since prior to this work, only two blended lines (105α and 121α) of He II had been reported. Our detected lines can be fitted simultaneously under assumption of local thermodynamic equilibrium (LTE). An abundance of C III and O III of 8.8×10−4 was obtained, avoiding the complexities of optical and infrared observations and the blending of RRLs of atoms. It is consistent with but approaches the upper bound of the value (10−4–10−3) estimated from optical and infrared observations. The effects of dielectronic recombination may contribute to enhancing the level populations even at large n. We expect future observations using radio interferometers could break the degeneracy between C and O, while also helping to reveal the ionization structure and dynamical evolution of various ionized regions.
Instantaneous frequency measurement of microwave signals is a fundamental functionality for applications including radar and electronic warfare. Photonic techniques have potential to enlarge the measurable frequency range, however, most of photonicsassisted solutions are lack of the ability to simultaneously achieve large measurement range, high measurement speed, and high measurement accuracy in a single system. Here, we propose a photonics-assisted nonscanning high-accuracy multiple frequency measurement scheme based on optical beating between double sideband carrier-suppressed signals and a detuning optical frequency comb at a narrow band photodiode. The frequencies of the incoming RF signals can be estimated by analyzing the frequency and power information of the beating notes. In a proof-of-principle experiment, single-and multiple-frequency measurements with a large measuring range from 2 to 12 GHz and low measuring error of less than 2 MHz was achieved. This scheme is suitable for instantaneous multiple-frequency measurement with high accuracy across a large frequency measurement range.
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