Rotational Spectroscopy of a Triatomic Molecular Anion

Abstract

Rotational transitions of the nonlinear triatomic molecular anion NH_{2}^{-} have been observed by terahertz spectroscopy in a cryogenic radio frequency ion trap. Absorption of terahertz photons has been probed by rotational state-dependent photodetachment of the trapped negative ions near the detachment threshold. Using this two-photon scheme, the two lowest rotational transitions for the asymmetric top rotor NH_{2}^{-} have been found. For the para nuclear spin configuration, the 1_{0}←0_{0} transition frequency was determined to be 933 954(2) MHz, and for the ortho configuration the 1_{+1}←1_{-1} transition frequency was determined to be 447 375(3) MHz. This result appears to preclude the recent tentative assignment of an interstellar absorption feature to NH_{2}^{-}.

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…72 Other notable achievements in high-resolution rotational spectroscopy using terahertz technology include the measurement of rotational transitions in molecular oxygen isotopologues, and highly-accurate measurements of the rotational transitions of the NH 2 anion, critical to the subsequent investigation of interstellar spectroscopic measurements made by the Herschel Space Observatory. [73][74][75] More recently, terahertz QCLs have been used to perform ultra-high resolution gas-phase spectroscopy, with a reported resolution of < 0.5 MHz (5 × 10 −7 THz), with some studies even suggesting that QCLs can simultaneously act as both the emitter and receiver module for high-resolution spectroscopy -enabling a compact spectrometer design that is suitable for space-based applications. 76,77 A second area of terahertz spectroscopy that was quickly recognized leveraged the time-domain nature of most terahertz spectrometers.…”

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…72 Other notable achievements in high-resolution rotational spectroscopy using terahertz technology include the measurement of rotational transitions in molecular oxygen isotopologues, and highly-accurate measurements of the rotational transitions of the NH 2 anion, critical to the subsequent investigation of interstellar spectroscopic measurements made by the Herschel Space Observatory. [73][74][75] More recently, terahertz QCLs have been used to perform ultra-high resolution gas-phase spectroscopy, with a reported resolution of < 0.5 MHz (5 × 10 −7 THz), with some studies even suggesting that QCLs can simultaneously act as both the emitter and receiver module for high-resolution spectroscopy -enabling a compact spectrometer design that is suitable for space-based applications. 76,77 A second area of terahertz spectroscopy that was quickly recognized leveraged the time-domain nature of most terahertz spectrometers.…”

Vibrational overtone spectroscopy of cold trapped hydroxyl anions

“…+ is a carrier of four diffuse interstellar bands [11]. Our group has performed rotational spectroscopy on the triatomic anion NH 2 − [12] precluding the tentative assignment that this species supports an interstellar absorption feature. The sensitivity in ion-molecule studies within Coulomb crystals has also allowed for the testing of non-Langevin behavior at ultracold temperatures [13].…”

“…More recently a double resonance IR-THz LIR [21] scheme has been presented which may be applied to the full range of cationic and anionic species. Probing schemes involving photodetachment of the extra electron have also been put to use in the case of trapped molecular anions [12,25]. The application of a neutral, inert buffer gas to a trapped ion setup allows for the cooling of both the translational and internal degrees of freedom [8,10].…”