Rotational Spectroscopy of a Triatomic Molecular Anion

Rotational transitions of the nonlinear triatomic molecular anion ${{\mathrm{NH}}_2}^{-}$ 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 ${{\mathrm{NH}}_2}^{-}$ have been found. For the para nuclear spin configuration, the $1_0\leftarrow 0_0$ transition frequency was determined to be 933 954(2) MHz, and for the ortho configuration the $1_{+1}\leftarrow 1_{-1}$ 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 ${{\mathrm{NH}}_2}^{-}$.

Figure 1

${{\mathrm{NH}}_2}^{-}$ and ${\mathrm{NH}}_2$ rotational energy-level diagram showing the allowed PD transitions for the $s$-wave electron ($A_1$ symmetry) and rotational excitations detected in THz spectroscopy. The levels and transitions corresponding to ortho states are indicated with dashed lines and arrows. The red and blue colors are used to distinguish transitions involved in the spectroscopy of para-${{\mathrm{NH}}_2}^{-}$ and ortho-${{\mathrm{NH}}_2}^{-}$, respectively.

Figure 2

Histograms of the ion intensity of ${{\mathrm{NH}}_2}^{-}$ after photodetachment as a function of the THz radiation frequency. Individual frequency points are combined in bins with a 1.8 MHz (para) or 1.6 MHz (ortho) width. The resonances are observed at $933953.3\pm 1.0\mathrm{MHz}$ for para-${{\mathrm{NH}}_2}^{-}$: $0_0\to 1_0$ [panel (a)] and at $447376.5\pm 0.3$ for ortho-${{\mathrm{NH}}_2}^{-}$: $1_{-1}\to 1_1$ [panel (b)]. The fitted linewidths of the two transitions are 6.1(5) MHz for para- and 6.4(7) MHz for ortho-${{\mathrm{NH}}_2}^{-}$ (FWHM).