Threshold photodetachment spectroscopy has been performed on the molecular anion CN − at both 16(1) K and 295(2) K in a 22-pole ion trap and at 295(2) K from a pulsed ion beam. The spectra show a typical energy dependence of the detachment cross section yielding a determination of the electron affinity of CN to greater precision than has previously been known at 31 163(16) cm −1 [3.864(2) eV]. Allowed s-wave detachment is observed for CN − , but the dependence of the photodetachment cross section near the threshold is perturbed by the long-range interaction between the permanent dipole moment of CN and the outgoing electron. Furthermore, we observe a temperature dependence of the cross section near the threshold, which we attribute to a reduction of the effective permanent dipole due to higher rotational excitation at higher temperatures.
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}^{-}.
We present a characterization of the ions’ translational energy distribution in a multipole ion trap. A linear mapping between the energy distribution of the trapped ions onto the ions’ time-of-flight (TOF) to a detector is demonstrated. For low ion temperatures, a deviation from linearity is observed and can be attributed to the emergence of multiple potential minima. The potential landscape of the trapped ions is modeled via the finite element method, also accounting for subtleties such as surface-charge accumulation. We demonstrate the validity of our thermometry method by simulating the energy distribution of the ion ensemble thermalized with buffer gas using a Molecular Dynamics (MD) simulation. A comparison between the energy distribution of trapped ions in different multipole trap configurations—i.e., with hyperbolic rods, cylindrical rods, and cylindrical wires—is provided. With these findings, one can map the temperature of the trapped ions down to the Kelvin regime using their TOF distributions. This enables future studies on sympathetic cooling and chemical reactions involving ions in multipole traps.
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