Threshold photodetachment spectroscopy of the astrochemical anion CN−

Abstract

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.

On the Role of the Electron-Dipole Interaction in Photodetachment Angular Distributions

“…Accurate control of the ion temperature is currently beyond the capability of our instrumentation. However, near threshold measurements of the overall detachment cross section (σ) for CN − have recently been reported at two temperatures, 16 K and 295 K. 17 The overall cross sections at a given photon energy are the sum of the cross sections for all rovibronic channels ( ℎ ′′ → ′…”

“…In Figure 5b, the 295 K results are represented using n = 0.7, a compromise between the values reported from two different sets of apparatus. 17 Figures 5c (16 K) and 5d (295 K) show the overall cross section results of the point dipole model. The dashed curves represent the variation in σ with photon energy for dipole moments, from D = 0.0 to D = 0.5 a.u.…”

On the Role of the Electron-Dipole Interaction in Photodetachment Angular Distributions

“…Accurate control of the ion temperature is currently beyond the capability of our instrumentation. However, near threshold measurements of the overall detachment cross section (σ) for CN − have recently been reported at two temperatures, 16 K and 295 K. 17 The overall cross sections at a given photon energy are the sum of the cross sections for all rovibronic channels (𝜎 ℎ𝜈 𝐽 ′′ →𝐽 ′ 𝑘 ), which are related to the photoelectron matrix elements as (Supporting Information, SI4)…”

“…In Ref. [17] the experimental data is fit to the expression The scaling factor, A, allows comparison with experiment, and the integrals of (9) for each channel are replaced with an n th power dependence of the channel's electron kinetic energy.…”

“…We further show that the model also offers an explanation for the recently reported temperature dependent changes in the overall detachment cross section for CN − . 17 Photoelectron images for CN − detachment were collected using photon energies ranging from 3.87 eV to 4.40 eV (Experimental and data processing/analysis details can be found in Supporting Information, SI1). Each image contains a single electronic band, from the anion ground state to the neutral ground state (X''…”