Photodetachment spectroscopy is a powerful spectroscopic technique for determining the internal state distribution of a molecular anion. Previously, our group studied the threshold photodetachment spectroscopy of CN⁻ at both 16 Kelvin and 295 Kelvin in a 22-pole ion trap and measured the electron affinity of CN with great precision (EA: 3.864(2) eV). The dicarbon anion, our current molecule of interest, is a well-studied system due to its stable electronic level structure and potential laser cooling transition. Here we present the threshold photodetachment spectroscopy study of C₂⁻ in a radiofrequency 16-pole ion trap at 8 Kelvin. We investigated the behavior of the cross section near the threshold for the ground state transition, C₂X¹Σ⁺g ← C₂⁻ X²Σ⁺g. We see a p-wave behavior for this ground state detachment. And we measured the electron affinity, which is consistent with the previously measured values."
We have conducted laser photodetachment spectroscopy near the detachment threshold of the electron affinity of
S2 in a 1.8-T field. The ions are prepared by dissociative electron attachment to carbonyl sulfide. The experiment is conducted in a Penning ion trap and with a narrow-band, tunable, Ti:sapphire laser. A hybrid model for photodetachment in an ion trap is fit to the data using the appropriate Franck-Condon factors. The observations reveal detachment from and to the first few vibrational levels of the anion and the neutral molecule, respectively. Evaporative cooling of the anion ensemble condenses the thermal distribution to the lowest initial vibrational states. The subsequent detachment spectroscopy yields results consistent with a vibrationally cooled anion population.
Photodetachment spectroscopy of OH(-) and H(3)O(2)(-) anions has been performed in a cryogenic 22-pole radiofrequency multipole trap. Measurements of the detachment cross section as a function of laser frequency near threshold have been analysed. Using this bound-free spectroscopy approach we could demonstrate rotational and vibrational cooling of the trapped anions by the buffer gas in the multipole trap. Below 50 K the OH(-) rotational temperature shows deviations from the buffer gas temperature, and possible causes for this are discussed. For H(3)O(2)(-) vibrational cooling of the lowest vibrational quantum states into the vibrational ground state is observed. Its photodetachment cross section near threshold is modelled with a Franck-Condon model, with a detachment threshold that is lower, but still in agreement with the expected threshold for this system.
Negative ions are unique quantum systems where electron correlation plays a decisive role in determining their properties. The lack of optically allowed transitions prevents traditional optical spectroscopy and the electron affinity is, therefore, for most elements, the only atomic quantity that can be determined with high accuracy. In this work, we present a high-precision experimental determination of the electron affinity of cesium. A collinear laser-ion beam apparatus was used to investigate the partial photodetachment cross section for the cesium anion, leaving the neutral atom in the 6p 2P3/2 excited state. A resonance ionization scheme was used to obtain final-state selectivity, which enabled the investigation of a sharp onset of the cross section associated with a Wigner s -wave threshold behavior. The electron affinity was determined to be 0.4715983(38)eV . Published by the American Physical Society 2024
The study of ion-molecule reactions plays a vital role in cold chemistry, thus implying the need of well-controlled ion ensembles in a cold environment. The internal and external degrees of freedom of molecular ions, trapped in multipole radio frequency ion traps, can be cooled via collisions with neutral atoms [1]. Usually the coolant undergoes collisions with a thermal shield mounted on a cryostat attaining temperatures of about 4 K. To overcome this lower temperature limit two approaches have been proposed. Firstly, using a laser-cooled buffer-gas localized at the center of the ion cloud [2]. Secondly, sympathetic cooling via a second laser cooled molecular anion like C
2− [3]. This, in addition to its potential role in forming carbon chain anions in the interstellar medium, makes C
2− an important molecule in anion research. In this contribution we present a newly developed 16-pole cryogenic radio frequency wire trap for spectroscopic and kinetic studies of molecular anions. First results of quenching rates of the first excited vibrational level of C
2− due to collisions with H
2 are presented. Furthermore, possible reaction pathways with H
2 are discussed and recent spectroscopic measurements of the second electronic excited state of C
2− are presented. References: [1] R. Wester , J. Phys. B 42, 154001 (2009) [2] B. Höltkemeier et al., Phys. Rev. A 94, 062703, (2016) [3] P. Yzombard et al., Phys. Rev. Lett. 114, 213001 (2015)
The absolute photodetachment cross section of OH- anions at a rotational and translational temperature of 170 K is determined by measuring the detachment-induced decay rate of the anions in a multipole radio-frequency ion trap. In comparison with previous results, the obtained cross section shows the importance of the initial rotational-state distribution. Using a tomography scan of the photodetachment laser through the trapped ion cloud, the derived cross section is model-independent and thus features a small systematic uncertainty. The tomography also yields the column density of the OH- anions in the 22-pole ion trap in good agreement with the expected trapping potential of a large field free region bound by steep potential walls.
Photodetachment thermometry on a beam of OH^{-} in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional frequency- and time-dependent photodetachment spectroscopy over 20 min of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J=1 natural lifetime (about 193 s) and determine the OH^{-} rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.