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Description
Negative ions are interesting quantum systems due to the nature of the force that binds the extra electron. Since the nucleus effectively screens the Coulomb attraction, the additional electron is bound by a dipole moment induced by electron-electron interactions. As a result, typical spectroscopic methods can not be applied and the only quantity that can be probed with high precision is the energy required to bind the extra electron, i.e. the electron affinity, defined as the energy difference between the lowest energy state of the neutral system and the negative ion. The standard method to obtain such values is through laser photodetachment threshold spectroscopy (LPTS), in which the extra electron is detached from the anion using a photon with sufficient energy [1,2].
With the applicability of infrared light sources to LPTS experiments, the determination of the binding energies and electron affinities of most transition metals were possible. These systems are of special interest due to their open d-shell, where electron correlation and relativistic effects must be taken into account. In particular, in 1998 Sheer et al. [3], obtained a higher precision measurement of the electron affinity of rhodium in comparison to the first measurement done by Feigerle et al. [4]. They also attempted to measure the binding energies of the fine-structure levels of this transition metal. Due to the small ion current, these measurements were not carried out, although the possible photon energies were available [3]. In 2019, measurements of the lifetimes of these states were performed at the Double ElectroStatic Ion Ring ExpEriment (DESIREE) facility in Stockholm, where an additional, previously unknown state was observed [5]. Subsequently, the binding energy of this state was measured at the Gothenburg University Negative Ion Laser Laboratory (GUNILLA).
It is well known that the spectral linewidth of the light source used in photodetachment experiments limits the precision of these measurements. At GUNILLA, a pulsed nanosecond laser of 90 GHz linewidth was recently purchased to perform LPT measurements. In order to reduce the linewidth to less than 10 GHz, an external air-gapped optical cavity has been designed and implemented downstream from the output of the laser [6,7]. A proof-of-principle of this cavity has been performed using photon energies between 1.77 eV and 3.04 eV.
Here, the lifetimes of the excited states of Rh- and the binding energy of the highly mixed excited state, as well as the design of an optical cavity for the reduction of the linewidth of our OPO laser system will be presented.
References:
[1] D. J. Pegg, Rep. Prog. Phys. 67 85 (2004).
[2] C. Ning and Y. Lu, J. Phys. Chem. Ref. Data 51, 021502 (2022).
[3] M. Scheer, et al. Phys. Rev. A 58, 2051 (1998).
[4] C. S. Feigerle et al., J. Chem. Phys. 74, 1580–1598 (1981).
[5] J. Karls et al., Lifetimes of excited states in Rh-. In preparation.
[6] E. Hecht, Optics. Wiley-Interscience, 2002.
[7] O. Svelto and D. C. Hanna, Principles of lasers. Plenum Publishing Corporation, 1998.
