MNRAS 514, 844–851 (2022)

Thermal radiative cooling of carbon cluster cations C_N⁺, N = 9, 11, 12, 17–27

Shimpei Iida¹, Wei Hu², Rui Zhang², Piero Ferrari³, Kei Masuhara¹, Hajime Tanuma¹, Haruo Shiromaru¹, Toshiyuki Azuma⁴ and Klavs Hansen^5,6,*

1 Tokyo Metropolitan University, Tokyo 192-0397, Japan

2 School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China

3 Quantum Solid-State Physics, Department of Physics and Astronomy, KU Leuven, B-3001 Leuven, Belgium

4 Atomic, Molecular and Optical Physics Laboratory, RIKEN, Saitama 351-0198, Japan

5 Center for Joint Quantum Studies and Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China

6 Lanzhou Center for Theoretical Physics, Key Laboratory of Theoretical Physics of Gansu Province, Lanzhou University, Lanzhou, Gansu 730000, China

* E-mail: klavshansen@tju.edu.cn

Abstract

The radiative cooling rates of C_N⁺ clusters (N = 9, 11, 12, 17–27) have been measured in the ultrahigh vacuum of an electrostatic storage ring to values on the order of 10⁴ s⁻¹. The rates were measured as a competing channel to unimolecular decay, and the rate constants pertain to the excitation energies where these two channels compete. Such high values can only be explained as photon emission from thermally excited electronic states, a mechanism that has also been seen in polycyclic aromatic hydrocarbon cations. The high rates have a very strong stabilizing effect on the clusters and the underlying mechanism gives a high energy conversion efficiency, with the potential to reach high quantum efficiencies in the emission process. The competing decay of unimolecular fragmentation defines upper limits for photon energies that can be down-converted to lower energy photons. Including previously measured cluster sizes provides the limits for all clusters C_N⁺⁠, N = 8–27, of values that vary from 10 to 14.5 eV, with a general increase with size. Clusters absorbing photons of energies below these limits cool down efficiently by emission of photons via electronic transitions and their fragmentation is strongly reduced, increasing their survival in HI regions.