Radiative cooling of carbon cluster anions C2n+1\u2212 (n\xa0=\xa03\u20135)

Mark H Stockett,James N Bull,Michael Gatchell,Jack T Buntine,Emma K Anderson,Eduardo Carrascosa,Henning Zettergren,Henning T Schmidt,Rodrigo F Nascimento,Henrik Cederquist,Magdalena Kaminska

doi:10.1140/epjd/e2020-10052-5

Abstract

Radiative cooling of carbon cluster anions C2n+1− (n = 3–5) is investigated using the cryogenic electrostatic ion storage ring DESIREE. Two different strategies are applied to infer infrared emission on slow (milliseconds to seconds) and ultraslow (seconds to minutes) timescales. Initial cooling of the ions over the millisecond timescale is probed indirectly by monitoring the decay in the yield of spontaneous neutralization by thermionic emission. The observed cooling rates are consistent with a statistical model of thermionic electron emission in competition with infrared photon emission due to vibrational de-excitation. Slower cooling over the seconds to minutes timescale associated with infrared emission from low-frequency vibrational modes is probed using time-dependent action spectroscopy. For C9− and C11−, cooling is evidenced by the time-evolution of the yield of photo-induced neutralization following resonant excitation of electronic transitions near the detachment threshold. The cross-section for resonant photo-excitation is at least two orders of magnitude greater than for direct photodetachment. In contrast, C7− lacks electronic transitions near the detachment threshold.Graphical abstract

Highlights

One of the hallmarks of the physics of atomic clusters is the dramatic variation of their properties and stability with size [1]
The model utilizes the vibrational density of states ρ computed with the Beyer–Swinehart algorithm and scaled harmonic vibrational mode frequencies νs calculated at the ωB97X-D//aug-cc-pVTZ level of Density Functional Theory (DFT) in Gaussian 16 [36,37,38]
The higher neutralization rate at early times is attributed to spontaneous neutralization of the small fraction of ions with internal energies that exceed the detachment threshold Φ

Summary

Introduction

One of the hallmarks of the physics of atomic clusters is the dramatic variation of their properties and stability with size [1]. This strategy allows investigation of cooling associated with infrared modes of lower frequency and modes with lower decay rates; it is not possible to study these dynamics in room-temperature ion storage rings. We applied the 2D near-threshold photodetachment action spectroscopy strategy to monitor cooling over the slower (seconds to minutes) timescale where only radiative cooling through infrared photon emission is important. Both sets of results are interpreted within the framework of the SHC radiative cooling model

Experiments

Modeling

Spontaneous decay

Action spectroscopy

Conclusions