Shape fluctuations and radiation from thermally excited electronic states of boron clusters

TL;DR

This study investigates how thermal shape fluctuations influence the fluorescence and radiation properties of boron cluster cations, revealing significantly broadened emission spectra and increased fluorescence rates compared to static models.

Contribution

It provides a detailed numerical analysis of thermal effects on electronic states and fluorescence in boron clusters, highlighting the importance of dynamic sampling over static approximations.

Findings

Broad emission spectrum down to 0.85 eV for B13+

Recurrent fluorescence rates are an order of magnitude higher than static predictions

Fast radiationless crossing justifies thermal population assumptions

Abstract

The effect of thermal shape fluctuations on the recurrent fluorescence of boron cluster cations, B$_N^+$ ($N=9-14$), has been investigated numerically, with a special emphasis on B$_{13}^+$. For this cluster, the electronic structures of the ground state and the four lowest electronically excited states were calculated using time-dependent density functional theory, and sampled on molecular dynamics trajectories of the cluster calculated at an experimentally relevant excitation energy. The sampled optical transition matrix elements for B$_{13}^+$ allowed to construct its emission spectrum from the thermally populated electronically excited states. The spectrum was found to be broad, reaching down to at least 0.85 eV. This contrasts strongly with the static picture, where the lowest electronic transition happens at 2.3 eV. The low-lying electronic excitations produce a strong increase in the rates of recurrent fluorescence, calculated to peak at $4.6 \times 10^{4}$ s$^{-1}$, with a time-average of $8 \times 10^{3}$ s$^{-1}$. The average value is one order of magnitude higher than the static result, approaching the measured radiation rate. Similar results were found for the other cluster sizes. Furthermore, the radiationless crossing between the ground-state and the first electronic excited state surfaces of B$_{13}^+$ was calculated, found to be very fast compared to experimental time scales, justifying the thermal population assumption.