Oxygen-rich anionic metal silicate clusters as nucleation seeds for noctilucent clouds

Abstract

Silicate-based meteoric smoke particles (MSPs) resulting from extraterrestrial dust impacting on the upper atmosphere are likely involved in seeding the formation of mesospheric ice-based noctilucent clouds (NLCs). However, the detailed physicochemical properties of MSP seeds remain elusive. Considering that ice formation is sensitive to MSP charging efficiency, here we focus on species which can efficiently capture charge. As in situ MSP characterisation is highly challenging, we mimic atmospheric MSP formation using laser ablation and collisional cooling to produce small anionic magnesium silicate clusters. Our detailed experimental and theoretical characterisation shows that these clusters have unusual oxygen-rich structures. Moreover, the corresponding neutral clusters have extraordinarily high adiabatic electron affinities and 3-18% charging efficiencies, which is remarkably consistent with estimates required by models to simulate atmospheric measurements. Overall, our highly oxidised anionic silicate clusters exhibit all necessary chemical, electronic and optical properties to be highly credible MSP-based seed candidates for NLC nucleation.

Keywords: Atmospheric chemistry; Atmospheric science; Climate sciences.

Fig. 1. Experimental IR-MPD spectra (in grey) of various Mg_xSi_yO_z⁻ together with the DFT-calculated spectra (in blue; PBE0 functional, relative energies in eV) of the lowest energy isomers. The red curves represent the O₃ modes of the shown structures obtained with the PBE functional.

The IR-MPD spectra were measured in two independent runs and are the average of five (650–1800 cm⁻¹) and six (225–700 cm⁻¹) individual spectra, respectively, and a 3-point adjacent average of the individual data points. To achieve intensity match of the two spectra, the experimental spectrum in the high wavenumber range was multiplied with a factor of 3.9. The chosen naming x,y,z − n denotes the elemental composition of Mg_xSi_yO_z and n = a,b,c indicates the isomer sorted in order of increasing energy (further isomers can be found in section S2 of the SI). Mg, Si, and O atoms are depicted as blue, yellow, and red spheres, respectively.

Fig. 2. Calculated absorption cross sections for anionic MgSi₂O₇⁻ (isomer 1,1,7-a; black curve), Mg₂SiO₇⁻ (isomer 2,1,7-a; red curve) and Mg₂SiO₇⁻ (isomer 2,1,7-b; blue curve) as a function of the wavelength.

The arrows indicate the electron photodetachment thresholds calculated as the energy difference between the anionic and neutral clusters, both in their relaxed geometries.