Wannier-Mott Excitons in Nanoscale Molecular Ices

Abstract

The absorption of light to create Wannier-Mott excitons is a fundamental feature dictating the optical and photovoltaic properties of low band gap, high permittivity semiconductors. Such excitons, with an electron-hole separation an order of magnitude greater than lattice dimensions, are largely limited to these semiconductors but here we find evidence of Wannier-Mott exciton formation in solid carbon monoxide (CO) with a band gap of >8 eV and a low electrical permittivity. This is established through the observation that a change of a few degrees K in deposition temperature can shift the electronic absorption spectra of solid CO by several hundred wave numbers, coupled with the recent discovery that deposition of CO leads to the spontaneous formation of electric fields within the film. These so-called spontelectric fields, here approaching 4×10^{7} V m^{-1}, are strongly temperature dependent. We find that a simple electrostatic model reproduces the observed temperature dependent spectral shifts based on the Stark effect on a hole and electron residing several nm apart, identifying the presence of Wannier-Mott excitons. The spontelectric effect in CO simultaneously explains the long-standing enigma of the sensitivity of vacuum ultraviolet spectra to the deposition temperature.

$^{13}$CO and $^{13}$CO$_2$ ice mixtures with N$_2$ in photon energy transfer studies

“…As Chen et al (2017) reported, N 2 and 13 CO energy levels only overlap in the low wavelength emission range of the MDHL. N 2 ice VUV absorption ranges from 120 nm to 145 nm, and 13 CO ice absorbs photons from 130 nm to 160 nm.…”

Assigning a structural motif using spontaneous molecular dipole orientation in thin films

“…With a simple infrared spectroscopic characterisation method available, the spontelectric nature of carbon monoxide (CO) thin films was demonstrated [10]. This turn furnished an explanation for the longstanding problem in the electronic spectroscopy of solid CO [11], and other species (e.g. ammonia [12]), of substantial band origin shifts with deposition temperature through the impact of the electronic Stark Effect on Wannier-Mott excitons within a spontelectric solid.…”

Characterizing Interstellar Medium, Planetary Surface and Deep Environments by Spectroscopic Techniques Using Unique Simulation Chambers at Centro de Astrobiologia (CAB)

“…This phenomenon leads to a constant desorption rate of photoproducts that are formed on the ice surface and desorb due to the excess energy of the reaction [33]. We also found the first evidence of excitons in ice exposed to UV irradiation [34]. The observation that only the photons absorbed in the top CO ice monolayers lead to a photodesorption event and the high value of the photodesorption rate suggested that the photodesorption quantum yield, expressed in molecules per absorbed photon, should be higher than unity.…”

“…We measured the absorption cross-sections of several ice species for the first time, which allowed to estimate the efficiency of photodesorption per absorbed photon in the ice [11,12,13]. We extended this study of CO ice UV-photodesorption to other molecular ice components, and started to explore the effect of soft X-ray irradiation in the photodesorption of ice analogues [14,15,33,34,35,36,37,38,39,40,41,42].…”