Our group is interested in the physics and chemistry of ice. We use various advanced surface science techniques to unravel the complex reaction dynamics and underlying mechanisms that occur in ice and at its surface. Using various modelling and simulations approaches, we strive to provide detailed molecular-level interpretations to help elucidate environmental chemistry problems such as important heterogeneous atmospheric chemistry phenomena recently reported by field campaigns.
Firstly, we use molecular beam techniques to study how molecules adsorb, dissolve, react and penetrate within ice. We perform in situ spectroscopic and thermal desorption mass spectrometry characterization of thin films as model for environmental interfaces. Using a simple classical optics model we developed, we are able to provide a detailed interpretation of the complex optical effects due to refraction, multiple reflections and interference which distort the reflection-absorption infrared spectra (RAIRS) of thin ice films (1). Such a quantitative understanding of infrared spectra is required in order to unravel the mechanism of ionic dissociation of such simple acid molecule as hydrogen fluoride.
Interesting, we recently made the surprising discovery that HF appears to be dissociated in amorphous ice at 40K (2), and also at its surface (3), highlighting the fact that it is a weak acid in liquid water owing mainly to entropic considerations. Furthermore, we demonstrated that proton-transfer reaction intermediates, and their respective highly energetic metastable solvent configurations, could be trapped in the disordered H-bonded network of binary mixtures of hydrogen fluoride and water (4) allowing us to study them spectroscopically (5). This suggests these cryogenic binary amorphous solids may be interesting model systems to investigate aqueous solutions behaviour. We are currently studying other important systems to atmospheric chemistry namely hydrogen chloride and nitric acid adsorption, dissolution and absorption in ice and are interested in developing detailed experimental investigations of heterogeneous photolysis of nitrate anions on ice.
Second, we are deploying advanced cryogenic transmission (TEM) and scanning (SEM) electron microscopy analytical techniques to probe mesoscopic structural and morphological properties of the thin ice films used as model environmental interfaces as well as those of natural samples. Namely, we are investing the wetting and adhesion behaviours of vapour deposited ice films as a function of growth conditions to better understand how this may affect the integrity and the reactivity of ice surfaces created in the laboratory. Furthermore, using electron diffraction, we also study the crystallization of pure and binary ice films to better understand their microstructure such as grain size as well as grain boundary density, connectivity and network morphology. Using these techniques, we discovered that doping amorphous ice with small amounts of methanol offsets the kinetic preference for the crystallization of amorphous ice to cubic ice Ic, favouring instead the thermodynamically most stable polymorph: hexagonal ice Ih (6). Acknowledging these structural and morphological features was crucial to provide a proper interpretation of the diffusive transport properties of polycrystalline ice Ih towards methanol.
Finally, we are gearing up to implant electron spectroscopic (UPS, XPS, as well as time-resolved femtosecond two-photon photoemission spectroscopy-2PPES) capabilities in order to probe the electronic structure of ice films and that of their surfaces. Photoemission studies will allow us to better understand how the electronic properties, and thus how the molecular orbitals, of water molecules at the ice surface differ from those in the bulk. They will allow us to better understand how the electronic structure of ice surfaces controls their reactivity. Time-resolved pump-probe studies will enable us to study the ultrafast photochemical reaction dynamics providing detailed insight in the complex nuclear dynamics involved in heterogeneous photolysis.
1 – F. Cholette et al., J. Phys. Chem. A 113, 4131-4140 (2009).
2 – P. Ayotte et al., J. Chem. Phys. 123 184501 (2005).
3 – P. Ayotte et al. J. Chem. Phys. 131, 124517 (2009).
4 – R. Iftimie et al. J. Am. Chem. Soc. 130, 5901-5907 (2008).
5 – P. Ayotte et al. Phys. Chem. Chem. Phys. 10, 4785-4792 (2008).
6 – P. Marchand et al. J. Phys. Chem. A 110, 11654-11664 (2006).