Dr Hendrik Nahler

Personal web pages

(email at n.h.nahler@durham.ac.uk)

Research Interests

Water ice surfaces are abundant in the polar regions of the Earth and on small ice particles in the atmosphere. When water freezes other species dissolved in it will be embedded in the resulting ice. Molecules can also be adsorbed on an ice surface. Therefore, it is evident that ice surfaces in the polar regions and ice particles in the atmosphere serve as a sink for trace gases and pollutants present in air. Molecules on these highly reactive surfaces undergo processes like physisorption, chemisorption, dissociation, recombination, phonon excitation and electron-hole pair generation.

Dynamics on water ice surfaces plays a major role in the understanding of environmental physical chemistry, especially in the atmospherically relevant temperature range (140–300 K). Recent discoveries have shown that pollutants are deposited in arctic snow and ice, a surface that, along with polar stratospheric and mesospheric clouds (PSC and PMC), is highly reactive. Photochemical processes, typically induced by sunlight, drive the production and chemical transformation of a variety of compounds within snow and ice, including halogens, carbonyl compounds, alkyl halides and nitrogen oxides. These molecules play a crucial role in atmospheric processes e.g. catalytic ozone depletion by destruction of the halogen reservoir on PSCs. It is even possible to extrapolate beyond atmospheric and environmental science, and investigate extraterrestrial systems where water-ice surfaces are seen to play a crucial role in the formation of organic molecules on interstellar ice coated dust grains. Today, the chemistry and dynamics in aqueous solutions are well understood on a molecular level. The understanding for reactions and reaction mechanisms on ice surfaces, despite their importance in atmospheric processes, is still very limited, mainly because they are yet to be fully characterized under varied environmentally relevant conditions.

Atmospheric field experiments discovered photochemical transformations of anthropogenic contaminants in ice. Many of these compounds - organic and inorganic - are toxic and are listed as possible carcinogens by European government agencies. The concerns over negative health effects provide an impetus for the study of atmospheric pollutants and their potential for chemical transformation into other more, or less, toxic compounds. Therefore, it is evident that we have to understand the desorption and photochemical decomposition and reactions of these environmentally dangerous species. However, only laboratory studies will reveal the mechanisms and dynamics of these reactions. The dynamics of molecules on the ice surface will be strongly influenced by the temperature and thereby the structure (amorphous, crystalline, quasi liquid like QLL) of the surface. Measurements in kinetics average the internal state-, energy- and angular distribution of desorbed molecules and dissociated products. Dynamics measurements will reveal the changed reactivity of these species in terms of their state- and energy distribution caused e.g. by specific surface-sites. The effects of UV photodissociation of water on ice producing H and OH are important for atmospheric chemistry but the subsequent chemical and physical processes are rather poorly understood. The mobility of these species at the ice surface determined by their kinetic energy and the nature of the surface will influence further reactions with coadsorbed atoms and molecules. It is thus apparent that studies of photochemical processes and dynamics at ice surfaces are urgently needed to better understand the adsorption and desorption processes as well as the mechanisms for photo-induced reactions on ice surfaces. The interest of our group lies in fully characterising the dynamics of the reaction products in order to understand their mobility, probability for recombination and reactivity. Key parameters to look at are the surface site-specificity, surface nature as well as the product internal state and energy distributions

The photodissociation and desorption dynamics at ice surfaces will be investigated with spectroscopic methods which are well established in gas-phase spectroscopy, namely cavity ring-down spectroscopy (CRDS) and velocity-map ion imaging (VMI). These two techniques yield high-quality results in the full characterisation of reaction products measuring quantities such as the ro-vibrational state, kinetic energy and angular distribution. In our group we will investigate reactions on ice surfaces in terms of site-specificity and surface nature as well as the influence of product internal state and energy distributions. We are specifically interested in:

• the surface temperature dependent desorption of water and pollutants from ice surfaces
• the photochemical behaviour of pollutants on ice surfaces
• the influence of the ice surface nature on the reaction pathway
• deploying novel spectroscopic techniques for processes at the gas-surface interface
• the comparison with experimental and theoretical studies of pure gas-phase and cluster photochemistry

Our studies will focus on systems relevant in atmospheric reactions such as halogen containing compounds, small organic molecules, NH₃, NO_x, H₂CO. Their photochemistry in the gas-phase is well understood which will allow to quantify the influence on their dynamics induced by the ice surface.

Biography

PhD (1999-2002) studying the photodissociation of hydrogen halides in cluster environments and working on the formation, orientation and dissociation of hydrogen-rare gas-halogen molecules (Max-Planck-Institut für Strömungsforschung, Göttingen with Prof U Buck). EU Marie-Curie Fellowship (2003-2005) at the University of Bristol in the group of Prof MNR Ashfold studying the photodissociation dynamics of state-selected molecular ions using velocity-map ion imaging. From 2005-2007 work on electronically non-adiabatic interactions of vibrationally excited molecules with reactive surfaces as Feodor-Lynen Fellow of the Alexander von Humboldt-Foundation (University of California Santa Barbara, Prof AM Wodtke). Since 2007 Royal Society University Research Fellow at Durham University in the Department of Chemistry establishing research on the photochemistry and dynamics on water ice surfaces.

Recent Publications

1. Vieuxmaire, O. P. J., Nahler, N. H., Dixon, R. N., Ashfold, M. N. R. “ Multiphoton dissociation dynamics of BrCl and the BrCl⁺ cation”. Phys. Chem. Chem. Phys., 9, 5531-5541 (2007). DOI
2. Ashfold, M. N. R., Nahler, N. H., Orr-Ewing, A. J., Vieuxmaire, O. P. J., Toomes, R. L., Kitsopoulos, T. N., Garcia, I. A., Chestakov, D. A., Wu, S. M., Parker, D. H. “ Imaging the dynamics of gas phase reactions”. Phys. Chem. Chem. Phys., 8, 26-53 (2006). DOI
3. Nahler, N. H., Farnik, M., Buck, U., Vach, H., Gerber, R. B. “ Photodissociation of HCl and small (HCl)_m complexes in and on large Ar_n clusters”. J. Chem. Phys., 121, 1293-1302 (2004). DOI
4. Nahler, N. H., Vieuxmaire, O. P. J., Jones, J. R., Ashfold, M. N. R., Eppink, A. T. J. B., Coriou, A. M., Parker, D. H. “ High-resolution ion-imaging studies of the photodissociation of the BrCl⁺ cation”. J. Phys. Chem. A , 108, 8077 -8083 (2004). DOI
5. Nahler, N. H., Baumfalk, R., Buck, U., Bihary, Z., Gerber, R. B., Friedrich, B. “ Photodissociation of oriented HXeI molecules generated from HI-Xe_n clusters”. J. Chem. Phys., 119, 224-231 (2003). DOI