Grecea, Mihail Laurentiu
Citation
Grecea, M. L. (2006, February 23). Light-induced molecular processes on ice. Retrieved from https://hdl.handle.net/1887/4322
Version: Corrected Publisher’s Version
License: Licence agreement concerning inclusion of doctoralthesis in the Institutional Repository of the University of Leiden
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Chapter 1
Introduction
1.1 Ozone in the Earth’s atmosphere
stratosphere, where it typically takes years for a molecule to cross the layer. The increase in temperature with altitude in the stratosphere is mainly due to the absorption of solar ultraviolet and visible radiation by the ozone layer. It is important to note that chemical and physical processes occurring at higher altitudes can influence the entire atmosphere.
The common picture of the atmospheric ozone (O3) is as being contained in an ozone layer about 20 km thick, centered at an altitude of 25-30 km. The stratospheric ozone represents in fact 90% of the atmospheric ozone [2], the rest being found in trace amounts throughout the atmosphere. Ozone has several decisive roles in the Earth’s atmosphere. First, it absorbs the solar ultraviolet (UV) radiation in the wavelength range of 240-290 nm, which otherwise would be transmitted to the Earth’s surface. This radiation can damage proteins and nucleic acids characteristic of living cells, and, as such, are potentially lethal to simple unicellular organisms and to the surface cells of more elevated plants and animals. Second, the upper atmospheric meteorology is significantly affected by the heating that accompanies absorption of UV and visible light by ozone. The resulting increase in temperature with altitude produces high stationary stability of the atmospheric air.
Until the 1960s, it was believed that the model launched by Chapman [3] could account for the atmospheric ozone abundance. In this model, which involves only oxygen as an active ingredient, atmospheric ozone is formed as a result of UV-induced dissociation of molecular oxygen (at λ < 240 nm), in the presence of an inert molecule (N2, O2). According to the same model, ozone can be depleted by both UV dissociation (at λ < 310 nm) and direct reaction with atomic oxygen. Later on, a catalytic, more efficient route of the latter was added to the model:
XO + O ⎯→ X + O2 (1.2) O3 + O ⎯→ 2O2 , (1.3) where X: H, OH, NO, Cl or Br. This extension followed the observation that the ozone concentration can be affected by trace atmospheric constituents reacting in a cyclic mode [1]. 150 200 250 300 20 40 60 80 100 Temperature (Kelvin) A ltitude ( k m )
Figure 1.1. Sketch of the Earth’s atmosphere, illustrating the four different
layers. A temperature-altitude profile is shown, corresponding to latitude 400 N
0 5 10 15 0 5 10 15
Ozone abundance (mPa)
0 5 10 15 20 25 30 A lti tu de (k m)
Figure 1.2. Antarctic and Arctic ozone distribution function of altitude, for
different periods. The picture is adapted from ref. [5].
In the stratosphere, UV radiation from the Sun can induce photodissociation of reactive halogen gases resulting in the halogen species active in ozone depletion. Among the halogens present, bromine is much more effective overall (about 45 times) on a per-atom basis than chlorine in chemical reactions that destroy stratospheric ozone [5]. In the Polar regions, the presence of polar stratospheric clouds (PSC) greatly increases the abundance of the reactive halogen gases owing to the heterogeneous reactions on PSC particles. The activation of halogen “reservoir compounds” into reactive halogen gases, otherwise very slow, occurs more rapidly when adsorbed on PSC particles. Examples of typical reactions occurring on the surface of PSC particles are
ClONO2(g) + HCl(ads) ⎯→ Cl2(g) + HNO3(ads) (1.4)
1.2 Ubiquitous ice
Water ice is omnipresent throughout the universe. Depending on the precise physical circumstances, water ice can be present as amorphous solid water (ASW) or one of the forms of crystalline ice (CI) [9]. In interstellar clouds, the predominant form of solid water is ASW, formed by reactions on silicate and carbonaceous particles at low temperature (~10 K) or by H2O vapor deposition onto the grain surface. ASW is also present in protostellar disks, comets and asteroids. Although ASW is predominant in the low temperature astronomical environments, crystalline phases of water ice have been detected in the circumstellar shells, moons, Saturn’s rings and comets [9,10]. This indicates that these environments are or have been at some point in time under higher temperature and/or pressure conditions, or that another process, such as high-energy irradiation, has led to crystallization of the amorphous phase [11]. In our solar system, many planets and moons are known to have ices on their surface and in their atmosphere. In the Earth’s atmosphere, icy particles containing crystalline ice phases are present, together with supercooled liquid particles resulting from rapid cooling from the liquid phase [1,9]. However, under conditions of rapid cooling in a cold air or cryogenic liquid flow, the supercooled liquid droplets can form a distinct phase of amorphous solid particles, namely hypercooled solid water (HSW) [9,12]. Although ASW and HSW constitute two distinct phases of water ice, many similarities may exist between ASW observed in extraterrestrial environments and HSW present in the Earth’s atmosphere [9].
it forms a viscous liquid [16]. The hexagonal CI, formed at temperatures higher than ~160 K, is thermodynamically the most stable form of water ice. ASW and CI layers on Pt(533) substrate are introduced in Chapter 3, along with a study of the interface water layer on the stepped Pt(533) surface, while dynamics of the laser-induced desorption of water from ASW are explored in Chapter 4.
1.3 Molecular processes on ice
In the context of the highly worrisome issue of ozone depletion, it is important to obtain fundamental insights into the adsorption, desorption and UV photochemistry of atmospherically relevant molecules on ice surfaces. UHV conditions ensure strict control over the conditions under which the processes are studied; the presence of impurities, for example, can alter the properties of ice, and one would therefore like to control this. In contrast to the laboratory model systems of supported ice layers or particles in UHV, in the stratosphere ice is present as isolated particles levitating within a particular temperature and pressure environment. Nonetheless, laboratory studies at low pressure can contribute, together with the atmospheric in situ measurements and theoretical simulations, to the unraveling of the real phenomenon of stratospheric ozone depletion.
CI. The interaction with these “dangling bonds” can result in specific adsorption energetics, function of the ability to form hydrogen bonds, the dipole moment and the polarizability of the adsorbate molecules. The adsorption/desorption behavior of bromoform (CHBr3) on ASW and CI surfaces, as well as the comparison with structurally similar chloroform (CHCl3), are studied in Chapter 5. CHBr3 is one of the contributors to the atmospheric bromine that has been detected in the stratosphere [20]. The UV-induced surface photochemistry of CHBr3 monolayer on ASW is described in Chapter 6, while Chapter 7 offers detailed insights into the adsorption and the UV photochemistry of CHBr3 multilayers. 1 2 3 4 5 6 ML 1 2 3 BL Proton acceptor water Proton donor water
Figure 1.3. Simulated side view of the ice lattice. The “dangling bonds” are
1.4 This thesis
The thesis Light-induced molecular processes on ice draws attention to two main issues: first, water ice layers (ASW and CI) grown on the stepped Pt(533) substrate, along with the dynamics of the laser-induced desorption of water from ASW. Second, various processes experienced by CHBr3 on ice surfaces, potentially relevant for stratospheric chemistry: adsorption, desorption, surface mobility and UV-induced photochemistry. This thesis is structured as follows:
Chapter 2 gives details of the UHV apparatus and the laser system, together with a brief description of the experimental techniques exploited in this thesis.
The adsorption of water on the Pt(533) surface is described in Chapter 3. In the first layer regime, the (100)-steps provide an additional stabilization of the water that subsequently adsorbs on the (111)-terraces (see Figure 3.1 for a schematic picture of the stepped Pt(533) surface). On the contrary, the ice multilayers on Pt(533) show vibrational spectroscopy and thermal desorption features similar with those reported on flat (111) metal surfaces.
The dynamics of the ultrafast laser-induced desorption of water from ASW layers grown on Pt(533) is described in Chapter 4. Electrons injected from the metal substrate into the water layer at the metal-water interface, result in water desorption from the water-vacuum interface. Surprisingly large mean free path of an electron-excited water molecule (~1 nm, corresponding to several water molecules) is determined.
Chapter 5 deals with the mobility of the haloform molecules CHCl3 and CHBr3 on ice surfaces, as is reflected by the desorption features from ASW and CI. Up to its desorption temperature, CHCl3 does not diffuse over the CI surface, whereas CHBr3 is found to be mobile at temperatures as low as 85 K.
ice-surface-irradiation of CHBr3 on ASW. The cross-section of CHBr3 depletion, ~5 × 10-20 cm2, is roughly two orders of magnitude lower than the value reported for gas phase CHBr3. Surface water species are involved in the photochemistry of CHBr3.
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