The chemical desorption of the O-H system

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The microwave cavities

Each beamline is equipped with a microwave cavity (a Surfatron cavity) and a microwave power supply delivering up to 300 W at 2.45 GHz. These two components are used for dissociating molecular gases such as hydrogen, deuterium, nitrogen or oxygen to produce atomic or molecular radicals. The molecular gases pass in a quartz pipe (diameter 4 mm, length 10-20 cm) traversing the microwave cavity with a pressure ranging from 0.3 to 4 mbar. Microwaves transfer energy to the gas by exciting and ionizing the species. They create a plasma in which electrons are accelerated. The dissociative electronic excitation of H2, D2, N2, or O2 produce atoms in the discharge zone. At the pressures used, the large number of collisions, especially on the walls of the pipe, favors the recombination of molecules after being dissociated. To minimize the spontaneous recombination of the atomic species and to reduce the discharge temperature the cavity is cooled by a ux of compressed air around the tube and by a water circuit cooling the metallic parts. We have estimated that the temperature connect the three stages, and a system of laser pointing allows beam direction adjusting. of the gas coming out from the pipe is lower than 450 K. Actually, due to multiple collisions the atoms generated in the discharge can thermalize upon impacts with the pipe internal surfaces. It is possible, however, that a gradient of temperature is present in the pipe. By considering the thickness of the pipe, the dierence between the outer and the inner part should be lower than 100 K. Hence, an upper limit for the translational temperature is 450 K. SubSec. 2.3 presents the method used to know the excitation state of dissociated species.

The sample holder and the sample

The sample holder is an oxygen-free high thermal conductivity (OFHC) copper cylinder, with radius equal to 5 mm. The sample holder is placed in the center of the main chamber, at the same height of the beamlines, and an electric resistance is able to heat the sample holder up to 400 K. The sample holder is in contact with a closed-cycle He cryostat. By throttling the ow of liquid helium the sample can be cooled to 6.5 K. A buckler, made of a mixture of copper and nickel, protects and isolates the sample holder, the 400 K interface and the second stage of the cryostat from IR radiation coming largely from the walls of the chamber (See Figure 2.5).
The sample holder is mounted on a translation plate, that allows us to move back and forth the sample holder with respect to the center of the chamber in order to allow a better positioning of the sample with respect to the QMS or the water vaporizer. The 400 K interface is connected to a controller (Lakeshore 340 ), that allows the reading of the dierent temperatures and, by varying the power of the heater, to regulate the sample temperature.
The sample is interchangeable by opening the main chamber. Two dierent sample were used to perform the experiments:
1. a non-porous amorphous olivine-type silicate
2. a HOPG (Highly Ordered Pyrolytic Graphite) slab.

Water ice deposition on the sample holder

Two dierent techniques of water ice growth are used in the FORMOLISM set-up: either the spray deposition or the background deposition. Both of them will be described in the following two sections. The deposition by spraying is a direct way to grow the amorphous solid water (ASW) ice layer quickly and it is used to deposit large amount of water molecules. During the spray deposition, the microchannel array doser is placed at 2 cm in front of the sample holder surface maintained at 110 K, as shown in Figure 2.2.
As it is shown in Sec.2.1.1, a small glass vial containing puried liquid water is connected with the water diuser located into UHV chamber via a leak valve. By opening this leak valve, the local pressure in the region between the diuser and the copper substrate reaches about 10-6 mbar, while the residual pressure into the chamber is 10-9 mbar. It has been evaluated that the mean free path of H2O molecules is about 1 m while the residual pressure is 10-6 mbar. Since the distance between the diuser and the cold surface is smaller than the mean free path, the majority of the water molecules will hit the sample holder and the cryoshield and stick on them. For this reason, water molecules sent into the chamber during the direct deposition contribute marginally to the UHV pressure. Spray deposition allows the growth of about a hundred monolayers of ASW within 5 minutes (i.e. 0.33 ML=s).
The background deposition method is performed by lling uniformly the entire volume of the chamber with water vapor. Now the microchannel doser is kept high above the sample holder surface.

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Mass spectroscopy: the dierent uses of the QMS

In subSec. 2.1.5, we have briey described the QMS operation. We have claimed that gas species are detected after their ionization. This occurs in the so called ionization zone through electron impacts. Here we pinpoint that electron impacts could produce dissociation of species in addition to ionization. Actually it is necessary to pay attention to the fragment distribution of ionic species which results from dissociation and ionization of multi-atomic molecules of any given species in the ionizer, the so called cracking pattern. The probabilities of ionization and dissociation depend on molecular geometry (and intra-molecular bonds energy), angle of impact between molecules and ionizing electrons of the QMS, and the energy of ionizing electrons. For these reason, signals (and peaks) at dierent masses can derive from the same molecule, reecting its partial fragmentation in the QMS head. Figure 2.8 shows the TPD curves of CH3OH and six major signals are presented. Table 2.1 reports the cracking pattern of some molecules: NO2, O3, H2CO, NH2OH, HCOOH, and CH3OH. The signals are normalized to the highest mass signal. Nitrogen dioxide is the simplest case listed in Table 2.1.

Table of contents :

Introduction
1 Theoretical background 
1.1 Surface Physics
1.1.1 Sub-monolayer and multilayer: surface and bulk
1.1.2 Adsorption
1.1.2.1 Physisorption and chemisorption
1.1.2.2 Sticking
1.1.3 Desorption
1.1.3.1 Thermal desorption
1.1.3.2 Non-thermal desorption
1.1.4 Surface migration: Thermal hopping and tunneling
1.1.4.1 An evaluation of the crossover temperature Ttn􀀀th
1.2 Gas Phase Chemistry
1.2.1 Chemistry in the gas phase
1.2.2 Exothermic and endothermic reactions
1.3 Surface Chemistry
1.3.1 Eley-Rideal mechanism
1.3.2 Langumuir-Hinshelwood
1.3.3 Hot atom mechanism
2 Experimental apparatus and methods 
2.1 Experimental apparatus
2.1.1 The main chamber
2.1.2 The beamlines
2.1.3 The microwave cavities
2.1.4 The sample holder and the sample
2.1.5 The Quadrupole Mass Spectrometer
2.1.6 The Infrared Spectrometer
2.2 Experimental methods
2.2.1 Water ice deposition on the sample holder
2.2.2 Mass spectroscopy: the dierent uses of the QMS
2.2.2.1 Cracking pattern
2.2.2.2 Residual gas analyzer: knowing beam composition
2.2.2.3 Knowing electronic state of atoms and molecules
2.2.2.4 TPD: Temperature Programmed Desorption
2.2.2.5 DED: During-Exposure Desorption
2.2.3 Calibration of the H2 and D2 beams
2.2.4 Calibration of the molecular beams
2.2.4.1 Determination of O3 monolayer and detection eciency .
2.2.5 Can reactions occur on gas phase?
3 Model 
3.1 Model
3.1.1 Initial condition
3.1.2 Flux: x
3.1.3 Reaction probability: r
3.1.4 Diusion coecient: kdiff
3.1.5 Desorption coecient: Nxdes
3.1.6 Conclusion
3.2 From model parameters to physical-chemical quantities
3.2.1 Reactivity: reaction probability and activation barrier
3.2.1.1 Teff evaluation
3.2.2 Mobility: kdiff and surface migration
3.2.3 Desorption: desorption probability and binding energy
4 Surface Physics 
4.1 Oxygen reactivity and diusion
4.1.1 Experimental
4.1.2 Oxygen reactivity
4.1.3 Oxygen diusion
4.1.3.1 The role of substrate
4.1.3.2 Model
4.1.3.3 Armophicity and diusion
4.2 Evaluation of desorption energies
4.2.1 Desorption energy of non-reactive species
4.2.2 Desorption energy of reactive species
4.2.3 Conclusion
4.3 Chemical desorption
4.3.1 The chemical desorption of the O-H system
4.3.1.1 Chemical desorption in O2+H experiments
4.3.1.2 Chemical desorption in O3+H experiments
4.3.1.3 Chemical desorption in O+H experiments
4.3.2 The inuence of the substrate: CD of O2
4.3.2.1 Experimental results
4.3.2.2 Model and discussion
4.3.3 Conclusion
5 Surface Chemistry 
5.1 Water formation via O2+H(D)
5.1.1 Experimental
5.1.2 Results and discussion
5.2 Nitrogen oxides chemistry
5.2.1 NO+O2
5.2.1.1 Experimental
5.2.1.2 Result and discussion
5.2.1.2.a Initial conditions and model
5.2.2 NO+O3
5.2.2.1 Experimental
5.2.2.2 Results and discussion
5.2.3 NO+O
5.2.3.1 Experimental
5.2.3.2 Results and discussion
5.2.3.2.a Dependence on surface temperature
5.2.4 NO2 reactivity
5.2.5 Conclusion
5.3 O/C/H chemistry
5.3.1 Carbon dioxide formation on cold surfaces
5.3.1.1 CO+O
5.3.1.1.a Experimental
5.3.1.1.b Results and discussion
5.3.1.1.c Model: evaluation of the CO+O barrier
5.3.1.1.d Conclusions
5.3.1.2 H2CO+O
5.3.1.2.a Experimental
5.3.1.2.b Results and discussion
5.3.1.2.c Model: evaluation of the H2CO+O barrier
5.3.1.3 Conclusion
5.3.2 The cycle of the CO-H chemistry
5.3.2.1 CO+H
5.3.2.2 H2CO+H
5.3.2.3 Chemical desorption pumping process
5.3.3 Preliminary results of CH3OH and HCOOH irradiation with H/O atoms
6 Conclusion 
6.1 Astrophysical conclusion
6.1.1 Astrophysical context
6.1.2 O diusion
6.1.3 Chemical Desorption
6.1.4 H/C/N/O chemical network
6.1.4.1 Interstellar ices
6.2 Perspectives
6.2.1 Ice growth: MonteCarlo Model
6.2.2 Further perspectives
Appendix A: atomic and molecular term symbols
Appendix B: list of publications
List of Acronyms
Acknowledgements
Remerciements
Ringraziamenti

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