Study Of Gas-surface Interaction

Contents

Introduction 1

I. The work context 2

1) The Heriot-Watt University 2

2) The Astrochemistry Group 3

The ICE-RIG 3

The Beam Rig 3

3) Problematic 4

II. The work 5

1) Technical explanations 5

The vacuum system 5

Monitoring pressure 5

The cooling system 6

The cold finger 7

Deposition of the samples 7

Temperature-Programmed Desorption 8

Infrared spectroscopy 8

2) Solving the temperature monitoring issue 9

Identification of the issue 9

Replacement of the sample heater 10

3) Evacuating the ICE-RIG under UHV 11

III. Results 14

1) Calibration 14

2) Estimation of the desorption parameters 15

Desorption order 16

Determination of the other kinetic parameters 18

3) TPD results 19

CONCLUSION: 20

ANNEXES: 21

Lexicon 21

Bibliography 21

Introduction

The InterStellar Medium (ISM) is the space separating stars, and it was originally thought to be empty. Over the past decades, the knowledge of the InterStellar Medium (ISM) has made important progress; observations performed with space-telescopes revealed that the ISM is not empty; it is mainly composed of gas (99%), and also of silica dust grains (1%). The gas phase is composed of 90.8% of hydrogen, 9.1% of helium and 0.1% of heavier elements4. Observations have also shown high concentrations of gas and dust in the ISM, named the “interstellar clouds”. These diffuse clouds can eventually collapse, becoming denser until the final formation of a star. In these clouds, many chemical reactions take place, leading to a surprising molecular complexity. The reactions occurring in the gas-phase have been extensively studied, and allow a good understanding of the interstellar chemistry.

However, gas-phase chemistry alone cannot account for the formation and abundances of some molecules of primary importance. For example, the formation of molecular hydrogen (H2, the most abundant molecule in the universe) after the recombination of two hydrogen atoms is extremely slow in the gas-phase; this reaction cannot explain the H2 concentrations observed in the ISM. Thus, one has to consider the reactions occurring at the surface of the interstellar dust grains, which are composed of silicate. The surface of the silicates acts as a third body, and opens new reactional pathways that accelerate or make possible the formation of new molecules. Actually, the low temperatures occurring in the dense interstellar clouds (typically 10 K) allow the adsorption and desorption of molecules on the surface of the dust grains. Then, the grains are coated with an icy mantle (mainly composed of H2O) in the dense regions of the clouds.

To study the gas-grain interactions in astronomically relevant conditions, Pr. McCoustra’s group in Heriot-Watt University developed a unique experimental setup called the “ICE-RIG”. This apparatus allows the study of the interaction between atoms/molecules and a silicate surface at cryogenic temperatures in Ultra High Vacuum conditions. The exchanges between solid and gas phases are studied by Temperature-Programmed Desorption (TPD) and infrared spectroscopy in grazing incidence (RAIRS). However, some of the experiments carried out previously on the ICE-RIG present discrepancies with the results established by other groups. This problem is thought to originate from a current leakage occurring near the sample heater, and creates a random shift in the TPD spectra. Consequently, the kinetic parameters deduced from the data analysis suffer an overestimation compared to the results obtained by other groups. Therefore, the first objective of this project is to replace the sample heater. Then, the apparatus has to be restarted and tested before any experiment can be achieved; indeed, the ICE-RIG had previously been stopped for a long time, and a leak test needs to be performed. The second objective is to realize a precise calibration of the thermocouples, to have a reliable indication of the sample temperature. Then, TPD experiments could be carried out to check the results obtained on the desorption of O2, CO and N2 from astronomically relevant surfaces.

I will present you the Heriot-Watt University and the astrochemistry group. Then, I will describe all my work during my training period. Finally, I will present my different results.

I. The work context

1) The Heriot-Watt University

The Heriot-Watt University is a teaching and research university based in Edinburgh, Scotland. It was named after George Heriot, minister of King Jacque VI of Scotland, and James Watt, the brilliant Engineer. It is the eighth oldest higher education institution in the United Kingdom, created in 1821 as the School of Arts of Edinburgh. In 1837, it became the Watt Institution and School of Art, then in 1885 the Heriot-Watt College. Finally in 1966 it became a university. The Heriot-Watt University possesses a real feminist tradition; indeed it accepts women since 1869, 20 years before the other universities. Prof. Susan Greenfield is the chancellor of the University.

Nowadays Heriot-Watt University is the most important in Scotland (elected Scottish University of the year 2011-2012 by the Newspaper The Sunday Times), with 11000 students from 150 countries. The University is divided into four campuses, three in Scotland and one in Dubai. It is a cosmopolitan university, which Business School is recognised all around the world.

The Heriot-Watt University is divided in eight schools: Built Environment, Engineering & Physical sciences, Life Sciences, Management & Languages, Mathematical & Computer Sciences, Textiles and Design, Institute of Petrol Engineering and Edinburgh Business School. The University proposes about 25 postgraduate formations in chemistry.

Moreover,

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