PALMS sujet these 2004

Administrator

Description

erAstrochimie Expérimentale - 1 sujet - Information : Ian SIMS - [e-mail] Tél. [phone] Laboratory : UMR 6627 - PALMS : Equipe d’Astrochimie Expérimentale - Université de Rennes 1 Adress of laboratory : Bat. 11c – Campus de Beaulieu – 35042 Rennes Cedex – France Web : http://www.palms.univ-rennes1.fr/ASTROEXP/ Tél [phone] - Fax [phone] Lab Director: Guy JEZEQUEL – [e-mail] Team leader : Bertrand ROWE – [e-mail] Proposed subject of PhD thesis : Laboratory astrochemistry – atomic and molecular collisions under extreme conditions Introduction Astrochemistry, or understanding both the synthesis of molecules and grains in space, in order to predict their abundances, and their subsequent chemical and physical behaviour, not only represents a major scientific challenge but is of vital importance to the field of astrophysics. Molecules play a crucial role not only in mapping the Universe via the radio astronomical observation of molecular emissions, but also by acting as vital cooling agents during the process of star formation. Astrophysical environments display a wide variety of exotic conditions, ranging in temperature from close to absolute zero up to several thousand Kelvin. The speciality of our group is the use of methods which rely on flow reactors under extreme conditions, for example the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or ...

Informations

• Publié par: Endrui
• Nombre de lectures: 59
• Langue: English

Extrait

Astrochimie Expérimentale

- 1

er

sujet -

Information : Ian SIMS - [e-mail] Tél. [phone]

Laboratory :

UMR 6627 - PALMS : Equipe d’Astrochimie Expérimentale -

Université de Rennes 1

Adress of laboratory : Bat. 11c – Campus de Beaulieu – 35042 Rennes Cedex – France

Web :

http://www.palms.univ-rennes1.fr/ASTROEXP/

Tél [phone] - Fax [phone]

Lab Director:

Guy JEZEQUEL –

[e-mail]

Team leader

: Bertrand ROWE –

[e-mail]

Proposed subject of PhD thesis :

Laboratory astrochemistry – atomic and molecular collisions under

extreme conditions

Introduction

Astrochemistry, or understanding both the synthesis of molecules and grains in space, in order to predict their

abundances, and their subsequent chemical and physical behaviour, not only represents a major scientific

challenge but is of vital importance to the field of astrophysics. Molecules play a crucial role not only in mapping the

Universe via the radio astronomical observation of molecular emissions, but also by acting as vital cooling agents

during the process of star formation. Astrophysical environments display a wide variety of exotic conditions, ranging

in temperature from close to absolute zero up to several thousand Kelvin. The speciality of our group is the use of

methods which rely on flow reactors under extreme conditions, for example the CRESU (Cinétique de Réaction en

Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) technique for chemical

reactions and energy transfer down to 7K which was awarded one of the first EU Descartes prizes in 2000. We

combine these with state-of-the-art laser techniques to initiate and probe reactive and inelastic processes within the

supersonic flow, using the exceptionally wide range of pulsed and cw lasers now available within our group. The

experience that will be acquired by the student in the areas of aerodynamics, laser techniques and chemical

physics will be widely applicable not only in academic subjects but also for applied science and engineering,

providing an excellent basis for future careers both in academic research and in industry.

Research topics

It is planned to develop three new areas:

(i) Reactions and relaxation of atomic free radicals

Reactions involving atomic free radicals are both of fundamental interest owing to their relative simplicity, and also

of potentially great importance in the extremely cold environment of dense ISCs where

their abundances are still

relatively high. Highly reactive atoms such as C(

3

P) are thought to play a major role in interstellar chemical

synthesis by the building up of carbon chains via successive insertion-elimination reactions. Astrochemically

important low temperature reactions of this and other atomic species such as N, P, O and H will be studied by the

technique of pulsed laser photolysis (PLP) – pulsed vacuum ultraviolet laser induced fluorescence. Relaxation

(energy transfer) of spin-orbit states of these atoms in collisions with He and H

2

at very low temperatures are also of

great astrochemical interest and will be studied by these same techniques.

(ii) Product branching ratios for reactions of astrochemical relevance

All but the simplest chemical reactions have more than one possible set of products. The so-called product

branching ratios between these different pathways are extremely difficult to measure using standard techniques,

and measurements below room temperature are almost non-existent. But without the vital information given by the

product branching ratios, the existing data cannot be used to model complex chemical environments such as

interstellar clouds and planetary atmospheres. We intend to make the first determinations of product branching

ratios for bimolecular reactions at temperatures below 200 K, down to ~10 K, using a ‘marker reaction’ technique

involving reactions producing atomic species in one of their product channels (with detection as above by VUV LIF).

(iii) Rotational energy transfer rates for key interstellar atoms and molecules

A third area that will be explored is rotational energy transfer in systems and at temperatures of astrophysical

relevance. Microwave emission from different rotational states of CO and NH

3

in interstellar clouds is detected by

radio telescopes, and the relative intensities are used to determine the temperature in the cloud. CO and NH

3

are

thus used as molecular thermometers, and yet very little is known of how they exchange energy with

H

2

(the

majority species) at the extremely low temperatures prevailing in these clouds. We have developed a completely

novel cold supersonic flow operating in pure H

2

below 10 K, and using tuneable infrared and vacuum ultraviolet

lasers we plan to investigate these

and other systems of astrophysical relevance, including the fundamentally

interesting system H

2

–

H

2

. Successful measurements have already been completed on CO + He energy transfer,

demonstrating the feasibility of the experimental approach.

Supervisors

Ian SIMS

E. mail :

[e-mail]

Sébastien LE PICARD

E. mail :

sebastien.le-

[e-mail]