Cuestionario Semiestructurado

A detailed study o f ion irradiation of ices is essential to understand the effect of energetic particle bombardment in the different astrophysical environments described above. Some of the basic principles of the interaction o f ions with matter will be described in the following chapter. The relevance to astrophysical ices will however be discussed here, and recapped in Chapters 4 and 5 where the relevance o f our laboratory work on ion irradiation o f ices will be discussed.

Ions impinging onto ice will produce a number o f effects due to both elastic and inelastic collisions with the constituent target molecules, and consequent loss in ion energy. The important effects and consequences o f ion irradiation o f ices may be briefly summarised as;

(i) Sputtering (Brown et.al, 1978; Pirronello, 1991; Johnson, 1998), results in loss

o f material and ejection o f atoms or molecules into the gas phase. A direct consequence of sputtering is the enrichment of the gas phase molecular composition, be it from grain mantles in the interstellar clouds, or cometary nuclei in the Oort cloud, or the surfaces

1 In t r o d u c t io n______________________________________________________________________________21 of icy satellites or planets (thus modifying or creating atmospheres). The sputtered material may comprise the initial ice compounds, fragmentation products, or new species formed by chemical reactions. Another important consequence o f sputtering is the physical surface turnover as material is eroded away, thus exposing the underlying layers.

(ii) Changes in the physical properties o f the ice, for example amorphisation (the

change of phase from ordered crystalline structure to a random amorphous network) of ice. Amorphisation has been observed in the laboratory (Baratta et.al, 1991; Hudson and Moore, 1992). Voids may be created by atomic/molecular dislocation as a result of elastic collisions. Direct ion collisions or chemical reactions will result in the formation of volatile species which will either remain trapped within the ice or leave the ice by migration through micropores. All o f these effects (which may also be linked to chemical processes) will result in density variation. This will also have direct consequences on the optical properties of the ice (see below). Furthermore, local recrystallisation may occur along the ion tracks where excess energy is available for molecular redistribution (Hudson and Moore, 1992).

(Hi) Changes in the chemical composition may occur as an ion traverses the ice and

leaves a track of ionised and excited species, resulting in the formation o f new species by direct dissociation or reactions between the excited, ionised or fragmented species. In a regime where the ion penetration depth is less than the thickness o f the ice, as may be the case on the surfaces of satellites or cometary nuclei, the ion will be implanted in the ice. If the ion is reactive (e.g. H^ or Jovian magnetospheric ions: S"^, O"^, etc.) it may react with the target atoms or molecules to form new species that are not native in the ice. The results of carbon ion implantation in pure H2O ice will be presented in Chapter

4, demonstrating the possible formation o f CO and CO2.

(iv) Changes in the optical properties o f the ice may occur as a result o f physical or

chemical alterations described above. Variations in the reflection and transmission properties o f ices are observed for example in the variation o f albedo as a function of latitude has been observed in the inner Jovian satellites as a result o f magnetospheric particle bombardment (Nelson, et.al 1987). Darkening o f surfaces may be a result of formation o f carbon-containing species. Furthermore, alteration o f the top layers of the ice and resultant variation of the transmission properties may have a protective or destructive effect on the underlying layers as refractory material is formed in the top

1 In t r o d u c t io n______________________________________________________________________________2 2 layers o f the ice. Strazzulla and Johnson (1991) suggested that the top layers o f a comet residing in the Oort cloud and subjected to galactic cosmic ray radiation may develop a ‘crust’ o f non-volatile material.

In any given environment many o f the effects described above will occur simultaneously. The extent of these effects is determined both by the properties o f the ices (composition, temperature and thickness) and the ions (energy, flux and type). Although the effects o f ion irradiation have been studied extensively with the application to metals, semiconductors and human tissue, it was only in the late 1970’s that interest in ices began (Brown et.al, 1978). Ion induced processes are now being studied in several laboratories either with specific relevance to cometary ices (e.g. Moore and Hudson, 1998; Brucato et.al 1997a; Kobayashi et.al 1995), grain mantles (Palumbo and Strazzulla, 1993), or icy satellites o f the outer planets (Gomis, et.al

2003; Moore and Hudson, 2000; Baragiola et.al 1999; Strazzulla, 1999). General studies have been carried out that can be applied to a variety o f astrophysical environments (e.g. Strazzulla et.al 2003; Strazzulla and Palumbo, 2001; Hudson and Moore, 2001; Gerakines et.al 2000; Hudson and Moore, 1999; Johnson, 1998; Strazzulla, 1998; Brucato et.al 1997b; Strazzulla et.al 1996, Strazzulla et.al 1995; Pirronello, 1991; Brown et.al, 1978), using different ice mixtures, temperatures and ion energies. Extensive reviews on ion chemistry of ices are also available, e.g. Johnson and Quickenden (1997), Delitsky and Lane (1998) and Strazzulla and Johnson (1991).

In this thesis I present a new research programme commenced in collaborations with Queens University Belfast. Results of our first ion irradiation experiments {Chapters 4

and J) and the new apparatus developed for these studies {Chapter 3) are described in

this thesis.