The SRIM Program

A program. The Stopping and Range o f Ions in Matter (SRIM 2003), has been developed by Zeigler et.al. (2003) and is widely used to calculate the stopping power and the range of ions in different materials (e.g. Cooper et.al. 2001; Gomis et.al. 2003). The program is available for download from their website: wmv.SRIM.org. The SRIM programs utilise a quantum mechanical treatment of ion-atom collisions. This treatment is extended to molecular targets using a Core-and-Bond model in which the stopping power o f compounds is determined by superimposing the stopping due to the atomic cores and the bonding electrons between the atoms. The atomic stopping is determined following the Bragg's Rule which states that the stopping power o f a compound may be determined by a linear combination o f its constituent atoms. A correction factor for the bonds is then introduced to the rule by SRIM. The bond stopping correction depends on

2 Mo l e c u l a r St r u c t u r e, Sp e c t r o s c o p yan d Ra d iaj i o n Ch e m is t r y 6 8

the type o f bond e.g. C-C, C-N, C=C, C=C, C=0, etc. Target compounds may be selected by specifying the relative atomic stoichometry o f each atom in the compound, the target density and the type o f bonding between the atoms. Alternatively a large collection of compounds already exist in the target dictionary that is available with the software. This may be edited if new compounds need to be added. An example of an entry for the H2O target is shown in Figure 2-24.

The bonding and appropriate corrections are suggested for each phase. Further detail on the special bonding corrections can be found in references provided with the software. Once the desired target and the ion is selected, with the appropriate density and compound correction specified a table is calculated containing the values for the electronic stopping (d E /d x )e , nuclear stopping (d E /d x )n , the projected range and the

longitudinal and transverse straggling for the selected ion range.

================== Water_Liquid ================== Stopping Correction for Target Chemistry and Phase

***** Correction assumes Ion = H(l) *****

SOLID Target Binding Corr. = 0.890 = - 11.00%

GAS Target Binding Corr. = 0.940 = - 6.00%

Target PHASE Correction = 0.920 = - 7.99%

TOTAL Target Correction (SOLID) = 0.819 = - 18.11%

Density = 1 g/cm3

Chemical Formula: H — O — H

There is about an 8% increase in the peak of the

stopping power for ions in water vapour relative to the liquid. (The peak of the stopping occurs at an energy of about 150 keV/amu times the 2/3 power of the ion's

atomic number.) Above the peak the phase difference begins to disappear. This calculation is for the LIQUID p h a s e .

======= TARGET COMPOSITION =======

Atom Atom Number M o l e c . Core

Name Numb Atoms Mass % Stopping

2. 0 0 1 . 0 0 11.19 88. 81 0 . 0 0 5 .45 === TARGET BONDS (per molecule)===

Bond Type Number Stopping

(H-0) 10.061

Figure 2-24: SRIM Compound Dictionary entry for H2O (Zeigler etMl 2003),

2 Mo l e c u l a r St r u c t u r e, Sp e c t r o s c o p yan d Ra d ia t io n Ch e m is t r y_______________________6 9

SRIM was used in this work for determination o f the stopping powers and ranges in ion irradiation experiments. The stopping powers and ranges were used primarily for comparison between different ions in irradiation o f similar targets.

2 .1 0 S

This chapter covers the theoretical background for the spectroscopic techniques used and the molecular interactions encountered in this work. An overview is given o f the molecular structure, together with a description o f the atomic and molecular orbital theory and molecular symmetry. Some o f the key theoretical concepts o f molecular spectroscopy are discussed, along with a description of the theoretical aspects of vibrational and electronic excitation and spectroscopy and associated selection rules. An introduction to radiation chemistry is given, distinguishing between the interaction of photons and ions with matter. The general processes involved in ion and photon interactions with molecules are described. An overview is given of the ion energy loss processes in bulk matter along with a description of the SRJM software used in this work.

70

3

Th e Ex p e r i m e n t a l A

p p a r a t u s

AND Pr o c e d u r e s

" M a k e ei'erythin<y as sirnf^le as possihL-, h u t n o t simfyler."

Albert Eimcem (18791955)

T h is ch a p ter c o n ta in s a d e ta ile d d e sc r ip tio n o f the d e v e lo p m e n t and c o n str u c tio n o f a n e w

app aratu s to p r o d u ce and stu d y a str o p h y sica l ic e a n a lo g u e s . T h e v a r io u s fea tu re s o f th is

ap p aratu s are d e sc r ib ed , as w e ll as the e x p e r im e n ta l p r o c e d u r e s im p le m e n te d u s in g th is

app aratu s for sa m p le d e p o s itio n , irradiation and s p e c tr o s c o p y . S o m e lim ita tio n s o f th is

app aratu s are a ls o d isc u s se d .

3.1

I

In the last three decades, advances in ground, air and space based infrared observations have triggered interest in astrochemical condensed phase processes which, in turn have led to the construction o f a number o f laboratory systems to study such processes.

A new apparatus has been built to study astrophysical ices and their evolution under ion, photon and electron processing. The apparatus was designed and built primarily with portability in mind, so that it may be transported and coupled to a variety o f radiation sources and fit within conventional UV-Vis or FTIR spectrometers. Hence, it was essential to ensure minimum size o f the sample chamber and mobility associated components whilst maintaining full functionality and a sufficient degree o f flexibility and compatibility.

3 Th e Ex p e r im e n t a l Ap p a r a t u s A N D PROCEDURES________________________________________71

In Chapter 1 we have discussed the different astrophysical environments that require

study, along with the complexity of the physical and chemical processes associated with them. Therefore, from a practical perspective, a working laboratory system should be as flexible as possible.

The key requirements for such an apparatus are:

(i) to create, monitor and control, within experimental limitations, laboratory environments analogous to that o f the astrophysical environments;

(ii) to prepare both layered and mixed ice samples o f varying compositions, structures and thicknesses, analogous to astrophysical ices in the environment in which they may be found;

(Hi) to process the samples by means of external radiation (photon, ion and electron) as a function o f energy flux and time;

(iv) to spectroscopically monitor and analyse the samples before, during and after irradiation;

(v) to remove the sample and clean the substrate effectively;

(vi) to be able to repeat each of the above under duplicate conditions, ensuring reproducibility o f results.

It is not possible to completely simulate identical conditions to those in astrophysical environments; however, approximate, or analogous conditions may be simulated. Table 3-1 compares the laboratory and the interstellar conditions (adapted from Hagen

et.al\919 and Schutte 1999).

As with many laboratory systems there are limitations which are inherent to the system:

(i) There is generally a lack of knowledge about the composition, density and temperature o f ices in various astrophysical environments. The information about the ices is inferred solely by observation. There are no samples available for examination in the laboratory.

(ii) The analogy to the grain surface in the laboratory is poor. The real compositions o f grain mantles are not known. Furthermore, the grain surfaces are very different in

3 Th e Ex p e r im e n t a l Ap p a r at u sa n d Pr o c e d u r e s 72

size and morphology. In this particular experiment an approximation is made, neglecting grain surface structure and surface effects. The ice layers deposited are thick enough to consider only the surface or bulk reactions, but thin enough for transmission spectroscopy. However we are not able to generate a close approximation to the surface area in the case of grain mantles. The ices created in the laboratory have very large planar surfaces in comparison with the grains which are sub-micron to micron sized particles that may have a high degree o f complexity in their supposedly ffactal-like surface structure. See Chapter 1.

Table 3-1: Comparison between the parameters in astrophysical environments of interest and laboratory simulations.

P a r a m e te r IS M G a lile a n S a ttelite s ' L ab o ra to ry

Ice temperature (K) - 1 0 -7 0 70-180 Variable variable but > 10

Ice thickness 0.02 - 0,2 pm 1 pm - several km 0.2 - few pm Condensable

m olecular species

CO, N2, O2, etc. C O , C O2 A ny m olecular species o f interest

Condensable atomic species

H, 0 , C, N , etc. - G enerally cannot be

prepared in the lab Condensation rate

(m olec. cm'^ s ')

lOf-lO? Variable depending on day/night temperatures - lO 'L lo '’ A m bient pressure (mbar) - 1 0 '^ - lO 'L io 'o U V flux (hv cm'^ s ') (>6 eV ) 10^ - 10* (>4.4 e V ) 4 x 1 0'°* i o '' - i o 's

Ions M eV & He^^ ^ n + ( n = l - 6 ) q n + ( n = l- 5 ) t t+ H e \ C ^ W , K ^ ^ ’and’ m olecular ions S0 2% H2 0\ H3 0\ 0H \ H2", H3"

M eV -keV : & He^ few keV: any ion + any charge state

Ion energy flux (keV cm'^ s ') (1 M eV H^) 10^-10^ (E >20 keV) 2.2 X 1 0 * - 7 .8 X 1 0'°* variable Equivalent time scales

10^ years variable depending on local environment and radiation

1 hour type

“ D elitsky a n d Lane (1998) * C ooper et.al. (2001)

(in) In the case where ice analogues o f the surface o f icy bodies in the solar system are studied, we can create a better approximation o f the ice surface. However the compositions of these ices, as inferred from reflectance spectra, are not completely known. This is partly owing to the limitation in the spectral and spatial resolution o f the

3 Th e Ex p e r im e n t a l Ap p a r a t u s A N D PROCEDURES________________________________________73

astronomical data and to the high complexity of physical and chemical processed involved in such environments.

(iv) There is no conceivable way to simulate the timescales at which reactions occur in the interstellar medium. For example 1 hour of irradiation in the laboratory is equivalent to 1000 years irradiation in the interstellar medium. It is difficult in the laboratory to simulate the flux of radiation comparable to that in the astrophysical environments. However, it is possible to obtain time-dependent data in the laboratory regimes, and in some cases to extrapolate these to astrophysical time scales.

(v) The pressures used in the laboratory can be 10^ times greater in the laboratory compared to interstellar clouds.

At present it follows that the astrophysical conditions that are possible to recreate in the laboratory are approximate, within the limitations of the laboratory system but should provide us with sufficient amount of information to understand the processes involved in ice chemistry. A compromise must be made between observation and experiment, and theoretical modelling is required to map and extrapolate one system to the other and vice versa, in order to obtain the required data with a certain degree o f confidence.

3 .2

A

A