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\chapter{Chapter 1 Introduction}
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\section{Background}
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\cite{monograph}.

Plasmas with a wide variety of densities ans temprets permeate most regious 
of our galaxy (see in Table 1.1) and interact with various solid objects and 
neutral gases. Therefore, an understanding of the interaction of energetic 
plasma particles with solids and gases may be required in order to interpret 
remote sensing observations both in our solar system and in the interstellar 
media. The nature of the energetic, charged-particle radiations in various 
regions of space can be obtained from in-situ spacecraft measurements of the 
solar wind, magnetospheric, and cosmic-ray plasmas along with observation of 
line spectra of ''hot'' plasmas and shocks intersecting interstellar molecular 
clouds. In addition, a recent increase in information on the magnetospheric 
plasmas of other planets from spacecraft and ground-based measurements has 
enhanced interest in the effects of energetic charged particles on the surface 
materials of objects embedded in plasmas. 

Much of the early interest in this area was related to the interaction of
charged particles with the gaseous envelopes of the planets and comets, and
to the effect of such radiation on man-made devices in space. Wehner et al.
(1963) suggested that, in addition, particle radiation may have modified the
surface materials of planetary bodies or their satellites over geologic
time. Such changes would be produced by bombardment of ions that
collisionally eject (sputter) atoms and molecules from the surface, by the
implantation of incident ions into the bombarded material, and by chemical
alterations induced in the materials by the incident ions. In fact,
neutralized solar wind helium has been observed as an atmospheric
constituent of Mercury (Broadfoot et al. 1976; Goldstein et al. 1981; Hunten
et al. 1988) and the Moon (Hoffman et al. 1973). The analysis of lunar
materials confirmed that grains on the lunar surface are modified, at the
micron level, by solar-wind ion bombardment (McDonnell 1977; Taylor 1982).
In addition, solar-flare and cosmic-ray particle tracks have been used to
determine the length of time that minerals in the lunar regolith have spent
at the surface, thereby giving estimates of the gardening rate of the
surface (Maurette and Price 1973), and to determine how long meteoritic
materials have been exposed (Caffee et al. 1988). It is therefore likely
that solar and cosmic ray ions have also modified materials on asteroid
surfaces, proto-planetary materials, interplanetary grains, and the zodiacal
light particles.

Matson et al. (1974) proposed that particle radiation associated with
Jupiter's magnetospheric plasma may sputter significant quantities of
surface material from the Jovian satellites. This suggestion was based on
the initially surprising discovery by Brown and Chaffee (1974) of a cloud of
neutral sodium atoms co-orbiting with lo about Jupiter. Subsequent
ground-based and spacecraft measurements have demonstrated that the
composition of the Jovian magnetospheric plasma in the vicinity of the
Galilean satellites is, in fact, determined primarily by those volatile
materials that are removed from the satellite surface or from its gaseous
envelope by particle bombardment (Cheng and Johnson 1989). That is, not only
do the particles in plasmas modify exposed surfaces (Cheng et al. 1986) or
atmospheres (Chamberlain and Hunten 1987), but also the material ejected,
which is eventually ionized, determines the nature of the plasma (Fanale et
al. 1982; Ip 1986'; Huang and Siscoe 1987; Johnson et al. 1989'). This
discovery has provided a new link between planetary geology and
magnetospheric physics in which observations of ions in the magnetosphere
can be used to determine the atomic composition of satellite surfaces an
exciting possibility for the Cassini mission. It has also stimulated
attempts to understand the interactions of plasma particles with the surface
and gaseous envelopes of the satellites. 

Concomitant with the above has been a recent increase in interest in plasma
erosion of grains in the interstellar medium (Draine and Salpeter 1979a, b).
Grains are known to initiate chemical processes in interstellar clouds by
providing surfaces for the formation of molecular hydrogen. The formation of
molecular hydrogen is then followed by the formation of a large variety of
molecules from the ambient atomic and ionic constituents in these clouds
(Irvine et al. 1985). In dark clouds the molecules so formed can condense on
the grains (Draine 1985; Greenberg 1982). In this condensed state, incident
radiations can lead to the formation of new materials in the grain mantles.
These new molecules can be added to the gas phase, for instance, by
sputtering by heavy cosmic ray nuclei (de Jong and Kamijo 1973; Leger et al.
1985; deVries et al. 1984b; Sundqvist 1989; Moore and Tanabe 1989). If the
cloud moves through a shocked region, the volatiles may be removed by the
plasma formed at the shock boundary and, indeed, the grains themselves may
be eroded (Draine et al. 1983). Because such regions are important for the
formation of solar systems (Elmegreen 1985), the effect of the hot plasmas
on the molecular composition of the cloud and the composition of the grains
is of some interest (Mendis 1979; Wdowiak et al. 1989). 

The excitement and interest in plasma interactions with materials in space
was further enhanced by the discovery of W.L. Brown, L.J. Lanzerotti and
co-workers (Brown et al. 1978) of a new sputtering mechanism which operated
for low temperature, condensed-gas solids such as low-temperature water,
carbon monoxide, nitrogen, methane, and sulfur-dioxide ices. These have been
shown to be important constituents of the surfaces of the outer solar system
satellites, cometary nuclei, and interstellar grains. Such materials behave
like insulators at the very low temperatures in the Jovian system ($<$ 140 K) 
and are highly responsive to the electronic energy
deposited in solids. It was found that surface material is ejected from such
solids because of electronic processes (Brown et al. 1982a, b), as well as
by direct momentum transfer collisions of the incident ions with atoms in
the solid. This meant that very energetic light ions, fast electrons, and
ultraviolet (UV) photons could also produce gas-phase species from exposed
surface materials. The discovery of this ejection mechanism also led to
renewed interest in sputtering within the solid-state physics community,
stimulated in a large part by the planetary and astrophysical observations
(Brown et al. 1982a). An expanded effort is also being made to describe the
relationship of the sputter ejection of molecules from icy surfaces in space
to other material alterations induced in solids by charged particle
radiations (Johnson et al. 1984a). As UV radiations also initiate electronic
excitations, differences between the nature of the alterations produced in
materials by particles and by UV radiation are considered here. 

The processes discussed above can each be represented by the diagram in Fig.
1.1, which forms a basis for the work presented. Various stimuli (plasma,
photons, collisions) eject atoms or molecules from the surfaces of solid
objects into the gas or plasma phase. The gas in turn can be ionized by the
plasma or by photons and the ions in the plasma neutralized by charge
exchange or by electron-ion recombination. Both neutrals and ions can impact
and stick to (condense on) the surfaces of a solid object thereby returning
to the solid phase. In addition, ion-molecule reactions (plasma/gas) can
enhance condensation by building charged clusters. 

The effects of electronic excitation and ionization processes induced in the
gaseous envelopes of planets by UV and cosmic rays have been studied for
years. It is, therefore, important to understand the effect of such
excitations in these same materials when they are condensed on will condense
on grains in dense interstellar clouds as discussed above; interplanetary
grains are thought to have icy mantles; comets and many of the satellites of
the distant planets are icy objects; and even the Earth and Mars have icy
polar caps. In all cases these materials are exposed to charged-particle
radiation. As volatile and nonvolatile materials often exist together (e.g.,
silicates, organics, and ices in the core and mantle surfaces in the colder
regions of space. This phenomenon is observed in several situations: gases
of interstellar grains) it is also useful to know the relative importance of
the effects produced in different materials by incident ions and to know the
ability of these ions to mix the materials. 

The physics of the relationship between these ''gaseous'' and solid phases
forms the important physics in many regions of space and is a primary topic
of this text. Therefore this text is intended to bring together two
separate, but clearly related, areas of physics needed to describe
charged-particle bombardment effects: (1) collisions of energetic particles
with atoms and molecules, and (2) charged-particle modification of solids.
In the last half century an enormous scientific effort has been made in both
areas. This has been stimulated almost as much by applications (e.g.,
atmospheric physics, radiation biology, production of electronic devices,
etc.) as it has been by the desire to test our understanding of the basic
theories of the physics of atoms, molecules, and solids. Because of the
wealth of data, I summarize in Chapters 2 and 3 only those ideas and
expressions which I feel are useful for interpreting astrophysical
phenomena. I hope that these discussions, however, are written in such a
manner that the reader will acquire the language and basic concepts needed
to understand the literature in the relevant field. Of primary importance is
my intent to indicate the limitations of the expressions and models
discussed. A considerable danger always exists that models of limited
validity are applied rather generally under conditions for which they are
not intended. In order to avoid this, I describe the development of physical
models and hope that this does not deter those astronomers and planetary
scientists who want to use the results. When appropriate, the details are
given in the Appendices. After describing the basic physics, I apply the
models discussed to certain astrophysical and solar system phenomena in
Chapter 4. This is done in a general way, emphasizing commonality of
processes rather than listing a series of objects and treating them. I feel
this is appropriate, as the information on the plasma compositions and the
target constituents is improving rapidly. Further, the most important
problems for understanding primitive solar system materials have only
recently begun to emerge from analysis of data from the spacecraft
encounters with Halley (Jesseberger et al. 1988a, b) and Uranus (T.V.
Johnson et al. 1987), and the recent encounter with Neptune. 

Primary emphasis in this text is given to fast ion or atom interactions with
solids and gases, that is, ions with energies much greater than the
''thermal'' energies of the neutral medium. Therefore, the very interesting
ion-molecule reaction, important for molecule formation in interstellar
clouds and for determining the nature of the ionosphere, is only briefly
mentioned. It is, in fact, covered extensively in other texts (see
Bibliography). Throughout the text, incident ions associated with various
solar system and astrophysical plasmas are emphasized, but in most instances
the results can also be applied to energetic neutral atoms and the
electronic effects produced are similar to those induced by fast electrons.
Finally, as objects exposed to charged-particle radiations are also exposed
to UV (and often X-rays or $\gamma $-rays) the relative importance of these
radiations is discussed. Before proceeding to the discussion of the physics
in Chapters 2 and 3, the character of the plasma environment in various
regions of space is considered in the following section. This is followed by
a brief discussion of the types of materials that are of interest. 

\section{Plasma Environment}

Objects in the interplanetary medium are exposed to the continuous expansion
of the solar corona, referred to as the solar wind (Hundhausen 1972). This
is of the order of 1 keV/amu (\symbol{126}400 km/s) at 1 AU (Fig. 1.2a).
Other constituents and their abundances are given in Table 1.1. In the
vicinity of 1 AU the flux is of the order of 2 x 10$^{8}$ part/(cm$^{2}$ s)
corresponding to a number density of ions (and electrons) of the order of
4-5 ions/cm$^{3}$. The flux decreases, roughly, like that from a point
source, i.e., 1/R$^{2}$, where R is the distance from the sun. The
temperature of this plasma is $\sim$ 7 eV at 1 AU and decreases with
increasing distance from the sun, so that the energy connected with the
expansion determines the bombardment energy of the ions. This outflowing
plasma carries with it a magnetic field, which in the ecliptic plane, at
distances $\sim$ 1 AU, lies roughly along the direction of flow
(Hundhausen 1972; Lanzerotti 1987). The solar wind flux is compared to the
solar Lyman-$\alpha$ flux in Fig. 1.2a. 

Superimposed on this flux are higher energy particles associated with active
solar regions, very energetic particles produced in solar flares and the
galactic cosmic-ray particles (Kennel et al. 1979; Simpson 1983). A typical
large flare event has a total fluence of protons with energies greater than
1 MeV of the order of 10$^{10}$ part/cm$^{2}$. Such events typically have an
energy spectrum $\propto $ E$^{-2}$, where E is the proton energy. Other
constituents are listed in Table 1.1. Assuming one solar flare event per
year, the flux of such particles is of the order of 10$^{4}$ lower than the
solar wind flux, but with a very different energy spectrum. Co-rotating
events associated with active solar regions have much steeper energy spectra
($\sim$ E$^{-4}$). If the occurrence rate is of the order of two events
per solar rotation ($\sim$ 27 days) with interplanetary space filled
with such particles one-third of the time and the spectra extrapolated to 10
keV, a flux of the order of 10$^{2}$ smaller than the solar wind flux is
obtained, but again involving more energetic particles. These effects
typically also decay as 1/R$^{2}$, as does the solar wind and UV radiation.
However, recently the Low Energy Charged Particle (LECP) instrument on the
Voyager spacecraft found that the flux of particles with energies $\sim$
0.5 MeV did not decay as 1/R$^{2}$ in the outer solar system, indicating
that acceleration processes are occurring (L.J. Lanzerotti, personal
communication). 

The description above pertains to the present conditions, that of a
quiescent Sun. During its evolution, however, a star, such as the Sun, may
have passed through phases in which the particle radiation has been much
more intense and, therefore, the plasma effects have been more important
(Strazzulla et al. 1983). In a T-Tauri phase, the solar flare rate is much
larger than that of our present Sun, and the particles are more energetic.
Worden et al. (1981) give an average flux from these flares roughly
equivalent to the solar wind flux, 2 x 10$^{8}$ ions/ (cm$^{2}$ s) at 1 AU,
for particles of energies, E $>$ 10 MeV. As the Sun may have gone
through such a phase, Feigelson (1982) deduced an average proton flux for
the young Sun of 4 x 10$^{6}$ ions/(cm$^{2}$ s) at 1 AU for E $>$ 6 MeV, 
which could have lasted anywhere from 10$^4$ to 10$^7$ years. 

Besides those particles ejected from our Sun, the interplanetary medium and
interstellar medium are filled with a background of Galactic (cosmic-ray)
particles (Simpson 1983). The energy in the cosmic-ray background is
primarily contained in the energetic ions (Fig. 1.2b) with smaller fractions
carried by $\gamma $ rays and electrons. Not only is the interstellar medium
exposed to the background cosmic ray radiation, but there are also many
regions in which shock-heated plasmas co-exist with grains and neutral gases
(Draine et al. 1983). Such regions can be generated, for instance, by
supernova remnants. Temperatures can vary enormously depending on the nature
of the shock. Typically densities and temperatures used (H, H$^+$, He, 
He$^+$,... ) range from 1/cm$^3$ at 10$^5$ K to 0.001/cm$^3$ at 10$^8$ K.

The earth and the outer planets have magnetospheres which can trap solar
particles for considerable times (Roederer 1970; Schultz and Lanzerotti
1974). Ions may also be supplied from atmospheres or surface within the
magnetospheres of objects and, in turn, these objects are subject to
bombardment by the plasma. Some values of the average fluxes experienced by
objects in the vicinity of the planets are given in Table 1.2 for comparison
with the above sources. The distances associated with the satellites are
also given, as these will be useful in our later discussions. It is seen
that the densities achieved, particularly near lo, can be many orders of
magnitude larger than the local solar wind density and the bombarding ions
may have significant energies. Of course the fluxes vary rapidly with height
above the satellite orbital plane. As Voyager 1 and 2 did not travel in this
plane, the data given in Table 1.1 are values roughly extrapolated to the
satellite orbital plane (Richardson and Sittler 1989). Models for the outer
planet magnetospheric plasmas are rapidly changing because of spacecraft
data and ground-based line spectra (Trauger 1984) so that many more data
will be available in the near future. As the magnetosphere rotates with the
rotational period of the planet, the ions also have an overall motion with
respect to objects which are gravitationally bound to the planet (Cheng et
al. 1986). Therefore, in addition to random bombardment, the satellites and
imbedded grains (e.g., E-ring at Saturn) are exposed to a streaming plasma.
This preferentially bombards, with rather high relative kinetic energies in
some instances, one hemisphere of a satellite which is phasedlocked to its
planet resulting in hemispherical differences in reflectance (Clark et al.
1984). 

The magnetospheric plasmas described above also have a hot component
(Krimigis et al. 1981 a, b). In Figs. 1.3a, 1.4, and 1.5 are given the ion
energy spectra measured by the LECP instrument on Voyager 1 at a few
distances from Jupiter, Saturn, and Uranus. Combining these with the low
temperature plasma in Table 1.1, the plasma can be quite complex, as
indicated by a composite spectra for the plasma and LECP instruments on
Voyager 1. This is shown in Fig. 1.3b at Io, an orbital radius at which the
low temperature part of the plasma dominates and at Dione and Rhea, Fig.
1.6. Both at Saturn and Jupiter this low temperature component was found to
consist of a number of ions of differing charges and may include even a
small fraction of molecular ions (Bagenal et al. 1985). Spacecraft data of
this type have been used to construct two-dimensional models of the plasmas
as in Fig. 1.7. Unfortunately, our ability to use the data is limited often
by a lack of knowledge of the composition of the plasma, particularly for
the LECP data from Voyager (hot component). 

What we know of the composition of both the Jovian and Saturnian plasmas
suggests that the satellites are sources of the heavy ions in the plasma
(Cheng and Johnson 1989) and the atmospheres of the planets are sources of H$%
^{+}$ (Shemansky and Smith 1982, 1986). The imbedded satellites as sources
present a particularly interesting situation. The species that bombard the
surface produce a sufficient amount of new plasma, compensating for
diffusive loss processes (Huang and Siscoe 1987) and thereby significant
densities accumulate. The material ejected from the surface or atmosphere
forms a giant neutral envelope of atoms and molecules which orbits with the
satellite. These species are eventually ionized and become part of the
plasma. They in turn bombard the surface, ejecting more material so that the
plasma is self-sustaining, as indicated in Fig. 1.8. Recent in-situ
spacecraft measurements close to comets also indicate that intense plasmas
of ions co-exist with the gas and dust and have a composition determined by
the comet material being sublimated (Mukai et al. 1987). These plasma and
solar photons bombard the escaping cometary dust (Combi 1987) and may
produce a spatial correlation between gas (A'Hearn et al. 1986), large
molecules (Mitchell et al. 1987; Heubner 1987) and dust. 

\section{Materials}

The materials bombarded by charged particles are diverse. Because we will be
considering bombardment of atmospheres, typical gases will be discussed
(e.g., H$_{2}$, O$_{2}$, CO, CO$_{2}$, CH4, NH$_{3}$, H$_{2}$O, SO$_{2}$,
etc.). Their relative abundances in the interstellar medium are given in
Table 1.3. In the colder regions of the solar system, the interstellar
media, and stellar nebulae, such molecules are also condensed on available
surfaces. Therefore, we will also be considering solids formed of these same
molecular species on small grains or on satellite surfaces. In Fig. 1.9
characteristic temperatures in the solar system are given. The solid line is
the temperature determined from simple black-body radiation equilibrium on a
rapidly rotating object with an albedo (reflectance coefficient) equal to
zero. The points indicate effective average temperatures of planets obtained
from measured albedos. Also indicated (on the right-hand scale) are the
melting points for various molecular species. Finally, the dashed lines
indicate the effective temperature of an object composed of a condensed gas
exposed to solar photon radiation and with temperature controlled by the
sublimation rate. This rate increases very rapidly with temperature; hence,
the change in effective temperature with distance from the sun is shown. At
distances close to the sun, the temperatures of such objects may not be very
high, but the mass loss rate may be enormous as, for instance, on a
solar-approaching comet. However, the recent fly-by of Halley showed that
comets have very heterogeneous surfaces (Keller et al. 1987). 

It is clear from the diagram that condensed-gas solids are stable,
particularly in the outer solar system. This is consistent with the
existence of icy interplanetary grains, icy ring particles at Saturn, icy
periodic and new comets, H$_{2}$O and CO$_{2}$ on the Martian polar caps,
condensed N$_{2}$ on Triton (Cruikshank et al. 1984; Broadfoot et al. 1990),
ice on the Jovian (Pilcher et al. 1972; Brown et al. 1986), Saturnian
(Johnson et al. 1975; Morrison et al. 1976), and Uranian (Cruikshank and
Brown 1981) satellites, condensed SO$_{2}$ on Io (Smythe et al. 1979), and
methane at Pluto (Cruikshank et al. 1976; Sawyer et al. 1987) and Triton
(Cruikshank and Salvaggio 1979; Broadfoot et al. 1990). It is also well
known that the solar system exhibits considerable chemical differentiation,
with the outer solar system dominated by the lighter gases (Lewis 1972).
This differentiation is, of course, a product of thermal and .gravitational
gradients which existed in the early solar nebula and these differentiations
assisted in the formation of icy bodies in the outer solar system. In
interstellar clouds the observation of the 3 $\mu$m band associated with the 
OH stretch (Soifer et al. 1979) and other IR bands (Tielens et al. 1984) is
used as evidence for the presence of ices and organics. Such a band has also
been seen in the nebulae of some young stars (H.L. Tau in Cohen 1975). 

One of the most interesting areas of endeavor at present is the
consideration of the relationship of outer solar system materials to
interstellar materials (Sagan and Khare 1979; Elmegreen 1985; Greenberg
1983; Tielens and Allamandola 1987). Interstellar grains are assumed to be
formed from silicates, crystalline or amorphous carbon, and iron (Donn et
al. 1981), which may or may not be covered with icy mantles and range in
size from $\sim$ 0.01 $\mu$m to $\sim$1 $\mu $m (Savage and
Mathis 1979). Greenberg (1982) suggests that comets are also composed of a
bird-cage-like structure of such grains. These grains have a core (e.g.,
silicates $\sim$ 0.1 $\mu$m), which is covered with an organic residue
assumed to be formed from UV-modified mixtures of condensed gases. Finally,
there is an icy mantle which accretes prior to the aggregation of the grains
into comets. Confirmation of a silicate-carbonaceous model for grains from
comet Halley has been recently given (Kissel and Krueger 1987) although the
region of formation remains unclear. 

One of the most extensively studied aspects of particle interactions with
refractory materials is the production of tracks (Fleischer et al. 1975) in
such materials due to exogenic processes, solar flare, and cosmic ray nuclei
bombardment, and endogenic processes such as the decay of radioactive
species. This has been used as a means for obtaining a record of the
irradiation history of materials, e.g., collected lunar surface materials
(Maurette and Price 1973) and interplanetary dust collected from lunar
samples and in our upper atmosphere (Bradley et al. 1984). 

The surfaces of Mercury (Ip 1986a; Cheng et al. 1987) and the Moon (Jull et
al. 1980; Taylor 1982) are exposed to solar particle radiation, as are
asteroidal and meteoritic materials. These are composed primarily of
silicates (e.g., basalts, feldspar, and glasses), and the meteorites can
contain volatile-rich carbonaceous chondrites. Like these chondrites, many
objects in the outer solar system contain organics which appear as dark
and/or red materials, as indicated by recent observations of comets (Keller
et al. 1987; Kissel et al. 1986), asteroids (Bell et al. 1985), and the
rings of Uranus (Pang and Nicholson 1986). Finally, the surface of to is
likely to be silicates coated with an SO$_{2}$, frost (Nash et al. 1980),
probably mixed with sulfur (Nash et al. 1986) and possibly H2S (Nash and
Howell 1989). These materials are replenished by the volcanic activity on
this satellite and contain important trace elements, such as Na, which have
been observed as neutrals around to (Brown 1974; Matson et al. 1974; Pilcher
et al. 1984). Na is also seen at Mercury (Potter and Morgan 1985) and the
Moon (Potter and Morgan 1988a, b; Tyler et al. 1988) which may be
''sputtered'' from the surface by ions or photons (McGrath et al. 1986;
Potter and Morgan 1988b). 

We will not treat each material separately. Rather, results will be
presented for classes of materials. For our purposes the solids are
classified according to their sublimation energy and band gap (roughly, the
level of the first group of excited states or bands). The size of the band
gap determines whether the material behaves in the bulk as an electrical
insulator, a semiconductor or a conductor (i.e., large, going to small band
gaps). The ion bombardment effects can often be related to these properties,
some of which are given in Table 1.3. Trace elements are treated according
to the nature of their binding in the solid. 

\section{Overview of Effects Produced}

For many aspects of charged-particle interactions with solids, certain
properties of the material are unimportant. For instance, the particle
penetration depths scale roughly as the nuclear charge of the incident ion
as well as with the atomic number density and some simple function of the
number of electrons per atom in the target material. Therefore, the
penetration depth, given as a column density of material, is nearly
identical in a gas or a solid of the same atomic composition. More
important, from the point of view of analysis, is the evolution of effects
produced by an incident ion which can be divided into rough time periods
differing by orders of magnitude. This allows a separation between causes
and effects, and indeed, a separation in treatment of types of effects. In
Table 1.4 is shown a rough breakdown in the history of events initiated by
an incident ion in a solid in which the time frames differ by orders of
magnitude. 

In Fig. 1.10 are shown those events initiated by fast ions incident on a
material, the physical stage in Table 1.4. It is seen that a number of
possible events are produced: excitation of atoms and molecules, ionization
resulting in the production of a moving electron (secondary electron), and
direct collisions resulting in moving target atoms (ions), also called
secondaries. Very energetic ions also produce nuclear reactions (Ryan and
Draganic 1986) and the resulting radiation rapidly produces excitations,
ionizations, and energetic secondary electrons and atoms. As the incident
ion and the secondary particles give up their energy, the electronic
excitations produced can be rapidly transferred between atoms in a solid. In
addition, some of the secondary electrons and atoms (ions) energized near
the surface can leave the solid (sputter). 

Following this stage of energy dispersal the resulting ''excited'' material
''relaxes'' via a number of processes. As excited states relax and electrons
recombine, molecules are dissociated, and photons may be eventually emitted
(luminescence). Because the time for photon emission is long ($\sim$ 10$%
^{-9}$ s), nonradiative processes often dominate in a solid. The production
of fragments by these processes results in an altered material, the nature
of which is determined in part by its temperature and also by the energy
density deposited by the incident radiation. 

For all cases of astronomical interest, the bombarding fluxes are small and
those processes which depend on the bombardment rate can be ignored.
Therefore, the electronic effects produced by incident photons in the first
stage of Table 1.4 are similar to those produced by fast ions and electrons.
Also, in the later stages any differences are only related to the
differences in average distribution of radicals. Therefore, to first order,
the effects of all radiations are the same. However, in the physico-chemical
stage, as ions and electrons produce multiple excitations locally, reactions
which are not likely at low background temperature may occur. Therefore,
certain products can be produced much more efficiently by ions than, for
instance, by a flux of UV radiation on a lowtemperature sample. This is well
established in radiation biology where one refers to the ''quality'' of the
radiation (Dertinger and Jung 1970). Most of the multiple excitation
products of fast incident ions and electrons are also produced by the
energetic secondary electrons. Hence, X-rays and $\gamma $-rays, which also
generate such electrons, can act somewhat differently from UV photons. These
differences are only important in the solid state, in which effects are
produced in close proximity in space and time. For highly ionizing
radiations (e.g., heavy energetic ions) new effects occur. For example,
large neutral molecules and molecular ions can be ejected from the solid to
the gas phase (Sundqvist 1989; Hedin et al. 1987; deVries et al. 1984a). \\
In the gaseous state, electronic effects produced by the dominant UV photons
are generally more important than those produced by plasma particles. As
indicated in Table 1.5, it is interesting to note that the solar photon flux
carries not only more energy than the ion flux and, hence, is a principal
source of heating of solids and gases, but also more total momentum.
Therefore, solar radiation pressure on materials can be important. However,
in the magnetospheres of outer solar system planets, the ions are often the
dominant agent, and at the orbits of Neptune and Pluto cosmic ray ions carry
more energy than the ionizing UV flux. Of course, ions always have the
unique capability of setting individual atoms in motion with relatively
large kinetic energies, a process which will be important in both gases and
solids. 

In the following chapters we discuss means of describing the effects
produced by the incident ions. The material in Chapter 2 deals with the
excitations and energy transfer produced by incident ions in collisions with
atoms and molecules, the early part of the physical stage. In Chapter 3 the
cascade of physical and chemical processes which evolve, producing an
altered material (e.g., eroded or chemically altered), are described.
Finally, Chapter 4 is concerned with the application of these materials and
effects to certain planetary and astrophysical problems. Useful planetary
parameters are given in Table 1.6, and some of these objects will be
explicitly discussed in Chapter 4. 

\section{Units}

In the material to be described, a number of scaled units are useful and are
often found in the literature. For example, I AU compares distances in the
solar system to the distance between the earth and the sun. Similarly,
distances of satellites from their parent planets are often given in terms
of the planet's radius (Table 1.2). At the other end of the size scale are
the atomic units (a.u.). These are used to scale various quantities by their
corresponding values for the hydrogen atom. That is, it is often useful to
set the size of the charge on the electron (e), the mass of the electron
(me), and Planck's constant over two pi ($\hbar $ = h /2$\pi $) all equal to
one. Then the average radius of the electron in a ground-state hydrogen atom
(a$_{o}$, the Bohr radius) is one, the average speed of the electron (v$_{o}$%
) is one, and the ionization energy of this electron (I$_{o}$ $\equiv $ - $%
\varepsilon _{o}$ = + $\hbar ^{2}$ e$^{2}$ /2m$_{e}$ = e$^{2}$/2a$_{o}$) is
one-half. This is useful because many of the atomic processes discussed
depend on the comparison of the speed of an ion colliding with an atom to
the speed of the electrons on the atom. The atomic units are avoided here so
as not to complicate the discussion, even though size comparisons are made
frequently, but the reader can find them helpful. Gaussian units are used in
order to describe electric quantities since they avoid the use of the
permeability of free space in describing the force between charges. Electron
volts are often used as an energy unit, for convenience, rather than the
standard SI units. Tables 1.7 and 1.8 give values of quantities and
conversion factors. In expressions containing the electric charge, e, it is
useful to remember that the ionization energy of the hydrogen atom is about
13.6 eV so that e$^{2}$ = (27.2 eV)a$_{0}$, an energy times a length. 

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