Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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480 Charged Particle and Photon Interactions with Matter

respectively. The (2 + 1) REMPI scheme via the E,F

∑

intermediate state was used to detect the

) products in the ground (X

∑

) state.

The LEEs are typically supplied using pulsed electron beams generated by means of commer-

cially available low-energy (5–1000 eV) electron guns. The typical full width at half maximum

(FWHM) of the electron energy distribution is <0.5eV. The focused beam size is normally ∼1 mm,

and varies little with the electron energy. Most experiments use a variable pulsed (20–200 Hz,

100 ns–25 μs) electron beam that gives electron doses of 10

−5

–10

−3

electrons/surface molecule/s.

This dose is quite small so that all desorption and dissociation events observed are due to direct

electronic transitions and have cross sections greater than ∼10

−23

. The electron ux is mea-

sured by picoammeters and is always kept below 10

electrons/cm

/s to eliminate or greatly

reduce any potential charging effects. The variation of the electron ux is performed by changing

the gun emission current; all desorption and dissociation events discussed in this chapter have a

linear dependence on the electron-beam ux. This indicates that the observed dissociation and

desorption channels arise solely from single scattering events. When necessary, the pulsed electron

beam can be focused on the target and controlled by x–y deection to scan the entire substrate

surface. Since the electron-beam spot size and ux are measured accurately, absolute cross sections

can be obtained.

18.3.3 poStirradiation teMperature-prograMMed deSorption

In addition to the measurement of accurate stimulated desorption and dissociation cross sections,

the subsequent chemistry initiated by the LEE inelastic energy-loss channels can be examined by a

technique known as postirradiation temperature-programmed desorption. This is a straightforward

approach that involves irradiating the sample with a xed dose or ux at a given electron impact

energy. After the irradiation, the lm is heated slowly (typically 8 K/min) and the desorbed prod-

ucts are detected with a QMS as a function of substrate temperature. These experiments are then

carried out for several different electron impingement energies and total doses. The results, when

compared to TPD studies of similar targets that have not been irradiated, yield information regard-

ing the electron-induced chemistry within the ice lms. A disadvantage of this technique is that it

is difcult to examine purely the radiation-driven processes, since the results are a convolution of

radiation and thermally induced reactions. Although this topic will not be discussed in detail in this

chapter, we note that an Fourier transform infra red (FTIR) spectroscopic study of the lms before,

during,

and after irradiation helps to overcome this problem.

18.3.4 poStirradiation gel electrophoreSiS

Another novel technique for the analysis of DNA damage is to remove the irradiated samples from

the UHV chamber and then apply conventional wet chemical methods to determine the extent of

damage. The technique was originally developed by Sanche and coworkers (Boudaiffa etal., 2000).

After electron irradiation, the biological materials (e.g., DNA) are removed from the chamber, redis-

solved into water, and examined ex situ using a technique known as agarose gel electrophoresis.

The supercoiled DNA single-strand breaks (SSBs) and double-strand breaks (DSBs) can easily be

separated spatially based on their different migrating abilities in agarose. Quantitative analysis of

SSBs and DSBs can be determined by integrating the light intensities of corresponding ethidium-

stained DNA bands, thus allowing the measurement of the SSB and DSB probabilities as a function

ofelectron-beam energy and dose. For the water–DNA damage studies, total electron doses of

only ∼10

electrons/DNA sample are used. This dose is two orders of magnitude lower than that

known to cause charging in DNA lms (Zheng etal., 2006) and assured a dose response in the

linear regime. The amount of DNA is quantied by measuring absorbance at 260 nm before and

after irradiation. Usually, more than 95% of the deposited mass of DNA is recovered. Three con-

trolled DNA samples are usually analyzed together with postirradiation samples: (1) an original

Low-Energy Electron-Stimulated Reactions in Nanoscale Water Films 481

plasmid DNA solution; (2) a DNA sample that was dried, transferred in vacuum, and recovered in

water

without irradiation; and (3) DSB DNA made by the restriction enzyme Sca I.

18.4 a useFul multiple sCattering Formalism

The theory we utilize to investigate the LEE-induced damage of water and water–DNA interfaces

was developed to examine diffraction effects in stimulated desorption (Sieger etal., 1999; Orlando

etal., 2004; Oh etal., 2006) and is a modication of the scattering theory used to describe photo-

electron diffraction (Chen etal., 1998) and XAFS (Rehr and Albers, 1990). A brief description of

the most relevant aspects of the calculation is given below. Specically, the electron wavefunction

is given by

φ φ φ φ φ( ) ( ) ( ) ( ) ( )r r r r r= + + + +

0 1 2



(18.1)

where

(r) and ϕ

(r) represent the non-scattered

Nth

order scattered components, respectively

The

rst-order scattering component can be represented as

φ π ρ

4( ) ( )

( )r R

k R

( )

⋅

∑∑

G t Y e

L i

i L





(18.2)

In order to solve the free-space electron propagator, G

LL′

, the separable representation has been

introduced

with the expression

L L

′

∑

( ) ( ) ( )





 

ρ ρ

Γ Γ

(18.3)

where λ is the matrix index for quasi-angular momentum. After some mathematical treatment, the

rst-order

scattering component can be expressed as

φ ρ ρ

( ) ( ) ( , )r M P

r r

k R

∑∑

⋅

 

(18.4)

where

λ λ

ρ π ρ( , ) ( )

( )

 

ji

t Yk k

ˆ ˆ

∑

4 Γ

(18.5)

is dened as the introduction matrix and M

r rλ λ

ρ ρ( ) ( )







i i

= Γ

is the termination matrix (Rehr and

Albers,

1990; Rehr etal., 1995).

18.5 low-energy eleCtron interaCtions with water interFaCes

Before we discuss the primary desorption and dissociation channels involved in LEE interactions with

nanoscale lms of water and water–DNA interfaces, a brief review of the electronic structure and

excited states of condensed-phase water is necessary. A more detailed description of the electronic

structure of water can be found elsewhere (Ramaker, 1983; Sieger etal., 1997; Herring etal., 2004).

482 Charged Particle and Photon Interactions with Matter

18.5.1 electronic Structure of water thin filMS

The real-space orbitals of water can be represented as a linear combination of the valence

molecular orbitals: the 1b

and 2a

orbitals are the primary constituents of the O–H bonds,

while the 1b

and a

make up the oxygen lone-pair orbitals (Jorgensen and Salem, 1973). The

four lowest unoccupied molecular orbitals are 4a

, 2b

, and 5a

. The 4a

and 5a

orbitals

are strongly antibonding, and occupying these states leads to breaking of the O–H bond. The

orbital mixes with the 3s Rydberg state in the gas phase, and is sometimes referred to as

3s4a

. Calculations and photoemission data show that the electronic structure of condensed

ice retains much of the gas-phase character with some broadening and shifting of the energy

levels. Since the molecular orbitals retain much of their gas-phase character, the peaks in the

ice valence-band density of states are usually labeled by the same spectroscopic notation as

free water. The molecular orbitals, the gas-phase photoelectron spectrum, and condensed-phase

photoemission data are shown schematically in Figure 18.5. The unoccupied 4a

orbital is in

the bandgap, spatially extended, and is mixed with the 2b

level. The conduction band of ice is

very narrow, with the band minimum less than 1 eV below the vacuum level. The Fermi level is

estimated to be 5 eV above the 1b

band maximum. The optical absorption spectra of ice show

a pronounced peak at ∼8.6 eV, corresponding primarily to a 1b

→ 4a

transition, well below

the photoelectric threshold (Kobayashi, 1983). In the solid state, the unoccupied molecular orbit-

als (i.e., the 4a

) maybe best described as Frenkel excitons, in which the Coulomb attraction

localizes the electron–hole pair on the water molecule. This may be particularly important at

surfaces and terminal sites.

The largest change in the condensed phase occurs at the levels of a

symmetry, which are

signicantly broadened (compared to the 1b

and 1b

levels), and are most affected by hydro-

gen bonding. Calculations of the ice band structure indicate that the a

bands have the most

dispersion, while the 1b

and 1b

bands are virtually dispersionless. One would then expect

that electronic excitations involving a

bands would be the most sensitive to changes in the

hydrogen-bonding environment. Specically, a reduction of the bandwidth should occur if the

–10

–20

–30

–40

SolidVapor

Energy(eV)

3s4a

Figure 18.5 Left frame: Comparative relationship between the gas-phase and condensed-phase photoelec-

tron spectra of water. Right frame: The similarity allows for extension of the gas-phase state labels to the solid

phase.

Molecular orbital density plots of the valence levels for the water molecule.

Low-Energy Electron-Stimulated Reactions in Nanoscale Water Films 483

hydrogen bonding is weakened, and the lifetimes of electrons and holes in these bands should

increase as a consequence.

The effect of temperature on the photoemission spectra of ice has been measured and is

shown in Figure 18.6. It is clear that the a

bands are the broadest; however, the broadening

associated with the 3a

level emerges into a reasonably well-dened splitting as the temperature

is increased. The 3a

molecular orbital is composed primarily of the oxygen lone-pair orbitals,

is strongly involved in hydrogen bonding, and is very sensitive to the local geometry of nearby

water molecules. The splitting and narrowing of the 3a

level is likely due to the reduced per-

turbation on the a

levels and the general return of the atomic P

character as the number of

hydrogen bonds drops with increasing temperature. For a symmetric pair of water molecules in a

unit cell of ice, the 3a

orbitals directly interact with each other giving rise to a bonding and anti-

bonding combination (Nordlund etal., 2008). Alternatively, this splitting can also be described

in terms of Davydov splitting due to the fact that ice has a nonzero entropy even at 0 K (Petrenko

and Whiteworth, 2002). Thus, adjacent unit cells in the lattice are symmetry-inequivalent and

can have nonzero overlap integrals allowing for mixing of the a

bands between cells (Winter

etal., 2004).

Though it is difcult to discern this in Figure 18.6, the bands, particularly the 2a

level, shift ∼1eV

to lower energy (relative to the Cu substrate) as the temperature increases. This can be attributed to

the change in the work function. Note that the orientation of water molecules in the surface dipole

layer collectively contributes to the work function. As the temperature increases, the net orientation

of these molecules changes, giving rise to many more water molecules with dangling OH bonds

pointing into the vacuum.

40 ML amorphous D

O/Cu(111)

hν = 55 eV

20 30 40 50

Electron kinetic energy (eV)

180 K

110 K

Cu d-bands

Temperature

Photoemission intensity (arb. units)

and 3a

narrow

with increasing temp

Figure 18.6 Temperature-dependent photoemission data of 40ML amorphous ice deposited on Cu(111)

from

110 to 180

484 Charged Particle and Photon Interactions with Matter

18.6 stimulated dissoCiation and desorption

F w

ater

hin

Films and i

nter

aCes

18.6.1 lee-StiMulated deSorption of anionS

TNI resonances result when the ground or excited electronic states of molecular collision partners

temporarily capture an incident electron during the electron–molecule collision. The simplest

TNI is produced when the incident electron is trapped by the centrifugal potential arising from

the interaction between the incident electron and the neutral molecule in its ground electronic

state. These are referred to as single-particle shape resonances. Shape resonances usually lie at

low energies (∼E

< 5 eV) and are energetically above the potential energy surfaces of the neu-

tral molecule ground states. These resonances typically decay via autodetachment leaving the

neutral moleculewith vibrational and rotational excitation. Another important decay pathway is

the DEA, which involves the formation of anionic and neutral fragments. Shape resonances can

also involve electronic excitations, and these core-excited shape resonances involve an attractive

interaction between the incident electron and the excited state of the target molecule. Since the

potential barrier is strongly dependent on the angular momentum value of the excited-state orbitals,

p-, d-, and f-waves are expected. These resonances also lie above the neutral excited-state ener-

gies and decay by autodetachment or DEA.

Another type of a core-excited resonance is referred to as the Feshbach (Type-I) resonance. This

involves coupling of the kinetic energy of the captured electron to nuclear motion. If the incident

electron excites a valence or shallow core level, a dynamic polarization is produced and the electron

is temporarily trapped in the dipole eld of the excited target molecule. Electron correlation and

reduced screening allow the incoming electron to couple to the excited electron and a slightly posi-

tive core, producing a one-hole, two-electron core-excited Feshbach resonance. These resonances

lie below the corresponding excited states (i.e., they have a positive electron afnity). The energy

gained in this correlation is referred to as the Feshbach decrement, and is typically ∼0.5eV. As

depicted in Figure 18.7, the transition is typically mediated by Franck–Condon factors and the cross

section is mediated by lifetimes against autodetachment and dissociative attachment (Ingolfsson

etal., 1996). The negative-ion fragments are often produced with an excess kinetic energy, which

can be partitioned into reactive scattering events with coadsorbed molecules or surface terminal

Potential energy

AB*

–

AB*

A + B

–

EA(B)

D(AB)

–

Anion yield

AB bond coordinate

Incident electron energy

Figure 18.7 Schematic of the potential energy surfaces accessed during DEA processes. Inelastic electron

scattering results in a transition to an excited (dissociative) potential surface that is asymptotic to the anionic

decay

channels.

Low-Energy Electron-Stimulated Reactions in Nanoscale Water Films 485

groups. As illustrated in Figure 18.7, the conservation of energy and momentum yields the most

probable

kinetic energy of the departing anion fragment, E

−

, as

E E

−

= − + − − −( ) [ ]1 EA(B) D(AB) D(AX)β ε

(18.6)

where

is the ratio of the mass of B

−

to that of AB

is the captured electron’s energy

EA(B) is the electron afnity of B

D(AB)

is the bond energy of AB

is the excitation energy of the fragments (Christophorou etal., 1984)

is apparent that DEA favors the loss of the light fragment.

The

effects of the condensed phase on the DEA and TNI resonances have been studied and several

aspects are well understood. Specically, the DEA and TNI resonances can be drastically altered due

to third-body interactions that open pathways for energy dissipation (Bass and Sanche, 1998, 2003a,b;

Sanche, 2000). These interactions can have an effect during the lifetime of the TNI as well as alter

products after the TNI dissociates. Most notably, dissociation from the low-energy shape resonances

is suppressed, while dissociation from the Feshbach resonances can be enhanced by the surrounding

medium.

Although there is no detection of anions from the low-energy shape resonance, dissociation

could still occur where the products simply do not gain sufcient kinetic energy to escape the surface

potential. Mechanisms for enhancement in dissociation can be explained by the effects of the medium

on the electron autodetachment probability. One mechanism for dissociation enhancement through the

Feshbach resonance is the lowering in energy of the anionic potential energy surface via the substrate

and surrounding medium polarization interaction (Balog etal., 2004). Due to the lowering of the ionic-

state potential, the wavepacket on the TNI potential spends less time traversing it, thus lowering the

autodetachment probability. Another explanation for enhanced dissociation in the condensed phase is

the more efcient conversion of an open-channel resonance into a Feshbach resonance due to interac-

tions with the medium (Balog etal., 2004). An open-channel resonance lies higher in energy than

the Feshbach resonance and only requires a one-electron transition to return to the associated neutral

molecule compared to the two-electron transitions needed for the Feshbach resonance. The electron

autodetachment probability is higher for the one-electron transition compared to the two-electron

transitions; thus, the Feshbach resonance is more likely to dissociate than autodetach.

The formation of H

−

) from the DEA of water H

O (D

O) was studied extensively in the gas

phase (Compton and Christophorou, 1967; Melton, 1972; Jungen etal., 1979; Curtis and Walker,

1992; Fedor etal., 2006), and some of these recent results from Fedor etal. are shown in Figure

18.8a (Fedor etal., 2006). There is a strong resonance with a threshold energy near ∼5 eV, a peak at

6.7eV, and a weak resonance with a peak near 9.7eV. The yield of H

−

) from ASW (i.e., nanoscale

multilayers of water) condensed on Pt(111) has also been studied over the past several years. Our

results from these experiments are shown in Figure 18.8b (Simpson etal., 1997). The main feature

peaking near 6.7eV remains although the peak shifts to ∼7.3eV in the condensed phase. The width

this feature is also broader, and there is a resolvable weak structure near 10

eV.

The rst feature observed in Figure 18.8a involves excitation of the 1b

electron into the mixed

3s4a

level, giving rise to a

, one-hole, two-electron (…1b

−1

3s4a

) core-excited Feshbach reso-

nance. Since the 4a

level is dissociative, this leads to facile bond breakage. The detailed dynamics

of the

DEA channel has been calculated using accurate potential surfaces (Haxton et al.,

2007a,b). The second, much weaker feature, may be due to the excitation of the 3a

electron into

the mixed 3s4a

level, giving rise to a

, one-hole, two-electron (…3a

−1

3s4a

) core-excited

Feshbach resonance. This state mixes with the lower energy

state via nonadiabatic curve

crossings, andalso dissociates due to the 4a

antibonding level and probably some admixture

containing the 2b

character.

486 Charged Particle and Photon Interactions with Matter

As can be seen in Figure 18.8, the yields of H

−

) in both the gas phase and the condensed

phase are very low for excitation energies above 10eV (Simpson etal., 1997). This is largely due to

the fact that the higher-energy resonances associated with excitations from the 3a

and 1b

levels

decay to primarily form O

−

and H

. The O

−

yield has been measured in the gas phase (Compton

and Christophorou, 1967; Melton, 1972; Jungen etal., 1979; Curtis and Walker, 1992; Fedor etal.,

2006), whereas the neutral molecular hydrogen channel was measured from multilayer water sam-

ples condensed at 90K. These are also shown in Figure 18.8c. The two resonances dominating

the O

−

yields in Figure 18.8 are one-hole, two-electron core-excited Feshbach resonances, and are

formally assigned to the

and

states. We have demonstrated that the molecular hydrogen

produced from water multilayers via the

and/or

resonances contains only even quanta of

vibrational energy (Kimmel and Orlando, 1996). Since the vibrational and rotational quantum-state

distributions of the departing H

are identical from E

= 8–14eV, it is very likely that the

and/or

resonances are strongly mixed, particularly in the condensed phase.

Based upon the β parameter discussed above, the O

−

is not expected to leave the surface of mul-

tilayer water ice. However, it is very probable that this product, as well as the OH product, reacts

within the ice or water directly at an interface or surface. In the case of water at a DNA interface,

this

can lead to facile DEA-induced damage of DNA.

18.6.2 lee-StiMulated deSorption of cationS

Figure 18.9 shows the cations produced and desorbed during electron impact of pristine ice.

Adetailed description of the cation yields from pristine ice can be found elsewhere (Herring-

Captain etal., 2005). Briey, the cation yield is dominated by H

with a much smaller yield of H

The H

and H

yields increase with increasing growth and substrate temperature with ratios of

1:2:3 (PASW:ASW:CI) for H

and 1:1:1.5 for H

. There is also a very reproducible protonated

3 4 5

(a)

(b)

6 7 8 9

(c)

(d)

–

from condensed D

O D

from condensed D

–

from gas-phase D

O O

–

from gas-phase D

10 11 12 13

Incident electron energy(eV) Incident electron energy(eV)

14 15 5 6 7 8 9 10 11 12 13 14 15

Figure 18.8 Comparison of DEA resonances for gas-phase water (Fedor etal., 2006) and ice lms (Simpson

etal., 1997). (a) and (b) show a highly correlated resonance structure for D

−

desorption. (c) and (d) show that

desorption from a condensed phase correlates with excitation to the

state (Kimmel and Orlando, 1996).

Low-Energy Electron-Stimulated Reactions in Nanoscale Water Films 487

cluster ion signal from PASW, ASW, and CI. The cluster yield from PASW is typically ve to

six times larger than CI, and the ASW cluster yield is about two to three times larger than CI

(Herring-Captain etal., 2005).

The threshold energies for electron-stimulated production and desorption of H

and H

from CI,

and H

O) from PASW and ASW are 22 ± 3 eV. There is also a H

yield increase near

40± 3 eV and an ∼70eV threshold for H

n=2–8

for all the phases of ice studied. These threshold

measurements are shown in Figure 18.10 and are very useful in determining the dominant excited

states

and excitations involved in cation desorption.

18.6.2.1

mechanisms

for lee

-stimulated

Cation d

esorption

Of the two-hole and two-hole, one-electron congurations, at least four are known to produce protons

from ice with kinetic energies ranging from 0 to >7eV (Sieger etal., 1997; Sieger and Orlando, 2000;

×50

×300

×150

×40

10 15

Time of ﬂight (μs)

20 25 30

×20

×50

80 K PASW

= 250 eV

110 K ASW

= 250 eV

140 K CI

= 250 eV

n = 1

n = 2

n = 3

n = 2

n = 3

n = 4

n = 5

n = 4

n = 3

n = 2

n = 1

n = 6

n = 7

n = 6

n = 5

n = 8

n = 5

n = 4

n = 6

Figure 18.9 Cation ion yields from crystalline, amorphous, and porous amorphous ice lms. Pulsed irradia-

tion stimulated desorption using a 250eV incident electron energy produce protonated water clusters H

The yields dramatically increase with the disorder in the lm structure.

488 Charged Particle and Photon Interactions with Matter

Herring etal., 2004; Herring-Captain etal., 2005). These congurationally mixed two-hole excited

states and two-hole, one-electron excited states have excitation energies from 21 to 70eV. The linear

dependence of the proton yield on dose as well as the threshold energy of 21–25eV indicated that

low-kinetic-energy (∼4eV) proton desorption is the result of two-hole states occurring at the surface

of the ice. These have been assigned to (3a

)

−1

(1b

)

−1

(4a

)

nal states of terminal water molecules

with

dangling bonds. There is a secondary threshold energy of ∼40

eV,

which may correspond to the

(3a

)

−2

(4a

)

and (1b

)

−2

(4a

)

states of water, the latter producing protons with kinetic energy up to 7eV.

These energetic protons (>7eV) can also be produced from the (2a

)

−2

conguration, which can be

produced at incident electron energies of >70eV.

All of these two-hole excited states and two-hole, one-electron excited states involve bands of a

symmetry. As previously discussed, levels with a

character should be sensitive to the local bond-

ing environment of the water molecule (Herring-Captain etal., 2005). Therefore, the temperature

dependence of the proton yield (not shown) gives insight into the local conguration of the surface

of the ice and the dangling bonds responsible for proton production. At low dosing temperatures, the

random orientation of the surface-water molecules leads to a low proton yield for PASW compared

to CI, most likely because the proton undergoes reactive scattering across the ice surface or into

the bulk (Sieger and Orlando, 1997). As the temperature increases, the degree of coupling between

water molecules decreases and the mobility of surface-water molecules increases (i.e., they have

sufcient energy to rotate and start to diffuse) (Akbulut etal., 1997). The reduced coupling and

emergence of a split 3a

level discussed above lead to band narrowing, increased excited-state life-

times,

and increased proton emission (Sieger etal., 1997; Herring etal., 2004).

desorption has similar threshold values compared to H

, but the yields are much less. Although

the branching ratio is small, some of the above-mentioned two-hole states and two-hole, one-

electron states responsible for proton desorption can decay to form H

. This has been observed in the

10 20 30 40

140 K Cl

80 K PASW

0 20 40 60 80 100

Electron energy (eV)

Ion count(arb. units)

120 140

(a)

n =2

(b)

Figure 18.10 Threshold energies for electron-stimulated desorption of protonated water cation clusters

from

crystalline and amorphous ice.

Low-Energy Electron-Stimulated Reactions in Nanoscale Water Films 489

time-correlated dissociation of doubly ionized states of water, most likely through a dissociative triplet

state (Tan etal., 1978). Photoionization near the O 1s → 2b

resonance (Piancastelli etal., 1999) was

reported to produce H

. The absorption of four photons to produce H

∑

( )

was also reported in

a recent multiphoton ionization study of gas-phase water (Rottke etal., 1998). The H

yield observed

from these ices is large relative to the gas phase studies. This indicates that many body interactions

and orientational defects may play a role in its production. Reactive scattering of the energetic protons

may also be a source of H

The mechanism for cluster ion desorption involves the production of two holes and a Coulomb

explosion resulting from these holes in neighboring water molecules. A detailed description of this

model can be found elsewhere (Herring-Captain etal., 2005). Briey, the weak ∼25eV threshold for

O) formation is most likely due to the reactive scattering of a proton (Bernholc and Phillips,

1986). The primary 70eV threshold for clusters with 1–7 water molecules may initially correspond

to the 2a

−2

state. Coupling of this doubly ionized water molecule to a neighboring water molecule

leads to a nal state containing one positive charge on each of the two water molecules, most likely

with a OH

…H

O) conguration. The surrounding water molecules respond to the charges

by reorienting in an attempt to solvate the charges, and nally the unstable conguration undergoes

Coulomb explosion, resulting in desorption of a protonated cluster.

The

temperature/phase dependence of the cluster ion yield provides insight into the local hydro-

gen bonding in the terminal water layers. While the proton yield reects the behavior of the surface

molecules in a direct line of sight to the vacuum, the clusters reect the behavior of the terminal

1–3 water layers. At low dosing temperatures, the water molecules are randomly oriented and there

is no long-range order. The localization of two holes can cause dissociation and proton transfer

(or hole hopping) along the hydrogen bond, forming a hydronium ion. The holes are then localized

on neighboring water molecules. Due to the lack of an ordered hydrogen-bonding network in PASW,

the holes are less likely to be able to hop to a screening distance (1–2nm) before a Coulomb explo-

sion occurs. This “localized hole hopping,” where a hole hops to a neighboring water molecule but

not far enough to be screened, occurs in the terminal 1–3 water layers of PASW and results in a

large cluster yield. As the temperature increases, a more crystalline structure is formed (Smith etal.,

1996; Sieger etal., 1997) and the holes are more likely to hop further away from each other through

the hydrogen-bonding network. CI has the most extensive hydrogen-bonding network of the samples

studied

here and the lowest cluster yield, due to the efcient hole hopping through this network.

18.6.3 lee-StiMulated deSorption of neutral fragMentS and productS

It is well known that ion desorption accounts for only a small fraction of the total mass loss during

electron bombardment of materials and surfaces. In ice and more strongly coupled materials, this

is due to hole hopping, efcient autoionization, and electron–hole recombination. In the latter case,

the lowest-energy “excitons” formed as a result of hole (ion)–electron recombination can localize at

the surface or in the near surface zone, and dissociate to produce atomic fragments such as H (

S) and

O(

D, etc.) (Kimmel and Orlando, 1995; Orlando and Kimmel, 1997) and molecular products

such as hydrogen (Orlando etal., 1999) and OH. The threshold energies for producing the neutral

atomic hydrogen and oxygen fragments are shown in Figure 18.11. The experimentally observed

value of ∼6.5eV (relative to the vacuum level) is lower than the 9.5 and 11.5eV thermodynamic ener-

gies required to produce O(

) + 2D(

S) and O(

D) + 2D(

S), respectively. The low threshold value

indicates that the formation of atomic oxygen must occur via a pathway that involves a molecular

elimination step. Since the molecular hydrogen is in the

∑

( )

ground state, the observation of

both singlet and triplet atomic oxygen demonstrates that singlet and triplet excitons are involved.

This is consistent with exciton formation via hole–electron recombination and the fact that the

triplet states can be directly excited during electron scattering since the transitions are not governed

optical selection rules.