Electronic sputtering of low temperature molecular solids

307

Nuclear Instruments and Methods in Physics Research Bl (1984) 307-314 North-Holland. Amsterdam

Section I?? Condensed gases ELECTRONIC

SPU’I-IERING

OF LOW TIZMPERATURE MOLECULAR

W.L. BROWN, W.M. AUGUSTYNIAK,

K.J. MARCANTONIO

SOLIDS

and E.H. SIMMONS *

Beli L&oratories, Murray Hill, New Jersey 07974, USA

J.W. BORING, R.E. JOHNSON and C.T. REIMANN Department of Engmeerrng Physics, Universtty of Virginia **, Charlottesville, Virginia 22903, USA

Electronically stimulated sputtering of insulating molecular gas solids is a remarkably efficient process at excitation densities accessible by MeV light ions and keV electrons. This paper concentrates on the cases of CO and H,O (D,O). The approximately quadratic dependence of sputtering yield on the excitation density along individual particle tracks observed earlier for incident MeV ions has also been found for incident keV electrons in the case of CO. Coupled with time-of-flight energy spectra of ejected D,O from solid D,O, this behavior leads to a picture of rapid electronic relaxation with molecular repulsion involving pairs of molecular ions. We also report a dependence of CO sputtering yield on incident angle for MeV He ions which varies as (cos f3)-‘.6, in qualitative

support of the multiple ion picture. In addition to ejection of the principal molecular species of a solid film, electronic excitation of molecular solids even at very low temperatures leads to formation of new molecular species by bond disruption and fragment rearrangement. activated.

1.

In the case of D,O the dominant

new molecules

Introduction

Conventional sputtering of metals is initiated by direct transfers of momentum in elastic collisions of incident ions with the atoms of a solid [l]. In metals and narrow band gap semiconductors this is the only mechanism that is effective. However, in insulators an additional path for sputtering exists through electronic excitations 121.The relatively large units of energy that are involved in electronic excitations in these materials, with electronic band gaps of l-10 eV, are one major factor in this new regime. A second is the relative localization of excitation in insulators in contrast to that of electrons in metals. Electronic sputtering has been observed in a number of insulating materials, including condensed molecular [2-101 and rare gases [ll-131, alkali halides [14-171 and an increasing number of other room temperature insulators [I&21] The studies have primarily been made using fast ions to introduce electronic excitation, but electrons have been used in several cases including one to be discussed in this paper. The critical question in all cases is how energy, initially introduced into the electronic system, is transferred to the kinetic energy of motion of the atoms of molecules required for ejection from the surfaces of their solids. Because of the low surface binding energy * Currently, Department of Physics, Harvard University. ** Work at University of Virginia was supported in part by NSF Grants AST-82-00477 and DMR-82-11555. 0168-583X/84/%03.00 0 Elsevier Science Publishers B.V. (Norm-Holl~d Physics Publis~ng Division)

are Dz and 0, whose ejection from the film is strongly thermally

of condensed molecular and rare gases (varying from - 0.08 eV for CO, to - 0.5 eV for H,O) they provide particularly sensitive cases for study. In almost all solids that have been examined, the sputtering yield (the number of atoms or molecules ejected per incident particle) varies superlinearly with the density of electronic excitation along the path of individual incident particles. In a number of cases, including those of H,04 and CO” to be discussed in this paper, the relationship is approximately quadratic. Some groups have primarily concentrated attention on sputtering due to very heavily ionizing high energy heavy ions for which the dependence of yield on deposited electronic energy density may be even steeper [5,19]. Others have used primarily hydrogen and helium ions at energies of the order of 1 MeV and a few have used l-5 keV electrons [13] which have electronic excitation densities along their tracks comparable to or less than those for 2 MeV protons. In some cases the sputtering yield by electrons has been reported to be approximately linear in the excitation density 1131. This paper will concentrate on the low ionization density regime. Electronic sputtering of insulators has connection to a diversity of other fields: in the low ionization density regime it is closely related to electron and photon stimulated desorption, ESD and PSD [22-243. ESD and PSD have attractive specificity in the electronic excitation they introduce, but they do not easily provide a means for observing the nonlinearity in desorption (or sputtering) associated with simultaneous excitation of IV. CONDENSED GASES

308

WL

Brown et al. / Electronic sputtering of molecular solids

more than a single site in the solid. Track formation in minerals was a major stimulation for studies of sputtering by high energy heavy ions in refractory insulators [25]. The formation of defects in crystalline insulators and semiconductors by electronic excitation (most notably in the alkali halides but more recently in many other materials) involves similar considerations of how the electronic energy is relaxed to provide new atomic structures not initially in a solid [26]. The changes in polymers due to electronic excitation are further examples in which molecular rearrangement and new radical and molecule formation occur as the electronic system relaxes [27,28]. Some of these cases have considerable technological importance, while others have scientific implications ranging from planetary science and astrophysics [29] to biochemistry. This paper presents recent results on electronic sputtering of CO and H,O (D,O) and discusses the insight these give concerning the mechanisms involved.

OEFINING APERTURE (4-2 mm OIA)

ANNULAR SILICON SURFACE BARRIER PARTICLE DETECTOR 1 11

/

EbICKSCATTEREO HELIUM ION

(al SPUTTERED PARTICLES

“ICE” FILM (.4-IptTI

THICK1

COLD SUBSTRATE

OFF AXIS DCDRPULSED MULTIPLIER

ASS SELECTOR

2. Experimental approach In all cases the solids being studied were prepared as thin films typically 0.1-l pm in thickness by condensation of room temperature gas streaming onto a cryogenically cooled metallic substrate in an ultrahigh vacuum chamber. We have used three types of measurements in this work. They are sketched in fig. 1. Fig. la illustrates Rutherford backscattering of MeV helium ions which is used to measure the absolute total sputtering yield by observation of the thickness (actually the number of atoms or molecules per square centimeter) of the film [2]. The ions responsible for eroding the film may be the same as those used to probe the thickness or they may be different in ion species or energy. These measurements give no information about the nature of the ejected particles. Fig. lb illustrates the use of a quadrupole mass spectrometer. The incident beam may be steady and the quadrupole used to measure the masses of the sputtered particles and the relative sputtering yields as a function of particle energy, film thickness, total fluence or temperature. Studies have concentrated on the sputtering of neutral particles (the dominant species) so an electron impact ionizer immediately precedes the quadrupole mass selector. This method is quite sensitive but does not easily provide absolute sputtering yields. The arrangement of fig. lb may alternatively involve a pulsed beam and time-of-flight measurements for determining the velocity (energy) spectra of the sputtered particles. A pseudo-random pulsing system is used on the incident ion beam to improve the signal-to-background [30] and cross-correlation techniques are employed to extract the time-of-flight spectra. Signal-tobackground in quadrupole measurements is often im-

SPUTTERED

PARTICLES

ICE* FILM (.4-4pLm THICK) OLD SUBSTRATE

Fig. 1. Schematic of the two experimental measurement arrangements: (a) for Rutherford backscattering to measure absolute total erosion yields by reduction in thickness of thin film “ices”. (b) For measurements of specific particle ejection and velocity distributions of ejected particles using a quadrupole mass spectrometer. For velocity distributions, time-of-flight (TOF) measurements are made using pseudo-random pulsed beams.

proved by using films formed from isotopically labeled molecular gases to place the sputtering particles at masses different from those of dominant background species in the ultrahigh vacuum. Hence D,O rather than Ha0 has been used in the experiments discussed in section 4. Sometimes singly or doubly labeled CO (13C?0 for example) is also used, but this was not necessary in obtaining the results of section 3.

3. Carbon monoxide ice The sputtering yield for carbon monoxide films at 10 K is shown in fig. 2 as a function of the electronic energy loss, (dE/dx),, of various incident particles. Rutherford backscattering measurements of absolute yield (fig. la) were made for helium and hydrogen ions

W.L. Brown et al. / Electronicsputtering of molecular solicis

(dE/dx), of 1 keV electrons (4.5 eV/lOt’ molecules/crna) [32]. The energy dependence of the electron yield is well described by the drawn line of slope 2 and we are led to question the tentative value of the single 2 keV point. A quadratic dependence of yield on electronic energy loss has been discussed from the point of view of several models. Perhaps the simplest of these is the sublimation model [3,33]. It assumes that there is a rapid (less than lo-” s) conversion of electronic excitation into heat. The model does not directly address the issue of the mechanism by which this rapid conversion takes place, nor what fraction of the total electronic excitation is rapidly converted. Cylindrical symmetry is needed to produce a quadratic dependence. In addition, the quadratic dependence of this model requires that as the “heat” associated with conversion of the electronic energy for each individual ionization event spreads out and joins with that from neighbors to approximate a hot line, the energy per molecule must be large compared to the sublimation energy of the material [34]. See ref. 33 for a full treatment of these assumptions. The geometry of this model is illustrated in fig. 3a. As the (dE/dx), decreases, at the low end of fig. 2, the separation between in~~du~ ionization events in-

co

4-3.5

I

1

rtev e-

I

II

4

8 I

40 (dE/dXIe

309

ev/4015

I

400

ri40LEcuLEs/cm2

Fig. 2. Sputtering yields of CO at 10 K as a function of electronic energy loss for He+, H+ and electrons. The 1-3.5 keV electron data have been normalized to the He+/H+ line at 1 keV.

[lo]. The straight line in this log-log plot is drawn with a slope of 2. It represents the four ion points relatively well, indicating that the sputtering yield is approximately proportional to the square of the electronic stopping power. Also shown in the figure is a single absolute (but tentative) yield value determined by Schou and Sorenson [31] for 2 keV electrons. It has been placed at a (dE/dx), value according to Berger [32]. This value is approximately 20% less than the {dE/dx), for protons of the same velocity at which this point was plotted in ref. 10. The point now falls well above the drawn line and suggests a possible transition to a linear rather than a quadratic dependence on (dE/dx),. Finally, there is a set of points taken with a quadrupole mass spectrometer using electrons of different energies between 1 and 3.5 keV. These are not absolute measurements; they have been normalized to the ion line at the

fC)

(b)

Fig. 3. Schematic illustration of models of the sputtering processes: (a) time expanding spheri& “hot spots” merge to form a hot cylinder. Transient sub~ation occurs from the te~nation of the cylinder at the surface. (b) Individual time expanding “hot spots” are separated too far to allow overlap as in (a). Transient sublimation occurs at the intersection of a “hot spot” with the surface. (c) Two ions couple to form a repulsive molecular state which may be viewed either as repulsion due to dissociative recombination or as Coulomb repulsion. IV. CONDENSED

GASES

310

W.L Brown et al. / Electronic sputtermg of molecular soliak

creases and the condition of overlapping to form a sufficiently hot cylinder becomes increasingly difficult to satisfy. The 3.5 keV electron point corresponds to a (dE/dx), of 1.95 eV/1015 CO molecules /cm2 [32]. With a value of - 34 eV as the average energy to create an ion- electron pair in CO, a 3.5 keV electron creates one pair approximately every seventeen molecular layers on the average, or approximately one pair each 50 A along the particle path. Even if all the electronic energy input were converted into heat in a cylinder of diameter 50 A, the energy per molecule would be only 0.01-0.02 eV, much less than the 0.08 eV sublimation energy of co. If the conditions for a hot cylinder are not met, transient sublimation can still occur due to a single hot spot close to the surface (fig. 3b). However, the expected result in this case is a sputtering yield linear with (dE/dx), [33]. There is no hint of a break toward a linear response with (dE/dx), at low (dE/dx), in the data of fig. 2. Thus transient sublimation of CO based on hot spots centered on individual ionization sites is unable to account for the quadratic dependence of S on (d E/dx),. It is clearly desirable to make measurements at still lower (dE/dx), using electrons with energies greater than 3.5 keV (or using protons with energies greater than 7 MeV) to see if and where a transition from quadratic to linear dependence does occur. Dissociative recombination is a possible way by which electronic excitation energy may be rapidly converted to atomic motion by an electronic transition to a repulsive (or antibonding) state. This case has been discussed by Johnson and Inokuti [35] for argon in terms of formation and decay of molecular excitons. The repulsive energy given to a molecule in such a case would be rapidly shared with neighboring molecules by collisions. This process could serve as one of the local heat sources for the sublimation model. Alternatively, if the repulsive event is sufficiently close to the surface, a molecule may be ejected either directly, or through a mini-cascade initiated by the repulsion without having to go through the statistical mechanisms of a quasi-thermal distribution. See Johnson and Brown [33] for a fuller treatment of this case. Applying the idea of repulsive electronic recombination at an isolated ionization site produces a yield linear in the electronic stopping power. The quadratic behaviour found for CO suggests that the appropriate repulsive configuration in solid CO requires two closely spaced excited or ionized molecules. This idea is sketched in fig. 3c. The quadratic dependence in this case arises from the simple statistical probability of having one ionization event adequately close to the surface and a second adequately close to the first. Fig. 3c actually looks like an illustration for a Coulomb explosion model of sputtering [33,36] in the limit of low ionization density. If the electrons ejected

from ions in ionizing collisions remained sufficiently separated from the ions for - lo-l2 s, Coulomb repulsion could push a molecule near the surface out of the solid [2]. Electrons are not likely to remain separated from the ions for that long, but when the electrons do return they will be in highly excited states, and hence will only partly shield the ionic charges. Two ions and their electrons interacting in this way is another description of a coupled 2-molecule repulsive state. If the coupling of two molecular ions provides an efficient dissociative recombination path (a local Coulomb explosion), the energy of the repulsive recombination could be a source for transient sublimation. The quadratic (dE/dx), dependence would then come, not from cylindrical geometry, but from the statistics of production of the two-ion configuration. In this limit of low ionization density [low (dE/dx),] the distinction between a spherical thermal “spike” and a mini-collision cascade, each initiated by repulsive electronic deexcitation, lies in the energy distributions of the ejected particles. To attempt to clarify the origin of the quadratic yield dependence on (d E/d x), , we have measured the angular dependence of the total sputtering yield, in this instance using 1.5 MeV helium ions corresponding to one of the high-yield points of fig. 2. The results are shown in fig. 4. The quadrupole mass spectrometer shows that the overwhelming dominant ejected species are CO. However, the data of fig. 4 are from total system pressure increases produced by CO sputtering and measured by an ionization gauge, not by the

TOTAL FROM

PRESSURE Y IELO CO ICE ; IOK

Fig. 4. Angular dependence of total CO ejection at 10 K by 1.5 MeV He+ ions measured by total pressure increases. Quadrupole measurements indicate that CO molecules are the overwhelmingly dominant ejected species.

311

W.L. Brown et al. / Elecironic sputtering of molecular solids

quadrupole. The dependence on target angle observed in the quadrupole is more complex because it includes both the dependence of sputtering yield on incidence angle and the angular distribution of the sputtered particles (fig. lb). A more complete treatment of this case is planned for separate publication. The fitted curve of fig. 4 has a dependence of (cos 19)-‘.~. An approximately (cos 19-i would be expected if sublimation from a hot cylinder were responsible for the sputtering yield since the area of the intersection of the cylinder with the surface increases by that factor. On the other hand, since all ionization events along a particle’s path are moved closer to the surface as B increases, if two molecular ions are required for a repulsion state to produce sputtering of CO, a (cos 8)-2 might have been expected. However, if only one ionization event must be close to the surface, and the second event only needs to be close to the first to participate in the two-ion repulsive state, the dependence might be expected to be (cos 0)-l. The intermediate angular dependence is a tantalizing indication that the ejection efficiency of the ejecting state has a more complex dependence on distance to the surface than either of these simple models but that a single ionization event is not enough. A (cos 8)-‘.6 dependence is similar to the angular dependence found for collision cascade sputtering in the nuclear stopping regime for incident heavy ions in light targets [1,37]. In that case, it arises because the secondary particles set in motion by primary collisions are directed forward in a cone about the track. These secondaries contribute increasingly to sputtering as the track is tilted away from the surface normal. Although there is some forward asymmetry in the distribution of electrons ionized by MeV helium ions, it is a small fraction of the total ionization. The contribution it makes to the angular dependence of electronically stimulated sputtering should thus also be small. Multiple scattering of the incident electrons to form a broadening cone with penetration is small for low 2 films unless ionizing events deep in the films are contributing to sputtering. This is not the case for CO and D,O, but may be significant in solid argon. Backscattering of the electrons plays no singular role (as it does in the nuclear collision regime) because electronic excitation, not momentum transfer, is responsible for sputtering.

4. H,O, D,O ice It has previously been shown that the absolute sputtering yield for water ice [4] also has an approximately quadratic dependence on electronic stopping power as determined using Rutherford backscattering measurements of film thinning. Those data are reproduced in fig. 5. No transition from quadratic to linear

loo-

/ /

YatdE/dr12 . ./

n

IO-

g

-

3 ;r

-

I

.

fB P

/

P:

2

.

I.0 -

HHE

AC 00 0 F

./ ‘0

. .

.

/ 0.1:

/

I -

/

001’

’

I

’

“I

IO

’

’

“’

loo

’

’

“1

no0

Fig. 5. Erosion yield of water ice as a function of electronic energy loss. The figure is reproduced from ref. 4.

dependence is apparent in the low (dE/dx), region in this case either, although the lowest (dE/dx), value at which the yield was measured was - 4 eV/1015 H,O molecules/cm2, not as low as for CO. However, because the sublimation energy of H,O is seven times larger than that of CO, the energy per molecule as it is shared to form a hot cylindrical volume would again be much less than the sublimation energy. The temperature dependence of the absolute sputtering yield of H,O ice has also previously been reported [38]. The published curve is reproduced in a curve (a) of fig. 6. A wide temperature independent region exists at temperatures below - 110 K with a gradual turn-up to higher yields at higher temperatures. The equilibrium vapor pressure of ice at 155 K, the highest temperature of the curve, is - 2 X lo-’ Torr. At this temperature, in addition to the particle induced sputtering, thermal sublimation produces film thinning at a rate of apIV. CONDENSED

GASES

312

W.L.. Brown et al. / Electrome sputtering of molecular sohds

proximately 2 X 1014 molecules/cm* sec. This contribution was directly measured and subtracted to produce the results in the figure. Because these temperature dependent yield data are from Rutherford backscattering they give no information on the identity of the sputtered particles. Quadrupole mass spectroscopy identified D,O (mass 20), D, (mass 4) and 0, (mass 32) as the primary ejected species from D,O ice [4]. D,O rather than H,O was used in the published studies (and in the present results) in order to improve the signal-to-background ratio of the measurements. The temperature dependence of the yield of these ejected molecules is also shown in fig. 6. (These results have been obtained from more refined measurements than those presented in ref. 4; see also refs. 39 and 40.) The mass 20 yield is remarkably constant below - 145 K and rises sharply above that temperature. This behavior is very similar to that reported for solid argon [12]. The mass 32 data rise continuously with temperature and the mass 4 data do so also except for a beam-dependent background contribution below 60 K. The mass 20 data have been normalized to the Rutherford backscattering data at low tem-

I- ’ ’ ’ ’ ’ ’ ’ ’ 4.5 McV He+ ON D20

t

peratures because of their common temperature independence. The mass 4 and 32 data have been normalized as follows: mass 4 is placed at twice the mass 32 value, since the net sputtering has been found not to change the stoichiometry of the ice [2]. The two curves have then been normalized with respect to the mass 20 curve so that the total yield (mass 20 plus mass 4) fits the Rutherford backscattering total yield data at 150 K. The curve that results from this addition accounts for the total yield temperature dependence reasonably well. The ejection of mass 4 and 32 that was measured (fig. 6) requires formation of D, and 0, molecules in the solid ice film. These masses cannot appear as a result of fragmentation of D,O by the electron ionizer of the quadrupole mass spectrometer. The temperature dependence of yield, fig. 6, may then be the temperature dependence of the molecular formation process in the film: D,O molecules break up followed by rearrangement of the fragment free radicals into new molecules. Alternatively, the temperature dependence may be that of diffusion of the new molecules to the surface where they can be ejected. We have determined the energy distribution of ejected D,O molecules from time-of-flight measurements of the velocity distribution (fig. lb). Spectra are shown in fig. 7 for three different projectile ions: 1.5 MeV He+, 1.5 MeV Ar+ and 50 keV Ar+. These three provide very different energy inputs to the film as shown in table 1. For 1.5 MeV He+ the energy loss in electronic excitation of the ice is a thousand times larger than the energy loss in elastic nuclear collisions: for 1.5 MeV Ar+ the electronic loss dominates by a factor of 10 and for 50

ABSOLUTE

6-

2 g

5-

& s4!z

D20

z

EJECTED

TARGET MASS

20

:3ti t -

g2 -l

.or’ 0

-’ 20

I

I

I

40

6b

00

I

I

I

I

1

400

420

440

460

480

-

1.5 MW

He+.

T.

--

1.5 MW

Ar+,

T= 12K

kev

Ar*,

T= 25K

-.-

50

. . ..-

COLLISION

l-

CASCADE

FIT

(Eb=.055eV)

T (K)

Fig. 6. Temperature dependence of erosion of Da0 ice. The absoluteyield deduced from Rutherford backscatteringis shown as curve (a). The partial yields measured by quadrupole mass spectrometry are shown for D,O (mass 20), 0, (mass 32) and Da (mass 4). See text for normalization of the partial yields.

12K

1 -4

I -3

I -2 LOG(E).

I -1 E

I 0

I 1

in ev

Fig. 7. Energy distribution of D,O molecules ejected from Da0 ice by different ions. Also shown is a calculated fit to collision cascade theory with a surface binding energy of 0.055 eV.

313

W. L. Brown et al. / Electronic sputtering of molecular solids Table 1 Electronic and nuclear energy losses for different projectile energies. Ion and

Electronic energy

Nuclear energy

energy

IOSS

1OSS

(X

He+ (1.5 MeV) Ar+ (1.5 MeV) Ar * (50 keV)

lo-r5 eV cm2)

59.3 214 39

(X lo-l5

eV cm’)

0.051 19.9 127

keV argon ions, the nuclear energy loss dominates by a factor of three. The three spectra of fig. 7 have been normalized at high energies where they all vary approximately as l/E’. Such a dependence is what might be expected in the high energy regime if ejection were due to collision cascades initiated by nuclear elastic collisions. Thompson [41] has given the spectra for such cases as N(E) = AE/(E + U) [3] where U is the surface binding energy. For 50 keV argon this might seem plausible, but for 1.5 MeV helium it is surprising. More can be said about each case. The total 50 keV Ar spectra can be quite well fitted with the Thompson expression, as shown by the dotted line in fig. 7. However, the binding energy of the fit is 0.055 eV, nearly 10 times smaller than the 0.52 eV sublimation energy of D,O molecules from ice. This discrepancy leads to recognition that although principal ejection by 50 keV Ar+ may be due to collision cascades initiated by nuclear elastic collisions, the electronic excitation in each particle track is very large ( > 10 21ionization events/cm for 50 keV Ar) and may drastically alter the intermolecular binding of molecules in the cascade. A similar result has been found for SO, ice [8] and is also present in published results for rare gas solids bombarded by slow heavy ions [42]. The energy spectrum for 1.5 MeV He” ions presents two special features. Since the total sputtering yield for ice is known, the absolute yield for the high energy portion of the spectra can be determined by appropriate integration of the spectrum. For energies greater than 0.52 eV, this yield turns out to be 0.31 D,O molecules/l.5 MeV helium ion. Calculating the absolute yield for E > 0.52 eV expected from the Thompson 1411 expression, with molecular cross-sections for molecule-molecule scattering in the cascade, gives 0.014 molecules/l.5 MeV helium ion, even if U= 0. Thus collision cascades initiated by nuclear elastic collisions cannot nearly account for the high energy part of the spectrum. These fast particles must come from the decay of electronic excitation, perhaps through minicascades initiated by dissociative recombination of ionized water molecules analogous to the case of solid argon. [35]. The low energy part of the 1.5 MeV He+ sputtered

spectrum

is also intriguing. Its energy dependence is approximately E-‘j2, similar to the case of laser stimulated ejection of ZnO 1431.Such a result could be due to the density of final states in electron-phonon decays as discussed by Stoneham 1441. Recalling that the overall yield dependence on is approximately quadratic, a picture is (dE/dx), emerging for D,O sputtering at low ionization densities. Two closely spaced ionized water molecules which electronically decay with a large repulsive energy initiate mini-cascades or undergo electron-phonon decay producing low kinetic energy particles.

5. Summary The study of electronic sputtering of insulating molecular gas solids is found to be a unique tool for exa~ning non-radiative electronic relaxation processes. These processes yield very different energy spectra for the ejected particles than are found in the nuclear eleastic energy loss region. For Hz0 and CO at low excitation densities the sputtering yield is approximately quadratically dependent on the electronically deposited energy along the tracks of incident fast ions or electrons. This dependence strongly suggests that the process is dominated by electronic relaxation of pairs of ionized or excited molecules along an incident particle track. The reason that two-event processes dominate in these solids down to low excitation densities remains a puzzle. In Hz0 (D,O) and also in SO, and CO, [38], a remarkable amount of low temperature chemistry is stimulated by electronic excitation in the solid, producing and ejecting secondary molecular species. This is in addition to ejection of the primary molecules of the solid. To clarify the still rather hazy picture of the processes that are responsible for sputtering and ionizationstimulated chemistry it will be important to study the energy spectra of ejected particles under more varied conditions of excitation and to study the molecular constituents of the film and the radiative processes of electronic deexcitation that occur in virgin films and films containing newly formed radicals and molecules. Infrared and ultraviolet spectroscopy [45,46] are important tools that have only begun to be applied. References [l] P. Sigmund, Phys. Rev. 184 (1969) 383; Phys. Rev. 187 (1969) 768. [2] W.L. Brown, W.M. Augustyniak, E. Brody, B. Cooper, L.J. Lanzerotti, A. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321. [3] J. Bettiger, J.A. Davies, J. l’Ecuyer, N. Matsunami and IV. CONDENSED

GASES

314

W.L., Brown et al. / Electronic sputtering of molecular sohds

R.W. Ollerhead, Rad. Eff. 49 (1980) 119. [4] W.L. Brown, W.M. Augustyniak, E. Simmons, K.J. Marcantonio, L.J. Lanzerotti, R.E. Johnson, J.W. Boring, C.T. Reimann, G. Foti and V. Pirronello, Nucl. Instr. and Meth. 198 (1982) 1. [5] B.H. Cooper. Ph.D Thesis, Cal Tech (1982); B.H. Cooper and T.A. Tombrello, in press. (61 L.J. Lanzerotti, W.L. Brown, W.M. August~i~, R.E. Johnson and T.P. Armstrong, Astrophys. J. 259 (1982) 920. [7] D.J. LePoire, B.H. Cooper, C.L. Melcher and T.A. Tombrello, Rad. Eff. 71 (1983) 245. [l?] J.W. Boring, J.W. Garrett, T.A. Cummings, R.E. Johnson and W.L. Brown, these Proceedings (REI-83), p. 321. [9] V. Pirronello, G. Strazzulla, G. Foti and E. Rimini, Astron. Astrophys. 96 (1980) 267. [lo] W.L. Brown, W.M. Augustyniak, L.J. Lanzerotti, K.J. Marcantomo and R.E. Johnson, in Desorption induced by electronic transitions, DIET-I, eds., N.H. Tolk et al. (Springer, New York, 1983) p. 234. [ll] R. Ollerhead, J. Bottiger, J.A. Davies, J. l’Ecuyer, H.J. Haugen and N. Matsunami, Rad. Eff. 49 (1980) 203. [12] F. Besenbacher, J. Bottiger, 0. Graversen and J.L. Hansen, Nucl. Instr. and Meth. 191 (1981) 221. [13] P. Borgesen, J. Schou, H. Sorensen and C. Claussen, Appl. Phys. A 29 (1982) 57. (141 P.D. Townsend and D.J. Elliott, in Atomic collision phenomena in solids, eds., D.W. Palmer et al. (North-Holland, Amsterdam, 1970) p. 328. [IS] J.P. Biersack and E. Santer, Nucl. Instr. and Meth. 132 (1976) 229. [16] R.D. Macfarlane and D.F. Torgerson, Phys. Rev. Lett. 36 (1976) 486. [17] P.D. Townsend, Nucl. Instr. and Meth. 198 (1982) 9. [18] J.E. Griffith, P.A. Weller, L.E. Seiberlittg and T.A. Tombrello, Rad. Eff. 51 (1980) 223. [19] L.E. Seiberling, C.K. Meins, B.H. Cooper, J.E. Griffith, M.H. Mendenhall and T.A. Tombrello, Nucl. Instr. and Meth. 198 (1982) 17. [20] Y. Qiu, J.E. Griffith and T.A. Tombrello, Rad. Eff. 64 (1982) 111. [21] T.R. Harrison, P.M. Mankiewich and A. Dayen, Appl. Phys. Lett. 41 (1982) 1102. [22] D.Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3311. [23] M.L. Knotek and P.J. Feibelman, Phys. Rev. Lett. 40 (1978) 964.

[24] T.E. Madey, F.P. Netzer, J.E. Houston, D.M. Hanson and R. Stockbauer, in Dcsorption induced by electron transitions DIET-I, eds., N.H. Tolk et al. (Springer, New York, 1982) p. 120. 1251 R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear tracks in solids, principles and applications (Univ. of California Press, Berkeley, 1975). [26] N. Itoh, Advan. Phys. 31 (1982) 491. [27] T.M. Hall, A. Wagner and L.F. Thompson, J. Vat. Sci. Technol. 16 (1979) 1889. [28] T. Venkatesan, W.L. Brown, B.O. Wilkens and CT. Reimann, these Proceedings (REI-83), p. 605. [29] W.L. Brown, L.J. Lanzerotti and R.E. Johnson, Science 218 (1982) 525. [30] H. Overeijnder, A. Hating and A.E. DeVries, Rad. Eff. 37 (1978) 205. 1311 J. Schou and H. Sorensen, private communication. 1321 M.J. Berger, private communication. 1331 R.E. Johnson and W.L. Brown, Nucl. Instr. and Meth. 198 (1982) 103; R.E. Johnson and W.L. Brown, Nucl. Instr. and Meth. 290/210 (1983) 469. [34] R.E. Johnson and R. Evatt, Rad. Eff. 52 (1980) 197. [35] R.E. Johnson and M. Inokuti, Nucl. Instr. and Meth. 206 (1982) 289. [36] P.K. Haff, Appl. Phys. Lett. 29 (1976) 473. [37] We are indebted to H.H. Andersen for suggesting this similarity. [38] W.L. Brown, W.M. Augustyniak, L.J. Lanzerotti, R.E. Johnson and R. Evatt, Phys. Rev. Lett. 45 (1980) 1632. 1391 J.W. Boring, R.E. Johnson, CT. Reimann, J.W. Garrett, W.L. Brown and K.J. Marcantonio, Nucl. Instr. and Meth. 218 (1983) 707. [40] CT. Reimann, J.W. Boring, R.E. Johnson, J.W. Garrett, K.R. Farmer, W.L. Brown, K.J. Marcantonio and W.M. Augusty~~, submitted for publication to Surf. Sci. [41] M.W. Thompson, Phil. Mag. 18 (1965) 377. [42] H.E. Roosendaal, R.A. Hating and J.B. Sanders, Nucl. Instr. and Meth. 194 (1982) 579. [43] T. Nakayama, N. Itoh, T. Kawai, K. Hahimoto and T. Sakata, Rad. Eff. Lett. 67 (1982) 129. [44] A.M. Stoneham. Advan. Phys. 28 (1979) 457. [45] W. Hagen, L.J. Allamandola and J.M. Greenberg, Astrophys. Am. Space Sci. 65 (1979) 215. [46] M.H. Moore and B. Donn, Astrophys. J. 257 (1982) L47.