4510
J . Phys. Chem. 1984,88,4510-4512
Synthesis and Sputtering of Newly Formed Molecules by Kiloelectronvolt Ions A. E. de Vries,* R. A. Haring, A. Haring, F. S. Klein,+A. C. Kumme1,Z and F. W. Saris FOM- Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands (Received: April 1, 1983)
Frozen layers of water, carbon monoxyde, ammonia, and some mixtures thereof have been bombarded with 3-keV Ar’ ions at about 20 K. The sputtered neutral particles were mass analyzed by means of a quadrupole mass spectrometer. In a few cases time-of-flight measurements were performed. Several mass/charge ratios were found, which could be attributed to either newly formed compounds during sputtering or clusters of the original molecules. In order to differentiate between these two possibilitieswe also studied fragmentation patterns of clusters in the mass spectrometer. Together with some other techniques we could thus definitely establish the formation and sputtering of newly formed compounds. Besides the fundamental interest the results may have astrophysical implications.
Introduction Sputtering is the release of particles under impact of energetic ions.’ It is a phenomenon which is of interest in various fields of physics and chemistry. In this paper we are concerned with sputtering in which the chemical nature of the sputtered particles differs from that of the original target material, simple molecules which are condensed at low temperatures of 10-30 K. The reason is twofold. Little is known about the mechanism of this phenomenon in which physics and chemistry go side by side. Measurements of the velocity distributions of sputtered particles, as were done in a few cases, will a t least give some clue to the mechanism. A second reason for this research is astrophysical: in interstellar space so-called dust grains are present. They are believed to consist mainly of silicates, covered with frozen layers of various simple molecules like the ones we study here and they are exposed to energetic particles.2 If, by such exposure, new molecules would be formed and sputtered, this might eventually lead to an explanation of the various more complicated molecules found in space by astrophysicist^.^ While an analysis by mass spectrometry is of a universal nature and therefore almost indispensible in a type of research like the present one, it contains one inherent difficulty. This is caused by the fact that clusters of the original target molecules may be sputtered also: Fragmentation patterns in a mass spectrometer thus can be due to either new molecules or clusters. Therefore, fragmentation patterns of some clusters have to be studied and it will be shown that our results definitely establish the sputtering of newly formed molecules. Experimental Section The apparatus has been described in detail in ref 5 . A beam of 3-keV Ar+ or He+ ions of about 10 pA/cm2 was aimed at a target of condensed gas at an angle of 60’ with respect to the normal. The target was formed by directing the gas or gas mixture through a small tube at the substrate. This tube was terminating in a 50-pm collimated hole structure (3 mm thick), which was located about 2 cm from the cold tip of an Air Displex CS208L closed-cycle helium cooling machine. The tip, on which a thin copper plate was mounted as a substrate, had a temperature of 10-30 K. The layer condensed on it was 0.05-0.5 mm thick. Neutral particles sputtered from this target in the normal direction reached the electron impact ionizer of a quadrupole mass spectrometer after traversing a flight path of 143 cm for experiments with Art and 37 cm for those with He+. When a gas was admitted to the target compartment with the cooling switched off and without an ion beam present, accurate measurements of the fragmentation patterns of the parent molecules could be made in situ. With a nozzle system mounted on t Visitor from the Weizmann Institute, Isotope Department, Rehovot, Israel. *Department of Chemical Engineering, Stanford University, Stanford, CA.
0022-3654/84/2088-4510$01.50/0
TABLE I: Neutral Particles Sputtered from Frozen Layers of Some Gases and Mixturesa gas
supersonic nozzle
“3
(NHSIx’, (“,)X-jH+
co
(CO),+, (CO),-,C+
h.0
(H,O),+, (H,O)x-lH+
CH4
(CH,),+, (CH,),-,H’ IX= 1-6)
H,O NH,
+ CO + CO
sputtering N,,b N,H4 (29-321, some clusters: 35 c o , , c,, o,, no 56 so no dimers O,,b HO,, OH, [O],
dimers: 19 not performed yet
CO,,b O,,b [HCO],HCOH, new OH, new CO, HO, N,,b HCN, N,H
a The middle column shows the fragment ions obtained from clusters of these gases in a mass spectrometer. 0 and HCO are in brackets because they may be fragments of OH and HCOH. Ion signal > 10% of the total signal.
top of the cooling machine fragmentation patterns were obtained for the van der Waals clusters. In all cases the beam under study was modulated in order to discriminate signal from background. For time-of-flight measurements we used the correlation technique.6 For mass spectrometry measurements a low modulation frequency of 2 Hz was chosen to collect all particles irrespective of their time-of-flight during one phase of the chopping cycle. When determining fragmentation patterns of either parent molecules or nozzleproduced van der Waals clusters the beam was modulated with a mechanical chopper. When sputtering was performed, the incident ion beam was electronically chopped. When the mass spectrum was sampled, the readout and the processing of the data were computer controlled. In the mass spectra in this paper only those signals are given which were in-phase with the modulation and which were statistically significant above background. Results and Discussion Results of the comparison between fragments from clusters and from sputtering are given in Table I, (1) A recent review of sputtering is given in “Sputtering by Particle Bombardment” in “Topics in Applied Physics”, R. Behrisch, Ed., Springer Verlag, Berlin The first part, Vol. 47 (sputtering by ions), and the second part, Vol. 52 (sputtering of alloys and compounds, electron and neutron sputtering, surface topography) have been published in 1981 and 1983, respectively. The third volume will be published in 1984. (2) M. Maurette, Nucl. Instrum. Meth., 132, 579 (1976). R. E. Johnson, L. J. Lanzerotti, and W. L. Brown, ibid, 198, 147 (1982). (3) E. Herbst and W. Klemperer, Phys. Today, 32, June 1976. (4) R. Pedrys, R. A. Haring, A. Haring, F. W. Saris, and A. E. de Vries, Phvs. Lett.. 82A.371 (19811. 1 5 ) M. Szymonski, H. OGereijnder, and A. E. de Vries, Radiat. Effects, 36, 189 (1978). (6) C. A. Visser, J. Wolleswinkel, and J. Los, J . Phys. E , 3, 483 (1970).
0 1984 American Chemical Society
Synthesis and Sputtering by keV Ions
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4511
/
a
,
0;
NH' 1
I
10
0 b
30
20
.:i": N'
N:,
,
LO
50 m/e
N2"
N;
510 1 1
attenuation
of D,O*reference
Figure 2. Attenuation of the DzO+, (D20)D+, and 02+signals as a function of the attenuation of a DzOt signal measured as a reference.
H 2 0 . From clusters we found peaks at mass/charge ratios of (H,O),+ and ( H 2 0 ) f l . The fragmentation pattern of water has been studied before9 and our findings are in agreement with this. Clusters are definitely present in the sputtered beam, as a peak at mass/charge ratio 19 is found in this case. Peaks at 32 and 33 are not present in a spectrum of the original gas and so these peaks in the sputtered beam can be ascribed to O2and H02. O H and 0 are obtained from water itself. However, these peaks as obtained in sputtering are much larger than for water vapor. Therefore, at least O H is also sputtered as such. It is not known if this is also the case for 0, since 0 might be a fragment from OH. Besides these measurements we have used another method to determine if different ions, found in the mass spectrum, originate from different parent molecules or if they are fragmentation products from the same molecule. This method consists of filling a compartment between the condensed layer and the ionizer of the mass spectrometer with a gas. If two fragment ions stem from the same parent molecule, the attenuation of the sputtered beam by the gas is the same, since it is the original molecule which collides with the gas. If the fragment ions show a different attenuation, they certainly originate from different parent molecules. We performed these attenuation experiments with a sputtered beam from D 2 0 on masses 20 (presumably D20), 22 (presumably D30), and 32 (presumably 02).D 2 0 was used because the background in the mass spectrometer at the corresponding signals for H 2 0 were larger. A vacuum compartment between the ice layer and the electron impact ionizer was filled with C 0 2 at different pressures. The attenuation of a signal A is given by I A =',I exp[-nul]
in which n = p / k T is the number density, I, and ZAo are the attenuated and unattenuated intensities, 1 is the length of the scattering volume, and u is the total cross section for removal of the parent molecule of fragment A from the beam. In Figure 2, In (zAo/I+) is plotted against In (ID,O+o/ID,O+). The + A represents slopes are then proportional to C T ~ / C T ~in, ~which either D20+, (D20)D+,or 02+.It is seen that the attenuation is clearly different for the three species, showing that these fragmentations originate from different parent molecules. It can (7) 'EPA/NIH Mass Spectral Data Base", US. Department of Commerce/NBS, Washington, DC, Suppl. 1, 1980, p 3977. "Eight Peak Index of Mass Spectra", Mass Spectrometry Data Centre, AWRE, Aldermaston, Reading, U.K., 1947, Vol. 1, p 2.
(8) J. J. C. Schats and A. H. W. Aten, Jr., Radiochim. Acta, 15, 46 (1 971). (9) J. Fricke, W. M. Jackson, and W. L. Fite, J . Chem. Phys. 57, 580 (1972).
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
4512
1031
*.
m (L
Q
v
x
3
d
\
lo, c
.\
O C
t
" 10-2
10"
1
\
\ 10
ENERGY (eV) Figure 3. Energy spectra of O2sputtered by 3-keV Art ions from consputtered by the same ions from a condensed H20(0)and C160180 densed mixture of CI80and H 2 l 6 0(0).The line indicates a behavior a
E2.
be concluded that sputtering of D 2 0 ice produces D,O (found at m l e 20), (D,O), leading to (D20)D+with m l e 22, and O2found a t m l e 32. CH4. We have not performed sputtering experiments with CH+ Electron bombardment of clusters gives the parent ions (CH4)x+ and ions of the form (CH4)rlH+. We have found these ions from clusters up to x = 6 . It seems to be a rather general rule that clusters containing H give parent cluster ions and cluster ions with one extra hydrogen which, of course, is formed from fragmentation of a larger cluster. H 2 0 CO. Three special features in the sputtering of this mixture, for which we have made use of isotopically labeled C l 8 0 or H2I80,deserve mentioning. First, the fraction of COz from this target is much larger than from pure CO. Second, OH, CO, and COz are sputtered, in which the atoms in one sputtered molecule stem from two different types of molecules. Third, we even have found C1602from C I 8 0 H2160. H C O might be a fragmentation product of HCOH. NH, + CO. Here we have found N,, HCN, and N2H as newly formed and sputtered molecules. It has to be mentioned that in the last two cases we have not been successful in making supersonic nozzle beams containing clusters of mixed molecules. Although we cannot be absolutely certain that fragments would be formed from such mixed clusters,
+
+
de Vries et al. the mass/charge ratios found in sputtering are very likely due to the tabulated molecules. Time-of-Flight Measurements. The sputtered fluxes obtained for O2 and CO, were large enough to be able to make time-offlight measurements, which were converted into energy distributions of sputtered molecules. They are given in Figure 3. It is obvious from the high energies present that these newly formed gases do not just evaporate as in that case the intensity would drop off much faster with energy. In fact the energy distribution is very reminiscent of a so-called collision cascade mechanism10)in which the momentum of the incoming ion is (partially) transferred to a target particle, eventually leading to a cascade of colliding particles. When this cascade reaches the surface, particles with sufficient energy are emitted with an energy distribution having an asymptotic E-2 dependence (see Figure 3). The incoming ion may produce freely moving atoms with considerable energy by momentum transfer, provided the energy transfer is enough to break chemical bonds. These atoms are then chemically active when penetrating and slowing in the target matrix. Such processes resemble the situation in hot atom chemistry where an energetic atom is produced inside a matrix by a nuclear reaction. This atom then may end up in a chemical bond different from the original one. It is interesting to note that N2H4 has been found in hot atom reactions in condensed NH3.* Another possibility is that the incoming ion causes electronic excitation, for instance by a charge transfer reaction,l' of atoms at or near the surface, thereby starting a chemical reaction. Products might be emitted by collision cascades, caused by further ion bombardment. In conclusion we have demonstrated that kiloelectonvolt ions hitting a target of condensed H20, CO, or N H 3 and mixtures thereof lead to the ejection of various molecules: 02,N,, COz, OH, HO,, N2H, NzH4, HCN, C2, HCON, and HCO have been found. In some cases, like N2 from NH3, 0,from H,O, CO, from a mixture of CO and HzO, the sputtered beam contains >lo% of the newly formed molecules. The synthesis and sputtering of newly formed molecules from condensed gases as presented here may prove to be important in the outer solar system and in interstellar clouds, where dust grains with mantles of condensed gases are exposed to bombardment by energetic particles and UV light.2 In the same regions various complex molecules have been detected as listed in ref 3. Acknowledgment. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research on Matter) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (Netherlands Organization for the Advancement of Pure Research). Registry No. Art, 14791-69-6; H20, 7732-18-5; CO, 630-08-0; NH3, 7664-41-7. (10) M. W. Thompson, Phil. Mag., 18, 377 (1968). (11) K. T. Gillen and A. W. Kleyn, Chem. Phys. Lett. 72, 509 (1980).
