SIMS of solid hydrogen

Vacuum/volume 34/numbers 1-2/pages 113 to 117/1984

0042-207X/8453.00 + .00 Pergamon Press Ltd

Printed in Great Britain

SIMS of solid hydrogen R Clampitt,

Oxford Applied Research, Crawley Mill, Witney, Oxfordshire, UK

Secondary ion mass spectrometry (SIMS) is now a weE-established technique. More recently interest has been focused on "molecular" or "cluster" SIMS with potential for the analysis of complex materials. In order to gain understanding of SIMS of condensed matter some workers have investigated relatively simple materials such as solidified rare gases, water and methanol. One of the simplest condensed molecular solids is hydrogen. We report here secondary ion mass spectra obtained by electron excitation of solid crystalline hydrogen, termed electron-induced SIMS. A remarkable feature of the spectra is the ion/induced-dipole clustering of H= molecules around a central H ; nucleus which is produced by dissociative ionization of H= followed by the recombination reaction: H+ + H= + H=--~H; + H=. Electron-induced SIMS of solid deuterium has also been obtained and is compared with that of H=. The phenomenon of field emission of ions from solid hydrogen in very low ( < 100V . cm- 1) fields is reported. Data is given on the ejection of H - from solid hydrogen and, for comparison, solid H=O. The variation of the normalized H- electron impact cross-section curves for these two solids are essentially identical with those of free molecules. Finally SIMS data is also reported for low energy alkali ion scattering from solid hydrogen and the effects of primary ion size on secondary ion cluster size is compared.

1. Introduction Much attention has been directed recently to secondary ion mass spectrometry (SIMS)of condensed films of organic and inorganic compounds, commonly called molecular or cluster SIMS ~'2 Some of the earliest reports of such cluster ion SIMS spectra were on condensed (solid) hydrogen 3.4, lithium ~ and water s. Already from these works there was information relating to the structure of both the condensed solid and of the ejected ions. We report here previously unpublished 7 data on SIMS of solid hydrogen which provide information on the mechanisms of secondary ion formation and ejection. For the most part we generate the primary ion (a proton) in situ by low energy electron bombardment of the condensed film. Energetic protons, possessing ample kinetic energy to escape from the surface, are produced in abundance by dissociative ionization of the hydrogen molecules. This technique is to be compared with related in situ ion generation methods using radioactive emitters s'9. However low energy electrons are less destructive than ions and thus provide a sensitive probe for SIMS of condensed materials.*

2. Experimental methods The experimental apparatus has been reported previously 1°. Hydrogen gas is condensed on a copper surface in a vacuum of ~-10-1 o tort. The temperature of the surface can be controlled accuratdy between 2 and 4.2 K by means of a liquid helium cooled cryostat. * The process of ion generation at surfaces by primary electrons is called by some workers 'Electron Stimulated Desorption'. In the present context it would seem more appropriate to use the term 'Electron-Induced SIMS'.

For electron-induced SIMS a focused electron beam of typically ~<70eV and 2 x 1 0 - S A is swept over the surface by sawtooth voltages on X and Yplates, so as to distribute the charge accumulated during the experiment. For ion-induced SIMS a beam of H~" ions ( 4 1 keV) of ,,- 10-1o A can be directed at the target or alkali metal ions from nearby coated filaments. Accumulated positive charge at the surface is neutralized with electrons from another filament. Secondary ions emerging from the surface are focussed into a quadrupole mass filter for spectral analysis.

3. Electron-induced SIMS A typical mass spectrum of positive ions ejected from the unflashed copper substrate at room temperature, prior to condensation of hydrogen, is shown in Figure I. It is seen that the surface is covered mostly with chemisorbed hydrogen atoms, and fluorine atoms derived from the bulk of the copper I°. The observed deuterons derive from an earlier experiment using deuterium gas. When molecular hydrogen is condensed onto this surface at 3 K, hydrogenic ions are ejected from the surface (Figure 2) and we have shown 11 that the most likely processes leading to these secondary ions are e+H2--.H+ +H H++H2+H2-,H~ +H2 H ~ + (Hz).--. H ~ (Ha) ._ I + H 2 where the proton gains kinetic energy from dissociative ionization of H 2. In the gas phase the major electron-induced ion-forming process at low energy is: e + H2--*H ] + 2 e . 113

R Clampitt: SIMS of solid hydrogen

H+ 4-

F 77eV eLec'trons Torge'~ 293°K j t I00-

K+

lo"

f

~OD+OHiO÷

[

so' ,~ o.

~

35Ct4

/

1

L i0 J

"~ U 50.

0

35

23

12

2

m/e

Figure 1. Ejectedion si~ctrum from coppersurfaces. H+

H +15

I000

4-

7" . >o- l x + m "T >.

% ,c_ 51

45

39

33

27

21

15

9

3

m/e Figure 2. Ions ejected from solid H2 by electrons.

However the molecular ions so formed in the solid do not gain sufficient kinetic energy to escape from the surface and in the absence of electric field gradients they remain trapped. The SIMS spectrum exhibits a prominent ion at Hfs which might well be H J (H2)6. Figure 3 shows the intensity distribution of ejected cluster ions for n > 10. It may not be coincidence that the break in the curve occurs at a value of n close to that for a hexagonal close-packed crystal (n= 13), which form hydrogen usually assumes when condensed from the gas phase ~2. Thus we might be observing here the ejection of a crystallite of hydrogen attached to an ion. A high current ( > 10 - s A) electron beam changes considerably this distribution and probably indicates disruption of the ordered lattice. An electron-induced SIMS spectrum of solid deuterium is shown in Figure 4. Here also D~~ is a particularly significant ion. The intensity distribution differs from that of solid hydrogen maybe because of differing polarizabilities of the nucleated molecules and ionic radii of the postulated nucleating centres, H ~" and D J . We could not obtain a reproducible characteristic ion spectrum as observed for hydrogen. Here the D~ is prominent and the D~'s ion clearly more abundant than D~'4 and D~'6. Figure 5 shows a deuterium ion spectrum obtained in a separate experiment.

114

o

~0

~15

20

25

30

35

40

n

Figure 3. H i intensity distribution for n> 10. Of the unlabelled ions in both deuterium spectra three of them are probably D + (H20), D + (HDO) and D~" (H20) at respectively role 20, 21 and 24. The ion at m/e 16 could be O ÷ from the water impurity though it is uncommonly abundant. It is not clear why there should have been much more water contamination here than in the hydrogen experiments. An electron-induced SIMS spectrum of solid D~/I-I 2 mixtures exhibits ion species at essentially every mass unit up to the limit (200 AMU) of the spectrometer. A curious effect of impurity gases on the secondary ion intensities from solid hydrogen has been observed. The efficiency of ion ejection by low energy electrons increases 10-20 times when neon or argon is condensed onto the solid during electron impact. Unfortunately trace impurities such as H 2 0 and CO were observed to be condensed along with rare gas*. These are ejected * These are generated in the ion pump of the apparatus when rare gases in particular are introduced.

R Clampitt: SIMS of solid hydrogen 4

D,

D;, ],o"

%

I01

3'o

A

3'o

2'6

,'8

~2

,'4

,'o

d

d

1Jlo= °

I000

1

500 ~ o.

~

m/e

Imql~e 4. Deuterium ions ©j~m:d from solid D2 by electrons. .

+

-I-

HD2 HD

i0'~ lOOn

I

+

D*7

D* 13

,

+

+

+

D9

D7

D~ L i0 2

~ 5o

rn/e

.-

Figm'e 5. Deuterium ions ejectedfrom solid D2 by ¢]~ctmns.

from the surface as H 30 * and HCO ÷ respectively. Consequently we cannot at present offer an unambiguous explanation of this phenomenon. There is no precedent for ion spectra of this sort and so the presence of impurities complicates enormously the identification of the secondary ion species. Indeed the complete and consistent absence of an ion at some particular value of m/e, such as role 34 in the spectra from solid H 2 contaminated with N2, should constitute a fundamental clue as to the composition of adjacent ionic species. Even with this information, however, we still cannot unequivocally assign identities to most impurity ions. Some contaminant-related ions are fairly common and readily identifiable, e.g. HCO* and H3CO+ from CO-doped H2; or the water cluster ions of the type H30*(I-I20), with n = 1-6. We have also studied secondary negative ion ejection. Prior to condensation of H 2 the 'bare' target surface exhibits a SIMS spectrum comprising principally the ions H - , O - , F - and CI-. The only major negative ion detected from solid H 2 is H - . An ion at role 2 of about one tenth the intensity o f H - was observed from solid H 2. This could not be readily attributed to D - impurity. The ion intensity was dependent on gas phase pressure of H2, in contrast to all the usual ions of surface origin. If the ion is H2, the existence of which has evoked much speculation ~3 then it would appear to be formed by a charge-exchange process near the

surface. The cross-section for the process H - + H 2 - , H 2 + H is not reported. That for the resonant charge-exchange process H-+HoH+His very high, 3 x 10-14 cm 2, at 1 eV t(. Alternatively it might be that the H 2 molecule, incident on the dectron-rich surface, captures an electron there to form a stable state of H~ of extremely low binding energy. It would be of considerable interest to repeat these experiments in order to verify or refute the observation of a stable state of H~. We have not detected ion clusters of the type H-(]'I2) . though this may be due to insufficient sensitivity of the detection system since the cross-section for H - production from H , is many orders lower than that for H *. The variation of H - ion intensity with electron energy is similar to that observed for the free molecule in the gas phase t5 and is shown in Figure 6. That this may not be pure coincidence is reinforced by a curve for H - from condensed H 2 0 which also shows remarkable similarity to that observed for H 2 0 in the gas phase te for the process e+ H20--*H- +OH.

115

R Clampitt: SIMS of solid hydrogen

40

|i

H- ( HzO; Li ÷

3O g

Li

a~

(H2) n

zo .c

L_

{p -,F*

I 19

I 15

L II

C

Z

10

A m

t,

v

J

No~(H z)n..~ ~',~o,-o

I

c

.o.- o o,o.o o,.O-O,.o"o'~°'~ I I

i0 15 20 E Lec't ron energy (eV 1 Figure 6. Variation of H - cross-section with energy. - - - - solid H2.

c

37

5

3t

23

ice;

4. Ion-induced SIMS

For 8 eV Li + ions (typically 2 × 10 - i ° A) incident at 45 ° onto a thick ( > 5 0 monolayers) layer of solid hydrogen the total reflection coefficient was approximately 0.2. Some of these scattered ions emerge not as Li + but as ion-induced dipole clusters of the type Li + (H2) . with n lying mostly between unity and six4. Other alkali ions emerge similarly clustered from the surface ~t. Figure 7 shows a comparison of alkali-hydrogen spectra. As the ionic size increases more molecules can be accommodated. SIMS spectra from Li ÷ impinging onto solid H 2 contaminated with residual gases exhibit many ions up to the mass limit of the quadrupole and interpretation of composition is too difficult at present. We could not detect secondary ions arising from H~" ions (of ~ 1 0 - l ° A at 70 eV) incident on solid hydrogen. However, in contrast to the alkali ions, H~ ions must undergo a multiplicity of resonant charge-exchange and other 'ion-trapping' reactions. 5. Field emission of ions from solid hydrogen

K (Hz) n

63

I 39

51 m/e

Figure 7. Alkali-hydrogen duster ion spectra. 2

d. ,J ~,

,c~

15j 3 r

19t7

i0 z

Low energy H I ions of 70 ¢V were trapped in solid hydrogen at 3 K by directing the ions onto the metal substrate during condensation of H 2 from the gas phase. Charge-neutralizing electrons of nominally ~ 1 eV (i.e. well below ionizing threshold) were simultaneously directed at the target. Subsequent application of a positive potential of 100 V (E ~ I00 V c m - ~) in the absence of the primary beams results in the evaporation of positive ions from the surface into the vacuum. The emission can be sustained for several minutes depending on the original ion dose: A mass spectrum of the field-evaporated ionic species is shown in Figure 8. These are cluster ions of the type observed in electroninduced SIMS. The mass distribution varies considerably during the emission period. It is interesting that H~ is prominent i.e. that some of these ions can survive in solid hydrogen without undergoing the reaction

The cross-section a for this process varies, at low ion velocities, as aocl/v and is reported by experimen¢ ~ to be 36 A 2 at 0.1 eV. The resonant charge-exchange process

H ~ + H2..-,H~" + H.

H~ +H2~H2+H~

116

c

Io

222~ 25

2t

15

9

3

m/e

Figure 8. Mass spectra of field evaporated ions.

R Clampitt: SIMS of solid hydrogen has not been measured at very low energies but an extrapolated theoretical value t . at 0.1 eV is about 10 A'. Our observations of Hi" indicate that a considerable fraction of H~" ions move through solid hydrogen by resonant charge-exchange and can emerge through the surface-vacuum interface without undergoing the bimolecular dissociative process. Deposition of up to 200 layers of H , onto the surface during field ion evaporation did not significantly alter the ion intensities which tends to reinforce the charge-hopping model. The ionic mass distribution was too erratic to detect any change on deposition of H2. At about 400 monolayers of overlaid H , ion evaporation ceased abruptly. The only previous work for comparison is the field evaporation of negative charge (mass unknown) from liquid helium 8. It has long been postulated that ions in liquid helium exist as cluster ions of perhaps 40-50 atoms ~9 but there is no previous experimental evidence that this is so. We have shown by mass-identification of the evaporated ions that in the case of solid hydrogen most are indeed clustered.

6. Conclusions We have demonstrated here a fundamental process not widely appreciated: that, at least in films of H atom-containing molecules, low energy primary electrons provide a sufficient and gentle means of generating very low energy protons in situ, for condensed film SIMS analysis generally in the form of ion-dipole clusters of the parent molecule. The technique will likely be useful for spatially-resolved imaging of beam-sensitive films, such as of biological materials. The use of scanned microfocussed electron beams (such as in an electron microscope) will be far less destructive and should offer greater potential than focused primary ion beams which destroy sensitive samples after only one or t w o scans :°.

Acknowledgements The author wishes to thank L Gowland and D K Jefferies for experimental assistance and UKAEA Culham Laboratory for permission to publish the work.

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