Secondary ion emission from UHV-deposited amino acid overlayers on metals

Surface Science 136 (1984) 419-436

North-Holland, Amsterdam

419

SECONDARY ION EMISSION FROM W-IV-DEPOSITED AMINO ACID OVERLAYERS ON METALS

W. LANGE, M. JIRIKOWSKY and A. BENNINGHOVEN

Physikalisches Instiiut, Universitiit Miinster, Domagkstrasse 75, 4400 Milnster, Fed. Rep. of Germany

Received 20 July 1983; accepted for publication 28 September 1983

The secondary ion emission from organic solids has been studied under UHV conditions in a

combined SIMS-AES instrument equipped with an evaporation source for volatile organic

compounds. The instrument allows controlled formation of amino acid overlayers in the submono-

layer, monolayer and multilayer range on metal surfaces, which can previously be characterized by

their secondary ion and Auger electron emission. Secondary ion emission from leucine, ghttamic

acid, methionine, phenylalanine and glycine overlayers on Ag, Au, Ni and Cu have been

investigated. In the submonolayer and monolayer range the emission strongly depends on the

chemical nature of the metal substrate. No influence in the multilayer range was observed. The

highest (M + H)+ yields were found for noble metal substrates. Protonated and deprotonated

molecules (M+ H)+ and (M-H)- show a completely different behaviour concerning their

intensity change during enhanced ion bombardment or heating: the damag6 cross section for

(M + H)+ emission from metal surfaces exceeds that for (A4 - H)- by a factor of about 5. During

heating of the sample, the (M + H)+ emission disappears at about 100 K below the disappearance

temperature of (M - H)-.

I. Introduction

Investigations on the emission of molecular secondary ions are important for a fundamental understanding of secondary ion formation as well as for its various analytical applications [l]. Molecular secondary ion emission has been studied for many years. So the strong emission of negatively charged anion complexes as SO;, NO;, etc. is well known for more than one decade [2]. Metal oxygen cluster ions Me,,,O, have been widely applied to the investigation of metal-oxygen interaction [3]. More recently the cluster ion emission (Me + M)+ (“M” stands for an organic compound) from metal adsorption systems Me-M has also been applied to the investigation of surface reactions [4-61.

Molecular secondary ion emission from organic compounds was first re- ported for organic anion complexes [2,7]. In a systematic investigation of 15 amino acids recently we found that even for these relatively complex organic molecules, secondary ions of the general composition (M f H) * and (M - COOH)+ are emitted with relatively high yields ranging from lo-*-lo-’ [8].

0039-6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

420 W. Lange er al. / Secondary ion emission

The further investigations of many other groups of organic compounds as peptides, drugs, vitamins, nucleotides, nucleosides, etc. have shown that molecular ions are emitted from all of these compounds with comparable intensities

t91. Today an increasing amount of experimental material is published in this

field concerning the preparation from an aqueous solution - see, for instance, refs. [lo-141. All the investigations, however, refer to samples, that have been prepared by drying the solution of the corresponding organic compound under atmospheric conditions without substrate control by surface sensitive experimental techniques. Similar problems are expected from a preparation in the (liquid) glycerol matrix [15-181.

The present state of organic secondary ion mass spectrometry therefore may be characterized by three main facts: - There exists a large body of experimental material on secondary ion emission for various groups of organic compounds. - All this experimental material refers to sample surfaces of relatively unknown composition: The metal substrate may be covered by oxide and hydroxide layers, by residues of the solvent and random contaminations. The organic compound of interest will be incorporated in a more or less unknown manner in this complex surface structure. This uncontrolled sample composition seems to be the main reason for the wide scattering of experimental results. - At present no generally accepted model exists for the interpretation of the experimental results [l]. This concerns the sputtering of large unfragmented organic molecules, as well as the ionization process resulting in the finally observed secondary ions.

In order to get more insight into the fundamental surface processes due to sample preparation, and the formation of large organic ions, experimental results under more defined and reproducible surface conditions are necessary. Corresponding experiments should start with a substrate surface, previously controlled by surface analytical techniques as AES, ESCA, or SIMS. Such a surface should then be covered by well-known amounts of an organic compound monitored by an additional quantitative technique as AES.

We set up an UHV secondary ion mass spectrometer equipped with an evaporation source for volatile organic compounds, and an additional AES device. With this instrument the secondary ion emission from amino acids deposited on clean metal surfaces was investigated.

2. Instrumental

The SIMS instrument (fig. 1) is a UHV (p < 10v9 mbar) quadrupole secondary ion mass spectrometer (mass range: l-300 an-m) of moderate

W. Lange et al. / Secondary ion emission 421

PI SOURCE

SI OPTIC MTH WADRUPOLE MASS FILTER

WLECULAR BEAM SOURCE WITH SHUTTER

AUGER SPECTROMETER

SAMPLE CAROUSEL

ELECTRON GUN

Fig. 1. Combined SIMS-AES instrument. The sample on the carousel (5) can be exposed to the

secondary ion mass spectrometer (1,2), the evaporation source (3) or the Auger spectrometer (4).

resolution and a single-ion counting device for positive and negative secondary ions. A central stop within the secondary ion optical system prevents neutrals from entering into the quadrupole system. The Cu-Be electron multiplier is mounted off axis (90”). Ions leaving the quadrupole system are postaccelerated to 3 keV. The overall transmission of this instrument was estimated to be 10P5.

The primary ion source is of the scanning/gating type. It works reliable in the range of lo-‘* to 5 x 10e6 A with the ion beam typically focussed on 0.5 mm*. Scanning of the ion beam and the combined gating of the single-ion counting device allows a detection of secondary ions emitted from a target area of 0.7 x 0.7 cm* without any crater effect. The energy range is 0.3-5 keV. All investigations were carried out with 3.5 keV argon primary ions. While “static” SIMS investigations were carried out with a beam current density lower than lop9 A/cm*, experiments on damage effects during ion bombardment were

usually performed with 10v7 A/cm*. The AES system is a conventional single pass CMA with an integrated

electron gun (energy range: O-3 keV; beam current: O-100 yA; beam diame- ter: 100 pm). ‘The d N/d E spectra were registrated by lock-in technique.

Various metal foils were used as substrate materials. They were mounted on a heatable copper-block on a target carousel and could be cleaned by combined heating (1 K/s, max. temperature: 1000 K) and additional ion bombardment (5 X lO-‘j A/cm2, 3.5 keV Ar+). The surface composition was controlled by monitoring the secondary ions and the Auger electron emission.

It is known from chemical analysis and chemical ionization (CI) mass spectrometry, that most of the amino acids are volatile as intact molecules at moderate temperatures and decompose in the range 500-550 K [20-221. So we

422 W. Lange et al. / Secondary ion emission

built up an evaporation source mainly consisting of a heated tube and several diaphragms, out of which the organic material can be deposited onto metal surfaces (fig. 1). The temperature within the tube was measured by Ni-NiCr thermocouples. The thermal equilibrium was reached after about 30 min. Prior to the evaporation the amino acids were degassed for some hours at 370 K. During the amino acid deposition, the target was positioned perpendicular to the molecular beam. By means of a shutter between source and target the exposition time could be varied. In addition the amino acid coverage could be controlled by Auger electron spectroscopy.

3. Experimental results

In this paper we report on investigations of secondary ion emission from amino acid overlayers deposited on different metals. Detailed studies were carried out for gold, silver, nickel and copper. We selected four amino acids for our experiments: leucine, glutamic acid, methionine and phenylalanine (fig. 2). In addition, some experiments described in the following sections were also performed with glycine.

3.1. Spectra of evaporated amino acids

In order to compare the preparation techniques we took mass spectra of the same organic material deposited in different ways on a silver target. The upper parts of figs. 3 and 4 represent the positive and negative secondary ion spectrum of phenylalanine deposited in aqueous solution on silver. Besides the

secondary ions (M f H) * and (M - COOH)+, typical for almost all amino acids [8,11], a large number of additional secondary ions especially in the low

NAME

Glycine

Leucine

Glutamic acid

FORMULA MOL. MASS

H,;-COOH 75

NH2 H3C-CH-CH2-CH-COOH 131

tH3 AH2

tkXlC-KH2)2-~H-COOH 147

NH2 Methionine

Phenylalanine

H3C-S-(CH2)2 - H-COOH E

149

(=&CH2 -$C:;H 165

Fig. 2. Table of the investigated amino acids.

W. Lunge et al. / Secondary ion emission 423

Fig. 3. Positive secondary ion spectrum of about one monolayer phenylalanine on Ag. Upper spectrum: deposited from the solution; lower spectrum: deposited by UHV evaporation. Primary

ions: 1 X 10T9 A, Ar+ on 1 cm2.

mass range are observed. This is quite different for the same substrate and the same amino acid, if deposited by evaporation on the clean metal surface under UHV conditions (lower parts of figs. 3 and 4).

The spectra prepared by molecular beam technique show molecular ions (M f H) * and typical fragment ions ((M - 45)+, C,Hl , etc.) as well as cluster ions of the form Ag + M and Ag + (M - 45). The substantial difference of these spectra, as compared to the former, is the strongly reduced background (note the logarithmic intensity scale!), as well as the absence of contaminations originated by the solvent (AgCl;, Cl-, etc.).

3.2. Behaviour of different secondary ions during increasing deposition time

Figs. 5 and 6 present the behaviour of characteristic secondary ions during the deposition of phenylalanine on Au and Ag at room temperature. The previously cleaned target surface was positioned in front of the evaporation source and exposed a certain amount of time to the molecular beam. Then the


e

I 105

‘iii =wk 2d a.

f f 103

102

10'

100

Fig. 4. Negative secondary ion spectrum of about one monolayer phenylaianine on Ag. Upper spectrum: deposited from the solution; lower spectrum: deposited by UHV evaporation. Primary

ions: 1 x low9 A, Ar+ on 1 cm*.

sample was subsequently returned into the measuring position and the SI emission was investigated. This procedure was repeated, so that the deposition time is given by the sum of the single exposition intervals.

At low deposition times all signals - molecular, metal and fragment ions - show a nearly linear increase of intensity. Depending on the evaporation source temperature a plateau is reached after about 100 s (in the case of phe/Au: 395 K) and about 20 s (in the case of phe/Ag: 400 K). A further increase of deposition time does not change the intensities of the observed ions. It should be mentioned that under these experimental conditions the observed SI yields do not change at least during several hours due to reevaporation or chemical reactions of the organic material (see also section 4.1). The resulting curves are presented in the left half side of figs. 5 and 6.

When starting at a higher evaporation source temperature (Au, 406 K; Ag, 433 K), already after the first deposition interval all formerly observed ions


Phenylalanine -Au

(=J C~~HCOOH

NY M.165

104 T=395K

-l . Al-

T=406K

1 101 102 ld 1 1 102 depositionl4ime [secl-

103

Fig. 5. Characteristic secondary ion emission from a Au surface during the deposition of

phenylalanine at two different temperatures of the evaporation source (395 K, 406 K). Target

temperature: 298 K. Primary ions: 5 X lo-” A/cm*, Ar+ on 0.5 cm*.

were detected with high intensities nearly identical to those of the plateau region. The region of linear increase is not resolved.

Going on to evaporate the amino acid at elevated temperatures, we observed

Id 1 101 deposition time [set]

Fig. 6. Characteristic secondary ion emission from a Ag surface during the deposition of

phenylalanine at two different temperatures of the evaporation source (400 K, 433 K). Target temperature: 298 K. Primary ions: 5 X lo-” A/cm*, Ar+ on 0.5 cm2.

426 W. Lange et al. / Secondaty ion emission

that after a decrease of the substrate signals all secondary ion intensities are strongly reduced (not O-). They show a second plateau and decrease finally after still increasing deposition times below the detection limit. Here the counting rates of the SI spectra fluctuate unsystematically, which can partly be overcome by means of an electron flood gun. This is a strong hint that charging effects occur, which impede a further investigation of evaporation experiments. These results are presented in the right half of figs. 5 and 6. However, the so prepared amino acid layer changes with time. After some

hours the substrate specific ions were detectable again. All investigated amino acids (leucine, glutamic acid, methionine and phenyl-

alanine) showed the same behaviour on Ag and Au which is demonstrated schematically in fig. 7. Differences effected by the substrate material are described more detailed in section 3.4.

Instead of depicting the secondary ion yields as dependent on the deposition time we used the particle dose to which the targets were exposed. So a higher evaporation source temperature can be transformed to a higher particle dose, and a combination of the two parts of figs. 5 and 6 then should be possible. Of course, the assignment of the temperature transition relative to the particle dose in fig. 7 is arbitrary. nevertheless, in this picture five different

regions (I-V) can be distinguished.

3.3. Change of AES signals during amino acid deposition

To get more insight into the mechanism of layer formation, we studied characteristical Auger transitions during deposition of a Ni target with

t I I

,/ 100 lol 102 3 r,

particle e$. density &units1 105

Fig. 7. Characteristic change in (M rt H) *, O-, and Me * emission during the UHV deposition of amino acid molecules on a metal surface. Five different regions (I-V) can be distinguished. Between region I and III the source temperature has been increased (7” z T1).


methionine. Methionine has the special advantage that it contains sulfur, which was not detected on the clean surface and does not show interferences with substrate AES lines.

Besides S and C, the AES substrate lines of the Ni target (61 and 848 eV) during a deposition of methionine are represented (fig. 8). It is remarkable, that - a steep decrease of the low energy substrate peak is followed by a sharp bend and a transition to a stationary region; - the sulfur shows a similar sharp increase in the same region, followed by a stationary behaviour as well; - the carbon line increases also but its behaviour is not so pronounced.

3.4. Secondary ion formation on different substrates

The formation of molecular secondary ions strongly depends on the substrate material in the regions I-III (see fig. 7). In fig. 9 some data are collected exemplifying this behaviour for leucine on different metals, which were covered up to region II. For the (M - H)- or (M - 45)+ emission there are remarkable differences in the yields and obviously no systematic dependence exists on the mass number of the metal. Most interesting is the fact that the formation of protonated ions occurs for the noble metals gold, platinum and silver only.

A more systematic investigation of the coverage dependence of protonated

60- I 1’ Methionlne

-i;;!io- -NI CH3SICH212~HCOOH

NH? M.lL9 L

= - f2

40 1 al 30-

s

%20-

E L 10

ji;::-‘--r--~:

0 100 200 300 400 deposihon tlme[secl-

Fig. 8. Change in some characteristic AES intensities during the deposition of phenylalanine on Ni. Monolayer completion (region II, fig. 7) occurs below 20 s. Deposition temperature: 410 K. Target temperature: 298 K.


Leucine-Me target temp.: lO,Scm~

Tm Ta W Pt Au

Fig. 9. Relative intensities of the secondary ions (M-k H)+, (M-H)- and (M-45)+ after deposition of a leucine layer (region II) on several substrates.

and deprotonated molecular ions was carried out for a limited number of metals: Ni, Cu, Ag and Au. Fig. 10 presents the results for the protonated and deprotonated molecular ions of phenylalanine deposited up to region II. A (M -t H)+ emission could not be detected for nickel and copper under the ion bombardment conditions of static SIMS (5 x lo-” A/cm’, Arf, 3.5 keV).

Phenylalanme

M-165

(M+H,+

0 21 CH CHCOOH

2

A;

Au

/' __.---

/ A!4 /

T.395K -

I 1 II .

1 10' 102 ld 1 Id 102 103 deposition time [secl

Fig. 10. Change in the emission of protonated and deprotonated molecular secondary ions (M f H) * during the deposition of phenylalanine (up to region II) on various chemically clean metal surfaces.


This pronounced difference between noble and reactive metals is not observed for the (M - H)- emission.

If the phenylalanine is deposited up to region IV, (M + H)+ emission is observed on all metals including the reactive ones. Furthermore, here the intensities of (M + H)+ and (M - H)- respectively are the same, independent of the underlying substrate. This behaviour holds for the other amino acids as well.

3.5. Enhanced primary ion bombardment

During enhanced ion bombardment of the particular deposition stages on different metals the emission of protonated molecular ions is distinct from that of deprotonated species. For a region-I deposition of phenylalanine on Ag a rapid decrease (three orders of magnitude) of the (M + H)+ intensity occurs (fig. 11). The deprotonated molecular ion (M - H)- shows a quite lower decrease. The disappearance cross sections were estimated from the linear parts of the curves to 3.8 X lo-l4 cm2 for (M + H)+ and 0.8 X lo-l4 cm2 for (M - H)- respectively. This large difference in the cross sections was found for all investigated amino acids.

Ion bombardment of a region-IV layer shows a relatively weak decrease for all secondary ions (fig. 12). The corresponding cross sections are evaluated to 1.2 X lo-l4 cm2 for (M + H)+ and 0.8 X lo-l4 cm2 for (M - H)-. The difference in the disappearance of the protonated and deprotonated ions is quite smaller than during region-I sputtering. The relative Ag intensity remains constant, but one should keep in mind, that the absolute intensity is smaller by two orders of magnitude (see fig. 6). This observation is a strong indication that the amount of deposited material did not change over the time of the

I Phenylalkine I/L -Ag 1sJ -

OS -

0.01 -

O,OOl-

submonolayer coverage

45- [M-HI-

Ag+

w4v

L I . I , 1 * I I

0 1 2 3 4 ion dose [As cmm21 -

5x10-

Fig. 11. Relative decrease of characteristic secondary ion emission during sputtering of a phenylalanine submonolayer on Ag (region-I coverage).

430 W. Lange et al. / Secondaty ion emission

t ’ I

III0 1.0 -

0.1 -

Phenylatanine

0.01 - -4

I

45-

Ag+ (M-HI-

II-45l+

iM+H)+

multilayer coverage

I

0 1 2 3 4 ion dose t As cm-2 l-

5x10-*

Fig. 12. Changes of characteristic secondary ion intensities during sputtering of a phenylalanine multilayer on Ag (region-IV coverage).

experiment. So layer changes due to reevaporation were obviously negligible and we really measured the irradiation damage. During enhanced ion bombardment, however, new mass lines appear in the range > M presumably

as a result of ion bombardment induced reactions of the amino acid molecules and fragments.

3.6. Variation of target temperature

During temperature increase, amino acid samples deposited on Ag show two clearly separated changes in their secondary ion emission. As an example,

I z

105

Y

+?lOk

2 2 E 103

10

I

10'

Phenylalanme

-ALI

I...#.. .I..,.‘., .1,,.,‘.,,,I I 300 350 400 450 500 550K

target temperature

Fig. 13. Changes of characteristic secondary ion intensities during temperature increase of a Ag target covered by a region-II layer of phenylalanine. The disappearance of M + H)+ and (M - H)- emission occurs at different temperatures, indicating the existence of different bonding situations of these two secondary ions.

W. Lange et al. / Secondary ion emission 431

in fig. 13 the corresponding emission behaviour of a region-II layer phenylalanine on silver is shown. Up to a temperature of about 320 K no consider- able change in secondary ion emission occurs. In the range from 320 to 350 K the intensity of the protonated ions completely disappears. The (M - 45)+ intensity is reduced by a factor of about 5, and the emission of deprotonated molecular ions, however, virtually does not change. Finally, after a ragion of constant secondary ion emission up to about 450 K, the (M - H)- disappears together with the ions (M - 45)+ and Ag+. The O- and OH- emissions change at both of these temperatures.

This different behaviour of protonated and deprotonated molecular ions at

I

Glycine -ku tq-COOH_ ~

NH2 M=75

Glycine-CL

target temperature IKI

Fig. 14. Changes in secondary ion emission during target heating of different substrates. A Au (upper part) and a Cu target (lower part) were previously deposited by a region-II layer of glycine; from ref. 1191.


various temperatures was found for all investigated samples. Always the protonated ion disappears first.

Fig. 14 depicts the results of heating experiments performed on Au and Cu, which were covered by glycine up to region II. In the case of Au all parent molecular ions (M k H) * and (M - 45)+ drop at about 330 K (the remaining (M - H)- and (M - 45)+ ion yield is due to desorption from the target holder).

Note the practically equal yields of (M + H)+ and (M - H)- at low temperatures. The same experiment performed with a Pt target gave similar results.

The main feature of glycine desorption from a copper target (covered up to region II as well) is the high (M - H)- yield up to 450 K and the drastic decrease around 480 K. (There is a little (M + H)+ emission observed as well, but this is not followed up here.) A Ni target covered by amino acid material showed comparable results.

4. Discussion

4.1. Formation of metal supported amino acid layers

In the range of lowest particle dose the yiuelds of all secondary ions are increasing nearly linearly, including the lines of the target metal. The increase of the substrate ions during adsorption on a clean metal surface is a well-known phenomenon in SIMS. This fact and the increasing molecular ion intensities indicate an increase of the coverage. In region II the intensities of all secondary ions remain constant. This means that the coverage of the substrate remains constant.

The increase of coverage is clearly seen in the AES investigations. It is known that the molecular structure of organic overlayers is destroyed by conventional AES control, according to the high electron current densities, but assuming that most of the organic fragments remain on the surface still a thickness control of the surface layers is possible. In fig. 8 a steep decrease of the substrate atomic Auger peak-to-peak intensities is observed in region I, as well as a corresponding increase of typical adsorbate specific atomic lines. Differences in time at the sharp bend of the intensity-time curves of the SIMS measurements on one hand and the AES results on the other are due to differences in the evaporation temperatures. In region II only small AES intensity changes of both are observed. The intensity change of the Ni line (62 eV) as a whole is typical for the change of AES intensities during monolayer completion of adsorbates. So region II can be characterized as monolayer-like though the particle dose is still increasing.

These observations can only be understood assuming a strong reevaporation due to the relatively high vapour pressures of the amino acids [21] and the missing bonds to the substrate. Thus at low evaporation temperatures sub-


monolayer- and monolayer-like adsorbate structures can be generated. (These conclusions are strongly confirmed by “static” AES investigations [23].)

The reevaporation can be compensated by an increase of the incoming particle flux produced by a temperature raise of the molecular beam source. When the evaporation rate is increased remarkable changes of SI yields are observed (regions III-IV of fig. 7). The yield of the molecular secondary ions drops drastically sometimes by orders of magnitude, and the metal ion emission disappears completely (region III).

All these observations may be explained by multilayer formation. In region IV only secondary ions of the adsorbed amino acid are detected, and no signal of the covered substrate is observed. Thus the emission of molecular secondary ions now corresponds to that of the compact material. The O- (III-V) emission remains nearly constant. This behaviour is not completely understood.

4.2. Bonding states of protonated and deprotonated molecular ions

Both ion bombardment and heating experiments (figs. 11, 13 and 14) show a different behaviour of (M + H)+ and (M - H)- concerning the disappearance cross section and the target temperature. This indicates that for these two secondary ions two different types of bonding states exist.

During target heating the (M + H)+ completely disappears below 350 K. In the same temperature region a corresponding drop of Ag+ (about a factor 5) is observed. This indicates a desorption of a certain amount of sample material from the surface. Above 350 K the state from which the protonated molecule is emitted does not exist anymore. Whereas the ions (M - H)-, (M - 45)+ and Ag+ decrease totally at a temperature region which is about 100 K higher (about 450 K). This indicates a second bonding state, which completely vanishes with the residual sample material.

Ion bombardment of a submonolayer (fig. 11) confirms this picture of the two different bonding states. The weaker bond of (M + H)+ is correlated with a higher cross section during submonolayer sputtering. Vice versa the lower cross sections of (M - H)- and (M - 45)+ may indicate a higher binding energy of these particles.

The disappearance cross sections of protonated and deprotonated molecular ions during multilayer sputtering (fig. 12) are slightly different but not so pronounced as in the case of a submonolayer. The existence of two bonding states cannot be concluded. This may be explained by the assumption that sputtering of amino acid molecules within a matrix of the same molecular species effects the formation of both secondary ions (M + H) * in a similar manner.


4.3. Influence of the substrate material

The behaviour of the protonated and deprotonated molecular ions discussed for silver in the previous section holds partly for the reactive metals Cu and Ni covered up to one monolayer as well. The small amount of (M + H)+ (about 1% of (M - H)-) desorbing between 300 and 350 K during heating of the copper target (fig. 14) may be due to the fact that in some districts of the surface slightly more than one monolayer was deposited. However, the main feature of reactive metals is the disappearance of (M - H)- and (M - 45)+ at temperatures higher than 450 K. This is in strong contrast to the results on the noble metals Au and Pt, where these two ion species desorb already at a temperature below 350 K. Thus following our concept of bonding states, for noble metals one can claim the existence of a weakly bound state only causing the emission of protonated and deprotonated ions in a similar manner. On the contrary, the reactive metals display primarily a strongly bound state. The silver may take a position between the noble and the reactive metals, but this is not completely clarified.

4.4. Model of amino acid-substrate interaction

It is known from the literature that amino acids in bulk are present in the zwitter-ionic form. We assume that this picture also holds for multilayers of

.PHY=XIlQN MULTILAYER

CHEMISCRPTION

Fig. 15. Some possible surface bonding states of amino acid molecules. Physisorption: proton exchange is possible between different amino acid molecules or between the metal surface and an ammo acid molecule. Chemisorption: a chemical bond is formed between the amino acid and the metal surface. The deprotonated molecule acts as an anion. Multilayer: ion formation occurs by proton transfer, which is possible between adjacent ammo acid molecules only.

W. Lnnge et al. / Secondary ion emission 435

amino acids and even for sub- and monolayers on noble metals (fig. 15). The detected molecular ions may then easily be formed by proton transfer between adjacent molecules. So one molecule releases a proton, becoming negatively charged, and an adjacent one incorporates the proton, getting the positive charge. We assume that this process and the resulting secondary molecular ion emission are strongly affected by the chemical nature of the noble metal substrate.

In the case of reactive metals we assume that amino acids are adsorbed by loss of hydrogen resulting in a polarized bond between (M - H)- and the

metal. Where the separated hydrogen finally goes is unknown. In this picture the formation of the deprotonated ion is the result of this bond break due to ion bombardment.

Furthermore, already the adsorption process of an organic molecule on a

metal surface may result in the fragmentation of the molecule and the formation of metal-fragment complexes. This was observed by SIMS for various compounds as formic acid on Ni-Cu alloy [24] or ethylene and acetylene on nickel [25]. However, in the case of amino acid adsorption on metal surfaces we assume that the molecules are adsorbed unfragmented. nevertheless on the basis of these experiments a clear decision cannot be made.

5. Conclusion

The secondary ion formation during sputtering of organic molecules strongly depends on the chemical composition and structure of the bombarded sample surface. The results of the present investigations on the secondary ion emission from UHV deposited amino acids on various metal surfaces and its change during heating or ion bombardment supply more detailed information on the interaction of these organic molecules in direct metal contact as well as in the environment of the same chemical species. Both of these surface compositions are strongly correlated to remarkable differences of the resulting secondary ion emission, especially of the molecular ions (M + H)+ and (M - H)-.

This kind of SIMS experiments with more defined and controlled surface compositions are a prediction for an improved understanding of the ion formation process as well as for an optimization of its analytical application.

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