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Heavy particle induced ion emission from Langmuir-Blodgett films: dependence on the charge state and the angle of incidence
Nuclear Instruments North-Holland
and Methods
in Physics
Research
121
B52 (1990) 121-128
Heavy particle induced ion e~ssion from ~angmuir-B~~dgett dependence on the charge state and the angle of incidence
films:
S. Della-Negra, J. Depauw, H. Joret, Y. Le Beyec, 1.53.Bitensky *, G. Bolbach * *, R. Galera * * and K. Wien * * * Institut de Physique Nucle!aire, 91406 - Orsay Cedex, France * Arifov Institute of Electronics, 700143 Tashkent, USSR * * Institut Curie, II rue P.&M. Curie, 75231 - Paris Cedex 05, France * * * Institute of Nuclear Physics, TH Darmstadt, W-6100 Darmstadt, Germany Received
15 May 1990 and in revised
form 24 August
1990
At the Orsay tandem accelerator beams of 32S and “‘I at energies of 1 MeV/u were used to investigate secondary ion emission from Langmuir-Blodgett films deposited on Au. The yields of negative parent ions, dimer and trimer ions were measured for various superposed fatty acid layers as function of primary ion charge state and angle of incidence. By means of TOF mass spectrometry the ejection of intact molecular ions was observed from layers as deep as 200 A underneath the surface. For grazing primary ion trajectories, the charge state dependence of yields are almost independent of the charge state of the primary ions (PI). The results are in fair agreement with theoretical calculations based on crater formation due to a shock wave expanding from the highly energized
zone along the nuclear track.
1. Introdudion It is well established that heavy ions in the 1 MeV/u energy regime are very efficient for ejecting intact molecular ions and neutral molecules from solid organic layers. The mechanisms involved in the “desorption” of these molecules are not completely understood; several theoretical approaches have been made [1,14]. The models have to describe four basic steps: 1) the initial electronic energy deposition and its spatial distribution as a function of time, 2) the transfer of this energy into forces and molecular momenta, 3) the dynamics of molecular motion into the gas phase leading to free and intact molecules, and 4) the ionization process. The treatment of these steps is particularly complex and experimental results are needed to highlight the possible processes of emission. In this work we present new results on the influence of the initial charge state of the projectiles and on the influence of the angle of incidence. Beams of lz71 and 32S at 1 MeV/u and several types of Langmuir-Blodgett targets have been used. It is shown that molecular ions are emitted from the surface but also from deep layers below the surface whatever the angle of incidence and the charge state of the primary particle. For normal angles of incidence a strong influence of the primary ion charge state on the secondary ion abundance is observed. For grazing angles this influence vanishes in agreement with the assumption of a large interaction depth 121. 0168-583X/90/$03.50
0 1990 - Elsevier Science Publishers
A shock wave model has been used to reproduce experimental data of secondary ion yields as a function of incident charge states and angles of impact. A large number of experimental data has been collected [3] and only part of them will be presented in the following.
2. Experimental Experiments have been performed at the Orsay tandem accelerator with ion beams of 32S and 1271 at energies of 32 and 127 MeV. The selection of a given projectile charge state is made with a large magnet after Rutherford elastic scattering of the beam through a thin carbon foil (10 pg/cm*). The reaction chamber containing the targets is mounted on the exit flange of the magnet. A low intensity beam (< 500 ions/s) can be obtained by moving the magnet (and therefore the reaction chamber) to a certain angle with respect to the incident beam before diffusion. In the reaction chamber (40 cm in diameter) several targets (or samples) can be kept under vacuum (vacuum lock) and positioned successively in the irradiation position at the center of the chamber. A collimator hole of 1 mm is put at the entrance of the chamber to define the spot size of the beam on the sample. The bottom of the chamber which supports the targets and a short distance time of flight mass spectrometer can rotate by 360 O, Therefore, the
B.V. (North-Holl~d)
122
S. Della-Negra
et al. / ion induced emission from Langmuir-Blodgett
angle of incidence of the beam with the target surface can be changed precisely without changing the other experimental conditions. We define the incident angle fl as the angle between the beam direction and the normal to the target surface. For practical reasons B has only been varied from 20 o (almost normal incidence) to 78 o (almost grazing incidence). A silicon detector has been used to monitor the energy and the intensity of the primary ions. Time of flight mass spectra of secondary ions were measured in a few minutes with the multiparameter data acquisition system described elsewhere 141. It was sometimes necessary to bombard the targets with projectiles having a charge state equal to the equilibrium charge state (4,) inside the solid. Since we have used in this work organic targets, we have chosen equilibrium charge state values experimentally found for carbon targets. It must be recalled that the mean equilibrium charge state inside solids differs [5,6] from that measured outside the target which can be determined for example with a magnet after the passage of ions through a foil. For ‘*‘I at 127 MeV and 32S at 32 MeV, the ( qeq) values are respectively 24+ and 11 t- [6]. It has been shown experimentally [3] that an equilibrium charge state of ‘*‘I at 127 MeV is reached after passage through 350 A of carbon. Langmuir-Blodgett films have been prepared according to the methods reported earlier 171. Monolayers of fatty acids were transferred onto thin films of gold (1000 A) vacuum deposited on glass disks. Due to the hydrophobic character of such gold surfaces, an even number of monolayers is normally transferred. In the present experiments, the samples consisted of superposed layers of 2 types of fatty acids: arachidic acid (MW = 312.3) and stearic acid (MW = 284.3). However the water subphase at pH = 5.7 contains Cd*’ counterions (CdCl,, lop4 M/l). Under these conditions, every monolayer consists of a mixture of non dissociated fatty acids and Cd salts of fatty acids (2(Mi-H)Cd, i = 1, 2). Since the pH is constant during the process of transfer it is assumed that the composition of the layers is the same as in the subphase, around 65% of Cd salt [S]. The upper layer is referred as M, and the underneath layer as M,. In the following, for the sake of simplicity, the notation nM,/mM, will refer to a composite film corresponding to n monolayers of M, deposited on top of m monolayers of M, on Au substrates. In a few cases, a third type of fatty acid - palmitic acid (M3, MW = 256.2) - was used as an underlayer of arachidate and stearate. The characterization of LB films was first checked during the transfer of the monolayers onto the metal substrate. The orientation of the meniscus for both the immersion and the emersion of the substrate showed that the surface of the substrate became hydrophobic during the emersion and hydrophilic during the immersion. The transfer ratio (area of transferred mole-
films
cule/area of the substrate surface) was measured and found to be 1 + 0.1, for both immersions and emersions. A second characterization was performed on stepped LB films using Nomarsky microscopy (93. These films obtained in the same experimental conditions than those used for desorption studies were composed of superposed 2, 4 and 6 ML; 25% of the substrate area were covered by 2 ML, 25% by 4 ML and 25% by 6 ML. The analysis by Nomarsky microscopy of such samples clearly showed the different steps Au-2 ML, 2 ML-4 ML,. . . This indicated that the pile up of the monolayers was exactly the pile up expected from the transfers. The Normarsky microscopy analysis of the region corresponding to a given coverage (2 ML or 4 ML or 6 ML,. . .) showed that about 95% of the surface area was homogeneous. Thus only 5% of the sample area were expected to be inhomogeneously covered (pinholes or dust particles). This inhomogeneity did not depend on the film thickness. It is due either to small holes or more likely to dust particles. Fig. 1 shows a negative ion mass spectrum of a composite film 2M,/6M,. The spectrum exhibits essentially the deprotonated molecular ion of the fatty acids (M,-II-, i = 1 and 2, the dimers (2Mi-H)-, (M,M,H)- and trimers containing Cd [3(M,-H) + Cd]-. Series of fragments are also observed but their origin cannot be assigned specifically to M, or M,. The ion desorption yield Y which is shown in the figures is obtained from the measured yield Y, which is the ratio between the number of detected secondary ions N,, and the number of primary N,, hitting the target. Np, is measured by the secondary electrons ejected from the target after impact. Assuming an efficiency of detection e = 0.6 and a Poisson dist~bution
12T124+ III ‘A-H)-
CnH-
B.SS*
127 Me’/
1600 -
ZMLX 6ML M1 ----_
Gold 1200
(t+Hl
(2M,-ii IMIMz-HII2 Mt- H’;l
I
It31Mz-Hl+Cd)
Fig. 1. Mass spectrum of a Langmuir-Blodgett film with 2 layers of M, cm 6 layers of M, (see text).
S. Delta-Negru
of the number can write: Y= -log{1
3. Resub
of secondary
- &Jm.
et at. / Ion induced emission from Langmuir-Blodgetl
ions emitted
by impact
films
123
we
1
and discussion
The increase of secondary ion yields with the thickness of samples has first been studied with fission fragments from *“Cf [lo-121. Langmuir-Blodgett (LB) films were used to obtain quantitative results as function of the sample thickness [Il-131. These results were consistent with a simple picture of emission from a conical volume, the depth of the crater was estimated to be about 200 A. To confirm these results, we have used 32S and ‘271 ions at 1 MeV/u having equilibrium charge states and superposed LB layers of nM, on 6M,. The number of top layers n has been varied from 2 to 12. Molecular ion yields from upper and underneath layers were measured simultaneously for each target. Fig. 2 shows the results obtained with “S”+. The yield of the molecular ions M, decreases rapidiy when the number n of M, layers increases. After 7-8 layers (175-200 A) the emission yield from the “bottom” layer is close to the background and remains constant. An opposite trend is observed for the yield of M, which increases with n and saturates after PI= 8. Fig. 3 pre-
I
3
I
1
,
Number
of
(
I
32MeV
Monotoyers
Mt
Fig. 2. Variation of the molecular ion yield (Ml-H)and (M,-Ii)as a function of the number of layers of M, deposited on top of 6 layers of M,. The projectiles are 32S ions at 32 MeV. The angle of incidence is 45 O.
Number
of MmoloyersM,
Fig, 3. Yariation of the molecular ion yield (M, -H- ) and (M,-H)as a function of the number of layers of M, deposited on top of 6 layers of M,. The projectiles are ‘271 ions at 127 MeV, the angle of incidence is 45”.
~~~~~~~lar results measured with the same targets and I projectiles. Above 10 layers, the yields of M, and M, are constant. The saturation depth is roughly the same with 32S and “‘I and therefore does not seem to depend strongly on the electronic stopping power of the projectiles (dE,/dx(‘“I) = 3dE/dx(32S) at 1 MeV/u). In an earlier paper 121 on the charge state dependence of ion yields an “interaction depth” din, was introduced (see also [16]), which is not equal to the emission depth observed experimentally here. dint is nearly proportional to dE/dx and would have at normal incidence the values 90 t 20 A for 32S’1+ and 300 & 80 A for 12?I14’, respectively. It reflects the effective length of the nuclear track (at normal incidence), from where an interaction causing desorption reaches the uppermost surface layer. Only this top layer contributes to desorption. As the present work proses, also molecules from layers as deep as 200 w underneath the surface appear in the TOF spectrum. Therefore, the picture of crater formation seems to be more appropriate, to describe particle desorption by heavy ion impact, than an effective interaction depth. An exception is one organic monolayer on top of a metal substrate since then only the organic layer is desorbed and the metal remains undestroyed. In this case, dint should be a useful parameter (see also the next section).
124
S. Della-Negra et al. / Ion induced emission from Langmuir-Blodgett films
The absolute secondary ion yield of emission is however almost 10 times larger with lz71 than with 32S. That is consistent with many other experiments, where a (dE/dx)” dependence of secondary ion yields with n = 2-3 is reported (see, for instance, the review article 1141). If we apply the simple picture of a conical ernission volume, the cone would have the same depth for 32S and **?I, but a larger surface cross section in case of ‘*? ions having the higher dE/dx value (see also the formalism in the next section). It should be noted that the elongated shape of the present molecules may give a nonuniversal picture of the desorption process. 3.2. Incident angle dependence of the electronic desorption yield Experimental results on the angle dependence have been reported by several authors [15,16,11]. These experiments were performed with MeV ions in their equilibrium charge state. It is known however that a distance larger than 150 A is necessary to achieve charge state equilibration. Calculation and measurements are in agreement on this point [6,17]. As shown in the preceeding section the emission depths can reach 200 A. If the fast incident ion is not in its equilibrium charge state the variation of yield with B depends on the incident charge state qi and on its equilibration function [q(x) = f( q,, x)]. The effects were first observed by Nieschler et al. 1161 with oxygen ions at 9 MeV with 2 s q. I 8. With 32S ions between 6+ and 14+ as well as with 127I between 18’ and 30+ the angle dependence of the SI yields is very sensitive to the primary ion charge state. Fig. 4 shows the yield variation of molecular ions for 32S8+ and 32S’4+ as a function of the angle of incidence. For 0, = 78” there is no effect of the charge state on the M, yield and on the M, yield whereas for @i = 30’ the charge state effect is clearly visible between Ss+ and S14+. The absence of charge state effects at large angles of incidence is also shown in fig. 5 which presents systematic studies with an LB target 12M,/6M2 bombarded by 12’1 ions at the angles 20*, 45”, 60°, 70 O, 78 O. At 20 o the slope of yield variation with qi is very steep. The deposited energy loss which contributes to the desorption depends on the charge state variations along the track. Qualitatively, at this angle, the interaction distance is at least of the order of the distance travelled for equilibration (200-300 A) (by reference to a previous work with Kr ions [6]). As B increases the influence of the primary ion charge state vanishes and the yield variation tends to be flat. The primary ions travel a long distance compared to the equilibrium charge state distance and the secondary emission depends mainly on the energy loss with the charge state ~uilibrated. If the m~mum depth of emission is
Fig. 4. Secondary molecular ion yields as a function of the angle of incidence. The primary ions are 32S at 32 MeV with the charge
states 8+ and 14+.
around 8-10 layers (200-250 A) then the primary ion travel distance could be around 1000 A! The geometrical shape of the volume (or “crater”) of
J 60’ 5
9.10-1
5 s
e.10-
.I -
D 7.10’ .m ; 6.10.?
,I -
5 :
I-
5.10-t
4.10-I
3.10-’
----
-
Gold
2.10”’ ,I;,
. 2o
J 30’ 12’Jq’ Charge
state
Fig. 5. Dependence of the (M,-H)yield from a mixed multilayer film on the angle of incidence. A beam of “‘1 ions at 127 MeV with charge states between 20+ and 30+ was used.
S. DebNegra
et ol. / Ian induced emission from Langmuir-Blodgett
emission is expected to vary considerably with the angle of projectile impact and is certainly very asymmetric for large angles. This is in agreement with the anisotropic angular distribution of emitted ions measured by the Uppsala and the Darmstadt groups [l&19]. Targets with a relatively thin upper layer of M, molecules 2M,/&I,/Au and 2M, on Au have also been prepared to study the influence of the charge state and the impact angle. Experimental results are shown in fig. 6. At 20°, for 2M,/Au the equilibrium charge state is not achieved in the organic layers but in the gold material. At 78”, the charge state is almost equilibrated in the two M, monolayers. A dependence of yield of M, on the charge state would be expected if only the energy loss in these 2 monolayers would be used for desorption. At this angle, the experimental results exhibit a flat dependence of yield with q. It is therefore concluded that the energy loss in the gold substrate, by projectiles in their equilibrium charge state, contributes to the desorption of M,. Energy is transferred in gold, from a certain distance, to the attached 2 monolayers above, although the depth of emission cannot be larger than 50 A (2Mr only). A similar behaviour is observed with the target 2M,/6M,/Au. The difference of SI yields between 20° and 78” is however not as large as with the 2 monolayers directly on the gold substrate. These results demonstrate that the energy distribution in the material can
20*
films
30+ La* ‘271q* Charge state
Fig. 7. Yields of (MI-H) and (M,H) ejected from 2 superposed LF3films (6M,/6M,) as a function of the charge state of *“I projectiles at a fixed incident angie 8 = 45O.
be transferred over a large distance and that the interface metal-insulating organic layer does not prevent the energy transfer. With a 6M,/6Mz sample, the same effects are displayed in fig. 7. The variation of SI yields, as a function of the charge state, for both molecular ions emitted from the two layers by r2’I projectiles is the same. The angle is 45 *. The equilibrium charge state is reached in the second part of the target (M,) but the rate of emission from M, is depending on the dist~bution of energy (variation of charge state) loss in the first part (M,). The memory of the incident charge state is kept (because a dependence of q, is observed for Y(M,)) and realized by the layers underneath. There is a certain flow of energy from the upper layers to the bottom layers and vice versa. The absolute yields of M, and M, are of course very different and in agreement with the results of fig. 3. 15’
20’
30’
20’ 1271’4+
30’
Chorqe state
Fig. 6. Variation of the (MI--N)- ion yield from 2 monolayers M, on GoId (right part) and from 2 monolayers of M, on top of 6 M, (with a gold substrate, left part). The Influence of the angle of incidence and of the primary I271 ion charge state is dispfayed.
4. Comparisons of yield dependence on the primary ion charge and the angle of incidence with cafcidations from a shock wave mode1 In recent papers 120,21] a shock wave mechanism has been developed for the sputtering of bio-organic mole-
S. Della-Negra
126
et al. / Ion induced emission from Langmuir-Blodgett
cules. Emission of molecules occurs from a crater produced by a shock wave, which originates from a high density energy spike in the vicinity of the heavy ion track. Crater formation is in agreement with the present results and the yield dependence on film thickness already mentioned [l&12]. Formulas derived in 120,211 will be utilized after some modifications concerning desorption from LB films. For grazing incidence of primary ions having equilibrium charge, the boundary of a crater with a surface is an ellipse (221. One of its semiaxes p,e depends on the angle of incidence 6, the other one (perpendicular to the beam direction) pa0 does not. If p, is the crater radius for 0 o impact (8 = 0), then 6320 =
PI0
Ps*
=
P,/COS
@*
(1)
Analogously to ref. [20] one can show that the semiaxes of the ellipse are given by Pli =
PlO
(4ilqeq
)
where qi differs from the equilibrium value qe9. x1 = X/plo and X is the charge equilibrium distance. pzi is obtained by means of a similar formula, where the index 1 is replaced by 2. For B = 0 the semiaxis pli and
A)
fihns
pzi are equal and approach the value p of eq. (31) in ref.
[21] for xX.2 >> 1. In the case of two monolayers of molecules M, on top of 6M,, the yield of the parent ions M, is given by a formula equivalent to eq. (24) in ref. [Zl]: ~(6)=4a1)-NmLpijpai
i
l+z
1
~[(1-~).(l-%)]1’2.
(3)
Here, n- is the probability to form and eject the negative parent ion (M,-H)-, Nm the number density of M, and pi the radius of the high density energy spike around the nuclear track. i is the index of the charge state q,. The yield y(e) is proportional to the surface area of the crater reduced by an area of destroyed molecules around the point of impact. The semiaxis bli, can be written as bli = p,/cos B + C(e) (figs. 8A and 8B). For 0 = 0, C(0) = ao, and for B = a/2, C(e) approaches L tg 8. Parameter a0 depends on the crater shape. For example, for cylindrical crater shape LIP= a (here a is the radius of linear molecules with length L) for spherical one a0 = L2/2ps (fig. 8C) and for conical one a0 = L tg +/2 where (h is a cone vertex angle. With
area of intact molecule emission
0 = 0 and spherical crater Fig. 8. Schemes showing the surface of emission in the shock wave model (see text).
S. Della-Negru
127
et al. / km induced emission from ~~~muir-Blodg~tt films
the assumption that C(e) 2=a0 + L tg 8 we obtain the semiaxis hn and bzi: bli=
&
+a,+LtgB
and
b,,=p,+a,.
(4
In fact, the actual boundary of the destruction area is difficult to estimate and, therefore, we used the value of a, as a fitting parameter depending on the crater shape at i? = O”. Fig. 9 shows the results of the calculation describing satisfacto~ly the expe~ment~l data (for B -Z 70 o ) of the yield dependence on the initial ion charge and the angle of incidence. For 8 = 7S0, eq. (3) overestimates the molecular ion yield because for large 8 the parameter xi = X/p,, becomes smaller than 1 and the approximations made to evaluate eq. (2) are not valid. The primary ion equilibrium charge state of ‘27I at 127 MeV is 24+ and this value has been taken as q,_, the other parameter had the values 4 = 100 A (crater radius), pi/p% = 0.17, ae= 24 A, h = 200 A. Q- has been adjusted to the expe~mental data for @= 20* and a,= 2@. It should be noted that the fitting value a,, = 24 A (a,, = L) shows that for B = 0 o we can approximate the crater as a cone with a vertex angle # = 90°. On the basis of the shock wave model of Biter&i et al. 1211, a spherical crater is obtained for fl = 0 o im-
. *
2 .
*
.
*
*
t
78’
70’
60’
* 2 ML 6ML
I 15+
20’
1%
-emGold
_
,lrlcllrru
30+ r271q* Charge
state
Fig. 9. Comparison of the calculated variation of yields with B and 4, (solid lines). The parameters are pS= 100 A, a = 25 A and h = 200 ,& (see text). The target is made out of 2 ML of M, on 6 ML of M, on a gold substrate.
6ML 6ML 2OML
_-_-Cold
5.10‘2 20’
30” 40 ‘*‘Iq*Charge state
Fig. 10. Yields of the (M,-H)-
parent ions, the dimer and the trimer as a function of the charge state of ‘271 at 127 MeV (8 = 45 “) (solid lines). The target consists of superposed layers 6M1/6M,/20 M, on Au.
pacts, the crater depth ps being equal to 100 A in the case of i2’1 at I MeV/u. On the other hand we have observed that the emission of intact molecules occurs from a depth larger than 150 A and a conical shape of crater was taken as a first approach to explain the data. It is however interesting to note that, with the shock wave model, the general trend of yield variations with t? and s can be reproduced. This model also qual~tativeiy reproduces the asymmetric distributions of secondary molecular ions observed ]18,19] for nonnormal angles of incidence. As it is seen in fig. 10, the dependence of the yield of the parent ion (M,-I-I)-‘, the dimers (2M,-H) and even the trimers (3(M,-H) + Cd)- can be described by eq. (3), but with a larger value of h = 300 A. The observed steeper yield dependence of the cluster ions (2M,-H)and (3(M,-II)+ Cd)- (compared with the curve of (M,-H)-) on the primary ion charge state is reproduced if a larger size of the molecules is used in the calculation. We choose a, = 50 A and L = 25 A or 50 A, respectively, for the dimer and the trimer. That means, the dimer molecule has the same length as M, whereas the trimer is twice as long. Two different values of h have been introduced as fitting parameters in figs. 9 and 10. This could be an indication that the exponential dependence of the charge state in ref. {20] is not an appropriate description of the charge variation in solids.
128
S. DeIIa-Negra et al. / Ion induced emissionfrom Langmuir-Blodgett films
In fact recent experiments have shown that this variation is more complex [23]. The steeper slope for the yield of large molecular ions indicates that they probably originate from a smaller depth. Dimers and trimers have a higher chance to escape without dissociation if they come from the upper layers. The calculations which have been presented can explain the experimental results within the framework of a shock wave model. E. Hilf and F. Kammer have also performed calculations by means of direct numerical evaluation of molecular equations of motion [24]. Their results give information about the magnitude of the contribution from different layers and are in qualitative agreement with the results on thickness effects.
5. Conclusions The influence of the PI incident angle and of the PI charge state has been systematically studied. The secondary ion yield of molecular ions forming LangmuirBlodgett films depends on both parameters and there is no simple relation between the yield and the angle of incidence. For grazing angles, a constant electronic stopping power (dE/dx a (q,,)*) is involved in the desorption processes. The memory of the projectile charge state is lost. A tentative explanation of these experimental results has been made by using a fluid dynamical model and the general trends of the secondary ion yield variations can be reproduced by such a model.
Acknowledgements We acknowledge the staff of the tandem accelerator, G. Andlauer for his kind assistance, D. Chatenay and B. Colin for their Nomarsky microscopy studies of the LB films and E. Davanture for taking care of the manuscript. One of us H.J. thanks the Delsi-Nermag Company for financial support. I. Bitensky thanks the Institut de Physique Nucltaire for inviting him as a visiting scientist for a one month period.
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and Methods
in Physics
Research
121
B52 (1990) 121-128
Heavy particle induced ion e~ssion from ~angmuir-B~~dgett dependence on the charge state and the angle of incidence
films:
S. Della-Negra, J. Depauw, H. Joret, Y. Le Beyec, 1.53.Bitensky *, G. Bolbach * *, R. Galera * * and K. Wien * * * Institut de Physique Nucle!aire, 91406 - Orsay Cedex, France * Arifov Institute of Electronics, 700143 Tashkent, USSR * * Institut Curie, II rue P.&M. Curie, 75231 - Paris Cedex 05, France * * * Institute of Nuclear Physics, TH Darmstadt, W-6100 Darmstadt, Germany Received
15 May 1990 and in revised
form 24 August
1990
At the Orsay tandem accelerator beams of 32S and “‘I at energies of 1 MeV/u were used to investigate secondary ion emission from Langmuir-Blodgett films deposited on Au. The yields of negative parent ions, dimer and trimer ions were measured for various superposed fatty acid layers as function of primary ion charge state and angle of incidence. By means of TOF mass spectrometry the ejection of intact molecular ions was observed from layers as deep as 200 A underneath the surface. For grazing primary ion trajectories, the charge state dependence of yields are almost independent of the charge state of the primary ions (PI). The results are in fair agreement with theoretical calculations based on crater formation due to a shock wave expanding from the highly energized
zone along the nuclear track.
1. Introdudion It is well established that heavy ions in the 1 MeV/u energy regime are very efficient for ejecting intact molecular ions and neutral molecules from solid organic layers. The mechanisms involved in the “desorption” of these molecules are not completely understood; several theoretical approaches have been made [1,14]. The models have to describe four basic steps: 1) the initial electronic energy deposition and its spatial distribution as a function of time, 2) the transfer of this energy into forces and molecular momenta, 3) the dynamics of molecular motion into the gas phase leading to free and intact molecules, and 4) the ionization process. The treatment of these steps is particularly complex and experimental results are needed to highlight the possible processes of emission. In this work we present new results on the influence of the initial charge state of the projectiles and on the influence of the angle of incidence. Beams of lz71 and 32S at 1 MeV/u and several types of Langmuir-Blodgett targets have been used. It is shown that molecular ions are emitted from the surface but also from deep layers below the surface whatever the angle of incidence and the charge state of the primary particle. For normal angles of incidence a strong influence of the primary ion charge state on the secondary ion abundance is observed. For grazing angles this influence vanishes in agreement with the assumption of a large interaction depth 121. 0168-583X/90/$03.50
0 1990 - Elsevier Science Publishers
A shock wave model has been used to reproduce experimental data of secondary ion yields as a function of incident charge states and angles of impact. A large number of experimental data has been collected [3] and only part of them will be presented in the following.
2. Experimental Experiments have been performed at the Orsay tandem accelerator with ion beams of 32S and 1271 at energies of 32 and 127 MeV. The selection of a given projectile charge state is made with a large magnet after Rutherford elastic scattering of the beam through a thin carbon foil (10 pg/cm*). The reaction chamber containing the targets is mounted on the exit flange of the magnet. A low intensity beam (< 500 ions/s) can be obtained by moving the magnet (and therefore the reaction chamber) to a certain angle with respect to the incident beam before diffusion. In the reaction chamber (40 cm in diameter) several targets (or samples) can be kept under vacuum (vacuum lock) and positioned successively in the irradiation position at the center of the chamber. A collimator hole of 1 mm is put at the entrance of the chamber to define the spot size of the beam on the sample. The bottom of the chamber which supports the targets and a short distance time of flight mass spectrometer can rotate by 360 O, Therefore, the
B.V. (North-Holl~d)
122
S. Della-Negra
et al. / ion induced emission from Langmuir-Blodgett
angle of incidence of the beam with the target surface can be changed precisely without changing the other experimental conditions. We define the incident angle fl as the angle between the beam direction and the normal to the target surface. For practical reasons B has only been varied from 20 o (almost normal incidence) to 78 o (almost grazing incidence). A silicon detector has been used to monitor the energy and the intensity of the primary ions. Time of flight mass spectra of secondary ions were measured in a few minutes with the multiparameter data acquisition system described elsewhere 141. It was sometimes necessary to bombard the targets with projectiles having a charge state equal to the equilibrium charge state (4,) inside the solid. Since we have used in this work organic targets, we have chosen equilibrium charge state values experimentally found for carbon targets. It must be recalled that the mean equilibrium charge state inside solids differs [5,6] from that measured outside the target which can be determined for example with a magnet after the passage of ions through a foil. For ‘*‘I at 127 MeV and 32S at 32 MeV, the ( qeq) values are respectively 24+ and 11 t- [6]. It has been shown experimentally [3] that an equilibrium charge state of ‘*‘I at 127 MeV is reached after passage through 350 A of carbon. Langmuir-Blodgett films have been prepared according to the methods reported earlier 171. Monolayers of fatty acids were transferred onto thin films of gold (1000 A) vacuum deposited on glass disks. Due to the hydrophobic character of such gold surfaces, an even number of monolayers is normally transferred. In the present experiments, the samples consisted of superposed layers of 2 types of fatty acids: arachidic acid (MW = 312.3) and stearic acid (MW = 284.3). However the water subphase at pH = 5.7 contains Cd*’ counterions (CdCl,, lop4 M/l). Under these conditions, every monolayer consists of a mixture of non dissociated fatty acids and Cd salts of fatty acids (2(Mi-H)Cd, i = 1, 2). Since the pH is constant during the process of transfer it is assumed that the composition of the layers is the same as in the subphase, around 65% of Cd salt [S]. The upper layer is referred as M, and the underneath layer as M,. In the following, for the sake of simplicity, the notation nM,/mM, will refer to a composite film corresponding to n monolayers of M, deposited on top of m monolayers of M, on Au substrates. In a few cases, a third type of fatty acid - palmitic acid (M3, MW = 256.2) - was used as an underlayer of arachidate and stearate. The characterization of LB films was first checked during the transfer of the monolayers onto the metal substrate. The orientation of the meniscus for both the immersion and the emersion of the substrate showed that the surface of the substrate became hydrophobic during the emersion and hydrophilic during the immersion. The transfer ratio (area of transferred mole-
films
cule/area of the substrate surface) was measured and found to be 1 + 0.1, for both immersions and emersions. A second characterization was performed on stepped LB films using Nomarsky microscopy (93. These films obtained in the same experimental conditions than those used for desorption studies were composed of superposed 2, 4 and 6 ML; 25% of the substrate area were covered by 2 ML, 25% by 4 ML and 25% by 6 ML. The analysis by Nomarsky microscopy of such samples clearly showed the different steps Au-2 ML, 2 ML-4 ML,. . . This indicated that the pile up of the monolayers was exactly the pile up expected from the transfers. The Normarsky microscopy analysis of the region corresponding to a given coverage (2 ML or 4 ML or 6 ML,. . .) showed that about 95% of the surface area was homogeneous. Thus only 5% of the sample area were expected to be inhomogeneously covered (pinholes or dust particles). This inhomogeneity did not depend on the film thickness. It is due either to small holes or more likely to dust particles. Fig. 1 shows a negative ion mass spectrum of a composite film 2M,/6M,. The spectrum exhibits essentially the deprotonated molecular ion of the fatty acids (M,-II-, i = 1 and 2, the dimers (2Mi-H)-, (M,M,H)- and trimers containing Cd [3(M,-H) + Cd]-. Series of fragments are also observed but their origin cannot be assigned specifically to M, or M,. The ion desorption yield Y which is shown in the figures is obtained from the measured yield Y, which is the ratio between the number of detected secondary ions N,, and the number of primary N,, hitting the target. Np, is measured by the secondary electrons ejected from the target after impact. Assuming an efficiency of detection e = 0.6 and a Poisson dist~bution
12T124+ III ‘A-H)-
CnH-
B.SS*
127 Me’/
1600 -
ZMLX 6ML M1 ----_
Gold 1200
(t+Hl
(2M,-ii IMIMz-HII2 Mt- H’;l
I
It31Mz-Hl+Cd)
Fig. 1. Mass spectrum of a Langmuir-Blodgett film with 2 layers of M, cm 6 layers of M, (see text).
S. Delta-Negru
of the number can write: Y= -log{1
3. Resub
of secondary
- &Jm.
et at. / Ion induced emission from Langmuir-Blodgetl
ions emitted
by impact
films
123
we
1
and discussion
The increase of secondary ion yields with the thickness of samples has first been studied with fission fragments from *“Cf [lo-121. Langmuir-Blodgett (LB) films were used to obtain quantitative results as function of the sample thickness [Il-131. These results were consistent with a simple picture of emission from a conical volume, the depth of the crater was estimated to be about 200 A. To confirm these results, we have used 32S and ‘271 ions at 1 MeV/u having equilibrium charge states and superposed LB layers of nM, on 6M,. The number of top layers n has been varied from 2 to 12. Molecular ion yields from upper and underneath layers were measured simultaneously for each target. Fig. 2 shows the results obtained with “S”+. The yield of the molecular ions M, decreases rapidiy when the number n of M, layers increases. After 7-8 layers (175-200 A) the emission yield from the “bottom” layer is close to the background and remains constant. An opposite trend is observed for the yield of M, which increases with n and saturates after PI= 8. Fig. 3 pre-
I
3
I
1
,
Number
of
(
I
32MeV
Monotoyers
Mt
Fig. 2. Variation of the molecular ion yield (Ml-H)and (M,-Ii)as a function of the number of layers of M, deposited on top of 6 layers of M,. The projectiles are 32S ions at 32 MeV. The angle of incidence is 45 O.
Number
of MmoloyersM,
Fig, 3. Yariation of the molecular ion yield (M, -H- ) and (M,-H)as a function of the number of layers of M, deposited on top of 6 layers of M,. The projectiles are ‘271 ions at 127 MeV, the angle of incidence is 45”.
~~~~~~~lar results measured with the same targets and I projectiles. Above 10 layers, the yields of M, and M, are constant. The saturation depth is roughly the same with 32S and “‘I and therefore does not seem to depend strongly on the electronic stopping power of the projectiles (dE,/dx(‘“I) = 3dE/dx(32S) at 1 MeV/u). In an earlier paper 121 on the charge state dependence of ion yields an “interaction depth” din, was introduced (see also [16]), which is not equal to the emission depth observed experimentally here. dint is nearly proportional to dE/dx and would have at normal incidence the values 90 t 20 A for 32S’1+ and 300 & 80 A for 12?I14’, respectively. It reflects the effective length of the nuclear track (at normal incidence), from where an interaction causing desorption reaches the uppermost surface layer. Only this top layer contributes to desorption. As the present work proses, also molecules from layers as deep as 200 w underneath the surface appear in the TOF spectrum. Therefore, the picture of crater formation seems to be more appropriate, to describe particle desorption by heavy ion impact, than an effective interaction depth. An exception is one organic monolayer on top of a metal substrate since then only the organic layer is desorbed and the metal remains undestroyed. In this case, dint should be a useful parameter (see also the next section).
124
S. Della-Negra et al. / Ion induced emission from Langmuir-Blodgett films
The absolute secondary ion yield of emission is however almost 10 times larger with lz71 than with 32S. That is consistent with many other experiments, where a (dE/dx)” dependence of secondary ion yields with n = 2-3 is reported (see, for instance, the review article 1141). If we apply the simple picture of a conical ernission volume, the cone would have the same depth for 32S and **?I, but a larger surface cross section in case of ‘*? ions having the higher dE/dx value (see also the formalism in the next section). It should be noted that the elongated shape of the present molecules may give a nonuniversal picture of the desorption process. 3.2. Incident angle dependence of the electronic desorption yield Experimental results on the angle dependence have been reported by several authors [15,16,11]. These experiments were performed with MeV ions in their equilibrium charge state. It is known however that a distance larger than 150 A is necessary to achieve charge state equilibration. Calculation and measurements are in agreement on this point [6,17]. As shown in the preceeding section the emission depths can reach 200 A. If the fast incident ion is not in its equilibrium charge state the variation of yield with B depends on the incident charge state qi and on its equilibration function [q(x) = f( q,, x)]. The effects were first observed by Nieschler et al. 1161 with oxygen ions at 9 MeV with 2 s q. I 8. With 32S ions between 6+ and 14+ as well as with 127I between 18’ and 30+ the angle dependence of the SI yields is very sensitive to the primary ion charge state. Fig. 4 shows the yield variation of molecular ions for 32S8+ and 32S’4+ as a function of the angle of incidence. For 0, = 78” there is no effect of the charge state on the M, yield and on the M, yield whereas for @i = 30’ the charge state effect is clearly visible between Ss+ and S14+. The absence of charge state effects at large angles of incidence is also shown in fig. 5 which presents systematic studies with an LB target 12M,/6M2 bombarded by 12’1 ions at the angles 20*, 45”, 60°, 70 O, 78 O. At 20 o the slope of yield variation with qi is very steep. The deposited energy loss which contributes to the desorption depends on the charge state variations along the track. Qualitatively, at this angle, the interaction distance is at least of the order of the distance travelled for equilibration (200-300 A) (by reference to a previous work with Kr ions [6]). As B increases the influence of the primary ion charge state vanishes and the yield variation tends to be flat. The primary ions travel a long distance compared to the equilibrium charge state distance and the secondary emission depends mainly on the energy loss with the charge state ~uilibrated. If the m~mum depth of emission is
Fig. 4. Secondary molecular ion yields as a function of the angle of incidence. The primary ions are 32S at 32 MeV with the charge
states 8+ and 14+.
around 8-10 layers (200-250 A) then the primary ion travel distance could be around 1000 A! The geometrical shape of the volume (or “crater”) of
J 60’ 5
9.10-1
5 s
e.10-
.I -
D 7.10’ .m ; 6.10.?
,I -
5 :
I-
5.10-t
4.10-I
3.10-’
----
-
Gold
2.10”’ ,I;,
. 2o
J 30’ 12’Jq’ Charge
state
Fig. 5. Dependence of the (M,-H)yield from a mixed multilayer film on the angle of incidence. A beam of “‘1 ions at 127 MeV with charge states between 20+ and 30+ was used.
S. DebNegra
et ol. / Ian induced emission from Langmuir-Blodgett
emission is expected to vary considerably with the angle of projectile impact and is certainly very asymmetric for large angles. This is in agreement with the anisotropic angular distribution of emitted ions measured by the Uppsala and the Darmstadt groups [l&19]. Targets with a relatively thin upper layer of M, molecules 2M,/&I,/Au and 2M, on Au have also been prepared to study the influence of the charge state and the impact angle. Experimental results are shown in fig. 6. At 20°, for 2M,/Au the equilibrium charge state is not achieved in the organic layers but in the gold material. At 78”, the charge state is almost equilibrated in the two M, monolayers. A dependence of yield of M, on the charge state would be expected if only the energy loss in these 2 monolayers would be used for desorption. At this angle, the experimental results exhibit a flat dependence of yield with q. It is therefore concluded that the energy loss in the gold substrate, by projectiles in their equilibrium charge state, contributes to the desorption of M,. Energy is transferred in gold, from a certain distance, to the attached 2 monolayers above, although the depth of emission cannot be larger than 50 A (2Mr only). A similar behaviour is observed with the target 2M,/6M,/Au. The difference of SI yields between 20° and 78” is however not as large as with the 2 monolayers directly on the gold substrate. These results demonstrate that the energy distribution in the material can
20*
films
30+ La* ‘271q* Charge state
Fig. 7. Yields of (MI-H) and (M,H) ejected from 2 superposed LF3films (6M,/6M,) as a function of the charge state of *“I projectiles at a fixed incident angie 8 = 45O.
be transferred over a large distance and that the interface metal-insulating organic layer does not prevent the energy transfer. With a 6M,/6Mz sample, the same effects are displayed in fig. 7. The variation of SI yields, as a function of the charge state, for both molecular ions emitted from the two layers by r2’I projectiles is the same. The angle is 45 *. The equilibrium charge state is reached in the second part of the target (M,) but the rate of emission from M, is depending on the dist~bution of energy (variation of charge state) loss in the first part (M,). The memory of the incident charge state is kept (because a dependence of q, is observed for Y(M,)) and realized by the layers underneath. There is a certain flow of energy from the upper layers to the bottom layers and vice versa. The absolute yields of M, and M, are of course very different and in agreement with the results of fig. 3. 15’
20’
30’
20’ 1271’4+
30’
Chorqe state
Fig. 6. Variation of the (MI--N)- ion yield from 2 monolayers M, on GoId (right part) and from 2 monolayers of M, on top of 6 M, (with a gold substrate, left part). The Influence of the angle of incidence and of the primary I271 ion charge state is dispfayed.
4. Comparisons of yield dependence on the primary ion charge and the angle of incidence with cafcidations from a shock wave mode1 In recent papers 120,21] a shock wave mechanism has been developed for the sputtering of bio-organic mole-
S. Della-Negra
126
et al. / Ion induced emission from Langmuir-Blodgett
cules. Emission of molecules occurs from a crater produced by a shock wave, which originates from a high density energy spike in the vicinity of the heavy ion track. Crater formation is in agreement with the present results and the yield dependence on film thickness already mentioned [l&12]. Formulas derived in 120,211 will be utilized after some modifications concerning desorption from LB films. For grazing incidence of primary ions having equilibrium charge, the boundary of a crater with a surface is an ellipse (221. One of its semiaxes p,e depends on the angle of incidence 6, the other one (perpendicular to the beam direction) pa0 does not. If p, is the crater radius for 0 o impact (8 = 0), then 6320 =
PI0
Ps*
=
P,/COS
@*
(1)
Analogously to ref. [20] one can show that the semiaxes of the ellipse are given by Pli =
PlO
(4ilqeq
)
where qi differs from the equilibrium value qe9. x1 = X/plo and X is the charge equilibrium distance. pzi is obtained by means of a similar formula, where the index 1 is replaced by 2. For B = 0 the semiaxis pli and
A)
fihns
pzi are equal and approach the value p of eq. (31) in ref.
[21] for xX.2 >> 1. In the case of two monolayers of molecules M, on top of 6M,, the yield of the parent ions M, is given by a formula equivalent to eq. (24) in ref. [Zl]: ~(6)=4a1)-NmLpijpai
i
l+z
1
~[(1-~).(l-%)]1’2.
(3)
Here, n- is the probability to form and eject the negative parent ion (M,-H)-, Nm the number density of M, and pi the radius of the high density energy spike around the nuclear track. i is the index of the charge state q,. The yield y(e) is proportional to the surface area of the crater reduced by an area of destroyed molecules around the point of impact. The semiaxis bli, can be written as bli = p,/cos B + C(e) (figs. 8A and 8B). For 0 = 0, C(0) = ao, and for B = a/2, C(e) approaches L tg 8. Parameter a0 depends on the crater shape. For example, for cylindrical crater shape LIP= a (here a is the radius of linear molecules with length L) for spherical one a0 = L2/2ps (fig. 8C) and for conical one a0 = L tg +/2 where (h is a cone vertex angle. With
area of intact molecule emission
0 = 0 and spherical crater Fig. 8. Schemes showing the surface of emission in the shock wave model (see text).
S. Della-Negru
127
et al. / km induced emission from ~~~muir-Blodg~tt films
the assumption that C(e) 2=a0 + L tg 8 we obtain the semiaxis hn and bzi: bli=
&
+a,+LtgB
and
b,,=p,+a,.
(4
In fact, the actual boundary of the destruction area is difficult to estimate and, therefore, we used the value of a, as a fitting parameter depending on the crater shape at i? = O”. Fig. 9 shows the results of the calculation describing satisfacto~ly the expe~ment~l data (for B -Z 70 o ) of the yield dependence on the initial ion charge and the angle of incidence. For 8 = 7S0, eq. (3) overestimates the molecular ion yield because for large 8 the parameter xi = X/p,, becomes smaller than 1 and the approximations made to evaluate eq. (2) are not valid. The primary ion equilibrium charge state of ‘27I at 127 MeV is 24+ and this value has been taken as q,_, the other parameter had the values 4 = 100 A (crater radius), pi/p% = 0.17, ae= 24 A, h = 200 A. Q- has been adjusted to the expe~mental data for @= 20* and a,= 2@. It should be noted that the fitting value a,, = 24 A (a,, = L) shows that for B = 0 o we can approximate the crater as a cone with a vertex angle # = 90°. On the basis of the shock wave model of Biter&i et al. 1211, a spherical crater is obtained for fl = 0 o im-
. *
2 .
*
.
*
*
t
78’
70’
60’
* 2 ML 6ML
I 15+
20’
1%
-emGold
_
,lrlcllrru
30+ r271q* Charge
state
Fig. 9. Comparison of the calculated variation of yields with B and 4, (solid lines). The parameters are pS= 100 A, a = 25 A and h = 200 ,& (see text). The target is made out of 2 ML of M, on 6 ML of M, on a gold substrate.
6ML 6ML 2OML
_-_-Cold
5.10‘2 20’
30” 40 ‘*‘Iq*Charge state
Fig. 10. Yields of the (M,-H)-
parent ions, the dimer and the trimer as a function of the charge state of ‘271 at 127 MeV (8 = 45 “) (solid lines). The target consists of superposed layers 6M1/6M,/20 M, on Au.
pacts, the crater depth ps being equal to 100 A in the case of i2’1 at I MeV/u. On the other hand we have observed that the emission of intact molecules occurs from a depth larger than 150 A and a conical shape of crater was taken as a first approach to explain the data. It is however interesting to note that, with the shock wave model, the general trend of yield variations with t? and s can be reproduced. This model also qual~tativeiy reproduces the asymmetric distributions of secondary molecular ions observed ]18,19] for nonnormal angles of incidence. As it is seen in fig. 10, the dependence of the yield of the parent ion (M,-I-I)-‘, the dimers (2M,-H) and even the trimers (3(M,-H) + Cd)- can be described by eq. (3), but with a larger value of h = 300 A. The observed steeper yield dependence of the cluster ions (2M,-H)and (3(M,-II)+ Cd)- (compared with the curve of (M,-H)-) on the primary ion charge state is reproduced if a larger size of the molecules is used in the calculation. We choose a, = 50 A and L = 25 A or 50 A, respectively, for the dimer and the trimer. That means, the dimer molecule has the same length as M, whereas the trimer is twice as long. Two different values of h have been introduced as fitting parameters in figs. 9 and 10. This could be an indication that the exponential dependence of the charge state in ref. {20] is not an appropriate description of the charge variation in solids.
128
S. DeIIa-Negra et al. / Ion induced emissionfrom Langmuir-Blodgett films
In fact recent experiments have shown that this variation is more complex [23]. The steeper slope for the yield of large molecular ions indicates that they probably originate from a smaller depth. Dimers and trimers have a higher chance to escape without dissociation if they come from the upper layers. The calculations which have been presented can explain the experimental results within the framework of a shock wave model. E. Hilf and F. Kammer have also performed calculations by means of direct numerical evaluation of molecular equations of motion [24]. Their results give information about the magnitude of the contribution from different layers and are in qualitative agreement with the results on thickness effects.
5. Conclusions The influence of the PI incident angle and of the PI charge state has been systematically studied. The secondary ion yield of molecular ions forming LangmuirBlodgett films depends on both parameters and there is no simple relation between the yield and the angle of incidence. For grazing angles, a constant electronic stopping power (dE/dx a (q,,)*) is involved in the desorption processes. The memory of the projectile charge state is lost. A tentative explanation of these experimental results has been made by using a fluid dynamical model and the general trends of the secondary ion yield variations can be reproduced by such a model.
Acknowledgements We acknowledge the staff of the tandem accelerator, G. Andlauer for his kind assistance, D. Chatenay and B. Colin for their Nomarsky microscopy studies of the LB films and E. Davanture for taking care of the manuscript. One of us H.J. thanks the Delsi-Nermag Company for financial support. I. Bitensky thanks the Institut de Physique Nucltaire for inviting him as a visiting scientist for a one month period.
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