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Fast heavy ion induced desorption: Dependence of the yield on the primary ion energy
Surface Science 145 (1984) 197-206 North-Holland, Amsterdam
197
FAST HEAVY ION INDUCED DESORPTION: YIELD ON THE PRIMARY ION ENERGY B. NEES, E. NIESCHLER, N. BISCHOF, W. TIERETH and H. VOIT Physikalisches
DEPENDENCE
H. FRijHLICH,
OF THE
K. RIEMER,
Instiiui der Uniuersitiit Erlangen - Niirnberg. D - 8520 Erlangen, Fed. Rep. of Germany
Received 13 March 1984: accepted for publication
22 May 1984
The dependence of the secondary ion yield on the energy of the fast primary ions has been investigated for several organic samples. We find different dependences for positive secondary ions desorbed from polar and ionic compounds as well as for positive and negative secondary ions desorbed from the same sample (polar compounds).
1. Introduction Fast heavy ions can be used to desorb secondary ions from dielectric samples. Even unfragmented molecular ions are desorbed with considerable yields from organic compounds consisting of large and fragile molecules (e.g. biomolecules) ‘[l]. This is rather surprising since energies of the order of lo*--lo3 eV are released within a depth of 1 A of the sample. The desorption mechanism is still not understood; it is currently studied both theoretically [2-41 and experimentally [5-91. Obviously fast heavy ion induced desorption depends on (i) the bonding state of the desorbed ion (or the parent ion/molecule) and (ii) the energy available at the molecular site. Point (i) concerns the chemistry of the molecule and the sample preparation (preionization and fragmentation in solvents etc.), (ii) is related to physical properties of the sample (substrate) like spatial energy transfer, energy transfer to the desorbed species and to the primary ion parameters determining the energy loss in the sample surface. The influence of the primary ion parameters on the desorption yield is currently studied in our laboratory. First results show that the desorption can be initiated only within a small layer (a few monolayers) at the sample surface [lo] and that the yield is correlated with the energy deposited by the primary ions [8,11]. This is most obvious from ref. [8] which also gives a simple connection between the yield and those parameters which determine the energy loss of the primary ions. Since only one sample (the amino acid valine) has 0039-6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
198
8. Nees et ai. / Fast heavy ion induced desorprion
been investigated so far we have performed additional measurements various samples, the results of which are presented in this paper.
for
2. Experimental procedure The samples investigated are the amino acid phenylalanine (m = 165), the dyes crystal violet and malachite green (cation masses m = 372 and 329, respectively), the antibiotic chloramphenicol (m = 322) and the antiarrhythmic verapamit (cation mass m = 455). In addition data were taken for the amino acid valine (m = 117) in order to compare the present with the previous results [8]. The samples were deposited on a thin aluminized mylar foil by means of the electrospray method [12]. Sample thicknesses were of the order of 30-50 pg cm -‘. One valine sample was prepared by vacuum evaporation. The energy dependence was studied with the 44 MeV oxygen beam from the Erlangen tandem accelerator. The energy range covers lo-42 MeV. This region is beyond the maximum of the electronic stopping power of oxygen ions. The experimental setup is essentially the same as that used in ref. [S]. The time-of-flight spectrometer was however slightly modified: in the present experiment we used an electron converter in addition with channel plates for the detection of the secondary ions. The converter consists of a es.1 coated stainless steel plate which is placed perpendicular to the spectrometer axis. Secondary electrons produced by the desorbed ions are bent in the channel plates by means of an electromagnet. The targets were positioned on a target ladder (5 positions) which allowed to change targets without breaking the vacuum.
3. Experimental results and discussion Figs. l-4 show mass spectra of positive and negative secondary ions. The spectra have been obtained using 60 MeV 32S ions or fission fragments from a 252Cf source which was positioned for this purpose in the beam axis. Primary ions of 32S and I60 as well as fission fragments yield the same spectra. Only small sections of the total spectra containing the mass peaks of interest have been analysed in the present investigation in order to avoid shadow effects due to the dead time of the electronic clock. The positive ion spectrum of phenylaIanine (fig. 1) exhibits two pronounced peaks which belong to the [M + II]+ and [M-COOHI+ ions, the negative ion spectrum is dominated by the [M-H]mass peak. These are features characteristic for other amino acids, too. Obviously the negative ion spectrum contains less fragment ions than the positive spectrum. A section of the total mass spectrum for positive and negative secondary
B. Nees et al. / Fast heavy ion induced desorption
199
ions desorbed from the chloramphenicol sample is shown in fig. 2. It is obvious from this figure that several mass peaks are lumped together to one mass group. This is due to the fact that the compound contains two Cl atoms with masses 35 and/or 37. The positive ion spectrum exhibits the mass group of the protonated molecular ion [M + H] + , the negative ion spectrum that of the deprotonated molecular ion [M-H]-. The positive ion spectra for the dyes are shown in fig. 3. They are dominated by a single peak which belongs to the cation of the compounds. The negative ion spectra of both dyes do not contain significant mass peaks except for small masses which have not been analyzed in this experiment. Fig. 4 shows two sections of the total positive ion spectrum for verapamilhydrochloride. They contain the cation peak and several peaks the origin of which becomes clear from the structural formula given in the figure. Again the negative spectrum does not show mass peaks in the mass range of the verapamil molecule. The desorption yield and its dependence on the primary ion energy E is shown in figs. 5-7. It is obvious from these figures that different energy dependences are observed both for positive secondary ions desorbed from
200
150
100
I
81
I
iM-COOHI+
100
120
1LO
160
180
mass Fig. 1. Positive and negative secondary ion spectra for the amino acid phenylalanine. are fission fragments from a 252Cf source.
Primary ions
200
B. Nees et al. / Fast heavy ran Induced desorption
different samples and for positive and negative secondary ions desorbed from the same sample (see figs. 1 and 2). The [M-H]yield depends much stronger on the energy than the [M f H]+ yield. The energy dependence of the cation yield is comparable with that of the [M-H]yield. The experimental curves can be reproduced (see figs. 5-7) by the expression Y(E)=av
-+CQS(E) 4
8,
(1)
where u and q are the velocity and the charge state of the primary ions, respectively. The function P~( E) is the weight of the charge state q within an equilibrium charge state distribution (primary ions hitting the sample surface have an equilibrium q-distribution due to the experimental setup used 181). The factor a is a constant. Weights ‘~4 as given in ref. [13] have been used, the reliability of which has been examined by comparison with more recent experimental data [14,15]. It should be noted that the parametrization with eq. (1) is only applicable for primary ion energies larger than the smallest energy studied in this work.
fM*CIlm= 357
iii
287
5
305
325
3-a
365
60
0 267
280
290
300
310
320
330 mass
Fig. 2. Positive and negative secondary ion spectra for chlorampbenicol (antibiotic). Primary ions are 60 MeV “S.
B. Nees et al. / Fast heavy ion induced desorption
201
This energy coincides approximately with the energy for which oxygen ions passing through matter suffer the largest energy loss. Yield data for smaller energies show a different behaviour [6,9,11] which cannot be reproduced with eq. (1). This is reminiscent of the situation concerning the theoretical description of the energy loss of charged particles in matter. The well known Bethe formula [16] describes the energy only for energies beyond the energy maximum, for smaller energies a totally different theory [17] has to be applied. It seems worthwhile to remind that the energy loss for charged particles predicted by the Bethe formula is (in its simplest version): dE ----_-----
b2)
dx
In
k”2
U2
(2)
’
where u is the velocity of the primary ion and k is a constant. squared charge (q2) of the ion is given by the expression [18]:
(q2)= i (p,q2,
The mean
(3)
q=l
2500 1250
mass Fig. 3. Positive secondary ion spectra for the dyes malachite green and crystal violet. Primary ions are 60 MeV 32S.
8. Nees etal./ Fast heauy
202
ion
induced
desorption
where ‘~4 and q have the same meaning as in eq. (1). Two different exponents n are necessary in order to reproduce the data with eq. (1): n = 2 for the yield of protonated molecular ions [M + HI+ from phenylalanine and chloramphenicol, n = 4 for the deprotonated molecular ions [M-H]of the same samples and n = 4 for the cations from the dyes and verapamilhydrochloride. The results for phenylalanine agree with the results for valine remeasured in this work (both for the evaporated and the electrosprayed targets) and with those reported in ref. [8]. The key for an understanding of the different dependences observed for positive secondary ions is most probably related with the fact that the different compounds investigated have different chemical properties. Chloramphenicol and the amino acids consist of polar molecules, moreover amino acid molecules have pronounced amphoteric features. The dyes on the other hand as well as verapamilhydrochloride consist of molecules with pronounced ionic character. These features give reason to believe that different surface bonding states exist for the two types of molecules: the ionic molecules probably are chemisorbed, the polar molecules physisorbed. The latter is supported by investiga-
I
1
I
I
,-r11
veropamllhydrochlor~de
,p:
33
) m=165
600
m=151 300
I _I
.-
m c
140
170
220
260
300
600 7
i
388
400
410
L20
430
440 450
460
mass Fig. 4. Sections of the positive Primary ions are 60 MeV ‘*S.
ion spectrum
of verapamilhydrochloride.
R = CH,,
R’= CH,.
203
B. Nees et al. / Fast heavy ion induced desorption
z
0 iii 5
~~~
x3-*15*-
;, lG3J lo”-
lG3-
e
10
20
E IMeW
30 &ev,
Fig. 5. Energy dependence of the yield for secondary ions desorbed by 160 primary ions from a phenylalanine sample. The solid lines are calculated with eq. (1) using the exponent n indicated in the figure. Fig. 6. See caption
of fig. 5. The sample investigated
is chioramphenicol.
Fig. 7. Energy dependence of the yield for cations desorbed by t60 primary ions from malachite green, crystal violet and verapamilhydrochloride (also for the verapamil fragment with mass 303). The solid lines are calculated with eq. (1) using n = 4.
B. Nees et al. / Fast heacy ran induced desorptron
204 tions
performed for various amino acids by Benninghoven et al. [19]. If these two different bonding states exist, it is reasonable to assume that the different energy dependences observed for positive secondary ions simply reflect the different strengths of the surface bonds: the energy variation of the primary ions and thus the variation of the energy deposited in the sample surface has less influence on the yield of weakly bound molecules compared with that of strongly bound species. This can be shown in a purely geometrical picture describing the radial dissipation of the energy deposited along the primary ion trajectory: the radius of the zone accessible to desorption decreases more rapidely for a strong bonding state compared with a weak one if
GY
0
It+PI-
/ fi
f
W CV,
surface interaction. m Coulomb attraction
interna vibration 1
(repulsion
Fig. 8. Schematic drawing of the assumed desorption processes for molecules or molecular ions physisorbed at the sample surface and for two different starting conditions: (i) preformed [M + H]+ and [M-H]- molecular ions exist at the surface before the primary ion hit the sample and experience different Coulomb forces after the passage of the primary ion (uppermost cases) and (ii) neutral molecules [Ml0 are physisorbed at the surface (all other examples). The symbols + , _ and 0 refer to ionized and neutral species, respectively. R and p stand for a molecular fragment and proton. respectively. The dashed curve shows the assumed distribution of the energy AE deposited by the primary ion.
B. Nees et al. / Fast heavy ion induced desorption
205
the energy deposited decreases assuming any reasonable functional form for the dissipation. In addition the desorption of strongly bound molecules is hindered compared to that of weakly bound states in regions of small energy deposits due to the quantum-mechanical reflection at the surface potential barrier. The different energy dependences observed for the [M + H]+ and [M-H]yield obtained from the same sample (containing polar molecules) touches upon the question of the nature of origin of the desorbed species. This includes predissociation in the solvent, dissociation and association processes at the surface, the surface selvedge and the vacuum as well as other possibilities. If the [M + H]+ and [M-H]molecular ions are preformed at the sample surface as assumed by Benninghoven et al. [19], one needs an additional process which enforces [M + H]+ desorption and hinders [M-H]desorption in order to explain the different energy dependences observed. This process could be Coulomb repulsion [20] (attraction) between the preformed species and positively charged atoms or molecules which exist abundantly within the primary ion track as a result of the energy transfer mechanism valid for fast heavy ions (electronic stopping). The same mechanism could explain the desorption of other preformed positive and negative charged entities, too. Without the above Coulomb effects it seems hardly possible to explain different energy dependences for [M + H]+ and [M-H]molecules unless one assumes that neutral molecules [Ml0 are physisorbed at the sample surface. These molecules may be desorbed as neutrals and undergo subsequently association and dissociation processes. The latter are energy consuming and introduce therefore a hindrance for the desorption of [M-H]ions in the sense discussed above. They are restricted to molecules with reasonable internal excitations (vibrational, rotational, electronic). Since the excitation energy available decreases with increasing primary ion energy E different dependences on E can e expected for protonated and deprotonated molecular ions. If the neutral surface molecules [Ml0 become ionized by the primary ions, processes combining the above mechanism with Coulomb explosion (attraction) effects may occur as sketched schematically in fig. 8. It is assumed in this schematic that the energy A E deposited by the fast I60 ions (via electronic excitations) is coupled into internal (vibrational, rotational, electronic) and external (vibrations relative to the surface) excitations of the surface molecules. In order to explain the desorption of unfragmented species one has to assume that electronic excitations couple frequently to continuum states of the surface potential. 4. Conclusion We have measured the dependence of the desorption yield of organic molecular ions on the energy of the fast primary ions (oxygen ions). Different
206
B. Nees et al. / Fast heuuy ion induced
desorpfion
samples have been investigated. The yield can be parameterized with parameters which determine the energy loss of the primary ions in the sample surface. Nevertheless it turns out that the desorption yield is not just proportional to the energy loss (electronic stopping power) in the energy range studied. Different energy dependences observed for positive secondary ions from various samples are probably related to the strength of the corresponding surface bonding states; different dependences for positive and negative secondary ions desorbed from the same sample (polar compounds) have to be related to the formation processes for the desorbed species. Explanations are given the confirmation of which needs further investigations which are planned in this laboratory.
Acknowledgement This work was supported Fed. Rep. of Germany.
by the Deutsche
Forschungsgemeinschaft,
Bonn,
References [l] R.D. Macfarlane, R.P. Skrowronski and D.F. Torgerson, Biochem. Biophys. Res. Commun. 60 (1974) 616. [2] F.R. Krueger, Surface Sci. 86 (1979) 246. [3] L.E. Seiberling, J.E. Griffith and T.A. Tombrello. Radiation Effects 52 (1980) 201. [4] C.C. Watson and T.A. Tombrello, to be published. [5] C.K. Meins, J.E. Griffith, Y. Qin, M.H. Mendenhall. L.E. Seiberling and T.A. Tombrello. Radiation Effects 71 (1983) 13. [6] P. H%kansson, I. Kamensky and B. Sundqvist, Nucl. Instr. Methods 198 (1982) 43. [7] W. Guthier, 0. Becker, S. Della Negra. W. Knippelberg, Y. LeBeyec. U. Weikert, K. Wien, P. Wiesner and R. Wurster, J. Mass Spectrom. Ion Phys. 53 (1983) 185. [8] B. Nees, E. Nieschler, N. Bischof, P. Duck, H. Frohlich. W. Tiereth and H. Voit, Radiation Effects 77 (1983) 89. [9] S. Della Negra, D. Jacquet, I. Lorthiois and Y. LeBeyec, J. Mass Spectrom. Ion Phys. 53 (1983) 215. [lo] H. Voit, B. Nees. E. Nieschler and H. Frohhch, J. Mass Spectrom. Ion Phys. 53 (1983) 201. [ll] A. Albers, K. Wien, P. Dick, W. Treu and H. Voit, Nucl. Instr. Methods 198 (1982) 69. [12] C.J. McNeaI, R.D. Macfarlane and E.L. Thurston, Anal. Chem. 51 (1979) 2036. [13] J.B. Marion and F.C. Young, Nuclear Reaction Analysis (North-Holland. Amsterdam, 1968). [14] R.B. Clark, I.S. Grant, R. King, D.A. Eastham and T. Joy, Nucl. Instr. Methods 133 (1976) 17. [15] R. Ambros and A.M. Kleinfeld, Nucl. Instr. Methods 99 (1972) 173. [16] H.A. Bethe, Phys. Rev. 89 (1953) 1256. [17] J. Lindhard, M. Scharff and H.E. Schiott, Kgl. Danske Vidensk. Selskab. Mat. Fys. Medd. 33 (1963) No. 14. [18] L.C. Northcliffe. Ann. Rev. Nucl. Sci. 13 (1963) 67. [19] A. Benninghoven, W. Lange, M. Jirikowsky and D. Holtkamp, Surface Sci. 123 (1982) L721. [20] P.K. Haff, Appl. Phys. Letters 29 (1976) 443.
197
FAST HEAVY ION INDUCED DESORPTION: YIELD ON THE PRIMARY ION ENERGY B. NEES, E. NIESCHLER, N. BISCHOF, W. TIERETH and H. VOIT Physikalisches
DEPENDENCE
H. FRijHLICH,
OF THE
K. RIEMER,
Instiiui der Uniuersitiit Erlangen - Niirnberg. D - 8520 Erlangen, Fed. Rep. of Germany
Received 13 March 1984: accepted for publication
22 May 1984
The dependence of the secondary ion yield on the energy of the fast primary ions has been investigated for several organic samples. We find different dependences for positive secondary ions desorbed from polar and ionic compounds as well as for positive and negative secondary ions desorbed from the same sample (polar compounds).
1. Introduction Fast heavy ions can be used to desorb secondary ions from dielectric samples. Even unfragmented molecular ions are desorbed with considerable yields from organic compounds consisting of large and fragile molecules (e.g. biomolecules) ‘[l]. This is rather surprising since energies of the order of lo*--lo3 eV are released within a depth of 1 A of the sample. The desorption mechanism is still not understood; it is currently studied both theoretically [2-41 and experimentally [5-91. Obviously fast heavy ion induced desorption depends on (i) the bonding state of the desorbed ion (or the parent ion/molecule) and (ii) the energy available at the molecular site. Point (i) concerns the chemistry of the molecule and the sample preparation (preionization and fragmentation in solvents etc.), (ii) is related to physical properties of the sample (substrate) like spatial energy transfer, energy transfer to the desorbed species and to the primary ion parameters determining the energy loss in the sample surface. The influence of the primary ion parameters on the desorption yield is currently studied in our laboratory. First results show that the desorption can be initiated only within a small layer (a few monolayers) at the sample surface [lo] and that the yield is correlated with the energy deposited by the primary ions [8,11]. This is most obvious from ref. [8] which also gives a simple connection between the yield and those parameters which determine the energy loss of the primary ions. Since only one sample (the amino acid valine) has 0039-6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
198
8. Nees et ai. / Fast heavy ion induced desorprion
been investigated so far we have performed additional measurements various samples, the results of which are presented in this paper.
for
2. Experimental procedure The samples investigated are the amino acid phenylalanine (m = 165), the dyes crystal violet and malachite green (cation masses m = 372 and 329, respectively), the antibiotic chloramphenicol (m = 322) and the antiarrhythmic verapamit (cation mass m = 455). In addition data were taken for the amino acid valine (m = 117) in order to compare the present with the previous results [8]. The samples were deposited on a thin aluminized mylar foil by means of the electrospray method [12]. Sample thicknesses were of the order of 30-50 pg cm -‘. One valine sample was prepared by vacuum evaporation. The energy dependence was studied with the 44 MeV oxygen beam from the Erlangen tandem accelerator. The energy range covers lo-42 MeV. This region is beyond the maximum of the electronic stopping power of oxygen ions. The experimental setup is essentially the same as that used in ref. [S]. The time-of-flight spectrometer was however slightly modified: in the present experiment we used an electron converter in addition with channel plates for the detection of the secondary ions. The converter consists of a es.1 coated stainless steel plate which is placed perpendicular to the spectrometer axis. Secondary electrons produced by the desorbed ions are bent in the channel plates by means of an electromagnet. The targets were positioned on a target ladder (5 positions) which allowed to change targets without breaking the vacuum.
3. Experimental results and discussion Figs. l-4 show mass spectra of positive and negative secondary ions. The spectra have been obtained using 60 MeV 32S ions or fission fragments from a 252Cf source which was positioned for this purpose in the beam axis. Primary ions of 32S and I60 as well as fission fragments yield the same spectra. Only small sections of the total spectra containing the mass peaks of interest have been analysed in the present investigation in order to avoid shadow effects due to the dead time of the electronic clock. The positive ion spectrum of phenylaIanine (fig. 1) exhibits two pronounced peaks which belong to the [M + II]+ and [M-COOHI+ ions, the negative ion spectrum is dominated by the [M-H]mass peak. These are features characteristic for other amino acids, too. Obviously the negative ion spectrum contains less fragment ions than the positive spectrum. A section of the total mass spectrum for positive and negative secondary
B. Nees et al. / Fast heavy ion induced desorption
199
ions desorbed from the chloramphenicol sample is shown in fig. 2. It is obvious from this figure that several mass peaks are lumped together to one mass group. This is due to the fact that the compound contains two Cl atoms with masses 35 and/or 37. The positive ion spectrum exhibits the mass group of the protonated molecular ion [M + H] + , the negative ion spectrum that of the deprotonated molecular ion [M-H]-. The positive ion spectra for the dyes are shown in fig. 3. They are dominated by a single peak which belongs to the cation of the compounds. The negative ion spectra of both dyes do not contain significant mass peaks except for small masses which have not been analyzed in this experiment. Fig. 4 shows two sections of the total positive ion spectrum for verapamilhydrochloride. They contain the cation peak and several peaks the origin of which becomes clear from the structural formula given in the figure. Again the negative spectrum does not show mass peaks in the mass range of the verapamil molecule. The desorption yield and its dependence on the primary ion energy E is shown in figs. 5-7. It is obvious from these figures that different energy dependences are observed both for positive secondary ions desorbed from
200
150
100
I
81
I
iM-COOHI+
100
120
1LO
160
180
mass Fig. 1. Positive and negative secondary ion spectra for the amino acid phenylalanine. are fission fragments from a 252Cf source.
Primary ions
200
B. Nees et al. / Fast heavy ran Induced desorption
different samples and for positive and negative secondary ions desorbed from the same sample (see figs. 1 and 2). The [M-H]yield depends much stronger on the energy than the [M f H]+ yield. The energy dependence of the cation yield is comparable with that of the [M-H]yield. The experimental curves can be reproduced (see figs. 5-7) by the expression Y(E)=av
-+CQS(E) 4
8,
(1)
where u and q are the velocity and the charge state of the primary ions, respectively. The function P~( E) is the weight of the charge state q within an equilibrium charge state distribution (primary ions hitting the sample surface have an equilibrium q-distribution due to the experimental setup used 181). The factor a is a constant. Weights ‘~4 as given in ref. [13] have been used, the reliability of which has been examined by comparison with more recent experimental data [14,15]. It should be noted that the parametrization with eq. (1) is only applicable for primary ion energies larger than the smallest energy studied in this work.
fM*CIlm= 357
iii
287
5
305
325
3-a
365
60
0 267
280
290
300
310
320
330 mass
Fig. 2. Positive and negative secondary ion spectra for chlorampbenicol (antibiotic). Primary ions are 60 MeV “S.
B. Nees et al. / Fast heavy ion induced desorption
201
This energy coincides approximately with the energy for which oxygen ions passing through matter suffer the largest energy loss. Yield data for smaller energies show a different behaviour [6,9,11] which cannot be reproduced with eq. (1). This is reminiscent of the situation concerning the theoretical description of the energy loss of charged particles in matter. The well known Bethe formula [16] describes the energy only for energies beyond the energy maximum, for smaller energies a totally different theory [17] has to be applied. It seems worthwhile to remind that the energy loss for charged particles predicted by the Bethe formula is (in its simplest version): dE ----_-----
b2)
dx
In
k”2
U2
(2)
’
where u is the velocity of the primary ion and k is a constant. squared charge (q2) of the ion is given by the expression [18]:
(q2)= i (p,q2,
The mean
(3)
q=l
2500 1250
mass Fig. 3. Positive secondary ion spectra for the dyes malachite green and crystal violet. Primary ions are 60 MeV 32S.
8. Nees etal./ Fast heauy
202
ion
induced
desorption
where ‘~4 and q have the same meaning as in eq. (1). Two different exponents n are necessary in order to reproduce the data with eq. (1): n = 2 for the yield of protonated molecular ions [M + HI+ from phenylalanine and chloramphenicol, n = 4 for the deprotonated molecular ions [M-H]of the same samples and n = 4 for the cations from the dyes and verapamilhydrochloride. The results for phenylalanine agree with the results for valine remeasured in this work (both for the evaporated and the electrosprayed targets) and with those reported in ref. [8]. The key for an understanding of the different dependences observed for positive secondary ions is most probably related with the fact that the different compounds investigated have different chemical properties. Chloramphenicol and the amino acids consist of polar molecules, moreover amino acid molecules have pronounced amphoteric features. The dyes on the other hand as well as verapamilhydrochloride consist of molecules with pronounced ionic character. These features give reason to believe that different surface bonding states exist for the two types of molecules: the ionic molecules probably are chemisorbed, the polar molecules physisorbed. The latter is supported by investiga-
I
1
I
I
,-r11
veropamllhydrochlor~de
,p:
33
) m=165
600
m=151 300
I _I
.-
m c
140
170
220
260
300
600 7
i
388
400
410
L20
430
440 450
460
mass Fig. 4. Sections of the positive Primary ions are 60 MeV ‘*S.
ion spectrum
of verapamilhydrochloride.
R = CH,,
R’= CH,.
203
B. Nees et al. / Fast heavy ion induced desorption
z
0 iii 5
~~~
x3-*15*-
;, lG3J lo”-
lG3-
e
10
20
E IMeW
30 &ev,
Fig. 5. Energy dependence of the yield for secondary ions desorbed by 160 primary ions from a phenylalanine sample. The solid lines are calculated with eq. (1) using the exponent n indicated in the figure. Fig. 6. See caption
of fig. 5. The sample investigated
is chioramphenicol.
Fig. 7. Energy dependence of the yield for cations desorbed by t60 primary ions from malachite green, crystal violet and verapamilhydrochloride (also for the verapamil fragment with mass 303). The solid lines are calculated with eq. (1) using n = 4.
B. Nees et al. / Fast heacy ran induced desorptron
204 tions
performed for various amino acids by Benninghoven et al. [19]. If these two different bonding states exist, it is reasonable to assume that the different energy dependences observed for positive secondary ions simply reflect the different strengths of the surface bonds: the energy variation of the primary ions and thus the variation of the energy deposited in the sample surface has less influence on the yield of weakly bound molecules compared with that of strongly bound species. This can be shown in a purely geometrical picture describing the radial dissipation of the energy deposited along the primary ion trajectory: the radius of the zone accessible to desorption decreases more rapidely for a strong bonding state compared with a weak one if
GY
0
It+PI-
/ fi
f
W CV,
surface interaction. m Coulomb attraction
interna vibration 1
(repulsion
Fig. 8. Schematic drawing of the assumed desorption processes for molecules or molecular ions physisorbed at the sample surface and for two different starting conditions: (i) preformed [M + H]+ and [M-H]- molecular ions exist at the surface before the primary ion hit the sample and experience different Coulomb forces after the passage of the primary ion (uppermost cases) and (ii) neutral molecules [Ml0 are physisorbed at the surface (all other examples). The symbols + , _ and 0 refer to ionized and neutral species, respectively. R and p stand for a molecular fragment and proton. respectively. The dashed curve shows the assumed distribution of the energy AE deposited by the primary ion.
B. Nees et al. / Fast heavy ion induced desorption
205
the energy deposited decreases assuming any reasonable functional form for the dissipation. In addition the desorption of strongly bound molecules is hindered compared to that of weakly bound states in regions of small energy deposits due to the quantum-mechanical reflection at the surface potential barrier. The different energy dependences observed for the [M + H]+ and [M-H]yield obtained from the same sample (containing polar molecules) touches upon the question of the nature of origin of the desorbed species. This includes predissociation in the solvent, dissociation and association processes at the surface, the surface selvedge and the vacuum as well as other possibilities. If the [M + H]+ and [M-H]molecular ions are preformed at the sample surface as assumed by Benninghoven et al. [19], one needs an additional process which enforces [M + H]+ desorption and hinders [M-H]desorption in order to explain the different energy dependences observed. This process could be Coulomb repulsion [20] (attraction) between the preformed species and positively charged atoms or molecules which exist abundantly within the primary ion track as a result of the energy transfer mechanism valid for fast heavy ions (electronic stopping). The same mechanism could explain the desorption of other preformed positive and negative charged entities, too. Without the above Coulomb effects it seems hardly possible to explain different energy dependences for [M + H]+ and [M-H]molecules unless one assumes that neutral molecules [Ml0 are physisorbed at the sample surface. These molecules may be desorbed as neutrals and undergo subsequently association and dissociation processes. The latter are energy consuming and introduce therefore a hindrance for the desorption of [M-H]ions in the sense discussed above. They are restricted to molecules with reasonable internal excitations (vibrational, rotational, electronic). Since the excitation energy available decreases with increasing primary ion energy E different dependences on E can e expected for protonated and deprotonated molecular ions. If the neutral surface molecules [Ml0 become ionized by the primary ions, processes combining the above mechanism with Coulomb explosion (attraction) effects may occur as sketched schematically in fig. 8. It is assumed in this schematic that the energy A E deposited by the fast I60 ions (via electronic excitations) is coupled into internal (vibrational, rotational, electronic) and external (vibrations relative to the surface) excitations of the surface molecules. In order to explain the desorption of unfragmented species one has to assume that electronic excitations couple frequently to continuum states of the surface potential. 4. Conclusion We have measured the dependence of the desorption yield of organic molecular ions on the energy of the fast primary ions (oxygen ions). Different
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samples have been investigated. The yield can be parameterized with parameters which determine the energy loss of the primary ions in the sample surface. Nevertheless it turns out that the desorption yield is not just proportional to the energy loss (electronic stopping power) in the energy range studied. Different energy dependences observed for positive secondary ions from various samples are probably related to the strength of the corresponding surface bonding states; different dependences for positive and negative secondary ions desorbed from the same sample (polar compounds) have to be related to the formation processes for the desorbed species. Explanations are given the confirmation of which needs further investigations which are planned in this laboratory.
Acknowledgement This work was supported Fed. Rep. of Germany.
by the Deutsche
Forschungsgemeinschaft,
Bonn,
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