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Analysis of the desorption of clusters of intact amino acid valine molecules induced by energetic monoatomic and cluster ions
Nuclear Instruments and Methods in Physics Research B 230 (2005) 502–506 www.elsevier.com/locate/nimb
Analysis of the desorption of clusters of intact amino acid valine molecules induced by energetic monoatomic and cluster ions G. Szenes
*
Department of General Physics, Eo¨tvo¨s University, P.O. Box 32, 1518 Budapest, Hungary
Abstract The experiments on the desorption of intact valine molecules and (nM + H)+ clusters are analyzed and a ln(Y/Se) 1/Se scaling is found where Y, and Se are the sputtering yield and the electronic stopping power, respectively. The scaling can be derived with the assumption of a thermal activation mechanism. In the plots the desorption is a uniform process without threshold value of Se, having different activation energies U in various charge states. The desorption of (nM + H)+ clusters proceeds in n steps with varying Coulomb contributions. Irradiation with C10 and C60 ions leads to higher Y, however, the increment is reduced with the increase of Se. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.82.Pr; 79.20.Rf; 82.80.Rt; 87.50.Gi Keywords: Biomolecule desorption; Valine clusters; Thermal activation; Energetic ions; Mass spectrometry
1. Introduction Ion-induced sputtering of biomolecules is often called desorption. The desorption of intact biomolecules is a kind of mysterious effect. It is quite unexpected that large biomolecules – amino acids, proteins etc. – are able to survive the impact of swift heavy ions when these molecules are easily
*
Tel.: +36 1 372 2821; fax: +36 1 372 2811. E-mail address: [email protected]
damaged by low energy electrons. Moreover, in the process of desorption they obtain kinetic energy up to 10 eV, as well [1]. Much experimental and theoretical effort has been devoted to these problems. However, up to now even the basic mechanism of the desorption process has not been revealed [2]. Recently, we proposed a thermal activation model of desorption, which provided a uniform description of the phenomenon for small and large molecules in various charge states and the validity of this model was demonstrated in a broad range
0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.091
G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
503
of energy deposition [3]. In the present paper we apply the results for the analysis of the experiments on the desorption of clusters of amino acid valine molecules.
2. Experimental Hakansson et al. performed irradiations with I, Cu, Ni, S, O and C ion beams of the same velocity (E = 0.71 MeV/nucleon). They studied the desorption of bovine insulin (molecular mass MW = 5733) and of amino acid valine (MW = 117) [4,5]. The molecules were dissolved in a solvent and electrosprayed onto an aluminium backing. The desorped ions were measured by a time-of-flight spectrometer. The desorption yield Y was measured by a relative method. Brandl et al. also measured Y for valine in a broader range of Se. The samples were made by evaporation [6]. Recently, we found a simple correlation between Y and the electronic stopping power Se Y x ¼ Bx S e expfAx =S e g;
ð1Þ
where x denotes the charge state and Ax and Bx are constants. Eq. (1) is fulfilled well for the desorption of various intact biomolecules – valine, leucine, insulin – in different charge states, at low and high values of Se [3]. The results are shown in Fig. 1. In this plot the slopes agree within experimental error for experiments performed in different laboratories. In the experiments on valine ions the desorption of clusters of n molecules with
Fig. 2. The desorption yield Y of clusters of amino acid valine molecules [4] plotted according to Eq. (1). The results of irradiations by C10 (Se = 45.6 and 91.4 MeV/(mg/cm2)) and C60 (Se = 242 MeV/(mg/cm2)) are denoted by full symbols [18]. Cf denotes fission fragments from a 252Cf source.
positive charge (nM + H)+ were also measured [4,5]. According to Fig. 2 the scaling is valid for valine clusters, as well. The slopes only slightly increase with n. Baudin et al. [7] performed irradiation experiments with C10 (4.2 and 20.2 MeV), C60 (20.2 MeV) ions and with fission fragments (ff) from a 252Cf source. The 100 nm thick valine samples were evaporated onto self-supporting thin carbon foils. They performed relative measurements of the yield Y of valine clusters up to clusters containing n = 11 valine molecules. In the conditions of the experiment the ff of a 252Cf source were characterized by Se = 5 keV/nm in average. In Fig. 2 we shifted the results of ff experiments at this Se to fit the data in [4,5]. Thus a scaling coefficient m = 2.5 was obtained and applied to the cluster irradiation. In Fig. 2 the yield is higher for clusters than for monoatomic irradiations but the difference is considerably reduced at high Se values.
3. Discussion
Fig. 1. The desorption yield Y of amino acid valine (V) and bovine insulin (I) molecular ions plotted according to Eq. (1).
We assume that the desorption of intact biomolecules is a thermally activated process and Yx is given by Z Z U Yx / P x ðT ; tÞ exp r dr dt; ð2Þ kT
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G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
where T, r, t are the local temperature, the radial distance from the spike center and the time, respectively, U is the activation energy, and Px(T, t) is the probability of formation and survival meaning the probability that a target molecule is in the given state (intact molecule, neutral, positive or negative ion) at its ejection and detection [8]. In previous models Px(T, t) = constant was supposed. To evaluate Eq. (2) we applied our thermal spike model [9] which assumes a Gaussian temperature distribution in the thermal spike. Previously we applied our model to the analysis of track evolution in insulators [9], in the applications to the problem of electronic mixing [10] and of hillocks induced by monoatomic and cluster ions on surfaces [11]. The derived equations were in good agreements with the experiments providing a justification of the assumptions of the model. In [3] we showed that Eq. (1) can be derived from Eq. (2) only if the probability Px(T, t) 5 constant. By comparison of Eqs. (1) and (2) we derived the general form of Px(T, t), and the relation Ax/ Se = U/kTpo, which provides a possibility to estimate the activation energy U from the experimental results [3]. In the expression Tpo is the initial peak temperature in the spike T po ¼
gS e ¼ RS e ; pqca2 ð0Þ
ð3Þ
where a(0) = 4.5 nm, c, q are the specific heat and the density, respectively and g is the efficiency determining the fraction of the deposited energy transferred to the thermal spike [9]. To estimate U one must know the value of R in Eq. (3). However, the specific heat c of the biomolecules is not known in the conditions of a thermal spike. This is an especially complex problem, since the stability of biomolecules in the spike is possible only if the excitation of the internal modes of the molecules is not in thermal equilibrium [12]. This may strongly reduce the specific heat. To avoid the consequences of the uncertainty in the estimate of c, we evaluated the activation energies relative to the sublimation energy Usb = 1.69 eV [13]. According to our conclusions, it is reasonable to use Usb for the activation energy of the desorption of (M–H) ions U (for details see [3]). As a
result we obtained the proportionality factor R = 730 in the relation Tpo = RSe where Se is measured in MeV/(mg/cm2) units. In the following we use this R value to estimate the activation energy Un of clusters (nM + H)+ of n valine molecules. We have shown that g = 0.4 for E < 2 MeV/nucleon [14] and obtained c = 0.14 J/g K from Eq. (3) for valine [3] compared to c = 1.44 J/g K at T = 300 K. A high specific heat indicates a high excitation of the internal modes. When the excitation is so intense at high temperatures, the survival of intact molecules is doubtful even in positions far from the center of the spike. On the other hand, the specific heat of monoatomic solids with atomic weight A = 117 is c = 0.21 J/g K according to the Dulong–Petit law. This is not far from the value which we derived from the experiments. In our estimate the low specific heat indicates that up to the moment of the ejection, the biomolecules behave like rigid spheres, and the excitation of vibrational modes is weak. This is in agreement with previous assumptions with respect of the origin of the stability of biomolecules in the spike [12]. In the experiments on valine samples large clusters were observed in the time of flight spectra [4,5]. Since the sputtering yield Y < 1 for valine ions, the probability of clustering of sputtered positive ions is very low. Y for neutrals is about 104 higher, if the results on the desorption of neutral amino acid leucine (MW = 131) molecules are qualitatively valid for valine, as well. However, we can exclude any contribution of neutral molecules to the formation of positive cluster ions as U+ U0 [3]. In Fig. 3 we plotted Un versus n for (nM + H)+ valine clusters. The activation energy for desorption of valine clusters increases with n up to n = 3, however, its value does not change for n = 4. This can be explained if we assume that the double layer structure extending parallel to the (0 0 1) planes in the crystals [15] is preserved in the samples, prepared by the electrospray method, as well. In valine crystals a molecule is bound to four neighbors by four strong lateral hydrogen bonds and the interlayer ones are much weaker van der Waals bonds with UW = 0.06 eV [16]. All four bonds must be broken for the desorption of a single molecule. Each internal bond in a cluster
G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
Fig. 3. Variation of the desorption energy of valine clusters Un and of the number of broken bonds Nbr versus the number of molecules in the cluster n. A closed (head-to-tail) and an open configuration are considered for n = 4.
reduces the number of broken bonds Nbr by two. The number of internal bonds may be three (open configuration) or four (closed or head-to-tail configuration) for n = 4, consequently, Nbr = 10 or Nbr = 8. We also plotted the number of broken bonds Nbr in various clusters in Fig. 3. The figure confirms the close relation between Un and Nbr and indicates that the cluster with n = 4 is a closed configuration of molecules. To explain quantitatively the experimental results we assume that the desorption of a cluster proceeds in several steps. After each step except the last one some bonds still fix the molecules to the surface while the distance between the ion charge and the partner charge gradually increases. Consequently, the Coulomb contribution gUCb changes with n. By using the values of Un, Usb UW and Nbr we found that UCb = 1.3 eV, g = 1, 1.5, 2.02, 2.05 for n = 1, 2, 3, 4, respectively. There is a systematic variation of Bx with n. We found that Bn = B0exp{1.1n} (x is omitted). The reduction of Bn with n is related to the variation of Nbr and of the shape of the desorped clusters. While the size of tracks induced by monoatomic and Cn ions in insulators can be scaled by Se for E < 2 MeV/nucleon [17], the results in Fig. 2 show that this is not the case for the desorption of (nM + H)+ ions. Compared to the track formation there are two important additional effects in the desorption of biomolecules, (i) direct damage of molecules, (ii) enhanced emission of H+ ions. Both processes are typical in the track core. Compared
505
to monoatomic projectiles, the track core of Cn ions is considerably narrower. Moreover the number of emitted protons is by about an order of magnitude higher for Cn ions than for ff fragments [18]. Since the track core diameter is smaller, the total number of intact molecules in the spike is higher but the total number of positive ions can be smaller. The balance of (i) and (ii) determines the variation of Y. Because of their low yields the valine cluster ions are less sensitive to the reduction of the available H+ ions, than the positive valine molecules. Thus, in the experiments with C10 of 4.2 MeV energy the increase of Y was by about five times for valine cluster ions with n = 2–4 and only by 2.1 times for (M + H)+. Se was higher in the experiments with 20.2 MeV C10 and C60 ions, and Y was also higher. However, this increase in Y was smaller than it could have been expected. This is demonstrated by the fall of the appropriate Y/Se values with increasing Se. Moreover, Y of (M + H)+ ions is even smaller for C60 irradiation than for a monoatomic ion with the same Se. According to this consideration, the yield of neutrals would increase much steeper because of the different sensitivity to the proton emission.
4. Conclusions The ion-induced desorption of valine clusters is a thermally activated process. The activation energy Un is closely related to the number of broken bonds. The desorption proceeds in n steps with an increase of the total Coulomb contribution in each step. The (4M + H)+ cluster is a closed configuration and U3 U4. Compared to monoatomic irradiations, the desorption induced by C10 and C60 cluster ions is higher. However, the increment decreases with increasing Se, which is attributed to the reduction of the number density of molecules with positive charge.
Acknowledgement This work was completed with the support of the National Scientific Research Fund (OTKA, Hungary) under Contract No. T031756.
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G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
References [1] K. Wien, Nucl. Instr. and Meth. B 65 (1992) 149. [2] C.T. Reimann, Det Kgl. Danske Videnskab. Selskab Mat. Fys. Medd. 43 (1993) 351. [3] G. Szenes, Phys. Rev. B 70 (2004) 094106. [4] A. Hedin, P. Hakansson, B. Sundqvist, R.E. Johnson, Phys. Rev. B 31 (1985) 1780. [5] P. Hakansson, I. Kamensky, M. Salehpour, B. Sundqvist, S. Widdiyasekera, Radiat. Eff. 80 (1984) 141. [6] D. Brandl, Ch. Schoppmann, R. Schmidt, B. Nees, A. Ostrowski, H. Voit, Phys. Rev. B 43 (1991) 5253. [7] K. Baudin et al., Nucl. Instr. and Meth. B 112 (1996) 59. [8] R.E. Johnson, Int. J. Mass Spectrom. Ion Proc. 126 (1993) 17.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
G. Szenes, Phys. Rev. B 51 (1995) 8026. G. Szenes, Appl. Phys. Lett. 81 (2002) 4622. G. Szenes, Nucl. Instr. and Meth. B 191 (2002) 27. R.D. Macfarlane, D.F. Torgerson, Science 191 (1976) 920. H.J. Svec, D.D. Clyde, J. Chem. Eng. Data 10 (1965) 151. G. Szenes, Phys. Rev. B 60 (1999) 3140. T. Kazuo, I. Yoichi, Acta Cryst. B 26 (1970) 1317. C.T. Reimann, Nucl. Instr. and Meth. B 95 (1995) 181. ´ . Pe´ter, A.I. Popov, Nucl. Instr. and G. Szenes, F. Pa´szti, A Meth. B 166–167 (2000) 949. A. Brunelle, S. Della-Negra, C. Deprun, J. Depauw, P. Hakansson, D. Jacquet, Y. le Beyec, M. Pautrat, Int. J. Mass Spectrom. Ion Proc. 164 (1997) 193.
Analysis of the desorption of clusters of intact amino acid valine molecules induced by energetic monoatomic and cluster ions G. Szenes
*
Department of General Physics, Eo¨tvo¨s University, P.O. Box 32, 1518 Budapest, Hungary
Abstract The experiments on the desorption of intact valine molecules and (nM + H)+ clusters are analyzed and a ln(Y/Se) 1/Se scaling is found where Y, and Se are the sputtering yield and the electronic stopping power, respectively. The scaling can be derived with the assumption of a thermal activation mechanism. In the plots the desorption is a uniform process without threshold value of Se, having different activation energies U in various charge states. The desorption of (nM + H)+ clusters proceeds in n steps with varying Coulomb contributions. Irradiation with C10 and C60 ions leads to higher Y, however, the increment is reduced with the increase of Se. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.82.Pr; 79.20.Rf; 82.80.Rt; 87.50.Gi Keywords: Biomolecule desorption; Valine clusters; Thermal activation; Energetic ions; Mass spectrometry
1. Introduction Ion-induced sputtering of biomolecules is often called desorption. The desorption of intact biomolecules is a kind of mysterious effect. It is quite unexpected that large biomolecules – amino acids, proteins etc. – are able to survive the impact of swift heavy ions when these molecules are easily
*
Tel.: +36 1 372 2821; fax: +36 1 372 2811. E-mail address: [email protected]
damaged by low energy electrons. Moreover, in the process of desorption they obtain kinetic energy up to 10 eV, as well [1]. Much experimental and theoretical effort has been devoted to these problems. However, up to now even the basic mechanism of the desorption process has not been revealed [2]. Recently, we proposed a thermal activation model of desorption, which provided a uniform description of the phenomenon for small and large molecules in various charge states and the validity of this model was demonstrated in a broad range
0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.091
G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
503
of energy deposition [3]. In the present paper we apply the results for the analysis of the experiments on the desorption of clusters of amino acid valine molecules.
2. Experimental Hakansson et al. performed irradiations with I, Cu, Ni, S, O and C ion beams of the same velocity (E = 0.71 MeV/nucleon). They studied the desorption of bovine insulin (molecular mass MW = 5733) and of amino acid valine (MW = 117) [4,5]. The molecules were dissolved in a solvent and electrosprayed onto an aluminium backing. The desorped ions were measured by a time-of-flight spectrometer. The desorption yield Y was measured by a relative method. Brandl et al. also measured Y for valine in a broader range of Se. The samples were made by evaporation [6]. Recently, we found a simple correlation between Y and the electronic stopping power Se Y x ¼ Bx S e expfAx =S e g;
ð1Þ
where x denotes the charge state and Ax and Bx are constants. Eq. (1) is fulfilled well for the desorption of various intact biomolecules – valine, leucine, insulin – in different charge states, at low and high values of Se [3]. The results are shown in Fig. 1. In this plot the slopes agree within experimental error for experiments performed in different laboratories. In the experiments on valine ions the desorption of clusters of n molecules with
Fig. 2. The desorption yield Y of clusters of amino acid valine molecules [4] plotted according to Eq. (1). The results of irradiations by C10 (Se = 45.6 and 91.4 MeV/(mg/cm2)) and C60 (Se = 242 MeV/(mg/cm2)) are denoted by full symbols [18]. Cf denotes fission fragments from a 252Cf source.
positive charge (nM + H)+ were also measured [4,5]. According to Fig. 2 the scaling is valid for valine clusters, as well. The slopes only slightly increase with n. Baudin et al. [7] performed irradiation experiments with C10 (4.2 and 20.2 MeV), C60 (20.2 MeV) ions and with fission fragments (ff) from a 252Cf source. The 100 nm thick valine samples were evaporated onto self-supporting thin carbon foils. They performed relative measurements of the yield Y of valine clusters up to clusters containing n = 11 valine molecules. In the conditions of the experiment the ff of a 252Cf source were characterized by Se = 5 keV/nm in average. In Fig. 2 we shifted the results of ff experiments at this Se to fit the data in [4,5]. Thus a scaling coefficient m = 2.5 was obtained and applied to the cluster irradiation. In Fig. 2 the yield is higher for clusters than for monoatomic irradiations but the difference is considerably reduced at high Se values.
3. Discussion
Fig. 1. The desorption yield Y of amino acid valine (V) and bovine insulin (I) molecular ions plotted according to Eq. (1).
We assume that the desorption of intact biomolecules is a thermally activated process and Yx is given by Z Z U Yx / P x ðT ; tÞ exp r dr dt; ð2Þ kT
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G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
where T, r, t are the local temperature, the radial distance from the spike center and the time, respectively, U is the activation energy, and Px(T, t) is the probability of formation and survival meaning the probability that a target molecule is in the given state (intact molecule, neutral, positive or negative ion) at its ejection and detection [8]. In previous models Px(T, t) = constant was supposed. To evaluate Eq. (2) we applied our thermal spike model [9] which assumes a Gaussian temperature distribution in the thermal spike. Previously we applied our model to the analysis of track evolution in insulators [9], in the applications to the problem of electronic mixing [10] and of hillocks induced by monoatomic and cluster ions on surfaces [11]. The derived equations were in good agreements with the experiments providing a justification of the assumptions of the model. In [3] we showed that Eq. (1) can be derived from Eq. (2) only if the probability Px(T, t) 5 constant. By comparison of Eqs. (1) and (2) we derived the general form of Px(T, t), and the relation Ax/ Se = U/kTpo, which provides a possibility to estimate the activation energy U from the experimental results [3]. In the expression Tpo is the initial peak temperature in the spike T po ¼
gS e ¼ RS e ; pqca2 ð0Þ
ð3Þ
where a(0) = 4.5 nm, c, q are the specific heat and the density, respectively and g is the efficiency determining the fraction of the deposited energy transferred to the thermal spike [9]. To estimate U one must know the value of R in Eq. (3). However, the specific heat c of the biomolecules is not known in the conditions of a thermal spike. This is an especially complex problem, since the stability of biomolecules in the spike is possible only if the excitation of the internal modes of the molecules is not in thermal equilibrium [12]. This may strongly reduce the specific heat. To avoid the consequences of the uncertainty in the estimate of c, we evaluated the activation energies relative to the sublimation energy Usb = 1.69 eV [13]. According to our conclusions, it is reasonable to use Usb for the activation energy of the desorption of (M–H) ions U (for details see [3]). As a
result we obtained the proportionality factor R = 730 in the relation Tpo = RSe where Se is measured in MeV/(mg/cm2) units. In the following we use this R value to estimate the activation energy Un of clusters (nM + H)+ of n valine molecules. We have shown that g = 0.4 for E < 2 MeV/nucleon [14] and obtained c = 0.14 J/g K from Eq. (3) for valine [3] compared to c = 1.44 J/g K at T = 300 K. A high specific heat indicates a high excitation of the internal modes. When the excitation is so intense at high temperatures, the survival of intact molecules is doubtful even in positions far from the center of the spike. On the other hand, the specific heat of monoatomic solids with atomic weight A = 117 is c = 0.21 J/g K according to the Dulong–Petit law. This is not far from the value which we derived from the experiments. In our estimate the low specific heat indicates that up to the moment of the ejection, the biomolecules behave like rigid spheres, and the excitation of vibrational modes is weak. This is in agreement with previous assumptions with respect of the origin of the stability of biomolecules in the spike [12]. In the experiments on valine samples large clusters were observed in the time of flight spectra [4,5]. Since the sputtering yield Y < 1 for valine ions, the probability of clustering of sputtered positive ions is very low. Y for neutrals is about 104 higher, if the results on the desorption of neutral amino acid leucine (MW = 131) molecules are qualitatively valid for valine, as well. However, we can exclude any contribution of neutral molecules to the formation of positive cluster ions as U+ U0 [3]. In Fig. 3 we plotted Un versus n for (nM + H)+ valine clusters. The activation energy for desorption of valine clusters increases with n up to n = 3, however, its value does not change for n = 4. This can be explained if we assume that the double layer structure extending parallel to the (0 0 1) planes in the crystals [15] is preserved in the samples, prepared by the electrospray method, as well. In valine crystals a molecule is bound to four neighbors by four strong lateral hydrogen bonds and the interlayer ones are much weaker van der Waals bonds with UW = 0.06 eV [16]. All four bonds must be broken for the desorption of a single molecule. Each internal bond in a cluster
G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
Fig. 3. Variation of the desorption energy of valine clusters Un and of the number of broken bonds Nbr versus the number of molecules in the cluster n. A closed (head-to-tail) and an open configuration are considered for n = 4.
reduces the number of broken bonds Nbr by two. The number of internal bonds may be three (open configuration) or four (closed or head-to-tail configuration) for n = 4, consequently, Nbr = 10 or Nbr = 8. We also plotted the number of broken bonds Nbr in various clusters in Fig. 3. The figure confirms the close relation between Un and Nbr and indicates that the cluster with n = 4 is a closed configuration of molecules. To explain quantitatively the experimental results we assume that the desorption of a cluster proceeds in several steps. After each step except the last one some bonds still fix the molecules to the surface while the distance between the ion charge and the partner charge gradually increases. Consequently, the Coulomb contribution gUCb changes with n. By using the values of Un, Usb UW and Nbr we found that UCb = 1.3 eV, g = 1, 1.5, 2.02, 2.05 for n = 1, 2, 3, 4, respectively. There is a systematic variation of Bx with n. We found that Bn = B0exp{1.1n} (x is omitted). The reduction of Bn with n is related to the variation of Nbr and of the shape of the desorped clusters. While the size of tracks induced by monoatomic and Cn ions in insulators can be scaled by Se for E < 2 MeV/nucleon [17], the results in Fig. 2 show that this is not the case for the desorption of (nM + H)+ ions. Compared to the track formation there are two important additional effects in the desorption of biomolecules, (i) direct damage of molecules, (ii) enhanced emission of H+ ions. Both processes are typical in the track core. Compared
505
to monoatomic projectiles, the track core of Cn ions is considerably narrower. Moreover the number of emitted protons is by about an order of magnitude higher for Cn ions than for ff fragments [18]. Since the track core diameter is smaller, the total number of intact molecules in the spike is higher but the total number of positive ions can be smaller. The balance of (i) and (ii) determines the variation of Y. Because of their low yields the valine cluster ions are less sensitive to the reduction of the available H+ ions, than the positive valine molecules. Thus, in the experiments with C10 of 4.2 MeV energy the increase of Y was by about five times for valine cluster ions with n = 2–4 and only by 2.1 times for (M + H)+. Se was higher in the experiments with 20.2 MeV C10 and C60 ions, and Y was also higher. However, this increase in Y was smaller than it could have been expected. This is demonstrated by the fall of the appropriate Y/Se values with increasing Se. Moreover, Y of (M + H)+ ions is even smaller for C60 irradiation than for a monoatomic ion with the same Se. According to this consideration, the yield of neutrals would increase much steeper because of the different sensitivity to the proton emission.
4. Conclusions The ion-induced desorption of valine clusters is a thermally activated process. The activation energy Un is closely related to the number of broken bonds. The desorption proceeds in n steps with an increase of the total Coulomb contribution in each step. The (4M + H)+ cluster is a closed configuration and U3 U4. Compared to monoatomic irradiations, the desorption induced by C10 and C60 cluster ions is higher. However, the increment decreases with increasing Se, which is attributed to the reduction of the number density of molecules with positive charge.
Acknowledgement This work was completed with the support of the National Scientific Research Fund (OTKA, Hungary) under Contract No. T031756.
506
G. Szenes / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 502–506
References [1] K. Wien, Nucl. Instr. and Meth. B 65 (1992) 149. [2] C.T. Reimann, Det Kgl. Danske Videnskab. Selskab Mat. Fys. Medd. 43 (1993) 351. [3] G. Szenes, Phys. Rev. B 70 (2004) 094106. [4] A. Hedin, P. Hakansson, B. Sundqvist, R.E. Johnson, Phys. Rev. B 31 (1985) 1780. [5] P. Hakansson, I. Kamensky, M. Salehpour, B. Sundqvist, S. Widdiyasekera, Radiat. Eff. 80 (1984) 141. [6] D. Brandl, Ch. Schoppmann, R. Schmidt, B. Nees, A. Ostrowski, H. Voit, Phys. Rev. B 43 (1991) 5253. [7] K. Baudin et al., Nucl. Instr. and Meth. B 112 (1996) 59. [8] R.E. Johnson, Int. J. Mass Spectrom. Ion Proc. 126 (1993) 17.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
G. Szenes, Phys. Rev. B 51 (1995) 8026. G. Szenes, Appl. Phys. Lett. 81 (2002) 4622. G. Szenes, Nucl. Instr. and Meth. B 191 (2002) 27. R.D. Macfarlane, D.F. Torgerson, Science 191 (1976) 920. H.J. Svec, D.D. Clyde, J. Chem. Eng. Data 10 (1965) 151. G. Szenes, Phys. Rev. B 60 (1999) 3140. T. Kazuo, I. Yoichi, Acta Cryst. B 26 (1970) 1317. C.T. Reimann, Nucl. Instr. and Meth. B 95 (1995) 181. ´ . Pe´ter, A.I. Popov, Nucl. Instr. and G. Szenes, F. Pa´szti, A Meth. B 166–167 (2000) 949. A. Brunelle, S. Della-Negra, C. Deprun, J. Depauw, P. Hakansson, D. Jacquet, Y. le Beyec, M. Pautrat, Int. J. Mass Spectrom. Ion Proc. 164 (1997) 193.