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Ionization of water and carbon dioxide molecules by electron impact near threshold
Nuclear Instruments and Methods in Physics Research B 233 (2005) 298–301 www.elsevier.com/locate/nimb
Ionization of water and carbon dioxide molecules by electron impact near threshold A.N. Zavilopulo, F.F. Chipev *, O.B. Shpenik Institute of Electron Physics, Ukrainian National Academy of Sciences, 88017 Uzhhorod, Ukraine Available online 10 May 2005
Abstract A measurement technique is described and relative cross-sections for direct and dissociative ionization of H2O and CO2 molecules by electron impact near the threshold are obtained. The experiment is performed by a setup with mass separation of the ions by a monopole mass spectrometer and their detection by a secondary electron multiplicator. The energy dependencies of the cross-sections for the formation of the main molecule ions and fragment ions resulting from dissociation are given in the incident electron energy range from 7 to 35 eV. Ó 2005 Published by Elsevier B.V. PACS: 32.80D; 34.80D Keywords: Dissociative ionization; Cross-sections; Fragment ions
1. Introduction Investigation of the specific features of single and dissociative ionization of multiatomic molecules near threshold is of essential interest in order to determine the role of the initial energy dissipation during the interaction of electrons with molecules. It is the near-threshold energy range where many aspects of the molecular and atomic structures are revealed as well as their influence on the energy dependence of the process is investi*
Corresponding author. E-mail address: [email protected] (F.F. Chipev).
0168-583X/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.03.125
gated. Systematic studies of the threshold behaviour of the reaction products formed in single and dissociative ionization of complex organic [1] and halogencontaining molecules [2], deuterated molecules [3] and molecules of some atmospheric gases [4,5] by electron impact are performed in our laboratory. Here we present the results obtained by a commercially available mass spectrometer (MS) with a monopole ion analyser: threshold dependencies of single and dissociative ionization of H2O and CO2 molecules by electron impact are investigated. The main attention was paid to the measurement and thorough analysis of the ionization thresholds of
A.N. Zavilopulo et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 298–301
2. Experimental For the experimental purposes a monopole mass spectrometer SELMI MX-7304A was used as an analytical device, being a unipolar version of quadrupole mass spectrometer. Residual gases were pumped out by oil-free technique. This type of mass spectrometer belongs to the class of dynamic devices and is in fact a band mass filter. The ions, having passed through the mass filter, are detected and registered by the measuring system. The beams of molecules under investigation were formed by a multichannel effusive source which provided a concentration of 1010–1011 cm3 in the area of interaction with the electron beam. The substance (or mixture) under study is supplied to the source by a dosage valve. The molecules of the analyzed substance are ionized in the ion source by electron bombardment. The ions, obtained by the collisions, are extracted out of the interaction range of the electron and the molecular beam and let to the input of the ion-optical system of the analyzer (mass filter). A special attention was paid to the energy scale calibration of the ionizing electron. The accuracy of the determination of the appearance and ionization potentials for certain fragments depends on the accuracy of the energy of the primary electrons. For this purpose we applied the method of matching the initial regions of the measured energy dependencies of the ionization cross-sections to the known ionization thresholds of Ar atoms, determined with high reliability [6]. We have chosen argon and krypton atoms as calibration gases and measured the initial regions of the ionization function by electron impact. The calibration procedure was repeated 4–5 times for each measurement cycle.
atoms were shown by Wannier [7] for hydrogen atom. He introduced the concept of three radial zones: the first one is located at small distances between particles (below 1 a0) where the interaction has to be calculated by quantum mechanics; the second one is located at the distance of the order of 100 a0 where Coulombic forces act; and the third one where the particles are practically free from interaction. Figs. 1 and 2 represent the measured initial regions of the cross-sections for single ionization of the molecules under investigation and cross-sections for formation of their fragments due to dissociative ionization by electron impact. In the figures the points plotted by various symbols denote the experimental data, while the solid lines were calculated with use of a procedure similar to that of [8]. For CO2 and H2O molecules a gradual growth of the cross-section for the yield of the fragment ions þ + + + (H+, OH+, O+, Hþ 2 , H2O , CO2 , CO , C etc.) formed by dissociative ionization can be observed with increasing electron energy. This growth depends on the binding type of the parent molecule as well as on the number of atoms in the molecule. For the exact determination of the ionization potentials of the molecules (or atoms) and the appearance energies of the ion fragments formed by dissociative ionization we applied a method
H2O+/H2O OH+/H2O O+/H2O 3
e- + H2O H+/H2O
I, arb.un.
the parent molecules and the threshold of the appearance of the dissociative ionization fragments (child ions).
299
2 3
1
12
H2+/H2O 4
0 10
12
14
16
18
20
22
24
26
E, eV
3. Results and discussion The main regularities of the threshold behaviour of the effective ionization cross-section of
Fig. 1. Threshold regions of the relative cross-section for ionization of water molecule and formation of ion fragments due to dissociative ionization by electron impact. H+ion: 1 Eap = 16.00 eV, 2 Eap = 16.95 eV, 3 Eap = 18.70 eV; O+ion: 4 Eap = 19.00 eV.
300
A.N. Zavilopulo et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 298–301
2
e- + CO2
+
CO+/CO2
C /CO2
+
I, arb.un.
CO2 /CO2 O+/CO2
1
2 1
0 10 12 14 16 18 20 22 24 26 28 30 32
E, eV Fig. 2. Threshold regions of the relative cross-section for ionization for carbon dioxide molecule and formation of ion fragments due to dissociative ionization by electron impact. C+ion: 1 Eap = 22.70 eV, 2 Eap = 27.8 eV.
worked out by Ma¨rk group [9,10]. For water and carbon dioxide molecules we have measured the energy dependencies of the formation of the fragments due to dissociative ionization. Now let us consider the dissociation channels for water: e þ H2 O ! Hþ þ OH þ 2e ðEap ¼ 16.95 eVÞ Hþ þ OH þ e ðEap ¼ 16.00 eVÞ Hþ þ OHðX2 PÞ þ 2e ðEap ¼ 18.70 eVÞ Hþ 2 þ O þ 2e ðE ap ¼ 20.70 eVÞ HOþ þ H þ 2e ðEap ¼ 18.11 eVÞ Oþ þ H2 þ 2e ðEap ¼ 19.00 eVÞ Oþ þ 2H þ 2e ðEap ¼ 26.80 eVÞ ð1Þ +
As can be seen, H ions can be formed in three channels at different energies Eap, O+ ions are + formed in two channels and only Hþ 2 and HO ions are formed at a single threshold energy. The threshold behaviour of the energy dependence of oxygen ion appearance for the dissociative ionization of water molecule is characterized by a very slow (overextended) increase of the cross-section with the ionizing electron energy (see Fig. 1). The results for the carbon dioxide molecule, shown in Fig. 2, are generally similar to those for the water molecule. However, here all the fragment ions are formed at a single threshold energy except
for the case of carbon ions where two fragment formation channels exist (see also the two last processes in (1)): e þ CO2 ! Cþ þ O2 þ 2e ðEap ¼ 22.70 eVÞ ð2Þ Cþ þ 2O þ 2e ðEap ¼ 27.80 eVÞ An important factor affecting the threshold behaviour of the ion fragment formation is the value of the energy of breaking the bond binding the atom or the molecule within the parent molecule. This can be traced especially clearly for carbon dioxide molecule. The maximal value of the bond breaking energy (10.9 eV) for the case of dissociative ionization corresponds to reaction (2) when carbon dioxide decays into C+ ion and molecular oxygen O2. Note that the main constituent of water and carbon dioxide molecules is oxygen. As mentioned above, the mass spectrum of CO2 lacks the peak corresponding to the O2 molecule what is related to the linear structure of this molecule, hence in the process of dissociative ionization O+ ion fragment is formed. From this point of view it would be interesting to compare the threshold regions of the cross-sections for the appearance of this O+ ion in interaction of electrons with molecules of oxygen, water and carbon dioxide. Such comparison is shown in Fig. 3. As one can see, the threshold dependencies of the dissociative ionization cross-section for O+/H2O and O+/CO2 are generally similar and, as mentioned above, this is in accordance with the fact that dynamics of the dissipation of energy depends on the reaction channel. Note that for both molecules the potentials of appearance Eap are very close. Quite different is the threshold behaviour of the dissociative ionization cross-section for O+/O2 (see Fig. 3). Here a sharp growth of the cross-section is observed immediately after the appearance potential. The obtained data on the relative cross-sections of the yield of positive ions due to direct ionization of the parent molecule and the yield of fragments due to the dissociative ionization give qualitative information on the processes under investigation. As already mentioned, in order to obtain absolute cross-sections besides the information on the number of molecules having participated in the collision process, one should also take into account
A.N. Zavilopulo et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 298–301
+ O /CO2 (Eap=19.05 eV)
I, arb.un.
30
+ O /O2 (Eap=18.69 eV) + O /H2O (Eap=19.00 eV)
20
10
0 14
16
18
20
22
24
26
28
E, eV Fig. 3. Comparison of the threshold regions of the dissociative ionization of oxygen, water and carbon dioxide molecule resulting in O+ ion fragment.
the angular distribution of the formed ion fragments. The analysis of the collision kinematics in the dissociative ionization processes is rather complicated. Due to the difference in the shares of the obtained kinetic energy, the angular distribution of the child ions can be different for the various reaction channels. This problem is especially urgent for mass spectrometric studies where a variety of factors should be taken into account. Therefore, in order to obtain the absolute ionization crosssections, special experiments are required, similarly to those of [11].
4. Conclusions A monopole mass spectrometer was applied for near-threshold experimental studies of the processes of both direct and dissociative ionization.
301
The high sensitivity of the spectrometer made it possible to carry out the measurements with sufficient accuracy. From the measured threshold dependencies of CO2 and H2O ionization cross-sections the appearance energies for the ion fragments of the parent molecule were determined. The non-monotonous behaviour of the crosssection for the formation of the ion fragments in a broad range of the threshold energies is to a certain degree related to the multichannel character of the process of the dissociative ionization of the molecule.
References [1] A.N. Zavilopulo, A.V. Snegursky, J.E. Kontros, I.O. Zapfel, Pisma Zh. Tekhn. Fiz. 22 (1996) 3. [2] A.V. Snegursky, A.N. Zavilopulo, F.F. Chipev, O.B. Shpenik, Rad. Phys. Chem. 68 (2003) 291. [3] A.N. Zavilopulo, A.V. Snegursky, in: V.I. Lapshin, V.M. Shulaev (Eds.), Vacuum Technologies and Equipment, Kontrast, Kharkiv, 2002, p. 16. [4] A.N. Zavilopulo, A.V. Snegursky, Techn. Phys. Lett. 28 (2002) 913. [5] A.N. Zavilopulo, F.F. Chipev, O.B. Shpenik, in: Program and Abstract EMSÕ2003 Prague, 2003, p. 200. [6] G. Mallard, P.J. Linstrom, NIST Standard Reference Database, Vol. 69, 2000, http://www.webbook.nist.gov. [7] G.H. Wannier, Phys. Rev. 90 (1953) 817. [8] A.N. Zavilopulo, F.F. Chipev, O.B. Shpenik, Techn. Phys. 4 (2005) 402. [9] T. Fiegele, G. Hanel, I. Torres, M. Lezius, T.D. Ma¨rk, J. Phys. B 33 (2000) 4263. [10] G. Hanel, B. Gstir, T. Fiegele, F. Hagelberg, K. Becker, P. Scheier, A. Snegursky, T.D. Ma¨rk, J. Chem. Phys. 116 (2002) 2456. [11] A.I. Zhukov, A.N. Zavilopulo, A.V. Snegursky, O.B. Shpenik, J. Phys. B 23 (1990) 2373S.
Ionization of water and carbon dioxide molecules by electron impact near threshold A.N. Zavilopulo, F.F. Chipev *, O.B. Shpenik Institute of Electron Physics, Ukrainian National Academy of Sciences, 88017 Uzhhorod, Ukraine Available online 10 May 2005
Abstract A measurement technique is described and relative cross-sections for direct and dissociative ionization of H2O and CO2 molecules by electron impact near the threshold are obtained. The experiment is performed by a setup with mass separation of the ions by a monopole mass spectrometer and their detection by a secondary electron multiplicator. The energy dependencies of the cross-sections for the formation of the main molecule ions and fragment ions resulting from dissociation are given in the incident electron energy range from 7 to 35 eV. Ó 2005 Published by Elsevier B.V. PACS: 32.80D; 34.80D Keywords: Dissociative ionization; Cross-sections; Fragment ions
1. Introduction Investigation of the specific features of single and dissociative ionization of multiatomic molecules near threshold is of essential interest in order to determine the role of the initial energy dissipation during the interaction of electrons with molecules. It is the near-threshold energy range where many aspects of the molecular and atomic structures are revealed as well as their influence on the energy dependence of the process is investi*
Corresponding author. E-mail address: [email protected] (F.F. Chipev).
0168-583X/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.03.125
gated. Systematic studies of the threshold behaviour of the reaction products formed in single and dissociative ionization of complex organic [1] and halogencontaining molecules [2], deuterated molecules [3] and molecules of some atmospheric gases [4,5] by electron impact are performed in our laboratory. Here we present the results obtained by a commercially available mass spectrometer (MS) with a monopole ion analyser: threshold dependencies of single and dissociative ionization of H2O and CO2 molecules by electron impact are investigated. The main attention was paid to the measurement and thorough analysis of the ionization thresholds of
A.N. Zavilopulo et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 298–301
2. Experimental For the experimental purposes a monopole mass spectrometer SELMI MX-7304A was used as an analytical device, being a unipolar version of quadrupole mass spectrometer. Residual gases were pumped out by oil-free technique. This type of mass spectrometer belongs to the class of dynamic devices and is in fact a band mass filter. The ions, having passed through the mass filter, are detected and registered by the measuring system. The beams of molecules under investigation were formed by a multichannel effusive source which provided a concentration of 1010–1011 cm3 in the area of interaction with the electron beam. The substance (or mixture) under study is supplied to the source by a dosage valve. The molecules of the analyzed substance are ionized in the ion source by electron bombardment. The ions, obtained by the collisions, are extracted out of the interaction range of the electron and the molecular beam and let to the input of the ion-optical system of the analyzer (mass filter). A special attention was paid to the energy scale calibration of the ionizing electron. The accuracy of the determination of the appearance and ionization potentials for certain fragments depends on the accuracy of the energy of the primary electrons. For this purpose we applied the method of matching the initial regions of the measured energy dependencies of the ionization cross-sections to the known ionization thresholds of Ar atoms, determined with high reliability [6]. We have chosen argon and krypton atoms as calibration gases and measured the initial regions of the ionization function by electron impact. The calibration procedure was repeated 4–5 times for each measurement cycle.
atoms were shown by Wannier [7] for hydrogen atom. He introduced the concept of three radial zones: the first one is located at small distances between particles (below 1 a0) where the interaction has to be calculated by quantum mechanics; the second one is located at the distance of the order of 100 a0 where Coulombic forces act; and the third one where the particles are practically free from interaction. Figs. 1 and 2 represent the measured initial regions of the cross-sections for single ionization of the molecules under investigation and cross-sections for formation of their fragments due to dissociative ionization by electron impact. In the figures the points plotted by various symbols denote the experimental data, while the solid lines were calculated with use of a procedure similar to that of [8]. For CO2 and H2O molecules a gradual growth of the cross-section for the yield of the fragment ions þ + + + (H+, OH+, O+, Hþ 2 , H2O , CO2 , CO , C etc.) formed by dissociative ionization can be observed with increasing electron energy. This growth depends on the binding type of the parent molecule as well as on the number of atoms in the molecule. For the exact determination of the ionization potentials of the molecules (or atoms) and the appearance energies of the ion fragments formed by dissociative ionization we applied a method
H2O+/H2O OH+/H2O O+/H2O 3
e- + H2O H+/H2O
I, arb.un.
the parent molecules and the threshold of the appearance of the dissociative ionization fragments (child ions).
299
2 3
1
12
H2+/H2O 4
0 10
12
14
16
18
20
22
24
26
E, eV
3. Results and discussion The main regularities of the threshold behaviour of the effective ionization cross-section of
Fig. 1. Threshold regions of the relative cross-section for ionization of water molecule and formation of ion fragments due to dissociative ionization by electron impact. H+ion: 1 Eap = 16.00 eV, 2 Eap = 16.95 eV, 3 Eap = 18.70 eV; O+ion: 4 Eap = 19.00 eV.
300
A.N. Zavilopulo et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 298–301
2
e- + CO2
+
CO+/CO2
C /CO2
+
I, arb.un.
CO2 /CO2 O+/CO2
1
2 1
0 10 12 14 16 18 20 22 24 26 28 30 32
E, eV Fig. 2. Threshold regions of the relative cross-section for ionization for carbon dioxide molecule and formation of ion fragments due to dissociative ionization by electron impact. C+ion: 1 Eap = 22.70 eV, 2 Eap = 27.8 eV.
worked out by Ma¨rk group [9,10]. For water and carbon dioxide molecules we have measured the energy dependencies of the formation of the fragments due to dissociative ionization. Now let us consider the dissociation channels for water: e þ H2 O ! Hþ þ OH þ 2e ðEap ¼ 16.95 eVÞ Hþ þ OH þ e ðEap ¼ 16.00 eVÞ Hþ þ OHðX2 PÞ þ 2e ðEap ¼ 18.70 eVÞ Hþ 2 þ O þ 2e ðE ap ¼ 20.70 eVÞ HOþ þ H þ 2e ðEap ¼ 18.11 eVÞ Oþ þ H2 þ 2e ðEap ¼ 19.00 eVÞ Oþ þ 2H þ 2e ðEap ¼ 26.80 eVÞ ð1Þ +
As can be seen, H ions can be formed in three channels at different energies Eap, O+ ions are + formed in two channels and only Hþ 2 and HO ions are formed at a single threshold energy. The threshold behaviour of the energy dependence of oxygen ion appearance for the dissociative ionization of water molecule is characterized by a very slow (overextended) increase of the cross-section with the ionizing electron energy (see Fig. 1). The results for the carbon dioxide molecule, shown in Fig. 2, are generally similar to those for the water molecule. However, here all the fragment ions are formed at a single threshold energy except
for the case of carbon ions where two fragment formation channels exist (see also the two last processes in (1)): e þ CO2 ! Cþ þ O2 þ 2e ðEap ¼ 22.70 eVÞ ð2Þ Cþ þ 2O þ 2e ðEap ¼ 27.80 eVÞ An important factor affecting the threshold behaviour of the ion fragment formation is the value of the energy of breaking the bond binding the atom or the molecule within the parent molecule. This can be traced especially clearly for carbon dioxide molecule. The maximal value of the bond breaking energy (10.9 eV) for the case of dissociative ionization corresponds to reaction (2) when carbon dioxide decays into C+ ion and molecular oxygen O2. Note that the main constituent of water and carbon dioxide molecules is oxygen. As mentioned above, the mass spectrum of CO2 lacks the peak corresponding to the O2 molecule what is related to the linear structure of this molecule, hence in the process of dissociative ionization O+ ion fragment is formed. From this point of view it would be interesting to compare the threshold regions of the cross-sections for the appearance of this O+ ion in interaction of electrons with molecules of oxygen, water and carbon dioxide. Such comparison is shown in Fig. 3. As one can see, the threshold dependencies of the dissociative ionization cross-section for O+/H2O and O+/CO2 are generally similar and, as mentioned above, this is in accordance with the fact that dynamics of the dissipation of energy depends on the reaction channel. Note that for both molecules the potentials of appearance Eap are very close. Quite different is the threshold behaviour of the dissociative ionization cross-section for O+/O2 (see Fig. 3). Here a sharp growth of the cross-section is observed immediately after the appearance potential. The obtained data on the relative cross-sections of the yield of positive ions due to direct ionization of the parent molecule and the yield of fragments due to the dissociative ionization give qualitative information on the processes under investigation. As already mentioned, in order to obtain absolute cross-sections besides the information on the number of molecules having participated in the collision process, one should also take into account
A.N. Zavilopulo et al. / Nucl. Instr. and Meth. in Phys. Res. B 233 (2005) 298–301
+ O /CO2 (Eap=19.05 eV)
I, arb.un.
30
+ O /O2 (Eap=18.69 eV) + O /H2O (Eap=19.00 eV)
20
10
0 14
16
18
20
22
24
26
28
E, eV Fig. 3. Comparison of the threshold regions of the dissociative ionization of oxygen, water and carbon dioxide molecule resulting in O+ ion fragment.
the angular distribution of the formed ion fragments. The analysis of the collision kinematics in the dissociative ionization processes is rather complicated. Due to the difference in the shares of the obtained kinetic energy, the angular distribution of the child ions can be different for the various reaction channels. This problem is especially urgent for mass spectrometric studies where a variety of factors should be taken into account. Therefore, in order to obtain the absolute ionization crosssections, special experiments are required, similarly to those of [11].
4. Conclusions A monopole mass spectrometer was applied for near-threshold experimental studies of the processes of both direct and dissociative ionization.
301
The high sensitivity of the spectrometer made it possible to carry out the measurements with sufficient accuracy. From the measured threshold dependencies of CO2 and H2O ionization cross-sections the appearance energies for the ion fragments of the parent molecule were determined. The non-monotonous behaviour of the crosssection for the formation of the ion fragments in a broad range of the threshold energies is to a certain degree related to the multichannel character of the process of the dissociative ionization of the molecule.
References [1] A.N. Zavilopulo, A.V. Snegursky, J.E. Kontros, I.O. Zapfel, Pisma Zh. Tekhn. Fiz. 22 (1996) 3. [2] A.V. Snegursky, A.N. Zavilopulo, F.F. Chipev, O.B. Shpenik, Rad. Phys. Chem. 68 (2003) 291. [3] A.N. Zavilopulo, A.V. Snegursky, in: V.I. Lapshin, V.M. Shulaev (Eds.), Vacuum Technologies and Equipment, Kontrast, Kharkiv, 2002, p. 16. [4] A.N. Zavilopulo, A.V. Snegursky, Techn. Phys. Lett. 28 (2002) 913. [5] A.N. Zavilopulo, F.F. Chipev, O.B. Shpenik, in: Program and Abstract EMSÕ2003 Prague, 2003, p. 200. [6] G. Mallard, P.J. Linstrom, NIST Standard Reference Database, Vol. 69, 2000, http://www.webbook.nist.gov. [7] G.H. Wannier, Phys. Rev. 90 (1953) 817. [8] A.N. Zavilopulo, F.F. Chipev, O.B. Shpenik, Techn. Phys. 4 (2005) 402. [9] T. Fiegele, G. Hanel, I. Torres, M. Lezius, T.D. Ma¨rk, J. Phys. B 33 (2000) 4263. [10] G. Hanel, B. Gstir, T. Fiegele, F. Hagelberg, K. Becker, P. Scheier, A. Snegursky, T.D. Ma¨rk, J. Chem. Phys. 116 (2002) 2456. [11] A.I. Zhukov, A.N. Zavilopulo, A.V. Snegursky, O.B. Shpenik, J. Phys. B 23 (1990) 2373S.