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Ionization and electron capture for H+ collisions at low keV energy with a water-vapor target
International Journal of Mass Spectrometry 446 (2019) 116214
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Ionization and electron capture for Hþ collisions at low keV energy with a water-vapor target pez-Patin ~ o b, B.E. Fuentes a, *, F.B. Yousif b, H. Martínez c J.R. Legorreta a, J. Lo noma de M Departamento de Física, Facultad de Ciencias, Universidad Nacional Auto exico, Mexico n en Ciencias, Universidad Auto noma Del Estado de Morelos, Cuernavaca, Morelos, Mexico Instituto de Ciencias, Centro de Investigacio c noma de M Instituto de Ciencias Físicas, Universidad Nacional Auto exico, Cuernavaca, Morelos, Mexico a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 April 2019 Received in revised form 16 August 2019 Accepted 1 September 2019 Available online 9 September 2019
We have investigated the total relative ionization and dissociative electron capture cross section for proton collisions with water-vapor molecules within the energy range of 2.5e10 keV. Time of flight mass spectroscopy was employed in the measurements of the atomic and molecular singly charged particles þ þ þ þþ , H2Oþþ, OHþþ, and the contriHþ, Hþ 2 , O , OH and H2O as well as the doubly charged particles O bution of the different channels was evaluated. Ionization was found to be the dominant reaction channel as expected. The contribution of the singly charged particles was found to be one order of magnitude higher than that of the doubly charged particles. © 2019 Elsevier B.V. All rights reserved.
Keywords: Experimental physics Mass spectrometry Ionization Electron capture Low energy collision physics
1. Introduction Particle interactions between water molecules and photons, metastable atoms, electrons, and ions are of importance not only in Physics and Astronomy [1e4], but also in Biology and Medicine [5,6]. Incident rays can damage living tissue both as a result of direct particle bio-molecule interactions and through processes initiated by secondary species, such as radicals formed by the dissociation of neighboring water molecules. Boudaïffa and co-workers [7], several years ago, demonstrated that a detailed knowledge of the ionization processes is necessary to achieve a full understanding of biological radiation damage on a microscopic scale. On planetary sciences, data received from space probes with respect to Saturn and Mars do require more detailed information on water dissociation in order to find answers regarding the presence of water on these planets and the implications on other regions of the Universe. For example, the oxygen-rich atmosphere of Europa is assumed to be a result from collision-induced dissociation of water [8]. Montenegro et al. [9] have studied the importance of the fragmentation of water molecules by heavy ions in
* Corresponding author. E-mail address: [email protected] (B.E. Fuentes). https://doi.org/10.1016/j.ijms.2019.116214 1387-3806/© 2019 Elsevier B.V. All rights reserved.
environments such as Europa and tumor treatment. Fundamental interest on this collision system continues to remain strong in a bid to properly test the wave functions and the potential energy curves of the water molecule in isolated form and in the liquid phase [10]. Toburen et al. [11] reported, in the energy range from 100 to 2500 keV, the first total cross sections (CSs) for one-electron capture and total one-electron loss for incident protons and neutral hydrogen atoms, respectively, on the water molecule. Rudd et al. [12] extended the data, measuring total cross sections for target positive ion production and one electron capture for incident protons over the energy range 7e4000 keV. Later, Werner et al. [13] employed a time-of-flight (TOF) coincidence and imaging detection techniques to measure individual water dissociation products during ionizing collisions with fast protons over the 100e350 keV incident energy range. In 2001, Gobet et al. [14] carried out dissociative and non-dissociative cross section measurements at lower energies, 20e150 keV, by proton impact using a time-of-flight coincidence technique to separately record the cross sections arising from ionizing and electron capture collisions. As will be discussed later, Gobet et al. [14,15] used large species-dependent correction factors to obtain their cross sections for channels leadþ þ ing to Hþ, Oþ 2 , O , and OH target ion formation.
2
J.R. Legorreta et al. / International Journal of Mass Spectrometry 446 (2019) 116214
1.1. In the present work, the reaction investigated is that of Hþ þ H2O / Hmþ þ H2Onþ þ (m þ n e1)e, for dissociative and non-dissociative ionization as well as dissociative and nondissociative electron capture when both the projectile final charge m is equal to 0 or 1 and the water target productions H2Onþ,with n taking the value of 1 or 2, and in which the fragment ions are Hþ, OHþ, and Oþ, OHþþ, and Oþþ. 1.2. Experimental apparatus and procedure The experimental apparatus is shown in Fig. 1. Ions are generated using a Colutron ion source at 180 mTorr with a mixture of 80/ 100 argon to hydrogen. Ions are accelerated up to 10 keV and are allowed to pass through a Wien velocity filter for ion selection. Water vapor was allowed to pass through a hypodermic needle jet of 0.1 mm diameter, where protons interact with the water vapor jet just below the needle end. A pulsed power supply PVX-4140 DEI (0e3000 V) with pulse duration of 500 ns was used to direct the ions out of the interaction region and into the TOF region. Typical TOF spectra is recorded for protons in collisions with H2O as shown in Fig. 2, where an acceptable resolution is observed showing the contribution from the singly and doubly channels as well as the ionization peak channels and the contribution from the background. Spectra of the TOF fragments were observed as a function of the extraction pulsed voltage. The area under the main peak of H2Oþ was monitored as a function of the pulsed voltage amplitude and the signal was plotted as in Fig. 3. It can be seen that the signal at voltage above 250 V remains the same within the uncertainty of the measurements indicating the total collection of the fragments. The proton beam is collimated after exiting the Wien filter by three collimators. Once the interaction has been defined, it is extremely difficult to have any horizontal or vertical shift in the
Fig. 2. TOF spectra showing the fragment peaks as well as the peak fit showing the contribution of each ion with background contribution removed.
position. The last of these collimators has a diameter of 3 mm compared to the target vapor orifice of 0.1 mm. Therefore, during the experimental measurements, the spatial overlap of the proton beam and vapor target jet was not expected to change as the proton beam of 3 mm in diameter is considerably larger than the gas expansion under the jet orifice at the interaction position just below the orifice of the jet; so, a complete overlap of the proton beam and jet is considered at all energies. Adding to that, the frequency of the extraction pulse was varied and the various peaks of the TOF spectra were observed and maintained at maximum over a sufficient frequency range above the 500 ns pulse duration. Further specific detailed description of the experimental apparatus and procedure are available in Ref. [16].
Fig. 1. Schematics of the experimental apparatus.
J.R. Legorreta et al. / International Journal of Mass Spectrometry 446 (2019) 116214
3
Hþ þ H2 O/H þ H þ þ H þ O þ e /H þ Hþ 2 þOþe /H þ H2 þ Oþ þ e
5 cross section for the following processes involving ionization have been considered
Hþ þ H2 O/Hþ þ H2 Oþ þ e /Hþ þ H þ þ H þ O þ e /Hþ þ H þ H þ Oþ þ e /Hþ þ Hþ 2 þOþe
Fig. 3. Signal intensity as a function of the extracting pulse voltage.
2. Results and analysis Measurements were made for a proton beam of 2.5e10 keV in collisions with H2O molecules; Fig. 2 shows the TOF spectra of water vapor fragments for a proton energy of 5 keV. An important result found is that a maximum signal is obtained by changing the pulse duration of the power supply responsible for ejecting the fragments into the TOF tube between 250 and 600 ns; at 500 ns, a maximum signal with respect to the intensity of all of the peaks of the detected fragment at all collisional energies is maintained. The background spectra were recorded and subtracted from the spectra recorded for the target fragments taking into account the duration of the measurements and the proton beam intensity. The area under the various peaks, after removing the background, were normalized to the same proton current and target pressure for all the collisional energies leading to relative cross sections. The overlapping tail of adjacent peaks, as seen in Fig. 2, was deconvoluted employing PeakFit software (version 4.12). Cross sections were determined for the following processes involving Hþþ H2O collisions: 1 non-dissociative electron capture
However, the experimental apparatus does not distinguish between the origins of the processes leading to each detected ion, therefore measurements are for the total ionization (pure ionization and transfer ionization) and for the total dissociative fragments CSs that includes dissociative ionization and dissociative electron capture CSs. Both, target pressure measurements and proton beam intensity, contribute to the uncertainties. The manufactures error in the pressure gauge and the variations of the gas pressure during each measurement is identified to be of ±5%. However, the target number density within the proton beam cross section would be influenced further by an estimated additional 10% due to target pressure fluctuations. The average proton beam intensity was determined employing a PC interface leading to the evaluation of the average current of protons at the end of each scan. Nevertheless, statistical uncertainties were about 20% of the lower energy and falling down to 10% at a higher collisional energy. An additional uncertainty of 10% due to the normalization procedure was introduced. Our measured relative cross sections were normalized to the total cross section of Rudd et al. [12] at 5 keV following the procedure outlined in Ref. [16]. The measured absolute total ionization and dissociative electron capture cross sections for Hþ on H2O are shown in Fig. 4, for the dissociative electron capture, and in general, the removal of the electron from the H2O, leading to the breakdown of the resulting H2Oþ which ends up mainly with the Hþ, Oþ, OHþ. Nevertheless, the doubly charged H2Oþþ ions and the OHþþ and Oþþ fragments are detected, at considerably smaller cross sections. Our present data show that the H2Oþchannel is the dominant channel and it can
Hþ þ H2 O/H2 Oþ þ H 2 dissociative electron capture
Hþ þ H2 O/H þ þ H þ O þ H /Hþ 2 þOþH /H þ H þ Oþ þ H
3 non dissociative transfer ionization
Hþ þ H2 O/H þ H2 Oþþ þ e /H þ OH þþ þ H þ e /H þ Oþþ þ H2 þ e 4 dissociative transfer ionization
Fig. 4. Total absolute ionization and dissociative electron capture cross sections for ionic fragments for H þ þ H2 Ocollisions.
4
J.R. Legorreta et al. / International Journal of Mass Spectrometry 446 (2019) 116214
be seen to be increasing slowly as the collisional energy is increased to a maximum at 5 keV. Following that, a decreasing behavior is observed. On the other hand, it can be seen that the production of Hþ in the electron capture dissociative ionization is about a factor of 5 lower than that for the H2Oþ. Fig. 4 also shows that the cross sections of the doubly charged ions Oþþ, OHþþ and H2Oþþ are about one order of magnitude lower than those of the singly charged ions. Similar observations were made by Ref. [17] at considerably higher collisional energies. In Fig. 5, the absolute cross sections for H2Oþ, Hþ, Oþ, OHþ are plotted together with the previously measured experimental data from several groups for the electron capture, ionization and dissociative ionization of Hþ colliding on H2O. Among those is the work of Werner et al. [12], who measured the multiple ionization and fragmentation of H2O in coincidence measurements, in the energy range of 100e350 keV; as well as Luna et al. [18], in which they measured the time-of-flight-based mass analysis of charged fragments at 15e100 keV and 500e3500 keV. Also included are the results of Gobet et al. [14], who measured the electron capture, ionization and dissociative ionization in coincidence measurements between 20 and 200 keV. The data of Luna et al. [18], Werner et al. [13] and Gobet et al. [14] seem to connect and overlap well in the case of H2Oþ at all energies higher than 100 keV; equally in the case of OHþ and Oþ. Nevertheless, there are differences in the case of Hþ, where the data of Luna et al. [18] and Gobet et al. [14] diverge at energies lower than 100 keV. The data of Luna et al. [18] for Hþ/H2Oþ is 0.85 at 15 keV, while it is 0.25 for Gobet et al. at 20 keV. Also included in Fig. 5 are the theoretical data of [19] for the absolute cross sections for H2Oþ, OHþ, Oþ, and Hþ within the energy range of 30e4000 keV, for comparison. The present measurements for the total absolute cross section for the sum of ionization and fragmentation are shown in Fig. 6, together with the data of Rudd et al. [12]. It can be seen that our present data are the only available data in the lower energy range below 10 keV. The data of Rudd et al. [12] and Werner et al. [13] seem to merge at the higher energy range, however deviate at energies lower than 200 keV. On the other hand, the data of Luna et al. [18] and Rudd et al. [12] agree within the energy range of 15e80 keV and merge reasonably well with our present results at about 1019 m2 as seen in Fig. 6 for the total cross section at 10 keV and below. Also shown are the data of Lindsay et al. [20] at 0.5, 1.5 and 5 keV for the absolute integral charge-transfer cross sections.
Fig. 6. Present and previously published data for the total and for the sum of ionization and fragmentation for Hþ on H2O.
Also included in Fig. 5 are the calculated data of Kimura et al. [21] for comparison. It can be seen that our present data show an increasing trend between 2.5 to 4 keV that is followed by a gradual decrease as the energy increases to 10 keV. The data of Toburen et al. [11] above 100 keV are significantly below the data of [12,13and22]. However, at higher collisional energies, the contribution from the electron capture cross section becomes less important, and as a result, it can be observed that there is a reasonable agreement between the results of [12,14and18] with the exception of those of [11], which are several orders of magnitude below those of [12,14and18]. The data of Dagnac et al. [23] which were obtained for Dþ in collisions with H2O are plotted at the equivalent proton energy, i.e., one half of the deuteron energy between 1.5 and 5 keV, and are in reasonable agreement with those of Lindsay et al. [20] as well as with our present data. The differences between the data in Refs. [11,12] could be attributed to the less accurate target pressure measuring equipment in 1968.
3. Conclusions
Fig. 5. Comparison of our present data for Hþ, Oþ, OHþ and H2Oþ with data of Gulyas et al. [19], Gobet et al. [14], Werner et al. [13] and Luna et al. [18].
Cross sections for ionization leading to H2Oþ, and the fragments of OHþ and Oþ are reported here as a function of the collisional energy for Hþ on H2O within the energy range of 2.5e10 keV as well as for the doubly charged fragments of H2Oþþ, OHþþ and Oþþ. The doubly charged fragments cross section was found to be one order of magnitude smaller than those of the singly charged fragments. The Hþ fragments show a cross section that is one order of magnitude smaller than those of the remaining singly charged fragments. Our present data for dissociative ionization leading to target fragments within our collisional energy seem to connect reasonably well with previously measured data at energies higher than 10 keV. Equally, our present total cross section data agree well enough with the few points measured previously by other groups within our present projectile energy and are in agreement with the theoretical available data. The Hþ/H2Oþ ratio measured in this work is 0.3 at 10 keV, which is close to the 0.25 measured by Gobet et al. [14] at 20 keV where both seem to converge, while the same ratio is about 0.8 measured by Luna et al. [18].
J.R. Legorreta et al. / International Journal of Mass Spectrometry 446 (2019) 116214
Acknowledgment We appreciate the participation and technical assistance of Dalila Martinez in this work. J. R. Legorreta thanks the postdoctoral scholarship granted by DGAPA-UNAM. This work was partially supported by DGAPA- PAPIIT- IN 115718. References [1] E.W. McDaniel, J.B.A. Mitchell, M.E. Rudd, Atomic Collisions, Wiley, New York, 1993. [2] C.S. Enos, A.R. Lee, A.G. Brenton, Electronic excitation of atmospheric molecules by proton impact, Int. J. Mass Spectrom. Ion Process. 104 (2) (1991) 137. [3] D.H. Katayama, R.E. Huffman, C.L. O’Bryan, Absorption and photoionization cross sections for H2O and D2O in the vacuum ultraviolet, J. Chem. Phys. 59 (8) (1973) 4309. [4] K. Mitsuke, T. Takami, K. Ohno, Kinetic energy dependence of partial cross sections for the collisional ionization of H2O, H2S, O2, and Ar with He(2 3S) metastable atoms, J. Chem. Phys. 91 (3) (1989) 1618. [5] G.H. Olivera, C. Caraby, P. Jardin, A. Cassimi, L. Adoui, B. Gervais, Multiple ionization in the earlier stages of water radiolysis, Phys. Med. Biol. 43 (8) (1998) 2347. [6] M. Dingfelder, M. Inokuti, H.G. Paretzke, Inelastic-collision cross sections of liquid water for interactions of energetic protons, Radiad. Phys. Chem. 59 (3) (2000) 255. [7] B. Boudaïffa, P. Cloutier, D. Hunting, M.A. Huels, L. Sanche, Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons, Science 287 (2000) 1658. [8] D.T. Hall, D.F. Strobel, P.D. Feldman, M.A. McGrath, H.A. Weaver, Detection of an oxygen atmosphere on Jupiter’s moon Europa, Nature 373 (1995) 677. [9] E.C. Montenegro, H. Luna, Primary radicals production from water fragmentation by heavy ions, Braz. J. Phys. 35 (4a) (2005) 927. [10] B.C. Garrett, D.A. Dixon, D.M. Camaioni, D.M. Chipman, et al., Role of water in electron-initiated processes and radical chemistry: issues and scientific advances, Chem. Rev. 105 (1) (2005) 355. [11] L.H. Toburen, M.Y. Nakai, R.A. Langley, Measurement of high-energy charge-
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5
transfer cross sections for incident protons and atomic hydrogen in various gases, Phys. Rev. 171 (1) (1968) 114. M.E. Rudd, T.V. Goffe, R.D. DuBois, L.H. Toburen, Cross sections for ionization of water vapor by 7-4000-keV protons, Phys. Rev. A 31 (1) (1985) 492. U. Werner, K. Beckord, J. Becker, H.O. Lutz, 3D imaging of the collision-induced coulomb fragmentation of water molecules, Phys. Rev. Lett. 74 (11) (1995) 1962. , M. Lezius, P. Scheier, F. Gobet, B. Farizon, M. Farizon, M.J. Gaillard, M. Carre T.D. M€ ark, Total, partial, and electron-capture cross sections for ionization of water vapor by 20e150 keV protons, Phys. Rev. Lett. 86 (17) (2001) 3751. F. Gobet, S. Eden, B. Coupier, J. Tabet, B. Farizon, M. Farizon, M.J. Gaillard, , S. Ouaskit, T.D. M€ M. Carre ark, P. Scheier, Ionization of water by (20e150)keV protons: separation of direct-ionization and electron-capture processes, Phys. Rev. A 70 (6) (2004), 062716. pez-Patin ~ o, B.E. Fuentes, F.B. Yousif, H. Martínez, Relative differential cross J. Lo section measurements in Hþ - CO2 low energy collisions, Int. J.Phys. Astron. 3 (2) (2015) 23. D. Oubaziz, R. Boulifa, Z. Aitelhadjali, C. Champion, Double ionization of water molecules by proton impact: the role of the direct ionization mechanism, Eur. Phys. J. D. 72 (6) (2018) 119. H. Luna, A.L.F. de Barros, J.A. Wyer, S.W.J. Scully, J. Lecointre, P.M.Y. Garcia, G.M. Sigaud, A.C.F. Santos, V. Senthil, M.B. Shah, C.J. Latimer, E.C. Montenegro, Water-molecule dissociation by proton and hydrogen impact, Phys. Rev. A 75 (5) (2007), 042711. L. Guly as, S. Egri, H. Ghavaminia, A. Igarashi, Single and multiple electron removal and fragmentation in collisions of protons with water molecules, Phys. Rev. A 93 (3) (2016), 032704. B.G. Lindsay, D.R. Sieglaff, K.A. Smith, R.F. Stebbings, Charge transfer of 0.5-, 1.5-, and 5-keV protons with H2O: absolute differential and integral cross sections, Phys. Rev. A 55 (5) (1997) 3945. M. Kimura, L. Pichl, Y. Li, H.-P. Liebermann, R.J. Buenker, I.F. Schneider, Steric effect in Oþ/H2 and Hþ/H2O collisions. Charge transfer in H2Oþ and H3Oþ collision intermediate systems, Eur. Phys. J. D 38 (2006) 85. H. Luna, E.C. Montenegro, Fragmentation of water by heavy ions, Phys. Rev. Lett. 94 (2005) 43201. R. Dagnac, D. Blanc, D. Molina, A study on the collision of hydrogen ions Hþ 1, þ Hþ 2 and H3 with a water-vapour target, J. Phys. B At. Mol. Phys. 3 (1970) 1239.
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Ionization and electron capture for Hþ collisions at low keV energy with a water-vapor target pez-Patin ~ o b, B.E. Fuentes a, *, F.B. Yousif b, H. Martínez c J.R. Legorreta a, J. Lo noma de M Departamento de Física, Facultad de Ciencias, Universidad Nacional Auto exico, Mexico n en Ciencias, Universidad Auto noma Del Estado de Morelos, Cuernavaca, Morelos, Mexico Instituto de Ciencias, Centro de Investigacio c noma de M Instituto de Ciencias Físicas, Universidad Nacional Auto exico, Cuernavaca, Morelos, Mexico a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 April 2019 Received in revised form 16 August 2019 Accepted 1 September 2019 Available online 9 September 2019
We have investigated the total relative ionization and dissociative electron capture cross section for proton collisions with water-vapor molecules within the energy range of 2.5e10 keV. Time of flight mass spectroscopy was employed in the measurements of the atomic and molecular singly charged particles þ þ þ þþ , H2Oþþ, OHþþ, and the contriHþ, Hþ 2 , O , OH and H2O as well as the doubly charged particles O bution of the different channels was evaluated. Ionization was found to be the dominant reaction channel as expected. The contribution of the singly charged particles was found to be one order of magnitude higher than that of the doubly charged particles. © 2019 Elsevier B.V. All rights reserved.
Keywords: Experimental physics Mass spectrometry Ionization Electron capture Low energy collision physics
1. Introduction Particle interactions between water molecules and photons, metastable atoms, electrons, and ions are of importance not only in Physics and Astronomy [1e4], but also in Biology and Medicine [5,6]. Incident rays can damage living tissue both as a result of direct particle bio-molecule interactions and through processes initiated by secondary species, such as radicals formed by the dissociation of neighboring water molecules. Boudaïffa and co-workers [7], several years ago, demonstrated that a detailed knowledge of the ionization processes is necessary to achieve a full understanding of biological radiation damage on a microscopic scale. On planetary sciences, data received from space probes with respect to Saturn and Mars do require more detailed information on water dissociation in order to find answers regarding the presence of water on these planets and the implications on other regions of the Universe. For example, the oxygen-rich atmosphere of Europa is assumed to be a result from collision-induced dissociation of water [8]. Montenegro et al. [9] have studied the importance of the fragmentation of water molecules by heavy ions in
* Corresponding author. E-mail address: [email protected] (B.E. Fuentes). https://doi.org/10.1016/j.ijms.2019.116214 1387-3806/© 2019 Elsevier B.V. All rights reserved.
environments such as Europa and tumor treatment. Fundamental interest on this collision system continues to remain strong in a bid to properly test the wave functions and the potential energy curves of the water molecule in isolated form and in the liquid phase [10]. Toburen et al. [11] reported, in the energy range from 100 to 2500 keV, the first total cross sections (CSs) for one-electron capture and total one-electron loss for incident protons and neutral hydrogen atoms, respectively, on the water molecule. Rudd et al. [12] extended the data, measuring total cross sections for target positive ion production and one electron capture for incident protons over the energy range 7e4000 keV. Later, Werner et al. [13] employed a time-of-flight (TOF) coincidence and imaging detection techniques to measure individual water dissociation products during ionizing collisions with fast protons over the 100e350 keV incident energy range. In 2001, Gobet et al. [14] carried out dissociative and non-dissociative cross section measurements at lower energies, 20e150 keV, by proton impact using a time-of-flight coincidence technique to separately record the cross sections arising from ionizing and electron capture collisions. As will be discussed later, Gobet et al. [14,15] used large species-dependent correction factors to obtain their cross sections for channels leadþ þ ing to Hþ, Oþ 2 , O , and OH target ion formation.
2
J.R. Legorreta et al. / International Journal of Mass Spectrometry 446 (2019) 116214
1.1. In the present work, the reaction investigated is that of Hþ þ H2O / Hmþ þ H2Onþ þ (m þ n e1)e, for dissociative and non-dissociative ionization as well as dissociative and nondissociative electron capture when both the projectile final charge m is equal to 0 or 1 and the water target productions H2Onþ,with n taking the value of 1 or 2, and in which the fragment ions are Hþ, OHþ, and Oþ, OHþþ, and Oþþ. 1.2. Experimental apparatus and procedure The experimental apparatus is shown in Fig. 1. Ions are generated using a Colutron ion source at 180 mTorr with a mixture of 80/ 100 argon to hydrogen. Ions are accelerated up to 10 keV and are allowed to pass through a Wien velocity filter for ion selection. Water vapor was allowed to pass through a hypodermic needle jet of 0.1 mm diameter, where protons interact with the water vapor jet just below the needle end. A pulsed power supply PVX-4140 DEI (0e3000 V) with pulse duration of 500 ns was used to direct the ions out of the interaction region and into the TOF region. Typical TOF spectra is recorded for protons in collisions with H2O as shown in Fig. 2, where an acceptable resolution is observed showing the contribution from the singly and doubly channels as well as the ionization peak channels and the contribution from the background. Spectra of the TOF fragments were observed as a function of the extraction pulsed voltage. The area under the main peak of H2Oþ was monitored as a function of the pulsed voltage amplitude and the signal was plotted as in Fig. 3. It can be seen that the signal at voltage above 250 V remains the same within the uncertainty of the measurements indicating the total collection of the fragments. The proton beam is collimated after exiting the Wien filter by three collimators. Once the interaction has been defined, it is extremely difficult to have any horizontal or vertical shift in the
Fig. 2. TOF spectra showing the fragment peaks as well as the peak fit showing the contribution of each ion with background contribution removed.
position. The last of these collimators has a diameter of 3 mm compared to the target vapor orifice of 0.1 mm. Therefore, during the experimental measurements, the spatial overlap of the proton beam and vapor target jet was not expected to change as the proton beam of 3 mm in diameter is considerably larger than the gas expansion under the jet orifice at the interaction position just below the orifice of the jet; so, a complete overlap of the proton beam and jet is considered at all energies. Adding to that, the frequency of the extraction pulse was varied and the various peaks of the TOF spectra were observed and maintained at maximum over a sufficient frequency range above the 500 ns pulse duration. Further specific detailed description of the experimental apparatus and procedure are available in Ref. [16].
Fig. 1. Schematics of the experimental apparatus.
J.R. Legorreta et al. / International Journal of Mass Spectrometry 446 (2019) 116214
3
Hþ þ H2 O/H þ H þ þ H þ O þ e /H þ Hþ 2 þOþe /H þ H2 þ Oþ þ e
5 cross section for the following processes involving ionization have been considered
Hþ þ H2 O/Hþ þ H2 Oþ þ e /Hþ þ H þ þ H þ O þ e /Hþ þ H þ H þ Oþ þ e /Hþ þ Hþ 2 þOþe
Fig. 3. Signal intensity as a function of the extracting pulse voltage.
2. Results and analysis Measurements were made for a proton beam of 2.5e10 keV in collisions with H2O molecules; Fig. 2 shows the TOF spectra of water vapor fragments for a proton energy of 5 keV. An important result found is that a maximum signal is obtained by changing the pulse duration of the power supply responsible for ejecting the fragments into the TOF tube between 250 and 600 ns; at 500 ns, a maximum signal with respect to the intensity of all of the peaks of the detected fragment at all collisional energies is maintained. The background spectra were recorded and subtracted from the spectra recorded for the target fragments taking into account the duration of the measurements and the proton beam intensity. The area under the various peaks, after removing the background, were normalized to the same proton current and target pressure for all the collisional energies leading to relative cross sections. The overlapping tail of adjacent peaks, as seen in Fig. 2, was deconvoluted employing PeakFit software (version 4.12). Cross sections were determined for the following processes involving Hþþ H2O collisions: 1 non-dissociative electron capture
However, the experimental apparatus does not distinguish between the origins of the processes leading to each detected ion, therefore measurements are for the total ionization (pure ionization and transfer ionization) and for the total dissociative fragments CSs that includes dissociative ionization and dissociative electron capture CSs. Both, target pressure measurements and proton beam intensity, contribute to the uncertainties. The manufactures error in the pressure gauge and the variations of the gas pressure during each measurement is identified to be of ±5%. However, the target number density within the proton beam cross section would be influenced further by an estimated additional 10% due to target pressure fluctuations. The average proton beam intensity was determined employing a PC interface leading to the evaluation of the average current of protons at the end of each scan. Nevertheless, statistical uncertainties were about 20% of the lower energy and falling down to 10% at a higher collisional energy. An additional uncertainty of 10% due to the normalization procedure was introduced. Our measured relative cross sections were normalized to the total cross section of Rudd et al. [12] at 5 keV following the procedure outlined in Ref. [16]. The measured absolute total ionization and dissociative electron capture cross sections for Hþ on H2O are shown in Fig. 4, for the dissociative electron capture, and in general, the removal of the electron from the H2O, leading to the breakdown of the resulting H2Oþ which ends up mainly with the Hþ, Oþ, OHþ. Nevertheless, the doubly charged H2Oþþ ions and the OHþþ and Oþþ fragments are detected, at considerably smaller cross sections. Our present data show that the H2Oþchannel is the dominant channel and it can
Hþ þ H2 O/H2 Oþ þ H 2 dissociative electron capture
Hþ þ H2 O/H þ þ H þ O þ H /Hþ 2 þOþH /H þ H þ Oþ þ H
3 non dissociative transfer ionization
Hþ þ H2 O/H þ H2 Oþþ þ e /H þ OH þþ þ H þ e /H þ Oþþ þ H2 þ e 4 dissociative transfer ionization
Fig. 4. Total absolute ionization and dissociative electron capture cross sections for ionic fragments for H þ þ H2 Ocollisions.
4
J.R. Legorreta et al. / International Journal of Mass Spectrometry 446 (2019) 116214
be seen to be increasing slowly as the collisional energy is increased to a maximum at 5 keV. Following that, a decreasing behavior is observed. On the other hand, it can be seen that the production of Hþ in the electron capture dissociative ionization is about a factor of 5 lower than that for the H2Oþ. Fig. 4 also shows that the cross sections of the doubly charged ions Oþþ, OHþþ and H2Oþþ are about one order of magnitude lower than those of the singly charged ions. Similar observations were made by Ref. [17] at considerably higher collisional energies. In Fig. 5, the absolute cross sections for H2Oþ, Hþ, Oþ, OHþ are plotted together with the previously measured experimental data from several groups for the electron capture, ionization and dissociative ionization of Hþ colliding on H2O. Among those is the work of Werner et al. [12], who measured the multiple ionization and fragmentation of H2O in coincidence measurements, in the energy range of 100e350 keV; as well as Luna et al. [18], in which they measured the time-of-flight-based mass analysis of charged fragments at 15e100 keV and 500e3500 keV. Also included are the results of Gobet et al. [14], who measured the electron capture, ionization and dissociative ionization in coincidence measurements between 20 and 200 keV. The data of Luna et al. [18], Werner et al. [13] and Gobet et al. [14] seem to connect and overlap well in the case of H2Oþ at all energies higher than 100 keV; equally in the case of OHþ and Oþ. Nevertheless, there are differences in the case of Hþ, where the data of Luna et al. [18] and Gobet et al. [14] diverge at energies lower than 100 keV. The data of Luna et al. [18] for Hþ/H2Oþ is 0.85 at 15 keV, while it is 0.25 for Gobet et al. at 20 keV. Also included in Fig. 5 are the theoretical data of [19] for the absolute cross sections for H2Oþ, OHþ, Oþ, and Hþ within the energy range of 30e4000 keV, for comparison. The present measurements for the total absolute cross section for the sum of ionization and fragmentation are shown in Fig. 6, together with the data of Rudd et al. [12]. It can be seen that our present data are the only available data in the lower energy range below 10 keV. The data of Rudd et al. [12] and Werner et al. [13] seem to merge at the higher energy range, however deviate at energies lower than 200 keV. On the other hand, the data of Luna et al. [18] and Rudd et al. [12] agree within the energy range of 15e80 keV and merge reasonably well with our present results at about 1019 m2 as seen in Fig. 6 for the total cross section at 10 keV and below. Also shown are the data of Lindsay et al. [20] at 0.5, 1.5 and 5 keV for the absolute integral charge-transfer cross sections.
Fig. 6. Present and previously published data for the total and for the sum of ionization and fragmentation for Hþ on H2O.
Also included in Fig. 5 are the calculated data of Kimura et al. [21] for comparison. It can be seen that our present data show an increasing trend between 2.5 to 4 keV that is followed by a gradual decrease as the energy increases to 10 keV. The data of Toburen et al. [11] above 100 keV are significantly below the data of [12,13and22]. However, at higher collisional energies, the contribution from the electron capture cross section becomes less important, and as a result, it can be observed that there is a reasonable agreement between the results of [12,14and18] with the exception of those of [11], which are several orders of magnitude below those of [12,14and18]. The data of Dagnac et al. [23] which were obtained for Dþ in collisions with H2O are plotted at the equivalent proton energy, i.e., one half of the deuteron energy between 1.5 and 5 keV, and are in reasonable agreement with those of Lindsay et al. [20] as well as with our present data. The differences between the data in Refs. [11,12] could be attributed to the less accurate target pressure measuring equipment in 1968.
3. Conclusions
Fig. 5. Comparison of our present data for Hþ, Oþ, OHþ and H2Oþ with data of Gulyas et al. [19], Gobet et al. [14], Werner et al. [13] and Luna et al. [18].
Cross sections for ionization leading to H2Oþ, and the fragments of OHþ and Oþ are reported here as a function of the collisional energy for Hþ on H2O within the energy range of 2.5e10 keV as well as for the doubly charged fragments of H2Oþþ, OHþþ and Oþþ. The doubly charged fragments cross section was found to be one order of magnitude smaller than those of the singly charged fragments. The Hþ fragments show a cross section that is one order of magnitude smaller than those of the remaining singly charged fragments. Our present data for dissociative ionization leading to target fragments within our collisional energy seem to connect reasonably well with previously measured data at energies higher than 10 keV. Equally, our present total cross section data agree well enough with the few points measured previously by other groups within our present projectile energy and are in agreement with the theoretical available data. The Hþ/H2Oþ ratio measured in this work is 0.3 at 10 keV, which is close to the 0.25 measured by Gobet et al. [14] at 20 keV where both seem to converge, while the same ratio is about 0.8 measured by Luna et al. [18].
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