Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 90 (2017) 391 – 398
Conference on the Application of Accelerators in Research and Industry, CAARI 2016, 30 October – 4 November 2016, Ft. Worth, TX, USA
Ionization and electron capture for ܪା collisions with ܱܥat low keV energy J. López-Patiñoa, B. E. Fuentesa*, F. B. Yousifb and H. Martínezc a
b
Facultad de Ciencias, Universidad Nacional Autónoma de México, Ciudad de México, México. Instituto de Ciencias, Centro de Investigación en Ciencias, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos, México. c Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, México.
Abstract We have investigated the total relative ionization and dissociative electron capture cross section for proton collisions with ܱܥ within the energy range of 2 keV to 10 keV. Time of flight mass spectroscopy was employed in the measurements ofܱܥା ,ܱ ା and ܥା and the contribution of the different channels was evaluated. Ionization was found to be the dominant reaction channel as expected, while the ܥା ionic fragments intensity was found to be higher than those ofܱ ା . © 2017 2017Published The Authors. Published Elsevier B.V.access article under the CC BY-NC-ND license © by Elsevier B.V.by This is an open Peer-review under responsibility of the Scientific Committee of the Conference on the Application of Accelerators in Research (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of the Conference on the Application of Accelerators in Research and Industry and Industry. Keywords: Experimental Physics, spectroscopy, ionization, Electron capture, low energy collision physics, Martian atmosphere.
1. Introduction Protons among other ions are the primary constituents of the solar wind and are important in diagnosing photon emission from distant objects. In order to model this emission from bodies exposed to the solar wind (e.g., comets and planetary upper atmospheres), relative, absolute and differential cross sections of fragment ions are needed for each incident solar-wind ion and charge state. Since ܱܥis one of the major constituents of the comet’s neutral atmosphere near the Sun, has a large ionization potential (14.0139 eV), and the solar photoionization rates are low, charge exchange in collisions of solar-wind ܪା ions with ܱܥis the main mechanism for the production ofܱܥା .
* Corresponding author. Tel.: +525556224845. E-mail address:
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1875-3892 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of the Conference on the Application of Accelerators in Research and Industry doi:10.1016/j.phpro.2017.10.001
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Due to its fundamental interest, the data are important for our understanding of the interaction of the solar wind with comet type atmospheres in order to have a clear image of the charge exchange observed in the interaction of protons with gases in comets. Charge-exchange cross sections have been reported previously and mostly for total electron capture. Nevertheless, when data are needed for a specific application, there is the need for new measurements when: either the energy range has not been covered, discrepancy among researchers, or the need for fragments channels and ratios. High energy measurements have been reported for the total cross section. For example, Rudd et al [1] at 5 keV to 150 keV, McNeal [2] for 1 keV to 25 keV and Kusakabe et al [3] for 0.2 keV to 4.2 keV. Kimura et al [4] reported the results of an ab initio calculation of the ܪା ܱܥreaction. In particular, the theory shows a slow broad plateau in the cross section within the energy range of 3 keV to 10 keV. Cadez et al [5] reported total proton–electron capture cross sections at energies between 2.5 keV and 7 keV onܱܥ, Lindsay et al [6] reported total electron capture for 1 keV to 5 keV for ܪା ܱܥ, and Gao et al [7] for 1.5 keV for ܪା ܱܥ. Berkner et al [8] measured the total cross section between 0.15 keV to 10 keV. We know of several experimental investigations for ionization and electron capture leading to ܱܥା , ܥା , and ܱା : the first is that of Browning and Gilbody [9] for 5 keV and 45 keV, the second is that of Shah and Gilbody [10] for 10 keV to 98 keV and the third is that of Afrosimov et al [11] for 5 keV to 50 keV. A similar investigation was undertaken by Knudsen et al [12], however, for the higher energy of 50 keV to 6000 keV. The present work illustrates detailed measurements of the relative dissociative electron capture for the ܪା ܱܥ collisions within the energy range of 2 keV to 10 keV. We have investigated previously the ionization and dissociative electron capture and ionization of ܪܥସ [15] and ܱܥଶ [14] by proton impact employing a pulsed interaction region technique and time of flight mass spectroscopy. In the present work, we use the same method to investigate the low keV energy ionization and dissociative electron capture of carbon monoxide by proton impact.
2. Experimental apparatus and procedure In our previous work [13-15] we have given details of the experimental apparatus as well as the experimental procedure. Both projectile ions and resulting neutrals as well as the pulsed and accelerated target ions and fragments are then detected using time of flight mass spectroscopy. The proton beam was constantly monitored and recorded. The mass scale of the spectrum is calibrated and the efficiency of detection was evaluated. Background signal was taken into account in evaluating the absolute cross sections after normalization to previously measured total cross sections. 3. Results and analysis A typical time of flight (TOF) spectra for protons in collisions with ܱܥis shown in figure 1, and it illustrates a satisfactory level of resolution as well as channels ܱܥା , ܥା , and ܱ ା . The observed TOF spectra include the contribution from the ܱܥionization and dissociation and the contribution from the background. The presence of ܰଶ and ܪଶ ܱ in the vacuum chamber contribute to the background signal. The background spectra is recorded and subtracted from the spectra of the ܱܥgas. The small contribution of ܰଶା and any signal from the presence of ܪଶ ܱ was found to be below the detection limit. The ܰଶା contribution to the total signal was taken into account in evaluating the relative total cross sections of the fragment ions. To obtain the relative cross sections, first the background was removed from all the peak areas; then, for all the collisional energies, a normalization was made to the same proton current and target gas pressure.
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2000 +
Signal (a.u.)
CO
1000
C
+ +
O
0 0
10
20
30
40
50
60
70
TOF (Ps)
Figure 1. TOF spectra showing the fragments with background contribution removed.
The contribution from the tail of the overlapping peaks of ܥା and ܱା as seen in figure 1, was de-convoluted employing a peak fitting software. Cross sections (CSs) have been determined for the following processes involving ܪା െ ܱܥcollisions and mainly leading to ionization of ܱܥand fragments of ܥା and ܱା ions: x
non-dissociative electron capture ܪା ܱܥ՜ ܱܥା ܪ
x
dissociative electron capture ܪା ܱܥ՜ ܥା ܱ ܪ ܪା ܱܥ՜ ܥ ܱ ା ܪ
x
dissociative transfer ionization ܪା ܱܥ՜ ܪ ܥା ܱ ା ݁
x
cross section for the following processes involving ionization have been considered ܪା ܱܥ՜ ܪା ܱܥା ݁ ; ܪା ܱܥ՜ ܪା ܥା ܱ ݁ ; ܪା ܱܥ՜ ܪା ܥ ܱା ݁
Other ions were not detected within our energy range.
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The errors considered in our experiment can be found in references [13-15] since the same apparatus was used. Mainly they are due to statistical uncertainties, to average pressure measurements, to the proton beam intensity and variations of the gas pressure. Adding to that, an estimated, error of 10 % due to the normalization procedure was introduced. As the experimental apparatus does not distinguish between the origins of the processes leading to the same detected ion, the measurements are for the total relative ionization (pure ionization and transfer ionization) and for the total relative dissociative fragments CSs, that includes dissociative ionization and dissociative electron capture CSs. Our measured relative cross sections were normalized to the total cross section of Browning and Gilbody [9] at 10 keV according to the following procedure: Since very few of the incident protons produce ions under single collision conditions, the total cross section is given by ߪሺܺሻ ൌ ܰ ሺܺሻΤܰ , where ܰ ሺܺሻ is the number of ܺ ions produced by a number ܰ of protons passing a distance ݈ through a uniform gas target of number density݊. Using the cross section of [9] at 10 keV, we obtain the ݈݊ value, so for a particular ionic species the cross section can be determined. This is done using the area under each peak for any particular ionic species. However, at the lower impact energies considered in the present work, a significant yield of secondary ions may arise through collisions involving both charge transfer and ionization [9]. Although it is not possible to make a precise assessment of the relative importance of these two processes, a comparison of the magnitude and energy dependence of our measured cross sections for the formation of these ions with well-established total charge transfer data [16,17], shows that charge transfer becomes dominant at the lowest energies considered in the present measurements. Figure 2 shows our measured absolute total ionization and dissociative electron capture cross section for ܪା ܱܥ. It can be seen that the ܱܥା channel is the dominant channel and the cross section is hardly changing within our energy range. A similar trend has been observed by Browning and Gilbody [9] and Shah and Gilbody [10]. Next in size are the cross sections for ܥା and ܱା . At energies below 5 keV, both are close in value and are about the same at 2 keV. It is well known that the production of atomic ions of ܥା and ܱା at low collisional energy, mainly comes from the dissociation of ܱܥା with the probability of ܥା ܱ higher than that of ܥ ܱ ା [18]. The excitation of the ܱܥା to a repulsive state leading to ܥା ܱ and ܥ ܱ ା requires the expenditure of energy. Therefore, in transitions that obey the Frank-Condon principle it is expected that the relative probability of the ܱܥା dissociation will be reduced as the collisional energy decreases. The ionization potential of the oxygen atom exceeds that of carbon atom by 2.36 eV, the discrete vibrational spectrum of ሺ ܥ ܱା ሻ bound state must overlap the continuous spectrum of ሺ ܥା ܱሻ [18]. This permits the predissociation processes that lead to the dominances of ܥା production at least at energies higher than 4 keV, as seen in figure 3 with the ratio of ܥା Τܱା decreasing from 1.6 to 1.2 as the energy decreases down to 3 keV taking into account the statistical uncertainty in the data.
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-16
2
Absolute cross section (10 cm )
10
+
C + O + CO Total
1
0.1
1
2
3
4
5
6
7
8
9
10
11
Energy (keV)
Figure 2. Total absolute ionization and dissociative electron capture cross sections for ionic fragments for ܪା ܱܥcollisions.
As seen in figure 2, our total absolute cross section for the sum of ionization and fragmentation shows a minor increase as the energy decreases from 10 keV to 2 keV. However, ܥା and ܱା data of Browning and Gilbody [9] show a flat region as the energy decreases down to 10 keV and falls down as the energy decreases to 5 keV. Equally Afrosimov et al [11] for 5 keV to 50 keV show a decrease in the cross section of ܥା and ܱ ା as the energy decreases below 10 keV. Nevertheless, our present data for ܥା and ܱା show a modest increase as the energy is decreased reaching a maximum at around 3 keV to 4 keV. Below that ܥା and ܱା show a decreasing trend. We have plotted our present absolute total cross section data in figure 3 with those of previously measured data for comparison. It can be seen that our present data fit very well and connect with data at higher energies. Our present data agree well with those of McNeal [2], Kusakabe et al [3], Cadez et al [5], and Lindsay et al [6] and connect with those of Afrosimov et al [11]. However, the present data as seen in figure 3, show some cross section fluctuation in a value between ͳʹ ൈ ͳͲିଵ cm2 and ͳͷ ൈ ͳͲିଵ cm2 which cannot be attributed totally to statistical errors. Similar resonances were observed in our previous investigations on ܰଶ െ ܱଶ [16],ܱܥଶ [17] and ܪܥସ [18]. Lindsay et al [6] observed such resonances for 1 keV ܪା on ܱܥ. Similar resonances were attributed to a combination of various interference effects by Gao et al [7]. Also differential cross section data calculations employing the molecular orbital (MO) approach at 1.5 keV ܪା by Kimura et al [4] on ܱܥshow strong resonances below 1 degree laboratory angle. Kimura et al [4] indicate that these resonances may arise from quantum and classical effects involving the attractive and repulsive parts of the potential surfaces relevant to collisions such as diffraction and rainbow scattering.
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Table 1: data from our previous publications [13-15] for ܰଶ , ܱଶ , ܱܥଶ and ܪܥସ targets for total, ionization and fragmentation cross sections in 10-16 cm2.
Energy (keV) 2 3 4 5 6 7 8 9 10
CO+ 14.9 12.9088 12.992 15.2 13.8 14.976 13.0112 13.2672 12.1792
C+ 0.224 0.32 0.4288 0.3008 0.2688 0.1856 0.16 0.1472 0.1728
O+ 0.11614 0.16 0.11584 0.16669 0.112 0.06754 0.04949 0.09581 0.09262
Energy (keV) 2 3 4 5 6 7 8 9 10
CH4+ 22.97 12.13 11.34 12.13 12.54 9.58 8.1 7.08 5.6
CH3+ 12.54 7.8 5.4 7.1 5.8 4.9 4.14 4.13 4.13
CH2+ 0.72 0.36 0.32 0.41 0.61 0.39 0.32 0.31 0.32
CH+ 0.096 0.086 0.12 0.15 0.21 0.19 0.18 0.12 0.14
C+ 0.036 0.031 0.028 0.047 0.052 0.068 0.049 0.068 0.056
H+ 0.0039 0.0015 0.0019 0.0019 0.0019 0.0025 0.0039 0.0097 0.0097
Total CS 36.4 20.44 17.23 19.82 19.21 15.14 12.8 11.73 10.26
Energy (keV) 2. 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5
CO2+ 14.88272 15.38125 14.88272 13.04504 10.35812 8.22463 7.95806 5.7242 7.70013 7.95806 6.74934 5.01739 6.74934 3.85482 2.68289 5.18546
CO+ 0.08436 0.12121 0.22668 0.19869 0.17416 0.14292 0.12527 0.09011 0.12527 0.1338 0.1098 0.10624 0.17999 0.1098 0.1028 0.1338
O2+ 0.028 0.041 0.067 0.065 0.061 0.038 0.034 0.032 0.041 0.037 0.036 0.026 0.041 0.025 0.014 0.038
O+ 0.02 0.029 0.039 0.037 0.04 0.021 0.023 0.017 0.024 0.029 0.028 0.018 0.033 0.021 0.019 0.019
C+ 0.003 0.005 0.004 0.004 0.003 0.0026 0.0035 0.0059 0.0063 0.0056 0.0063 0.0039 0.0069 0.003 0.0042 0.0045
CO2++
Total CS 15.02 15.6 15.2 13.35 10.64 8.4 8.2 5.9 7.9 8.2 6.9 5.2 7 4 2.8 5.4
Energy (keV) 2 3 4 5 6 7 8 9 10
O+ 1.82332 1.76225 2.1833 1.82332 1.54412 1.15099 1.66199 2.09638 2.45291
O2+ 8.96312 9.12204 10.92942 10.28983 11.92272 7.65139 5.89525 5.93943 5.35199
Total CS 10.78644 10.88429 13.11272 12.11315 13.46684 8.8854 7.65139 8.03181 7.71294
Energy (keV) 2 3 4 5 5.5 6 7 7.5 8 8.5 9 9.5 10
N+ 1.3693 1.93542 3.02108 2.70858 2.8942 2.12687 3.80535 2.41096 1.54393 2.38196 2.52831 1.8286 1.54393
N2+ 16.65579 15.61189 9.69677 8.51101 7.13207 7.0705 10.28302 9.98302 11.1615 8.74684 7.92245 7.19424 7.13207
Total CS 18.02509 17.54732 12.71785 11.21958 10.02627 9.19737 14.08837 12.39398 12.70543 11.1288 10.45076 9.02285 8.676
1
Total CS 15.1 13.3248 13.536 14.7 14.2 15.2256 13.216 13.504 12.4352
0.0025 0.0036 0.0044 0.0061 0.005 0.0047 0.0079 0.0087 0.0096 0.0152 0.012 0,0087 0.017
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Figure 3. Absolute cross section as a function of the proton energy.
In general, the total cross sections previously measured by other groups for proton impact, are basically a measure of the fast neutrals resulting from all the processes involved. This may obscure any structures that may arise from different dissociation paths in contrast to our present total cross section, which is the sum of the total absolute cross sections of the fragments and any structure in the cross section would be the result of interference between the competing paths. Nevertheless, our present apparatus does not distinguish between the origins of the processes leading to the same detected slow ion and each fragment ion measured would be the result of the total ions originating from all the processes leading to that particular ion. As a result, further investigations at energies below 5 keV would be needed to verify such observations. 4. Conclusions We report the fragmentation cross section for ܱܥin collisions with protons in the low energy region of 2 keV to 10 keV. TOF spectra show ionization to be the dominant reaction over all of the energy range of 2 keV to 10 keV, followed by descending intensity of fragments of ܥା and ܱା respectively. ܥା and ܱା fragments cross sections show a slow decrease above 4 keV collisional energy. The total cross section shows a minor decreasing trend as the collisional energy was increased. Our total cross sections show fluctuations in the cross section measurements between 4 keV and 7 keV which is explained as an interference in the fragments competing cross sections and confirms the observations made by theoretical and experimental differential cross sections of prior investigators. Our total cross sections overlap well with previously measured absolute total cross sections reported by other groups.
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Aknowledgment We appreciate the participation and technical assistance of Dalila Martinez in this work. This work was partially supported by DGAPA- PAPIIT- IN-116909, CONACyT Scholarship for the PhD student J. López-Patiño. In part by the Programa de Mejoramiento del Desarrollo del Profesorado (PRODEP).
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