- Email: [email protected]
Molecular ion collision chemistry in CH4 ionization and dissociation
International Journal of Mass Spectrometry 408 (2016) 38–41
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Molecular ion collision chemistry in CH4 ionization and dissociation A.T. Hasan a,∗ , T.J. Gray b a b
Department of Physics, American University of Sharjah, Sharjah, United Arab Emirates Department of Physics, Kansas State University, Manhattan, KS 66506, United States
a r t i c l e
i n f o
Article history: Received 16 June 2016 Received in revised form 29 August 2016 Accepted 29 August 2016 Available online 2 September 2016 PACS: 82.30.Fi 34.50.Fa 34.70.+e
a b s t r a c t The use of the 90◦ hemispheric electrostatic high resolution analyzer in conjunction with the time-offlight (TOF) technique allowed us to identify various species associated with the ionization of methane molecular gas by a pulsed fast 19 MeV F4+ beam. We were able to measure the yields of single and double + + + 2+ + 2+ + ionization-dissociation species of H+ , H+ 2 , C , C , CH , CH2 , CH3 , CH4 and CH4 . We reported the single+ + + and total ionization cross sections of CH+ and the daughter ionsCH , CH , CH and C+ . Our results show 4 3 2 + that the single ionization cross sections of CH+ 4 and CH3 are in good accord with those found in literature, + + however, the cross sections of CH+ 2 , CH and C are in disagreement with the ones found in literature. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Although the phenomena of ion-molecular collisions have been the subject of extensive research, still much more can be learned about these complex and variant processes. Further experimental and theoretical studies of ion-molecule collision will provide a better understanding of the atomic structure, collision phenomena, and molecular ion formation. The interactions of atomic and molecular ions with molecules lead to a variety of processes which are important from the viewpoint of gaining a better, more quantitative insight into the structure of the molecular targets and molecular product ions themselves, and the mechanisms and the collision dynamics involved in their ionization and dissociation processes. These processes are of interest as they are most likely to have impact on the fields of astrophysics, atmospheric sciences [1,2], low- and high-temperature laboratory plasmas [3], testing theoretical models and understanding the dynamics of these reactions. Atmospheric molecular ions and their dissociation products produced from ionization of CH4 [4–11], H2 O, CO2 [12–14] and N2 [15] gases are found to be important constituents of the earth’s upper atmosphere. The ionization and dissociation processes of the simplest hydrocarbon methane lure considerable experimental and theoretical attention. Ward et al. [10] measured the cross sections of ionization and dissociation events by bombarding methane with energetic electrons. Ben-Itzhak et al. [5] and Luna et al. [8] reported the cross
∗ Corresponding author. E-mail addresses: [email protected] (A.T. Hasan), [email protected] (T.J. Gray). http://dx.doi.org/10.1016/j.ijms.2016.08.009 1387-3806/© 2016 Elsevier B.V. All rights reserved.
sections of proton-methane ionization. Campeanu et al. [4] studied the ionization of CH4 by positron. Sharifi et al. [9] addressed the theoretical and experimental aspects of ionization and dissociation of methane using a high-power laser and Malhi et al. [6] studies the ionization and dissociation processes of methane utilizing several energetic beams. To place our measurements of ionization and dissociation of methane in this context, we planned to: measure the total cross section of ionization of methane and the relative cross sections for formation of H+ , H+ , C+ , C2+ , CH+ , CH+ , CH+ to CH+ utilizing 2 2 3 4 a pulsed fast 19 MeV F4+ beam, and compare our measurements with the experimental and theoretical model calculations available in literature, and to test our ion recoil system.
2. Experiment The experiment is carried out in the recoil ion source apparatus [16]. A schematic diagram of this apparatus is shown in Fig. 1. A 10-pA, 250-ps pulsed F4+ beam, of energy 1 MeV/amu and a diameter of approximately 2 mm with a divergence of 5 mrad from the Kansas State University Tandem Van de Graaff accelerator is impinged on the recoil ion source (RIS) containing CH4 molecular gas. The collisions that take place in the RIS between the fast F4+ pulsed beam and the CH4 gas generate recoil ions that are extracted by the voltage gradient across the RIS electrodes. Upon exiting the RIS, these ions travel through a 5.8 cm long first acceleration unit, a seven-plate apparatus, which only the fifth plate is not grounded. This plate (Einzel lens) focuses and drifts the primary ions toward the secondary pressurized gas-target cell, which is 31.8 cm away from the detector. Following the ionization of CH4 molecular gas,
A.T. Hasan, T.J. Gray / International Journal of Mass Spectrometry 408 (2016) 38–41
39
Fig. 1. A schematic experimental set-up for the recoil ion source.
single and double ion species of H+ , H+ , C+ , C2+ , CH+ , CH+ , CH+ , 2 2 3 + and CH4 were detected. These ions travelled through a second acceleration unit, which is identical to the first acceleration unit. The focused low-energy ions then drifted to the detector through double-plate electrostatic analyzer. A 0.5 mTorr CH4 gas pressure was measured by MKS type 90 capacitance manometers and controlled by servo-assisted gashandling system, which regulated a Granville-Philips variable leak. The methane gas pressure was kept sufficiently low to minimize a second collision. The background pressure of the beam line is maintained at about 5 × 10−7 Torr. The time-of-flight (TOF) and the final charge state of the all ions created were the basis of our measurements, thus a fast coincidence technique was used. By applying an appropriate voltage at the analyzer, selected charge ions are allowed to reach the detector. The use of this electrostatic analyzer in conjunction with TOF techniques allow the identification of various events associated with the charged molecular ions. The analyzer voltage and the time-of-flight of the ions created are digitized and stored in a two-dimensional coincidence spectrum. The separation of the various ion species that created between the point of creation and the point of detection was done by the help of time-of-flight and the analyzer voltage. The time-of-flight was calibrated using the observed ion species. The mass-to-charge ratio was determined from the time-of-flight and the length of path traveled by these ions. 3. Result and discussion The two-dimensional coincidence spectrum depicting raw data of the analyzer voltage versus the time-of-flight is shown in Fig. 2. The density of the points and the size of the species are proportional to the yields of the detected ions. Ionization and dissociation + events are labeled. The ion species of CH+ 5 and CH6 could occur following the ionization of methane. These ions arise from CH+ ions 4 by picking one and two hydrogen atoms, respectively. Taking into consideration the formation of OH+ and H2 O+ ions, with the same + mass to charge ratio as CH+ 5 and CH6 respectively, ascending from the impurity of the methane gas. These ions became the dominant species in the spectrum. Now, we focus our attention to the CH+ products. The top hot 4 spot of the spectrum are labeled from left to rightCH+ , CH+ , CH+ , 4 3 2 + + CH , and C , respectively. These are primary ions which are created in the RIS during the early stages of the collision between the fast beam and methane gas. These ions travelled from the RIS to
Fig. 2. A two-dimensional coincidence spectrum of the analyzer voltage versus TOF produce by bombarding methane gas with 19 MeV F4+ beam.
the detector without further interaction or modification. These ions were created at exact analyzer voltage and their time-of-flights are proportional to square root of the mass to charge ratio. The spots directly below the primary-parent ions represent are the daughter species. They were created by losing one or more hydrogen atom by the parent ions. These daughter ions were observed of having the time-of-flight as their parent ions but created at different analyzer voltages. The creation of the primary ions following the single ionization of the methane gas can be described as follows: CH+ → CH+ + H 4 3
(1)
CH+ → CH+ + H 3 2
(2)
CH+ 2
(3)
+
+
→ CH + H +
CH → C + H
(4)
Detailed theoretical discussion for ionization and dissociation of CH+ ions has been addressed by Sharifi et al. [9] and Gray et al. 4 [17]. A time-of-flight spectra depicting various ion species is shown in Fig. 3. The present time-of-flight spectrum is, in many aspects,
40
A.T. Hasan, T.J. Gray / International Journal of Mass Spectrometry 408 (2016) 38–41
5
6
Time-of-flight (μs) 8
9
10
11
12
4+
F + CH4 CH+2
104
105
F4+ + CH4
Counts
C
CH2+
CH +
104
+ 2
+
OH
H
5
C2+
CH2+ 2
CH2+ 4
101
100
0 0
+ 2
10
C2+ H2+
H+
CH
+
O+2
CH22+ CH2+ 4
103
103
+
2
H2O
+
C+
N
H+
Counts
+ CH+3 CH4
105
CH+3 CH4+
0
10
15
20
25
30
1
2
6
7
35
mass/charge Fig. 3. High resolution time-of-flight spectrum depicting the ionization and dissociation ions produced by 19 MeV F4+ beam on CH4 .
8
12
13
14
15
16
mass/charge Fig. 4. The yields of ionization and dissociations species produces by 19 MeV F4+ on CH4.
3.1. Analysis of single ionization of methane identical with the ones obtained by Sharifi et al. [9] and Ben-Itzhak et al. [5]. These ions are identified according to the mass to charge ratio. Strong showing of N+ and O+ 2 ions arise from the residual vacuum. 2 OH+ and H2 O+ ions are also identified. Furthermore, low intensities H+ , H+ , C2+ , CH2+ , and CH2+ ions are detected. These ions are 2 2 4 ascending from the double ionized of CH4 gas: Dissociation:
(5) CH2+ → CH+ + H+ 4 2 2
(6)
(7) The abundance of the ionization and dissociation yields is shown in Fig. 4. It is clearly observed that the yields of CH2+ and CH2+ are 4 2 very little compared to the rest of the observed ions.
The single ionization cross section of the methane gas was calculated following the procedure mentioned in Ref. [16], taking into consideration the number of detected ions, gas target pressure, diameter of the extractor aperture, detection efficiency, and the target gas density. The overall uncertainty in the present measurements was determined and found to be in the order of 20%. This uncertainty was determined by the quadrature combination of the uncertainties due counting statistics, uncertainties in the procedure for summing up the yield, the detection efficiency and the determination of the effective length of the target cell. The single ionization cross sections produced by F4+ and H+ on CH4 are tabulated in Table 1. Fig. 5 depicts the present measurements in concurrence with Malhi et al. [6] results of F4+ on CH4 . Overall, the present result for 19 MeV F4+ incident on CH4 is partially comparable with Malhi et al. [6] measurements. Malhi et al. results repeatedly overestimated the single ionization cross sections of CH+ , CH+ and C+ . It is worth to note that the single 2 ionization cross sections of CH+ n (n = 0–4) ions produced by 1 MeV H+ energy incident on CH4 gas were measured by Luna et al. [8] and Malhi et al. [6] and reported in Table 1. The results of these two measurements are in good accord in estimating the cross secand CH+ , but again Malhi et al. [6] overestimated tions of CH+ 4 3 the single ionization cross sections of CH+ , CH+ and C+ ions. Ben2 Itzhak et al. [5] reported the abundances of singles relative to CH+ 4 yields produced by 4-MeV proton. Their results for CH+ and CH+ 4 3 are comparable with those of found by Malhi et al. [6], however, their results of CH+ , CH+ and C+ are in disagreement with Malhi 2 et al. [5] results. However, it is worth saying that the total ionization cross sections CH+ n (n = 0–4) are good accord with their partners and with the theoretical model calculations [18–20] sited by Malhi et al. [6]. The present measurement of the total ioniza-
Table 1 The measurements of single ionization cross sections (10−16 cm2 ) produced by 19 MeV F4+ and 1 MeV H+ on CH4 are reported. Ion
F4+ + CH4 19 MeV Present
F4+ + CH4 19 MeV Ref. [6]
H+ + CH4 1 MeV Ref. [6]
H+ + CH4 1 MeV Ref. [8]
CH+ 4 CH+ 3 CH+ 2 + CH + C
11.2 ± 2.2 8.7 ± 1.7 0.9 ± 0.2 0.4 ± 0.08 0.31 ± 0.0.6
9±3 7.9 ± 2.8 2.2 ± 0.8 1.4 ± 0.5 1.02 ± 0.4
0.8 ± 0.3 0.66 ± 0.2 0.17 ± 0.06 0.056 ± 0.02 0.013 ± 0.004
0.97 ± 0.02 0.855 ± 0.02 0.14 ± 0.01 0.039 ± 0.003 0.013 ± 0.002
A.T. Hasan, T.J. Gray / International Journal of Mass Spectrometry 408 (2016) 38–41 -14
10
2 Cross section (cm )
4+
F + CH4 -15
10
Present Malhi et al
41
By developing the systematics and a more general understanding of the population of specific states in ionization and fragmentation of methane gas, we hope to learn more about the role of atomic structure in the decay of the ionized gas. This should also enhance our understanding of the molecular ion formation, and charge transfer processes which are extremely important in many research area in physics and chemistry. Acknowledgements
-16
10
The authors are gratefully acknowledges the experimental support of the accelerator crew of the J.R. Mc Donald laboratory at Kansas State University for providing a high quality beam during the course of the experiment.
-17
10
+
CH4
+
CH3
+
CH2
+
CH
+
C
Species CH+ , CH+ , CH+ and C+ ions are shown Fig. 5. Single ionization cross sections for CH+ 4 3 2 in concurrence with the result of Ref. [6].
tion cross section was found to be (2.15 ± 0.43) × 10−15 cm2 and Malhi et al. [6] reported (2.13 ± 0.7) × 10−15 cm2 , meanwhile Malhi et al. [6] reported (1.7 ± 0.5) × 10−16 cm2 and Luna et al. [8] reported (2.01 ± 0.03) × 10−16 cm2 for 1 MeV H+ + CH4 . 4. Conclusion In this work we presented and analyzed the time-of-flight spectrum produced by 19-MeV F4+ incident on CH4 molecular gas. The spectrum contains the ionization and dissociation of methane gas. We reported the single- and total ionization cross sections of CH+ n (n = 0–4) and compared them to those found in literature. Information ionization and dissociation of methane gas extracted from this measurement fit partially to the experimental data and the model calculations found in literature. There are some discrepancies between the present measurements and the cited ones. These discrepancies may result from the limitations related to the efficiency and the resolution of the detector and the presence of non-statistical yields. Various species were recognized in the high resolution time-offlight spectrum. The fact that we are able to detect these species indicates that the life time of each ion detected is larger than the instrumental TOF, which is in the order of 2 s.
References [1] M.R. Swain, G. Vasisht, G. Tinetti, Nature 452 (2008) 329. [2] P.R. Mahaffy, Science 308 (2005) 969. [3] H.C. Straub, D. Lin, B.G. Lindsay, K.A. Smith, R.F.M. Stebbings, J. Chem. Phys. 106 (1997) 4430. [4] R.I. Campeamu, V. Chis, L. Nagy, A.D. Stauffer, Nucl. Instrum. Methods B 24 (2006) 758. [5] I. Ben-Itzhak, K.D. Carnes, S.G. Ginther, D.T. Johnson, P.J. Norris, O.L. Weaver, Phys. Rev. A 47 (1993) 3748. [6] N.B. Malhi, I. Ben-Itzhak, T.J. Gray, J.C. Legg, V. Needham, K. Carnes, J. McGuire, J. Chem. Phys. 87 (1987) 11. [7] H. Knudsen, U. Mikkelsen, K. Paludan, K. Kirsebom, S.P. Moller, E. Uggerhoj, J. Slevin, M. Charlton, E. Morenzon, J. Phys. B 28 (1995) 3569. [8] H. Luna, E.G. Cavalcanti, J. Nickles, G.M. Sigaud, E.C. Montenegro, J. Phys. B 36 (2003) 4717. [9] M. Sharifi, F. Kong, S.L. Chin, H. Mineo, Y. Dyakov, A.M. Mebel, S.D. Chao, M. Hayashi, S.H. Lin, J. Phys. Chem. A 111 (2007) 9405. [10] M.D. Ward, S.J. King, S.D. Price, J. Chem. Phys. 134 (2011) 024308. [11] S. Mondal, R. Shanker, Nucl. Instrum. Methods B 246 (2006) 297. [12] O. Abu-Haija, E.K. Kamber, S.M. Ferguson, N. Stolterfoht, Phys. Rev. A 72 (2005) 042701. [13] A. Hasan, O. Abu-Haija, J. Harris, T. Elkafrawy, A. Kayani, E.Y. Kamber, Phys. Scr. T156 (2013) 014041. [14] O. Abu-Haija, A. Hasan, A. Kayani, E.Y. Kamber, EPL 93 (2011) 13003. [15] M. Charlton, T.C. Griffith, G.R. Heyland, G.L. Wright, J. Phys. B 13 (1980) L353. [16] A. Hasan, T.J. Gray, Nucl. Instrum. Methods B 198 (2002) 1. [17] T.J. Gray, J.C. Legg, V. Needham, Nucl. Instrum. Methods B 10 (1985) 253. [18] R.H. Madison, E. Merzbacher, Academic (1975) 1. [19] R.G. Glauber, Lect. Theor. Phys. 1 (1959) 315. [20] J.H. McGuire, Phys. Rev. A 26 (1982) 143.
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Molecular ion collision chemistry in CH4 ionization and dissociation A.T. Hasan a,∗ , T.J. Gray b a b
Department of Physics, American University of Sharjah, Sharjah, United Arab Emirates Department of Physics, Kansas State University, Manhattan, KS 66506, United States
a r t i c l e
i n f o
Article history: Received 16 June 2016 Received in revised form 29 August 2016 Accepted 29 August 2016 Available online 2 September 2016 PACS: 82.30.Fi 34.50.Fa 34.70.+e
a b s t r a c t The use of the 90◦ hemispheric electrostatic high resolution analyzer in conjunction with the time-offlight (TOF) technique allowed us to identify various species associated with the ionization of methane molecular gas by a pulsed fast 19 MeV F4+ beam. We were able to measure the yields of single and double + + + 2+ + 2+ + ionization-dissociation species of H+ , H+ 2 , C , C , CH , CH2 , CH3 , CH4 and CH4 . We reported the single+ + + and total ionization cross sections of CH+ and the daughter ionsCH , CH , CH and C+ . Our results show 4 3 2 + that the single ionization cross sections of CH+ 4 and CH3 are in good accord with those found in literature, + + however, the cross sections of CH+ 2 , CH and C are in disagreement with the ones found in literature. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Although the phenomena of ion-molecular collisions have been the subject of extensive research, still much more can be learned about these complex and variant processes. Further experimental and theoretical studies of ion-molecule collision will provide a better understanding of the atomic structure, collision phenomena, and molecular ion formation. The interactions of atomic and molecular ions with molecules lead to a variety of processes which are important from the viewpoint of gaining a better, more quantitative insight into the structure of the molecular targets and molecular product ions themselves, and the mechanisms and the collision dynamics involved in their ionization and dissociation processes. These processes are of interest as they are most likely to have impact on the fields of astrophysics, atmospheric sciences [1,2], low- and high-temperature laboratory plasmas [3], testing theoretical models and understanding the dynamics of these reactions. Atmospheric molecular ions and their dissociation products produced from ionization of CH4 [4–11], H2 O, CO2 [12–14] and N2 [15] gases are found to be important constituents of the earth’s upper atmosphere. The ionization and dissociation processes of the simplest hydrocarbon methane lure considerable experimental and theoretical attention. Ward et al. [10] measured the cross sections of ionization and dissociation events by bombarding methane with energetic electrons. Ben-Itzhak et al. [5] and Luna et al. [8] reported the cross
∗ Corresponding author. E-mail addresses: [email protected] (A.T. Hasan), [email protected] (T.J. Gray). http://dx.doi.org/10.1016/j.ijms.2016.08.009 1387-3806/© 2016 Elsevier B.V. All rights reserved.
sections of proton-methane ionization. Campeanu et al. [4] studied the ionization of CH4 by positron. Sharifi et al. [9] addressed the theoretical and experimental aspects of ionization and dissociation of methane using a high-power laser and Malhi et al. [6] studies the ionization and dissociation processes of methane utilizing several energetic beams. To place our measurements of ionization and dissociation of methane in this context, we planned to: measure the total cross section of ionization of methane and the relative cross sections for formation of H+ , H+ , C+ , C2+ , CH+ , CH+ , CH+ to CH+ utilizing 2 2 3 4 a pulsed fast 19 MeV F4+ beam, and compare our measurements with the experimental and theoretical model calculations available in literature, and to test our ion recoil system.
2. Experiment The experiment is carried out in the recoil ion source apparatus [16]. A schematic diagram of this apparatus is shown in Fig. 1. A 10-pA, 250-ps pulsed F4+ beam, of energy 1 MeV/amu and a diameter of approximately 2 mm with a divergence of 5 mrad from the Kansas State University Tandem Van de Graaff accelerator is impinged on the recoil ion source (RIS) containing CH4 molecular gas. The collisions that take place in the RIS between the fast F4+ pulsed beam and the CH4 gas generate recoil ions that are extracted by the voltage gradient across the RIS electrodes. Upon exiting the RIS, these ions travel through a 5.8 cm long first acceleration unit, a seven-plate apparatus, which only the fifth plate is not grounded. This plate (Einzel lens) focuses and drifts the primary ions toward the secondary pressurized gas-target cell, which is 31.8 cm away from the detector. Following the ionization of CH4 molecular gas,
A.T. Hasan, T.J. Gray / International Journal of Mass Spectrometry 408 (2016) 38–41
39
Fig. 1. A schematic experimental set-up for the recoil ion source.
single and double ion species of H+ , H+ , C+ , C2+ , CH+ , CH+ , CH+ , 2 2 3 + and CH4 were detected. These ions travelled through a second acceleration unit, which is identical to the first acceleration unit. The focused low-energy ions then drifted to the detector through double-plate electrostatic analyzer. A 0.5 mTorr CH4 gas pressure was measured by MKS type 90 capacitance manometers and controlled by servo-assisted gashandling system, which regulated a Granville-Philips variable leak. The methane gas pressure was kept sufficiently low to minimize a second collision. The background pressure of the beam line is maintained at about 5 × 10−7 Torr. The time-of-flight (TOF) and the final charge state of the all ions created were the basis of our measurements, thus a fast coincidence technique was used. By applying an appropriate voltage at the analyzer, selected charge ions are allowed to reach the detector. The use of this electrostatic analyzer in conjunction with TOF techniques allow the identification of various events associated with the charged molecular ions. The analyzer voltage and the time-of-flight of the ions created are digitized and stored in a two-dimensional coincidence spectrum. The separation of the various ion species that created between the point of creation and the point of detection was done by the help of time-of-flight and the analyzer voltage. The time-of-flight was calibrated using the observed ion species. The mass-to-charge ratio was determined from the time-of-flight and the length of path traveled by these ions. 3. Result and discussion The two-dimensional coincidence spectrum depicting raw data of the analyzer voltage versus the time-of-flight is shown in Fig. 2. The density of the points and the size of the species are proportional to the yields of the detected ions. Ionization and dissociation + events are labeled. The ion species of CH+ 5 and CH6 could occur following the ionization of methane. These ions arise from CH+ ions 4 by picking one and two hydrogen atoms, respectively. Taking into consideration the formation of OH+ and H2 O+ ions, with the same + mass to charge ratio as CH+ 5 and CH6 respectively, ascending from the impurity of the methane gas. These ions became the dominant species in the spectrum. Now, we focus our attention to the CH+ products. The top hot 4 spot of the spectrum are labeled from left to rightCH+ , CH+ , CH+ , 4 3 2 + + CH , and C , respectively. These are primary ions which are created in the RIS during the early stages of the collision between the fast beam and methane gas. These ions travelled from the RIS to
Fig. 2. A two-dimensional coincidence spectrum of the analyzer voltage versus TOF produce by bombarding methane gas with 19 MeV F4+ beam.
the detector without further interaction or modification. These ions were created at exact analyzer voltage and their time-of-flights are proportional to square root of the mass to charge ratio. The spots directly below the primary-parent ions represent are the daughter species. They were created by losing one or more hydrogen atom by the parent ions. These daughter ions were observed of having the time-of-flight as their parent ions but created at different analyzer voltages. The creation of the primary ions following the single ionization of the methane gas can be described as follows: CH+ → CH+ + H 4 3
(1)
CH+ → CH+ + H 3 2
(2)
CH+ 2
(3)
+
+
→ CH + H +
CH → C + H
(4)
Detailed theoretical discussion for ionization and dissociation of CH+ ions has been addressed by Sharifi et al. [9] and Gray et al. 4 [17]. A time-of-flight spectra depicting various ion species is shown in Fig. 3. The present time-of-flight spectrum is, in many aspects,
40
A.T. Hasan, T.J. Gray / International Journal of Mass Spectrometry 408 (2016) 38–41
5
6
Time-of-flight (μs) 8
9
10
11
12
4+
F + CH4 CH+2
104
105
F4+ + CH4
Counts
C
CH2+
CH +
104
+ 2
+
OH
H
5
C2+
CH2+ 2
CH2+ 4
101
100
0 0
+ 2
10
C2+ H2+
H+
CH
+
O+2
CH22+ CH2+ 4
103
103
+
2
H2O
+
C+
N
H+
Counts
+ CH+3 CH4
105
CH+3 CH4+
0
10
15
20
25
30
1
2
6
7
35
mass/charge Fig. 3. High resolution time-of-flight spectrum depicting the ionization and dissociation ions produced by 19 MeV F4+ beam on CH4 .
8
12
13
14
15
16
mass/charge Fig. 4. The yields of ionization and dissociations species produces by 19 MeV F4+ on CH4.
3.1. Analysis of single ionization of methane identical with the ones obtained by Sharifi et al. [9] and Ben-Itzhak et al. [5]. These ions are identified according to the mass to charge ratio. Strong showing of N+ and O+ 2 ions arise from the residual vacuum. 2 OH+ and H2 O+ ions are also identified. Furthermore, low intensities H+ , H+ , C2+ , CH2+ , and CH2+ ions are detected. These ions are 2 2 4 ascending from the double ionized of CH4 gas: Dissociation:
(5) CH2+ → CH+ + H+ 4 2 2
(6)
(7) The abundance of the ionization and dissociation yields is shown in Fig. 4. It is clearly observed that the yields of CH2+ and CH2+ are 4 2 very little compared to the rest of the observed ions.
The single ionization cross section of the methane gas was calculated following the procedure mentioned in Ref. [16], taking into consideration the number of detected ions, gas target pressure, diameter of the extractor aperture, detection efficiency, and the target gas density. The overall uncertainty in the present measurements was determined and found to be in the order of 20%. This uncertainty was determined by the quadrature combination of the uncertainties due counting statistics, uncertainties in the procedure for summing up the yield, the detection efficiency and the determination of the effective length of the target cell. The single ionization cross sections produced by F4+ and H+ on CH4 are tabulated in Table 1. Fig. 5 depicts the present measurements in concurrence with Malhi et al. [6] results of F4+ on CH4 . Overall, the present result for 19 MeV F4+ incident on CH4 is partially comparable with Malhi et al. [6] measurements. Malhi et al. results repeatedly overestimated the single ionization cross sections of CH+ , CH+ and C+ . It is worth to note that the single 2 ionization cross sections of CH+ n (n = 0–4) ions produced by 1 MeV H+ energy incident on CH4 gas were measured by Luna et al. [8] and Malhi et al. [6] and reported in Table 1. The results of these two measurements are in good accord in estimating the cross secand CH+ , but again Malhi et al. [6] overestimated tions of CH+ 4 3 the single ionization cross sections of CH+ , CH+ and C+ ions. Ben2 Itzhak et al. [5] reported the abundances of singles relative to CH+ 4 yields produced by 4-MeV proton. Their results for CH+ and CH+ 4 3 are comparable with those of found by Malhi et al. [6], however, their results of CH+ , CH+ and C+ are in disagreement with Malhi 2 et al. [5] results. However, it is worth saying that the total ionization cross sections CH+ n (n = 0–4) are good accord with their partners and with the theoretical model calculations [18–20] sited by Malhi et al. [6]. The present measurement of the total ioniza-
Table 1 The measurements of single ionization cross sections (10−16 cm2 ) produced by 19 MeV F4+ and 1 MeV H+ on CH4 are reported. Ion
F4+ + CH4 19 MeV Present
F4+ + CH4 19 MeV Ref. [6]
H+ + CH4 1 MeV Ref. [6]
H+ + CH4 1 MeV Ref. [8]
CH+ 4 CH+ 3 CH+ 2 + CH + C
11.2 ± 2.2 8.7 ± 1.7 0.9 ± 0.2 0.4 ± 0.08 0.31 ± 0.0.6
9±3 7.9 ± 2.8 2.2 ± 0.8 1.4 ± 0.5 1.02 ± 0.4
0.8 ± 0.3 0.66 ± 0.2 0.17 ± 0.06 0.056 ± 0.02 0.013 ± 0.004
0.97 ± 0.02 0.855 ± 0.02 0.14 ± 0.01 0.039 ± 0.003 0.013 ± 0.002
A.T. Hasan, T.J. Gray / International Journal of Mass Spectrometry 408 (2016) 38–41 -14
10
2 Cross section (cm )
4+
F + CH4 -15
10
Present Malhi et al
41
By developing the systematics and a more general understanding of the population of specific states in ionization and fragmentation of methane gas, we hope to learn more about the role of atomic structure in the decay of the ionized gas. This should also enhance our understanding of the molecular ion formation, and charge transfer processes which are extremely important in many research area in physics and chemistry. Acknowledgements
-16
10
The authors are gratefully acknowledges the experimental support of the accelerator crew of the J.R. Mc Donald laboratory at Kansas State University for providing a high quality beam during the course of the experiment.
-17
10
+
CH4
+
CH3
+
CH2
+
CH
+
C
Species CH+ , CH+ , CH+ and C+ ions are shown Fig. 5. Single ionization cross sections for CH+ 4 3 2 in concurrence with the result of Ref. [6].
tion cross section was found to be (2.15 ± 0.43) × 10−15 cm2 and Malhi et al. [6] reported (2.13 ± 0.7) × 10−15 cm2 , meanwhile Malhi et al. [6] reported (1.7 ± 0.5) × 10−16 cm2 and Luna et al. [8] reported (2.01 ± 0.03) × 10−16 cm2 for 1 MeV H+ + CH4 . 4. Conclusion In this work we presented and analyzed the time-of-flight spectrum produced by 19-MeV F4+ incident on CH4 molecular gas. The spectrum contains the ionization and dissociation of methane gas. We reported the single- and total ionization cross sections of CH+ n (n = 0–4) and compared them to those found in literature. Information ionization and dissociation of methane gas extracted from this measurement fit partially to the experimental data and the model calculations found in literature. There are some discrepancies between the present measurements and the cited ones. These discrepancies may result from the limitations related to the efficiency and the resolution of the detector and the presence of non-statistical yields. Various species were recognized in the high resolution time-offlight spectrum. The fact that we are able to detect these species indicates that the life time of each ion detected is larger than the instrumental TOF, which is in the order of 2 s.
References [1] M.R. Swain, G. Vasisht, G. Tinetti, Nature 452 (2008) 329. [2] P.R. Mahaffy, Science 308 (2005) 969. [3] H.C. Straub, D. Lin, B.G. Lindsay, K.A. Smith, R.F.M. Stebbings, J. Chem. Phys. 106 (1997) 4430. [4] R.I. Campeamu, V. Chis, L. Nagy, A.D. Stauffer, Nucl. Instrum. Methods B 24 (2006) 758. [5] I. Ben-Itzhak, K.D. Carnes, S.G. Ginther, D.T. Johnson, P.J. Norris, O.L. Weaver, Phys. Rev. A 47 (1993) 3748. [6] N.B. Malhi, I. Ben-Itzhak, T.J. Gray, J.C. Legg, V. Needham, K. Carnes, J. McGuire, J. Chem. Phys. 87 (1987) 11. [7] H. Knudsen, U. Mikkelsen, K. Paludan, K. Kirsebom, S.P. Moller, E. Uggerhoj, J. Slevin, M. Charlton, E. Morenzon, J. Phys. B 28 (1995) 3569. [8] H. Luna, E.G. Cavalcanti, J. Nickles, G.M. Sigaud, E.C. Montenegro, J. Phys. B 36 (2003) 4717. [9] M. Sharifi, F. Kong, S.L. Chin, H. Mineo, Y. Dyakov, A.M. Mebel, S.D. Chao, M. Hayashi, S.H. Lin, J. Phys. Chem. A 111 (2007) 9405. [10] M.D. Ward, S.J. King, S.D. Price, J. Chem. Phys. 134 (2011) 024308. [11] S. Mondal, R. Shanker, Nucl. Instrum. Methods B 246 (2006) 297. [12] O. Abu-Haija, E.K. Kamber, S.M. Ferguson, N. Stolterfoht, Phys. Rev. A 72 (2005) 042701. [13] A. Hasan, O. Abu-Haija, J. Harris, T. Elkafrawy, A. Kayani, E.Y. Kamber, Phys. Scr. T156 (2013) 014041. [14] O. Abu-Haija, A. Hasan, A. Kayani, E.Y. Kamber, EPL 93 (2011) 13003. [15] M. Charlton, T.C. Griffith, G.R. Heyland, G.L. Wright, J. Phys. B 13 (1980) L353. [16] A. Hasan, T.J. Gray, Nucl. Instrum. Methods B 198 (2002) 1. [17] T.J. Gray, J.C. Legg, V. Needham, Nucl. Instrum. Methods B 10 (1985) 253. [18] R.H. Madison, E. Merzbacher, Academic (1975) 1. [19] R.G. Glauber, Lect. Theor. Phys. 1 (1959) 315. [20] J.H. McGuire, Phys. Rev. A 26 (1982) 143.