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The Astrophysical Journal, 679:1661Y1664, 2008 June 1 # 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.

ABSOLUTE SINGLE AND MULTIPLE CHARGE EXCHANGE CROSS SECTIONS FOR HIGHLY CHARGED C, N, AND O IONS COLLIDING WITH CH 4 N. Djuric´, S. J. Smith, J. Simcic, and A. Chutjian Atomic and Molecular Physics Group, Jet Propulsion Laboratory/Caltech, Pasadena, CA 91109 Received 2007 December 10; accepted 2008 February 7

ABSTRACT Absolute charge exchange (CE) cross sections have been measured for single and multiple exchanges between the highly charged ions (HCIs) C q + (q ¼ 3Y 6), N q + (q ¼ 4Y7) and O q + (q ¼ 5Y7) colliding with the target methane (CH 4 ) present in comet and planetary atmospheres. Measurements were made at collision energies of 7 ; q keV (1.8Y3.5 keV amu
1), representative of the fast component of the solar wind. The HCIs are produced in a 14 GHz electron cyclotron resonance ion source. Absolute cross sections are measured using the retarding potential analyzer method, with knowledge of the target length, density, and projectile currents. Comparison is made with results of the classical trajectory Monte Carlo method, and the classical overbarrier model. Subject headingg s: atomic data — comets: general — solar wind

1. INTRODUCTION

2. EXPERIMENTAL METHODS

The charge-exchange (CE) collision of highly charged ions ( HCIs) with neutral atomic and molecular species is a fundamental astrophysical process that manifests itself through EUV and X-ray emissions that occur in, for example, solar wind collisions with the atmospheres of Venus (Gunnell et al. 2007) and Mars ( Dennerl 2006) and with cometary gases ( Bodewits & Hoekstra 2007; Bodewits et al. 2007; Lisse et al. 2005; Cravens 2002), and in energetic magnetospheric O+ and S+ ion precipitation into the Jovian upper atmosphere, followed by stripping and charge exchange (Bhardwaj et al. 2007; Branduardi-Raymont et al. 2007; Kharchenko et al. 2006). Charge exchange of lower charge state ions within the Enceladus Kronian toroidal atmosphere is the principal source of ionization (Johnson et al. 2006), and is also responsible for the production of energetic neutral atoms ( ENAs) through CE of fast N+ ions with the gases and plumes of Titan and Enceladus (Garnier et al. 2007; Hansen et al. 2006). The solar wind projectile ions and their relative abundances have been given by Schwadron & Cravens (2000). The molecular targets have been identified by, for example, Gibb et al. (2003), Biver et al. (2000), and especially in the Deep Impact studies of Comet 9P/Tempel 1 by Mumma et al. (2005), Biver et al. (2007), and Cochran et al. (2007). The comet compositions have been reviewed by Biver et al. (2002) and Huebner (2002). For simulating the effect of the solar wind HCIs on comet atmospheres ( Kharchenko et al. 2003), absolute single and multiple CE cross sections (and also X-ray emission spectra) have been presented for HCI collisions with molecular species such as those generated by comets as they approach the Sun (Greenwood et al. 2000a, 2000b, 2001; Cˇadezˇ et al. 2003; Mawhorter et al. 2007). Presented in this paper are absolute single and multiple CE cross sections for the species CH 4. These data provide needed experimental inputs for comet and planetary emission modeling codes, and also provide benchmarks for results of theoretical and semiempirical calculations. To this end, the experimental data are compared here with recent classical trajectory Monte Carlo (CTMC) calculations (Otranto et al. 2006), as well as to the classical overbarrier model (OBM; Ryufuku et al. 1980; Mann et al. 1981; Niehaus 1986).

The electron cyclotron resonance (ECR) ion source and the charge exchange (CE)/X-ray ion beam line at the Jet Propulsion Laboratory Highly Charged Ion Facility were used in the measurements. Details of the CE gas cell, retarding-potential geometry, system calibration, and errors have been reported previously (Cˇadezˇ et al. 2003; Greenwood et al. 2000a, 2000b, 2001; Mawhorter et al. 2007). In order to maximize the ion collection angle, measurements in Mawhorter et al. (2007) were taken with a large gas-cell exit aperture diameter (4.09 mm), consistent with maintaining adequate target density within the cell, as well as minimizing pressure gradients at the ion entrance and exit apertures to minimize CE outside the geometric limits of the cell, and hence to reduce the uncertainty in the effective path length of the cell. This uncertainty is one of the components in the quadrature error of the final cross section. As described in Cˇadezˇ et al. (2003) and Mawhorter et al. (2007), a number of tests of metastable content of the HCI beam were carried out. (1) The ECR plasma chamber was operated at low pressure (2 ; 10
5 Pa) and high pressure (7 ; 10
5 Pa), and measurements in the two modes compared to one another. Using a second beam line, the particular HCI was trapped in the Kingdon ion trap on the lifetime beam line. The emission of photons from any metastable level was monitored and, when present, was reduced or eliminated by adjustment of the ECR operating conditions of gas density, microwave power, extraction voltage, etc. (2) A quenching gas ( N2 or Ar) was introduced into a long section of the beam line between the 90 charge/mass selection magnet and the electrostatic ‘‘Y’’ deflector (Chutjian et al. 1999), and the metastable emission intensity monitored as a function of beam-line pressure. In both (1) and (2) one may adjust the ECR and beam-line gas pressures so as to reduce, below detection limit, the metastable-level photon emission from the HCIs trapped in the Kingdon ion trap. The tests provided both a means of detecting the HCI beam metastable content, and of reducing its presence to below the level that could affect the CE cross sections. The error analysis in the measurements is described in Cˇadezˇ et al. (2003). Error propagation through the measurements was performed by adding 2
statistical errors, taking into account the number of measurements for each ion-molecule pair, in quadrature with the errors in measuring the gas density, ion current ratios, absolute 1661

TABLE 1 Absolute Single and Multiple Charge Exchange Cross Sections for C(3,4,5,6 )+, N (4,5,6,7 )+, and O(5,6,7 )+ Ions Colliding with CH 4 Projectile Target, IP CH 4, 12.61 eV

C 3+

C4+

C5+

C6+

N4+

N 5+

N6+

N 7+

O5+

O6 +

O7+


q; q
1 ..........................
q; q
2 ..........................
q; q
3 ..........................

1.55  0.11 1.19  0.09 ...

3.53  0.26 0.96  0.07 0.41  0.05

5.13  0.38 0.65  0.05 0.12  0.02

5.94  0.44 1.42  0.11 ...

3.89  0.29 0.79  0.06 0.32  0.03

4.58  0.34 0.69  0.05 0.34  0.04

5.96  0.57 0.78  0.07 0.06  0.01

7.46  0.58 2.18  0.17 ...

5.08  0.38 1.07  0.08 0.20  0.02

4.95  0.38 1.56  0.12 0.23  0.02

7.70  0.57 1.11  0.08 0.10  0.01

Note.—For ion charge q, the HCI energies are 7.0 ; q keV, corresponding to velocities of the fast solar wind. The ionization potential (IP) for CH 4 is listed in the first column. Errors are given at the 95% (2
) confidence level, and cross sections are in units of 10
15 cm 2.

CNO-CH 4 COLLISIONAL CHARGE EXCHANGE

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Fig. 2.—Absolute double CE cross sections
q; q
2 (the sum of single capture and autoionizing multiple capture events) for highly charged ions colliding with CH 4. Squares show C q +, circles show N q +, and triangles show O q +.

Fig. 1.—Absolute single CE cross sections
q; q
1 (the sum of single capture and autoionizing double capture events) for highly charged ions colliding with CH 4 (Ip ¼ 12:61 eV ). Squares show C q +, circles show N q +, and triangles show O q +. Some data points have been slightly displaced to either side of the charge axis for clarity. Comparisons are given with results in the classical overbarrier model (solid line), and with the CTMC approach (long -dashed line, with an extrapolated portion, shown by the short-dashed line, from ion charge 4 to 3; Otranto et al. 2006). All values in Figs. 1Y 3 are listed in Table 1.

currents, ion beam instabilities, and the effective gas-cell collision length, including a correction for gas streaming through the entrance and exit apertures. 3. RESULTS AND DISCUSSION A variety of single and multiple CE cross sections have been measured for the projectiles C (3,4,5,6 )+, N (4,5,6,7 ) +, and O(5,6,7 ) + colliding with CH 4. Absolute single, double, and triple CE cross sections are listed in Table 1, together with the convoluted 95% (2
) confidence level errors for each target-projectile pair. The errors averaged to 8% (
q; q
1 ), 8% (
q; q
2 ), and 12% (
q; q
3 ). All results are plotted in Figures 1Y3; for the single exchanges, also shown are the calculated values from the CTMC approach and the semiempirical overbarrier model. The OBM can sometimes serve as a guide toward estimating magnitudes of single CE cross sections in terms of only the target’s ionization potential IP . For hydrogenic systems, the cross section is given terms of the ion charge q and the crossing radius Rc (Ryufuku et al. 1980), Rc ¼

q
1 ;
IP

q 2 =2n 2

one net electron captured. This channel contributes to the true single capture cross section, whereas the CTMC includes only the direct single-electron capture and hence provides a lower bound to the sum of all direct and indirect paths leading to
q; q
1 . (An additional difference lies in the assumption that there is only one active electron, and that the classical approach of the CTMC is adequate at these low collision energies.) For example, estimates by Mawhorter et al. (2007) using the extended overbarrier model (EOBM) for the case of O6 + + H2O show that about 30.5% of the total single-exchange cross section is due to double transfer followed by single autoionization. If one multiplies their total singleexchange cross section of (52:9  4) ; 10
16 cm 2 by (1
0.305), then the result of (36:8  3) ; 10
16 cm 2 agrees well with the direct single-capture cross section of (34  8) ; 10
16 cm2 measured by Bodewits & Hoekstra (2007). Double-transfer, single autoionization would also tend to raise the CTMC results for CH 4 (Fig. 1), although the percentage change could be slightly different than that estimated for H2O. Absolute multiple CE results
q; q
2 and
q; q
3 are shown in Figures 2 and 3, respectively. One sees that, for the limited number of targets and projectiles studied here, the ratio of single to double CE cross sections is about 5.0, and that of double to triple about 6.0, a trend also seen in the results of Mawhorter et al. (2007). Multiple CE can proceed through many-electron transfers followed by electron autoionizations to stabilize the projectile. Application of the EOBM (Niehaus 1987) in this case was not possible as it was for CO (Mawhorter et al. 2007), since there were no data available on multiple ionization cross sections for CH 4. Multiple transfers and autoionizations can lead to excited-state populations that can be quite different from those resulting from

ð1Þ

where n is the final principal quantum number. The CE cross section is just Rc2 . Referring to the single exchanges
q; q
1 given in Figure 1, one sees a trend of increasing cross section with q up to q ¼ 7, the highest level studied. The trend of increasing single-exchange cross section with increasing charge is also seen in the CTMC results, here and in other HCI-target partners ( Mawhorter et al. 2007). There is a consistent positive offset of all measured data above the CTMC by typically factors of 1.5Y2.5. This was also observed in earlier CTMC comparisons with HCI data (Mawhorter et al. 2007; Otranto et al. 2006). Differences may lie in the fact that present experiments and those of Mawhorter et al. include effects of autoionizing multiple captures to adjacent energy levels, with only

Fig. 3.—Absolute triple CE cross sections
q; q
3 (the sum of single capture and autoionizing multiple capture events) for highly charged ions colliding with CH 4. Squares show C q+, circles show Nq +, and triangles show O q +.

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DJURIC´ ET AL.

direct single capture. This in turns alters X-ray emission intensities and solar wind densities (Ali et al. 2005). Thus, the multipleexchange data are applicable in cases where high accuracy is required in calculating, for example, plasma ionization fractions, X-ray emissions in planetary and cometary systems, and solar wind abundances.

We thank R. Mawhorter for several helpful discussions. This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and was supported through agreement with the National Aeronautics and Space Administration. N. D. and J. S. are grateful for support through the National Research Council and the NASA Postdoctoral Program, respectively.

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