Experimental and theoretical state-selective X-ray spectra resulting from single-electron capture in charge exchange (CX) collisions of Ne^10+ with He, Ne, and Ar are presented for a collision velocity of 933 km s^-1 (4.54 keV nucleon^-1), comparable to the highest velocity components of the fast solar wind. The experimental spectra were obtained by detecting scattered projectiles, target recoil ions, and X-rays in coincidence; with simultaneous determination of the recoil ion momenta. Use and interpretation of these spectra are free from the complications of non-coincident total X-ray measurements that do not differentiate between the primary reaction channels. The spectra offer the opportunity to test critically the ability of CX theories to describe such interactions at the quantum orbital angular momentum level of the final projectile ion. To this end, new classical trajectory Monte Carlo calculations are compared here with the measurements. The current work demonstrates that modeling of cometary, heliospheric, planetary, and laboratory X-ray emission based on approximate state-selective CX models may result in erroneous conclusions and deductions of relevant parameters.
X-ray and extreme ultraviolet (EUV) emission has been detected from more than 20 comets since the first observation by Lisse et al. (1996). The charge exchange (CX) mechanism between highly charged solar wind (SW) minor heavy ions and cometary neutrals suggested by Cravens (1997) is now recognized as the primary process responsible for the observed emission (see, e.g., Lisse et al. 2001;Krasnopolsky & Mumma 2001;Krasnopolsky et al. 2002;Beiersdorfer et al. 2003;Kharchenko et al. 2003;Willingale et al. 2006;Bodewits et al. 2007;Lisse et al. 2007, and references therein). In the SWCX mechanism, electrons are captured from cometary neutrals by SW ions into excited states of the product ions, which may then decay radiatively and in the process emit X-ray radiation. The SWCX mechanism has been invoked with various degrees of sophistication to model and interpret cometary X-ray and EUV emission spectra (Häberli et al. 1997;Wegmann et al. 1998;Schwadron & Cravens 2000;Kharchenko et al. 2003;Otranto et al. 2007) and has been the subject of numerous reviews (Cravens 2002;Krasnopolsky et al. 2004;Bhardwaj et al. 2007;Dennerl 2008). It has been argued that cometary X-rays represent a potential tool to monitor not only cometary activity, but also the composition, velocity, and flux of the SW in regions that spacecraft cannot reach (Cravens 1997;Dennerl et al. 1997;Schwadron & Cravens 2000;Beiersdorfer et al. 2001).
It is now also recognized that heliospheric Xray emission due to SWCX with H and He interstellar neutrals (see, e.g., Cox 1998;Cravens 2000;Pepino et al. 2004;Robertson et al. 2009, and references therein), and X-ray generation throughout the terrestrial magnetosheath due to SWCX with geocoronal neutrals (see, e.g., Dennerl et al. 1997;Cox 1998;Cravens et al. 2009, and references therein) contribute to the soft X-ray background (SXRB). SWCX with H and O has also been proposed to account for the first definite detection of X-ray emission from the exosphere of Mars (Dennerl et al. 2006).
Understanding and accurately predicting these and related phenomena require novel experiments which simultaneously measure detailed (i.e., charge and quantum state-resolved) collision parameters in coincidence with consequent atomic energy de-excitation to elucidate the underlying chain of mechanisms in this CX induced X-ray emission, and ultimately, development of theoretical methods capable of broadly treating such interactions.
To this end, several experimental groups have carried out laboratory studies of relevant collision systems (see, e.g., Beiersdorfer et al. 2000Beiersdorfer et al. , 2001Beiersdorfer et al. , 2003;;Greenwood et al. 2001;Hasan et al. 2001;Gao & Kwong 2004;Ali et al. 2005;Bodewits et al. 2006;Mawhorter et al. 2007;Allen et al. 2008;Djurić et al. 2008, and references therein). Of particular importance for accurate modeling is the ability to predict the nℓ-state-selective CX cross sections (i.e., to account for the distributions of the principal n and angular momentum ℓ quantum numbers of the product projectile ions). All previous modeling attempts to simulate cometary or heliospheric X-ray spectra (Häberli et al. 1997;Wegmann et al. 1998;Rigazio et al. 2002;Beiersdorfer et al. 2003;Kharchenko et al. 2003;Otranto et al. 2007;Otranto & Olson 2008) have adopted simple nℓ empirical relations, scalings from related collision systems, or fits to laboratory non-coincident total X-ray spectra. It should be noted, however, that non-coincident laboratory spectra contain contributions from a variety of reaction channels such as single electron capture (SEC) and autoionizing and nonautoionizing multiple-electron capture (MEC). A superposition of several reaction channels is also likely to occur in cometary, planetary, and heliospheric spectra. Therefore, a technique which is capable of differentiating between the primary reaction channels is required for the interpretation of such spectra.
In this letter, we report an experimental investigation of the n-state-selective hydrogen-like ion X-ray spectra following SEC in collisions of Ne 10+ with He, Ne, and Ar neutral targets at a laboratory frame collision velocity v of 933 km s -1 (4.54 keV nucleon -1 ). This velocity is at the upper end of the SW ion velocities. The present interactions are close analogs of the interactions of heavy minor, multiply charged, SW ions with cometary, planetary, and heliospheric neutrals. Specifically, the dominantly molecular constituents of cometary and planetary atmospheres (e.g., H 2 O, CO 2 ) are simulated by gases of similar ionization potential and multielectron character (i.e., Ar, Ne). Helium, being the second most abundant (15%) interstellar neutral (Koutroumpa et al. 2009), is of direct relevance to heliospheric X-ray emission. The present spectra are free from complications arising from the inability of previously employed non-coincident total X-ray spectra to differentiate between the primary reaction channels. Consequen
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