X-ray-emission measurements following charge exchange between C6+and H2

Abstract

Lyman x-ray spectra following charge exchange between C 6+ and H 2 are presented for collision velocities between 400 and 2300 km/s (1-30 keV/amu). Spectra were measured by a microcalorimeter x-ray detector capable of fully resolving the C VI Lyman series emission lines though Lyman-δ.The ratios of the measured emission lines are sensitive to the angular momentum l-states populated during charge exchange and are used to gauge the effectiveness of different l-distribution models in predicting Lyman emission due to charge exchange. At low velocities, we observe that both single electron capture and double capture autoionization contribute to Lyman emission and that a statistical l-distribution best describes the measured line ratios. At higher velocities single electron capture dominates with the l-distribution peaked at the maximum l.Charge exchange (CX) between highly charged ions and atomic and molecular targets can exhibit large cross sections (10 −15 cm 2 ) at certain collision velocity regimes, making it one of the dominant processes in plasma environments. The resulting emission lines due to cascading captured electrons can provide temperature, density and relative abundance information about the interaction environment. In laboratory magnetic confinement plasmas, CX is a common diagnostic tool used in combination with puffed gas and neutral beams injection [1][2][3]. In astrophysical settings, the identification of x-ray emission from comets has been linked to CX between solar wind ions and ablated cometary neutral gases [4][5][6].These same solar wind ions interact with neutrals of planetary atmospheres and from the heliosphere. These applications have made CX modeling of paramount interest to both the laboratory plasma and astrophysics communities.A successful model of emission spectra due to CX relies on being able to map the specific distribution of principal, n, and angular momentum, l, quantum states in which the transferred electron is captured on the ion. Several successful tools have been developed to estimate the n-state of capture, however, the state-selective n, l CX cross sections are highly dependent on collision velocity and several models, of various effectiveness, have been put forth to estimate the l-distributions in CX over different collision velocity regimes.A number of previous theoretical and experimental studies helped to establish methods of determining CX cross sections and the principle quantum state n of capture for given interaction pairs and energies. In the regime where the collision velocity is approximately equal to the orbital velocity of the captured electron, the most successful tool has been the classical over-the-barrier model (CBM) [7]. At higher collision velocities, the classical trajectory Monte Carlo (CTMC) technique has shown to be successful [8]. At lower collision velocities, the CX process involves complex trajectories and interaction dynamics between many states and is best modeled by atomic orbital (or molecular orbital) close-coupling (AOCC, MOCC) methods [9...

A High Spectral Resolution Study of the Soft X-Ray Background with the X-Ray Quantum Calorimeter

“…As noted by a previous analysis of Mission 36.223, the Ly-γ/Ly-α ratio of H-like C VI and the G-ratio of He-like O VII can be used to constrain the allowed thermal contribution to the observed emission from these respective ions (Crowder et al 2012). Although the ratios for SWCX spectra depend on both collision velocity and electron donor (e.g hydrogen versus helium), upper limits can be used to conservatively estimate the minimum possible contribution from SWCX (Greenwood et al 2001;Koutroumpa 2007;Defay et al 2013;Fogle et al 2014). For an observed line ratio R obs and predicted ratios for thermal and SWCX emission, R therm and R swcx , the inferred fraction due to thermal emission, F therm , is given by Equation 1.…”

Line ratios for soft-x-ray emission following charge exchange betweenO8+and Kr

“…The sketch of the experimental setup can be seen in Fig. 1 and is identical to that used in other measurements [16][17][18].…”

“…X-ray spectra were measured using a X-ray quantum microcalorimeter (XQC) from the University of Wisconsin and Goddard Space Flight Center [17,24]. The sketch of the experimental setup can be seen in Fig.…”

“…X-ray spectra and line ratios measurements can be used to test our understanding of CX collision mechanisms. For example, our previous line ratio measurements [16,17] for C 6+ on He and H 2 collisions have been compared to recent theoretical line ratios constructed from calculations using the TC-BGM method. The satisfactory agreement with our line ratio measurements confirm the predictions that a statistical l distribution occurs at low velocities, while the l distribution shifts to maximum l at high velocities [10].…”

Radiance line ratios Ly-β/Ly-α, Ly-γ/Ly-α, Ly-δ/Ly-α, and Ly-ε/Ly-α for soft X-ray emissions following charge exchange between C6+ and Kr

“…Moreover, it is worthy to investigate if Kr might be a very good surrogate for H because its second and higher ionization potentials, which are respectively 24.4 and 37.0 eV, should be sufficiently high to limit double electron captures (DEC) and other multi-electron processes. From comparing the CX absolute cross section for C 6+ + H and the one for C 6+ + H 2 that were measured by Meyer et al [9], Fogle et al [10] recently concludes that double capture autoionization (DCAI), which most measurements cannot distinguish from SEC, might be a significant process in low energy CX with H 2 , limiting the possibility of using H 2 as a proxy for H at high energies only.…”