Collisional excitation and photoexcitation of Ca IV including a strong 3.2 $\upmu$m emission line

Abstract

Graphic Abstract

We report a detailed study of features of electron impact excitation (EIE) of Ca IV (Ca IV + e $\rightarrow$ Ca IV* + $e' \rightarrow$ Ca IV + h$\nu + e'$), for the first time using the relativistic Breit–Pauli R-matrix method with a large close-coupling wave function expansion of 54 fine structure levels belonging to $n=2,3,4$ complexes. Calcium lines in the infrared (IR) region are expected to be observed by the high-resolution James Webb Space Telescope. Our study predicts the presence of a strong 3.2 $\upmu$m emission line in IR formed due to EIE of $3p^{5~ 2}P^o_{3/2}-3p^{5~2}P^o_{1/2}$ in Ca IV. The EIE collision strength ($\Omega$) for the transition shows extensive resonance with enhanced background (top panel, Figure below), resulting in an effective collision strength ($\Upsilon$) of 2.2 at about 10$^4$ K that increases to 9.66 around 3$\times 10^5$ k (lower panel, Figure below). The present results include $\Omega$ for all 1431 excitations among the 54 levels and $\Upsilon$ for a limited number of transitions of possible interest. We found extensive resonances in the low-energy region of $\Omega$, and convergence of the resonances and of the partial waves with the 54-level wave function. At high energy, $\Omega$ decreases beyond the resonance region for forbidden transitions and is almost constant or decreases slowly for dipole-allowed transitions with low oscillation strength (f-values) and increases with Coulomb–Bethe behavior of ln(E) to almost a plateau for transitions with high f-values. The wave function of Ca IV was obtained from optimization of 13 configurations $3s^23p^5, 3s3p^6, 3s^23p^43d, 3s^23p^44s, 3s^23p^44p, 3s^23p^44d, 3s^23p^44f, 3s^23p^45s, 3s3p^53d, 3s3p^54s, 3s3p^54p, 3p^63d, 3s3p^43d^2$, each with the core configuration of $1s^22s^22p^6$, using the SUPERSTRUCTURE atomic structure program. They produce 387 fine structure levels. We report transition parameters—oscillator strength, line strength (S) and A-values-for a total of 93,296 electric dipole (E1), quadrupole (E2), octupole (E3), magnetic dipole (M1) and quadrupole (M2) transitions among these levels. The lifetimes of these levels are also presented.

Graphical abstract