Radiative Transition Probabilities and Photoionization Cross Sections for Cr I

We have investigated radiative lifetimes, transition probabilities, and photoionization cross sections of neutral chromium from the ground and excited states in the low-energy region from the first ionization threshold at 6.77 eV to 30 eV. Transition probabilities and partial photoionization cross sections of iron-peak elements are important for the determination of abundances in the late-type stars and nebular objects. Accurate descriptions of the initial bound states of Cr I and the final residual Cr II ionic states have been obtained in the multiconfiguration Hartree-Fock method together with adjustable configuration expansions and term-dependent non-orthogonal orbitals. The B-spline R-matrix method has been used for the calculation of total and partial photoionization cross sections. The 194 LS final ionic states of Cr II 3d⁴4s, 3d³4s², 3d⁵, 3d⁴4p, and 3d³4s4p principal configurations have been included in the close-coupling expansion. The inclusion of all terms of these configurations has significant impact on the near-threshold resonance structures. Total photoionization cross sections from the ground 3d⁵4s a⁷S and excited 3d⁵4s a⁵S, 3d⁴4s² a⁵D, 3d⁵4p z⁵P, and 3d⁴4s4p y⁵P states of Cr I have been compared with other available R-matrix calculation to estimate the likely uncertainties in photoionization cross sections. Our calculations clearly demonstrate the importance of Rydberg series of resonances converging to the higher excited states not considered in previous R-matrix calculation. We analyzed the partial photoionization cross sections for leaving the residual ion in various states to identify the important scattering channels, and noted that 3d electron ionization channel becomes dominant at higher energies. The present transition probabilities for transitions in Cr I have been compared with measured values determined from Fourier transform spectra combined with time-resolved laser-induced fluorescence lifetimes measurements. There is a good agreement between the calculated and measured values for many transitions indicating accurate bound states description in our calculations.