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Branching Ratios and Atomic Transition Probabilities of Fe II in the Vacuum Ultraviolet using Fourier Transform Spectroscopy

Published onJun 01, 2020
Branching Ratios and Atomic Transition Probabilities of Fe II in the Vacuum Ultraviolet using Fourier Transform Spectroscopy

The relatively high abundance of Fe II and its dense spectrum makes Fe II atomic reference data essential for interpreting the spectra of a wide variety of astrophysical objects. There are over 13,000 measured spectral lines in Fe II, of which roughly 290 have measured oscillator strengths. Most of these are in the visible and near-UV regions, but in the Vacuum Ultraviolet region, only twenty-one lines of the 3,000 have measured oscillator strengths. This limits the interpretation of spectra from STIS on the HST, where Fe II lines frequently obscure lines of other elements. To address this shortcoming, we are using the high precision Fourier transform spectroscopy facilities at the National Institute for Standards and Technology (NIST) to measure the relative intensities of Fe II lines in the region from roughly 155 nm to 350 nm. The spectrum is excited in a high current hollow cathode lamp, and radiometrically calibrated using deuterium standard lamps. These relative intensities are used to determine the branching fractions for our observed Fe II lines. We then combine these branching fractions with a combination of theoretical and experimental energy level lifetimes to report oscillator strengths. We will be presenting preliminary values and our estimated uncertainties for a select subset of Fe II lines. We will also present comparisons between our early results and the previously available reference data.

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