Fourier-transform infrared emission spectroscopy of Si II

Introduction:
Precise atomic data serve as an indispensable diagnostic probe across a wide range of studies, including atomic physics, astrophysics, cosmology, and industrial plasma physics. Specifically, such data for singly ionised silicon (Si II) in the IR region are crucial for determining the elemental abundances in hot A- and B-type stars, and to probe cooler gas components that do not emit strongly at visible wavelengths. The currently available data of Si II in the NIST Atomic Spectra Database (ASD) lack the desired accuracy and precision; hence, their applications are limited. This work aimed to procure a more accurate and at least ten times more precise spectral data for Si II in the near-IR to mid-IR region using Fourier transform spectroscopy (FTS).

Methods:
To enhance the quality of existing spectral data, we have conducted a comprehensive analysis of 8 high-resolution Si spectrograms, covering a broad IR wavenumber region of 7850–54000 Å (1852–12737 cm⁻¹), recorded on a 1 m FT spectrometer at KPNO, AZ, USA. These spectrograms were meticulously analysed using the XGREMLIN software package to determine various line parameters precisely. A rigorous wavenumber calibration was performed for each spectrogram using well-known low-excitation lines of the buffer gases (Ne I, Ar I-II) and molecular CO.

Results and Discussion:
In this work, we report 37 unique Si II lines in the IR region, whose accurately determined measurements were obtained from different spectrograms recorded under different experimental conditions/sources. A direct comparison of our data with the NIST ASD shows that our measurements offer at least a ten-fold improvement in accuracy.

Conclusion:
The present work on spectral data of Si II in the IR region provides significantly improved precision and reliability for astrophysical and laboratory applications.