Construction and Analysis of a Microwave-induced Plasma Lamp for Precision Spectroscopy

Abstract

Spectroscopy is one of the most powerful techniques to analyze light of astronomical objects. New instrumentation strives to extend high-precision spectroscopy from the optical to the near-infrared spectral range. This development is driven by research on cool low-mass stars and the search for low-mass extrasolar planets with the radial velocity technique. A crucial requirement for precise spectroscopic measurements is the wavelength calibration that maps the pixels of a spectrograph’s detector to the corresponding wavelengths. In the optical, hollow cathode lamps and gas absorption cells are established wavelength calibrators, but these techniques cannot be readily transferred to the near-infrared because they do not provide enough spectral lines over a broad spectral range. Many spectral lines in the near-infrared are present in discharge spectra of molecules, such as nitrogen or CN. In this thesis, I investigate whether discharge spectra of these two molecules are potential wavelength references for astrophysical spectrographs. An experimental setup with a microwave-induced plasma lamp has been constructed. Characteristics of this lamp are the electrodeless design and the relatively inexpensive equipment. The plasma is sustained within a sealed glass tube so that measurements can be repeated with the same gas mixture and gas pressure. In addition, the sealed cell allows for a compact setup with no requirements for gas supply during operation. Spectra of different gas discharges, using two sealed gas cells, are recorded with a high-resolution Fourier transform spectrometer. One cell is filled with pure nitrogen gas, while the second cell is filled with a gas mixture producing emission from CN molecules. In the context of wavelength calibration, the analysis of the spectra focuses on line density, relative line intensities, wavelength stability and aging behavior. The nitrogen discharge provides a spectrum with densely-spaced emission lines over the whole spectral range 4500-11000 cm^-1 (0.9-2.2 µm). In the spectrum of the second discharge cell, about 4500 lines of CN and about 26000 lines of molecular nitrogen are detected (4500-10000 cm^-1). Both species combined provide about four times more lines as uranium from hollow cathode lamps in this spectral range. The wavelength stability of the spectra is measured with a precision of about 1 m/s over 24 hours. These properties make the microwave-induced plasma lamp an interesting candidate for wavelength calibration of future high-resolution spectrographs. However, the number of spectral lines usable for calibration will be reduced due to inhomogeneities regarding line spacing and line intensities, depending on the characteristics of a spectrograph (e.g., resolution and detector response). The demonstrated operational time of a single gas cell is about 180 hours, which is about 4-6 times shorter than typical lifetimes of hollow cathode lamps. Application of the microwave-induced plasma lamp for astrophysics is not limited to the task of wavelength calibration. An example is the laboratory study of molecular spectra which can be used to analyze magnetic fields in cool stars. First measurements with the discharge lamp operated in a magnetic field are presented. A magnetic flux density of B = (0.130 +/- 0.003) T is determined using the Zeeman-splitting of three argon lines. Measurements of molecular species, such as CN, in a calibrated field could be used in the future to identify magnetically-sensitive lines and to determine Landé factors.