From raw data to processed spectra: A step-by-step guide

Abstract

Optical spectroscopy is an important and widely used technique, for instance, to characterize new materials and to identify unknown compounds. Spectra are typically reported as a function of the wavelength of light, yet the information extracted from such spectra can be misleading. In contrast, spectra represented as a function of the frequency (or photon energy) allow for a more direct extraction of the intrinsic quantum-mechanical properties of the materials under investigation. Here we discuss this conversion for absorption, fluorescence and fluorescence excitation spectra. We show step-by-step the different factors that lead to a rescaling of the measured absorption and fluorescence signals. This paper will assist instructors who aim at developing an (under-)graduate lab to introduce into the methodology and terminology of spectroscopic experiments and to provide clear, step-by-step guidelines for data analysis and representation.

Figures (5)

• Figure 1: (Color online) A schematic of a spectroscopic experiment. The output of a white-light source ('bulb') is filtered by a monochromator with a slit to select light of a specific wavelength interval (blue wavy arrow). This light induces transitions between energy levels of a molecule (for details, see text). The outgoing light (green/red wavy arrow) represents either transmitted light for absorption spectroscopy or emitted light for fluorescence spectroscopy. This light is recorded as a function of its wavelength using a combination of a spectrograph and a detector (Det.).
• Figure 2: (Color online) Quantum efficiency (black, solid) and spectral responsivity (red, dashed) for a scientific CMOS camera. Data taken from Ref.Zyla
• Figure 3: Schematics of spectroscopic techniques. a) Absorption spectroscopy. b) Fluorescence spectroscopy. c) Fluorescence excitation spectroscopy. Light paths are indicated with solid arrows for incident light, dashed arrows for detected light, and dotted arrows for light from a reference measurement. The double-headed arrows above $\lambda$ indicate that it is a function of the (excitation or fluorescence) wavelength; the crossed line indicates that the signal is taken at a fixed wavelength (interval). For further details see text.
• Figure 4: (Color online) Fluorescence (left) and absorption (right) spectra of the POPOP molecule (see right inset for the chemical structure) with successive application of all corrections discussed in this work. For the fully corrected spectra (solid), the y-axis represents a normalized dipole strength. The raw spectra (dashed) have been taken from Ref. Taniguchi2017
• Figure 5: Step-by-step conversion of the data file for the absorption spectrum of the POPOP molecule. In c) we have also added a column with the values of the refractive index $n(\Tilde{\nu_i})$ of the solvent.Kozma2005 For details see text.