An Experiment in Electronic Spectroscopy: Information Enhancement Using Second Derivative Analysis

Abstract

Second derivative (SD) analysis is extremely effective in enhancing and extracting information from experimental data where the information sought lies hidden. This paper describes an interesting experiment in molecular electronic spectroscopy in which information about the excited state vibrational levels of a molecule, contained in the absorption spectrum in latent form, is extracted by the SD analysis. The efficacy of the SD method is first demonstrated using synthetic data where the positions of hardly-noticeable gaussian features, buried obscurely in a strong, rising background, are recovered quantitatively. The method is then applied to the absorption spectrum of trimethylamine (TMA), which contains two absorption bands (arising from the nN --> 3sN and nN —> 3pN transitions) that show subtle undulations due to the presence of underlying vibronic features corresponding to the out-of-plane bending motion of the molecule. The positions of more than 30 vibrational features are determined using the SD analysis. The constancy of the spacing between adjacent features in each absorption band in the spectrum suggests that the excited state out-of-plane bending potential of TMA is considerably harmonic.