Empirical Low State Energies from Temperature Analysis of Cooled Cell and Supersonic Jet Spectra

In this paper we presented a method for determination the lower state energies, and hence the rotational quantum number, in the overtone spectra of methane (the Icosad polyad, between 7076 to 7291 cm). The method explores the temperature dependence of rovibrational transition intensities. In the first part of this report this approach is used with spectra measured at a) 81 K in liquid nitrogen cooled cell [Kassi et al., 2008] and b) 25 K in supersonic jet expansion. In the second part we demonstrate how the temperature-dependent line intensities can be directly extracted from the pulsed supersonic expansion experiment. Introduction Highly vibrationally excited polyatomic molecules play an important role in a wide range of phenomena, including the plasma chemistry. Information about those high vibrational states is important in industrial applications, astrophysical applications and also as benchmarks for quantum calculations. Theoretical prediction of such spectra is complicated because normal vibrational modes are no longer independent due to mutual interactions and experimental measurements are also difficult due to weak oscillator strength in the overtone region. As a result the high vibrational states are in general not well understood even for small molecules. Methane serves as a prototypical polyatomic molecule and plays an important role in a wide range of systems such as terrestrial and planetary atmospheres, combustion and others. From the fundamental point of view it serves as a prototypical molecule for which the theoretical calculations could be tested. Despite its small size and high symmetry the vibrational spectroscopy is surprisingly complicated. Methane molecule has four fundamental vibrational modes, stretching modes ν1 and ν3 and bending modes ν2 and ν4. Due to approximate resonance condition ν1 ~ ν3 ~ 2ν2 ~ 2ν4 the vibrational states group into polyads marked with polyad number n (n = 2ν1+2ν3+ν2+ν4). Due to the symmetry of the system each vibrational band splits into several sublevels. The number of levels (labelled N(n)) and sublevels rises steeply with the polyad number n: for n = 1 having 2 levels and 2 sublevels while for n = 4 it forms 14 levels and 60 sublevels [Boudon et al., 2006]. Polyads are often named in the form “N(n)-ad” with N(n) is expressed through the corresponding Greek prefix. Lower energy polyads up to Tetradecad region (6300 cm) have been mostly analysed in past years [Albert et al., 2009]. However in the Icosad region (6400 7700cm, n = 5, 20 levels and 134 sublevels) only some localised structures of bands have been assigned so far [Hippler & Quack, 2002]. Best theoretical predictions in this region have been made by [Wang & Carrington, 2003] but even this work can not be directly used for assignment of the vibrational modes. At this situation new experimental methods that would guide assignments are needed. Simplification of measured rotational-vibrational spectra is achieved when spectra are measured at low temperatures. When the molecules are cooled, only low energy states are populated and with transitions only from these states the spectra are less complicated. It has been furthermore shown that comparing spectra at different temperatures can be used for determination of lower state energies of transitions [Sciamma-O'Brien et al., 2009] [Kassi et al., 2008]. 243 WDS'09 Proceedings of Contributed Papers, Part II, 243–249, 2009. ISBN 978-80-7378-102-6 © MATFYZPRESS