Accurate determination of low state rotational quantum numbers (J < 4) from planar-jet and liquid nitrogen cell absorption spectra of methane near 1.4 micron

Abstract

An improved procedure for accurate determination of empirical lower state rotational quantum numbers from molecular absorption spectra is demonstrated for methane. We combine the high resolution absorption spectra in the 7070-7300 cm(-1) frequency range obtained in liquid nitrogen cooled cryogenic cell (T = 81 K) and in supersonic planar jet expansion (T(R) = 25 K). Empirical lower state energies of 59 transitions are determined from the ratio of the absolute absorption line strengths at 25 and 81 K. The procedure relies on the realistic description of rotational state populations in the supersonic jet expansion where non-equilibrium nuclear spin isomer distributions are generated due to the rapid cooling. The accuracy of the experimental determination of the lower state energies with J < or = 3 is found to considerably improve the results of the same approach applied to spectra at 296 and 81 K. The 59 transitions with determined lower J values provide a good starting point for the theoretical interpretation of the highly congested icosad region of methane. In particular, the centres of nine vibrational bands are estimated from the transitions with J = 0 lower state rotational quantum number.

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81K: (II) The icosad region (1.49–1.30μm)

“…Yet, the lowest rotational transitions are the most valuable for the unambiguous determination of the vibrational band origins and to provide initial estimates of the spectroscopic constants. We have recently applied the two temperature method to our LNT intensities combined to absolute line intensities of methane cooled down to 25 K in a planar supersonic expansion [9]. The lower energy values of 59 transitions ranging between 7070 and 7300 cm -1 were determined.…”

“…[6] but according to the theoretical calculations of Ref. [9] an assignment to  1 +3 4 is possible. At RT, this band is partly obscured by the rotational structure of some stronger bands of the icosad centred at higher energy.…”

Observation of methane nuclear spin isomers in gas phase at low temperature

“…Firstly one could try to make the measurements at more than two temperatures. For example Votava et al [17] used the spectra of supersonic jet expansion (around 25 K) to improve the WKMC identification in the 7070-7300 cm À1 region. Another possibility is to perform the measurements on the same experimental setup (L a /L b ?…”

The high sensitivity absorption spectrum of ozone (18O3 and 16O3) near 7800cm−1: Identification of the 3A2(000)–X(110) hot band superimposed to very weak vibrational bands

“…We have extensively used this so-called 'two temperatures' method to derive the empirical lower energy levels of the methane transitions [34][35][36].…”