OH-stretch overtone of methanol: empirical assignment using a two temperature technique in a supersonic jet

Abstract

This paper describes a novel approach for empirical lower state assignments in complex high resolution ro-vibrational overtone spectra of molecules with low rotational constants and complex intramolecular dynamics. Methanol, CH3OH, was chosen as a representative of such molecules - it is an asymmetric top with two non-hydrogen nuclei and hindered internal rotation leading to dense and disordered rotational structure of vibrational overtone bands. We report the first rotationally resolved methanol spectra of the OH-stretch overtone 2ν1 band using sub-Doppler diode laser spectroscopy in a supersonic jet, and describe how the combination of two temperature analysis (TTA) and analysis by ground state combination differences (GSCDs) is used to reliably identify spectral lines that originate from lowest rotational states. In the first step of the analysis, the TTA was utilized to obtain a set of possible rotational assignments for each spectral line using the line intensity variation between two different temperatures in the supersonic jet (13, and 56 K respectively). Thereafter, the GSCDs were used to confirm specific lower state assignment for those spectral lines that have been identified to have low rotational ground states by the TTA. We show that the TTA pre-selection leads to fast and reliable confirmation by GSCDs and avoids false assignments due to accidental GSCD matches. The procedure yields an important subset of reliably assigned spectral lines in the complex ro-vibrational structure that provides a convenient starting point for subsequent application of traditional spectral analysis techniques.

Assignment of the methanol OH-stretch overtone spectrum using the pattern recognition method

“…Even at low temperatures ( T rot = 26 K), the assignment of such complicated spectrum appeared to be quite challenging. 21,22 To tackle this difficulty, the technique suggested by Rakvoský 22 was used, consisting in the detection of patterns in the position of the molecular transitions. Due to the large number of pattern detections, intensity information was included in the construction of the reference patterns.…”

“…In the process known as beam injection, 31 a small flow of methanol was introduced ballistically inside the cold cell by passing it through a hole in its lid, using helium as a carrier gas. The rotational temperature was evaluated using a two-temperature analysis, 21,28,32 utilising the twelve transitions that were unambiguously assigned by Svoboda et al 21 as a reference. The rotational temperature was estimated to be 26 AE 12 K. The translational temperature of methanol was evaluated to 28 AE 4 K by assuming that the only contribution to the line broadening of these twelve methanol transitions is the Doppler effect.…”

“…2OH stretching mode, at the center of this work has already been studied at high resolution by the group of Ondrej Votava. 21,22 Experimentally, their spectrum was recorded using a pulsed slit expansion and double-pass laser diode absorption spectroscopy. The analysis of this spectral signature represents a challenge in itself with on the order of one thousand transitions over 60 cm À1 .…”

“…This band represents a formidable playground to test original approaches for spectroscopic assignment without the use of any modeling of the excited vibrational state. The group of Ondrej Votava followed this track and used first a two-temperature analysis (TTA), leading to the assignment of 15 transitions 21 and then developed a generalization of the ground state combination differences technique, i.e. the pattern recognition method.…”