INTERACTIONS IN SYMMETRIC TOP MOLECULES BETWEEN VIBRATIONAL POLYADS: ROTATIONAL AND ROVIBRATIONAL SPECTROSCOPY OF LOW-LYING STATES OF PROPYNE

Abstract

$^{a}$P. Pracna, G. Graner, J. Cosl\'eou, J. Demaison, G. Wlodarczak, V.-M. Horneman, and M. Koivusaari, J. Mol. Spectrosc. 206, (2001) 150-157; H. S. P. M\""uller, P. Pracna, et al., unpublished $^{b}$H. S. P. M\""uller, P. Pracna, and V.-M. Horneman, J. Mol. Spectrosc. 216, (2002) 397-407Author Institution: I. Physikalisches Institut, Universit\""at zu K\""oln; I. Physikalisches Institut, Academy of Sciences of the Czech Republic; I. Physikalisches Institut, 90014 University of Oulu; I. Physikalisches Institut, Justus-Liebig-Universit\""atA large body of very accurate (mostly 10-20 kHz) rotational transitions within the $10 {\mu}m$ tetrad of the strongly prolate symmetric rotor propyne, $H_{3}C-C\equiv CH$, had been recorded in order to analyze some weaker $resonances.^{a}$ The analysis, in particular for higher K transitions, indicated new problems: Interactions of members of the $10 {\mu}m$ tetrad ($v_{5} = 1, v_{9} = v_{10} = 1, v_{10} = 3$, and $v_{8} = 1$) with those of the $15 {\mu}m$ dyad ($v_{9} = 1$ and $v_{10} = 2$). More than 300 rotational transitions within $v_{10} = 1 (E_{vib} = 330.9 cm^{-1})$ have been recorded with $J^{\prime\prime} \leq 53$ and $-11 \leq K \cdot l_{10} \leq +16$ for a more systematic investigation into the lower excited vibrational states of propyne. Even the inclusion of many high order terms prevented the highest K transitions to be fit within experimental uncertainties, in particular those having $K \cdot l_{10} \leq 0$. Inclusion of $\nu_{10}$ IR transitions from a reanalysis of the spectrum that was used in the study of the $10 {\mu}m$ $tetrad^{a}$ confirmed these findings. It turned out that the first order energies of the $K \cdot l_{10} = -12$ levels coincide to within $1 cm^{-1}$ with those of the overtone $v_{10} = 2, K = 12$ of the $l_{10} = +2$ substate. A weaker Coriolis interaction occurs between $v_{10} = 1, K \cdot l_{10} = -11$ and $v_{9} = 1, K \cdot l_{9} = +10.^{b}$ Meanwhile, the investigation of $v_{10} = 1$ has been completed, and those of $v_{9} = 1$ and $v_{10} = 2$ have begun. Fermi-type interactions occur between the states $v_{10} = 2$ and 3 and between $v_{9} = 1$ and $v_{9} = v_{10} = 1$. Among the Coriolis resonances, the one between $v_{10} = 2, l_{10} = -2$ and $v_{5} = 1$ permitted intervibrational transitions to be detected in the submillimeter region. These interactions are expected to improve $A, D_{K}$, etc. for $v = 0$. Selected details of our ongoing analyses will be presented. In addition, the relevance of these types of interactions for other symmetric top molecules will be discussed