A novel feature of intramolecular vibrational redistribution in propargyl alcohol and propargyl amine

Intramolecular vibrational redistribution (IVR) from the terminal acetylene mode $νHC$ has been studied for four molecules: $H–CC–CH3$ (propyne), $H–CC–CH2Cl$ (propargyl chloride), $H–CC–CH2OH$ (propargyl alcohol), and $H–CC–CH2NH2$ (propargyl amine). The experiments were performed with the room-temperature gases. The transition $∣0⟩→∣1⟩$ in the mode $νHC$ was pumped by a short laser pulse. Anti-Stokes spontaneous Raman scattering was used as a probe. The measured parameters were the de-excitation rate $W$ and the dilution factor $σ$ defined as the relative level of the residual energy in the $νHC$ mode at long pump-probe delay times. The pair of these values ${W,σ}$ allowed us to determine the density $ρeff$ of those vibrational-rotational states, which are involved in IVR from state ∣1⟩. For two molecules, $HCCCH3$ and $HCCCH2Cl$⁠, the experimental results were consistent with the suggestion that all close vibrational-rotational states with the same total angular momentum $J$ and symmetry participate in the IVR regardless of the other rotator quantum number $K$ (in the case of $HCCCH3$⁠) or $Ka$ (in the case of $HCCCH2Cl$⁠) and the vibrational quantum numbers as well. For the other two molecules, $HCCCH2OH$ and $HCCCH2NH2$⁠, this effect was also present, yet the experimental results revealed certain restrictions. We have obtained a satisfactory theoretical fit with the assumption that the low-frequency torsion vibration of the hydrogen atom in the hydroxyl group (in the case of $HCCCH2OH$⁠) or hydrogen atoms in the amine group (in the case of $HCCCH2NH2$⁠) does not participate in the IVR. This assumption can be treated as a challenge to future studies of these molecules by high-resolution spectroscopy and various double-resonance and pump-probe techniques.