Collisional Energy Transfer from Vibrationally Excited Hydrogen Isocyanide

Abstract

Collisional deactivation of vibrationally excited hydrogen isocyanide (HNC) by inert gas atoms was characterized using nanosecond time-resolved Fourier transform infrared emission spectroscopy. HNC, with an average nascent internal energy of 25.9 ± 1.4 kcal mol −1 , was generated following the 193 nm photolysis of vinyl cyanide (CH 2 CHCN) and collisionally deactivated with the series of inert atomic gases: He, Ar, Kr, and Xe. Time-dependent IR emission allows simultaneous experimental observation of the ν 1 NH and ν 3 NC stretch emissions from vibrationally excited HNC. Subsequent spectral fit analysis enables direct determination of the average energy of HNC in each spectrum and therefore a measure of the average energy lost per collision, ⟨ΔE⟩, as a function of internal energy. Collisional deactivation of excited HNC is shown to be relatively efficient, exhibiting ⟨ΔE⟩ values more than an order of magnitude larger than comparably sized molecules at similar internal energies. Furthermore, the lighter inert gases are shown to be more efficient quenchers. Both observations can be qualitatively explained by the momentum gap law modeled through the repulsive force dominated vibration-to-translation energy transfer mechanism. The feasibility of efficient collisional deactivation as a contributing factor to the observed overabundance of astrophysical HNC is discussed.

Collisional Relaxation of Highly Vibrationally Excited Acetylene Mediated by the Vinylidene Isomer

“…It is important to note that the use of the SSH­(T) model here is simply to examine the relative trends of the energy transfer plots, in particular, the high-energy elbow. As has been demonstrated previously, compared to experiment, SSH­(T) theory often underpredicts ⟨Δ E ⟩ values by orders of magnitude. , Indeed, as compared to the 1–20 cm –1 measured values for acetylene (Figure ), the <1 cm –1 calculated ⟨Δ E ⟩ values (Figure a) are typical of SSH­(T) predictions. In Figure c, the weighted SSH­(T) energy transfer rate depicts a striking similarity to the enhanced energy transfer elbow as well as other features observed in the experimental data depicted in Figure .…”

“…Through decades of experimental and theoretical work, collisional relaxation of highly vibrationally excited molecules can now be well described, both in terms of the mechanisms and the magnitude of the energy transferred. − Beyond factors associated with the specific collider and the driving force for the energy transfer, a critical variable associated with the excited molecule that can significantly influence the energy transfer rate is the complexity of its potential energy surface (PES). In particular, the existence of low-lying excited electronic states or structural isomers on the PES has previously been shown to enhance the rate of energy transfer. ,,− …”

Collisional excitation of HNC by He found to be stronger than for structural isomer HCN in experiments at the low temperatures of interstellar space

“…In fact, no experiments of this kind have been performed on HNC, presumably owing to its instability in the laboratory making it impossible to obtain pure samples. While vibrational spectroscopy has been used to examine these isomers in collisional or photodissociative environments, 19,20 only recently has rotational spectroscopy become sensitive enough to allow for the detection of these species through transient experiments. 6,21 Here we describe an experiment to investigate the collisional properties of HCN and HNC with He at low temperatures, combining simultaneous in situ generation of HCN and HNC by laser photolysis in continuous uniform supersonic flows of cold He with time-resolved chirped pulse Fourier transform millimeter wave (CPFTmmW) spectroscopy.…”