High Resolution Overtone Spectroscopy of HNC$^+$ and HCN$^+$

TL;DR

The paper reports high-resolution rotationally resolved overtone spectroscopy of HNC+ and HCN+ in the $6200-6800 cm^{-1}$ region using a cryogenic ion trap and two action schemes (LIR and LOS). It delivers band origins, spectroscopic constants, and radiative lifetimes for the observed states, and reveals strong vibronic coupling in HCN+. The work combines experimental measurements with VPT2/CCSD(T) band-origin predictions to guide assignments and supports future astronomical detection and benchmarking of quantum-chemical methods. It also demonstrates how action-spectroscopy can probe ion–molecule interactions and energy flow in cold environments, opening routes to study radiative processes and V-T transfer in simple molecular ions.

Abstract

Rotationally resolved spectra of the HNC$^+$ and HCN$^+$ molecular ions have been recorded in the spectral range between 6200 and 6800 \rcm\ using a cryogenic ion trap instrument. The rovibrational transitions were probed using two different action spectroscopy schemes, namely laser-induced reaction (LIR) and leak-out spectroscopy (LOS). Various vibrational bands of HNC$^+$ and HCN$^+$ were measured with high resolution for the first time. For HNC$^+$, the $\text{X}~^2Σ^+~(20^00)-(00^00)$ overtone band was recorded using LIR, while LOS was used to probe the $\text{X}~^2Π~(000)^1-(210)^0μ$ combination band and the $\text{X}~^2Π~(000)^1-\text{A}~^2Σ^+~(10^00)$ vibronic band of HCN$^+$. Spectroscopic constants, band origins and radiative lifetimes for the observed states have been determined. The effective fit for the HCN$^+$ spectra revealed the presence of strong vibrational couplings leading to perturbations of the rovibrational levels of the excited states. The two action spectroscopy schemes are compared and their potential use to explore ion-molecule interactions is discussed.

Figures (4)

• Figure 1: Schematic overview of the lowest vibronic states of HCN+ and HNC+. The vibrational energies and the zero point energies (ZPE) were taken from refs. Kraemer1992Taroni2001. Only a subset of vibrational levels assigned by Taroni2001 are plotted. The laser-induced reaction probing schemes involving proton transfer to CO for HNC+ and charge transfer in reaction with Kr for HCN+ are denoted by arrows. Potential energy minima and curve crossing for the X $^2\Pi$ and A $^2\Sigma^+$ states of HCN+ are not to scale to improve readability.
• Figure 2: Comparison of the measured central wavenumbers for transitions in the $(20^00)$ band of HNC+ (upper trace) and of the spectrum generated by PGOPHER Western2017 using the spectroscopic constants listed in Table \ref{['t_constants_HNCp']} (lower trace). The displayed experimental line intensities were normalized to the number of primary ions and power of the laser.
• Figure 3: Comparison of the measured central wavenumbers for transitions of the bands X $^2\Pi$$(210)^0\mu$ and A $^2\Sigma^+~(10^00)$ of HCN+ (upper trace) and of the spectrum simulated by PGOPHER Western2017 using the spectroscopic constants in Table \ref{['t_constants_HCNp']} (lower trace). The displayed experimental intensities were normalized to the number of primary ions and power of the laser. LOS intensity is scaled 3 times to match the LIR intensity on the same transitions. The inset is showing a section of the Q branch rovibronic transitions, the red line depicts the fitted function determining the transition centers (sticks). Note that not the entire area has been continuously scanned (see text).
• Figure 4: Absorption line profiles for the unresolved transitions $^\mathrm{r}\mathrm{R}_{11}(4.5)$/$^\mathrm{r}\mathrm{R}_{22}(3.5)$ of the X $^2\Sigma^+~(00^00)-(20^00)$ band of HNC+ obtained by LIR and LOS techniques. In both cases, the constant background signal was subtracted to enable comparison of line intensities. The data is normalized to the number of primary ions. All the remaining parameters (laser power, irradiation period, temperature, rf amplitude, etc.) were kept constant.