Hyperfine-resolved rotational spectroscopy of HCNH⁺ Free

Protonated hydrogen cyanide (HCNH⁺) is a linear, closed-shell molecular ion that plays an important role in the chemistry of the interstellar medium (ISM), being the main precursor for the formation of neutral HCN and HNC.¹ HCNH⁺ has been extensively studied in both the laboratory and in space. In the laboratory, HCNH⁺ was investigated by rotationally resolved infrared spectroscopy,² followed by pure rotational spectroscopic studies spanning from the microwave³ to the sub-millimeterwave^4,5 spectral region. In space, HCNH⁺ has been detected across several interstellar regions based on its pure rotational fingerprints.^6–8 In the observations toward the Taurus molecular cloud (TMC-1), a dense and cold region in the ISM, Ziurys et al.⁷ observed the three ¹⁴N quadrupolar hyperfine components of the J = 1 → 0 transition around 74 GHz for the first time. A value for the quadrupole coupling constant eQq = −0.49(7) MHz was derived from these observations. Up to date, no hyperfine splittings could be resolved for any transitions of HCNH⁺ in the laboratory.

In this communication, we report the first hyperfine-resolved rotational spectrum of HCNH⁺ measured in the laboratory. Six transitions were recorded between 74 and 445 GHz using double-resonance spectroscopy. Hyperfine splittings due to the ¹⁴N quadrupolar nucleus were resolved for transitions up to J = 4 ← 3. The measurements presented here were carried out using the 4 K cryogenic ion trap instrument called COLTRAP.^9,10 The HCNH⁺ ions were created inside a storage ion source via electron impact ionization (⁠$Ee−$ = 50 eV) of methyl cyanide (CH₃CN) vapor, mass selected, and transferred to the cold ion trap. In the trap, the pure rotational transitions of HCNH⁺ were measured employing a double-resonance vibrational-rotational spectroscopic scheme. Trap-based rotational techniques have been thoroughly reviewed,¹¹ and the particular scheme applied here was recently developed^12,13 and has already been applied to molecular ions of astrophysical interest.^14,15

An example of a rotational transition (J = 3 ← 2) recorded for HCNH⁺ displaying partially resolved hyperfine splittings is shown in Fig. 1. While recording the rotational line, the wavenumber of the IR beam (red arrow in Fig. 1) is kept fixed on resonance with a rovibrational transition starting from a specific rotational level in the ground vibrational state. Then, millimeterwave radiation (blue arrow in Fig. 1) is used to excite a pure rotational transition starting or ending in the rotational quantum state probed by the IR laser, thus increasing or decreasing the signal counts. For the IR excitation, selected rovibrational transitions within the fundamental ν₂ C–H stretch band were used, and they were readily identified based on the previous report by Altman et al.² The transition in Fig. 1 as well as all other rotational transitions were recorded in several individual measurements, in which the millimeterwave frequency was scanned back and forth in a given frequency window in constant steps. The step size was fixed in each measurement and was typically 3–5 kHz. Care has been taken to lower the mm-wave power as much as possible to minimize power broadening effects. The baseline in Fig. 1 was normalized following a frequency-switching procedure, where the HCNH⁺ ion counts monitored in the frequency window of interest are divided by the HCNH⁺ counts at an off-resonance frequency position. Therefore, the baseline in the spectrum of Fig. 1 is close to unity.

Transition frequencies were determined by adjusting the parameters of an appropriate line function, typically a three-component Gaussian, in a least-squares procedure. In total, we measured six rotational transitions, with the first four exhibiting resolved or partially resolved hyperfine structures. Their frequencies and uncertainties given in Table I are obtained from the weighted averaging of all available measurements. To obtain the accurate spectroscopic parameters reported in Table II, a global fit of our observed lines and those at higher frequencies previously measured by Amano et al.⁵ (also shown in Table I) was carried out using a standard linear top Hamiltonian with a single quadrupolar nucleus as implemented in Western’s PGOPHER program.¹⁶ We also performed a similar fit using Pickett’s SPFIT/SPCAT program suite,¹⁷ and the obtained values for the spectroscopic parameters match well with those from PGOPHER in Table II within the error bars. The details of the SPFIT fit, along with spectral predictions from SPCAT, are provided as supplementary material. The spectroscopic parameters in Table II are considerably refined in this work and will certainly be useful for future astronomical observations. In particular, the eQq value is now improved and based on a terrestrial measurement. In addition, the nuclear spin-rotation interaction constant, C_I, is determined for the first time.

The PGOPHER and SPFIT/SPCAT fit files are available as supplementary material, as are figures of the four lowest rotational lines.

This work has been supported by an ERC advanced grant (Missions: 101020583) as well as by the Deutsche Forschungsgemeinschaft (DFG) via Collaborative Research Center 1601 (Project No. 500700252, sub-project C4) and “Schmid 514067452.” W.G.D.P.S. acknowledges the Alexander von Humboldt Foundation for support through a postdoctoral fellowship.

The authors have no conflicts to disclose.

Weslley G. D. P. Silva: Formal analysis (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (equal). Luis Bonah: Formal analysis (equal); Investigation (equal); Writing – review & editing (equal). Philipp C. Schmid: Writing – review & editing (equal). Stephan Schlemmer: Funding acquisition (equal); Project administration (equal). Oskar Asvany: Conceptualization (lead); Formal analysis (equal); Funding acquisition (supporting); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (lead); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.