Mid- and Far-IR Spectra of H₅⁺ and D₅⁺ Compared to the Predictions of Anharmonic Theory

Abstract

H5(+) is the smallest proton-bound dimer. As such, its potential energy surface and spectroscopy are highly complex, with extreme anharmonicity and vibrational state mixing; this system provides an important benchmark for modern theoretical methods. Unfortunately, previous measurements covered only the higher-frequency region of the infrared spectrum. Here, spectra for H5(+) and D5(+) are extended to the mid- and far-IR, where the fundamental of the proton stretch and its combinations with other low-frequency vibrations are expected. Ions in a supersonic molecular beam are mass-selected and studied with multiple-photon dissociation spectroscopy using the FELIX free electron laser. A transition at 379 cm(-1) is assigned tentatively to the fundamental of the proton stretch of H5(+), and bands throughout the 300-2200 cm(-1) region are assigned to combinations of this mode with bending and torsional vibrations. Coupled vibrational calculations, using ab initio potential and dipole moment surfaces, account for the highly anharmonic nature of these complexes.

Rotation/Torsion Coupling in H5+, D5+, H4D+, and HD4+ Using Diffusion Monte Carlo

“…We also compare our results to those obtained from multiconfigurational time-dependent Hartree (MCTDH) calculations, 31 vibrational configuration interaction (VCI) calculations obtained using the reaction path version of MULTIMODE, 32 and full CI (FCI) calculations. 33 We believe that the 33 cm −1 shift in the zero-point energy calculated using the basis set DMC approach reflects an incomplete treatment of the coupling between the torsion and the other vibrational degrees of freedom.…”

Modelling rotations, vibrations, and rovibrational couplings in astructural molecules – a case study based on the H+5 molecular ion

“…An example of such an astructural molecule, and one which recently attracted considerable experimental and theoretical attention [18,[26][27][28][29][30][31], is H + 5 . This molecular ion with five hydrogen atoms lacks the usual central atom that can form multiple strong bonds, which is typical of virtually all other small molecular species.…”

“…These reduced-dimensional Hamiltonians replace the full-dimensional (12D) rovibrational Hamiltonian and are used to provide physical insight into the motions of the atoms of H + 5 . The vibrational and rovibrational levels of H + 5 have been computed both by variational nuclear motion [29,30,32,33] and diffusion Monte Carlo (DMC) [31,34,35] techniques. The full-and reduced-dimensional variational computations performed by three of the present authors [18] utilised the GENIUSH algorithm and code [36][37][38].…”

Dissociative recombination and excitation of D5+by collisions with low-energy electrons

“…The next stable ion in the H 2n + 1 family, H 5 + , was discovered by mass spectrometry in a glow discharge in 1962 [8] along with its isotopologue D 5 + , but just as for H 3 + the spectroscopic discovery came much later [9,10], in particular the medium resolution spectroscopy of H 5 + and D 5 + [11,12]. The spectroscopic work and the fact that H 5 + is * Corresponding author.…”

“…The spectroscopic work and the fact that H 5 + is * Corresponding author. Email: mats.larsson@fysik.su.se the simplest molecule containing five atoms and the simplest system with two neutrals combined by a proton have stimulated much theoretical work [11][12][13][14][15][16][17][18][19].…”