Electron spin resonance study on H6+, H5D+, H4D2+, and H2D4+ in solid parahydrogen

Communication: Infrared photodissociation spectroscopy of the H6+ cation in the gas phase

The H₆⁺ cation was generated in a pulsed-discharge supersonic expansion of hydrogen and mass-selected in a time-of-flight spectrometer. Its vibrational spectrum was measured in the region of 2050-4550 cm^-1 using infrared photodissociation with a tunable OPO/OPA laser system. The H₆⁺ photodissociates, producing H₅⁺, H₄⁺, and H₃⁺ fragments; each of these fragment channels has a different spectrum. Computational studies identify two low-lying isomers described in previous work, whose energies were evaluated at the CCSD(T)/cc-pVTZ//MP2/cc-pVTZ level of theory. A D_2d species having an H₂⁺ cation bridging between two perpendicular H₂ molecules is the global minimum structure. A C_s structure with an H₃⁺ core ion bound to both H₂ and an H atom lies 4.0 kcal mol^-1 higher in energy. Anharmonic vibrational spectra were computed for each of these isomers with second-order vibrational perturbation theory (VPT2) in combination with density functional theory at the B2PLYP/cc-pVTZ level. The comparison between experimental and predicted spectra confirms the presence of both the D_2d and C_s structures and explains the spectra in different fragmentation channels. Although we find reasonable agreement between the experiment and the spectra predicted by VPT2 computations, a more sophisticated computational approach is needed to better understand this complex system.

Non-Born-Oppenheimer potential energy curve: Hydrogen molecular ion with highly accurate free complement method

Although the concept of a potential energy curve (PEC) originates from the outgrowth of the Born-Oppenheimer (BO) approximation, we propose the application of analysis methods for the physical PEC with non-Born-Oppenheimer (non-BO) wave functions. A numerical examination was performed with the highly accurate non-BO vibronic wave functions of hydrogen molecular ion, which were obtained in our previous studies with the free complement method. The reduced density function integrated over the electron coordinates plays an important role in understanding nuclear motion dynamics, since it corresponds to the wave function density of the vibrational and rotational motions. The maximum positions of this density indicate the high existence probability of nuclei and can be considered as a discrete representation of the PEC. Whereas an ordinary PEC with the BO approximation is obtained as a numeric curve after multiple electronic state calculations at fixed nuclear coordinates, we propose a new analytical expression of the PEC from a non-BO wave function.

Formation of even-numbered hydrogen cluster cations in ultracold helium droplets

Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered H(n)(+). The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered H(n)(+). The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered H(n)(+) up to n approximately = 120 by their mass excess that distinguishes them from He(n)(+), mixed He(m)H(n)(+), and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n=6, pronounced drops in the abundance of even-numbered cluster ions are seen at n=30 and 114, which suggest icosahedral shell closures at H(6)(+)(H(2))(12) and H(6)(+)(H(2))(54). Possible isomers of H(6)(+) are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory.

Electron spin resonance spectroscopy of molecules in large precessional motion: a case of H6(+) and H4D2(+) in solid parahydrogen

We have measured electron spin resonance (ESR) spectra of H6+ and H4D2(+) ions produced in gamma-ray irradiated solid parahydrogen. Anisotropic hyperfine-coupling constants for H6(+) and H4D2(+) determined by the analysis of ESR lines at 4.2K were -0.06 and -0.12 mT, respectively, which were opposite in sign to and much smaller than theoretical results of 1.17-1.25 mT. Although no change was observed in H6(+), the constant for H4D2(+) increased to be 1.17 mT at 1.7 K, which is very close to the theoretical value. We concluded that H6+ both at 4.2 and 1.7 K and H4D2(+) at 4.2K should be in a large precessional motion with the angle of 57-59 degrees, but the precession of H4D2(+) is stopped at 1.7 K.