A new ab initio potential energy surface for the NH–He complex

Abstract

We present a new three-dimensional potential energy surface (PES) for the NH(XΣ)-He van der Waals system, which explicitly takes into account the NH vibrational motion. The NH-He PES was obtained using the open-shell single- and double-excitation coupled cluster approach with non-iterative perturbational treatment of triple excitations. The augmented correlation-consistent aug-cc-pVXZ (X = Q, 5, 6) basis sets were employed, and the energies obtained were then extrapolated to the complete basis set limit. Using this new PES, we have studied the spectroscopy of the NH-He complex and we have determined a new rotational constant that agrees well with the available experimental data. Collisional excitation of NH(XΣ) by He was also studied at the close-coupling level. Calculations of the collisional excitation cross sections of the fine-structure levels of NH by He were performed for energies up to 3500 cm, which yield, after thermal average, rate coefficients up to 350 K. The calculated rate coefficients are compared with available experimental measurements at room temperature, and a reasonably good agreement is found between experimental and theoretical data.

Low-temperature reaction dynamics of paramagnetic species in the gas phase

“…The calculations identified a preference for maintaining the orientation of the electron spin-with transitions that conserved electron spin exhibiting larger cross sections. These findings are consistent with other studies on the NH molecule; 116,117 calculations at higher collision energies, in addition to further experimental work, are anticipated in the coming years.…”

Collisional excitation of NH(3Σ−) by Ar: A new ab initio 3D potential energy surface and scattering calculations

“…The predicted bound energy levels indicate that the coupling of the electron spin to the rotational motion of the complex is very weak. As a consequence, energy levels of NH-Ar are very similar to those obtained by neglecting the fine structure, as already found for the NH-He complex 10 . The dissociation energy is thus not significantly impacted by the fine structure.…”

“…As previously mentioned, the NH molecule exhibits a fine structure because of the coupling between the rotational angular momentum and the electronic spin. The BOUND program was modified to include this fine structure of the NH molecule 10 . Table II presents the bound state energies for the first total angular momentum J.…”

“…In addition, the magnetic moment of its 3 Σ − electronic ground state, makes NH suitable for studies of ultracold molecules 1,2 , because it can be easily thermalized at low temperatures through collision with cold buffer gas atoms. In the past, NH has been subject of many the-oretical and experimental collisional studies in different electronic states and with a variety of perturbers, such as the rare gases He [3][4][5][6][7][8][9][10] and Ne [11][12][13] .…”

“…How-ever, the NH bond length was kept frozen at 1.96 bohr. Recent studies 10,13,18,19 have proven that the use of a 3D PES which takes into account molecular vibration leads to more accurate results when employed in collisional excitation studies of light hydrides by rare gases. Moreover, inclusion of the bond vibrational motion makes it possible to comprise excited vibrational states.…”