Rotational excitation of C2(X1Σg +) by para- and ortho-H2

A new four dimensional (4D) potential energy surface for the C2(X1Σg +)-H2 van der Waals system is generated. The potential was obtained from a multi-reference internally contracted configuration-interaction method including the Davidson correction (MRCI+Q). The four atoms were described using the augmented correlation-consistent quadruple zeta (aug-cc-pVQZ) basis sets. Both molecules were treated as rigid rotors. Close-coupling calculations of the inelastic integral cross sections of C2 in collisions with para-H2(j H2 = 0) and ortho-H2(j H2 = 1) were also carried out at low energies. After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5 to 100 K. The rate coefficients for collisions with ortho-H2 are significantly larger than the rate coefficients for collisions with para-H2.

Hints of a rotating spiral structure in the innermost regions around IRC +10216

The Atacama Large Millimeter/submillimeter Array (ALMA) is allowing us to study the innermost regions of the circumstellar envelopes of evolved stars with un-precedented precision and sensitivity. Key processes in the ejection of matter and dust from these objects occur in their inner zones. In this work, we present sub-arcsecond interferometric maps of transitions of metal-bearing molecules towards the prototypical C-rich evolved star IRC +10216. While Al-bearing molecules seem to be present as a roughly spherical shell, the molecular emission from the salts NaCl and KCl presents an elongation in the inner regions, with a central minimum. In order to accurately analyze the emission from the NaCl rotational lines, we present new calculations of the collisional rates for this molecule based on new spectroscopic constants. The most plausible interpretation for the spatial distribution of the salts is a spiral with a NaCl mass of 0.08M☉. Alternatively, a torus of gas and dust would result in similar structures as those observed. From the torus scenario we derive a mass of ~ 1.1 × 10-4M☉. In both cases, the spiral and the torus, the NaCl structure presents an inner minimum of 27 AU. In the case of the torus, the outer radius is 73 AU. The kinematics of both the spiral and the torus suggests that they are slowly expanding and rotating. Alternative explanations for the presence of the elongation are explored. The presence of these features only in KCl and NaCl might be a result of their comparatively high dipole moment with respect to the Al-bearing species.

State-to-State Inelastic Scattering of O2 with Helium

Molecular oxygen (O2) is extremely important for a wide variety of processes on and outside Earth. Indeed, O2–He collisions are crucial to model O2 abundance in space or to create ultracold O2 molecules. A crossed molecular beam experiment to probe rotational excitation of O2 due to helium collisions at energies of 660 cm–1 is reported. Velocity map imaging was combined with state-selective detection of O2(X3Σg–) by (2+1) resonance-enhanced multiphoton ionization. The obtained raw O2+ images were corrected from density to flux and the differential cross sections (DCS) were then extracted for six O2 final states. Exact quantum mechanical calculations were also performed. A very good agreement between experimental and theoretical DCSs was found by using an initial O2 beam population ratio of 80% for the first rotational state and 20% for the first excited state. The agreement demonstrates our ability to model inelastic processes between O2 molecules and rare gas both theoretically and experimentally.

A new ab initio potential energy surface for the collisional excitation of N2H(+) by H2

We compute a new potential energy surface (PES) for the study of the inelastic collisions between N2H(+) and H2 molecules. A preliminary study of the reactivity of N2H(+) with H2 shows that neglecting reactive channels in collisional excitation studies is certainly valid at low temperatures. The four dimensional (4D) N2H(+)-H2 PES is obtained from electronic structure calculations using the coupled cluster with single, double, and perturbative triple excitation level of theory. The atoms are described by the augmented correlation consistent triple zeta basis set. Both molecules were treated as rigid rotors. The potential energy surface exhibits a well depth of ≃2530 cm(-1). Considering this very deep well, it appears that converged scattering calculations that take into account the rotational structure of both N2H(+) and H2 should be very difficult to carry out. To overcome this difficulty, the "adiabatic-hindered-rotor" treatment, which allows para-H2(j = 0) to be treated as if it were spherical, was used in order to reduce the scattering calculations to a 2D problem. The validity of this approach is checked and we find that cross sections and rate coefficients computed from the adiabatic reduced surface are in very good agreement with the full 4D calculations.

New ab initio potential energy surfaces for the ro-vibrational excitation of OH(X2Π) by He

A new, three-dimensional potential energy is presented. Values for integral and differential cross sections, and for inelastic rate coefficients were obtained. The results agree and significantly extend previous studies on OH(X) + He collisions.