Rotational excitation and de-excitation of PN molecules by He atoms

¹ Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warszawa, Poland
² Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
³ Department of Chemistry, Oakland University, Rochester MI 48309, USA e-mail: francois.lique@obspm.fr

Received: 22 February 2007
Accepted: 14 March 2007

Abstract

Context.Observations of molecular emission at millimeter and infrared wavelengths, supplemented by careful and detailed modeling, are powerful tools for investigating the physical and chemical conditions of astrophysical objects. Modeling of molecular emission requires the calculation of rates for excitation by collisions with the most abundant species.

Aims.The present paper focuses on the calculation of rate coefficients for rotational excitation of the PN molecule in its ground vibrational state in collision with He, based on a new two-dimensional potential energy surface from highly correlated ab initio calculations.

Methods.Calculations of pure rotational (de)excitation of PN by He were performed for the first rotational levels using the essentially exact close-coupling method for the lower levels and the coupled-states approximation for higher levels.

Results.Cross sections for transitions among the 31 first rotational levels of PN were calculated for kinetic energies up to 2500 cm^-1. These cross sections were used to determine collisional rate constants for temperatures ranging from 5 K to 300 K. A propensity for even  transitions is observed, which reflects the nearly symmetric character of the PN-He potential energy surface.