Laser excited N 2+ in a 22-pole ion trap: : Experimental studies of rotational relaxation processes

Abstract

The laser induced charge transfer N 2 + + Ar → Ar + + N 2 has been employed to measure rotationally resolved excitation spectra of N 2 +, held in a 22-pole ion trap. Detection of the Ar + product ions proved to facilitate an almost background free spectroscopic method. Rotational temperatures and Doppler temperatures determined from these spectra show that the internal and translational degrees of freedom of the parent ions are very well coupled to the trap temperature via collisions with the Ar buffer gas. Laser excitation starts a complex reaction kinetics of the finite ensemble of N 2 + ions. Various elementary reaction steps have been distinguished and followed in detail by varying laser duration, storage time, and target gas density. For a fixed temperature (90 K), specific rate coefficients have been determined. Pumping only one particular quantum state of ortho-N 2 + ( X 2∑ g , v″ = 0, J″ = 6.5) shows that rate coefficients for the mixing of fine structure states ( F 1/ F 2) as well as nuclear spin states (ortho/para) in collisions with Ar are negligible (smaller than 10 −12 cm 3 s −1). The rate coefficient for transitions between the rotational states of one subset, e.g. ortho-N 2 + ( F 1), are surprisingly small. The results are discussed in the framework of possible dynamical constraints, e.g. imposed by the potential energy surface.