Laser induced reactions in a 22-pole ion trap:C_2H_2~++hv_3+H_2- C_2H_2+H

摘要

A sensitive experimental method for ion spectroscopy and state specific reaction dynamics is described,briefly called laser induced reactions (LIR).The technique is based on (i) trapping ions over a long time in a cold 22-pole rf ion trap followed by mass spectrometric detection,(ii) providing a suitable low density gas environment for colisions,(iii) modifying the low temperature chemical kinetics using selective excitation via a tunalbe radiation source.In this paper,H-atom transfer reaction C_2H_2~+ (v_3=1,J)+H_2->C_2H_3~+H,is used tomonitor the infrared excitation of acetylene ions.Rotationally resolved spectra are presented for the antisymmetric C-H stretching vibration.For recording a spectrum,it is sufficient tofill the trap with a few thousand parent ions.Differences with respect to conventinal IR spectroscopy are discussed,especially the processes which influence the LIR signal.From the measured intensities and their dependence on parameters coefficients has been obtained at an ambient temperature of 90K.Based on a model simulating the kinetics,rate coefficients for various inelastic and reactive collisions are derived.Vibrational excitation of C_2H_2~+(v_3=1,J) increases the rate of the title reaction by more than three orders of magnitude,while rotation hinders the reaction.The fine-structure state of the parent ion does not affect its reactivity.Ways are pointed out to apply the method to various classes of moelcular ions.