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Abstract No.: ThP-005
Session: Activation and Dissociation
Presentation date: Thu, Aug 31, 2006
Presentation time: 09:50 – 11:20

Probing Collisional Excitation in Ion-molecule Collisions by Fluorescence Detection

Clement Poon1, Paul M. Mayer1

1 University of Ottawa, Ottawa, Canada

Correspondence address: Clement Poon, University of Ottawa, Department of Chemistry, 10 Marie Curie, Ottawa, Ontario, K1N 6N5 Canada.

Keywords: Charge Transfer; Collision Cell; Collision(s); Excited States.

Novel aspect: The novel aspect is to use fluorescence detection with mass spectrometry to probe ion-molecule collisions at various ion translational energies.


High energy ion-molecule collisions are common in the atmosphere. Species such as oxygen, nitrogen and neon fluoresce upon collisions with energetic charged particles from the magnetosphere leading to aurora. In mass spectrometry, keV ion-molecule collisions are used in CID to cause fragmentation of ions resulting in useful structural information. The deposition of internal energy in the ions during keV collisions, however, is not well understood. A possible way to study this is by coupling fluorescence detection with mass spectrometry.

The goal of this project is to investigate how ions and molecules are excited by observing the emission of photons from excited-state species. Ion-molecule collisions are carried out under normal CID conditions in a modified VG ZAB mass spectrometer. A spectrograph and a CCD camera for collecting photon emissions are installed above a collision cell. The spectra give information on the electronic state of both precursor and fragment species that are formed upon collisional excitation. A set of electrostatic lenses installed before and after the collision cell allow the ion translational energy to be varied between 800 and 8000 eV.

The fluorescence spectrum (190-1020 nm) of an 8 kV N2+. ion beam colliding with He results in several types of emissions: N2+. B2Σu+ --> X2Σg+ (ΔV = 2, 1, 0, -1, -2) emission band and several emission lines from He, N. and N+. Preliminary studies of photon emissions with respect to ion translational energies show that the relative intensities of the N2+. B2Σu+ --> X2Σg+ emission band is not altered by ion translational energy. The fluorescence spectrum of an 8 kV He+ ion beam colliding with N2 also shows the N2+. B2Σu+ --> X2Σg+ emission band as a result of favourable charge transfer (ΔH = -9.01eV). The relative intensities of the emission bands, however, are remarkably different from the reverse experiment described above. Collisions of N2+. with O2 also result in favourable charge transfer reaction (ΔH = -3.51eV). This results in much lower intensities from the N2+. emissions. Interestingly, we also observe the b4Σg- --> a4πu emission band from O2+.. The neutralization energy balance is not enough to excite O2+. to the b state in this case. The rest of the energy must result from the conversion of ion translational energy.

The above cases indicate the possibility of studying ion-molecule collisions including charge-transfer reaction by fluorescence detection on a mass spectrometer.