DETERMINATION OF POSITIONS, VELOCITIES, AND KINETIC ENERGIES OF RESONANTLY EXCITED IONS IN THE QUADRUPOLE ION-TRAP MASS-SPECTROMETER BY LASER PHOTODISSOCIATION

The effects on ion motion caused by the application of a resonance AC dipole voltage to the end-cap electrodes of the quadrupole ion trap ar e described. An excimer laser is used to photodissociate benzoyl ions, and its triggering is phase locked to the AC voltage to follow the mo tion of the ion cloud as a function of the phase angle of the AC signa l. Resonantly excited ions maintain a coherent motion in the presence of He buffer gas, which dissipates energy from the ions via collisions . Maximum ion displacements, which depend upon the potential well dept h (q(z) value), occur twice each AC cycle. Axial components of ion vel ocities are determined by differentiating the displacements of the dis tributions with respect to time. The experimental data show that these velocities are maximized when the ion cloud passes through zero axial displacement, and they compare favorably with results calculated usin g a simple harmonic oscillator model. Axial components of ion kinetic energies are low (<5 eV) under the chosen experimental conditions. At low values of q(z) (approximately 0.2), the width of the ion distribut ion increases as the ion cloud approaches the center of the trap and d ecreases as it approaches the end-cap electrodes. This effect is creat ed by compaction of the ion trajectories when ion velocities are decre ased.