Scanning Microscopy


Multiphoton ionization (MPI) by pulsed, tunable lasers provides a sensitive means for detection of neutral atoms, resulting from the high probability achievable in both the ionization and subsequent detection steps. Substantial selectivity is achieved by excitation between energy levels of the atom of interest. This resonant MPI technique can access all atomic states including ground and metastable levels.

The high efficiency of MPI technique permits detailed sputtering data to be obtained with minimal target damage. The goal is to obtain velocity and angular distributions for each energy level of every sputtered species. In practice, two types of experimental configurations have been employed. In one method, the photoionized atoms are allowed to strike a spatially resolved detector near the target, with extraction fields that preserve the angular distribution information. Velocity information is obtained by time of flight (TOF). This method is most suitable for majority species in the sputtered flux. In the case of minority species (either very dilute surface constituents or highly excited states produced), additional noise reduction is necessary. A suitable configuration involves extraction of the photoions into a sector-field TOF mass spectrometer. In standard, isochronous operation, energy and angular spreads at the point of ionization are compensated in flight to produce sharp TOF mass spectra. Noise sources (photons, metastable and scattered atoms) escaping through transparent grids are strongly suppressed. Angular distributions can be mapped "pointwise" by varying the relation between the point of ion beam impact and the photoionization volume. Velocity data can be obtained from the TOF spectra or by Doppler scanning on any resonant step of the laser excitation. Recent data are discussed.

Included in

Biology Commons