Institut für Physikalische Chemie und Elektrochemie I

Professor Karl Kleinermanns Research Group

Spectral Hole Burning (SHB)

A large geometry change in the ion may lead to a vertical ionization threshold far above the adiabatic threshold so that fragmentation of cluster ions can remain a problem even for soft two-color ionization. If only one single optical transition can safely be assigned to a cluster of specific size, then all other observed optical transitions of this cluster can be assigned as well via spectral hole burning. A strong burn laser lowers the number of molecules in a specific state. The ion signal from a weaker analysis laser decreases ("hole"), if burned and analysed transition share a common ground state level. Hence this method can be used to distinguish between different cluster sizes, conformers, and "hot" vibrational states. No hot bands or fragment peaks from higher clusters or from mixed clusters with the seedgas congest the spectrum. S1-S0 hole burning is not obstructed by short S1 lifetimes, which may even enlarge the hole depth. This is a major advantage of SHB compared to Laser Induced Fluorescence (LIF) and R2PI. In practice, the burn laser is scanned while the analysis laser frequency is kept constant at the wavelength of the cluster size assigned optical transition. Then the analysis laser ion signal is monitored and a depletion is noticed if both transitions start from the same ground state level. Ions, produced by the intense burn laser, perturb the experiment in two ways: they distort the electric acceleration field and saturate the MCP detector. Both effects lead to a decrease in detection efficiency of the analysis laser ions. The undesired burn laser ions are removed by reversing the polarity of the extraction field from shortly before until 100 ns after the burn laser was fired, using a fast push-pull high voltage switch. Due to inhomogenities of the laser pulses final spatial adjustments are carried out to optimize for maximum hole depth.

To the PC I Homepage
To the Research Group
To the HHU Homepage
If you have any comments, suggestions or remarks, please EMail to: pc1@uni-duesseldorf.de
last changed: 22.01.1998 (Wolfgang Roth)
number of accesses to this page: