Filter
Top Products
124 PI-MAX
®
4 System Manual Issue 4
7.4.2 Bracket Pulsing in Nanosecond Pump Probe Experiments
Some nanosecond pump-probe experiments combine a nanosecond or faster pump with a
flashlamp probe. The duration of the probe flash can be 10-50 µs and a gate is used to select
the specific nanosecond-scale time slice to be observed within the much longer probe flash.
In these absorbance experiments, accurate measurement of absorbance values depends
critically on the lack of stray light contamination, particularly at moderate to high optical
density levels. Selecting a 5 ns time window out of a 10 µs pulse is already one part in
2,000. If UV leakage gives an on/off ratio of only 20,000:1, contamination could be 10% of
higher. This would limit the optical density to 1.0, and it could make linear quantitation
difficult beyond 0.1 OD.
MCP bracket pulse gating can substantially improve the on/off ratio in such an experiment.
Even with a 1 µs MCP pulse, the rejection of flash-lamp leakage can add more than an order
of magnitude of range, to 2.0 OD.
7.4.3 Limitations of Bracket Pulse Gating
MCP bracket pulse gating is most useful in rejecting background that lasts microseconds up
to CW. Fast transient backgrounds can be in the form of stray laser light scattering (Raleigh,
MIE, Raman,) or unwanted fast fluorescence. Because these usually fall below the MCP
bracket pulsing 35 ns delay restriction, these measurements cannot be improved much by
MCP bracket pulsing in the PI-MAX4.
Electrically, gating the MCP will only reduce leakage at wavelengths where the MCP has
photoelectric response (primarily in the UV.) Thus, for visible and NIR wavelengths where
leakage is primarily optical, the improvement will be minimal (although the on/off ratio is
already very good in these regions.) Note that in some spectroscopic applications, visible
leakage may appear to be reduced by MCP pulsing. This is because the second order UV
spectrum overlays the first order visible spectrum in a grating spectrograph. MCP pulsing
can eliminate unwanted sensitivity to CW or quasi-CW second order UV, causing the
apparent improvement.
Also, keep in mind that MCP bracket pulsing adds 10 ns delay to the photocathode gating.
Even though the bracket timing is controlled automatically by the software, in an
experiment where it is necessary to delay the arrival of the laser pulse at the sample, this
will mean inserting an additional delay of 10 ns (minimum gate delay = ~25 ns, with
bracket = 35 ns,) to accomplish coincidence at the camera. MCP bracketing should only be
used in experiments where it is going to make a difference.
Also, MCP bracket gating limits the repetition rate to 6.25 kHz, without it the gate
repetition rate can be up to 1 MHz (more with option boards.) This is not a significant
limitation in most cases. Note that with a 10 ns gate at 6.25 kHz, the on/off ratio = 16,000,
close to the non-bracket value.
Note that background light need not be the limiting factor in measurements where MCP
bracket pulsing is unable to provide the required degree of rejection. In such measurements,
the option remains of installing an external shutter ahead of the PI-MAX4.