86 PI-MAX/PI-MAX2 System Manual Version 5.F
Bracket Pulsing in LIF Measurements
Most experiments using laser-induced fluorescence to probe combustion flows are
performed with UV probe/lasers. Atomic emission from flames also has significant UV
content. If the flame is continuous, the UV background will also be continuous. Even
where a flame is transient (e.g. internal combustion engine), its lifetime can be many
milliseconds, compared to the nanosecond time scale of the laser used. This background
can be a million times as long. If the background is bright, then a UV on/off ratio of
20,000:1 will be overwhelmed by the duty cycle and will not be adequate for extracting a
signal of 10
-5
. In high dynamic range quantitative measurements, backgrounds must be
kept to an absolute minimum. MCP bracket pulse gating dramatically improves the
rejection of CW and even millisecond time-scale background.
An alternative to suppressing background for imaging has been the use of very narrow
spectral bandpass filters. In the UV, these filters are expensive and they can have low
transmission at their central wavelength. An additional filter is required for each
wavelength to be imaged. The use of electronic temporal rejection of CW or quasi-CW
background may make it unnecessary to use these filters, thus increasing the optical
throughput, sensitivity, and quantitative precision of the measurement.
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.
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 1 µs delay restriction, these measurements cannot be improved
by MCP bracket pulsing in the PI-MAX.
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 is very much slower than 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
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