I'm setting up my first LIF experiment and I'm having some trouble. I'm measuring the nitrogen first positive system as has been done by numerous groups. The excitation beam is ~618 nm (v=0->4) and the detected fluorescence is around 676 nm (v=4->1). The plan is to use a barrier discharge (RF for now) to create a plasma (~5 torr). I am using an oscilliscope and PMT with a 680/10 nm bandpass filter. The problem I'm running into is that I"m getting some signal/scatter into the PMT. The filter is sealed very well so I can't see any light getting through it yet the signal goes away when I block the beam. Note that this happens with no plasma. The windows of the discharge cell, which are actually UV windows, seem to create a lot of scatter, but the bandpass should not be allowing any meaningful amount of light through. If I add an additional longpass filter that blocks anything under 650 nm, the result doesn't seem to change. When I use two bandpass filters the measured signal decreases as you would expect. The signal varies from pulse to pulse (at 10 Hz) but that may just be because of the variation in beam power. This is my first experiment but nobody seems to be able to help me out. Any ideas? Thanks so much for any help.