Grid-gated photomultiplier photometer with subnanosecond time response

A grid-gated photomultiplier has been used in the construction of a subnanosecond-time-resolved photometer. The photocathode is gated on during a sampling window which occupies 4 ns of an 8-ns mode-locked ion laser cycle. The 0.5-ns rise-time sampling window is derived from the mode-locker drive signal and can be adjusted in phase relative to the incident laser light pulses. By resolving scattered light pulses from long-lived fluorescence, the photometer affords a 20-fold reduction in scattered light interfering with photobleaching recovery measurements of low-level fluorescence. Moreover, it has permitted the time-domain measurement with dc circuitry of fluorescence lifetimes as short as 0.7 ns. This time response is achieved using a photomultiplier which, in the absence of grid gating, exhibits an anode pulse rise time of 18 ns. Additional applications of this device and of a related narrow-window sampling photometer currently under development are discussed.