Excited State Lifetime Measurement By EEL-CL Coincidence

Abstract

The lifetime of an excited state can be measured by detecting both the excitation (by EELS) and the de-excitation (by catho- doluminescence), and measuring the delay between the two events. We have adapted this technique for the measurement of lifetimes in an electron microscope.The experimental set-up is shown in Fig. 1. The arrival time and the energy loss of single electrons is monitored by the EELS (Gatan 607), and the arrival time and wavelength of single photons is monitored by the CL spectrometer. Pulses corresponding to the two events are fed to a time-to-amplitude converter (TAC), which outputs a variable height pulse proportional to the delay between the events. If no second (stop) pulse is detected within a preset time interval, the TAC recognizes a “false start”, does not output anything, and starts looking for a “start” pulse again. Since the count rate in the CL channel was typically 10 to 100 times weaker than in the EEL channel, we minimized the false starts by using the CL signal as the start pulse and the EEL signal, suitably delayed, as the stop pulse. This yields a “reversed time” spectrum, but minimizes the dead time of the electronics.