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SR430 Multichannel Scaler/Averager Fluorescence Decay of Ruby

The absorption band at 550 nm overlaps the emission line ofa green LED. In this example experiment, a pulsed green LED is used to quickly populate the excited state, and decays from the excited state are seen through a band pass interference filter centered on ruby's 694.3 nm emission line. Apparatus This experiment is typical of time resolved photoncounting experiments. Apulsed light source is used to pump atoms to an excited state. Fluorescent decay from the excited state is observed, allowing the lifetime of the upperstate to be measured.The energy level diagram of ruby is shown below. There areabsorption bands around 400 nm and 550 nm. The Cr The experimental set-up is shown below. The green LED isglued to the edge of a 1 cm diameter, 3 mm thick, ruby disk. The ruby disk is viewed through the band-pass interference filter by a Hamamatsu R928 PMT. This side-on PMTwas selected for its high gain, fast rise time, and good red sensitivity. Experiment Setup Ω, is used to further reduce anoderinging and reduce the fall time of the output current pulse. The phototube base uses a tapered voltage divider with about3 Ω resistors are used between the dynodesand their bypass capacitors to reduce ringing in the anode signal. Asnubber network, consisting of a ten inch piece of RG174 terminated into 20 × the normal interstage voltage between the photocathodeand the first dynode. This helps to narrow the pulse height spectrum for single photon events. The lower dynodes are bypassed, and 100 Operation The PMTis operated at the maximum rated high voltage(1250 VDC). The output pulses have a mean amplitude of 20 mVinto 50 Ω . To increase the pulse height to 100 mV, oneamplifier in the SR445Apreamp provides a gain of 5 with a 350 MHz bandwidth. The discriminator threshold is set to 20 mV. When viewed with a 300 MHz oscilloscope, it is
+++ ions,which absorb light at these wavelengths, decay in about 50 ns to the upper state of the well known laser transition. This state has a lifetime of about 3.5 ms, and decays to the ground stateby emitting a photon at 694.3 nm. ABSORPTIONBAND50 NS DECAYUPPER LEVEL OFLASER TRANSITIONSPONTANEOUS DECAYAT 694.3 NMWITH 3.5 MS LIFETIMEGROUND STATEGREENLEDENERGY LEVELS FOR RUBY Av=5 1/4 SR445AR928 PMT694.3 NM BANDPASSINTERFERENCE FILTER RUBY DISKGREENLED20 TO SR430 APPARATUS FOR DEMONSTRATION EXPERIMENT 50 Ω SNUBBER10" RG174 Ω

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