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Nanosecond Excited State Lifetimes
The measurement of excited state lifetimes reinforces the connection between transition rates and the selection rules introduced in undergraduate quantum mechanics. Monitoring time-dependent luminescence is a common method for measuring excited state lifetimes. However, measurement of luminescence lifetimes on short (i.e., nanosecond) timescales is challenging when the temporal response of the detection system is comparable to the luminescence lifetime. This situation is encountered for excited states that decay by allowed transitions, e.g., spin-allowed decay of fluorescent molecular excited states. In such cases, the instrument temporal response must be deconvolved from the observed luminescence decay to order extract the actual excited state lifetime.
In this Immersion, we will explore two different techniques, a purely analog method and time-correlated single photon counting (TCSPC), for measuring fluorescence lifetimes when the time scale of excited state decay is comparable to the temporal response of the detection system. Fluorescence lifetimes of the molecules perylene and fluorescein, which decay single exponentially on a nanosecond time scale, will be measured. The skills that will be covered include
A schematic of the analog method apparatus is shown below. For TCSPC, the silicon photomultiplier detector (SiPM) is replaced with a fiber-coupled single photon counting module (SPCM), and the oscilloscope is replaced with the TCSPC electronics. The optical setups for both methods are mostly identical, with additional light collection optics required for efficient coupling of fluorescence photons into the SPCM for TCSPC.
Literature will be provided prior to the immersion so that participants can familiarize themselves with both methods, the properties of the SiPM and SPCM detectors, and incorporation of the instrument temporal response into the data analysis.
What you should bring for data analysis
A laptop with your preferred curve fitting software that supports user-defined fit functions. Igor Pro (www.wavemetrics.com) will be used by the immersion mentor to demonstrate the data analysis procedure.
We will be using a Class 3B laser. Laser safety glasses will be provided and worn when appropriate, and best practices for safe laser beam alignment will be used.
Equipment required for implementation at your institution
For both methods
For the analog method
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