User:NotKlayman/Single Molecule Spectroscopy

To put it simple, to achieve SMS at any temperature, one must (a) guarantee that only one molecule is in resonance in the volume probed by the laser, (b) provide that the signal-to noise ratio (SNR) for the single-molecule signal is greater than the background for reasonable averaging time. Guaranteeing that only one molecule is present in the detection volume is generally achieved by dilution. For example, at room temperature one needs to work with roughly 10-10 mole/liter concentration with a probed volume of about 10mm3. Achieving the required SNR is be done by several methods: To obtain a large a signal as possible, one needs a combination of small focal volume, large absorption cross-section, high photo-stability, weak bottle necks into dark states such as triplet states, operation below saturation of the molecular absorption and a high fluorescence quantum yield if fluorescence is detected. For absorption methods, achieving a low noise level from background effects follows careful reduction of residual signals and operation at a power level sufficient to reduce the relative contribution from laser shot noise. For fluorescence methods, one has to rigoursly exclude fluorescent impurities, minimize the volume probed to avoid Raman scattering and reject any scattered radiation at the pumping wavelenght.