How this calculator works
For a molecule at the centre of a circular Gaussian continuous beam, convert power to peak intensity using the intensity FWHM diameter, then divide by photon energy. Use the event-cross-section convention R_event = delta Phi^2; 1 GM = 10^-50 cm4 s photon^-1. Publications using photon-removal cross-sections can differ by a factor of two. The weak-excitation estimate excludes saturation, bleaching and molecular relaxation. Pulsed beams require the time integral of squared instantaneous flux, not squared average power.
Formula
Phi_peak = [4 ln(2) P/(pi d_FWHM^2)] lambda/(hc); mean events = delta Phi^2 t. Poisson probability = 1 - exp(-mean events).
Worked example
At delta = 100 GM, Phi = 10^24 photons cm^-2 s^-1 and t = 0.001 s, the mean is 0.001 event per molecule. Doubling flux gives 0.004.
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Frequently asked questions
Can I use average pulsed laser power?
Not directly. Two-photon excitation depends on squared instantaneous intensity, so pulse shape and repetition rate are required.
Does the result apply to every molecule in the focus?
It is the on-axis peak estimate. Off-axis molecules see lower intensity.
Can mean events exceed one?
Mathematically yes, but the weak-excitation model then needs scrutiny; the Poisson readout is not a saturation model.