What is the percentage of light that is engaged by the sample after a path length of 1 µm?

Calculate the number of photons per second absorbed by the sample.

The absorption cross section of a molecule is the effectual area that captures all incoming photons under low illumination conditions (like an idealized solar cell that would capture all light photons hitting its surface). At room temperature, this corresponds roughly to the molecular area exposed to the light beam. If you calculate it from the molar absorptive, imagine that all molecules interacting with the light are arranged periodically in a plane perpendicular to the incoming light beam.