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Mass diffusion is a central process in many biological, chemical, and physical systems, and although the mathematics of diffusion has long been an important component of an undergraduate physics education, experimental measurements of diffusion are not very common in the undergraduate laboratory. We describe here an experiment that employs the interference of laser beams to allow measurement of molecular diffusion on micron length scales. The interference fringes of two intersecting “pump” beams within a dye solution create a sinusoidal distribution of long-lived molecular excited states. A third “probe” beam is incident at a wavelength at which the indices of refraction of the ground and excited states are different, so that the probe beam diffracts from the spatially periodic excited-state pattern. After the pump beams are switched off, the excited-state periodicity washes out as the system diffuses back to equilibrium, and the molecular diffusion constant is easily obtained from the rate constant of the exponential decay of the diffracted beam. It is also possible to measure the excited-state lifetime. The experiment provides hands-on insight into fluid dynamics, random walks, and coherent optics.

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The article appeared in (2011) American Journal of Physics, 79 (7), pp. 747-751.

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