Minimum intensity of the laser beam during each pulse.

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Chapter 29, Problem 69GP

To determine

Minimum intensity of the laser beam during each pulse.

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Fluorescence microscopy, discussed in Section 29.8, is an important tool in modern cell biology. A variation on this technique depends on a phenomenon known as two-photon excitation.If two photons are absorbed simultaneously (i.e., withinabout 10-16 s), their energies can add. A molecule that is normallyexcited by a 350 nm photon can be excited by two photonseach having half as much energy. For this process to beuseful, photons must illuminate the sample at the very high rateof at least 1029 photons/m2 # s. This is achieved by focusing alaser beam to a small spot and by concentrating the power ofthe laser into very short 110-13 s2 pulses that are fired 108 timeseach second. Suppose a biologist wants to use two-photon excitationto excite a molecular species that would be excited by500 nm light in normal one-photon fluorescence microscopy.What minimum intensity 1W/m2 2 must the laser beam haveduring each pulse?

Fluorescence microscopy, discussed is an important tool in modern cell biology. A variation on this technique depends on a phenomenon known as two-photon excitation. If two photons are absorbed simultaneously (i.e., within about 10-16 s), their energies can add. A molecule that is normallyexcited by a 350 nm photon can be excited by two photons each having half as much energy. For this process to be useful, photons must illuminate the sample at the very high rate of at least 1029 photons/m2 ⋅ s. This is achieved by focusing a laser beam to a small spot and by concentrating the power of the laser into very short (10-13 s) pulses that are fired 108 timeseach second. Suppose a biologist wants to use two-photon excitationto excite a molecular species that would be excited by 500 nm light in normal one-photon fluorescence microscopy. What minimum intensity (W/m2) must the laser beam have during each pulse?

Exposure to a sufficient quantity of ultraviolet light will redden the skin, producing erythema—a sunburn. The amount of exposure necessary to produce this reddening depends on the wavelength. For a 1.0 cm2 patch of skin, 3.7 mJ of ultraviolet light at a wavelength of 254 nm will produce reddening; at 300 nm wavelength, 13 mJ are required.a. What is the photon energy corresponding to each of these wavelengths?b. How many total photons does each of these exposures correspond to?c. Explain why there is a difference in the number of photons needed to provoke a response in the two cases.