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Out-of-class Questions: Instrumental Setup of a Spectrophotometer

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    The following set of questions is given as an out-of-class exercise. There are two ways to consider having the students answer them. One is to provide the text that accompanies this unit and have the students read the text and answer the questions. The other is to send them to a resource such as the Analytical Sciences Digital Library or more broadly the internet, to see what they can come up with as answers to the questions. Having worked on the assignment, we then spend a class period developing the different topics covered by the questions in the assignment.

    Notes are provided below for questions I ask in class to prompt their consideration and our discussion of the topics.

    Sources

    1. Describe the desirable features of a radiation source for a spectrophotometer.
    2. Plot the relative intensity of light emitted from an incandescent light bulb (y-axis) as a function of wavelength (x-axis). This plot is a classic observation known as blackbody radiation. On the same graph, show the output from a radiation source that operated at a hotter temperature.

    I find it desirable to explain something about the origin of blackbody radiation and Planck’s explanation of blackbody radiation.

    1. Examining the plots above, what does this suggest about the power that exists in radiation sources for the infrared portion of the spectrum?
    2. Explain the advantages of a dual- versus single-beam spectrophotometer.

    Lasers

    1. Why is it impossible to create a 2-level laser?

    Whether or not the students have read the textbook or found other sources, the concept of the population of energy states, a saturated transition and a population inversion are usually quite new and foreign to them. I find that it is necessary to take time to explain each of these terms, as well as the process of stimulated emission. Understanding absorption and stimulated emission, the students can then examine and explain why it is impossible to create a 2-level laser.

    1. Using your understanding of a 2-level system, explain what is meant by a 3-level and 4-level system. 3- and 4-level systems can function as a laser. How is it possible to achieve a population inversion in a 3- and 4-level system?

    It is helpful to discuss the concept of excited state lifetimes and ask the students to consider the relative lifetimes needed to create a 3- and 4-level laser.

    1. Which of the two (3- or 4-level system) is generally preferred in a laser and why?

    Having considered the 2- and 3-level systems and the corresponding populations, the students can usually explain why a 4-level laser is preferable.

    Monochromators

    1. Explain in general terms the mechanism in a prism and grating that leads to the attainment of monochromatic radiation. Compare the advantages and disadvantages of each type of device. What is meant by second order radiation in a grating? Describe the difference between a grating that would be useful for the infrared region of the spectrum and one that would be useful for the ultraviolet region of the spectrum.
    2. Explain the significance of the slit width of a monochromator. What is the advantage(s) of making the slit width smaller? What is the disadvantage(s) of making the slit width smaller?

    This has already been discussed in class.

    Detectors

    1. Explain how a photomultiplier tube works. What are any advantages or disadvantages of a photomultiplier tube?
    2. Describe a photodiode array detector. What advantages does it offer over other detection devices?

    This page titled Out-of-class Questions: Instrumental Setup of a Spectrophotometer is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Thomas Wenzel via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.