Another method of spectroscopy using a spectrometer is to point the spectrometer at a lamp filled with an element. Figure 1 shows the spectrum of a helium discharge lamp collected using a commercial spectrometer. The spectrum is not continuous like that of a blackbody. Instead, we observe individual spikes at very specific wavelengths. And, we already know the source of these spikes! Atoms have energy levels, and the atoms will only absorb and emit at specific wavelengths. Filling a lamp with a specific element, or the combination of a few elements, and collecting the emitted light with a spectrometer was the most common form of spectroscopy before the invention of the laser. After the spectrum was collected, physicists had to be very clever to back out the energy levels from all of those lines .
Whether you use a white light source and look for lost photons or you collect light from a discharge lamp, you gain the same information, which is the wavelengths of light needed to have an atom go from one energy level to another.
In the lab, spectrometers are still really useful tools, but they are not great for certain applications. Below are a few pros and cons to using spectrometers for spectroscopy.
Pros:
– You get lots of data all at once.
– You get lots of data very quickly
Cons:
resolution is limited. You need to get different wavelengths far enough apart to distinguish between them. For example, let’s see how far apart 450.334 nm is
Fig. 1 Spectrum collected from a helium discharge lamp collected using a spectrometer with an accuracy of 0.5 nm
from 450.335 nm using Eq. 2.2 with L = 10 cm, m =+1, and d = 1000 nm:
That is small! A possible solution is to make a bigger spectrometer, but then you will have to worry about thermal drifts, how well you can uniformly space the grating slits, and how well you can measure d. In short, it becomes really hard if you want really high resolution.
– The data can be hard to interpret. Getting all the data at once means you are getting all transitions from your atom or molecule all at once. A spectroscopist has to figure out what line comes from what transition (pair of energy levels), which is not an easy task!
For high precision spectroscopy, most spectroscopy groups use a laser to study a single transition instead of collecting light from many transitions at once... having atoms moving around (i.e., a hot gas) is a bad thing if we want to measure something like the energy difference between two energy states to really high precision. Clever physicists developed a neat technique to still use hot atoms to measure properties to high precision ..., but we need multiple lasers to do so.
