Hydrargyrum: the other white light.

An important day today: I actually did some spectroscopy. By means of a personal introduction, we took spectra of two different mercury light sources. The student I was working with, Justin, took point on the first one, and then allowed me to lead on the second. Here's what we did:

The source has to be arranged so as to shine its light into the tube which functions as the spectrograph's "input." Mechanically, the tube isolates the light produced by the source - what we're interested in - from the ambient light. Black cloth can be used to help ensure a relatively "tight" seal against unwanted light. Here's where things get a little technical, so bear with me. The light travels down the tube, and strikes a diffraction grating, which is a piece of material - depending on the year of manufacture, it can be made of photosensitive gel or silica - ruled with upwards of 120,000 lines per inch. When the light passes through the grating, it is split into its constituent wavelengths - similar in concept to when the ink from a black marker is wetted, and splits into several colors. The spectrograph then scans through a range of wavelengths, and when the split light's constituent wavelengths match with the wavelength being scanned, the light passes through and into a photomultiplier tube (PM tube).

To proceed, we must first understand the construction of a photomultiplier tube. It consists of a cathode, an anode, and mulitiple dynodes sandwiched between. When a photon strikes the cathode of the tube, an electron is ejected via the photoelectric effect, meaning that the energy 'hν' of the photon must be greater than the work function 'ϕ' of the cathode. In essence, the work function is how "hard" it is to eject electrons from the material. This can be increased or decreased by respectively lowering or raising the voltage across the PM tube. If the photon is of sufficient energy, it may eject an electron from the cathode. This electron will then strike other electrons on its way to the anode through the dynodes. These electrons may eject other electrons, and so on, so that a theoretically exponential increase in electron count occurs. When these electrons reach the anode, we can detect them as discreet, but variable "pulses," or as an absolute current. To detect these, we use either a "pulse counter" or a "picoammeter." For now, I'll restrict myself to discussing the picoammeter and its uses, since the pulse counter requires a more involved discussion, and this post is long enough already.

The ejected electrons form a current. If there is a greater intensity of light, there are more photons, and those photons create more electrons which equals more current. This current is detected by the picoammeter, and we can analyze that reading to get useful data. To permanently record this, though, we require some means of transcribing the current we detect onto paper. To accomplish this, we make use of a device similar to a polygraph - though without the implications of criminality. The output from the PM tube is routed through into the device, a simple printer which translates the current much as a seismograph translates oscillation in the earth, or a polygraph records the tell-tales of the body: into simple lines whose height indicates the magnitude of the current. This print-out can then be analyzed to determine what the constituent wavelengths are, how "strong" they are, whether the spectrum is continuous, and other valuable information.