Observing the Sodium D Lines
with a Compact Commercial Spectrometer

Rachel Losacco and John Noé

Laser Teaching Center
Department of Physics and Astronomy
Stony Brook University

Scientists use spectroscopy to separate a light sources spectrum and identify its chemical composition based on strong emission or absorption lines. When a gaseous element is heated, photons are absorbed by exciting electrons and causing them to momentarily transition to a higher energy orbit. When the electrons return to the ground state, a photon of the same wavelength and energy is re emitted: multiple emitted photons create a detectable emission line, and multiple states of absorbed energy create multiple emission lines, which are specific to the element. When a strong intensity of light passes through cold gas, the emitted photons travel in a random direction, usually not the same direction the photon entered, so the absorbed spectrum lines are weaker than the continuum of the light source. The resulting dark lines of the continuum are absorption lines. In 1814, Joseph von Fraunhofer classified the strong absorption lines of the Sun using letters A through K, denoting a particularly strong yellow spectral line as D-line. Today we know this line comes from sodiums valence electron transitioning from the 3s level to the 3p level, and that the line is actually two lines of wavelength 588.995nm (D1) and 589.592nm (D2). When absorbing energy, the electron can change orbit level as well as spin, both of which contribute to an increase in its total angular momentum j. Therefore, the 3p level is split into two based, where j = 1/2 and j = 3/2 ( j = 1/2 in 3s). The energy difference in 0.0021eV and the wavelength difference is 0.597nm.

This projects objective is to split the two D-lines while understanding the chemistry, physics, and quantum physics of the spectral lines and the mechanics of the spectrometer. We will use ThorLabs CCS100 Compact CCD Spectrometer and software to collect data from a low pressure sodium-vapor lamp. After further research on the properties of low pressure sodium-vapor lamps, we expect the lines to broaden due to a non-standardized pressure, the intensity of D1 to be almost twice the intensity of D2 due to the spontaneous decay rate of electrons from the respective levels, and a possible helium spectral line at 587.5618nm.