Optical Spectrometers Filters and Light Sources
In this laboratory exercise, students will learn the basics of spectrometer design and function. The frequency separating and intensity monitoring abilities of two spectrometer systems will be studied. These spectrometers will be used to take absorption and emission spectra.
Since the optically active transitions of atoms and molecules occur at unique frequencies, we may study these species by examining the frequencies of the transitions and how the microscopic environments surrounding the ion affect these transitions. To separate the light due to different transitions, several spectrometer systems have been developed. In this exercise, we focus on one type (Czerny-Turner) of optical spectrometer pictured below.
Light entering the spectrometer through the entrance slit falls on a mirror that collimates the light. The collimated light strikes a diffraction grating mounted on a rotating table. The light reflects from the grating, satisfying the conditions for interference
nl=2d sin q
where n is an integer, called the order of the diffraction, l is the wavelength of the light, d is the distance between parallel grooves on the grating, and q is the angle the light makes with the normal to the grating surface. Thus, we see the light is separated into its wavelength, or frequency, components. The light diffracted from the grating strikes a second mirror and is refocused on an exit slit. A photomultiplier tube, sensitive to the intensity of light, is mounted on the exit slit of the spectrometer. The detector measures the intensity of light incident on it while the grating is rotated. The rotation of the grating varies the angle q, thus allowing a measurement of the intensity of light as a function of grating position, which can be converted into a measure of the frequency of light.
The Ocean Optics USB4000 series spectrometers used for the following exercises vary from the basic design pictured above in that they contain a fixed grating and a detector, which can collect data from a wide spectrum rather than the narrow exit slit. This design eliminates the need to rotate the grating in order to obtain data of intensity as a function of frequency. However, the resolution and sensitivity to low light levels is reduced.
The USB4000 is a newly developed spectrometer based on fiber optic coupling and integrated computer design. The detector in the USB4000 is a CCD (charge-coupled device) array capable of measuring the intensity of light over a wide range of grating angles. Thus the USB4000 takes a "snapshot" of the spectrum without the need to rotate the grating. Since there is no exit slit in this type of spectrometer, the width of the pixels composing the diode array determines the minimum value for this parameter.
Prior to measurement on your sample it is still necessary to take a reading of the Dark current and Reference spectrum.
Begin by running the control software. An icon labeled SpectraSuite can be found on the desktop or in the Program Menu. Double click this icon to bring up the control software. The control software is a user-friendly interface with push buttons that enable the experimenter to set the acquisition parameters and acquire spectra in a variety of ways. Of importance to this experiment, there are buttons for acquiring dark current, reference, and data spectra. The dark current should be acquired prior to switching on the white light source or opening the light shutter.
After switching on the white light source, the reference spectrum should be acquired with the appropriate parameters. Be careful not to saturate the detector, as this will make your reference measurement useless.
Experiment I: Emission of Incandescent and Fluorescent Bulbs
Collect an emission spectrum from a tungsten filament lamp.
Experiment II: Emission of Gas Discharge Lamps
Collect emission spectra from the hydrogen gas-discharge lamp provided and examine the resolution of the spectrometer.
The spectrum observed is produced by the discrete transitions of the atoms in the gas as they "hop" from one energy level to another after being excited by colliding with electrons accelerated between the electrodes of the electrical discharge. The lifetimes of the excited states are on the order of 1ns.
Emission spectra are valuable measurements because they show researchers how energy leaves a system. Once a collection of ions is excited (in our case by the collisions with fast electrons) the energy they receive must somehow escape. If we detect light from the excited ions we know that the energy is carried away by photons. The energy, or frequency, of the emitted photons tells specifically which atomic transitions are responsible for the creation of the photons.
In the following experiment, we will examine absorption spectra. Absorption spectra tell researchers which energy levels of materials are capable of being coupled by photons. By measuring the frequency, and thus energy, of the photons which couple the levels, we can precisely place the levels in energy.
Experiment III: Colored Glass, Neutral Density, and Interference Filters
Examine the light transmitted through a variety of glass filters.
When a filter is placed in the optical path, the filter will attenuate the reference spectrum according to the filter's profile. The filter profile can be found by dividing the output spectrum by the reference spectrum. This gives the fraction of the spectrum that is transmitted as a function of wavelength.