Objective:

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=2dsinq

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 S2000 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.

Apparatus: The S2000 is a newly developed spectrometer based on fiber optic coupling and integrated computer design. The detector in the S2000 is a CCD (charge-coupled device) array capable of measuring the intensity of light over a wide range of grating angles. Thus the S2000 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 OCEAN OPTICS 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.

Collect an emission spectrum from a tungsten filament lamp.

· Switch on the tungsten lamp and the S2000 control computer, and run the software.

· Turn off the room lights and collect a dark spectrum (press the grey light bulb button).

· With the same background conditions, collect the emission spectrum of the tungsten lamp.

· Now switch off the tungsten lamp and measure the emission spectrum of the fluorescent room lights by reflecting light into the fiber optic probe.

· Estimate the temperature of the tungsten filament and compare it to your estimate in the Blackbody Lab.

· Present your two spectra and comment on the differences between the two. How can objects appear so similar to our eyes when being illuminated by such different sources?

Collect emission spectra from the hydrogen gas-discharge lamp provided and examine the resolution of the spectrometer.

· Load the gas bulb into the lamp base and switch the power on.

· Place the collection end of the fiber optic near the lamp so that the spectrum of the emitted light is seen on the computer screen.

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.

· Compare the values for the hydrogen lines to the accepted values.

· Zoom in (Use Set Scale in View Menu) on a few of the spectral lines (one at a time) and observe the width of the line. Does the width of the line accurately portray the shape expected from the uncertainly principle? When the resolution of the spectrometer (a function of the size of the spectrometer, the coarseness of the grating, the detector, and the entrance slits) distorts the width of a spectral line, the data is said to be "resolution limited" by the spectrometer.

· Based on the manufacturers specifications for the spectrometer, what is the best resolution we can expect? Estimate the width of the emission line from the gas discharge lamp. Based on your calculations, would you have expected the data to be resolution limited?

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.

Examine the light transmitted through a variety of glass filters.

· Allow the tungsten lamp to warm up (10 minutes)

· Close the lamp shutter, turn off the room lights, and store a dark spectrum.  You should subtract it from any signal you collect.

· Open the lamp shutter.  View and store the reference spectrum. The reference spectrum tells you how strong the lamp is and how sensitive the detector is at each frequency separated by the grating.  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.

· Place an orange long pass filter in the sample space and record the transmission spectrum.

· Place additive and subtractive interference filters in the sample space and record their spectrum. Summarize your findings and write an explanation about additive and subtractive colors.  Information on the color filters can be found at Edmund Industrial Optics site.

· Next, collect absorption spectra of individual and combinations of ND filters (0.15, 0.3, 0.6). Determine how well the equation %T = 10^-D describes the filters. T is the transmission and D is the optical density.  Comment on the "neutral" nature of these filters.  

· Hold one of the colored interference filters up to your eye and observe transmitted room light.  Tilt the filter with respect to your eye.  Describe and explain what  you see.