MODERN PHYSICS (PHY 320) LAB

Spectrometers

OBJECT:

To investigate the design, operation, optical properties and performance of a grating spectrometer.

APPARATUS:

• The HR-320 MONOCHROMATOR from Instruments SA, Inc. and the associated Model 1020-MS Microprocessor Scan system.

• The detector will be a Photomultiplier Tube R928 with an appropriate high voltage supply.  The maximum rating for the PMT Anode to Cathode Voltage is (1250 Vdc). 

• Signals from the PMT will be observed using an appropriate Pico-Ammeter.  The maximum rating for the average Anode Current is (0.1 mA) and the data analysis will be accomplished using the PASCO Science Workshop Interface with a computer.

• White light source, Sodium light source,  associated filters, crystals, lenses and apertures.

• Apparatus diagram:

BACKGROUND INFORMATION:

The basic theory for these operations is found in The Optics of Spectroscopy, a tutorial by J.M. Lerner and A. Thevenon provided from Instruments S.A. Inc.

EXPERIMENT:

A.    Set up the system with a white light source focused on the entrance slit of the spectrometer.  This part should have the shutter closed in front of the PMT, and thus the high voltage supply and the pico-ammeter can remain in the OFF position.   Remove the top cover (normally secured by the thumb screws) from the HR-320 housing.

CAUTION:   The optical surfaces of the grating and mirrors are easily damaged.  They should never be touched with any objects. Special care should be exercised to prevent any change in the delicate alignment of the instrument.

Place a white card just in front of the exit slit. The continuous spectrum produced on the card represents  the dispersion of the white light by the diffraction grating. Observe the effect on the spectrum resulting from changes in the entrance slit (height and width) and the focus of the beam on the slit.  Next, set the Microprocessor Controller to scan the visible wavelengths (400 nm to 700nm) while you observe what happens to the spectrum on the card.   The controller settings should be set up as follows:

1. L Key :      This key should read the same wavelength as the monochromator. Press L, press CLR to clear the register, press the number keys to enter the monochromator reading (in Angstroms!) in the format (0000.0).

2. L₁ Key :    The register for this key will store the lower limit wavelength for your scan.
   Press L₁, press CLR to clear the register, press the number keys to enter 4000.0 for this part.

3. L₂ Key :    The register for this key will store the upper limit wavelength for your scan.
   Press L₂, press CLR to clear the register, press the number keys to enter 7000.0 for this part.

4. SPD Key:  This sets the scan speed in Å/min.  The maximum scan speed is 1800  Å/min.  For greater scan settings the monochromator readings will not match the controller readings. 
   Set the speed to 300 Å/min. for this part.

5. CYL Key:  Allows user to enter the desired number of scans for repetitive scanning.

   FWD Key:    This causes the system to scan in the Forward direction to increasing  L.
   REV Key:    This causes the system to scan in the Reverse direction to decreasing L.
   STP Key:    This causes the system to Stop scanning (after a backlash correction).
   STA Key:    This causes the system to Start repetitive scanning for the number of cycles chosen by the CYL Key..
   
   First scan to change L to match L₁ at the lower limit 4000.0 Å.  
   Then, scan FWD and note the changes of the colored spectrum on the white card.

B.    Plot the dark current vs. PMT voltage and determine its functionality.

C.    Determine the zero opening readings on the micrometers for the entrance and the exit slits.  We will use these values in the future.