We will obtain the frequencies of the emission lines of I2 vapor following excitation with a Spectra-Physics model 2020 Ar+ laser at 514.5 nm. The I2 will be contained in an ordinary fluorescence cell which is place in front of the entrance slit of a 0.5 m SPEX monochromator. The fluorescence cell is evacuated with a mechanical pump and contains a small crystal of solid iodine which sublimes to provide I2 gas. Fluorescence from the laser excited gas is detected with the use of a photomultiplier tube situated at the exit slit of the monochromator. High voltage is supplied to the PM tube to provide electronic amplification of the photoelectrons induced by light hitting the photocathode of the PM tube. The amplified light signal is viewed on an oscilloscope and recorded by preamplification followed by A/D conversion and logging via a microcomputer.
The monochromator is equipped with a scan unit which changes the grating angle in such a fashion as to increment the transmitted light wavelength at a constant rate (fixed change in transmitted wavelenght per unit time). The computer 'reads' the intensityt of the fluoresence light as detected by the PM tube at regular intervals. Thus the data array acquired by the computer is a rendition of the fluorescence intensity versus waveltnght spectrum.
Suggested (initial) Operating Parameters
Photomultiplier Tube Voltage: -750 V (DO NOT EXCEED -900V!)
Stripchart
Scan Rate 20 cm/min
Sensitivity 100 mV/div
Monochromator
Start 5000
End 8500
Rate 5.25 Angstroms/s {This records the spectrum at 40 Angstroms/inch}
Repeat 2 times
Entrance Slit 0.050 mm
Exit Slit 0.050 mm
Ar+ laser
Tube current 18 Amps
Line 5145 Angstroms
Power 140 mW without cell, 100mW transmitted through cell
Record the barometric pressure, temperature, and humidity to calculate the index of refraction of the air in the lab. Is this needed to convert wavelength (which is determined by the grating in the monochromator) to frequency (which is proportional to energy)? The pressure of the cell has been controlled at the time of manufacture. A low pressure (I2 neat)and a high pressure (1 torr N2 gas added) cell are available for your use, and their spectra are different!
Remembering that our data acqusition system records photoelectrons (which are negative), your low pressure spectrum should look like this:
