Instytut Fizyki Doświadczalnej Wydział Matematyki, Fizyki i Informatyki

UNIWERSYTET GDAŃSKI

Instytut Fizyki Doświadczalnej 1.

Experiment 18 : Determination of dissociation energy based on iodine absorption spectra

I. Background theory.

1. Movement of nuclei in a diatomic molecule - oscillations and rotations. 2. Internal energy of diatomic molecules. 3. Energy structure of diatomic molecules.

4. Potential energy curves of diatomic molecules. 5. Absorption spectrum of an enharmonic oscillator. 6. Electron states in diatomic molecules. 7. Oscillatory structure of band spectra.

8. Absorption of radiation by molecules. 9. Franck – Condon principle. 10. Electron transition probabilities:

a) transition moment ; b) Einstein coefficients; c) selection rules.

11. Decay processes in molecular excited states.

12. Stokes’s rule. 13. Molecular dissociation through electron excitations. 14. Molecular band spectra.

15. Particle absorption J2 – transition 𝐽2�𝐵�3Π0,𝑢+ � ← 𝐽2�𝑋�1Σ𝑔+�.

16. Molecular dissociation J2 and the associated decay processes of molecular excited states J2.

17. Structure of the halogens’ absorption spectra. 18. Determination of the molecular dissociation energy of J2 based on its absorption spectrum. 19. Construction of mesh spectrograph. 20. Spectrographic parameters :

a) spectral region; b) angular and linear dispersion; c) spectrograph luminosity; d) spectrograph resolving power.

II. Experimental tasks.

1. Familiarise yourself with the setup shown in Picture 1. The SR-500i spectrograph is also used in other experiments.

2. Turn on the iodine heating tubes (8 in Picture 1). To do this, turn on the power supply temperature controller (3 in Picture 1) and set the temperature knob on the temperature controller (7 in Picture 1) to between 2 and 3.

The tube of iodine must be heated for 30 minutes before measurements in order to obtain adequate iodine vapor pressure.

Instytut Fizyki Doświadczalnej 2.

Experiment 18 : Determination of dissociation energy based on iodine absorption spectra

3. Turn on the incandescent lamp (6 in Picture 1) by turning on the power supply (4 in Picture 1) (the switch is found on the rear of the device) and set the current to 4 A.

4. Turn on the power strip to which the CCD camera and spectrograph are connected, ensuring that the switch on the camera power supply housing is set to position II.

5. Turn on the computer and the spectrograph power supply (the switch is on the right side of the housing).

6. Start the program “Andor Solis for Spectroscopy” using icon 1 in Picture 2.

Picture 1. View of the screen after starting the computer: 1 – “Andor Solis for Spectroscopy” icon.

Picture 1. View of the setup for recording iodine absorption spectra: 1 – CCD camera; 2 – spectrograph; 3 – heating tube power supply; 4 – lamp power supply; 5 –

computer; 6 – incandescent lamp; 7 – temperature regulator; 8 – test tube with iodine; 9 – mirror.

Instytut Fizyki Doświadczalnej 3.

Experiment 18 : Determination of dissociation energy based on iodine absorption spectra

7. Before taking measurements, ensure that the temperature of the CCD camera (1 in Picture 3) is set to -60 ⁰C.

Picture 2. View of the CCD camera and spectrograph control software: 1 – CCD camera temperature indicator (red – camera has not attained the desired temperature, blue – camera has reached the

desired temperature); 2 – dialogue box found in the Hardware menu for setting temperature control parameters; 3 – spectral range slider; 4 – tabs for setting a relative shift between the detector and

diffraction grating; 5 – toolbar; 6 – diffraction grating selection tab; 7 – CCD camera mode selection button and results display.

8. Remove the spectrograph’s entrance slit cover and slightly open the slit. The number of counts should be less than 6∙105.

9. Use the diffraction grating rotor icon (6 in Picture 3) to select a diffraction grating with 100 grooves/mm and then use slider 3 in Picture 3 to set the spectral range to 450 to 700 nm centred at 550 nm.

10. Click Acquisition → Setup Acquisition and set the exposure time to 0,1 s.

11. Click Acquisition and select Setup Data Type → % Abortance and then choose OK. 12. Tilt the mirror (9 in Picture 1) into the optical path, and record a background reading by clicking

Acquisition → Take Background.

During the experiment keep in mind that with increased exposure time and gap width comes an increase in the background signal, and therefore the spectrum of the background must be recorded each time you change one of the above parameters.

Instytut Fizyki Doświadczalnej 4.

Experiment 18 : Determination of dissociation energy based on iodine absorption spectra

13. Insert the mirror into the beam and record the incandescent lamp’s reference spectrum by selecting Acquisition → Take Reference.

14. Insert the iodine tube into the beam and record the absorption spectrum by clicking Acquisition → Take Signal and RT from the main menu (the green icon on the toolbar, 5 in Picture 3).

15. Identify the absorption spectrum of iodine in the resulting waveform. 16. Set the range of the absorption spectrum by clicking the X-axis and selecting the desired

wavelengths.

17. Save the resulting waveform in ASCII format by clicking: File → Export as ASCII → Signal. 18. Make a rough reading of the wavelengths corresponding to the limits of the absorption

spectrum of iodine. 19. Stop the program by clicking on the red icon in the toolbar (5 in Picture 3). 20. Use the diffraction rotor to select a grating with 1200 grooves/mm.

21. Remove the iodine tube from the beam and re-register background and lamp reference signals. 22. Insert the tube with iodine vapour in the beam and record a signal (Take Signal). 23. Stop the program and choose Image Mode for the CCD camera operating mode (7 in Picture 3). 24. Removing the tube with iodine and record a background and reference signal. 25. Insert the tube and measure the signal for the new grating. 26. Find and interpret the characteristics of the structure of the iodine absorption spectrum and

the transverse Deslandres series by scanning through the spectrum using slider 3 in Picture 3: a) search from the beginning of the infrared bands and read off the wavelength of the “0 – 0”

transition; b) read off the wavelength corresponding to the band convergence boundary; c) determine the differences between adjacent oscillatory bands in each Deslandres series; d) calculate the dissociation energies of iodine molecules in the ground and the excited states; e) discuss errors; f) compare results with the given tabulated data.

27. Cover the spectrograph entrance before turning off the apparatus.

III. Apparatus.

1. Andor CCD camera, model iDus 420A. 2. Shamrock spectrograph, model SR – 500i.

3. Constant current power supply. 4. Incandescent lamp power supply. 5. Computer. 6. Incandescent lamp.

7. Temperature regulator. 8. Test tube with iodine. 9. Mirror.

Instytut Fizyki Doświadczalnej 5.

Experiment 18 : Determination of dissociation energy based on iodine absorption spectra

IV. Literature.

1. F. Mayinger, O. Feldmann – “Optical Measurements”, Springer, 2001. 2. A. P. Arya – “Fundamentals of Atomic Physics”, Allyn&Bacon, Inc., Boston 1971. 3. G.M. Barrow – “Introduction to Molecular Spectroscopy”, McGraw – Hill, N.Y. 1962.

4. P. Suppan – “Chemistry and Light”, The Royal Society of Chemistry, 1994. 5. J.P. Simons – “Photochemistry and Spectroscopy”, Wiley, 1971. 6. W. Demtröder – “Laser Spectroscopy. Basic Concepts and Instrumentation”, Springer, 1988. 7. H. Haken, H. Ch. Wolf – “Molecular Physics and Elements of Quantum Chemistry”, Springer,

2010.