Instytut Fizyki Doświadczalnej Wydział Matematyki, Fizyki i Informatyki UNIWERSYTET GDAŃSKI

Instytut Fizyki Doświadczalnej Wydział Matematyki, Fizyki i Informatyki UNIWERSYTET GDAŃSKI

Experiment 20 : Studying light absorption in terphenyl I. Background theory. 1. 2. 3. 4. 5. 6. 7. Electromagnetic radiation: nature, characteristics, spectrum. Classification of molecular spectra. Spectroscopy and division by spectral range. Spectroscopic equipment. Construction and operation of a spectrophotometer. Molecular internal energy (division, simple models of molecules, energy states). Transitions between molecular energy states: a) spectra corresponding to these transitions; b) selection rules; c) electronic transitions. 8. Light absorption in solutions: a) light absorption law; b) absorption coefficient; c) molar extinction coefficient; d) oscillator power and its relationship with the absorption coefficient; e) relationship of the absorption spectrum with the average lifetime of the molecule. 9. Effect of a solvent on the absorption spectra (response field). 10. Preparing solutions of a specific concentration. II. Experimental tasks. 1. Refer to the setup shown in Picture 1. Picture 1. Setup for measuring absorption spectra: 1 computer; 2 spectrophotometer UV-1240. Instytut Fizyki Doświadczalnej 1.

2. Prepare five solutions of p-quaterphenyl in cyclohexane with concentrations ranging from 2,5 10-4 mol/l do 2,5 10-5 mol/l. 3. Refer to the instructions found in the Appendix. 4. Measure the absorption spectrum in the range 220 420 nm according to the instruction manual. 5. Based on the experimental results for each concentration of the solutions, plot a set of graphs: absorbance vs. wavelength; molar extinction coefficient vs. wavelength; molar extinction coefficient vs. wave number. 6. Tabulate your results including columns for maximum absorption wavelength, absorbance, molar absorption coefficients, molar extinction coefficients, integrals of the molar extinction coefficient and spectrum half-width. 7. Calculate: oscillator power of the observed electron transition; natural life span in an excited state. 8. Analyse the results for possible deviations from the absorption laws. III. Apparatus. 1. Spectrophotometer Shimadzu UV mini - 1240. 2. Computer. IV. Literature. 1. G.M. Barrow Introduction to Molecular Spectroscopy, McGraw Hill, New York 1962. 2. J.P. Simons Photochemistry and Spectroscopy, Wiley, 1971. 3. H. Haken, H. Ch. Wolf Molecular Physics and Elements of Quantum Chemistry, Springer, 2010. Instytut Fizyki Doświadczalnej 2.

Appendix Instruction manual for measuring absorption spectra I. Turning on the spectrophotometer. 1. Turn on the computer and monitor. 2. Turn on the spectrophotometer (switch is located on the back of the spectrophotometer). The power-on initialization process will begin, which tests each individual component. After completing the initialization, the spectrophotometer screen should appear as in Figure 2. Figure 2. Spectrophotometer screen after initialization. For optimal working conditions, you should wait at least 10 minutes after initialization of the spectrophotometer. II. Setting the measurement parameters. From the mode menu (Figure 3), select option 2.Spectrum. Figure 3. Spectrophotometer screen ready for work. Instytut Fizyki Doświadczalnej 3.

Experiment 20 : Studying light absorption in terphenyl The spectrophotometer screen will look as in Figure 4. Figure 4. Screen for setting measurement parameters for measuring spectra Using the spectrophotometer s keyboard, enter the following parameters: 1.Meas. mode : ABS 2.λ range : 450 nm ~ 250 nm 3.Rec. range : 0.00A ~ 2.00A 4.Scan speed : Slow 5.No. of scans : 1 6.Display mode : Sequential III. Baseline measurements. Baseline measurements must be made because of the need to take into account the spectral properties of both the cuvette (which must be identical to the cuvette used in performing spectral measurements of the prepared solutions) and the solvent in which the test substance is dissolved. To do this, insert the cuvette with pure solvent in the cuvette holder, as shown in Picture 5. Picture 5. View of the open spectrophotometer sample chamber: 1 cuvette holder. Instytut Fizyki Doświadczalnej 4.

Experiment 20 : Studying light absorption in terphenyl After placing the cuvette in the holder and closing the spectrophotometer chamber, run the baseline measurement by pressing F1 (BaseCorr). The measurement should be stopped by pressing START/STOP, when you reach a wavelength of 250 nm. The area on the spectrophotometer LCD showing the wavelength is shown in (1) in Picture 6. Picture 6. Spectrophotometer screen while measuring the baseline: 1 area displaying wavelength. After the baseline measurement, remove the pure solvent cuvette from the cuvette holder. IV. Measuring absorption spectra. Place the cuvette with the test solution in the holder inside the spectrophotometer sample chamber (Picture 5). After closing the chamber of the spectrophotometer, start the absorption spectrum measurement by pressing START/STOP. A view of the screen during the measurement of the absorption spectrum is shown in Picture 7. Picture 7. Spectrophotometer Screen while measuring an absorption spectrum. Instytut Fizyki Doświadczalnej 5.

V. Recording the data to the computer. After completing measurements, the data should be sent to the computer. To do this, the spectrophotometer must be set in file transfer mode. This requires changing the mode of the spectrophotometer (by pressing RETURN twice). The screen should then look as in Figure 3. In this mode, press F3 (FileTrans) to prepare the spectrophotometer to transfer files. The spectrophotometer screen should appear as follows (Picture 8): Picture 8. Spectrophotometer screen in file transfer mode. To exit the file transfer mode after finishing the transmission, press RETURN. Start the UV Data Manager program on your computer by double clicking the mouse on the program icon (Picture 9). Picture 9. View of the desktop. Instytut Fizyki Doświadczalnej 6.

After starting the program, the following window will appear (Picture 10): Picture 10. UV Data Manager program window. Connect the computer with the spectrophotometer by clicking once on Connect. This brings up a window with a question about the choice of data port (Picture 11). Picture 11. Data transmission port selection window. After selecting COM4 (by pressing OK), data will be transferred to the computer and the window will look as in Picture 12. Picture 12. UV Data Manager window after data acquisition from the spectrophotometer. Instytut Fizyki Doświadczalnej 7.

Recorded data (absorption curves in the form of a table of wavelengths and photometric values and measurement parameters) are always saved in the file Original (shown in the window Photometer ) which should be saved to the computer and marked with the selected channel Ch, as shown in Picture 12. The folder Dane MUST be chosen as the target in the Directories window. The pop-up window Target must have the property Data. After clicking Save, a dialogue box opens enabling the user to save a file to the disk. From the list File Type, choose Text (*.csv) and enter a filename according to the scheme described below. For the sake of order in the Dane folder, and to make it easier to identify your own data, use the following naming convention: XY_Substance_Solvent_concentration_range_date.CSV where: XY - Your initials Substance - substance abbreviation e.g. POPOP, PPO, 2MPOPOP, STILBEN Solvent - name of the solvent: MeOH, CH (cyclohexane), HX (hexane), etc. concentration - 2-5e-4 = 2,5 10-4 mol/l range - e.g. 250-450 nm date - 2011-02-15. Because of the way data is saved by the spectrophotometer and the data transmission format between the spectrophotometer and the computer, every measurement must be transferred to your computer and saved in accordance with the instructions above. Currently, it is not possible to make multiple measurements with the spectrophotometer and transfer them to the computer later. Each absorption spectrum measurement should be sent to the computer as soon as it is recorded (after setting the spectrophotometer to file transfer mode as detailed above) by clicking Update in the UV Data Manager program window (Picture 12). When importing data into the Origin software package, use the CSV format. Data is saved in a CSV file with the following format: 500,0, 0,0022 499,5, 0,0017 499,0, 0,0026 498,5, 0,0026 498,0, 0,0026 497,5, 0,0027 497,0, 0,0027 etc. Instytut Fizyki Doświadczalnej 8.