UV-Vis optical fiber assisted spectroscopy in thin films and - PDF document

UV-Vis optical fiber assisted spectroscopy in thin films and solutions Description UV-Visible absorption and transmission spectra provide fundamental information for all experiments related to the attenuation of a beam of light after it passes through a sample (absorption and transmission). Both approaches are suited for thin films measurements but the first type is mainly devoted to the measurement of solutions. They serve to identify the excited species in the sample, the presence of specific chromophores and their potential interactions, to verify the sample purity, to identify the bathochromic and hypsochromic shift effect. Spectra usually obey the golden rules given below. Goal To familiarize the student with an optical (fiber) spectrometer, to introduce the concepts of absorption and transmission spectra, to introduce the concept of reference and dark spectra, and to illustrate the Golden Rules. Introduction This exercise involves recording of absorption and transmission spectra and verification of the Golden Rules. The reason for the Golden Rules may be found in the reference text. These rules are: 1. The intensity of the spectra is proportional to the number of the active molecules (concentration), directly for absorption and inversely for transmission spectra. 2. The shape of the absorption/transmission spectrum does not change when the concentration of the sample is changed. 3. The intensity of the spectra is proportional to the length of the optical path (thickness of the thin film), directly for absorption and inversely for transmission spectra. 4. The shape of the absorption/transmission spectrum changes when the thickness of the film is changed. 5. When absorption maximum (λ max ) of a compound shifts to longer wavelength, it is known as bathochromic shift or red shift . The effect is due to a heteroatom-carbon double bond or a negatively charged heteroatom which delocalizes more effectively than the lone pair of electrons, when situated in the vicinity of a carbon-carbon double bond (conjugation is favorized). 6. When absorption maximum (λ max ) of a compound shifts to shorter wavelength, it is known as hypsochromic shift or blue shift . The effect is due to the presence of a group which causes removal of conjugation or by change of solvent. Safety issues High voltage is produced by the lamp power supplier; normally, students will have no need to have access to this circuitry. The samples used in the exercise are not toxic but wearing disposable gloves during the experiment is recommended. Materials Samples set 1: Thin films of Rhodamine 6G coated on the top of ultrathin glass cover slips ( 0.17 mm thickness) 1

1A 0.2M Rhodamine 6G (R6G) in MeOH glass cover slip 0.17 mm thickness d1 1B 40 mM Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 1C 20 mM Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 1D 10 mM Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 1E glass cover slip 0.17 mm thickness Samples set 2: 2A c2M Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d1 (recipe R2)d1 2B c2M Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d2 (recipe R1)d2 2C c1M Rhodamine 6G (R6G) in MeOH on glass cover slip 0.17 mm thickness d2 (recipe R2)d3 2D glass cover slip 0.17 mm thickness Samples set 3: Blank 1 mm quartz cuvette empty 3A Acetone Blank 1 mm quartz cuvette with pure ethanol 3B 1mM 3-benzophenone in ethanol 3C 1 mM 3-benzophenone in ethanol with 0.01N HCl pH 3 3D 1 mM 3-benzophenone in ethanol with 0.01N NaOH pH 11 UV-Vis optical fibers of 400 µm and 200 µm diameters, absorption spectra of deuterium and tungsten lamps, absorption spectra of samples 1D(Rh6G), 3A(acetone), 3B(3-benzophenone), and transmission spectra of samples 1D(Rh6G). Procedures Taking an Absorption and a Transmission Spectrum in thin films Rules 1 and 2 1. Turn on the light source and the computer 2. Configure the experimental setup connecting the light source and other sampling optics with spectrometer and computer. If follow the previous steps and start SpectraSuite application, the spectrometer is already acquiring data in Scope Mode. Even with no light in the spectrometer, SpectraSuite should display a dynamic trace in the bottom of the graph window. If the light is allowed into the spectrometer, the graph trace should rise with increasing light intensity. This indicates that the software and hardware are correctly installed. How can you check this if our light source has constant power? Compare the graph traces recorded with the optical fiber end oriented to the day light or any other light source before and after placing in the light path a neutral density filter or your own hand. 3. Disconnect the optical fiber from the light source. Then insert a visit card in front of the lamp exit having both the deuterium lamp and tungsten lamp on. Note the color of the light. 2

Repeat the same procedure with the deuterium lamp turned off and tungsten lamp turned off, respectively. Note the color of the light in either case. How this color changes when the card contains a chromophore (fluorophore) absorbing in UV range? Place a quartz cuvette filled in with acetone (sample 3A) in the light source path and observe the color of the light spot on the visit card, then replace the sample 3A and observe the color of the light spot on the visit card. Note the light spot color in each case. Reconnect the optical fiber to the light source and click again on the Scope Mode icon in the graph toolbar. Calibrate the correct integration time (IT) so that the maximum intensity to be around 14000 counts or a little bit less. Visualize the spectrum of each deuterium and tungsten lamp, then the entire light source spectrum. Save the spectra as text file and OOI Binary Format. 4. Use the set 1, where samples 1A – 1D are Rhodamine 6G thin films of decreased concentrations from 0.2M to 10mM (in methanol, MeOH) coated on ultrathin glass cover plate. Sample 1E (blank sample) is similar to samples 1A-1D but without Rhodamine 6G. 5. Insert the sample 1E (blank) and recalibrate the integration time (IT) so that the maximum intensity of the signal to be 14000 counts or a little bit less. Set the number of scan to average and the boxcar width so that the signal-to-noise ratio and the spectral resolution of the spectrum requirements to be fulfilled. 6. Store a reference spectrum and save it as a reference spectrum in OOI Binary Format on the disk. 7. Turn off the lamp to store a dark spectrum and save it as a dark spectrum in OOI Binary Format on the disk. If the light source is built in with a triggering system, it is recommended to turn off the shutter instead the lamp. 8. Turn on the lamp/shutter and replace sample 1E with sample 1A. 9. Start an absorption measurement to record the spectrum under the same conditions. Select File/New/Absorption and set the same acquisition parameters. Load first the stored/saved Reference spectrum, then load the stored/saved Dark spectrum and press Finish. Save the absorption spectrum of the sample 1A as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. “ S ame conditions” mean same acquisition settings. There are several reasons for doing this (e.g., potential artifacts); do you know them? Lower or higher integration times (ITs) affect the position of the graph trace: upper or downer than the baseline position. Scan to average changes affect the signal-to-noise ratio of the spectrum and boxcar width changes will bring losses in the spectral resolution. 10. Replace sample 1A with sample 1B and record the spectrum under the same conditions. Save the absorption spectrum of the sample 1B as a processed spectrum in OOI Binary Format but also in Text file Format on the disk. 11. Compare then the spectra of the samples 1A and 1B. How do they differ? (Golden Rule 1) 12. Apply the same procedure for the samples 1C and 1D. Save the absorption spectra of the samples 1C and 1D as processed spectra in OOI Binary Format but also in Text file 3