Podcast
Questions and Answers
What is the basis of electronic transitions in UV-Vis spectroscopy?
What is the basis of electronic transitions in UV-Vis spectroscopy?
Which factor does NOT lead to spectral deviations in UV-Vis spectroscopy?
Which factor does NOT lead to spectral deviations in UV-Vis spectroscopy?
How does molar absorptivity relate to the concentration of a solution in UV-Vis spectroscopy?
How does molar absorptivity relate to the concentration of a solution in UV-Vis spectroscopy?
What is the primary application of measuring the bandgap of a material using UV-Vis spectroscopy?
What is the primary application of measuring the bandgap of a material using UV-Vis spectroscopy?
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According to Beer-Lambert Law, how does absorbance relate to concentration?
According to Beer-Lambert Law, how does absorbance relate to concentration?
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Which component is NOT part of a typical UV-Vis spectrophotometer setup?
Which component is NOT part of a typical UV-Vis spectrophotometer setup?
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Which statement about the non-destructive nature of UV-Vis spectroscopy is true?
Which statement about the non-destructive nature of UV-Vis spectroscopy is true?
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What is a primary use of thermal imaging in relation to UV-Vis spectroscopy?
What is a primary use of thermal imaging in relation to UV-Vis spectroscopy?
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Which electronic transitions are associated with exciton formation in carbon nanotubes?
Which electronic transitions are associated with exciton formation in carbon nanotubes?
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How does the diameter of carbon nanotubes relate to the absorption spectrum?
How does the diameter of carbon nanotubes relate to the absorption spectrum?
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Which statement correctly describes Beer-Lambert's Law?
Which statement correctly describes Beer-Lambert's Law?
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According to Beer-Lambert's law, what happens to the intensity of light as it passes through an absorbing medium?
According to Beer-Lambert's law, what happens to the intensity of light as it passes through an absorbing medium?
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What is the formula used to represent the relationship outlined in Beer-Lambert’s Law?
What is the formula used to represent the relationship outlined in Beer-Lambert’s Law?
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What is the significance of Van-Hove singularities in relation to carbon nanotubes?
What is the significance of Van-Hove singularities in relation to carbon nanotubes?
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Which parameter is also referred to as the molar absorptivity in Beer-Lambert's law?
Which parameter is also referred to as the molar absorptivity in Beer-Lambert's law?
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What effect does the presence of defects have on the absorption spectrum of carbon nanotubes?
What effect does the presence of defects have on the absorption spectrum of carbon nanotubes?
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Which of these transitions corresponds to the wavelength of 982 nm in a carbon nanotube absorption spectrum?
Which of these transitions corresponds to the wavelength of 982 nm in a carbon nanotube absorption spectrum?
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What is the primary reason why the Beer-Lambert law may deviate under non-ideal conditions?
What is the primary reason why the Beer-Lambert law may deviate under non-ideal conditions?
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Which variable in the absorbance equation represents the concentration of the sample?
Which variable in the absorbance equation represents the concentration of the sample?
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Which of the following factors can directly change the molar absorptivity of a substance?
Which of the following factors can directly change the molar absorptivity of a substance?
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In the Tauc relation, what does the variable $E_g$ signify?
In the Tauc relation, what does the variable $E_g$ signify?
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What is implied by a linear plot in the context of the Beer-Lambert law?
What is implied by a linear plot in the context of the Beer-Lambert law?
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What does the variable $γ$ represent in the Tauc relation?
What does the variable $γ$ represent in the Tauc relation?
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Which of the following conditions will NOT likely affect absorbance measurements?
Which of the following conditions will NOT likely affect absorbance measurements?
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What is the significance of the condition $SBW ext{ } hicksim ext{ } 1/10th ext{ } NBW$ in absorbance measurements?
What is the significance of the condition $SBW ext{ } hicksim ext{ } 1/10th ext{ } NBW$ in absorbance measurements?
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For a direct bandgap material, what kind of electronic transition is expected?
For a direct bandgap material, what kind of electronic transition is expected?
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What effect does high concentration have on the absorbance behavior of molecules in solution?
What effect does high concentration have on the absorbance behavior of molecules in solution?
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What key characteristic distinguishes direct bandgap materials in UV-Vis absorption spectroscopy?
What key characteristic distinguishes direct bandgap materials in UV-Vis absorption spectroscopy?
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In measuring the energy bandgap using UV-Vis absorption, what does the intercept with the energy axis on the Tauc plot represent?
In measuring the energy bandgap using UV-Vis absorption, what does the intercept with the energy axis on the Tauc plot represent?
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What can indicate spectral deviations during absorbance measurements?
What can indicate spectral deviations during absorbance measurements?
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Which of the following elements in the Tauc relation is NOT a physical quantity?
Which of the following elements in the Tauc relation is NOT a physical quantity?
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Study Notes
Course Content
- X-Ray Methods of Analysis - TEST I
- UV, Vis, IR and THz spectroscopy
- Nuclear Magnetic resonance, Thermal Analysis Techniques - TEST I
- Raman Spectroscopy
- Electron Methods of Analysis - TEST II
- STM, TEM, AFM Techniques
- Mass spectroscopy
- Polarimetry Techniques - TEST III
Electromagnetic Spectrum
- The electromagnetic spectrum shows the different types of radiation, categorized by their frequency and wavelength.
- Gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves are all part of the spectrum.
- Each type of radiation has a specific range of frequencies and wavelengths.
- The energy of a photon is inversely proportional to its wavelength.
Instrumentation Methods
- Ultraviolet-visible (UV-VIS) spectroscopy
- Fluorescence spectroscopy
- Pump-probe spectroscopy
- Near infrared (NIR) spectroscopy
- Fourier-transform infrared (FTIR) spectroscopy
- Terahertz spectroscopy
Non-Ionization Processes
- Absorption: Interaction of resonant light with atoms/molecules.
- Spectroscopy, Imaging, Sensors, Detectors, Emitters.
- NMR, UV-Vis, THz, Interferometry, Optical Microscopy
- Reflection/Transmission: Imaging, Scattering phenomena, Low-energy neutron scattering experiments
- Electromagnetic Field Meters: Cell phones, Wi-Fi routers, and power lines
- Thermal Imaging: Infrared cameras, temperature sensors.
UV-Vis Spectroscopy
- First method for exploring atomic and molecular structure.
- Measures light absorbed by a sample.
- Atomic absorption is a simple example.
- Joseph Fraunhofer observed atomic absorption in the solar spectrum in 1814.
Features of UV-Vis Spectroscopy
- Analytical techniques using the EM band from 200-700nm.
- Non-destructive
- Qualitative and quantitative analysis
- High sensitivity, nanomolar-picomolar concentration determination
- Essential tool in analytical chemistry
Spectrophotometer
- An instrument that measures the ratio of the radiant power of two EM beams over a wavelength region.
- It utilizes dispersing elements (Prisms/Gratings) instead of filters to scan a large wavelength region.
Ideal Source Requirements
- Stable, no fluctuations
- Continuous spectrum, high and uniform intensity over the entire wavelength region
- Sufficient intensity for detection at the end of the optic path
- No fatigue on continued use
Tungsten Halogen Lamp
- Broadband source similar to a household lamp
- Tungsten filament in an evacuated bulb filled with inert gas and halogen (iodine).
- Maintains the filament temperature below the sublimation point.
- Ideal for polychromatic light
Hydrogen Discharge Lamp
- Encloses electrodes in glass tube (silica/quartz for UV radiation).
- Hydrogen gas inside
- High voltage creates an electron discharge which excites H molecules and emits strong UV radiation.
- Emits sharp lines at 410, 434, 486, and 656 nm.
- Monochromatic lines
- Useful for near UV region
Mercury Discharge Lamp
- Alumina arc tube. Sodium-mercury amalgam. A.C. voltage, ballast
- Shows characteristic sharp lines.
- Useful for UV and visible spectra.
Xenon Discharge Lamp
- Uses tungsten electrodes filled with Xe gas under pressure.
- High intensity arc.
- Wavelengths: 250-800nm.
- Suitable for continuous and discrete
Slits
- Important devices in resolving polychromatic radiation into monochromatic radiation.
- Entrance slit and exit slit are used.
- Width of slit and resolution.
Monochromators
- Devices that isolate specific wavelengths.
- Commonly use filters, prisms, or gratings.
Filters
- Selective absorption of unwanted radiation.
- Transmits the desired radiation.
- Examples: Glass, Gelatin, and colored filters.
Filter Merits
- Simple construction
- Cheaper
- Easy filter selection
Filter Demerits
- Less accurate (20-30nm band pass)
Interference Filters
- Works on interference, rejects unwanted wavelengths by selective reflection
- Uses parallel glass plates with dielectric material for different refractive indices.
- Bandpass of 10-15nm, transmittance of 40-60%.
Interference Filters Merits
- Provide greater transmittance and narrower band pass.
- Inexpensive
Prisms
- Made of glass, quartz, or fused silica.
- Dispersion of polychromatic light into rainbow.
- Rotation of prism to select specific wavelengths.
- Effective wavelength depends on dispersive power of prism material and optical angle.
Gratings
- Effective in converting polychromatic light to monochromatic light.
- Two types: Diffraction grating and transmission grating
- Diffraction grating.Reflection.
Diffraction Gratings
- Highly refined dispersion of light.
- Parallel lines (grooves), often 15000-30000/inch on aluminum surface.
- Replicas made from master gratings using epoxy resin and removed after setting.
Transmission Gratings
- Based on refraction, not reflection.
- Radiation transmitted through grating reinforces with partially refracted radiation.
Advantages of Gratings
- Higher and linear dispersions.
- Usable over wide wavelength ranges
- Constructed with materials for atmospheric moisture resistance
- Provides light of narrow wavelengths.
- No energy loss from absorption.
Comparison of Prisms vs. Gratings
- Prisms:
- Non-linear dispersion
- Limited wavelengths
- Not sturdy
- Gratings:
- Linear dispersion
- Wide range of wavelengths
- Sturdy
Detectors
- Devices converting light energy into electrical signals.
- Displayed on readout devices.
- Determines intensity of absorbed radiation.
- Types used in absorption spectrophotometers:
- Barrier layer cell/Photovoltaic cell
- Phototubes/Photoemissive tube
- Photomultiplier tube
Detectors Requirements
- Quantitative response
- High sensitivity, low noise level
- Short response time.
Barrier Layer Cells/Photovoltaic Cells
- Thin film metallic layer (silver/gold)
- Metal base plate
- Semiconductor layer (selenium)
- When light falls on Se, electrons move, hence current.
- Intensity is proportional to light.
Silicon and InGaAs Charge Coupled Devices (CCDs)
- Liquid-nitrogen cooled Si or InGaAs array.
- CCD array detectors with monochromator
- Suitable for UV-VIS and NIR regions with different sensors.
Instrument Designs
- Depending on monochromator type (filters/dispersing device):
- Photometer
- Spectrophotometer (single or double beam)
- Spectrophotometers measure percentage transmittance of light through a sample.
- Single or double beam optical systems.
Single Beam Photospectrophotometer
- Light from source carried through lenses or aperture.
- Passes through color-specific filter.
- Sample solution in cuvettes.
Double Beam Photospectrophotometer
- Forms two beams using a chopper (circular disk with opaque/transparent/mirrored sections).
- Splits monochromatic light into equal intensity beams.
Calibration Curve
- Absorbance in a sample is proportional to the concentration.
- Using standards, a curve plots absorbance vs. concentration to ascertain sample concentration
UV-Vis Absorption Spectroscopy of Carbon Nanotubes
- Absorption spectrum varies with nanotube diameter (n,m).
- Larger (n,m) = longer wavelengths of absorption.
- Spectroscopy helps identify nanotube species in mixed solutions.
Beer-Lambert's Law
- Light absorption is proportional to concentration and path length.
- Absorbance (A) = εCL (ε = molar absorption coefficient, C = concentration, L = path length)
- Deviation from linearity can occur.
Deviation from Beer-Lambert's Law
- Chemical interactions (high concentrations).
- Dilution (refractive index changes).
- Speed of light changes.
- Fluorescence/phosphorescence emissions.
- Instrumental factors (slit width, polychromatic light)
Measuring Bandgap Using UV-Vis Absorption Spectroscopy
- Tauc relation: (ahv)y = A(hv – Eg).
- Linear behavior in Tauc plot allows determination of the band gap energy.
More Examples
- Different materials show different UV/Vis absorption patterns, allowing for qualitative or quantitative identification.
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