unit 2: spectrophotometry Flashcards
Spectrum
display of intensity of radiation emitted, absorbed, or scattered by a sample as a
function of photon energy (typically in wavelength, frequency, or wavenumber)
Spectroscopy
study of the interaction between light and matter
Electromagnetic spectrum and quantum transitions
different atomic and molecular
transitions result from the interaction of a sample with light of different energies; know these
transitions and region of the EM spectrum causes them
Beer’s law
gives a quantitative relation between the attenuation of light and the concentration
of the absorber
Relevant equations:
T = P/P0
A = -log T
A = εbc
Atotal = A1 + A2 + A3 + …
limiations:
Real – applies only to dilute solutions
Instrumental – polychromatic radiation; mismatched cells; and stray light (𝐴′ = log (𝑃0+𝑃𝑠/𝑃+𝑃𝑠))
Chemical – any reaction that will change the absorptivity
Jablonski (energy level) diagram
understand, reproduce, and interpret these diagrams for
excitation by UV, Vis, and IR radiation, and the resulting quantized transitions (figure 18.16 in 10th
edn of Harris and in the lecture; I will not reproduce it here)
Select terms to know: absorbance, luminescence, fluorescence, phosphorescence, nonradiative relaxation, radiative relaxation, intersystem crossing, internal conversion, singlet
and triplet states
Excitation
Molecular absorbance spectrum – band spectrum
Gaseous samples – many lines observed due to electronic, vibration, and rotational
energy levels
Sample in solvent with minimal interactions – exhibit electronic transitions, but vibrational
and rotational structure is lost
Sample in solvent with strong IM forces – exhibits condensed phased broadening, or
interactions with solvent molecules and/or collisions that spread the energies of the
quantum states, resulting in a wider energy range of transitions and a broader
absorption band
Atomic absorbance spectrum – line spectrum
Only electronic transitions are applicable; spectra exhibit very narrow absorption lines
emission
𝐼 = 𝑘𝑃𝑜𝑐
Phosphorescence – caused by intersystem crossing to a different electronic state (triplet) and
subsequent radiative relaxation to the ground electronic state
Fluorescence – caused by radiative relaxation from the singlet state to the ground electronic state
Atomic fluorescence – emitted wavelength is identical to that of the excitation process,
called resonance fluorescence
Molecular fluorescence – more available energy states due to vibration and rotational
energy levels
*Note: Non-radiative relaxation back to ground electronic state can (and does!) also occur
Basic Components of Spectrophotometers:
(1) light source, (2) wavelength selector(s), (3)
sample container(s), (4) radiation detector, and (5) signal processor and data display
sources
choice dictated by application
- Continuum - emit nearly uniform intensity over a range of wavelengths
UV – H2 and D2 (110 – 400 nm)
UV/Vis/near IR – tungston/halogen lamp (320 – 2500 nm)
IR – Nernst glower (400 – 20,000 nm); globar (10,000 – 40,000 nm)
Fluorescence – Xe arc lamp (175 – 1000 nm) - Line – emit a limited number of wavelengths
Low pressure Hg arc lamps (253.7 nm, LC-UV), hollow cathode, and lasers - Continuous – constantly emit radiation over time
- Pulsed – emit radiation in burst (bursts can be very short)
cells
sample holder in spectroscopy; often specifically a cuvette
Cell(s) must be optically transparent over the spectral region of interest:
Visible Many materials transparent, so often go with the cheapest option: plastic
or glass
UV Fused silica or quartz
IR NaCl, AgCl, KBr optics (often plates) or diamond
(as in the ATR (attenuated total reflection)
attachment on the IRs in the 205 lab)
Wavelength selectors
Resolution – the difference between two just separated bands (i.e. can be distinguished from one
another). For resolved spectral peaks, this is Δλ and typically measured in nm
Note: Resolution in spectrophotometry is dependent on both spectral bandwidth (below)
and the natural absorbance bandwidth of absorbing species.
Resolving power – a measure of the ability to separate two wavelengths 𝑅𝑃 =𝜆/Δ𝜆= 𝑛𝑁
Grating – reflective component of a grating monochromator with closely ruled lines
𝑛𝜆 = 𝑑(𝑠𝑖𝑛𝜃 + 𝑠𝑖𝑛𝜙)
Dispersion of grating – separation of adjacent wavelengths through the difference in angle Δ𝜙/Δ𝜃=
𝑛/𝑑𝑐𝑜𝑠𝜙
Spectral Band Width (SBW) – wavelength range of light passing through the slit on a
monochromator 𝑆𝐵𝑊 =𝑊𝑑/ nF
Filters (wavelength selectors)
absorb/reflect radiation from a source and only allow select wavelengths to pass
Band Pass – allows the specified wavelength to pass and rejects others
Band stop/band rejection – stops the specified wavelength, transmits others
(notch filters have a very narrow stop band)
Interference filters (Fabry-Perot or dielectric filters) – employ optical interference to allow narrow
(10 - 100 nm) bands of radiation with a high transmittance within that band
Holographic filters – produce very narrow stop bands, used in notch filters
Absorbance filters – composed of colored glass that removes incident light via absorption
Monochromator (wavelength selectors)
rely on diffraction and optical interference to select wavelength
Key components: (1) entrance slit, (2) collimating lens or mirror, (3) prism or grating, (4) focusing
element, and (5) exit slit (see figure in Harris and lecture; image not
reproduced here)
detectors
Radiation transducer – a detector that converts light into an electrical signal; often divided into
photon transducers (convert photons to electricity, limited by shot noise) and thermal transducers
(convert temp due to the power of incident radiation to electrical signals, limited by thermal noise)
Photon Transducers
Photomultiplier tube (PMT) –based on the photoelectric effect; especially adept at
measuring low-powered radiation in the UV/Vis regions (image in Harris and class notes,
not reproduced here)
Photodiode and photodiode array (PDA) – employs a reverse-based p/n junction; less
sensitive than PMTs or CCDs and employed in UV to near IR regions (image in Harris and
class notes, not reproduced here)
Charge coupled device (CCD) – relies on promoting electrons from the valance band of
Si to the conducting band; enables sensitive detection of UV/Vis radiation
Thermal transducers – e.g. thermocouples, pyroelectric transducers, etc. – radiation is absorbed,
and the resulting change in temperature is measured and converted into an electrical signal,
enables detection of IR radiation
Fluorescence
Typically occurs from lowest vibrational state of the lowest excited electronic state to
various vibrational E levels of S0
- Fluorescence can be related to concentration by:
𝐼 = 𝑘Φ𝑃𝑜𝑐 (𝑘 ≈ 𝜀𝑏)
- Quantum yield – describes the efficiency of fluorescence
Φ = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑒𝑚𝑖𝑡𝑡𝑒𝑑/𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 =𝑘𝑟𝑎𝑑/k𝑛𝑜𝑛−𝑟𝑎𝑑 + 𝑘𝑟𝑎𝑑
- Competition between relaxing non-radioactively and luminescence (fluorescence and
phosphorescence)
