Karteikarten
Raman spectroscopy and chapter 18
Karten in diesem Stapel (38)
- 1what is scatteringenergy of the electromagnetic
wave is not lost but redistributed to other
waves in all directions by particles in the
path of the wave. - 2Scattering depends uponwavelength
and size of the scattering particle - 3An object will strongly scatter a wave ifthe
wavelength and object are close to the
same size. - 4Raman scattering involves a combination of (why two types of scattering)elastic scattering (Raleigh) and
inelastic scattering - 5what causes stokes shiftphoton energy
is lost to the molecule - 6what causes anti-stokes shiftenergy is transferred from the molecule to the photon
- 7elastic scattering occurs when there isno transfer of energy
- 8inelastic scattering results froma transfer of energy
- 9formula for energy of scattered EMR, the electric fielda=polarizability, Eo=average energy, Vo=frequency, t=time
- 10Label stokes, antistokes, rayleigh, IR, fluorescence, virtual state line
- 11formula for polarizability , alphaq=distance beyond equilibrium bond length
- 12Raman scattered radiation is in thein the visible or near-IR regions
- 13What is a virtual state?Because the excitation wavelength is usually well away from the absorption band, the molecule is said to enter a virtual state when it is excited by the light energy, usually represented by a dashed line
- 14Advantage of Raman-selection rules are different
-vibrations are strong is the bond is nonpolar
-Resonance Raman selectively enhances
vibrations of a chromophore
- Only small sample needed since laser beam < 2 mm in diameter
-water is a weak raman scatterer - 15scattering involves a momentary distortion of the electrons distributed around a bond in a molecule, followed byreemission of the radiation as the bond returns to its normal state.
- 16mutual exclusion principleFor molecules with a center of symmetry, such as CO2, no IR active transitions are in common with Raman active transitions
- 17Disadvantages of raman• Laser used can cause local heating or
photodecomposition of the sample.
• Some compounds fluoresce when irradiated with the laser beam.
• It’s more difficult to observe rotational vibrational spectra with high resolution Raman.
• Raman systems cost more than
conventional FT-IR spectrometers. - 18raman scattering is collected at a ___ degree angle90, or 180* with a curve mirror
- 19polarizability isability to disturb electron orbitals so electrons are pulled to one side of the molecule
- 20it is more difficult to pull electrons away from anelectronegative atom
- 21polarizability changes with ______ and ______bond length and as the bond vibrates
- 22how can raman scattering can measure temperature bythe ratio of Stokes to antiStokes gives a boltzmann distribution of temperature, so this can be used as a thermometer
- 23in a fiber optic, what determines the acceptance cone filterNumerical aperture, it describes the range of angles the cone can accept
- 24The intensity or radiant power of a normal Raman band depends onthe polarizability of the molecule, the intensity of the source, and the concentration of the active group
- 25the intensity of scattering is proportional tothe frequency of light ^4
- 26how do you get the best raman signaluse the highest power laser for excitation your sample can withstand
- 27why would you not want to use lasers with high frequencies (small wavelengths) that occur in the visible regionbecause raman would be competing with fluorescence
- 28to mitigate fluorescence, you shoulduse NIR laser, or differential raman
- 29in deep uv raman, fluorescence is not observed in the same part of the spectra as the raman, whya stokes shift occurs from the excitation frequency, causung a red shift in the fluorescence to a longer wavelength
- 30at excitation wavelengths less than 250 nm, the raman and fluorescence can be spatially separated byfiltering(grating) or spectrograph
- 31surface enhanced raman spectroscopy (SERS)-Nano-metallic substrates(metal nanoparticeles) locally amplify
electromagnetic fields at or near particle
surfaces.
• > 10^6 enhancement over normal Raman
• Quicker response times & reduction in
fluorescence - 32what advantages do NIR lasers have over shorter wavelength lasers?-NIR lasers can be operated at a higher power without causing photo decomposition, and they are not energetic enough to promote fluorescence
- 33why is it good to use more than one source or a source that contains multiple wavelengths in raman?colored samples and some solvents can absorb the incident radiation or the Raman-scattered radiation
- 34what type of wavelength selectors are ideal for ramana notch filter and a high-quality grating monochromator are found in most modern raman spectrometers, a high quality wavelength selector is needed to separate the relatively weak Raman lines from the intense Rayleigh-scattered radiation.
- 35Many newer Raman instruments have replaced the single-wavelength output monochromator withspectrograph and an array detector.
- 36(a) Under what circumstances would a helium-neon laser be preferable to an argon-ion laser?1. The sample absorbs at 488.0 nm but not at 632.8 nm.
2. The helium-neon laser has lower energy than argon-ion laser. This can avoid the photochemical decomposition and background fluorescence - 37(b) Under what circumstances would a diode laser be preferable to an argon-ion or helium-neon laser?1. To avoid photo decomposition
2. To avoid fluorescence
3. when high power is required - 38(c) Why are UV emitting sources avoided?1. To avoid photo decomposition
2. To avoid fluorescence
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- GRE instrumental analysis Karteikarten am Raman spectroscopy and chapter 18, erstellt von Ashlin Deatherage am 21/03/2023.
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