Optical Spectroscopy Flashcards
(30 cards)
Emission spectroscopy
Emitted light
of excited molecules/atoms
observed
Absorption spectroscopy
Observed
how light is absorbed
Energy of a photon equation
E = hv
E = hc/λ
E = hcṽ
Absorption interaction of light with molecules
Photon E
absorbed by molecule
Molecule promoted
from lower E ground state
to higher E excited state
ΔE = hv
Spontaneous emission interaction of light with a molecule
Molecule in excited state
may spontaneously lose excitation
releasing E of photon
ΔE = hv
Induced emission interaction of light with a molecule
Molecule in excited state
& already photon present w/ right E
may be stimulated to loose excitation
releasing E in photon
ΔE = hv
Beer-Lambert Law equation
I = I0 exp(-σ C l)
General features of molecular spectroscopy
Rotational:
Steps are small
Microwave spectroscopy
Vibrational:
Steps are intermediate
IR spectroscopy
Electronic:
Steps are big
UV-vis spectroscopy
Microwave spectroscopy energy equation
E = B J (J + 1)
B = rotational contant
J = rotational quantum no.
I = moment of inertia
μ = reduced mass
B = h(^2) / 8 π(^2) I
I = μ R(^2)
Microwave spectroscopy selection rules
Molecule needs a permanent dipole
Vibrational spectroscopy energy equation
E = h𝜈0 (v + 1/2)
𝜈0 = (1 / 2π)(k / μ)^1/2
Identification from vibrational spectroscopy
Almost always
comparison of spectrum
to database
Vibrational spectroscopy energy at room temp.
ΔE»_space; kT
Only v = 0 quantum level
significantly populated
IR vibrational spectroscopy selection rules
Electric dipole moment
must change by vibration
(IR active)
Rule of thumb:
Each additional quantum change
makes IR transition 10x weaker
IR spectra appearance for different phases
Gas
Often well resolved
Fine structure apparent
Liquid & solid
Often broad
Fine structure washed out & not clear
IR vibrational spectroscopy limitations
Need to identify band from component of interest
that isn’t overlapping w/ other components
Usually okay for simple mixtures
Near-infrared region investigation pros
Can use glass
Fibre optical cables exist
don’t absorb into region
∴ remote sensing possible
NIR cons
Only weak overtones & combination bands
Spectra hard to interpret
Increase sensitivity FTIR white cell
Increase absorption path length
use v. long cell
10 - 20 cm
more not practical
FTIR problem sampling liquids and solids
Liquid cells v. short path length (<1 mm)
∵ strong absorbances
difficult to fill without air bubbles
Solids pressed into KBr pellets
problem w/ reproducibility
extensive prep required
UV-vis uses
Sample usually liquid
Follow changes colour/composition
quite rapidly
UV-vis limitations
Doesn’t work if not absorbed in UV region
Not good if sample contains
several species that absorb in UV
AAS advantages
Sensitive to wide range elements
High accuracy
if care taken over prep. & calibration
AAS disadvantages
Some solids difficult to get solution form
Need hollow cathode lamp
for sharp monochromatic lines
for each element
Difference atoms require
different flame temp.
to achieve reliable results
