general principles of spectroscopy Flashcards
basic features of all spectroscopy
sample is exposed to electromagnetic radiation.
energy of this gradually changes.
when the energy corresponds to the difference between energy levels in the sample, it is absorbed.
what is absorption spectroscopy
when sample is exposed to EMR which changes in energy.
if energy of EMR matches a gap between different energy levels of the sample, EMR is absorbed.
what is the lowest energy state of a molecule
ground state
what is the state of a molecule when it absorbs radiation and increases in energy
excited state
energy absorbed by a molecule corresponds toooo
difference between energy levels
what can the graphs for absorbance spectroscopy show
show absorbance against energy
- absorption is seen by a hill
aka it points up from baseline
show transmittance against energy
- transmittance is seen as an icicle
aka it points down from baseline
what do the positions of the peaks show on absorption spec, remember that it’s abs/trans plotted against energy of radiation
peak corresponds to the energy of transitions
THE ENERGY BETWEEN ENERGY LEVELS.
2 diff types of absorption spec
single beam
double beam
what is used in a single beam absorption spec
a monochromator
what does a monochromator do
selects specific wavelengths of electromagnetic radiation.
allows one narrow band of wavelengths to hit the sample by splitting radiation
diffraction grating
what happens to the monochromator over time
is selects and allows a range of different wavelengths to hit the sample.
allows the detector to plot abs/trans for a series of different wavelengths.
problem of single beam absorption spectra
background absorption in IR SPEC such a CO2 and H2O in the atmosphere
what is double beam arrangement
radiation passes through a monochromator then a beam splitter that splits it evenly (identical beams)
one goes through sample, one doesn’t (reference) difference is recorded allowing background abs/trans to be ruled out.
how is the spectrum recorded
(dispersive instrument)
vary the wavelength of radiation passing into the same
record the output from the detector as a function of wavelength
what does the position of the peak correspond to
energy or transition
aka energy difference between two energy levels in the molecule
what determines peak size
intensity
what does intensity depend on
number of absorptions that take place, depends on:
- conc of sample
- path length
- amount molecules in the correct energy state to absorb/emit at this frequency
- likelihood of transition occurring
incident radiation
I0
radiation that shines onto sample
intensity of transmitted radiation
I
(reduced in energy due to some of I0 being absorbed by the sample)
transmittance equation and units
I0 / I
incident radiation / transmitted radiation
no units
Absorbance equation and units
-log (T)
T is transmittance
no units
units of conc in beer lambert
moldm-3
units of molar extinction coefficient in beer lambert
mol-1 dm3 cm-1
or m2mol-1
another equation for absorbance
A = -log(I0/I)
what acc is the molar absorbance conc
how well/effectively the molecules can acc absorb radiation
beer lambert equation (long one)
-log(I0/I) = Ecl = A
when using the SI units for molar extinction coefficient, what must we convert
conc : moldm-3 x1000 = molm-3
length: cm x100 = m
units = m2 mol-1
what else can determine the intensity of peaks in absorption and emission spec
the population of molecules at certain energy levels
in absorption: abs of photon allows promotion to higher energy level, more molecules on lower level increases chance that one will abs photon and become promoted
what is the boltzmann constant Kb
rate constant / avogadros number
(1.381 x10 ^-23)
what does the boltzmann distribution describe
the population of molecules in the energy levels
the population of molecules at the higher energy level depends
the energy difference between the 2 levels.
equation to find population of molecules at energy levels
N upper / N lower = Gup/ Glow x
e ^ (-🔺E / T x Kb)
where T is temp in K
-🔺E is in J
what if Gup and Glow
aka the degeneracies of both states is 1
means they are they are similar
larger 🔺E value means
ratio of population is close to 0,
small population at upper state
pop of higher level can increase with temp tho
what if 🔺E is small
population is similar to degeneracy
if degeneracy is 1, the population at both states are similar
remembering the graphs for absorption and emission spectra, what population level affects the intensity peaks
absorption: lower energy population
as they are abs + being promoted
emission: higher energy population as they are emitting + going down
what do selection rules tell us
tell us which transitions are allowed
allowed = yes transition
forbidden= low probability of transition, intensity is close to 0 bc there’s no abs or trans
what does a large number of energy levels mean
there can be a large amount of transitions possible
however not all transitions give rise to abs or trans
gross selection rules general statement
say what properties molecules need in order to absorb radiation
specific selection rules general statement
which levels transitions are allowed between
what does laser stand for
light amplification by stimulated emission of radiation
useful properties of lasers
monochromatic ( photons have same energy)
coherent (all photons are in the same phase)
laser uses
surgery
spectroscopy
bar code readers
cutting materials aka metals and plastics.
process of getting to an excited state
molecule absorbs a photon
electron in the molecule is promoted to a higher energy level (transitions)
excited state is reached.
spontaneous emission
molecules at an excited state release a photon and return back to ground state
simulated emission
excited state molecule interacts with a photon with the energy of the energy gap of the current energy level and the lower energy level and moves down to ground state.
what leads to the amplification of light intensify
increased number of photons with the same energy.
what is special aboht the photons emitted at stimulated emission spec
that have the same energy and are in the same phase
probability of stimulated emission increasing increases
as the number of photons increases
aka more photons = more probability that one will have the energy gap of the current and lower energy level = more probability of stimulated emission
how is stimulated emission sustained??
greater population of molecules in excited state than lower state.
use of pumping : flash of light or electrical discharge is applied.
allows for non equilibrium conditions (high pop of excited state)
pg: 464 chem3
spontaneous emission and pumping story line
energy applied via pumping to get molecule to excited state, 4
reaches excited state and decays rapidly to level 3 (no emission - radiation less- energy is transferred to vibrational motion aka heat) level 3 to level 2 uses spontaneous spec, level 2 to ground state also decays quick and its radiation less.
what does the pumping story line tell us about population at diff energy levels
level 3 has a greater pop as molecules decay from 4 to 3 very fast and from 2 to ground state very fast.
3 to 2 occurs via spontaneous emission and emission of photon. (laser transition)
population inversion ; high pop in 3 but not 2, laser transition can occur.
