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physical 3 - spectroscopy > general principles of spectroscopy > Flashcards

general principles of spectroscopy Flashcards

1
Q

basic features of all spectroscopy

A

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.

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2
Q

what is absorption spectroscopy

A

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.

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3
Q

what is the lowest energy state of a molecule

A

ground state

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4
Q

what is the state of a molecule when it absorbs radiation and increases in energy

A

excited state

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5
Q

energy absorbed by a molecule corresponds toooo

A

difference between energy levels

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6
Q

what can the graphs for absorbance spectroscopy show

A

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

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7
Q

what do the positions of the peaks show on absorption spec, remember that it’s abs/trans plotted against energy of radiation

A

peak corresponds to the energy of transitions

THE ENERGY BETWEEN ENERGY LEVELS.

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8
Q

2 diff types of absorption spec

A

single beam
double beam

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9
Q

what is used in a single beam absorption spec

A

a monochromator

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10
Q

what does a monochromator do

A

selects specific wavelengths of electromagnetic radiation.

allows one narrow band of wavelengths to hit the sample by splitting radiation
diffraction grating

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11
Q

what happens to the monochromator over time

A

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.

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12
Q

problem of single beam absorption spectra

A

background absorption in IR SPEC such a CO2 and H2O in the atmosphere

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13
Q

what is double beam arrangement

A

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.

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14
Q

how is the spectrum recorded
(dispersive instrument)

A

vary the wavelength of radiation passing into the same

record the output from the detector as a function of wavelength

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15
Q

what does the position of the peak correspond to

A

energy or transition
aka energy difference between two energy levels in the molecule

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16
Q

what determines peak size

A

intensity

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17
Q

what does intensity depend on

A

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

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18
Q

incident radiation

A

I0
radiation that shines onto sample

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19
Q

intensity of transmitted radiation

A

I
(reduced in energy due to some of I0 being absorbed by the sample)

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20
Q

transmittance equation and units

A

I0 / I

incident radiation / transmitted radiation

no units

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21
Q

Absorbance equation and units

A

-log (T)

T is transmittance

no units

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22
Q

units of conc in beer lambert

A

moldm-3

23
Q

units of molar extinction coefficient in beer lambert

A

mol-1 dm3 cm-1

or m2mol-1

24
Q

another equation for absorbance

A

A = -log(I0/I)

25
Q

what acc is the molar absorbance conc

A

how well/effectively the molecules can acc absorb radiation

26
Q

beer lambert equation (long one)

A

-log(I0/I) = Ecl = A

27
Q

when using the SI units for molar extinction coefficient, what must we convert

A

conc : moldm-3 x1000 = molm-3
length: cm x100 = m

units = m2 mol-1

28
Q

what else can determine the intensity of peaks in absorption and emission spec

A

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

29
Q

what is the boltzmann constant Kb

A

rate constant / avogadros number

(1.381 x10 ^-23)

30
Q

what does the boltzmann distribution describe

A

the population of molecules in the energy levels

31
Q

the population of molecules at the higher energy level depends

A

the energy difference between the 2 levels.

32
Q

equation to find population of molecules at energy levels

A

N upper / N lower = Gup/ Glow x
e ^ (-🔺E / T x Kb)

where T is temp in K
-🔺E is in J

33
Q

what if Gup and Glow

aka the degeneracies of both states is 1

A

means they are they are similar

34
Q

larger 🔺E value means

A

ratio of population is close to 0,

small population at upper state

pop of higher level can increase with temp tho

35
Q

what if 🔺E is small

A

population is similar to degeneracy

if degeneracy is 1, the population at both states are similar

36
Q

remembering the graphs for absorption and emission spectra, what population level affects the intensity peaks

A

absorption: lower energy population
as they are abs + being promoted

emission: higher energy population as they are emitting + going down

37
Q

what do selection rules tell us

A

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

38
Q

what does a large number of energy levels mean

A

there can be a large amount of transitions possible

however not all transitions give rise to abs or trans

39
Q

gross selection rules general statement

A

say what properties molecules need in order to absorb radiation

40
Q

specific selection rules general statement

A

which levels transitions are allowed between

41
Q

what does laser stand for

A

light amplification by stimulated emission of radiation

42
Q

useful properties of lasers

A

monochromatic ( photons have same energy)

coherent (all photons are in the same phase)

43
Q

laser uses

A

surgery
spectroscopy
bar code readers
cutting materials aka metals and plastics.

44
Q

process of getting to an excited state

A

molecule absorbs a photon
electron in the molecule is promoted to a higher energy level (transitions)
excited state is reached.

45
Q

spontaneous emission

A

molecules at an excited state release a photon and return back to ground state

46
Q

simulated emission

A

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.

47
Q

what leads to the amplification of light intensify

A

increased number of photons with the same energy.

48
Q

what is special aboht the photons emitted at stimulated emission spec

A

that have the same energy and are in the same phase

49
Q

probability of stimulated emission increasing increases

A

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

50
Q

how is stimulated emission sustained??

A

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

51
Q

spontaneous emission and pumping story line

A

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.

52
Q

what does the pumping story line tell us about population at diff energy levels

A

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.

53
Q
A
54
Q
A

physical 3 - spectroscopy (12 decks)

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