general principles of spectroscopy

Question 1

basic features of all spectroscopy

Answer 1

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.

Question 2

what is absorption spectroscopy

Answer 2

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.

Question 3

what is the lowest energy state of a molecule

Answer 3

ground state

Question 4

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

Answer 4

excited state

Question 5

energy absorbed by a molecule corresponds toooo

Answer 5

difference between energy levels

Question 6

what can the graphs for absorbance spectroscopy show

Answer 6

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

Question 7

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

Answer 7

peak corresponds to the energy of transitions

THE ENERGY BETWEEN ENERGY LEVELS.

Question 8

2 diff types of absorption spec

Answer 8

single beam
double beam

Question 9

what is used in a single beam absorption spec

Answer 9

a monochromator

Question 10

what does a monochromator do

Answer 10

selects specific wavelengths of electromagnetic radiation.

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

Question 11

what happens to the monochromator over time

Answer 11

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.

Question 12

problem of single beam absorption spectra

Answer 12

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

Question 13

what is double beam arrangement

Answer 13

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.

Question 14

how is the spectrum recorded
(dispersive instrument)

Answer 14

vary the wavelength of radiation passing into the same

record the output from the detector as a function of wavelength

Question 15

what does the position of the peak correspond to

Answer 15

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

Question 16

what determines peak size

Answer 16

intensity

Question 17

what does intensity depend on

Answer 17

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

Question 18

incident radiation

Answer 18

I0
radiation that shines onto sample

Question 19

intensity of transmitted radiation

Answer 19

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

Question 20

transmittance equation and units

Answer 20

I0 / I

incident radiation / transmitted radiation

no units

Question 21

Absorbance equation and units

Answer 21

-log (T)

T is transmittance

no units

Question 22

units of conc in beer lambert

Answer 22

moldm-3

Question 23

units of molar extinction coefficient in beer lambert

Answer 23

mol-1 dm3 cm-1

or m2mol-1

Question 24

another equation for absorbance

Answer 24

A = -log(I0/I)

Question 25

what acc is the molar absorbance conc

Answer 25

how well/effectively the molecules can acc absorb radiation

Question 26

beer lambert equation (long one)

Answer 26

-log(I0/I) = Ecl = A

Question 27

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

Answer 27

conc : moldm-3 x1000 = molm-3 length: cm x100 = m units = m2 mol-1

Question 28

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

Answer 28

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

Question 29

what is the boltzmann constant Kb

Answer 29

rate constant / avogadros number (1.381 x10 ^-23)

Question 30

what does the boltzmann distribution describe

Answer 30

the population of molecules in the energy levels

Question 31

the population of molecules at the higher energy level depends

Answer 31

the energy difference between the 2 levels.

Question 32

equation to find population of molecules at energy levels

Answer 32

N upper / N lower = Gup/ Glow x e ^ (-🔺E / T x Kb) where T is temp in K -🔺E is in J

Question 33

what if Gup and Glow aka the degeneracies of both states is 1

Answer 33

means they are they are similar

Question 34

larger 🔺E value means

Answer 34

ratio of population is close to 0, small population at upper state pop of higher level can increase with temp tho

Question 35

what if 🔺E is small

Answer 35

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

Question 36

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

Answer 36

absorption: lower energy population as they are abs + being promoted emission: higher energy population as they are emitting + going down

Question 37

what do selection rules tell us

Answer 37

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

Question 38

what does a large number of energy levels mean

Answer 38

there can be a large amount of transitions possible however not all transitions give rise to abs or trans

Question 39

gross selection rules general statement

Answer 39

say what properties molecules need in order to absorb radiation

Question 40

specific selection rules general statement

Answer 40

which levels transitions are allowed between

Question 41

what does laser stand for

Answer 41

light amplification by stimulated emission of radiation

Question 42

useful properties of lasers

Answer 42

monochromatic ( photons have same energy) coherent (all photons are in the same phase)

Question 43

laser uses

Answer 43

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

Question 44

process of getting to an excited state

Answer 44

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

Question 45

spontaneous emission

Answer 45

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

Question 46

simulated emission

Answer 46

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.

Question 47

what leads to the amplification of light intensify

Answer 47

increased number of photons with the same energy.

Question 48

what is special aboht the photons emitted at stimulated emission spec

Answer 48

that have the same energy and are in the same phase

Question 49

probability of stimulated emission increasing increases

Answer 49

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

Question 50

how is stimulated emission sustained??

Answer 50

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

Question 51

spontaneous emission and pumping story line

Answer 51

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.

Question 52

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

Answer 52

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.