advanced molecular spectroscopy pt 2 (raman & derivative)

Question 1

In raman, incident photons are scattered by molecules with what?

Answer 1

both an increased or decreased energy which is characteristic of sample

Question 2

Scattering of incident photons are at what angles?

Answer 2

right angles

Question 3

What is the energy change equal to?

Answer 3

difference in energy between vibrational energy levels

Question 4

what does the energy change correspond to?

Answer 4

wavelengths in mid-infrared

Question 5

what 3 ways can light interact with substance?

Answer 5

absorbed, transmitted, scattered

Question 6

what are the ways that radiation can be scattered?

Answer 6

elastically or inelastically

Question 7

If it is scattered elastically, how does this affect wavelength? and what is it called?

Answer 7

no change in wavelength, called rayleigh scattering

Question 8

what is it called when radiation is scattered inelastically?

Answer 8

raman effect

Question 9

Where does rayleigh scattering occur? how intense is the peak?

Answer 9

at exact wavelength of excitation - most intense peak

Question 10

Between stokes and antistokes lines, which has greater and smaller energy than rayleigh peak? and which has longer/shorter wavelengths?

Answer 10

stokes = smaller, longer antistokes = greater, shorter

Question 11

how intense are the stokes and antistokes lines?

Answer 11

stokes less intense than rayleigh, antistokes even weaker

Question 12

What are virtual states?

Answer 12

assumed state of photon and molecule during short time period

Question 13

A molecule will only emit monochromatic light and produce raman shifted lines (stokes and antistokes) when what occurs?

Answer 13

when molecule vibrates so that polarizability is changed during vibration

Question 14

Raman ………… shift = infrared ………… peak frequency

Answer 14

frequency, absorption

Question 15

Scattering involves a momentary distortion of what?

Answer 15

electrons distributed around a bond

Question 16

What happens to the molecule in its distorted form? 2

Answer 16

it is temporarily polarised, ie develops a momentary induced dipole

Question 17

This deforms molcules, leading to what?

Answer 17

vibrations with characteristic frequency

Question 18

When there is no dipole moment in equilibrium position or when stretched, what does this mean?

Answer 18

no infrared absorption

Question 19

If the polarizability varies, what does this mean?

Answer 19

it is raman active

Question 20

In symmetric and anti symmetric modes of stretching, what happens to the dipole moment?

Answer 20

symmetric = no dipole moment, antisymmetric = dipole moment fluctuates

Question 21

Are symmetricand antisymmetric modes IR active or inactive?

Answer 21

symmetric - inactive, antisymmetric - active

Question 22

Describe the polarizability of symmetric and anti-symmetric stretching modes?

Answer 22

symmetric = polarizability fluctuates in phase with vibration, antisymmetric = polarizability of one bond increases as other decreases

Question 23

Are symmetric and antisymmetric stretching modes raman active or inactive?

Answer 23

symmetric = raman active, antisymmetric = raman inactive

Question 24

does raman require a special sample prep method?

Answer 24

no

Question 25

Why does water show up on raman and not usually on FTIR?

Answer 25

no background problem in raman

Question 26

what types of cell might be used in raman?

Answer 26

inexpensive glass or quartz

Question 27

what types of probes ideally used? 2

Answer 27

non-invasive and immersion probes

Question 28

Which spectra is less cluttered, raman or FTIR?

Answer 28

raman

Question 29

What makes this technique well suited for both organic and inorganic species?

Answer 29

spectral range is below 400cm-1

Question 30

What can be used to do a remote analysis in raman?

Answer 30

fibre optic cables

Question 31

A major limitation of raman is background signals arising - what causes this?

Answer 31

signal arising from fluorescence of analyte or impurities

Question 32

Is raman strong or weak effect?

Answer 32

weak

Question 33

incident flux of … photons will give 1 raman ……

Answer 33

10^8, photon

Question 34

A ppm impurity with a quantum yield of …. will give ….. fluorescence photons for the same flux

Answer 34

0.1, 10

Question 35

What range are the detection limits in?

Answer 35

parts per thousand

Question 36

How can this problem be eliminated?

Answer 36

using FT-Raman

Question 37

It uses a high power Nd-YAG laser at what wavelength?

Answer 37

1064nm

Question 38

Is raman more/less expensive than FTIR?

Answer 38

more, due to expensive laser and filters

Question 39

What are 2 advantages of FT-Raman over dispersive?

Answer 39

1064nm eliminates fluorescence and photodecomposition, superior frequency precision

Question 40

What is a disadvantage of FT-Raman over dispersive?

Answer 40

Water absorbs in the 1000nm region

Question 41

What 2 ways can raman be enhanced?

Answer 41

resonance raman effect, surface-enhanced raman spectroscopy (SERS)

Question 42

In resonance raman effect, how are the raman signals greatly enhanced?

Answer 42

by exciting with wavelength near electronic absorption peak

Question 43

how does this differ from fluorescence?

Answer 43

no vibrational relaxation

Question 44

how is this limited?

Answer 44

due to fluorescence of analyte or impurities

Question 45

Where are samples adsorbed in SERS?

Answer 45

on surface of collodial metal particles

Question 46

When combined with resonance raman, how much is enhancement increased by?

Answer 46

100x

Question 47

Name 2 applications of raman?

Answer 47

fingerprinting, paint analysis

Question 48

What is derivative spectroscopy used for?

Answer 48

removing spectral interferences and increasing selectivity