6. Infrared and Raman

Question 1

what are the two models of vibrational spectroscopy?

Answer 1

harmonic and anharmonic spectroscopy

look at image

Question 2

what are the mechanical models based on?

Answer 2

the potential energy curve as the nuclear positions are changed

for example, for a diatomic molecule, only one internuclear distance to consider

Question 3

what is the harmonic approximation?

Answer 3

its for a mass fixed to the surface with a spring. like a classical spring

basically this one is the symmetrical curve of the displacement

Question 4

why is the anharmonic model importnat?

Answer 4

because the harmonic model is over simplified - molecules are not classcial!!!

potential energy is not harmonic

Question 5

what is the dipole moment? what occurs to it during IR?

Answer 5

the moment of charge seperation in a molecule.

during IR, it must change to absorb!

Question 6

what is dynamic dipole - three points

Answer 6

1. vibration of molecules
2. must change to be absorbed by IR
3. when there is assymetry in the molecule

Question 7

what is a permanent dipole? 2 points

Answer 7

1. electronegatively dosent absorb IR
2. when theres symmetry in the molecule

Question 8

what happens if there is a symmetric stretch in a molecule and absorbance of IR? how wouldthis compare if theres an assymetric stretch?

Answer 8

in a symmetric stretch - theres no absorbance of IR and its in permanent dipole

in an assymetric stretch - theres absorbance of IR because its in a dynamic dipole

Question 9

what are 3 problems when working with infrared?

Answer 9

1. infrared detectors are inefficent
2. noise from the detector means we need many photons to see anything above the noise
3. IR sources are weak

Question 10

how can the 3 problems be solved?

Answer 10

1. sample wavelengths simultaneously - more photons
2. cool detector for less thermal noise
3. use a hot efficent emmiter

Question 12

what is FTIR?

Answer 12

obtains the infrared spectrum of absorbption OR emission of a solid, liquid or gas.

Question 13

what is a grating based spectrometer?

Answer 13

it includes IR

Question 14

what are the advatages from using a interferometer instead of a grating based instrument?

Answer 14

1. less noise - all wavelengths are sampled at all posistions of the mirrors.
2. more light -no slits to limit amount of light
3. precise - a laser is used

Question 15

what is ATR?

Answer 15

a sampling technique which can be used with FTIR

(ATR: attenuated total reflectance)

(FTIR - fourier transforms infrared)

Question 16

why is ATR the most popular technique of measure IR for a solid sample

Answer 16

because ATR can be used with FTIR.

no further preperation of solids or liquids is necessary!!

Question 17

what are the differences between strokes and anti-stokes raman scattering?

Answer 17

strokes:

scattered light (looses energy equivalent to molecule vibration)

antistrokes:

gains energy equivalent to molecule vibration