electric field on Y axis

magnetic field on X axis

because of wave partical duality

thus, we often need a model of light as a corpuscular particle

photons

the energy (E) of a photon is related to the wave properties of 

• wavelength
• frequency
• wavenumber

wavelength

symbol give : lamba

measured in metres

frequency

symbol given: V

measured in cycles per second = Hz

wavenumber

symbol given: V with a line on top.

measured in metres^-1

2.998 x 10⁸ m^-1

is which constant?

the speed of light baby!

it is given the symbol C

6.63 x 10^-34 Js

is which constant?

is it plancks constant

it is given the symbol 'h'

E = hv

E = plancks constant x frequency

E = hc/λ

E= plancks x speed of light / wavelength

E = hcv^-

E= plancks x speed of light x wavenumber

v = c/λ

frequency = speed of light / wavelength

v^- = 1/λ

wavenumber = 1/wavelength

photons from different regions of the spectrum have very different energies - their wavelengths and frequencies vary according.

name the electro magnetic spectrum from highest energy to the lowest energy (7)

gamma rays, xrays, UV, visible, Infra-red, microwave, radiowave

low energy radiowaves have longer wavelengths that high energy gamma rays

400-700nm

red light has a longer wavelength and lower energy than blue light

10³ Hz is how many kHz?

10⁶ Hz is how many MHz?

10⁹ Hz is how many GHz?

10³ Hz is 1 kHz

10⁶ Hz is 1 MHz?

10⁹ Hz is 1 GHz?

1. no photons are absorbed - the substance transmits light of that wavelength

2. some or all of the photons are absorbed - molecules in the sample take up energy

it is always conserved

hence when a photon is absorbed the molecule undergoes an energy change that is equal to the energy of the photon.

true or false?

a molecule can be partially absorbed by a photon?

False

a molecule CANNOT be partially absorbed by a photon; It s an all or nothing event

its energy must be  exactly equal the energy difference between the old and the new molecular energy states.

the same applies to the emission of a photon when a molecule in an excited state moves to a lower energy state

internal energy

translational energy

energy stored within a molecule from the result of its rotation, vibration or electrical distribution

absortion/emission of a photon changes internal energy of a molecule by an equal amount to the energy of the photon

this is the motion of the whole molecule through space

translational energy cannot normally couple with electromagnetic radiation. changes in trasnlational energy result only from collisions between molecules

true or false?

translational energy can be coupled with electromagnetic radiation?

false

translational energy cannot normally couple with electromagnetic radiation. changes in translation energy result from collisions between molecules.

name the 3 ways internal energy can be classified according to how it is stored

these forms of energy are quantised what does this mean?

rotational energy

vibrational energy

electronic energy

quantised means that the molecules can only lose or gain certain 'well defined' amounts of internal energy. it cannot gain or lose arbitary amounts

the approx energy gap between Rotational energy levels is:

ΔE ~10^-19 - 10^-22 J

which region of the electromagnetic spectrum has energies in this range?

Microwave region have energy in this range

λ 1- 10^-3 m

vibrational energy levels ~ 10^-19 - 10^-22 J

which region has energies in this range?

Infra red

when  say vibrational energy 

you should think infra red

here is a easy thing to help you remember the molecular energy levels and their relationship to the elctro magnetic spectrum

EU

MR

IV

Electronic energy - UV/visible photons

Microwave photons - Rotational energy

Infra-red photons - Vibrational energy

a radiation source

a sample cell

a dispersion element

a light intensity detector

modern spectrometers use a beam splitter thater than a dispersion device and all wavelengths are collected simultaneously. this > speed and sensitivity

what does an absorbtion spectrum show?

how much light has been absorbed by the sample as a function of the wavelength of the light.

the amount of light absorbed at each wavelength is usually displayed as absorbance (A)

a logarithmic scale

no light absorbed? = absorbance 0

90% light absorbed? = absorbance 1

99% light absorbec? = absorbance 2 etc

in IR spectra what is commonly used instead of absorbance?

what affect does this have on the graph peaks?

% transmittance is used more commonly that absorbance

this is the % of incident light passing through the sample

the absorbtion peaks are inverted when showing transmission

the internal energy states of molecules are .......... in ways that depend on the structure of the molecule

........ gives direct information about the energy seperation between these molecular states because a ....... can only be absorbed if it has ..... the right energy to excite the molecule to a higher .......... energy state. Hence ........ information can be deduced from the energies of the photons that a molecule...........

quantized, spectroscopy, photon, exactly, internal, stuctural, absorbs

the internal energy states of molecules are quantised in ways that depend on the structure of the molecule. Spectroscopy gives direct information about the energy seperation between these molecular states because a photon can only be absorbed if it has exactly the right energy to excite the molecule to a higher internal energy state. Hence stuctural information can be deduced from the energies of the photons that a molecule absorbs

Rotational levels

vibrational levels are ~1000 more closely spaced than electronic levels

in order of closely spacedness name the 3 molecular energy levels.

Hint (RVE) Riding is Very Energetic

Translational and Rotational energy. At all ordinary temperatures (0-500K) virtually all molecules are in their lowest (or ground) vibrational and electronic state

any molecules excited to higer vibrational or electroni states by photon absorbtion will have short life times.

different spectroscopic techniques are the study of different types of molecular transition

what do each of these measure?

microwave spectroscopy

infra red spectroscopy

UV/visible spectroscopy

microwave spec - rotational transitions

infra red spec - vibrational transitons

uv/visible spec - electronic transitions

name the two ways spectra can be recorded.

do they give similar or different information?

which one is much more widely used?

can be recorded as absorbtion or emission

they give similar information but absorbtion spectroscopy is much more widely used