spectroscopy

The interaction of light with matter including absorption, emission and scattering of electromagnetic radiation by atoms and molecules

The exchange of energy between light and matter

By packets (quanta) of radiation. Enough energy to cause a change in energy level

Quantisations at the distance a.k.a. the energy that's required for this electron to move is fixed it has a set value. It can’t be too much or not enough

If the photon energy is a little lower or a little higher the photo will not be absorbed by this particle the protons energy is indivisible so the particle cannot partially absorb the photon

Electronic

vibrational (relative position to other molecules changes)

rotational (molecule rotates around an axis)

translational

The speed of light is 2.998x10^8 ms-1 or 2.998x10^10 cm s-1

Frequency is the number of wavelengths per second this is measured in hertz or S-1. Wavelength is the length of the wave between two identical points

A mole of photons

It is used to give a manageable X axis in IR spectroscopy (units reciprocal to wavelength)

J=0 pre rotation, j=1 after

I0 = light intensity on the sample

I = Light intensity transmitted

The thickness of the Sample (l) which is related to the concentration

A is proportional c.l (concxlength)

At very high concentrations there are more interactions between molecules

At low concentrations The signal intensity is low compared to the noise

Photo electron spectroscopy ionises atoms (by removing electrons from them). It uses the photo electric effect (emission of electrons with EM radiation).

It is an example of light acting as a particle as the atoms only absorb photons with the correct amount (quanta) of energy. Whereas wave theory states that energy is uniformly distributed across the wavefront and is dependent only on the intensity of the beam.

Energy of photon = hv = ionisation energy + KE (of the leaving electron). We can use fixed photon energy (x ray) and measure the KE to calculate the ionisation energy (fingerprint for every atom)

Identifying an element and its local environment (eg c-cl vs c-f), its quantitative (you can have a measure of the number of atoms present), it is surface sensitive (x rays can’t penetrate fully) and the surface: bulk is 1:10^8.

X rays can cause the emission of electrons XPS. It can also move electrons between orbitals (low energy x rays). Light is released when the electrons return to a lower level. The electron can sit at ANY level not just 2.

It can cause the H2 to split and en electron is promoted to a higher energy level.

Use the rydberg formula

A point in the Uv-Vis spectrum which indicates direct (no intermediate) change from 1 molecule to another

Aka IR spectroscopy

All molecules vibrate and in IR spec, incident photons are absorbed to increase vibrations of a molecule

take an unknown spectra and compare it to a known one

Ke is the smallest as it is at rest. Bands come from providing energy to change the rate of oscilation of the masses

The quantized idea that molecules can only vibrate/ oscillate at particular energies/ speeds. They require photons that are the right quanta to move between energy levels

The lowest energy is not at 0, molecule always has some vibrational energy

The masses have to be for 1 atom

The mass changes between isotopes. This changes the fundamental frequency and therefore Ve.

(Mass bigger, w smaller, e smaller)

Gross selection rule (property of a molecule to absorb radiation): to interact with Infrared radiation, a vibration must involve a changing dipole

Specific selection rule: the change in vibrational energy numbers can be + or - 1

Eventually you cannot compress or stretch the bond further. The potential energy is not a parabola. Real energy levels are not equally spaced.

As it's not a perfect curve, there's not a set difference between each and every energy level

The change in energy level can be +/- 1,2 or 3. (You can only have +/-1 with harmonic).

The fundamental vibration (the big peak on an IR spec is from going from vibrational energy levels 0 to 1). Overtones are v0->v2 ect

The total number of variables used to define the motion of a molecule completely

Vibration, translation and rotation

A molecule consisting of N atoms has 3N degrees of freedom

Vibration (bend or stretch) is the only movement changing the shape of the bond.

Linear and diatomic molecule one vibration is possible.

Triatomic molecules can have three vibrations (an asymmetrical or symmetrical stretch, or scissoring bend). Symmetric stretches don't cause a change in dipole (so they don’t abide the gross selection rule).

Non-linear: 3N-6

Linear: 3N-5

An atom can move in three Independent directions in correspondence to the Cartesian axis (x,y,z)

Microwave radiation has enough energy to cause rotation however it doesn't have enough to cause oscillation

The spectrum has a series of regular spaced absorptions Whose intensities rises passes through a maximum and then falls

How much energy is required to turn an object

GSR: A molecule must have a permanent fixed type to show “pure” rotational spectrum.

“Pure” meaning excited by MW radiation alone. IR Can also cause rotations, but this doesn't focus on that

SSR:

The position of a peak in the spectrum is determined by the energy of the transition, i.e. the difference between two energy levels in a molecule

E=hc (1/wavelength)

Beer lambert equation:

Concentration, high concentration Means more absorbance

Path length

How many molecules in the sample are at the right state to absorb or emit at this frequency

How likely is it for the transition to occur

Microwaves have a larger wavelength therefore a smaller wave number this means that KT is smaller. It is therefore not big enough in comparison to the change energy to cause excitation

It is not to do with absorption and transmission, Instead, there is a beam stop and we look at the scattered light

You can do it on samples that contain water you can't do this with infrared spectroscopy

In a Strong magnetic field, a spectral line normally at one wavelength splits into two or three. The amount of splitting depends on how strong the field is.

One electron is split into two in a magnetic field

If you measure how many electrons are in the high energy state with the magnetic field versus the low. It is about equal, i.e. they are in resonance

Nuclear spin interacting with magnetic field

There are different Spin values a nucleus can have. I: 0.1,2,3,0.5,1.5, 2.5

A nucleus with spin I has 2I+1 Total directions

Ml (Magnum quantum number)= I,I-1..,-I