Spectroscopy and Electromagnetic Spectra

Briefly state what the three main types of spectroscopy are

• electronic spectroscopy  

  •  how electrons move between orbitals  

• vibrational spectroscopy 

  • how molecules vibrate 

• rotational spectroscopy  

  • how molecules rotate 

What are transitions

Transition are when a molecule changes from one energy level / state to another 

• because energy levels are quantized, a transition involves the intake or release of a discrete amount of energy 

Describe energetics of transitioning

• When a transition occurs, the energy released is Delta E = hv

• So the light emitted will have frequency v = ΔE/h 

• for a transition to occur, the energy provided must be ΔE 

• the energy of a photon is hv 

How can we describe electromagnetic radiation

Can be describe as a wave of oscillating electric and magnetic fields. 

Give equation for frequency and wave number

Quantification of properties 

• Speed of Light (in a vacuum), c, ms^-1 

• Wave length (length of one wave), λ, m 

• Frequency (Number of waves per second), v, s^-1 = Hz 

• Wave number (Number of waves in 1 cm) v ~ = cm^-1 

Describe what a photon is

Electron magnetic radiation is also describe as a stream of particles called photons 

• Each carries a specific amount of energy (a quantum of energy) 

• An intense beam of light will contain many photons

Give equation for One photon and One mole of photons

Quantification of properties 

• Speed of Light (in a vacuum), c, ms^-1 

• Planck Constant, h 

• Avogadro constant, N_A 

• Boltzmann constant, k 

Describe how wave particle duality applies to photons

• At the atomic level, radiation or matter can appear to be waves or particles depending on how we observe them 

  • So wave and photon descriptions are used interchangeably 

• High Frequency and High Wave Number = High Energy 

  • Energy given in cm-1 units 

  • Low wave length = Low Energy 

Describe what is meant by the electromagnetic spectrum

• The range over which electromagnetic radiation is typically observed and used 

• Different regions of the spectrum interact with matter in different ways  

  • Different types of spectroscopy 

Draw out the wave spectrum detailing type of spec/analysis

What does a spectrum refer too

• A spectrum refers to a plot of absorption, emission, or scattering versus the wavelength, wavenumber, or frequency 

• Shows the specific energies that a molecules can take in or release 

Draw out a basic absorption spectrometer set up

Describe what is measured in an absorption spectrometer

• to record a spectrum 

  • scan the monochromator across a range of λ 

• regions where a molecule absorbs / emits / scatters are typically called 

  • LINES if they are narrow 

  • BANDS if they are broad 

  • PEAKS generally 

• outside these regions is - the BASELINE 

• the appearance of the spectrum depends on various spectrometer settings that can often be set by the user, including 

  • the RESOLUTION , smallest peak separation that can be distinguished (in λ, ν ~ , or v) e.g. determined by the monochromator slit width 

Draw out a quantitative absorption set up

Give equations for transmittance %transmittance and absorbance

Draw out an absorption band for % transmittance and absorbance

Draw out an emission spectrum set up and an emission band

Why do we need selection rules for transition states

• A molecule has many energy states, so many transitions are possible between them 

• Quantum theory tells us which transition can actually occur, and give us, selection rules stating whether transition state are allowed (strong band) or disallowed ( no strong band) 

How does number of molecules within energy levels effect transition

The greater the number of molecules in the level from which the transition starts the stronger the band arising from this transition  

Describe widely spaced energy levels

Describe closley spaced energy levels

How do we increase the number of molecules encountered by a spectrometer