Spectroscopy

Spectroscopy involves the study of the interaction of electromagnetic radiation with matter.

It is of paramount importance in biology, see for instance “Biological Spectroscopy”, Iain D Campbell and Raymond A Dwek, Benjamin/ Cummings, 1984.

Oscillating electric and magnetic fields that travel through empty space (vacuum) at

c = 3.00 x 108 m s-1 ,

the speed of light.

The frequency, ν, is the number of cycles per second, the unit is Hertz,

1 Hz = 1 s-1

In emission spectroscopy a molecule undergoes a transition from a high energy E1 to a low energy E2 emitting the excess energy as a photon.

In absorption spectroscopy the net absorption of energy incident on the molecule is monitored as the frequency of the radiation is swept.

The frequency, ν, of the photon is given by the Bohr frequency condition, ∆E = E1 - E2 = hν

Absorption spectroscopy is used to study electronic spectra in the UV/visible region and vibrational spectra in the infrared

The intensity of electromagnetic radiation transmitted, I, through a sample at a given wavelength or wavenumber decreases exponentially with sample thickness, l, and concentration, c. Double either and I is halved.

I = I0 x 10-εcl

where I0 is the incident intensity, I is the transmitted intensity, ε is the molar absorption coefficient, c is the concentration in mol dm-3 and l in cm.

Dimensions of ε are length/concentration, thus the usual units are dm3 mol-1 cm-1 or cm2 mol-1

Ratio of transmitted intensity to incident intensity at a given frequency is called the transmittance,

T = I/I0