W5 Molecular Absorption Spectroscopy

Question 1

what is electromagnetic radiation

Answer 1

light waves of perpendicularly oscillating electric and magnetic fields (plane polarised light)

Question 2

what are the three energy levels that molecules or atoms can possess

Answer 2

electronic: energy associated with electrons in the atom/molecule

vibrational: quantised states related to the vibrations of atoms within the molecule

rotational: energy associated with rotation of the whole atom/molecule itself

electronic > vibrational > rotational

Question 3

where is the radiation for rotational transition at

Answer 3

microwave to far-infrared region

(microwave: wavelengths longer than infrared but shorter than radio waves)

Question 4

where is the radiation for vibrational transition at

Answer 4

mid and far infrared regions

Question 5

where does electronic transition occur

Answer 5

in the UV or visible (UV-Vis) range of electromagnetic spectrum

Question 6

what are the two types of excitation sources for spectrometers

Answer 6

1. continuum source: provides a broad distribution of wavelengths within a particular spectral range
2. a line source: emits a limited number of spectral lines, each of which spans a very limited wavelength range (laser)

Question 7

most common continuum source

Answer 7

tungsten-halogen lamp (UV-Vis absorption)

wavelengths of 320nm to 2500 nm provided

operates at temperature of 3500K

Question 8

what range of light does deuterium lamp emit

Answer 8

in Uv region > emit light from 160 to 400nm

Question 9

what is a monochromator

Answer 9

consists of entrance and exit slits, mirrors and a grating > to isolate specific wavelengths to analyse light at one particular wavelength

Question 10

how does a monochromator work

Answer 10

polychromatic light enters the entrance slit > mirror collimates the light > different wavelengths diffracted at different angles after reflecting off the grating > mirror focuses light of different wavelength on focal plane > rotation of grating allows different wavelengths to pass through exit slit

Question 11

what is effective bandwidth

Answer 11

the width of the band of radiation in wavelength units at a half-peak height

depends on size and quality of grating, slit widths and focal length of monochromator