W5 Molecular Absorption Spectroscopy

Question 1

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

Answer 1

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 2

where is the radiation for rotational transition at

Answer 2

microwave to far-infrared region

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

Question 3

where is the radiation for vibrational transition at

Answer 3

mid and far infrared regions

Question 4

where does electronic transition occur

Answer 4

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

Question 5

what are the two types of excitation sources for spectrometers

Answer 5

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 6

most common continuum source

Answer 6

tungsten-halogen lamp (UV-Vis absorption)

wavelengths of 320nm to 2500 nm provided

operates at temperature of 3500K

Question 7

what range of light does deuterium lamp emit

Answer 7

in Uv region > emit light from 160 to 400nm

Question 8

what is a monochromator

Answer 8

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

Question 9

how does a monochromator work

Answer 9

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 10

what is effective bandwidth

Answer 10

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

Question 11

components of an absorption spectrophotometer

Answer 11

excitation source: source of continuous radiation over the wavelengths of interest

monochromator: to select a narrow band of wavelengths from the source spectrum

sample: a sample cell to contain the molecules in solution

detector: to convert radiant energy into electrical energy

readout recorder: device to read out the response of the detector

Question 12

wavelengths at which deuterium lamp provides

Answer 12

200 to 400nm

Question 13

how does photomultiplier tube (PMT) detector work

Answer 13

detects light by converting photons into electrons through photoelectric effect at photocathode > electrons accelerated and amplified through series of dynodes > each collision generates more electrons > create amplified signal > final electron current collected at anode > produce measurable electrical signal proportional to light intensity