Lecture 8

DetectorsSpectrophotometer designBackground correctionWidth of the spectral peak

Ch. 20 Harris

Light Source

Sample

Monochromator(filter,

wavelength selector)

Detector

SpectrometerData Processing

- +90 V

A phototube

Wavelength, nm

GaAsK/Cs/Sb

Ag/O/Cs

Photoemissive cathode

Light, h

Quarzenvelope

Anode

Several electronsfor each photon

Several electrons for each incident electron

Photomultiplier

photon electron

Photoelectromultiplier

dynode

dynode

Single beam

Double beam

Double beam with the beams separated in time

Single beam design with 210-element area detector

Baseline

Peak

I

wavelength

I0

Ipeak

Ipeak – I0

Intensities at two different wavelengthes

Flat background

Baseline

Peak

I

Wavelength ()

I0

IpeakFlat background

0)( II

III

signal

backgroundsignaltotal

constant

signalII

First derivative eliminates flat background

Wavelength ()

I

Background

Linear Background

I1

I2

Ipeak

Ipeak – (I1+I2) / 22

21 IIII peaksignal

Wavelength ()

I

Background

Linear Background

Ibackground = K+b 10)( III

III

signal

backgroundsignaltotal

1III signal

2

2

2

2

signalII

First:

Second:

Second derivative eliminates linear background

4 2 0 2 40

0.1

0.2

0.3

.4

0

f x( )

44 x

4 2 0 2 4

0.2

0

0.2

.3

0.3

f x( )

44 x

4 2 0 2 4

0.4

0.2

0

0.2.2

0.5

f x( )

44 x

4 2 0 2 4

0.5

0

0.5

1

1.3

.8

f x( )

44 x

Original

First

Second

4th

Almost featureless:little information

Some peaks visible

Well defined peaks and valleys

Low noise

High noise

Only photons of one specified wavelength areabsorbed

hchE

Ground state

Excited state

Width

Doppler cv

Speed of light 300,000,000 m/s

Velocity of a particle >1000 m/s

collisions Approximately as Doppler

Both effects increase with the temperature and

decrease with the size of the molecule/atom

Multiple energy levels

Width is around 0.01-0.001 nm in visible area

Energy levels of ground state

Energy levels of excited state

Energy sublevels

A primitive rule:

Molecular spectra - wide bands

Atomic spectra - narrow lines