Spectroscopy Intro Ducat Ion

8/14/2019 Spectroscopy Intro Ducat Ion

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Spectroscopy

Absorption |Emission |Scattering

Introduction

Spectroscopy is the use of the absorption, emission, or scattering ofelectromagneticradiation by atoms or molecules (or atomic or molecular ions) to qualitatively or

quantitatively study the atoms or molecules, or to study physical processes. The

interaction of radiation with matter can cause redirection of the radiation and/ortransitions between the energy levelsof the atoms or molecules. A transition from a

lower level to a higher level with transfer of energy from the radiation field to the atom or

molecule is calledabsorption. A transition from a higher level to a lower level is calledemission if energy is transfered to the radiation field, or nonradiative decay if no

radiation is emitted. Redirection of light due to its interaction with matter is called

scattering, and may or may not occur with transfer of energy, i.e., the scattered radiationhas a slightly different or the same wavelength.

Absorption

When atoms or molecules absorb light, the incoming energy excites a quantized structureto a higher energy level. The type of excitation depends on the wavelengthof the light.

Electrons are promoted to higher orbitals by ultraviolet or visible light, vibrations are

excited by infrared light, and rotations are excited by microwaves.

An absorption spectrum is the absorption of light as a function of wavelength. The

spectrum of an atom or molecule depends on its energy level structure, and absorptionspectra are useful for identifying of compounds.

Measuring the concentration of an absorbing species in a sample is accomplished by

applying the Beer-Lambert Law.

Emission

Atoms or molecules that are excited to high energy levels can decay to lower levels byemitting radiation (emission or luminescence). For atoms excited by a high-temperature

energy source this light emission is commonly called atomic or optical emission (see

atomic-emission spectroscopy), and for atoms excited with light it is called atomic

fluorescence (see atomic-fluorescence spectroscopy). For molecules it is calledfluorescence if the transition is between states of the same spin and phosphorescence if

the transition occurs between states of different spin.
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The emission intensity of an emitting substance is linearly proportional to analyte

concentration at low concentrations, and is useful forquantitating emitting species.

Scattering

When electromagnetic radiation passes through matter, most of the radiation continues in

its original direction but a small fraction is scattered in other directions. Light that is

scattered at the same wavelength as the incoming light is called Rayleigh scattering.Light that is scattered in transparent solids due to vibrations (phonons) is called Brillouin

scattering. Brillouin scattering is typically shifted by 0.1 to 1 cm-1 from the incident light.

Light that is scattered due to vibrations in molecules or optical phonons in solids is calledRaman scattering. Raman scattered light is shifted by as much as 4000 cm-1 from the

incident light.

Electromagnetic Radiation

Introduction

Electromagnetic radiation is an energy wave that is composed of an electric fieldcomponent and a magnetic field component. The electric and magnetic fields are

orthogonal to each other and orthogonal to the direction of propogation of the wave.

Schematic of an electromagnetic wave

The wavelength is the length of one complete oscillation and the frequency is the number

of oscillations per second. Electromagnetic waves travel through a vacuum at

2.99792x108

m/s, which is known as the speed of light. The relation between speed oflight (c), wavelength (lambda), and frequency (nu) is:

c = lambda * nu

Wave-particle duality
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Electromagnetic radiation shows both wave and particle characteristics depending on

how the radiation is observed. Einstein first postulated that the energy of radiation is

quantized and that radiation is composed of energy packets that were later namedphotons. The energy (E) of a photon depends on its frequency (or wavelength):

E = h * nu = h * c / lambda

where h is Planck's constant (6.62618x10-34 Js), nu is the frequency of the radiation (Hz),

c is the speed of light (2.99792x108 m/s), and lambda is wavelength (m).

de Broglie equation

Analogous to radiation, particles; such as electrons, protons, and neutrons have wave

properties as determined by the de Broglie equation:

lambda = h/p

where lambda is wavelength, h is Planck's constant, and p is the momentum of the

particle.

Beer-Lambert Law

Introduction

The Beer-Lambert law (or Beer's law) is the linear relationship between absorbance andconcentration of an absorbing species. The general Beer-Lambert law is usually writtenas:

A = a(lambda) * b * c

where A is the measured absorbance, a(lambda) is a wavelength-dependent absorptivitycoefficient, b is the path length, and c is the analyte concentration. When working in

concentration units of molarity, the Beer-Lambert law is written as:

A = epsilon * b * cwhere epsilon is the wavelength-dependent molar absorptivity coefficient with units of

M-1 cm-1.

Instrumentation

Experimental measurements are usually made in terms of transmittance (T), which is

defined as:

T = I / Iowhere I is the light intensity after it passes through the sample and Io is the initial light

intensity. The relation between A and T is:

A = -log T = - log (I / I o).


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Absorption of light by a sample

Modern absorption instruments can usually display the data as either transmittance, %-

transmittance, or absorbance. An unknown concentration of an analyte can be determined

by measuring the amount of light that a sample absorbs and applying Beer's law. If theabsorptivity coefficient is not known, the unknown concentration can be determined

using aworking curve of absorbance versus concentration derived from standards.

Derivation of the Beer-Lambert law

The Beer-Lambert law can be derived from an approximation for the absorption

coefficient for a molecule by approximating the molecule by an opaque disk whose cross-

sectional area,sigma, represents the effective area seen by a photon of frequency w. If thefrequency of the light is far from resonance, the area is approximately 0, and ifw is close

to resonance the area is a maximum. Taking an infinitesimal slab, dz, of sample:

Io is the intensity entering the sample at z=0, Iz is the intensity entering the infinitesimal

slab at z, dI is the intensity absorbed in the slab, and I is the intensity of light leaving the

sample. Then, the total opaque area on the slab due to the absorbers issigma * N * A *dz. Then, the fraction of photons absorbed will besigma * N * A * dz / A so,

dI / Iz = -sigma * N * dz
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Integrating this equation from z = 0 to z = b gives:

ln(I) - ln(Io) = -sigma * N * b

or - ln(I / Io) =sigma * N * b.

Since N (molecules/cm3) * (1 mole / 6.023x1023 molecules) * 1000 cm3 / liter = c

(moles/liter)

and 2.303 * log(x) = ln(x)

then - log(I / Io) =sigma * (6.023x1020 / 2.303) * c * b

or - log(I / Io) = A = epsilon * b * c

where epsilon =sigma * (6.023x1020 / 2.303) =sigma * 2.61x1020

Typical cross-sections and molar absorptivities are:

sigma (cm2)

epsilon (M-1 cm-1)

absorption - atoms 10-12 3x108

molecules 10-16 3x104

infrared 10-19 3x10

Raman scattering 10-29 3x10-9

Limitations of the Beer-Lambert law

The linearity of the Beer-Lambert law is limited by chemical and instrumental factors.Causes of nonlinearity include:

deviations in absorptivity coefficients at high concentrations (>0.01M) due to

electrostatic interactions between molecules in close proximity

scattering of light due to particulates in the sample

fluoresecence or phosphorescence of the sample changes in refractive index at high analyte concentration

shifts in chemical equilibria as a function of concentration

non-monochromatic radiation, deviations can be minimized by using a relativelyflat part of the absorption spectrum such as the maximum of an absorption band

stray light

Quantitative Fluorimetry


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Introduction

Light emission from atoms or molecules can be used to quantitate the amount of the

emitting substance in a sample. The relationship between fluorescence intensity and

analyte concentration is:

F = k * QE * Po * (1-10[-epsilon*b*c]

)where F is the measured fluorescence intensity, k is a geometric instrumental factor, QE

is the quantum efficiency (photons emitted/photons absorbed), Po is the radiant power of

the excitation source, epsilon is the wavelength-dependent molar absorptivity coefficient,b is the path length, and c is the analyte concentration (epsilon, b, and c are the same as

used in the Beer-Lambert law).

Expanding the above equation in a series and dropping higher terms gives:F = k * QE * Po * (2.303 * epsilon * b * c)

This relationship is valid at low concentrations (


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to avoid interferences. LIF is useful to study the electronic structure of molecules and to

make quantitative measurements of analyte concentrations. Analytical applications

include monitoring gas-phase concentrations in the atmosphere, flames, and plasmas; andremote sensing using light detection and ranging (LIDAR). Because of the differences in

the nature of the energy-level structure between atoms and molecules, the discussion on

atomic fluorescence spectroscopy is in a separate document.

Instrumentation

The excitation source for molecular LIF is typically a tunable dye laserin the visiblespectral region. Studies in the near-ultraviolet and near-infrared are becoming more

common as near-infrared lasers and frequency-doubling methods improve. High-

resolution studies require cooling of the molecules to remove spectral congestion and to

reduce the Doppler width of the transitions. A separate document on high-resolutionspectroscopy describes cooling methods such as molecular beams, free-jet expansions,

and cryogenic glass or crystalline matrices.

Atomic Transitions - Theory

Introduction

The probability that an atomic spectroscopic transition will occur is called the transitionprobability or transition strength. This probability will determine the extent to which an

atom will absorb light at a resonance frequency, and the intensity of the emission lines

from an atomic excited state. The spectral width of a spectroscopic transition depends on

the widths of the initial and final states. The width of the ground state is essentially adelta function and the width of an excited state depends on its lifetime.

Specific Documents

Transition strengths

Excited-state lifetime and the natural linewidth

Transition lineshapes and broadening

Spectroscopic Transition Strengths

Introduction

An atom or molecule can bestimulated by light to change from oneenergy stateto

another. An atom or molecule in an excited energy state can also decay spontaneously to

a lower state. The probability of an atom or molecule changing states depends on the
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nature of the initial and final state wavefunctions, how strongly light can interact with

them, and on the intensity of any incident light. This document discusses some of the

practical terms used to describe the probability of a transition occuring, which iscommonly called the transition strength. To a first approximation, transitions strengths

are governed by selection ruleswhich determine whether a transition is allowed or

disallowed. Practical measurements of transitions strengths are usually described in termsof the Einstein A and B coefficients or theoscillator strength (f).

Selection Rules

1. The parity of the initial and final wavefunctions must be different.

2. The spin can not change, deltaS = 0.

3. The change in orbital angular momentum can be deltaL = 0, 1, but L=0 to L=0

transitions are not allowed.4. The change in total angular momentum can be deltaJ = 0, 1, but J=0 to J=0

transitions are not allowed.

Transition Probability

The transition probability is R2 with units of J cm3, where R is the transition moment

given by:

R = and u is the dipole moment operator. Basically what this equation indicates is that the

strength of a transition is relative to how strongly the dipole moment of a resonance

between energy states can couple to the electric field of a light wave.

Einstein coefficients

For a two-level system (ground-state level i and upper level j), the rate of an upwardstimulated transition (absorption, -dNi/dt or dNj/dt) is:

where Ni is the number density of atoms in the ground state, Uv is the light intensity, andthe proportionality factor Bij is the Einstein B coefficient for absorption:

For stimulated emission the Einstein coefficient becomes:

where gi and gj are the degeneracies of the ground and excited states, respectively.
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Atoms in the excited state can decay without the presence of an external light field due to

stimulation due to "zero-point fluctuations." Zero-point fluctuations are the dynamic

variations in the shape of an electronic orbital at any instant in time. These instantaneousorbitals can be described by a linear combination of the wavefunctions of the system,

which provides the mechanism for transitions between different states of the system. The

spontaneous decay rate (-dNj/dt or dNi/dt) is:-dNj/dt = Nj * Ajiwhere Aji is the Einstein coefficient for spontaneous emission:

Since atoms in the upper level can decay by both spontaneous and stimulated emission,

the total downward rate (-dNj/dt or dNi/dt) is given by:

Oscillator strength

The oscillator strength of a transition is a dimensionless number that is useful forcomparing different transitions. It is defined as the ratio of the strength an atomic or

molecular transition to the theoretical transition strength of a single electron using a

harmonic-oscillator model. For absorption:

and for emission:fji = fij gi/gj

Oscillator strengths can range from 0 to 1, or a small integer. A strong transition will

have an f close to 1. Oscillator strengths greater than 1 result from the degeneracy of realelectronic systems.

Tabulations in the literature often use gf, where gf = g i fij = gj fji

Atomic-Absorption Spectroscopy (AA)

Introduction

Atomic-absorption (AA) spectroscopy uses the absorption of light to measure the

concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyteatoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb
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ultraviolet or visible light and make transitions to higher electronic energy levels. The

analyte concentration is determined from the amount of absorption. Applying theBeer-

Lambert law directly in AA spectroscopy is difficult due to variations in the atomizationefficiency from the sample matrix, and nonuniformity of concentration and path length of

analyte atoms (in graphite furnace AA). Concentration measurements are usually

determined from a working curveafter calibrating the instrument with standards ofknown concentration.

Schematic of an atomic-absorption experiment

Instrumentation

Light source

The light source is usually a hollow-cathode lamp of the element that is being measured.Lasersare also used in research instruments. Since lasers are intense enough to excite

atoms to higher energy levels, they allow AA and atomic fluorescence measurements in a

single instrument. The disadvantage of these narrow-band light sources is that only oneelement is measurable at a time.

Atomizer

AA spectroscopy requires that the analyte atoms be in the gas phase. Ions or atoms in a

sample must undergo desolvation and vaporization in a high-temperature source such as a

flame or graphite furnace. Flame AA can only analyze solutions, while graphite furnaceAA can accept solutions, slurries, or solid samples.

Flame AA uses a slot type burner to increase the path length, and therefore to increase the

total absorbance (seeBeer-Lambert law). Sample solutions are usually aspirated with the

gas flow into a nebulizing/mixing chamber to form small droplets before entering theflame.

The graphite furnace has several advantages over a flame. It is a much more efficient

atomizer than a flame and it can directly accept very small absolute quantities of sample.

It also provides a reducing environment for easily oxidized elements. Samples are placeddirectly in the graphite furnace and the furnace is electrically heated in several steps to

dry the sample, ash organic matter, and vaporize the analyte atoms.
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Light separation and detection

AA spectrometers use monochromators and detectors for uv and visible light. The main

purpose of the monochromator is to isolate the absorption line from background light dueto interferences. Simple dedicated AA instruments often replace the monochromator with

a bandpass interference filter. Photomultiplier tubes are the most common detectors for

AA spectroscopy.

Picture of a flame atomic-absorption spectrometer:

Picture of a graphite-furnace atomic-absorption spectrometer:
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Atomic Emission Spectroscopy (AES,

OES)

Introduction

Atomic emission spectroscopy (AES or OES) uses quantitative measurement of theoptical emissionfrom excited atoms to determine analyte concentration. Analyte atoms in

solution are aspirated into the excitation region where they are desolvated, vaporized, and

atomized by a flame, discharge, or plasma. These high-temperature atomization sourcesprovide sufficient energy to promote the atoms into high energy levels. The atoms decay

back to lower levels by emitting light. Since the transitions are between distinct atomic

energy levels, the emission lines in the spectra are narrow. The spectra of multi-elementalsamples can be very congested, and spectral separation of nearby atomic transitions

requires a high-resolution spectrometer. Since all atoms in a sample are excitedsimultaneously, they can be detected simultaneously, and is the major advantage of AEScompared to atomic-absorption (AA) spectroscopy.

Schematic of an AES experiment

Instrumentation

As in AA spectroscopy, the sample must be converted to free atoms, usually in a high-

temperature excitation source. Liquid samples are nebulized and carried into theexcitation source by a flowing gas. Solid samples can be introduced into the source by a

slurry or by laser ablation of the solid sample in a gas stream. Solids can also be directly

vaporized and excited by a spark between electrodes or by a laser pulse. The excitation

source must desolvate, atomize, and excite the analyte atoms. A variety of excitationsources are described in separate documents:

Direct-current plasma (DCP)

Flame

Inductively-coupled plasma (ICP)

Laser-induced breakdown (LIBS)

Laser-induced plasma
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Microwave-induced plasma (MIP)

Spark or arc

Since the atomic emission lines are very narrow, a high-resolutionpolychromatorisneeded to selectively monitor each emission line.Picture of an inductively-coupled

plasma atomic emission spectrometer

Direct-Current Plasma Excitation Source

Introduction

A direct-current plasma (DCP) is created by an electricaldischarge between two electrodes. A plasma support gas isnecessary, and Ar is common. Samples can be deposited on

one of the electrodes, or if conducting can make up one

electrode. Insulating solid samples are placed near thedischarge so that ionized gas atoms sputter the sample into

the gas phase where the analyte atoms are excited. This

sputtering process is often referred to as glow-discharge

excitation.

Flame Excitation Source

Introduction
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A flame provides a high-temperature source for desolvating and vaporizing a sample to

obtain free atoms for spectroscopic analysis. In atomic absorption spectroscopy ground

state atoms are desired. For atomic emission spectroscopy the flame must also excite theatoms to higher energy levels. The table lists temperatures that can be achieved in some

commonly used flames.

Temperatures of some common flamesFuel Oxidant Temperature (K)

H2 Air 2000-2100

C2H2 Air 2100-2400

H2 O2 2600-2700

C2H2 N2O 2600-2800

Introduction

The figure shows a total consumption burner in whichthe sample solution is directly aspirated into the flame.

This flame design is common for atomic emission

spectroscopy. All desolvation, atomization, andexcitation occurs in the flame. Other flame designs

nebulize the sample and premix it with the fuel and

oxidant before it reaches the burner. Atomic-absorptioninstruments almost always use a nebulizer and also use a

slot burner to increase the path length for the sample

absorption.

Inductively-Coupled Plasma (ICP)

Excitation Source

Introduction

An inductively coupled plasma (ICP) is a very high temperture (7000-8000K) excitationsource that efficiently desolvates, vaporizes, excites, and ionizes atoms. Molecular

interferences are greatly reduced with this excitation source but are not eliminatedcompletely. ICP sources are used to excite atoms foratomic-emission spectroscopyandto ionize atoms formass spectrometry.
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Instrumentation

The sample is nebulized and entrained in

the flow of plasma support gas, which is

typically Ar. The plasma torch consists of

concentric quartz tubes. The inner tubecontains the sample aerosol and Ar

support gas and the outer tube contains

flowing gas to keep the tubes cool. Aradiofrequency (RF) generator (typically

1-5 kW @ 27 MHz) produces an

oscillating current in an induction coilthat wraps around the tubes. The

induction coil creates an oscillating

magnetic field, which produces anoscillating magnetic field The magnetic

field in turn sets up an oscillating currentin the ions and electrons of the support gas (argon). As the ions and electrons collide withother atoms in the support gas

Laser-Induced Breakdown Excitation

Source

Introduction

When a high-energy laser pulse is focused into a gas or liquid, or onto a solid surface, it

can cause dielectric breakdown and create a hot plasma. For solids the laser pulse alsoablates material into the gas phase. The energy of the laser-created plasma can atomize,

excite, and ionize analyte species, which can then be detected and quantified byatomic-

emission spectroscopyormass spectrometry.

Laser-Induced Plasma Excitation Source

Introduction

A high-power CO2 laser that is focused into a support gas, such as Ar, can maintain a hotplasma. The energy of the plasma can atomize, excite, and ionize analyte species present

in the support gas, which can then be detected and quantified by atomic-emission
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spectroscopy ormass spectrometry. It can also be used in a glow-discharge mode to

sputter analyte atoms off of a solid surface for analysis in the plasma.

Microwave-Induced Plasma Excitation

Source

Introduction

A microwave-induced plama consists of a quartz tube surrounded by a microwave

waveguide or cavity. Microwaves produced from a magnetron (a microwave generator)fill the waveguide or cavity and cause the electrons in the plasma support gas to oscillate.

The oscillating electons collide with other atoms in the flowing gas to create and maintain

a high-temperature plasma. As in inductively coupled plasmas, a spark is needed to create

some initial electrons to create the plasma. Atomic emission is measured from excitedanalyte atoms as they exit the microwave waveguide or cavity.

Spark and Arc Emission Sources

Introduction

Spark and arc excitation sources use a current pulse (spark) or

a continuous electical discharge (arc) between two electrodes

to vaporize and excite analyte atoms. The electrodes are eithermetal or graphite. If the sample to be analyzed is a metal, it can

be used as one electrode. Non-conducting samples are ground

with graphite powder and placed into a cup-shaped lowerelectrode. Arc and spark sources can be used to excite atoms

foratomic-emission spectroscopy or to ionize atoms formass

spectrometry. Arc and spark excitation sources have beenreplaced in many applications with plasma or laser sources, but

are still widely used in the metals industry.

Atomic-Fluorescence Spectroscopy (AFS)
http://elchem.kaist.ac.kr/vt/chem-ed/spec/atomic/aes.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/ms/ms-intro.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/ms/ms-intro.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/spec/atomic/aes.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/spec/atomic/aes.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/ms/ms-intro.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/ms/ms-intro.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/spec/atomic/aes.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/ms/ms-intro.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/spec/atomic/aes.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/spec/atomic/aes.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/ms/ms-intro.htmhttp://elchem.kaist.ac.kr/vt/chem-ed/ms/ms-intro.htm


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Introduction

Atomic fluorescence is the optical emission from gas-phase atoms that have been excited

to higher energy levels by absorption ofelectromagnetic radiation. The main advantage

of fluorescence detection compared to absorption measurements is the greater sensitivity

achievable because the fluorescence signal has a very low background. The resonantexcitation provides selective excitation of the analyte to avoid interferences. AFS is

useful to study the electronic structure of atoms and to make quantitative measurements.

Analytical applications include flames and plasmas diagnostics, and enhanced sensitivityin atomic analysis. Because of the differences in the nature of the energy-level structure

between atoms and molecules, discussion oflaser-induced fluorescence (LIF) from

molecules is found in a separate document.

Instrumentation

Analysis of solutions or solids requires that the analyte atoms be desolvated, vaporized,

and atomized at a relatively low temperature in a heat pipe, flame, or graphite furnace. Ahollow-cathode lamp orlaserprovides the resonant excitation to promote the atoms to

higher energy levels. The atomic fluorescence is dispersed and detected bymonochromators and photomultiplier tubes, similar to atomic-emission spectroscopy

instrumentation.

Electron Paramagnetic Resonance (EPR,

ESR) Spectroscopy

Introduction

When an atom or molecule with an unpaired electron is placed in a magnetic field, the

spin of the unpaired electron can align either in the same direction or in the opposite

direction as the field. These two electron alignments have different energies andapplication of a magnetic field to an unpaired electron lifts the degeneracy of the 1/2

spins of the electron.

Electron-paramagnetic-resonance (EPR) or electron-spin-resonance (ESR) spectroscopy

measures the


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5. Triplet-state molecules

Instrumentation

A klystron tube generates monochromatic microwave radiation (~9500 MHz) in an EPRinstrument. The microwave radiation travels down a waveguide to the sample which is

held between magnets.

Spectra are obtained by measuring the absorption of the microwave radiation whilescanning the magnetic-field strength. EPR spectra are usually displayed in derivative

form to improve the signal-to-noise ratio.