Introduction toIntroduction toIR spectroscopyIR spectroscopy

Yongsik LeeYongsik Lee2004. 62004. 6

IR spectrumIR spectrum

16A theory of IR ABS16A theory of IR ABS

► Energy of IR photon insufficient to cause electronic excitation But can cause vibrational or rotational excitation

► Fundamentals Molecular electric field (dipole moment) interacts with IR photon ele

ctric field (both dynamic)► Magnitude of dipole moment determined by

charge separation of charge

► Vibration or rotation causes varying separation

Dipole moment changeDipole moment change

►Molecule must have change in dipole moment due to vibration or rotation to absorb IR radiation

► Absorption causes increase in vibration amplitude/rotation frequency

►Molecules with permanent dipole moments (µ) are IR active HCl, CO,HCl, CO, H2, N2, H2, N2, CO2CO2

IR active/inactiveIR active/inactive

Electric Dipole momentElectric Dipole moment

Types of Molecular Vibrations

►Stretching change in bond length Symmetric /

asymmetric

►bending change in bond angle symmetric scissoring asymmetric wagging rocking twisting/torsion

Molecular vibrationMolecular vibration

Classical vibrational motion

►Mechanical modelMechanical model Two massesTwo masses A springA spring Simple harmonic motionSimple harmonic motion

Energy of the Hooke’s lawEnergy of the Hooke’s law

►Negative sign force Negative sign force Restoring forceRestoring force Direction of the force is opposite to the Direction of the force is opposite to the

displacementdisplacement

Harmonic Ocsillator PotentialHarmonic Ocsillator Potential

► Potential highPotential high When the spring is When the spring is

compressed or compressed or stretchedstretched

► Parabola functionParabola function E=(1/2)kxE=(1/2)kx22

Minimum at Minimum at equilibrium positionequilibrium position

Maximum at max Maximum at max amplitude Aamplitude A

Classical vibrational frequencyClassical vibrational frequency

► F = ma = m(dF = ma = m(d22y/dty/dt22))► F=-kyF=-ky► Solution of differential equationSolution of differential equation

Y = A cos (2Y = A cos (2t)t) DD22y/dty/dt22 = -4 = -42222A cos (2A cos (2t)t) Reduced mass for two massesReduced mass for two masses

Quantum treatment of Quantum treatment of vibrationsvibrations

Anharmonic OscillatorAnharmonic Oscillator

►Must modify harmonic oscillator potential for electron repulsion

►steeper at small distances

dissociation ►bond breaks at large distances

anharmonicityanharmonicity

►Harmonic at low n►E becomes smaller at

high n broadens band

►Selection rule fails n = ±1 and n = ±2... fundamentals overtones

Vibrational Normal modesVibrational Normal modes► Number of possible vibrations in a polyatomic molecule

2 atoms (H2) - 1 vibration (stretch ) 3 atoms (H2O) - 3 vibrations ( s, as, ) 3 atoms (CO2) - 4 vibrations ( s, as, ) 4 atoms (H2CO) - 6 vibrations ( s, as, , (CH2) (C=O)) 5 atoms ... 3N - 6 Non - linear molecule 3N - 5 Linear molecule

► 3N degrees of freedom for N atoms 3 translation 3(or 2) rotation – rotation about the bond axis is not possible Orhters are "Normal modes"

Fewer experimental peaksFewer experimental peaks

► Fewer peaksFewer peaks Symmetry of the moleculeSymmetry of the molecule degenracydegenracy

►Energies of two or more vibrations are identicalEnergies of two or more vibrations are identical►Or nearly identicalOr nearly identical

Undetectable low absorption intensityUndetectable low absorption intensity Out of the instrumental detection rangeOut of the instrumental detection range

►More peaksMore peaks OvertoneOvertone Combination bandsCombination bands

Applications of FT-IRApplications of FT-IR

►Chemical Analysis: Match spectra to known databases

► Identifying an unknown compound, Forensics, etc. Monitor chemical reactions in-situ

►Structural ideas: Can determine what chemical groups are in a specific co

mpound► Electronic Information:

Measure optical conductivity►Determine if Metal, Insulator, Superconductor, Semiconductor

Band Gaps, Drude model

Vibrational couplingVibrational coupling►Coupling of different vibrations shifts frequencies► Energy of a vibration is influenced by coupling►Coupling likely when

common atom in stretching modes common bond in bending modes common bond in bending+stretching modes similar vibrational frequencies

►Coupling not likely when atoms separated by two or more bonds symmetry inappropriate

Carbon dioxideCarbon dioxide

► C=O bondC=O bond In methanol 1034 cmIn methanol 1034 cm-1-1

In ethanol 1053 cmIn ethanol 1053 cm-1-1

In butanol 1105 cmIn butanol 1105 cm-1-1

In CO2 In CO2 ►Asym 2330 cmAsym 2330 cm-1-1

► sym cmsym cm-1-1

►Degenerate bending 667 Degenerate bending 667 cmcm-1-1

Water vibrationsWater vibrations

► Non-linear waterNon-linear water 3 x 3 – 6 = 3 vib3 x 3 – 6 = 3 vib Stretching 3650 and 37Stretching 3650 and 37

6060 Bending 1595Bending 1595

► Positive identification oPositive identification of a specific compoundf a specific compound

Global climate changeGlobal climate change

► Recent years have seen a huge rise in the number Recent years have seen a huge rise in the number of abnormal weather events. of abnormal weather events.

► Meteorologists agree that these exceptional Meteorologists agree that these exceptional conditions are signs that Global Climate Change is conditions are signs that Global Climate Change is happening already.happening already.

► Scientists agree that the most likely cause of the Scientists agree that the most likely cause of the changes are man-made emissions of the so-called changes are man-made emissions of the so-called ""Greenhouse GasesGreenhouse Gases" that can trap heat in the " that can trap heat in the earth's atmosphere in the same way that glass earth's atmosphere in the same way that glass traps heat in a greenhouse. traps heat in a greenhouse.

► Although there are six major groups of gases that Although there are six major groups of gases that contribute to Global Climate Change, the most contribute to Global Climate Change, the most common is Carbon Dioxide (CO2). common is Carbon Dioxide (CO2).

Greenhouse gas level hits Greenhouse gas level hits record highrecord high

► The level of the major greenhouse gas, carbon dioxide, in the Earth's atmosphere has hit a record high.

► The new data from the US National Oceanic and Atmospheric Administration also suggest that the rate of increase of the gas may have accelerated in the last two years.

► Carbon dioxide emissions, mainly from burning fossil fuels, are thought to be a principle cause of global warming.

► Recordings from a volcano-top observatory, NOAA's Mauna Loa Observatory on Hawaii, showed carbon dioxide levels had risen to an average of about 376 parts per million (ppm) for 2003.

► This is 2.5 ppm up from the average for 2002. It is not the highest leap in year-on-year atmospheric carbon dioxide levels recorded by NOAA. But it is the first to be sustained, with 2002 levels up 2.5 ppm from 2001.

► http://www.newscientist.com/news/news.jsp?id=ns99994802

Greenhouse effect is realGreenhouse effect is real

Day after tomorrow?Day after tomorrow?

► 20022002 年年 88 月ミニアルバム月ミニアルバム『『 day after tomorrowday after tomorrow 』でデ』でデビュー。ビュー。 misonomisono （（ 1818 歳京歳京都府出身）・北野正人（都府出身）・北野正人（ 2828歳大阪府出身）・鈴木大輔歳大阪府出身）・鈴木大輔（（ 2424 歳神奈川県出身）の歳神奈川県出身）の 33人組。楽曲全て、作詞は人組。楽曲全て、作詞は mismisonoono 、サウンドプロデューサ、サウンドプロデューサーは元ーは元 Every Little ThingEvery Little Thing の五の五十嵐充が担当。十嵐充が担当。 1010 月には韓月には韓国で開催された「国で開催された「 Japan FestJapan Festival in Koreaival in Korea 」に出演、初海」に出演、初海外外 LIVELIVE を行った。 を行った。

The day after tomorrowThe day after tomorrow

16B IR sources and detectors16B IR sources and detectors

►Heated inert solidHeated inert solid 1500 – 2200 K1500 – 2200 K Blackbody radiationBlackbody radiation Max at 5000 – 5900 cm-1Max at 5000 – 5900 cm-1

Globar sourceGlobar source

► GlobarGlobar Silicon carbide rodSilicon carbide rod Diameter 5 mm x length 50 mmDiameter 5 mm x length 50 mm Heated 1300-1500 KHeated 1300-1500 K Water cooling to prevent arcingWater cooling to prevent arcing

► Compare to Nernst glowerCompare to Nernst glower Spectral energies are comparableSpectral energies are comparable At < 5 At < 5 m, Globar provides a greater m, Globar provides a greater

outputoutput

Carbon dioxide laserCarbon dioxide laser

Carbon dioxide (CO2 ) gas laserCarbon dioxide (CO2 ) gas laser

► One of the most One of the most versatile for materials versatile for materials processing applicationsprocessing applications

► emits IR radiation with emits IR radiation with a wavelength between a wavelength between 9 and 11µm, although 9 and 11µm, although emission at 10.6µm is emission at 10.6µm is the most widely used. the most widely used.

► TypesTypes Sealed tubeSealed tube WaveguideWaveguide TEATEA

TEA COTEA CO22 Lasers Lasers

► Discharge instabilities prevent operation of CW CO2 lasers at Discharge instabilities prevent operation of CW CO2 lasers at pressures above about 100mbar. pressures above about 100mbar.

► Pulses in the nanosecond to microsecond duration range can Pulses in the nanosecond to microsecond duration range can be produced by passing a pulsed current transversely through be produced by passing a pulsed current transversely through the lasing gas. the lasing gas.

► TEA (transversely excited atmospheric) lasers operate at gas TEA (transversely excited atmospheric) lasers operate at gas pressures of one atmosphere and above in order to obtain pressures of one atmosphere and above in order to obtain high energy output per unit volume of gas. high energy output per unit volume of gas.

Home built CO2 gas laserHome built CO2 gas laser

► J&K Laser Productions J&K Laser Productions

IR TransducersIR Transducers

► General typesGeneral types Thermal – thermocouple, bolometerThermal – thermocouple, bolometer PyroelectricPyroelectric Photoconducting (PC)Photoconducting (PC)

16C IR instruments16C IR instruments

► IR instrumentsIR instruments Dispersive grating IR instrumentsDispersive grating IR instruments Multiplex instruments using FTMultiplex instruments using FT

►Most widely used since 1980’sMost widely used since 1980’s Nondisperive photometersNondisperive photometers

►Quantitative determinationQuantitative determination

FT instrumentsFT instruments

► Multiplex instrument TypesMultiplex instrument Types Interferogram (FT) typeInterferogram (FT) type

► Tabletop sizeTabletop size► Reliable, ReproducibleReliable, Reproducible► MaintenaceMaintenace► Cheaper ($15000 – 20000)Cheaper ($15000 – 20000)► Computer cost downComputer cost down► Most widely usedMost widely used► High S/N ratios - high throughputHigh S/N ratios - high throughput► Rapid (<10 s)Rapid (<10 s)► High resolution (<0.1 cm-1)High resolution (<0.1 cm-1)

Handamard transform typesHandamard transform types► Dispersive instrumentDispersive instrument► Moving mask at the focal plane of a monochromatorMoving mask at the focal plane of a monochromator

Bio-Rad FTS-40 FT-IRBio-Rad FTS-40 FT-IR

FT advantageFT advantage

►Jaquinot or throughput advantage. Few optics, no slits mean high intensity Usually to improve resolution, decrease slit widt

h but less light makes spectrum "noisier" - signal to noise ratio (S/N)

►Fellget or multiplex advantage. Simultaneously measure all spectrum at once sa

ves time

Dispersive IRDispersive IR► Similar to UV-Vis spectrophotometer

BUT sample after source and before monochromator in IR (sample after monochromator in UV-Vis - less incident light)

► Grating 10-500 lines per mm► Single beam and double beam (in time and space)

Double beam is much more useful eliminates atmospheric gas interference

HomeworkHomework

►Questions and problemsQuestions and problems 16-2, 16-7, 16-8, 16-1016-2, 16-7, 16-8, 16-10