Microwaves are not just for Cooking!

Nicholas R. WalkerUniversity of Bristol

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30th January, 2009. 13879.0 13879.5

Electronic SpectroscopyMovement of electrons between levels

Vibrational SpectroscopyCause molecules to bend, stretch and twist.

Microwave SpectroscopyCause molecules to rotate about the centre of mass.

H = E

Schrodinger Equation predicts Quantization of Energy

Lines in absorption and emission spectra provide a means of probing energy levels in atoms and molecules.

Radio and Radar

Frequency Wavelength Propagation

MF 300-3000 kHz 1 km - 100 mLine of sight + ionosphere refractionHF 3-30 MHz 100 m – 10 m

VHF 30-300 MHz 10 m – 1 m

UHF 300-3000 MHz 100 cm – 10 cmLine of sight only

SHF 3-30 GHz 10 cm – 1 cm

• Light of short wavelength is most directional (less divergent);

sinan Where a is the width of the slit, n is an integer and is the wavelength.

• In principle, short wavelengths are better for radar applications (more directional, not refracted by ionosphere). BUT THERE’S A PROBLEM……

• Wavelengths below ~1.25 cm are efficiently absorbed by H2O vapour.

The sweep absorption (CW) experiment

19501954 – Invention of the Maser (Gordon, Zeiger and Townes).

1946 - First high resolution spectroscopic measurements using microwaves (B. Bleaney).

1968 – First polyatomic molecule identified in space is NH3.

1960

1970

1980

1990

2000

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RadioastronomyThe Atacama Large Millimeter Array (ALMA) is an international collaboration between Europe and N. America to build an array of radio telescopes operating at millimeter and submillimeter wavelengths high in the Andes.- dry, clear skies, minimal interference from Earth’s atmosphere.

L.M. Ziurys and co-workers (Uni. Of Arizona)Millimetre-wave spectroscopy + radioastronomy• Laboratory studies use an oven to generate metal

atoms. • Diatomics (metal hydrides, oxides, nitrides),

hydroxides, cyanides, methylidines, amides.• NaCN, MgCN, AlF observed in circumstellar

envelopes.

The pulsed emission (FT) experiment

Computer technology;

Efficient vacuum pumps and fast pulsed gas expansion nozzles;

Compatible with pulsed lasers

Animation : Prof. Wolfgang Jäger, Dept. of Chemistry, University of Alberta, Edmonton, AB, CANADA, T6G 2G2.

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Fourier Transform Microwave Spectroscopy

• Involves many microwave components.• Big vacuum chamber to accommodate cavity.• Reliable, high sensitivity, high resolution.

19501954 – Invention of the Maser (Gordon, Zeiger and Townes).

1946 - First high resolution spectroscopic measurements using microwaves (B. Bleaney).

1968 – First polyatomic molecule identified in space is NH3.

1960

1970

1980

1990

2000

2002 – rotational spectra of OCS in He droplets

1981 – cavity FT-MW spectroscopy (Balle and Flygare).

Pre-reactive complexes

Hydrogen and van der Waals bonding.

Explore intermolecular potentials.

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House of Thomas Jefferson, MonticelloBrooks Pate, University of Virginia

Matt Muckle, Justin NeillGordon Brown

Chirped pulse 6.5-18.5 GHz

20 Gs/s Arb. Waveform Generator

3.96 GHz PDRO

10 MHz Rb oscillator

TWT Amplifier

18.99 GHz PDRO

12 GHz oscilloscope (40 Gs/s)

Valve/TWTA Trigger

Scope Trigger

Free induction decay (0.5-11.5 GHz)

Microwave Irradiation

Chirped Pulse FTMW Spectroscopy

• A high intensity, chirped microwave pulse rotationally excites molecules.

• The free induction decay from the molecular emission is Fourier transformed (can be done in “real” time).

• The data are summed to obtain the broadband microwave spectrum.

• Instrument is simpler than existing FTMW instruments because the design benefits from latest technology.

S C Oa

b

cLinear molecules (e.g. OCS)•Effectively no moment of inertia about the a axis.•Moments of inertia about b and c axes are equal. •Only one rotational constant is required to fully describe the rotation of the molecule.

Asymmetric rotors (e.g. H2O)•Three rotational coordinates needed to fully describe the rotational spectrum.•Three distinct rotational constants are defined. •Spectra are more complicated than linear rotors and become increasingly complicated with increasing number of atoms and bonds.

OH H

a

bc

Knowing the molecular structure requires measurement of rotational constants.

If rotational constants can be measured for different isotopologues of the same molecule, structure can be established.

I

hB

20 8π

aI

hA

20 8π

bI

hB

20 8π

cI

hC

20 8π

Ground StateVibrationalFrequency

v=0

v=1

v=2

J=8

76 }Rotational

Levels

1S Ground

Electronic

State

}v=0,1,2,3 vibrational levels

J=8

76

Molecules must have permanent dipole moment.

For closed-shell linear molecule (in absence of external magnetic field):Pure rotation : J=1

To have rotational spectra….

No zero point energy !!

Level spacing increases with J (J’-J” transitions spaced by 2B)

HRot.=B0J(J+1)–DJJ2(J+1)2

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1-hexanal 1-heptene

• Rotational spectra of small alkanes and alkenes extensively studied by rotational spectroscopy.

• As the length of the hydrocarbon chain increases, the spectra become increasingly complex as a consequence of the multitude of different conformers and isotopologues in the molecular beam.

• CP-FTMW allows studies of complex spectra owing to increased bandwidth and accurate intensity profiles.

• Comparing the conformational properties of 1-hexanal and 1-heptene allows the role of hydrogen bonding involving the terminal oxygen atom to be explored.

11000 11200 11400 11600 11800 12000

Frequency / MHz8000 12000 16000

Frequency / MHz

10,000 nozzle pulses (~half an hour)

11750 11760 11770 11780

Frequency / MHz

11759 11760 11761Frequency / MHz

FWHM = 125 kHz

8836 MHz10386 MHz

Assigned ‘a’ and ‘b’ type transitions of conformer 1.

Assigned ‘a’ and ‘b’ type transitions of conformer 1.Assigned ‘b’ and ‘c’ type transitions of conformer 3.

8836 MHz 10386 MHz

Six conformers of 1-hexanal assigned and rotational constants determined

The Bristol CP-FTMW Spectrometer

Circuit componentsDecember ‘09

TablesSeptember ‘09

Chamber and pumpOctober ‘09

December ‘09

Final detailsJanuary ‘10

Recent Developments in CP-FTMW Spectroscopy

International Symposium on Molecular Spectroscopy, Ohio State University, 2008.

Stark Effect Measurements (WF12) – L. Alvarez-Valtierra et al. Low Frequency, 2-8 GHz Operation (WF08) – S. T. Shipman et al.Application to Biomolecules (TA01) – R. G. Bird et al.Room temperature, high pressure measurements in a waveguide cell (WF11) – S.T. Shipman et al.

Application to detect chemical warfare agents - Int. J. High Speed Electronics and Systems 18 31-45 (2008), J.J. Pajski et al.

Measuring Picosecond Isomerisation Kinetics via Broadband Microwave Spectroscopy – B.C. Dian et al., Science, 320 pp. 924-928, 16th May 2008.

CP-FTMW Spectrometers constructed at U. Pittsburgh, U. North Texas, Purdue

People

Bristol MicrowaversAnthony LegonSusanna StephensVictor MikhailovFelicity RobertsSophia To

University of VirginiaBrooks PateGordon BrownJustin NeillStephen Shipman

Financial Support