Microwaves are not just for Cooking! Nicholas R. Walker University of Bristol by 1 30 th January,

Electronic Spectroscopy Movement of electrons between levels Vibrational Spectroscopy Cause molecules to bend, stretch and twist. Microwave Spectroscopy Cause 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 FrequencyWavelengthPropagation MF kHz1 km m Line of sight + ionosphere refraction HF3-30 MHz100 m – 10 m VHF MHz10 m – 1 m UHF MHz100 cm – 10 cm Line of sight only SHF3-30 GHz10 cm – 1 cm Light of short wavelength is most directional (less divergent); 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 H 2 O vapour.

The sweep absorption (CW) experiment

– Invention of the Maser (Gordon, Zeiger and Townes) First high resolution spectroscopic measurements using microwaves (B. Bleaney) – First polyatomic molecule identified in space is NH

Radioastronomy The 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. 7

Fourier Transform Microwave Spectroscopy Involves many microwave components. Big vacuum chamber to accommodate cavity. Reliable, high sensitivity, high resolution.

– Invention of the Maser (Gordon, Zeiger and Townes) First high resolution spectroscopic measurements using microwaves (B. Bleaney) – First polyatomic molecule identified in space is NH – 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. 3

House of Thomas Jefferson, Monticello Brooks Pate, University of Virginia Matt Muckle, Justin Neill Gordon Brown

Chirped pulse GHz 20 Gs/s Arb. Waveform Generator 3.96 GHz PDRO 10 MHz Rb oscillator TWT Amplifier GHz PDRO 12 GHz oscilloscope (40 Gs/s) Valve/TWTA Trigger Scope Trigger Free induction decay ( 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.

SCO a b c Linear 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. H 2 O) 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. O HH a b c 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.

 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) H Rot. =B 0 J(J+1)–D J J 2 (J+1) 2 2

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.

10,000 nozzle pulses (~half an hour)

FWHM = 125 kHz

8836 MHz MHz Assigned ‘a’ and ‘b’ type transitions of conformer 1.

Assigned ‘b’ and ‘c’ type transitions of conformer MHz MHz

Six conformers of 1-hexanal assigned and rotational constants determined

The Bristol CP-FTMW Spectrometer Circuit components December ‘09 Tables September ‘09 Chamber and pump October ‘09 December ‘09 Final details January ‘10

Recent Developments in CP-FTMW Spectroscopy International Symposium on Molecular Spectroscopy, Ohio State University, 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 (2008), J.J. Pajski et al. Measuring Picosecond Isomerisation Kinetics via Broadband Microwave Spectroscopy – B.C. Dian et al., Science, 320 pp , 16 th May CP-FTMW Spectrometers constructed at U. Pittsburgh, U. North Texas, Purdue

People Bristol Microwavers Anthony Legon Susanna Stephens Victor Mikhailov Felicity Roberts Sophia To University of Virginia Brooks Pate Gordon Brown Justin Neill Stephen Shipman Financial Support