An Acoustic Demonstration Model for

CW and Pulsed Spectroscopy Experiments

Torben Starck, Heinrich Mäder

Institut für Physikalische Chemie

Christian-Albrechts-Universität zu Kiel, Germany

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Trevor Trueman, Wolfgang Jäger

Department of Chemistry

University of Alberta, Edmonton, Canada

OSU International Symposium on Molecular Spectroscopy, 64th meeting 2009, Paper RH01

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Overview

• Introduction

Principles of CW and pulsed (FT) spectroscopy experiments

Some remarks on the history of CW- and FT-MW spectroscopy

• An acoustic demonstration model

The sweep absorption experiment (CW spectroscopy)

The emission experiment (FT spectroscopy)

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CW- vs. FT-techniques

CW: continuous wave excitation

: slowly varying

sample

FT: pulsed excitation

: fixed

sample

() : absorption signal

S(t) : emission signal

t

Fourier transformation (FT)

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Spectroscopic applications

• CW- and FT-NMR spectroscopySample : molecules containing nuclei with spin > 0

Radiation frequency : RF (several 100 MHz)

Type of interaction : magnetic-dipole interaction

Sample response : macroscopic magnetic dipole moment (FID)

• CW- and FT-MW spectroscopySample : polar molecules in a static gas or a supersonic beam

Radiation frequency : MW (GHz to THz)

Type of interaction : electric-dipole interaction

Sample response : macroscopic electric dipole moment (transient emission)

For NMR and MW spectroscopy, FT-techniques hold considerable advantagesin both resolution and sensitivity over CW-techniques.

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MW spectroscopy: some remarks on its history

CW-techniques (sweep absorption experiments):

In the first thirty years of microwave spectroscopy, starting after World War II, rotational spectra of polar molecules were recorded only as absorption spectra, employing powerful modulation techniques, such as Stark-modulation (Hughes and Wilson, 1947). Further important developments of absorption spectrometers were particularly achieved in the mm- and sub-mm wavelength range, e.g. the FASSST spectrometer (de Lucia et al., 1997).

Initially, spontaneous emission signals were not considered to be strong enough to be usable for spectroscopic applications. Even in a modern textbook on Molecular Physics (2004), one can find the sentence :The rotational spectra of molecules are observed almost exclusively as absorption spectra, because the emission probability is very small as a result of low transition frequencies.This argument is based on the ν3-dependence of (incoherent) spontaneous emission probability.

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MW spectroscopy: some remarks on its history

• Early work on pulse techniques in MW spectroscopy Dicke and Romer (1955)

• Stark switching techniques Harrington (1968), Macke et al. (1972), Brittain et al. (1973), Flygare et al. (1974)

• First demonstration of FTMW spectroscopy McGurk, Mäder, Hofmann, Schmalz and Flygare (1974)

• Pulse-induced waveguide (WG)-FTMW- spectroscopy Ekkers and Flygare (1976)

• Molecular beam (MB)-FTMW- spectroscopy Balle and Flygare (1981)

FT-techniques (emission experiments) :The pulse-induced emission experiments are based on coherent spontaneous emission of the molecular sample, originating from a macroscopic polarization.

• Broadband chirped pulse (CP)-FTMW- spectroscopy Pate et al. (2005)

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FTMW spectroscopy: some remarks on its history

First demonstration of FTMW spectroscopy:

J. Chem. Phys. 61 , 3759 (1974)

Referee's comment :

J,K

3,3

2,1

3,1

4,1

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FTMW spectroscopy: laboratories all over the world

... and more than 2000 papers published

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The acoustic demonstration model

Schematic:

CW- and FT-acoustic spectroscopy

Sample : any acoustically resonant object, e.g. a tuning fork Sound waves, generated by a speaker : 20 Hz to 20 kHzType of interaction : excitation of object’s natural frequencies of vibrationSample response : mechanical vibrations of the objectDetector : microphone

In our experimental setup, the speaker and the microphone are both controlled by a computer sound card and the whole setup is housed in a plexiglass box, which serves as resonator.

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The acoustic demontration model

Experimental setup

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The sweep absorption (CW) experiment

Screen display at start of experiment

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The sweep absorption (CW) experiment

Frequency sweep without sample (background spectrum)

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The sweep absorption (CW) experiment

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The sweep absorption (CW) expermient

Frequency sweep with beer glass

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The sweep absorption (CW) experiment

Result for empty beer glass

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The pulsed emission (FT) experiment

Screen display at start of experiment

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The pulsed emission (FT) experiment

Realignment of the setup

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The pulsed emission (FT) experiment

Pulse excitation experiments with different pulse carrier frequencies

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The pulsed emission (FT) experiment

Screen shots, showing results for different pulse lengths

80 ms

t

200 ms

tpulse spectrum

sample spectrum

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The pulsed emission (FT) experiment

Screen shots, showing results for different recording times

t3 s

t1 s

Time domain signal

Spectrum

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The pulsed (FT) experiment: "beer resonances"

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The pulsed (FT) experiment: "beer resonances"

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The pulsed (FT) experiment: "beer resonances"

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The pulsed (FT) experiment: "testing beer content"

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Danke ! Thanks !

- to the workshops

of the Institut für Physikalische Chemie

at the University of Kiel and

of the Chemistry Departmentat the University of Alberta

- for funds

from the Land Schleswig-Holstein and

from the Natural Sciences and Engineering Research Council

of Canada