1. An Acoustic Demonstration Model for CW and Pulsed Spectroscopy Experiments Torben Starck, Heinrich Mäder Institut für Physikalische Chemie Christian-Albrechts-Universität zu Kiel, Germany 1 Trevor Trueman, Wolfgang Jäger Department of Chemistry University of Alberta, Edmonton, Canada OSU International Symposium on Molecular Spectroscopy, 64th meeting 2009, Paper RH01

2. 2 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)

3. 3 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)

4. 4 Spectroscopic applications CW- and FT-NMR spectroscopy Sample : 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 spectroscopy Sample : 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 advantages in both resolution and sensitivity over CW-techniques.

5. 5 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 ν 3 -dependence of (incoherent) spontaneous emission probability.

6. 6 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)

7. 7 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

8. 8 FTMW spectroscopy: laboratories all over the world... and more than 2000 papers published

9. 9 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 kHz Type of interaction : excitation of object’s natural frequencies of vibration Sample response : mechanical vibrations of the object Detector : 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.

10. 10 The acoustic demontration model Experimental setup

11. 11 The sweep absorption (CW) experiment Screen display at start of experiment

12. 12 The sweep absorption (CW) experiment Frequency sweep without sample (background spectrum)

13. 13 The sweep absorption (CW) experiment

14. 14 The sweep absorption (CW) expermient Frequency sweep with beer glass

15. 15 The sweep absorption (CW) experiment Result for empty beer glass

16. 16 The pulsed emission (FT) experiment Screen display at start of experiment

17. 17 The pulsed emission (FT) experiment Realignment of the setup

18. 18 The pulsed emission (FT) experiment Pulse excitation experiments with different pulse carrier frequencies

19. 19 The pulsed emission (FT) experiment Screen shots, showing results for different pulse lengths 80 ms t 200 ms t pulse spectrum sample spectrum

20. 20 The pulsed emission (FT) experiment Screen shots, showing results for different recording times t 3 s t 1 s Time domain signal Spectrum

21. 21 The pulsed (FT) experiment: "beer resonances"

22. 22 The pulsed (FT) experiment: "beer resonances"

23. 23 The pulsed (FT) experiment: "beer resonances"

24. 24 The pulsed (FT) experiment: "testing beer content"

25. 25 Danke !Thanks ! - to the workshops of the Institut für Physikalische Chemie at the University of Kiel and of the Chemistry Department at the University of Alberta - for funds from the Land Schleswig-Holstein and from the Natural Sciences and Engineering Research Council of Canada