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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
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
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
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.
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 ν3-dependence of (incoherent) spontaneous emission probability.
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
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
FTMW spectroscopy: laboratories all over the world
... and more than 2000 papers published
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 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.
10
The acoustic demontration model
Experimental setup
11
The sweep absorption (CW) experiment
Screen display at start of experiment
12
The sweep absorption (CW) experiment
Frequency sweep without sample (background spectrum)
13
The sweep absorption (CW) experiment
14
The sweep absorption (CW) expermient
Frequency sweep with beer glass
15
The sweep absorption (CW) experiment
Result for empty beer glass
16
The pulsed emission (FT) experiment
Screen display at start of experiment
17
The pulsed emission (FT) experiment
Realignment of the setup
18
The pulsed emission (FT) experiment
Pulse excitation experiments with different pulse carrier frequencies
19
The pulsed emission (FT) experiment
Screen shots, showing results for different pulse lengths
80 ms
t
200 ms
tpulse spectrum
sample spectrum
20
The pulsed emission (FT) experiment
Screen shots, showing results for different recording times
t3 s
t1 s
Time domain signal
Spectrum
21
The pulsed (FT) experiment: "beer resonances"
22
The pulsed (FT) experiment: "beer resonances"
23
The pulsed (FT) experiment: "beer resonances"
24
The pulsed (FT) experiment: "testing beer content"
25
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