ULTRAVIOLET - CHIRPED PULSE FOURIER TRANSFORM MICROWAVE (UV-CPFTMW) DOUBLE-RESONANCE SPECTROSCOPY...
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Transcript of ULTRAVIOLET - CHIRPED PULSE FOURIER TRANSFORM MICROWAVE (UV-CPFTMW) DOUBLE-RESONANCE SPECTROSCOPY...
ULTRAVIOLET - CHIRPED PULSE FOURIER TRANSFORM MICROWAVE (UV-CPFTMW) DOUBLE-RESONANCE
SPECTROSCOPY
Brian C. Dian, Kevin O. Douglass, Gordon G. Brown, Jason J. Pajski, and Brooks H. Pate
Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904
Kevin O. Douglass
Introduction
• UV – Chirped Pulse FTMW spectroscopy– Measure entire 7.5 – 18.5 GHz MW spectrum
as laser is actively scanned
• UV – Cavity FTMW spectroscopy– Enhanced sensitivity when monitoring single
line– Multiple MW pulse techniques:
background free
Laser-FTMW Double-Resonance
UV Scan
V=0,J=3
V=0,J=2
S1,J=3
MW Probe
UV MW
Timing
Detect
Ground State Depletion
Transfer population before MW pulse
Positive and negative peaks
Coherence Method
Destroy Coherence of molecular FID
Negative peaks onlyUVMW
Timing
Detect
Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo, Rev. Sci. Instrum. 73, 165 (2002).
8000 10000 12000 14000 16000 18000
0.0000
0.0003
0.0006
0.0009
0.0012
0.0015
0.0018
Inte
nsi
ty (
V)
Frequency (MHz)
Single Shot
100 Shots
10000 Shots
100 Shots:20 s acquisition~ 2 mol sample consumption
Pure Rotational Spectrum of Suprane20 s of FID Acquisition (80 kHz linewidth, FWHM)
10000 shots 20 μs gate: 45 min. acquisitionB-F Equivalent
0.1% Suprane in He/Ne
Choose Your Sensitivity
8000 10000 12000 14000 16000 18000
0.0000
0.0003
0.0006
0.0009
0.0012
0.0015
0.0018
Inte
nsi
ty (
V)
Frequency (MHz)
Single Shot
100 Shots
10000 Shots
100 Shots:20 s acquisition~ 2 mol sample consumption
Pure Rotational Spectrum of Suprane20 s of FID Acquisition (80 kHz linewidth, FWHM)
0.1% Suprane in He/Ne
Choose Your Sensitivity
~500:1 S/N in 20 secondsCavity has moved 5 MHz
Benzonitrile Multiplexed UV-CPFTMW
UV MW
Timing
Detect
UV Scan
V=0,J=2
V=0,J=1
S1,J=2
MW Probe
Internal Reference Coherence Method
time (s)
0 2 4 6 8 10
Inte
nsi
ty (
V)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
FT gate 1 (laser off)
Laser pulse FT gate 2 (laser on)
Monitor: (FT gate 2*scale factor) - FT gate 1Signal ~ 0 mVEquivalent to laser on – laser off for the same valve and MW pulse
Benzonitrile UV-CPFTMW(internal referenced coherence method)
UVMW
Timing
DetectDetect
UV Scan
V=0,J=2
V=0,J=1
S1,J=2
MW Probe
UV-CPFTMW Double Resonance Spectroscopy
• Implemented both Ground State Depletion (GSD) and Dual-Gate Coherence Method of Endo
• Lower single-shot sensitivity for CP-FTMW spectroscopy requires higher number of spectrum averages than cavity spectrometer BUT gives multiplexed DR scans.
• Competitive sensitivity is reached when the CP-FTMW measurement reaches about 100:1 signal-to-noise ratio
• This limit is determined by the typical pulsed valve signal stability
Comparisons between cavity FTMW and CP-FTMW spectrometers
These comparisons between cavity and CP-FTMW spectrometer performance have been made obsolete by the development of a double-pulse method for laser-FTMW spectroscopy.
Double-Pulse FTMW – Laser Spectroscopy
A Background Free Detection Technique with Order-of-Magnitude Sensitivity Improvement
Narrowband FTMW cavity Spectrometer
T.J. Balle and W.H. Flygare, Rev. Sci. Instrum. 52, 33 (1981).
MW Synthesizer
ν0
ν0
Free Induction Decay(30 MHz Carrier)
5 Gs/s Oscilloscope
R.D. Suenram, J.U. Grabow, A. Zuban, and I. Leonov, Rev. Sci. Instrum. 70, 2127 (1999)
2 Gs/s AFG
v0 + 30 MHzSingle Sideband
Pulsed 1 watt ampDye laser
Nd:YAG
Continuum
10 Hz rep. rate
200 mJ/p 532 nm
5 mJ/p UV0.025 cm-1
bandwidth
Front PanelKnob Control:
0.01o Phase 1 mV / 1 V Amplitude
Bloch Vector Model for a Resonant Double-Pulse MW Excitation Scheme
“ / 2” “- / 2”
- “- / 2” pulse used to counteract M-dependence of transition moment
Demonstration of Double-Pulse MW Excitation
MW Pulse(s) FID FT
Bloch Vector Model for a Resonant Double Pulse MW Excitation Scheme
“ / 2” Laser Pulse “- / 2”
How do we describe the interaction of the laser pulse with the coherent superposition of rotational levels created by the first MW pulse?
The Effects of Selective Laser Excitation Pulse
With the laser ON RESONANCE, the Bloch vector rotates about the x-axis (lower rotational level excited):
Laser Pulse“ / 2” “- / 2”
With laser excitation, the second pulse leaves the laser-induced population change in the x-y plane for background free detection.
Implications of the Mechanism
• For resonant laser excitation, there is a 180o phase shift for laser excitation of the lower and upper rotational levels (phase sensitive detection).
• Off-resonance the Bloch vector rotates around the pseudo-vector:
zxRabi ˆˆ
This gives rise to a phase shift in the FID as the laser is scanned across a resonance.
The technique measures the susceptibility of the laser transition giving both the real (dispersion) and imaginary (absorption) components via the FTMW spectrum.
The Effects of Selective Laser Excitation Pulse
With the laser ON RESONANCE, the Bloch vector rotates about the (-)x-axis (upper rotational level excited):
Laser Pulse“ / 2” “- / 2”
With laser excitation, the second pulse leaves the laser-induced population change in the x-y plane for background free detection.
Phenylacetylene Phase Information
R(3)
R(4)
Phenylacetylene Phase Shift Across Resonance
Phenylacetylene Phase Shift Across Resonance
Phenylacetylene Phase Shift Across Resonance
Phenylacetylene Phase Information
Phenylacetylene UV-FTMWBackground Free
UV Scan
V=0,J=2
V=0,J=1
S1,J=2
MW Probe
UV MW
Timing
DetectMW
Phenylacetylene UV-FTMW GSD vs. Background Free
UV Scan
V=0,J=2
V=0,J=1
S1,J=2
MW Probe
UV MW
Timing
DetectMW
Background Free
Previous Technique
Propyne IR-FTMW
IR Scan
V=0,J=1
V=0,J=0
V=1,J=1
MW Probe
IR MW
Timing
Detect
Propyne IR-FTMWBackground Free
IR Scan
V=0,J=1
V=0,J=0
V=1,J=1
MW Probe
IR MW
Timing
DetectMW
Imaginary FT (absorption)
Real FT (dispersion)
Pyridine UV-FTMWBackground Free
UV Scan
V=0,J=2
V=0,J=1
S1,J=2
MW Probe
UV MW
Timing
DetectMW
Previous Technique
Background Free
Conclusions
• UV – Chirped-Pulse FTMW Spectroscopy Demonstrated– Ability to monitor multiple transitions (conformers)
simultaneously
• UV – Cavity FTMW– Increased sensitivity for measuring single transition– Double MW pulse technique for zero-background laser scanning
Acknowledgements
Pate Lab Group Members
Funding:• NSF Chemistry• NSF MRI Program (with Tom Gallagher, UVa Physics)• John D. and Catherine T. Macarthur Foundation• SELIM Program
• University of Virginia
2 Pulse Background Free Technique
UVMW
Timing
DetectDetect
MW Pulse 1
MW Pulse 2
Adjustable phase and amplitude
UV Laser
500 ns 500 ns
50 ns
Molecular FID FT
WI02