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
31
Embed
ULTRAVIOLET - CHIRPED PULSE FOURIER TRANSFORM MICROWAVE (UV-CPFTMW) DOUBLE-RESONANCE SPECTROSCOPY Brian C. Dian, Kevin O. Douglass, Gordon G. Brown, Jason.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• 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.