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