Transcript low frequency.ppt

Design and performance of a direct digital chirpedpulse Fourier transform microwave (CP-FTMW)
spectrometer operating from 2 – 8 GHz
Steven T. Shipman,1 Leonardo Alvarez-Valtierra,1 Justin L. Neill,1
Brooks H. Pate,1 Alberto Lesarri,2 and Zbigniew Kisiel 3
1
University of Virginia
2 Universidad de Valladolid
3 Polish Academy of Sciences
I
II
Overview
• Chirped-Pulse Fourier Transform Microwave
Spectroscopy (CP-FTMW)
• Iodobenzene and its Ne complex
• Interlude: Strawberry aldehyde (C12H14O3)
• MW-MW Double Resonance
• Low frequency Stark measurements
True Broadband Rotational Spectroscopy
The Goal:
• Acquire a multi-GHz spectrum with every valve shot to gain
a multiplex advantage for use with other techniques
Two issues to resolve:
• Generating broad (in frequency), short (in time) pulses
• Getting these pulses in and out of the spectrometer
Arbitrary Waveform Generator
MW Synthesizer Replacement
(Image from www.testequity.com)
Standard Gain Horn
Mirror Replacement
(Image from www.atmmicrowave.com)
Chirped Pulse Excitation
(Linear Frequency Sweep)
J.C. McGurk, T.G. Schmalz, and W.H. Flygare, “Fast passage in rotational spectroscopy: Theory and experiment”,
J. Chem. Phys. 60, 4181 (1974).
E (t )  E0 e
i (0t  ( t ))
Instantaneous Frequency:
d
  (0t   (t ))
dt
 (t ) 

2
t2
Chirped Pulse
Need: 11,000 MHz/1ms
Synthesizer: 300 MHz/1ms
   0  t


t pulse
 = sweep rate
Use arbitrary
waveform
generator as the
frequency source
2 – 8 GHz Direct Detect CP-FTMW Spectrometer
Horns
2.5 – 7.5 GHz, 15 dBi gain
Amplifier
300 W TWTA
4 W SSA
Repetition rate
5 Hz (20 GS/s, 20 ms FID)
20 Gs/s
Oscilloscope
Freq. accuracy
Better than 4 kHz
Note: hard to completely
isolate from 2.4 GHz
wireless band.
2 – 8 GHz Pulse spectrograms
Pre-amplification
Post-amplification
The 300 W TWTA output has some 2nd and 3rd harmonic character.
Solid state amplifiers are cleaner, but at the expense of peak power.
2 – 8 GHz Iodobenzene Spectrum
1 atm of He/Ne
buffer gas flowing
over liquid
reservoir.
This spectrum
took about 3 days
to acquire.
Dorosh, O. et al., J. Mol. Spec. 246 (2007), 228.
2 – 8 GHz Iodobenzene Spectrum
J = 4 – 3 inset
Dorosh, O. et al., J. Mol. Spec. 246 (2007), 228.
Iodobenzene 13C Isotopomers
Ne-Iodobenzene Cluster
The quadrupole tensor of the cluster is indicative of an
11.43(2)º rotation of the a-axis with respect to the C-I bond.
Equivalent Sensitivity to 8 – 18 GHz CP-FTMW
See RH06 for more!
Data in each region scaled to match noise in overlap,
giving signals in good agreement with SPCAT.
4 W vs. 300 W Amplifier
300 W TWTA, 4 ms
Scale TWTA by
0.237 to match SSA
4 W SSA, 20 ms
300 W TWTA for 4 ms = 1200 mJ delivered to sample
4 W SSA for 20 ms = 80 mJ delivered to sample
Theoretical ratio = sqrt(1/15) = 0.253
4 W SSAs are much cheaper than 300 W TWTAs!
Spectrum of a Large Molecule:
Strawberry aldehyde (C12H14O3)
I
III
II
IV
1 nozzle, heated to 120°C
Future: Use multiple nozzles in 2-8 and 8-18 GHz.
Assigned 5th, haven’t gotten its complement yet.
Strawberry Aldehyde Rotational Constants
I
II
III
IV
A (MHz)
728.0954(6)
1214.7279(3)
723.1401(6)
1330.9496(13)
B (MHz)
628.6905(3)
287.76523(12)
581.4101(6)
293.4824(4)
429.84709(24) 269.46230(12)
421.9610(9)
281.8650(4)
C (MHz)
J (kHz)
0.069(3)
0.0144(3)
0.3083(10)
0.0178(11)
JK (kHz)
-0.062(14)
-0.0771(18)
-0.75(3)
-0.091(5)
K (kHz)
0.078(17)
0.905(19)
0.553(22)
0.96(7)
dJ (kHz)
0.0237(18)
0.00291(14)
0.125(5)
0.0031(5)
dK (kHz)
-0.030(7)
-0.271(25)
-0.091(23)
0.41(15)
# lines
77
92
40
33
OMC (kHz)
5.1
6.7
4.8
6.7
MW-MW Double Resonance Scheme
505
404
303
515
414
313
20 Gs/s
Oscilloscope
202
212
Multiple isolators needed to keep reflected TWTA power from blowing up SSA!
413
MW-MW Double Resonance
First (chirped) pulse polarizes rotational
transitions over a large bandwidth.
1
A second narrowband pulse pumps a single
transition. This destroys coherences with
connected levels, giving intensity
modulations in the detected FID.
2
1
2
MW-MW Double Resonance on
Iodobenzene
MW-MW Double Resonance on
Iodobenzene
MW-MW DR quickly establishes level connectivity, greatly facilitating assignments.
Low Frequency Stark Spectra (WF12)
Inhomogeneity leads to weak / missing lines, but there are many usable lines.
The best fit used 329 lines at 4 fields to give a dipole moment of 1.60070(68) D.
Field strengths were calibrated with TFP (2.319 D), which was in turn calibrated
with OCS in the 8 – 18 GHz spectrometer.
Future Directions
Short Term:
• Start taking data with multiple heated nozzles
• Stark effect measurements on strawberry aldehyde conformers
Long Term:
• Laser ablation sources for larger molecules
• Construct a 2 – 18 GHz direct detect spectrometer
Acknowledgements
The Pate Lab
Leonardo Alvarez-Valtierra
Matt Muckle
Justin Neill
Sara Samiphak
Collaborators
David Pratt (Pittsburgh)
Rick Suenram (UVa)
Lu Kang (Union College, KY)
Nick Walker (University of Bristol, UK)
Funding
NSF Chemistry CHE-0616660
NSF CRIF:ID CHE-0618755
Special Thanks: Tom Fortier and Tektronix
Iodo and Ne-Iodo Constants
Iodo
A (MHz)
5668.83(33)
20Ne-Iodo
22Ne-Iodo
1818.7033(214) 1731.661(145)
B (MHz)
750.41649(194) 605.64614(126)
598.7791(85)
C (MHz)
662.63353(183) 537.55376(112)
524.3020(66)
J (kHz)
0.0247(43)
1.1910(68)
1.335(54)
JK (kHz)
0.202(38)
18.292(44)
15.59(43)
K (kHz)
Not included
16.39(284)
Not included
dJ (kHz)
Not included
0.2985(52)
0.3490(196)
dK (kHz)
Not included
13.16(35)
12.44(306)
caa (MHz)
-1892.0388(45)
-1779.270(43)
-1754.84(29)
cbb (MHz)
978.816(11)
801.06(17)
776.55(49)
ccc (MHz)
913.222(16)
978.21(24)
978.29(64)
cab (MHz)
-
543.92(76)
597.74(39)
225
119
40
# of lines
Dorosh, O. et al., J. Mol. Spec. 246 (2007), 228.
Stark Fit Histogram
Iodobenzene K1 Stark Anomalies
MW-MW Double Resonance on
Iodobenzene
MW-MW Double Resonance on
Iodobenzene
MW-MW Double Resonance on
Iodobenzene
Extension to Very Low Frequency
0.8 – 2.0 GHz CP-FTMW (Suprane)
849.52 MHz
515 - 505
1618.56 MHz
101 - 000