Molecular Stark Effect Measurements in
Broadband Chirped-Pulse Fourier Transform
Microwave (CP-FTMW) Spectrometers
Leonardo Alvarez-Valtierra,1 Steven T. Shipman,1 Justin L. Neill,1
Brooks H. Pate,1 and Alberto Lesarri 2
1
University of Virginia
2 Universidad de Valladolid
Overview
• CP-FTMW Spectrometer and Stark Cage
• OCS, isotopomers and clusters
• Suprane and hexanal
7.5 – 18.5 GHz CP-FTMW Spectrometer
1) AWG generates a chirped
pulse that is upconverted to
7.5 – 18.5 GHz and amplified.
2) The pulse is broadcast into
the vacuum chamber where it
interacts with molecules in a
pulsed jet.
3) The FID is amplified, mixed
down, and finally digitized on
a fast oscilloscope.
For more information, see:
Rev. Sci. Inst. 79, 053103 (2008).
Why Stark and Why CP-FTMW Stark?
Added information to help connect assigned spectra to structures from ab initio.
Particularly important for conformationally rich systems.
With knowledge of dipoles, intensities in spectra can be converted into
population differences, allowing for tests of conformational energy ordering.
Chirped-pulse FTMW measurements are highly multiplexed, allowing:
1) simultaneous dipole measurements on multiple species
2) efficient Stark fits for each species (many lines followed at once)
II
I
III
The Stark Cage
The cage is a voltage divider – many small voltage drops rather than a single large one.
Expansion is unaffected and nozzle is in a low voltage region.
The cage can be left in the instrument at all times!
Two high voltage power supplies required (+ and – voltage).
Emilsson, T.; Gutowsky, H.S.; de Oliveira, G.; Dykstra, C.E., J. Chem. Phys. 112, 1287 (2000).
Effects of Field Inhomogeneity – OCS
0.2% OCS in He/Ne
4k shots, 20 ms FID
Molecules in inhomogeneous region do not contribute to FID at long times.
Intensity decrease is roughly linear with shift from field-free conditions.
Parallel Plates vs. Cage – OCS
0.2% OCS in He/Ne
4k shots, 20 ms FID
The cage vastly improves the field homogeneity relative to parallel plates.
Simultaneous Isotopomer Measurements
Ne-OCS Clusters
All of these come from the same data set, 4k shots (~15 minutes) per field strength.
Normal species OCS is used as an internal field calibrant.
Dipole moments of OCS species
Species
m
# of lines
Std. Dev. (kHz)
OCS
0.71519
-
-
OC34S
0.7153(9)
7
4.0
O13CS
0.7140(10)
7
4.6
OC33S
0.7146(7)
19
4.7
18OCS
0.7150(10)
7
4.5
O13C34S
0.7121(7)
7
3.0
OC36S
0.7156(25)
6
5.4
OCS (100 vib)
0.6939(12)
7
5.2
OCS (200 vib)
0.678(7)
5
6.7
OC34S (100 vib)
0.6948(21)
7
9.0
Dipole moments fit with QSTARK using data at 1, 2, 3, 4, 6, 8, and 12 kV.
Trifluoropropyne Dipole Calibration
Best fit slope:
0.09728(4) MHz / (V/cm)
R2 = 0.99976
Calc. dipole: 2.319 D
Lit. dipole: 2.317 D *
312 – 211
MF = 2
313 – 212
MF = 2
OCS gives the local field
strength, used to determine
TFP’s dipole moment from
its first-order shifts.
TFP is used as a calibrant
for the 2 – 8 GHz
spectrometer (WF08).
First-order shifts in 8 – 18
GHz help to reduce field
strength uncertainty.
* Kasten, W.; Dreizler, H., Z. Naturforsch. A, 39, 1003 (1984).
Suprane Stark
845 – 744
12991.05 MHz
836 – 735
12998.16 MHz
Overall fit included 132 transitions, with an OMC of 8.9 kHz.
Fit: mA = 1.4756(4) D, mB = 0.7584(22) D, mC = 0.23502(21) D
NIST (51 lines): mA = 1.483(2) D, mB = 0.761(2) D, mC = 0.242(4) D
NOTE: Uncertainties are fit uncertainties; we seem to be systematically low (~1%).
Hexanal – Field-Free Spectrum
(x75)
Only the dominant 6 conformers are shown in the simulations.
To date 10 conformers have been assigned, along with 22 13C species.
More “deep averaging” spectra of hexanal, 1-heptene, and suprane in RH06.
Hexanal – Field-Free Spectrum
(x5000)
Only the dominant 6 conformers are shown in the simulations.
To date 10 conformers have been assigned, along with 22 13C species.
More “deep averaging” spectra of hexanal, 1-heptene, and suprane in RH06.
Hexanal Conformers
II
III
I
180, 180, 180, 0
54 cm-1
IV
180, 180, 71, 6
0 cm-1
V
64, 176, 180, 0
259 cm-1
175, 64, 175, 0
249 cm-1
VI
63, 176, -72, -6
247 cm-1
64, 175, 71, 6
217 cm-1
Stark Spectrum of Hexanal – 154.75 V/cm
600k shots
154.75 V/cm
V
II
III
I
Stark Spectrum of Hexanal – 154.75 V/cm
I
III
II
IV
Hexanal Dipoles – mA, mB, mC
mA (D)
(exp)
mB (D)
(exp)
I
1.2738
(27)
II
mA (D)
(calc*)
mB (D)
(calc*)
mC (D)
(calc*)
# of
lines
OMC
(kHz)
2.2882 0
(21)
1.171
2.726
0
81
15.9
0.5151
(22)
2.292
(5)
1.012
(7)
0.526
2.561
1.234
53
16.7
III
1.918
(8)
1.651
(6)
0.877
(7)
1.976
2.080
0.954
35
12.8
IV
0.983
(13)
2.370
(10)
0.715
(15)
0.741
2.806
0.752
31
12.8
V
0.0461
(22)
2.251
(10)
0.833
(15)
0.044
2.698
0.919
11
9.6
VI
0.581
(7)
2.469
(8)
0.19
(4)
0.669
2.807
0.097
12
8.9
* MP2 / 6-31G(d)
mC (D)
(exp)
Hexanal Dipoles – mT, q, f
mT (D)
(exp)
q (º)
(exp)
f (º)
(exp)
mT (D)
(calc*)
q (º)
(calc*)
f (º)
(calc*)
# of
lines
OMC
(kHz)
I
2.619
60.9
90
2.967
66.8
90
81
15.9
II
2.558
77.3
66.7
2.891
78.4
67.4
53
16.7
III
2.678
40.7
70.9
3.023
46.5
71.6
35
12.8
IV
2.664
67.5
74.4
2.998
75.2
75.5
31
12.8
V
2.401
88.8
69.7
2.851
89.1
71.2
11
9.6
VI
2.544
76.8
85.7
2.887
76.6
88.1
12
8.9
Note: Assumes dipole is in first octant as signs are not accessible experimentally.
* MP2 / 6-31G(d)
MP2/6-31G(d) Dipoles
Dipole moments are not as bad as they might seem.
Convert (mA, mB, mC) measured and predicted into (mT, q, f).
mT,meas  87.7% of mT,pred
qmeas  qpred – 3.4º
fmeas  fpred – 0.8º
Need to see how well this holds up with more testing!
Future Directions
• Measurements on 1-heptene, strawberry aldehyde, (FA)3
• New cage design to accommodate multiple pulsed nozzles
• Measurements on laser-prepared excited states (Ar-DF)
Acknowledgements
The Pate Lab
Leonardo Alvarez-Valtierra
Matt Muckle
Justin Neill
Sara Samiphak
Collaborators
Lu Kang
Zbigniew Kisiel
Rick Suenram
Nick Walker
Li-Hong Xu
Funding
NSF Chemistry CHE-0616660
NSF CRIF:ID CHE-0618755
Special Thanks: Tom Fortier and Tektronix