Molecular Stark Effect Measurements in Broadband Chirped-Pulse Fourier Transform

Microwave (CP-FTMW) SpectrometersLeonardo Alvarez-Valtierra,1 Steven T. Shipman,1 Justin L. Neill,1

Brooks H. Pate,1 and Alberto Lesarri 2

1 University of Virginia2 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

For more information, see:Rev. Sci. Inst. 79, 053103 (2008).

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.

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 species2) efficient Stark fits for each species (many lines followed at once)

III

III

The Stark Cage

Emilsson, T.; Gutowsky, H.S.; de Oliveira, G.; Dykstra, C.E., J. Chem. Phys. 112, 1287 (2000).

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).

Effects of Field Inhomogeneity – OCS

Molecules in inhomogeneous region do not contribute to FID at long times.

Intensity decrease is roughly linear with shift from field-free conditions.

0.2% OCS in He/Ne4k shots, 20 s FID

Parallel Plates vs. Cage – OCS

The cage vastly improves the field homogeneity relative to parallel plates.

0.2% OCS in He/Ne4k shots, 20 s FID

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 # 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 4.718OCS 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 DLit. dipole: 2.317 D *

* Kasten, W.; Dreizler, H., Z. Naturforsch. A, 39, 1003 (1984).

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.

313 – 212

MF = 2

312 – 211

MF = 2

Suprane Stark845 – 744

12991.05 MHz836 – 735

12998.16 MHz

Overall fit included 132 transitions, with an OMC of 8.9 kHz.

Fit: A = 1.4756(4) D, B = 0.7584(22) D, C = 0.23502(21) DNIST (51 lines): A = 1.483(2) D, B = 0.761(2) D, C = 0.242(4) D

NOTE: Uncertainties are fit uncertainties; we seem to be systematically low (~1%).

Hexanal – Field-Free Spectrum

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.

(x75)

Hexanal – Field-Free Spectrum

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.

(x5000)

Hexanal Conformers

III

III

IV V VI

180, 180, 180, 054 cm-1 180, 180, 71, 6

0 cm-1

175, 64, 175, 0249 cm-1

64, 176, 180, 0259 cm-1

63, 176, -72, -6247 cm-1

64, 175, 71, 6217 cm-1

Stark Spectrum of Hexanal – 154.75 V/cm

600k shots154.75 V/cm I

III

II

V

Stark Spectrum of Hexanal – 154.75 V/cm

I II

III IV

Hexanal Dipoles – A, B, C

A (D)

(exp)

B (D)

(exp)

C (D)

(exp)

A (D)

(calc*)

B (D)

(calc*)

C (D)

(calc*)

# of lines

OMC

(kHz)

I 1.2738

(27)

2.2882

(21)

0 1.171 2.726 0 81 15.9

II 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)

Hexanal Dipoles – T, , T (D)

(exp)

(º)

(exp)

(º)

(exp)

T (D)

(calc*)

(º)

(calc*)

(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

* MP2 / 6-31G(d)

Note: Assumes dipole is in first octant as signs are not accessible experimentally.

MP2/6-31G(d) Dipoles

Dipole moments are not as bad as they might seem.

Convert (A, B, C) measured and predicted into (T, , ).

T,meas 87.7% of T,pred

meas pred – 3.4º

meas pred – 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

Special Thanks: Tom Fortier and Tektronix

The Pate LabLeonardo Alvarez-Valtierra

Matt MuckleJustin Neill

Sara Samiphak

CollaboratorsLu Kang

Zbigniew KisielRick SuenramNick WalkerLi-Hong Xu

FundingNSF Chemistry CHE-0616660NSF CRIF:ID CHE-0618755