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 For more information, see: Rev. Sci. Inst. 79, (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 species 2) efficient Stark fits for each species (many lines followed at once) I II 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/Ne 4k 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/Ne 4k 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 linesStd. Dev. (kHz) OCS OC 34 S0.7153(9)74.0 O 13 CS0.7140(10)74.6 OC 33 S0.7146(7) OCS0.7150(10)74.5 O 13 C 34 S0.7121(7)73.0 OC 36 S0.7156(25)65.4 OCS (100 vib)0.6939(12)75.2 OCS (200 vib)0.678(7)56.7 OC 34 S (100 vib)0.6948(21)79.0 Dipole moments fit with QSTARK using data at 1, 2, 3, 4, 6, 8, and 12 kV.

Trifluoropropyne Dipole Calibration Best fit slope: (4) MHz / (V/cm) R 2 = Calc. dipole: D Lit. dipole: 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 – 2 12 M F = – 2 11 M F = 2

Suprane Stark 8 45 – MHz 8 36 – MHz Overall fit included 132 transitions, with an OMC of 8.9 kHz. Fit:  A = (4) D,  B = (22) D,  C = (21) D NIST (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 C 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 C species. More “deep averaging” spectra of hexanal, 1-heptene, and suprane in RH06. (x5000)

Hexanal Conformers I II III IV V VI 180, 180, 180, 0 54 cm , 180, 71, 6 0 cm , 64, 175, cm -1 64, 176, 180, cm -1 63, 176, -72, cm -1 64, 175, 71, cm -1

Stark Spectrum of Hexanal – V/cm 600k shots V/cm I III II V

Stark Spectrum of Hexanal – 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 (27) (21) II (22) (5) (7) III1.918 (8) (6) (7) IV0.983 (13) (10) (15) V (22) (10) (15) VI0.581 (7) (8) 0.19 (4) * 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 II III IV V VI * 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 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 NSF CRIF:ID CHE