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Beam Action Spectroscopy via Inelastic Scattering BASIS Technique Bobby H. Layne and Liam M. Duffy Department of Chemistry & Biochemistry, the University of North Carolina at Greensboro, Greensboro, North Carolina 27402 Hans A Bechtel, Adam H. Steeves and Robert W. Field Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
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J. Phys. Chem. A (online)
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Current Research Using mm-Waves to probe: Photodissociation of atmospheric molecules: –characterizing quantum state distribution of products –hyper-rovibronic detail Crossed molecular beams: reactive and inelastic scattering dynamics
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Current Photodissociation Study Chlorine Dioxide h Chlorine Monoxide Oxygen Cl O O O O Parent Molecule Products
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OClO is an reservoir molecule for Cl radicals in the atmosphere A. Wahner, G. S. Tyndall, and A. R. Ravishankara, J. Phys. Chem. 91, 2734 (1987).
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mm-Wave: 1.Source Module 2.Amplifier 3.Multiplier 4.Horn Pulsed Slit Nozzle Teflon Window & Lens Top view of vacuum chamber with diffusion pump below InSb Hot Electron Bolometer ULN6 preamp Tunable UV from doubled OPO 43 2 1 Corner Reflector Multipass Cell Photodissociation Setup Current Available Range: 50-330 GHz
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Mm-Wave Source Module “Armadillo” Microwave Synthesizer Mm-Wave Amplifier & Tripler Frequency Resolution 10 Hz at 100 GHz or 1 part in 10 10 Power > 1 mW
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Parent Molecule
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Photodissociation of Parent Molecule
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Laser is fixed while mm-waves are stepped UV Laser fixed to OClO (X 2 B 1 A 2 A 2 (15, 0, 0)) O 35 ClO hyperfine lines Cl O O
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Products Probed in Hyper-rovibronic Detail Cl O
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Millimeter-wave absorption time trace centered on a single hyperfine line of O 35 ClO (268797.6550 MHz: N=6 5, J=6½ 5½, K -1 =3 2, K +1 =4 3, F=8 7) Problem ! The “hole” shows a 50% depletion of the parent. 8.4% is expected from the product of laser fluence and UV cross-section.
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“Hole burning” spectrum of O 35 ClO (14,0,0) (15,0,0) (16,0,0) (17,0,0) ? BASIS spectrum of O 35 ClO and O 37 ClO
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Parent Signal BASIS Signal
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SO 2 Parent Hans Bechtel & Adam Steeves (Field Group at MIT) Low rotational transition 3 13 - 2 02 High rotational transition 8 17 - 8 08 artistic simulation
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O C S O C S O C S O C S O C S O C S O C S O C S O C S O C S O C S O C S O C S Ar O Cl O Ar
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UV BASIS Signals of OClO OCS reporter & fixed UV J = 6 J = 25
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T rot =22K Raw time responses Amplitudes from yellow cursors
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T rot = 22 K T rot = 25 K Intensity Intensity diff.
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Transformed Data (see settings) Depletion Buildup Signals Inverted
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J = 6 J = 25 Dynamic range 1:10 5 Observable rotational temperature shifts as small as 200 K. OCS acts as a “virtual bolometer”
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Infrared BASIS Hans Bechtel & Adam Steeves (Field Group at MIT) H-C C-H + IR H-C C-H (vib. cold)(vib. hot) Reporting Molecule H-C C-H + OCS H-C C-H + OCS (vib. cold)(vib. hot)(rot. hot)(rot. cold) collision near nozzle
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IR-BASIS Spectrum of Acetylene monitored via OCS
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BASIS Advantages Gain: –energy deposited in beam is large –e.g. 6x over traditional hole burning ( f ) –analogous to optothermal technique (but simpler) –may extend the sensitivity of direct-IR absorption General Method: –should work for any rotationally resolved molecular beam spectroscopic technique
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Best BASIS Conditions Cold rotational distribution High density region: –reporting molecule needs to stay in the beam –near nozzle for IR BASIS –down stream possible for slit jet photodissociation Reporting molecule chosen: –largest rotational line intensity –does not photodissociate –large RT collision cross section?
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Possible BASIS Experiments UV, Vis, IR – BASIS Dark states Surface BASIS: –scattering off of optically excited SAMs Side Implications Slit-jet densities are so large that fragments are entrained even 10 cm downstream Pump-probe time delay important, particularly in CW experiments –Lifetime broadening may be collisional broadening. (e.g. OClO near Frank-Condon max)
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Acknowledgements UNCG Undergraduate: Bobby H. Layne Hans A Bechtel, Adam H. Steeves and Robert W. Field H.A.B. acknowledges the Donors of the American Chemical Society Petroleum Research Fund for support, and A.H.S. acknowledges the Army Research Office for a National Defense Science and Engineering Graduate Fellowship. The work at MIT was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy Helpful conversations & BASIS acronym: Prof. Robert M. Whitnell
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