1. 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

2. J. Phys. Chem. A (online)

3. 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

4. Current Photodissociation Study Chlorine Dioxide h Chlorine Monoxide Oxygen Cl O O O O Parent Molecule Products

5. 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).

6. 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

7. 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

8. Parent Molecule

9. Photodissociation of Parent Molecule

10. 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

11. Products Probed in Hyper-rovibronic Detail Cl O

12. 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.

13. “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

14. Parent Signal BASIS Signal

15. 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

16. 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

17. UV BASIS Signals of OClO OCS reporter & fixed UV J = 6 J = 25

18. T rot =22K Raw time responses Amplitudes from yellow cursors

19. T rot = 22 K T rot = 25 K Intensity Intensity diff.

20. Transformed Data (see settings) Depletion Buildup Signals Inverted

21. J = 6 J = 25 Dynamic range 1:10 5 Observable rotational temperature shifts as small as 200  K. OCS acts as a “virtual bolometer”

22. 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

23. IR-BASIS Spectrum of Acetylene monitored via OCS

24. 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

25. 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?

26. 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)

27. 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