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 Hans A Bechtel, Adam H. Steeves and Robert W. Field Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

J. Phys. Chem. A (online)

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

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

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).

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 Corner Reflector Multipass Cell Photodissociation Setup Current Available Range: GHz

Mm-Wave Source Module “Armadillo” Microwave Synthesizer Mm-Wave Amplifier & Tripler Frequency Resolution 10 Hz at 100 GHz or 1 part in Power > 1 mW

Parent Molecule

Photodissociation of Parent Molecule

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

Products Probed in Hyper-rovibronic Detail Cl O

Millimeter-wave absorption time trace centered on a single hyperfine line of O 35 ClO ( 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.

“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

Parent Signal BASIS Signal

SO 2 Parent Hans Bechtel & Adam Steeves (Field Group at MIT) Low rotational transition High rotational transition artistic simulation

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

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

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

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

Transformed Data (see settings) Depletion Buildup Signals Inverted

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

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

IR-BASIS Spectrum of Acetylene monitored via OCS

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

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?

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)

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