1. A New Spectroscopic Window on Hydroxyl Radicals and their Association Reactions of Significance in the Atmosphere Marsha I. Lester University of Pennsylvania National Science Foundation Department of Energy Molecular Spectroscopy Symposium June 18, 2012  Association reactions of OH with atmospheric partners  New photoionization scheme for OH detection

2. Detection of [OH] in atmospheric field measurements, in situ combustion diagnostics, and laboratory studies relies on OH A-X laser-induced fluorescence (LIF) measurements OH Central role of OH in the troposphere O3O3 O( 1 D) O( 3 P) CO CO 2 HONO 2 NO 2 h H2OH2O MM, O 2 SO 2 HSO 3 H 2 SO 4 H2SH2S HS SO 2 CxHyCxHy C x H y-1 CO 2, H 2 O HX X O3O3 XO NH 3 NH 2 OH H 2 O + NO 3 NO 2

3. Weakly bound association products / complexes: HO-OO, OH-HONO 2, HO-ONO Association reactions of OH with O 2, HONO 2, NO 2 Murray et al., Acc. Chem. Res. 42, 419 (2009)

4. HONO 2 in 20% O 2 /Ar UV probeIR pump photolysis x/D ~ 15 pump predissociation probe ≥ D 0 HOOO X 2 A″ OH X 2  + O 2 X 3  g – + M Infrared action spectroscopy E avl 3548 cm –1 6923 cm –1 3521 cm –1 6871 cm –1 1 OH stretch B3LYP/cc-pVTZ OH A 2   LIF

5. IR action spectra of HOOO Probe OH A–X (1,0) P 1 (4) transitionProbe OH A–X (1,1) P 1 (4) transition 1 11 Structured bands simulated with FTMW rotational constants for trans-HOOO r O-O = 1.688 Å Suma et al., Science 308, 1885 (2005) McCarthy et al., J. Chem. Phys. 136, 034303 (2012) Unstructured features attributed to cis-HOOO Total HOOO simulation Derro et al., J. Phys. Chem. A 111, 11592 (2007)

6. Anharmonic frequencies from B3LYP/cc-pVTZ Fabian et al. Theo. Chem. Acc. 114 182 (2005) 1 OH stretch Fundamental: 3569 cm -1 Overtone: 6974 cm -1 6 HOOO torsion 129 cm -1 (169 cm -1 ) 2 terminal OO stretch (1341 cm -1 ) 3 HOO bend 998 cm -1 (1202 cm -1 ) 4 OOO bend 482 cm -1 (651 cm -1 ) 5 central OO stretch 244 cm -1 (454 cm -1 ) trans-HOOO normal modes

7. HOOO survey spectrum Observed several vibrational features, both structured and unstructured Combination band assignments based on vibrational frequency, transition type and isotopic shift upon deuteration HOO bendOO strOH strOOO bendtorsion Derro et al., J. Phys. Chem. A 111, 11592 (2007); J. Chem. Phys. 128, 244313 (2008)

8. IR spectrum of HOOO in He nanodroplets Sequential doping with O 2 and OH radicals from pyrolysis source to produce HOOO RI09 T. Liang, P. Raston & G.E. Douberly (2012) He n O2O2 OH Observe trans-HOOO exclusively in He droplets  trans-HOOO lowest energy conformer  No barrier to HOOO formation from OH + O 2 T=0.37 K

9. Start with EOMIP-CCSD* potential, then scale by factor of 1.35 (  ) to obtain eigenvalues in excellent agreement with observed torsional frequencies  Narrower cis well raises 6 frequency; broader trans well lowers 6 frequency  Both cis and trans conformers of HOOO of potential atmospheric importance Torsional potential for HOOO ~340 cm -1 ~60 cm -1 75  Use experimental vibrational frequencies to tune ab initio torsional potential Beames et al, J. Chem. Phys. 134, 044304 (2011)

10. D 0 ≤ 1856 cm -1 (5.3 kcal mol -1 ) HOOO dissociation dynamics: OH product distribution 3521 cm –1 6871 cm –1 pump predissociation probe HOOO X 2 A″ OH X 2  + O 2 X 3  g – + KE 3548 cm –1 6923 cm –1 E avl OH A 2   LIF

11. Fractional composition of HOOO in atmosphere 1800 cm -1 1400 cm -1 1000 cm -1 Atmospheric abundance of HOOO changes dramatically with D 0 f HOOO Indirect CRESU [OH] kinetic loss measurements suggest much smaller binding energy D 0 ≤ 1856 cm -1 IWM Smith and coworkers, Science 328, 1258 (2010)

12. New spectroscopic window on OH radicals Still needed: State-selective ionization method for OH ion manipulation and collection Opens up possibility of new dynamical measurements: velocity map imaging of OH X 2 Π Target R-OH systems: HO-CO HO-OO HO-ONO HO-OH HO-NO 2 OH X 2 Π R-OH h v, J, F i, Λ, KE, I(Θ ) 1+1 REMPI Determine kinetic energy release using VMI to obtain binding energies (D 0 ) and barrier heights; Insight on correlated fragment

13. 2+1 REMPI3+1 REMPI1+1′ REMPI X 2 Π (v′′,J′′,F i ) D 2 Σ - (v′,J′) 3 2 Σ - (v′,J′) X 3 Σ - (v +,J + ) 10.14 eV 10.87 eV 13.01 eV X 2 Π (v′′,J′′,F i ) 13.01 eV 3 2 Π (v′,J′) 12.1 eV X 2 Π (v′′,J′′,F i ) D 2 Σ - (v′,J′) 10.14 eV 13.01 eV X 3 Σ - (v +,J + ) Photoionization schemes for OH radicals Greenslade et al., J. Chem. Phys. 123, 074309 (2005)

14. 2+1 REMPI3+1 REMPI1+1′ REMPI X 2 Π (v′′,J′′,F i ) D 2 Σ - (v′,J′) 3 2 Σ - (v′,J′) X 3 Σ - (v +,J + ) 10.14 eV 10.87 eV 13.01 eV X 2 Π (v′′,J′′,F i ) 13.01 eV 3 2 Π (v′,J′) 12.1 eV X 2 Π (v′′,J′′,F i ) D 2 Σ - (v′,J′) 10.14 eV 13.01 eV X 2 Π (v′′,J′′,F i ) A 2 Σ + 4.73 eV 4.38 eV X 3 Σ - (v +,J + ) Photoionization schemes for OH radicals (v′=1,2,J′) Beames et al., J. Chem. Phys. 134, 241102 (2011)

15. Experimental setup allows for nearly simultaneous LIF and REMPI detection Experimental setup 355 nm UV VUV Photolysis HNO 3 in He/Ar

16. (1,0)(2,0)OH A-X + 118 nm 1+1 photoionization of OH radicals P 1 (1) Q 1 (1) R 1 (1) S R 21 (1) P 1 (1) Q 1 (1) R 1 (1) S R 21 (1) Absence of OH + signal with A-X (0,0) excitation suggests ionization threshold 2 1 0 A2A2 X 2  (v=0) VUV UV OH 

17. UV + VUV ionization of OH radicals A-X 118 nm OH A 2   OH + A 3  T. G. Wright, J. Dyke and coworkers, J. Chem. Phys. 110, 345 (1999) Tunable UV Fixed VUV 2 1 0 OH + X 3  –

18. Photoionization and LIF of OH radicals  OH A-X (1,0) spectra recorded using 1+1′ REMPI and LIF detection  State-selective excitation for a wide range of rotational and fine-structure levels  Different line intensities observed depending on the technique REMPI intensities are ‘enhanced’ relative to LIF for many transitions TI09 J. M. Beames, Fang Liu & M. I. Lester (2012)

19. Photoionization and LIF of OH radicals OH A-X (1,0) P 1 lines recorded using 1+1′ REMPI and LIF detection Distinctively different line intensities in REMPI and LIF Two methods share common OH A-X step Intensity variation must arise from VUV photoionization process

20. Enhancement of REMPI vs. LIF for OH radicals Enhancement peaks at total energy of 14.9 eV for UV+VUV photoionization FWHM  170 cm -1 Lifetime ≥ 30 fs

21. Thoughts on the autoionization mechanism T. Wright, J. Dyke and coworkers, J. Chem. Phys. 110, 345 (1999)  Enhancement in 1+1 REMPI via OH A 2  + (v=1) coincides with CIS feature assigned to A 3 Π (3d) v=0 Rydberg state  Breadth of Rydberg peak suggests rapid autoionization (sub-ps) CIS v + =0CIS v + =1 A 3 Π (3d) OH A 2   (v,J) + VUV 4d v=1 v=2 v=0 Previous photoelectron spectra (PES) of OH – recorded in constant ionic state (CIS) mode with tunable VUV excitation – reveal Rydberg states that autoionize into OH + X 3   (v + =0,1) channels

22. Sensitivity of 1+1 photoionization method HONO 2 + h (193 nm)  OH + NO 2  33%  HONO + O67%  OH + NO + O NO + 118 nm  NO + TOF mass spectrum of HONO 2 photolysis products OH A 2   (v=1,J=5/2) + 118 nm  OH + IP = 9.26 eV  118 = 2.4 x 10 -18 cm 2 Estimate photoionization cross section for OH A 2   (v=1)  10 -17 cm 2 !

23. Recent direct observation of Criegee intermediate Atmosphere: Ozonolysis of alkenes Laboratory: Low pressure synthesis in flow cell with tunable VUV photoionization detection Taatjes and coworkers, Science 335, 204 (2012) CH 2 I 2 + hv (248 nm) CH 2 I + I CH 2 I + O 2 CH 2 OO + I Isomer-specific threshold for photoionization 118 nm aldehydes, ketones, OH radicals, aerosols, … O3O3

24. Generation of Criegee intermediate UV VUV 118 nm 248 nm photolysis CH 2 I 2 TOF-MS 20% O 2 / Ar 20 psi Laboratory: High pressure synthesis in pulsed supersonic expansion CH 2 OO + Photoionization mass spectrum induced by photolysis

25. Taatjes and coworkers predict CH 2 OO B-X electronic spectrum based on FC overlaps Speculate that CH 2 OO spectrum may already have been seen*, but misassigned as CH 2 IOO Preliminary CASSCF electronic structure calculations along R OO coordinate reveal similarity to isoelectronic O 3 potentials Spectroscopy of Criegee intermediate X (A) A (A  ) B (A) C (A  ) B-X CH 2 IOO ? CH 2 ClOO CH 2 BrOO * Heard and coworkers, ChemPhysChem 11, 3928 (2010) CH 2 OO ? CH 2 OO

26. Ongoing Efforts Focus on spectroscopy and dynamics of simplest Criegee intermediate CH 2 OO and development / utilization of photoionization schemes for substituted Criegees New 1+1 REMPI scheme via OH A 2  + (v=1) enables quantitative detection of OH X 2  (v=0-2) by photoionization for variety of applications including HOOO Collaborative efforts underway to measure the kinetic energy and angular distributions of the photoelectrons Setting up tunable VUV to probe Rydberg states directly

27. People at Penn: Tim Sechler, Julia Lehman, Craig Murray,* Logan Dempsey, MIL, Pesia Soloveichik, Bridget O’Donnell, Erika Derro [Joe Beames, Fang Liu] Acknowledgements * Now a Lecturer at U. Glasgow