Current team Mikhail Ryazanov Dr. Chirantha Rodrigo Overtone-induced dissociation and isomerization of the hydroxymethyl (CH 2 OH) radical First team: Lin Feng Dr. Jie Wei, Boris Karpichev Support: US Department of Energy CH 2 OH + 4 OH  CH 2 O + H OSU 66 th MSS, June 2011 Theory collaborators Dr. Eugene Kamarchik, Prof. Anna Krylov, Prof. Joel Bowman

CH 2 OH radical: dissociation on the ground state 11,600 ~ 15,000 cm -1 ~ 14,000 ~3,000 0 Wavenumbers  10 3 cm (X 2 A″)CH 2 OH CH 3 O H + CH 2 O (X) ~ ~ ~ O-H IR 10,484 13,600 ~16,000 7,158 3,675

Previous work on the hydroxymethyl radical Characterized ionization and excited Rydberg states, including conical intersections Studied excited state photochemistry Demonstrated the breaking of the weak O-H bond in CH 2 OH following IR overtone excitation Today: direct dissociation dynamics vs. isomerization CH 2 OH IE = 7.56 eV Feng, Wei, and Reisler, J. Phys. Chem. A, 108, 7903 (2004). Wei, Karpichev, and Reisler, J. Chem. Phys. 125 (3): (2006).

Challenges – OH-stretch is the reaction coordinate for direct OH bond cleavage: How high can we go? – Isomerization vs. direct O-H fission; relative barrier heights – OH-overtone pumping is always hard: CH 3 OH, NH 2 OH, HOOH, etc. (Crim, Rizzo, Perry, etc.); – Radical is a minor species and hard to make – CH 2 OH has low-lying electronic states: Will we get substantial vibrationally mediated dissociation?

Detection of CH 2 OH via 1+1 REMPI Wavenumbers  10 3 cm -1 (X 2 A″) ~ CH 2 OH CH 3 O H + CH 2 O (X) ~  14,000 11,000 15,000 cm -1 H + CH 2 O (X) ~ CsCs 3p z ( 2 A") 3p x 3s = 26,000 cm -1 Barrier heights: Radom, Schaeffer; Harding, Klippenstein; Temps; Walch…

OH stretch vibration: 1-3 OH 3 OH ; cm -1 ; linewidth = 0.4 cm -1 2 OH ; 7158 cm -1 ; linewidth = 0.8 cm- 1 1 OH ; 3678 cm -1 ; linewidth = 0.4 cm -1 T = K

Simulation of 2 nd overtone spectrum: a/b–type transition Wei, Karpichev, Reisler, J. Chem. Phys.(2006) No H atom detected 3 OH ; Laser linewidth: 0.1 cm -1 Observed linewidth = 0.4 cm -1

New imaging arrangement: Slice imaging of H photofragments Lenses in field free region control VMI conditions and image size 5 ns FWHM pulser allows slicing of H fragment Design based on: A. Suits, K. Liu

Br Br* 3685 cm -1

5ns X10 improvement in resolution and sensitivity

CH 2 OH H-photofragment yield spectrum in the region of 4 OH Accidental resonance between 4 OH and 3 OH + 1 asymCH [3 OH + 1 asymCH ]* nm cm -1 [4 OH ]* 735 nm cm -1 Methanol overtone spectrum shows a similar accidental resonance between 5 OH and 4 OH + 1 asymCH. [Rizzo, Perry, Boyarkin]

Ground Excited A /cm – B /cm – C /cm – % : 10% : 0% (a:b:c) T: 13 K Linewidths: Laser: 0.1 cm -1 Lorenzian width (FWHM) Strong band: ~ 1.6 cm –1 Weak band: ~ 0.6 cm –1

bright state dark states molecular eigenstates spectrum Coupling between a zeroth-order bright state and a dark state

H product Ground Excited A /cm – B /cm – C /cm – % : 20% : 0% (a:b:c) T: 13K D product (small) D signal is is very small 4 OH =13621 cm -1 No other bands observed Lorenzian Linewidths: Both bands: ~ 1.6 cm –1 H and D products from CD 2 OH

From maximum KER : D 0 = 10,166 ± 70 cm -1 Compared to calculated D 0 = 10,188 cm -1 (Marenich and Boggs) Vibrational labels of CH 2 O co-fragment are marked. Excited levels = 8%. Rotational temperature of CH 2 O co-fragment: ~ 140 K CH 2 OH: Slice image of H-photofragments from 4 OH peak

CH 2 OH + h → H + CH 2 O, = cm  1 KER of H-photofragments from 3 OH + 1 asymCH peak Vibrational labels of CH 2 O co-fragment are marked. Excited levels = 13%.

CD 2 OH: Monitoring H + CD 2 O h = cm 

CH 2 OH CD 2 OH Monitoring H 0.04 Formaldehyde cofragment: No CH stretch Ratio of CO str/CHH(D) bends is larger in CD 2 OH

Higher rotational and CHD vibrational excitation; No CO stretch CD 2 OH: Monitoring D + CHDO h = cm  1

Summary The OH overtone spectrum of CH(D) 2 OH has been characterized in the 4 OH region. Mixed levels (most likely involving levels 4 OH and 3 OH + 1 asymCH ) are seen in CH 2 OH but not in CD 2 OH. Dissociation, probably by tunneling through the O—H barrier, has been demonstrated. The observation of formaldehyde levels with skeletal CH(D)excitation appears to be associated with isomerization, whereas C=O stretch excitation accompanies O-H bond fission. Isomerization yield is < 10%. It appears to be the result of statistical IVR due to weak high-order resonance couplings. Electronic structure and dynamical calculations are in progress in collaborative work with Dr. Eugene Kamarchik, Joel Bowman (Emory) and Anna Krylov (USC).

10 0 Wavenumbers  10 3 cm -1 H + CH 2 O (X) ~ ~ Calculated Geometries; CCSD(T)/aug-cc-pVTZ Dr. Eugene Kamarchik

355nm Photolyticaly initiated reaction Cl 2  2Cl Cl + CH 3 OH  CH 2 OH + HCl k 1 = 3.7x10 13 (cm 3.mole -1.s -1 ) Radical production CH 2 OH + Cl  CH 2 O + HCl k 2 = 4x10 14 (cm 3.mole -1.s -1 ) ….. CH 3 OH/Cl 2 /He CH 2 OH 355nm

CH 2 OH diagnostics: 3p z REMPI or H-photofragment spectrum 0 nl m π co * 3p z (2 2 A”) 1 2 A” C-O Stretch ( 6 ) Progression C-O

Detection schemes for IR spectroscopy Double Resonance CH 2 OH (X 2 A") CH 2 OH + 3p z Probe Pump n OH Scheme 2 Or: Look directly at H atom when the radical dissociates

Birge-Sponer plot OH stretchA B, Anharmonicity CH 3 OH CH 2 OH NH 2 OH HOOH potential Anharmonic oscillator /  = A – B   Dissociation barrier > 4ν 1 H-atom generated via tunneling

Tunneling: Eckart Potential Linewidth of 1.3 cm -1 corresponds to a tunneling probability of 0.2%; Imaginary frequency= 1712 cm -1 (Larry Harding) Dissociation barrier height of 15,200 cm -1 Theoretical estimation: – cm -1 CH 2 OH 9557 cm cm -1 ? 1712 cm -1 CH 2 O + H

Next goal: Excite above barrier to dissociation 11,000 16,000cm -1 14,000 ~2,000 0 Wavenumbers  10 3 cm (X 2 A″)CH 2 OH CH 3 O H + CH 2 O (X) ~ ~ ~ O-H IR 11,000 14,600 18,000 7,158 3,675

735 nm 732 nm mode descr. freq. rel. pop. (fit) - ground C-H sym C=O CH2 bend * CH2 wag C-H asym * CH2 rock % excited (fit) % excited (int.) T_rot (fit), K

bright state dark states molecular eigenstates spectrum Coupling between a zeroth-order bright state and a bath of dark states

Exciting the 3rd overtone, 4 1 : Monitoring H photofragments from CH 2 OH and CD 2 OH CH 2 OH and CD 2 OH have the same linewidth: 1.3 cm -1 Could not detect deuterium from CD 2 OH Isomerization is at best a minor channel.