A New Spectroscopic Window on Hydroxyl Radicals and their Association Reactions of Significance in the Atmosphere

Marsha I. LesterUniversity of Pennsylvania

National Science FoundationDepartment of Energy

Molecular Spectroscopy SymposiumJune 18, 2012

Association reactions of OH with atmospheric partners New photoionization scheme for OH detection

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

O3 O(1D)

O(3P)

CO

CO2

HONO2NO2h H2O

MM, O2

SO2

HSO3

H2SO4

H2S

HS

SO2

CxHy

CxHy-1

CO2, H2O

HXX

O3

XO

NH3

NH2

OHH2O + NO3

NO2

Weakly bound association products / complexes: HO-OO, OH-HONO2, HO-ONO

Association reactions of OH with O2, HONO2, NO2

Murray et al., Acc. Chem. Res. 42, 419 (2009)

HONO2 in 20% O2/Ar

UV probeIR pump

photolysisx/D ~ 15

pump

predissociation

probe

≥ D0

HOOO X2A″

OH X2 + O2 X3g–

+ M

Infrared action spectroscopy

Eavl

3548 cm–1

6923 cm–1

3521 cm–1

6871 cm–1

1 OH stretch B3LYP/cc-pVTZ

OH A2

LIF

IR action spectra of HOOOProbe OH A–X (1,0) P1(4) transition Probe OH A–X (1,1) P1(4) transition

1 1

Structured bands simulated with FTMW rotational constants for trans-HOOO rO-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 simulationDerro et al., J. Phys. Chem. A 111, 11592 (2007)

Anharmonic frequencies from B3LYP/cc-pVTZFabian et al. Theo. Chem. Acc. 114 182 (2005)

1 OH stretchFundamental: 3569 cm-1

Overtone: 6974 cm-1

6 HOOO torsion129 cm-1 (169 cm-1)

2 terminal OO stretch(1341 cm-1)

3 HOO bend998 cm-1 (1202 cm-1)

4 OOO bend 482 cm-1 (651 cm-1)

5 central OO stretch244 cm-1 (454 cm-1)

trans-HOOO normal modes

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 str OOO bendtorsion

Derro et al., J. Phys. Chem. A 111, 11592 (2007);J. Chem. Phys. 128, 244313 (2008)

IR spectrum of HOOO in He nanodropletsSequential doping with O2 and OH radicals from pyrolysis source to produce HOOO

RI09 T. Liang, P. Raston & G.E. Douberly (2012)

Hen

O2 OH

Observe trans-HOOO exclusively in He dropletstrans-HOOO lowest energy conformerNo barrier to HOOO formation from OH + O2

T=0.37 K

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)

D0 ≤ 1856 cm-1 (5.3 kcal mol-1)

HOOO dissociation dynamics: OH product distribution

3521 cm–1

6871 cm–1

pump

predissociation

probe

HOOO X2A″

OH X2 + O2 X3g– + KE3548 cm–1

6923 cm–1

Eavl

OH A2

LIF

Fractional composition of HOOO in atmosphere

1800 cm-1

1400 cm-1

1000 cm-1

Atmospheric abundance of HOOOchanges dramatically with D0

fHOOO

Indirect CRESU [OH] kinetic loss measurements suggest much smaller binding energy

D0 ≤ 1856 cm-1

IWM Smith and coworkers, Science 328, 1258 (2010)

New spectroscopic window on OH radicalsStill needed: State-selective ionization method for OH ion manipulation and collection

Opens up possibility of new dynamical measurements: velocity map imaging of OH X2Π

Target R-OH systems:HO-COHO-OOHO-ONOHO-OHHO-NO2

OH X2Π

R-OH

h

v, J, Fi, Λ, KE, I(Θ)

1+1 REMPI

Determine kinetic energy release using VMI to obtain binding

energies (D0) and barrier heights;Insight on correlated fragment

2+1 REMPI 3+1 REMPI 1+1 REMPI′

X 2Π (v ,J ,F′′ ′′ i)

D 2Σ- (v ,J )′ ′

3 2Σ- (v ,J )′ ′

X 3Σ- (v+,J+)

10.14 eV

10.87 eV

13.01 eV

X 2Π (v ,J ,F′′ ′′ i)

13.01 eV

3 2Π (v ,J )′ ′12.1 eV

X 2Π (v ,J ,F′′ ′′ i)

D 2Σ- (v ,J )′ ′10.14 eV

13.01 eVX 3Σ- (v+,J+) X 3Σ- (v+,J+)

Photoionization schemes for OH radicals

Greenslade et al., J. Chem. Phys. 123, 074309 (2005)

2+1 REMPI 3+1 REMPI 1+1 REMPI′ 1+1 REMPI′

X 2Π (v ,J ,F′′ ′′ i)

D 2Σ- (v ,J )′ ′

3 2Σ- (v ,J )′ ′

X 3Σ- (v+,J+)

10.14 eV

10.87 eV

13.01 eV

X 2Π (v ,J ,F′′ ′′ i)

13.01 eV

3 2Π (v ,J )′ ′12.1 eV

X 2Π (v ,J ,F′′ ′′ i)

D 2Σ- (v ,J )′ ′10.14 eV

13.01 eV 13.01 eV

X 2Π (v ,J ,F′′ ′′ i)

A 2Σ+

4.73 eV

4.38 eV

X 3Σ- (v+,J+) X 3Σ- (v+,J+) X 3Σ- (v+,J+)

Photoionization schemes for OH radicals

(v =1,2,J )′ ′

Beames et al., J. Chem. Phys. 134, 241102 (2011)

Experimental setup allows for nearly simultaneous LIFand REMPI detection

Experimental setup

355 nm

UV

VUV

Photolysis

HNO3 in He/Ar

(1,0) (2,0)OH A-X+ 118 nm

1+1 photoionization of OH radicals

P1(1)Q1(1)

R1(1)

SR21(1)

P1(1)

Q1(1)

R1(1)

SR21(1)

Absence of OH+ signal with A-X (0,0) excitation suggests ionization threshold

210

A2

X2 (v=0)

VUV

UV

OH

UV + VUV ionization of OH radicals

A-X

118 nm

OH A2

OH+ A3

T. G. Wright, J. Dyke and coworkers,J. Chem. Phys. 110, 345 (1999)

Tunable UV

Fixed VUV

21

0

OH+ X3–

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 transitionsTI09 J. M. Beames, Fang Liu & M. I. Lester (2012)

Photoionization and LIF of OH radicals

OH A-X (1,0) P1 lines recorded using 1+1′ REMPI and LIF detection

Distinctively different line intensities in REMPI and LIFTwo methods share common OH A-X step

Intensity variation must arise from VUV photoionization process

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

Thoughts on the autoionization mechanism

T. Wright, J. Dyke and coworkers,J. Chem. Phys. 110, 345 (1999)

Enhancement in 1+1 REMPI via OH A2+ (v=1) coincides with CIS feature assigned to A3Π (3d) v=0 Rydberg state

Breadth of Rydberg peak suggests rapid autoionization (sub-ps)

CIS v+=0 CIS v+=1

A3Π (3d)

OH A2 (v,J) + VUV

4d

v=1

v=2

v=0

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+ X3 (v+=0,1) channels

Sensitivity of 1+1 photoionization methodHONO2 + h (193 nm) OH + NO2

33%

HONO + O 67%

OH + NO + O

NO + 118 nm NO+

TOF mass spectrum of HONO2 photolysis products

OH A2 (v=1,J=5/2) + 118 nm OH+

IP = 9.26 eV118 = 2.4 x 10-18 cm2

Estimate photoionization cross section for OH A2 (v=1) 10-17 cm2 !

Recent direct observation of Criegee intermediate

Atmosphere: Ozonolysis of alkenes

Laboratory: Low pressure synthesis in flow cell with tunable VUV photoionization detectionTaatjes and coworkers, Science 335, 204 (2012)

CH2I2 + hv (248 nm) CH2I + I

CH2I + O2 CH2OO + I

Isomer-specific threshold for photoionization

118 nm

aldehydes,ketones,OH radicals,aerosols, …

O3

Generation of Criegee intermediate

UV

VUV118 nm

248 nm photolysis

CH2I2TOF-MS

20% O2 / Ar20 psi

Laboratory: High pressure synthesisin pulsed supersonic expansion

CH2OO+

Photoionization mass spectrum induced by photolysis

Taatjes and coworkers predict CH2OO B-X electronic spectrum based on FC overlaps

Speculate that CH2OO spectrum may already have been seen*, but misassigned as CH2IOO

Preliminary CASSCF electronic structure calculations along ROO coordinate reveal similarity to isoelectronic O3 potentials

Spectroscopy of Criegee intermediate

X (A)

A (A)

B (A)

C (A)

B-X

CH2IOO ?

CH2ClOO

CH2BrOO

* Heard and coworkers, ChemPhysChem 11, 3928 (2010)

CH2OO ?

CH2OO

Ongoing Efforts

Focus on spectroscopy and dynamics of simplest Criegee intermediate CH2OO and development / utilization of photoionization schemes for substituted Criegees

New 1+1 REMPI scheme via OH A2+ (v=1) enables quantitative detection of OH X2 (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

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