Reaction Forensics: Using a variety of spectroscopic techniques

to elucidate reaction mechanismsSteve Rowling, Brianna Heazlewood and Scott KABLE

guest starring Paul Houston, David Osborn, Arthur Suits, Mike Ashfold, Peter Loock, Meredith Jordan (and groups)

University of SydneySydney, Australia, 2006

Cartoon surfaces for photodissociationh

Dissociation

Bound

h

Dissociation

Repulsive

h

Dissociation

Barrier

Real chemistry is more complex…

h

S1

(excited singlet)

ABC(S0, ground state)

3 or more challenges:- 1 PES… multiple products

S0* (excited ground state)

IC

h

S1

(excited singlet)

ABC(S0, ground state)

3 or more challenges:- 1 PES… multiple products

S0* (excited ground state)

A + BC

B + ACTS

IC

ACB

TS

h

S1

(excited singlet)

ABC(S0, ground state)

3 or more challenges:- 1 PES… multiple products

T1 (triplet)

S0* (excited ground state)

A + BC

B + ACTS

TS

ISC

IC

ACB

TS

h

S1

(excited singlet)

ABC(S0, ground state)

3 or more challenges:- 1 PES… multiple products- multiple PES’s… 1 product

T1 (triplet)

S0* (excited ground state)

A + BC

B + ACTS

TS

ISC

IC

ACB

TS

h

S1

(excited singlet)

H2CO(S0, ground state)

3 or more challenges:- 1 PES… multiple products- multiple PES’s… 1 product- 1 PES… 1 product…

multiple pathways

TS2?

T1 (triplet)

S0* (excited ground state)

H + HCO

H2 + COTS

TS

ISC

IC

h

S1

(excited singlet)

H2CO(S0, ground state)

simple molecular

simple radicalH2CO exhibits all of these complexities!

T1 (triplet)

S0* (excited ground state)

H + HCO

H2 + COTS

TS

ISC

IC

h

S1

(excited singlet)

H2CO(S0, ground state)

simple molecular

simple radicaltriplet

H2CO exhibits all of these complexities!

T1 (triplet)

S0* (excited ground state)

H + HCO

H2 + COTS

TS

ISC

IC

h

S1

(excited singlet)

H2CO(S0, ground state)

simple molecular

simple radicaltriplet

roaming

H2CO exhibits all of these complexities!

“roaming”

Multiple PES’s – 1 productthreshold

phofex

LIF

triplet threshold

LIF spectrum of nascent HCOPump = 2241

Distributions from Different Excitation

30500

31000

31500

32000

32500

3300024

041

0

23

043

0

11

021

041

0

21

061

0

PhofexLIF

11

041

0 & 51

0

21

042

061

0 E

xcita

tion

ener

gy (c

m-1

)

23

041

0

42

051

0

22

043

0

22

061

0

22

041

0

21

041

061

021

043

0

0

500

1000

1500

2000

2500

Eav

ail (c

m-1

)

2261

2243

2341

2441

112141

PST

244

1

Eav = 2612 cm-1

Eav = 1114 cm-1

0 5 10 15 20

PST

226

1

Re

lativ

e P

op

ula

tion

N

PST

244

1

Eav = 2612 cm-1

Eav = 1114 cm-1

0 5 10 15 20

PST

226

1

Re

lativ

e P

op

ula

tion

N

Fingerprinting with Phofex

Rel

ativ

e P

opul

atio

n

N

2261

Eavail = 11142261

statistical, or S0

2243

non-statistical, or T1

1)12,10(

)1,0(

N

N

32)12,10(

)1,0(

N

N

2441

Eavail = 2612

N(0,1) N(10,12)

30500

31000

31500

32000

32500

3300024

041

0

23

043

0

11

021

041

0

21

061

0

PhofexLIF

11

041

0 & 51

0

21

042

061

0 E

xcita

tion

ener

gy (c

m-1

)

23

041

0

42

051

0

22

043

0

22

061

0

22

041

0

21

041

061

021

043

0

0

500

1000

1500

2000

2500

Eav

ail (c

m-1

)

2261

2243

2341

2441

112141

Triplet dominates

Singlet exclusively

?

Zoom in

Fingerprinting with Phofex

N = 0 + 1

2243

Eavail ~ 1220 cm-1

Fingerprinting with Phofex

N = 0 + 1

2243

Eavail ~ 1220 cm-1

N = 10 + 12

220+221220, 221

303303+000

220, 221, 303 S0

all others T1

Rydbergtagging experiments

all 2243

Scott Hopkins, Peter Loock, Brid Croonan, Michael Nix, Adam Devine, Richard Dixon, Mike Ashfold, JCP, (in press).

Comparison

N = 0 + 1

N = 10 + 12

Rydberg tagging experiments

Scott Hopkins, Peter Loock, Brid Croonan, Michael Nix, Adam Devine, Richard Dixon, Mike Ashfold, JCP, (in press).

Ka = 0 1 2 3 4 5Excellent Ka resolution

Partial N resolution

0 1 2 3 4 5 60.00

0.25

0.50

0.75

1.00

1.25

PST

"singlet"

"triplet"

Re

lativ

e P

op

(K

a)

Ka

Comparison of Ka populations

LIF

Comparison of Ka populations

LIF

Comparison of N distributions

0 5 10 15 20 250

2

4"singlet-like" distribution

224

3 22

N

Rydberg tagging LIF

T1 (triplet)

S0* (excited ground state)

H + HCO

H2 + CO

Signatures of each pathway

TS

Triplet HCO: Low NLow KHigh trans

Singlet HCO:Statistical NStatistical KStatistical trans

TS

T.S. CO: High J(CO)Modest vib(H2)High trans

Roaming CO: v. low JHigh vib(H2)Low trans

Now probe roamingchannel with phofex…

Probe J(CO)=15(which has been associated with roaming)

Townsend et al, Science., 306, 1158 (2004)

Experiments: Lahankar and Rowling

Fingerprinting with Phofex

N = 0 + 1

2243

Eavail ~ 1220 cm-1

N = 10 + 12

Fingerprinting with Phofex

2243

Eavail ~ 1220 cm-1

N = 10 + 12

singlet HCO

Roaming scales very closely with S0 HCO!

roaming

how far can H-atom roam? can other moieties roam? what are the requirements for

roaming in larger systems?

Questions from “Roaming atom” mechanism…

Obvious next candidate: CH3CHO

T1 (triplet)

S0* (excited ground state)

CH3 + HCO

CH4 + CO

TS

TS

ISC

IC

CH3COHTS

h

S1

(excited singlet)

CH3CHO(S0, ground state)

CH2CHOHCH2CH2O

TS TS

H2 + CH2CO

H + CH3CO

TS

TS

GOAL = ROAMING??

TS

CH2 + H2CO

T1 (triplet)

S0* (excited ground state)

CH3 + HCOISC

IC

h

S1

(excited singlet)

CH3CHO(S0, ground state)

QCT theoryExperimen

tHCO & CH3

Known acetaldehyde dynamics

T1 (triplet)

S0* (excited ground state)

CH3 + HCO

CH4 + CO

ISC

IC

h

S1

(excited singlet)

CH3CHO(S0, ground state)

TS

QCT theoryExperimen

tHCO & CH3

Known acetaldehyde dynamics

CO PSD

ROAMING??

CO product state dist’ns

Roaming TS

CO(J) v.low high

CO(tr) v.low high

CO(v,J) anisotropicv ┴ J

T1 (triplet)

S0* (excited ground state)

CH3 + HCO

CH4 + CO

ISC

IC

h

S1

(excited singlet)

CH3CHO(S0, ground state)

TS

ROAMING??

, known

New experiments

Tune h 1

2

Acetaldehyde action spectra

1 = LIF

2 = Phofex(HCO)

B. Heazlewood, S. RowlingTriplet threshold

HCO formed below T1 barrier= produced on S0

T1 (triplet)

S0* (excited ground state)

CH3 + HCO

CH4 + CO

ISC

IC

h

S1

(excited singlet)

CH3CHO(S0, ground state)

TS

ROAMING??

1, , known

2, CH4

These experiments

FTIR emission

3000

2500

2000

1500

1000

500

0

4500400035003000250020001500

Wavenumber (cm-1)

Tim

e (

s)

Slice

Slice

Brianna Heazlewood, Talitha Selby and David Osborne

FTIR emission

3000

2500

2000

1500

1000

500

0

4500400035003000250020001500

Slice

CH4COHCO

2000 2250 2500 2750 3000 3250 3500

IR emission Max. ent. fit

Wavenumber (cm-1)

Maximum entropy fit

2000 2250 2500 2750 3000 3250 3500

IR emission Max. ent. fit

Wavenumber (cm-1)

• > 3,000,000 CH4 vib. states

• anharmonic (Morse)

oscillators

• harmonic linestrengths

• bin into 10 cm-1 emission

bins

•deconvolve with 400 K

rotational profile

• let all states within a bin be

equally likely

• vary populations to fit

experiment

Maximum entropy pop. distribution

0 5000 10000 15000 20000 25000 30000 35000 40000

Po

pu

la

tio

n (a

rb

.)

Energy (cm-1)

0 100 200 300 400 Energy (kJ/mol)

Emax

Summary of new CH3CHO dynamics

CH3 + HCO channel IS open on S0

HCO is born with low Etrans and higher Erot

CH4 is born extremely hot (vibrationally)

evidence of two distributions of vib. energy

supports a second mechanism for CH4 formation (roaming?)

What is “roaming”?

Acetaldehyde dynamics on a “grown” PES

Movie courtesy Meredith Jordan and Brianna Heazlewood

Roaming?

Roaming might not be so simple in larger systems.

Might there be a continuum between TS and roaming?

Spectroscopic techniques used:• laser induced fluorescence• IR emission• photofragment excitation• photofragment imaging• REMPI• Rydberg tagging• (ab initio + QCT)

Experiments on photodissociation dynamics–Brianna Heazlewood (Hons)

–Klaas Nauta (post doc)

–Steve Rowling (PhD)

–Hongming Yin (post doc)

Theoretical description–Joel Bowman (Emory U.)

–Meredith Jordan (U. Syd.)

–George McBane (Grand Valley U.)

–Ondrej Votava (Heyrovsky Inst.)

$$Funding$$–ARC

–Sydney Univ.

–APAC

Acknowledgements…

Collaborators–Paul Houston (Georgia Tech)

–Arthur Suits (Wayne State)

–Sridhar Lahankar (Wayne State)

–David Osborn (Sandia)

–Talitha Selby (Sandia)

–Mike Ashfold (Bristol)

–Hans-Peter Loock (Queens, CA)

Current laser spectroscopy group (April 2007)…

Tim SchmidtKlaas Nauta

Steve RowlingBrianna Heazlewood