Reaction Forensics: Using a variety of spectroscopic techniques to elucidate reaction mechanisms Steve Rowling, Brianna Heazlewood and Scott KABLE guest starring Paul Houston, David Osborn, Arthur Suits, Mike Ashfold, Peter Loock, Meredith Jordan (and groups) University of Sydney Sydney, Australia, 2006

Cartoon surfaces for photodissociation h Dissociation Bound h Dissociation Repulsive h Dissociation Barrier Real chemistry is more complex…

h S 1 (excited singlet) ABC (S 0, ground state) 3 or more challenges: - 1 PES… multiple products

S 0 * (excited ground state) IC h S 1 (excited singlet) ABC (S 0, ground state) 3 or more challenges: - 1 PES… multiple products

S 0 * (excited ground state) A + BC B + AC TS IC ACB TS h S 1 (excited singlet) ABC (S 0, ground state) 3 or more challenges: - 1 PES… multiple products

T 1 (triplet) S 0 * (excited ground state) A + BC B + AC TS ISC IC ACB TS h S 1 (excited singlet) ABC (S 0, ground state) 3 or more challenges: - 1 PES… multiple products - multiple PES’s… 1 product

T 1 (triplet) S 0 * (excited ground state) A + BC B + AC TS ISC IC ACB TS h S 1 (excited singlet) H 2 CO (S 0, ground state) 3 or more challenges: - 1 PES… multiple products - multiple PES’s… 1 product - 1 PES… 1 product… multiple pathways TS2?

T 1 (triplet) S 0 * (excited ground state) H + HCO H 2 + CO TS ISC IC h S 1 (excited singlet) H 2 CO (S 0, ground state) simple molecular simple radical H 2 CO exhibits all of these complexities!

T 1 (triplet) S 0 * (excited ground state) H + HCO H 2 + CO TS ISC IC h S 1 (excited singlet) H 2 CO (S 0, ground state) simple molecular simple radical triplet H 2 CO exhibits all of these complexities!

T 1 (triplet) S 0 * (excited ground state) H + HCO H 2 + CO TS ISC IC h S 1 (excited singlet) H 2 CO (S 0, ground state) simple molecular simple radical triplet roaming H 2 CO exhibits all of these complexities! “roaming”

Multiple PES’s – 1 product threshold phofex LIF triplet threshold

LIF spectrum of nascent HCO Pump =

Distributions from Different Excitation

Fingerprinting with Phofex Relative Population N E avail = statistical, or S non-statistical, or T E avail = 2612 N(0,1) N(10,12)

Triplet dominates Singlet exclusively ? Zoom in

Fingerprinting with Phofex N = E avail ~ 1220 cm -1

Fingerprinting with Phofex N = E avail ~ 1220 cm -1 N = , , 2 21, 3 03  S 0 all others  T 1

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

Comparison N = N =

Rydberg tagging experiments Scott Hopkins, Peter Loock, Brid Croonan, Michael Nix, Adam Devine, Richard Dixon, Mike Ashfold, JCP, (in press). K a = Excellent K a resolution Partial N resolution

Comparison of K a populations LIF

Comparison of K a populations LIF

Comparison of N distributions

T 1 (triplet) S 0 * (excited ground state) H + HCO H 2 + CO Signatures of each pathway TS Triplet HCO:Low N Low K High trans Singlet HCO:Statistical N Statistical K Statistical trans TS T.S. CO:High J(CO) Modest vib(H 2 ) High trans Roaming CO:v. low J High vib(H 2 ) Low trans

Now probe roaming channel 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 = E avail ~ 1220 cm -1 N =

Fingerprinting with Phofex E avail ~ 1220 cm -1 N = singlet HCO Roaming scales very closely with S 0 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: CH 3 CHO

T 1 (triplet) S 0 * (excited ground state) CH 3 + HCO CH 4 + CO TS ISC IC CH 3 COH TS h S 1 (excited singlet) CH 3 CHO (S 0, ground state) CH 2 CHOH CH 2 CH 2 O TS H 2 + CH 2 CO H + CH 3 CO TS GOAL = ROAMING?? TS CH 2 + H 2 CO

T 1 (triplet) S 0 * (excited ground state) CH 3 + HCO ISC IC h S 1 (excited singlet) CH 3 CHO (S 0, ground state) QCT theory Experiment HCO & CH 3 Known acetaldehyde dynamics

T 1 (triplet) S 0 * (excited ground state) CH 3 + HCO CH 4 + CO ISC IC h S 1 (excited singlet) CH 3 CHO (S 0, ground state) TS QCT theory Experiment HCO & CH 3 Known acetaldehyde dynamics CO PSD ROAMING??

CO product state dist’ns RoamingTS CO(J) v.lowhigh CO(tr) v.lowhigh CO(v,J) anisotropicv ┴ J

T 1 (triplet) S 0 * (excited ground state) CH 3 + HCO CH 4 + CO ISC IC h S 1 (excited singlet) CH 3 CHO (S 0, ground state) TS ROAMING??, known New experiments Tune h 1 2

Acetaldehyde action spectra 1 = LIF 2 = Phofex (HCO) B. Heazlewood, S. Rowling Triplet threshold HCO formed below T 1 barrier = produced on S 0

T 1 (triplet) S 0 * (excited ground state) CH 3 + HCO CH 4 + CO ISC IC h S 1 (excited singlet) CH 3 CHO (S 0, ground state) TS ROAMING?? 1,, known 2, CH 4 These experiments

FTIR emission Wavenumber (cm -1 ) Time (  s) Slice Brianna Heazlewood, Talitha Selby and David Osborne

FTIR emission Slice CH 4 CO HCO

Maximum entropy fit > 3,000,000 CH 4 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 E max

Summary of new CH 3 CHO dynamics CH 3 + HCO channel IS open on S 0 HCO is born with low E trans and higher E rot CH 4 is born extremely hot (vibrationally) evidence of two distributions of vib. energy  supports a second mechanism for CH 4 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)

n Experiments on photodissociation dynamics –Brianna Heazlewood (Hons) –Klaas Nauta (post doc) –Steve Rowling (PhD) –Hongming Yin (post doc) n Theoretical description –Joel Bowman (Emory U.) –Meredith Jordan (U. Syd.) –George McBane (Grand Valley U.) –Ondrej Votava (Heyrovsky Inst.) n $$Funding$$ –ARC –Sydney Univ. –APAC Acknowledgements… n 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 Schmidt Klaas Nauta Steve Rowling Brianna Heazlewood