reaction forensics: using a variety of spectroscopic techniques to elucidate reaction mechanisms...
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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