current team mikhail ryazanov dr. chirantha rodrigo overtone-induced dissociation and isomerization...
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Current teamMikhail Ryazanov Dr. Chirantha Rodrigo
Overtone-induced dissociation and isomerization of the hydroxymethyl (CH2OH) radical
First team:Lin Feng Dr. Jie Wei, Boris Karpichev
Support: US Department of Energy
CH2OH + 4nOH CH2O + H
OSU 66th MSS, June 2011
Theory collaboratorsDr. Eugene Kamarchik, Prof. Anna Krylov, Prof. Joel Bowman
CH2OH radical: dissociation on the ground state
11,600
~ 15,000 cm-1
~ 14,000
~3,000
0
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2
1
(X 2A″)CH2OH
CH3O
H + CH2O (X)~
H + CH2O (X)~
~
1
2
3
4
5
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 CH2OH following IR overtone excitation
Today: direct dissociation dynamics vs. isomerization
CH2OH IE = 7.56 eV
Feng, Wei, and Reisler, J. Phys. Chem. A, 108, 7903 (2004).Wei, Karpichev, and Reisler, J. Chem. Phys. 125 (3): 34303-34303 (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: CH3OH, NH2OH, HOOH, etc. (Crim, Rizzo, Perry, etc.);
– Radical is a minor species and hard to make
– CH2OH has low-lying electronic states: Will we get substantial vibrationally mediated dissociation?
Detection of CH2OH via 1+1 REMPI
10
30
40
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(X 2A″)~
CH2OH
CH3OH + CH2O (X)
~
14,000
11,000
15,000 cm-1
H + CH2O (X)~
Cs3pz (
2A")
3px
3s = 26,000 cm-1
Barrier heights:Radom, Schaeffer;Harding, Klippenstein;Temps;Walch…
OH stretch vibration: 1-3nOH
-10 -5 0 5 10 Energy - 0 (cm-1)
OH
= 3674.8 cm-1, = 0.4 cm-1
3OH
= 10484.3 cm-1, = 0.4 cm-1
2OH
= 7158.0 cm-1, = 0.8 cm-1
3nOH ; 10484 cm-1; linewidth = 0.4 cm-1
2nOH ; 7158 cm-1; linewidth = 0.8 cm-1
1nOH; 3678 cm-1; linewidth = 0.4 cm-1
T = 10-13 K
10470 10475 10480 10485 10490 10495 10500
Simulation qP(N") qQ qR(N")
5 4 3 2 1 0 1 2 3 4 5(b)
Inte
nsi
ty (
a.u.
)
Pump Frequency (cm-1)
Pump on Pump off
(a)
CH
2O
H+ (
a.u
.)
Simulation of 2nd overtone spectrum:a/b–type transition
Wei, Karpichev, Reisler, J. Chem. Phys.(2006)
No H atom detected
3OH ;
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
CH2OHH-photofragment yield spectrum in the region of 4nOH
Accidental resonance between 4nOH and 3nOH + 1nasymCH
[3nOH + 1nasymCH ]*731.75 nm13662 cm-1
[4nOH]*735 nm13602 cm-1
Methanol overtone spectrum shows a similar accidental resonance between 5nOH and 4nOH + 1nasymCH . [Rizzo, Perry, Boyarkin]
300
250
200
150
100
50
H+ s
igna
l (co
unts
)
13680136601364013620136001358013560
vac (cm
–1)
GroundExcited
A /cm–1 6.51 6.0
B /cm–1 1.01 1.01
C /cm–1 0.88 0.88
90% : 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
moleculareigenstates
spectrum
Coupling between a zeroth-order bright state and a dark state
H product
GroundExcited
A /cm–1 3.78 3.56
B /cm–1 0.84 0.83
C /cm–1 0.69 0.73
80% : 20% : 0% (a:b:c)
T: 13K
D product (small)
H+ s
igna
l (co
unts
)
13680136601364013620136001358013560
vac (cm
–1)
300
250
200
150
100
50
H+ s
igna
l (co
unts
)
13680136601364013620136001358013560
vac (cm
–1)
D signal is is very small4nOH =13621 cm-1
No other bands observed
Lorenzian Linewidths: Both bands: ~ 1.6 cm–1
H and D products from CD2OH
0 500 1000 1500 2000 2500 3000 35000
5
0
C–H symC=O
CH2 scis
CH2 wag
C–H asym CH2 rock
C–H sym
C=O
CH2 scis
CH2 wag
C–H asym
CH2 rock
Inte
nsi
ty,
a.u
.
KER, cm
0.0
0.5
From maximum KER :D0 = 10,166 ± 70 cm-1
Compared to calculated D0 = 10,188 cm-1
(Marenich and Boggs)
Vibrational labels of CH2O co-fragment are marked. Excited levels = 8%.Rotational temperature of CH2O co-fragment: ~ 140 K
CH2OH: Slice image of H-photofragments from 4nOH peak
0
C–H symC=O
CH2 scis
CH2 wag
C–H asym CH2 rock
C–H sym
C=O
CH2 scis
CH2 wag
C–H asym
CH2 rock
0 500 1000 1500 2000 2500 3000 35000
1
Kinetic Energy Release, cm-1
Inte
nsit
y, a
rb. u
nits
0.0
0.1
CH2OH + hn → H + CH2O, n = 13662 cm-1
KER of H-photofragments from 3nOH + 1nasymCH peak
Vibrational labels of CH2O co-fragment are marked. Excited levels = 13%.
0
C–D symC=O
CD2 scisC–D asym
CD2 rock
CD2 wag
0
C–D
sym
C=
O
CD
2 scis
C–D
asym
CD
2 rock
CD
2 w
ag
0 500 1000 1500 2000 2500 3000 35000
1
Intensity, arb. units
Kinetic Energy Release, cm1
CD2OH: Monitoring H + CD2O hn = 13621 cm-1
0.04
0
C–H symC=O
CH2 scis
CH2 wag
C–H asym CH2 rock
C–H sym
C=O
CH2 scis
CH2 wag
C–H asym
CH2 rock
0 500 1000 1500 2000 2500 3000 35000
1
Kinetic Energy Release, cm1
In
ten
sity
, arb
. u
nits
0.00
0.05
CH2OH
0
C–D symC=O
CD2 scisC–D asym
CD2 rock
CD2 wag
0
C–D
sym
C=
O
CD
2 scis
C–D
asym
CD
2 rock
CD
2 w
ag
0 500 1000 1500 2000 2500 3000 35000
1
Intensity, arb. units
Kinetic Energy Release, cm1
CD2OH
Monitoring H
0.04
Formaldehyde cofragment:No CH stretchRatio of CO str/CHH(D) bends is larger in CD2OH
Higher rotational and CHD vibrational excitation; No CO stretch
0
C–H C–D
C=O
CHD scis
CHD rock
CHD wag
0 500 1000 1500 2000 2500 3000 35000
1
In
ten
sity
, arb
. u
nits
Kinetic Energy Release, cm1
CD2OH: Monitoring D + CHDO hn = 13621 cm-1
Summary
The OH overtone spectrum of CH(D)2OH has been characterized in the 4nOH
region. Mixed levels (most likely involving levels 4nOH and 3nOH + 1nasymCH ) are seen in CH2OH but not in CD2OH.
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).
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H + CH2O (X)~
H + CH2O (X)~
Calculated Geometries; CCSD(T)/aug-cc-pVTZ Dr. Eugene Kamarchik
355nm
Photolyticaly initiated reaction
Cl2 2Cl
Cl + CH3OH CH2OH + HCl k1 = 3.7x1013 (cm3.mole-1.s-1)
Radical production
CH2OH + Cl CH2O + HCl k2= 4x1014 (cm3.mole-1.s-1)…..
+
CH3OH/Cl2/He CH2OH
355nm
CH2OH diagnostics:3pz REMPI or H-photofragment spectrum
0nlmπco*
3pz (22A”) 12A”
C-O Stretch (n6) Progression00 1
60
260
C-O
Detection schemes for IR spectroscopy
Double Resonance
CH2OH (X2A")
CH2OH+
3pz
Probe
PumpnOH
Scheme 2
Or: Look directly at H atom when the radical dissociates
39800 40000 40200 40400 40600
60
1
80
2
13
390
2
Pump on Pump off
CH
2OH
+ sig
nal (
a.u.
)
2-photon probe frequency (cm-1)
Birge-Sponer plot
OH stretch AB, Anharmonicity
CH3OH 3769 86.1
CH2OH 3766 91.4
NH2OH 3743 90.6
HOOH 3701 90.5
potential
Anharmonic oscillator
/ = A – B
0 1 2 3 4 53300
3400
3500
3600
3700
/
(cm
-1)
Vibrational level,
Ground state
A=3766.3±2.7cm-1
B=91.4±1.0 cm-1
Dissociation barrier > 4ν1H-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: 14000 – 16000 cm-1
CH2OH
9557
cm-
1
1359
8 cm
-1 ?1712 cm-1
CH2O + H
13000 13500 14000 14500 15000 15500
1E-3
0.01
0.1
1E=E(4
1),13600 cm-1
D0(CH
2O-H)=9557 cm-1,
c=1712 cm-1
Tunneling probability
P=0.5, E0=13 600 cm-1
P=2*10-3, E0=15 200 cm-1
Tu
nn
elin
g p
rob
ab
ility
Barrier, E0, (cm-1)
Next goal: Excite above barrier to dissociation
11,000
16,000cm-1
14,000
~2,000
0
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2
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(X 2A″)CH2OH
CH3O
H + CH2O (X)~
H + CH2O (X)~
~
1
2
3
4
5
O-H
IR
11,000
14,600
18,000
7,158
3,675
735 nm 732 nmmode descr. freq. rel. pop. (fit)
- ground 0 1 11 C-H sym. 2783 - -2 C=O 1746 0.0203 0.02923 CH2 bend 1500 0.0187 0.02934* CH2 wag 1167 0.0291 0.05615 C-H asym. 2843 - -6* CH2 rock 1249 0.0166 0.0348
% excited (fit) 7.8 13% excited (int.) 7.7 12.8
T_rot (fit), K 139 158
bright state
dark states
moleculareigenstates
spectrum
Coupling between a zeroth-order bright state and a bath of dark states
Exciting the 3rd overtone, 4n1: Monitoring H photofragments from CH2OH and CD2OH
13600 13610 13620 1363013580 13590 13600 13610Pump frequency (cm-1)
= 1.3 cm-1
H+
sign
al (
a.u.
)
Pump frequency (cm-1)
Pump-on Simulation
CH2OH
= 1.3 cm-1
Pump-on Simulation
CD2OH
CH2OH and CD2OH have the same linewidth: 1.3 cm-1
Could not detect deuterium from CD2OHIsomerization is at best a minor channel.
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