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IR spectra of Methanol Clusters (CH3OH)n Studied by IR Depletion and VUV Ionization Technique with TOF Mass Spectrometer
Department of Applied ChemistryNational Chiao Tung University
TaiwanHui-Ling Han and Yuan-Pern Lee
2
Outline1. Introduction
Importance Previous work on methanol clusters
2. Experimental setup VUV ionization and IR depletion technique
3. Results and discussion TOF spectra and action spectra IR spectra of methanol monomer and clusters
4. Summary and future work
3
Significance of Methanol Cluster
(CH3OH)n(H2O)n
n=2 n=3
F. N. Keutsch and R. J. Saykally, PANS, 98 10533 (2001)
n=4 n=5
two OH bonds one OH bond
U. Buck, J. G.Siebers, R. J. Wheatley, J. Chem. Phys. 108, 20 (1998)
4
Provencal et al., J. Phys. Chem. 110, 4258 (1999)
IR-CRLAS
Previous Work on Methanol Clusters ─ OH region
FTIR
Häber et al., Phys. Chem. Chem. Phys. 1, 5573 (1999)
jet‐cooledmethanol (0.1 % in He)
IR predissociation spectra
IR spectra shows severe overlap for the size larger than trimer
Reviews : M. A. Suhm, Chem. Rev. 100, 3863 (2000) M. A. Suhm, Adv. Chem. Phys. 142, 1(2009)
Huisken’s and
Buck’sgroups
momentum transfer method
in a scattering
experiment
J. Chem. Phys. 108, 33 (1998)J. Chem. Phys. 95, 3924 (1991)
5
Previous Work on Methanol Clusters ─ CH stretching region
Hu et al., J. Chem. Phys. 125, 154306 (2006).
They reported the variation in intensity of (CH3OH)nH+ generated by VUV laser ionization upon IR irradiation at various wavenumber.
Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomer and clusters
monitor
(CH3OH)+ (CH3OH)H+
(CH3OH)2H+ (CH3OH)3H+
(CH3OH)7H+
(CH3OH)6H+
(CH3OH)5H+ (CH3OH)4H+
6
Pulsed valve
time
TOF spectrum
sign
al
Cluster detection② Clusters were ionized by VUV at 118 nm without fragmentation.③ Cluster ions were detected by TOF MS.
Cluster formation ① Methanol clusters were formed through supersonic expansion
Experiments
(CH3OH)n
Ion detector
TOFMS
Cluster ions
1700 V
1500 V①
②
③
IR dissociation of clusters ④ Variation of intensity of each m/z peak
Action spectra of clusters ⑤ Scan IR frequency while monitoring
all m/z peaks simultaneously
time
TOF spectrum
sign
al
skimmer
VUV
0 V
IR
④
⑤
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ResultsTime‐of‐Flight Mass Spectra
0.0
0.1
0.2
0.3
0.0
0.1
0.2
0.3
10 12 14 16 18 20 22 24 26
0.0
0.1
0.2
0.3
(CH3OH)
2H+ (a) VUV (118 nm)
m/z 32
(CH3OH)+
m/z 129
(CH3OH)
4H+
m/z 161
(CH3OH)
5H+
m/z 65
m/z 33
(CH3OH)H+
m/z 64
(CH3OH)
2
+
(c) IR(3150 cm-1) + VUV
m/z
31
TOF time (s)
(b) IR(2950 cm-1) + VUV
Ion inte
nsity, arb
. units
m/z 97
(CH3OH)
3H+ (10.5 eV)
(CH3OH)n+1 + 10.5 eV → (CH3OH)nH+ + CH3O +e ‐
(n≥2)
CH3OH+ 10.8 eV → CH3OH+
8
0.0
0.1
0.2
0.3
0.0
0.1
0.2
0.3
10 12 14 16 18 20 22 24 26
0.0
0.1
0.2
0.3
(CH3OH)
2H+ (a) VUV (118 nm)
m/z 129
(CH3OH)
4H+
m/z 161
(CH3OH)
5H+
m/z 65
m/z 33
(CH3OH)H+
m/z 64
(CH3OH)
2
+
(c) IR(3150 cm-1) + VUV
m/z
31
TOF time (s)
(b) IR(2950 cm-1) + VUV
Ion inte
nsity, arb
. units
m/z 97
(CH3OH)
3H+
ResultsTime‐of‐Flight Mass Spectra
CH3OH+
CH excitation
OH excitation for n ≥ 4
(CH3OH)n+1 + 10.5 eV → (CH3OH)nH+ + CH3O +e ‐
(n≥2)
IR+VUV ionization
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Mechanism of Ionization and Dissociation
(CH3OH)4 (CH3OH)3 (CH3OH)2 (CH3OH)IR IR IR
VUVVUV
VUV VUVIR+VUV
(CH3OH)3H+
(CH3OH)2H+
(CH3OH)H+ (CH3OH)2+
(CH3OH)+
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TOF and Action SpectraMonitoring percentage variation in intensity of all mass channel while scan the IR frequency to obtain the action spectra
TOF spectrum
Action spectra
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Analysis of the Action SpectraAssumptions:
1. The cluster size only decreases by 1 during IR dissociation.
2. No multi-photon dissociation small IR cross section (~10-19 cm2/molecule)
3. The PIE for formation of (CH3OH)nH+ (n=2-5) at 10.5 eV, set to be 1.Set correction factors for formation of (CH3OH)H+ and (CH3OH)2
+ due to smaller PIE, 0.4 and 0.055, respectively.
A : absorption spectrum : action spectrum (% change)
-(Δ[(CH3OH)4H+]) = A[(CH3OH)5]
-(Δ[(CH3OH)3H+]) = A[(CH3OH)5]-A[(CH3OH)4]
Kostko et al., J. Phys. Chem. A, Vol. 112, No. 39, 2008
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IR Spectra of (CH3OH)n n =1−5
O −H O −H
PAPD
cooperative effect ↑
367535
7636
85
C −H C −H
PD
PA
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IR Spectra of (CH3OH)n in CH stretching region
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Oventone: 2ν4, 2ν10,
Combination: ν4+ν10, ν4+ν5, ν5+ν10,
Fundamental:ν2, ν9, ν3
Experiment Calculation
ANO1-DFT-B3LYP-VPT2
By H. Witek’s group
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Summary1. The IR-VUV-TOF system has been successfully implemented.
2. IR Spectra of (CH3OH)n (n≤5) have been determined.
3. Spectra support that (CH3OH)n (n≥3) are cyclic and (CH3OH)2 has a chain structure.4. The C-H excitation also leads to photodissociation, indicating rapid energy redistribution.
Tunable VUV laser light is going to be implemented to study the IR spectra of free radicals. Ex: CH3S, IP= 9.26 eV
Future work
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Thanks for your attention.
Acknowledgment
Thank C. Y. Ng’s group in UC Davis
Taiwan
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M = methanol (CH3OH)
PIE curves for protonated methanol monomer and methanol clusters ((CH3OH)nH+) for size n=2-6
Kostko et al., J. Phys. Chem. A, Vol. 112, No. 39, 2008
ν1 ν2 ν9 ν3
PA
PD
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Vibrations of methanol dimer