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

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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)

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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)

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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+

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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+

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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