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Chapter 10. REMPI, ZEKE, and MATI Spectroscopies

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy involves more than one photons in the ionization process. In general, the REMPI process occurs by a resonant m-photon excitation from a ground electronic state to an excited (ro)vibronic state. from a ground electronic state to an excited state and n photons from the neutral excited state to and ionic state. More (n) additional photons are then absorbed and the molecule is ionized. The probability of ionization is enhanced by the fact that the first m photons are resonant with an intermediate state.

10.1 REMPI spectroscopy

Most commonly used is resonance-enhanced two-photon ionization, termed (1+1’) R2PI.

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The great advantages of the REMPI technique, compared to other approaches such as laser-induced fluorescence (LIF) are its (1) mass selectivity and (2) its state selectivity.

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1C and 2C-REMPI spectra of phenol•N2

E2 = 31521 cm-1 Ref.: K. Müller-Dethlefs, J. Chem. Phys. 109, 9244 (1998).

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Goals• EE, IE, vibrations in the S1 and D0 states• D-substitution effect on transition energy and vibration• site-specific electronic transition

IAMS, Academia Sinica, Taiwan, 台灣 中研院原分所

MATI spectroscopy of aniline isotopomers

What we have known• S1 ← S0 at ~ 294 nm (4.19 eV), IE = 7.720 eV• theoretic prediction: (a) S1 ← S0 ~ ring, (b) ionization ~ the removal of an electron from the amino part (experimental evidence is not yet available)• cation data of deuterated species are not yet availableApproaches• preparation of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, C6D5ND2 • 1C-R2PI and MATI experiments

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C6H

5NH

2

+, D0

(7.7206 eV)

(293.87 nm)

IE = 62,271 cm-1

E = 34,029 cm-1

C6H

5NH

2, S

0

C6H

5NH

2

*, S1

Energy Level of Aniline

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N

H

H

HH

HH

H

Preparation of C6H5NHD and C6H5ND2

+ D2O N

H

D

HH

HH

H

N

D

D

HH

HH

H

(Mass 93)

(Mass 95)

(Mass 94)

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N

H

H

HH

HH

H

Preparation of C6H5NHD and C6H5ND2

+ D2O N

H

D

HH

HH

H

N

D

D

HH

HH

H

(Mass 93)

(Mass 95)

(Mass 94)

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92 96 100 104 108

100

(c)

9998(b)

94

93

(a)

Rel

ativ

e In

tens

ity

Mass / amu

TOF spectra of deuterium substituted aniline isotopomers

λ = 293.94 nm

λ = 292.54 nm

λ = 292.48 nm

93 ↔ C6H5NH2+

94 ↔ C6H5NHD+

98 ↔ C6D5NH2+

99 ↔ C6D5NHD+

100 ↔ C6D5ND2+

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34000 34200 34400 34600 34800 35000 35200

34038 cm-1

120

1

10

1I

0

2

6a0

100

0(c)

34031 cm-1 120

1

10

1I0

2

6a0

100

0(b)

34029 cm-1 120

1

10

1

I0

2

6a0

1

00

0(a)

Rel

ativ

e In

tens

ity

(a) C6H5NH2

(b) C6H5NHD

(b) C6H5ND2

1C-R2PI spectra of deuterium substituted aniline isotopomers

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1C-R2PI spectra of deuterium substituted aniline isotopomers

Relative Wavenumber / cm-1

Rel

ativ

e In

tens

ity

0 200 400 600 800 1000

34202 cm-1

120

1

I0

26a0

10

0

0(c)

(a)

34195 cm-1

120

1

I0

2

6a0

100

0(b)

34193 cm-1

120

1

I0

2

6a0

1

00

0

(a) C6D5NH2

(b) C6D5NHD

(c) C6D5ND2

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10.2 ZEKE spectroscopyRecall that, in photoelectron spectroscopy (PES) a high-energy photon ionizes a molecule and the kinetic energy of the resulting photoelectron is analyzed to reveal the energy levels of the corresponding ion. A typical resolution of PES is 10 meV (80 cm-1). Threshold photoelectron spectroscopy (TPES) is an improved version of PES. It detects electrons emitted only at the threshold of a specific ionic eigenstate. Zero-kinetic energy (ZEKE) photoelectron spectroscopy was developed in 1984 by K. Müller-Dethlefs and E.W. Schlag. In this scheme, the system (molecule) is photoexcited to a high-n (n > 150) Rydberg state, and then after a time delay of several microseconds, ionization of the Rydberg neutral is induced by a pulsed electric field. The process is often referred to as ZEKE-pulsed field ionization (PFI). The best resolution of ZEKE spectroscopy is 0.15 cm-1, whereas a typical resolution is 3–5 cm-1.

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(1) a molecule (M) is prepared in S0 state by molecular beam methods.(2) M is excited by the first laser to a particular vibrational level in the electronically excited S1 state (M*).(3) M* is excited by the second laser to a high-n (n > 150) Rydberg state (M**).(4) M** is ionized by PFI, and ZEKE electrons and ZEKE ions are generated simultaneously.(5) ZEKE photoelectron spectroscopy detects the ZEKE electrons.

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10.3 MATI spectroscopy

Mass analyzed threshold ionization (MATI) spectroscopy was developed in 1991 by P. Johnson. This method involves detection of ZEKE ions. One of the major advantages of MATI over ZEKE is that it provides mass information. Thus, MATI spectroscopy is suitable for spectroscopic and dynamics studies of isotopomers, radicals, clusters, etc.In the MATI experiments, the prompt ions, ZEKE electrons, and Rydberg neutrals are formed simultaneously. About 50 ns after the occurrence of the laser pulses, (ZEKE electrons are gone) a pulsed electric field of -1.0 V/cm is switched on to reject the prompt ions. After about 8-10 microsecond later, a second pulsed electric field of +400 V/cm is applied to field-ionize the Rydberg neutrals. These threshold (MATI) ions are then accelerated and detected by an ion detector.

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MATI spectra of deuterium substituted aniline isotopomers

Ion Internal Energy / cm-1

Rel

ativ

e In

tens

ity

0 400 800 1200 1600

62233 cm-1

8a1

9a1121

10b211

0+ (c)

62253 cm-1

8a1

9a112110b2 11

0+ (b)

62271 cm-1

9a1

121

11

16a2

10b2

0+ (a)

via S100

via S100

via S100

34029 cm-1

34031 cm-1

34038 cm-1

(a) C6H5NH2

(b) C6H5NHD

(c) C6H5ND2

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Rel

ativ

e In

tens

ity

via S100

via S100

via S100

34193 cm-1

34195 cm-1

34202 cm-1

(a) C6D5NH2

(b) C6D5NHD

(c) C6D5ND2

MATI spectra of deuterium substituted aniline isotopomers

0 400 800 1200 1600

62214 cm-1

6a1

121

118a1

0+ (c)

62237 cm-1

6a1

18a1

18a1

11

8a10+ (b)

6a1

62258 cm-1

121

18a1

11

8a1

0+ (a)

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Measured electronic transition and ionization energies (in cm-1)

a This work. b Fung, Selzle, Schlag, JCP 87, 5113 (1983)

S1 S0 Shift IE Shift

aniline a

C6H5NH2 34029 0 62271 0

C6H5NHD 34031 2 62253 -18

C6H5ND2 34038 9 62233 -38

C6D5NH2 34193 164 62258 -13

C6D5NHD 34195 166 62237 -34

C6D5ND2 34202 193 62214 -57

benzene b

C6H6 [via 601] 38608 0 74557 0

C6D6 [via 601] 38787 179 74584 27

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• MATI spectra of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, and C6D5ND2

precise IE, cation vibrations • IEs of deuterated species are lower than that of undeuterated aniline• site-specific electronic transition: S1 ← S0, ~ ring

ion ← S1, ~ amino

• deuteration on vibrational frquency depends on the pattern

IAMS, Academia Sinica, Taiwan, 台灣 中研院原分所

Summary (aniline isotopomers)