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Chapter 10. REMPI, ZEKE, and MATI Spectroscopies

Chapter 10. REMPI, ZEKE, and MATI Spectroscopies. 10.1 REMPI spectroscopy.

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Chapter 10. REMPI, ZEKE, and MATI Spectroscopies

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  1. Chapter 10.REMPI, ZEKE, and MATI Spectroscopies 10.1 REMPI spectroscopy 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. Most commonly used is resonance-enhanced two-photon ionization, termed (1+1’) R2PI.

  2. 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.

  3. 1C and 2C-REMPI spectra of phenol•N2 E2 = 31521 cm-1 Ref.: K. Müller-Dethlefs, J. Chem. Phys. 109, 9244 (1998).

  4. 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 available • Approaches • preparation of C6H5NH2,C6H5NHD,C6H5ND2,C6D5NH2, C6D5NHD, C6D5ND2 • 1C-R2PI and MATI experiments • 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

  5. Preparation of C6H5NHD and C6H5ND2 (Mass 94) (Mass 93) (Mass 95)

  6. Preparation of C6H5NHD and C6H5ND2 (Mass 94) (Mass 93) (Mass 95)

  7. TOF spectra of deuterium substituted aniline isotopomers λ = 293.94 nm 93 ↔ C6H5NH2+ 94 ↔ C6H5NHD+ λ = 292.54 nm Relative Intensity 98 ↔ C6D5NH2+ 99 ↔ C6D5NHD+ λ = 292.48 nm 100 ↔ C6D5ND2+ Mass / amu

  8. 1C-R2PI spectra of deuterium substituted aniline isotopomers (a) C6H5NH2 (b) C6H5NHD Relative Intensity (b) C6H5ND2 One photon energy / cm-1

  9. 1C-R2PI spectra of deuterium substituted aniline isotopomers (a) C6D5NH2 (b) C6D5NHD Relative Intensity (c) C6D5ND2 Relative Wavenumber / cm-1

  10. 10.2 ZEKE spectroscopy Recall 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.

  11. (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.

  12. 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.

  13. MATI spectra ofdeuterium substituted aniline isotopomers (a) C6H5NH2 via S100 34029 cm-1 (b) C6H5NHD via S100 Relative Intensity 34031 cm-1 (c) C6H5ND2 via S100 34038 cm-1 Ion Internal Energy / cm-1

  14. MATI spectra ofdeuterium substituted aniline isotopomers (a) C6D5NH2 via S100 34193 cm-1 (b) C6D5NHD via S100 Relative Intensity 34195 cm-1 (c) C6D5ND2 via S100 34202 cm-1 Ion Internal Energy / cm-1

  15. Measured electronic transition and ionization energies (in cm-1) a This work. b Fung, Selzle, Schlag, JCP 87, 5113 (1983)

  16. Summary (aniline isotopomers) • MATI spectra of C6H5NH2,C6H5NHD,C6H5ND2,C6D5NH2, C6D5NHD, and C6D5ND2precise IE, cation vibrations • IEs of deuterated species are lower than that of undeuterated aniline • site-specific electronic transition:S1 ← S0, ~ ringion ← S1, ~ amino • deuteration on vibrational frquency depends on the pattern IAMS, Academia Sinica, Taiwan, 台灣 中研院原分所

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