wbt1 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 REMPI spectroscopy Most commonly used is resonance-enhanced two-photon ionization, termed (1+1’) R2PI.

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3 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.

wbt4 1C and 2C-REMPI spectra of phenolN 2 E 2 = cm -1 Ref.: K. Müller- Dethlefs, J. Chem. Phys. 109, 9244 (1998).

wbt5 Goals EE, IE, vibrations in the S 1 and D 0 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 S 1 ← S 0 at ~ 294 nm (4.19 eV), IE = eV theoretic prediction: (a) S 1 ← S 0 ~ 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 C 6 H 5 NH 2, C 6 H 5 NHD, C 6 H 5 ND 2, C 6 D 5 NH 2, C 6 D 5 NHD, C 6 D 5 ND 2 1C-R2PI and MATI experiments

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7 Preparation of C 6 H 5 NHD and C 6 H 5 ND 2 (Mass 93) (Mass 95) (Mass 94)

wbt8 Preparation of C 6 H 5 NHD and C 6 H 5 ND 2 (Mass 93) (Mass 95) (Mass 94)

wbt9 Relative Intensity Mass / amu TOF spectra of deuterium substituted aniline isotopomers λ = nm λ = nm λ = nm 93 ↔ C 6 H 5 NH ↔ C 6 H 5 NHD + 98 ↔ C 6 D 5 NH ↔ C 6 D 5 NHD ↔ C 6 D 5 ND 2 +

wbt10 One photon energy / cm -1 Relative Intensity (a) C 6 H 5 NH 2 (b) C 6 H 5 NHD (b) C 6 H 5 ND 2 1C-R2PI spectra of deuterium substituted aniline isotopomers

wbt11 1C-R2PI spectra of deuterium substituted aniline isotopomers Relative Wavenumber / cm -1 Relative Intensity (a) C 6 D 5 NH 2 (b) C 6 D 5 NHD (c) C 6 D 5 ND 2

wbt 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.

wbt13 (1) a molecule (M) is prepared in S 0 state by molecular beam methods. (2) M is excited by the first laser to a particular vibrational level in the electronically excited S 1 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.

wbt 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.

wbt15 MATI spectra of deuterium substituted aniline isotopomers Ion Internal Energy / cm -1 Relative Intensity via S cm cm cm -1 (a) C 6 H 5 NH 2 (b) C 6 H 5 NHD (c) C 6 H 5 ND 2

wbt16 Ion Internal Energy / cm -1 Relative Intensity via S cm cm cm -1 (a) C 6 D 5 NH 2 (b) C 6 D 5 NHD (c) C 6 D 5 ND 2 MATI spectra of deuterium substituted aniline isotopomers

wbt17 Measured electronic transition and ionization energies (in cm -1 ) a This work. b Fung, Selzle, Schlag, JCP 87, 5113 (1983) S 1  S 0 ShiftIEShift aniline a C 6 H 5 NH C 6 H 5 NHD C 6 H 5 ND C 6 D 5 NH C 6 D 5 NHD C 6 D 5 ND benzene b C 6 H 6 [via ] C 6 D 6 [via ]

wbt18 MATI spectra of C 6 H 5 NH 2, C 6 H 5 NHD, C 6 H 5 ND 2, C 6 D 5 NH 2, C 6 D 5 NHD, and C 6 D 5 ND 2 precise IE, cation vibrations IEs of deuterated species are lower than that of undeuterated aniline site-specific electronic transition: S 1 ← S 0, ~ ring ion ← S 1, ~ amino deuteration on vibrational frquency depends on the pattern IAMS, Academia Sinica, Taiwan, 台灣 中研院原分所 Summary (aniline isotopomers)