Chapter 10. REMPI, ZEKE, and MATI Spectroscopies.ppt

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1、wbt,1,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)vibr

2、onic 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

3、an intermediate state.,10.1 REMPI spectroscopy,Most commonly used is resonance-enhanced two-photon ionization, termed (1+1) R2PI.,wbt,2,wbt,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 st

4、ate selectivity.,wbt,4,1C and 2C-REMPI spectra of phenolN2,E2 = 31521 cm-1,Ref.: K. Mller-Dethlefs, J. Chem. Phys. 109, 9244 (1998).,wbt,5,GoalsEE, IE, vibrations in the S1 and D0 statesD-substitution effect on transition energy and vibrationsite-specific electronic transition,IAMS, Academia Sinica,

5、 Taiwan, 台灣 中研院原分所,MATI spectroscopy of aniline isotopomers,What we have knownS1 S0 at 294 nm (4.19 eV), IE = 7.720 eVtheoretic 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

6、not yet available,Approachespreparation of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, C6D5ND2 1C-R2PI and MATI experiments,wbt,6,wbt,7,Preparation of C6H5NHD and C6H5ND2,(Mass 93),(Mass 95),(Mass 94),wbt,8,Preparation of C6H5NHD and C6H5ND2,(Mass 93),(Mass 95),(Mass 94),wbt,9,Relative Intensity,Ma

7、ss / amu,TOF spectra of deuterium substituted aniline isotopomers, = 293.94 nm, = 292.54 nm, = 292.48 nm,93 C6H5NH2+,94 C6H5NHD+,98 C6D5NH2+,99 C6D5NHD+,100 C6D5ND2+,wbt,10,One photon energy / cm-1,Relative Intensity,(a) C6H5NH2,(b) C6H5NHD,(b) C6H5ND2,1C-R2PI spectra of deuterium substituted anilin

8、e isotopomers,wbt,11,1C-R2PI spectra of deuterium substituted aniline isotopomers,Relative Wavenumber / cm-1,Relative Intensity,(a) C6D5NH2,(b) C6D5NHD,(c) C6D5ND2,wbt,12,10.2 ZEKE spectroscopy,Recall that, in photoelectron spectroscopy (PES) a high-energy photon ionizes a molecule and the kinetic e

9、nergy 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 ei

10、genstate.,Zero-kinetic energy (ZEKE) photoelectron spectroscopy was developed in 1984 by K. Mller-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

11、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 35 cm-1.,wbt,13,(1) a molecule (M) is prepared in S0 state by molecular beam methods. (2) M is excited

12、 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 spectroscop

13、y detects the ZEKE electrons.,wbt,14,10.3 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 spectros

14、copy 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

15、 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.,wbt,15,MATI spectra of deut

16、erium substituted aniline isotopomers,Ion Internal Energy / cm-1,Relative Intensity,via S100,via S100,via S100,34029 cm-1,34031 cm-1,34038 cm-1,(a) C6H5NH2,(b) C6H5NHD,(c) C6H5ND2,wbt,16,Ion Internal Energy / cm-1,Relative Intensity,via S100,via S100,via S100,34193 cm-1,34195 cm-1,34202 cm-1,(a) C6D

17、5NH2,(b) C6D5NHD,(c) C6D5ND2,MATI spectra of deuterium substituted aniline isotopomers,wbt,17,Measured electronic transition and ionization energies (in cm-1),a This work. b Fung, Selzle, Schlag, JCP 87, 5113 (1983),wbt,18,MATI spectra of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, and C6D5ND2 precise IE, cation vibrations IEs of deuterated species are lower than that of undeuterated anilinesite-specific electronic transition: S1 S0, ring ion S1, aminodeuteration on vibrational frquency depends on the pattern,IAMS, Academia Sinica, Taiwan, 台灣 中研院原分所,Summary (aniline isotopomers),