Resonant excitation multiphoton ionization REMPI

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

The vibration spectrum of the first excited state of guanine was measured using laser desorption jet-cooled resonance-enhanced multiphoton ionization (REMPI) spectrometry <1999JA4896>. The millimeter wave spectrum of purine was collected using a free jet spectrometer, and the observed rotational spectrum was assigned to the N(9)-H tautomer <1996CPL189>. 

Photolysis of the dimer, reaction (44), proceeds primarily via generation of Cl + ClOO (Cox and Hayrnan, 1988 Molina et al., 1990). For example, Molina et al. (1990) reported the quantum yield for this channel at 308 nm to be unity, with an uncertainty of 30%. Okumura and co-workers (Moore et al., 1999) and Schindler and co-workers (Schmidt et al., 1998) have reported that the quantum yield is less than 1.0. For example, Schmidt et al. (1998) used resonance-enhanced multiphoton ionization (REMPI) with time-of-flight (TOF) mass spectrometry to follow the production of oxygen and chlorine atoms as well as CIO in vibrational levels up to v" = 5 in the photolysis of the dimer. At a photolysis wavelength of 250 nm, the quantum yield for chlorine atom production was measured to be 0.65 + 0.15, but CIO was not observed. Assuming that all of the excited dimer dissociates, this suggests that the production of CIO in vibrational... 

Resonance-enhanced multiphoton ionization (REMPI) has proved to be a versatile ionization technique for MS offering a number of advantageous features in the field of chemical analysis. Since it makes use of substance-specific excited states for the ionization process, it involves UV spectroscopy of the molecule to be ionized. Thus, it enables ionization of preselected compounds, control of the degree of fragmentation and, for a large number of substances, a high ionization efficiency. These features require that the excited molecular state(s) involved in the REMPI process not be significantly depleted... 

Recently, a similar approach has beep-used to investigate van der Waals complexes of Al( P) with Ar and In( P) with Ar, Kr, and Xe." Laser vaporization of the refractory atom followed by supersonic expansion in rare gas carrier was to sinthesize van der Waals complexes. Gardner and Lester" then used mass resolved resonance-enhanced multiphoton ionization (REMPI) for obtaining rovibration excitation spectra... 

Infrared diode laser absorption spectroscopy has been employed to detect ground-state SiH2 and allowed the first observation of its high-resolution IR spectrum by Yamada and coworkers in 1989 . State-selective multiphoton IR excitation of SiH2 has been examined computationally . The resonance-enhanced multiphoton ionization (REMPI) spectrum of SiH2 was observed for the first time by Robertson and Rossi and was employed to measure the SiH2 sticking coefficient on a silicon surface . ... 

Removal of the Iti electron from NH(X S ) leads to the ionic ground state X removal of the 3a electron to the excited ionic states a A B A, and C Only a few experimental data for the first, third, and fourth ionization potentials Ej of gaseous NH are available. Resonance-enhanced multiphoton ionization (REMPI) of NH coupled with photoelectron spectroscopy (PES) yielded the most accurate results so far [1] and confirmed the values for the first E, obtained by electron-impact mass spectrometry (EIMS) [2] and by He I PES of NH [3]. Values for the second and third Ej to be observed in the He I PES of NH were predicted [3] from the optical emission spectra of NH [4]. Adiabatic and vertical Ej s (in eV) are compared in the following table ... 

Resonance-enhanced multiphoton ionization (REMPI) of NH and ND and detection of the NH or ND ions with a time-of-flight (TOF) mass spectrometer [3 to 8] or TOF detection of the photoelectrons (REMPI-PES) [5, 9, 10] enabled highly excited states of the NH (ND) radical (cf. pp. 83/7) and vibrational and rotational populations of the resulting NH or ND ions to be characterized. 

The above techniques are restricted to molecttles which fluoresce. A more general technique is multiphoton ionization (MPI) in which a molecule absorbs several photons sufficient in energy to produce a molecttlar ion. This technique is very sensitive since single ions can be detected. The process may involve a single laser and several photons, or two (or more) lasers with various corribinations of photons, e.g. 1+3, 2+2. One of the lasers can be adjusted so as to involve an intermediate excited state in which case the sensitivity is considerably enhanced and the process is known as resonance enhanced multiphoton ionization (REMPI). The detection of the resrtltant ion with a mass-spectrometer further refines the specificity of the method, and allows individual mass peaks, and isotopic species, to be monitored. The introduction of ZEKE (zero electron kinetic energy [42]) considerably increases the resolution which is beginning to approach the hmit imposed by the widths of the laser. 

Figure 1 Illustration of (A) sequential and (B) simultaneous two-photon excitation from state A to state B. Also shown in (B) are three possible fates of the excited state B fluorescence, dissociation and further photon absorption that ionizes the molecule. This latter process it termed 2+1 resonance-enhanced multiphoton ionization (REMPI).

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