Stepwise photoionization

A universal way to overcome this difficulty and make the multistep resonance photoionization method actually applicable to the detection of rare isotopes was suggested by Kudriavtzev and Letokhov (1982). The idea of the method is based on the collinear stepwise photoionization of a beam of accelerated atoms. The acceleration of the atoms in the form of ions under a given potential difference U with subsequent neutralization of the ions into atoms leads to bunching of the longitudinal ionic velocities, and hence to narrowing of the Doppler width of all the spectral lines of the given atomic species (if viewed in a collinear fashion), as compared with the initial Doppler width at... 

The earliest experiments on the resonance stepwise photoionization of molecules (H2CO) were conducted by Andreyev et al. (1975) in an ionization chamber without using any mass separation of the photoions produced. The next natural step was the two-stage resonance photoionization of molecules by the scheme of Fig. 10.1, involving mass analysis of the photoions produced (Antonov et al. 1977, 1978). The experimental setup for studying the stepwise photoionization of polyatomic molecules in a mass spectrometer consisted of a static magnetic mass spectrometer and time-synchronized... 

The stepwise resonance ionization of molecules differs materially from the resonance photoionization of atoms by, first, the small resonance excitation cross sections of the electronic states in polyatomic molecules, owing to their distribution among many rotational-vibrational levels, and, second, the fact that the absorption bands of many molecules occur in the UV region of the spectrum. For this reason, the soft resonance stepwise photoionization technique has proved unsuitable for the majority of... 

Antonov, V. S., Knyazev, I. N., Letokhov, V. S., Mat3uik, V. M., Movshev, V. G., and Potapov, V. K. (1977). Mass-spectrometer with selective stepwise photoionization of molecules with laser radiation. Journal of Technical Physics Letters, 3(23), 1287-1291 (in Russian). 

Since the second laser ionizes the excited atom in DLI, this step may be accomplished by an off-resonant photon (see Fig. 1). If a nitrogen laser-pumped dye laser provides the resonant photon, a fraction of the nitrogen laser beam can conveniently ionize the atom from the laser-excited state 14,15). It is theoretically possible to photoionize every atom in the periodic table except helium and neon using five RIS ionization schemes involving stepwise and/or multiphoton excitation4). Presumably these... 

Buettner, G.R., Hall, R.D., Chignell, C.F., and Motten, A.G. (1989) The stepwise biphotonic photoionization of chlorpromazine as seen by laser flash photolysis, Photochem. Photobiol., 49, 249-256. 

Table I. Representative stepwise laser photoionization threshold results...

Techniques of stepwise laser excitation and photoionization have been applied to study spectroscopic properties of neutral atoms of lanthanides and actinides. The spectroscopic properties that can be determined include the ionization potential, energy levels, isotope shifts, hyperfine structure, lifetimes of energy levels, branching ratios and oscillator strengths. We discuss the laser methods used to obtain these properties (with emphasis on ionization potentials) and give examples of results obtained for each. The ionization potentials obtained by laser techniques are in eV Ce, 3.3387(4) ... 

Laser SNMS requires the operation with properly selected duty cycles that control the delay times between the primary ion pulse, a pulsed extraction voltage for separating the secondary ions from post-ionized neutrals, and the firing of the postionizing laser pulse. Such duty cycles have, in addition, to be synchronized with the stepwise motion of the pulsed primary ion beam across the sample surface in the microprobe mode of laser SNMS. The selection of appropriate duration and decay times of the ion and laser pulses, of the laser intensity, and beam shape is important to make the photoion yields independent on the sputtered particle velocities. The detection volume must be matched to the entrance ion optics of the TOP such that it becomes independent of the individual ionization process. Usually, laser intensities in the range from 10 to lO Wcm are applied. While the particle density in the detection volume is monitored at small laser intensities, the particle flux is measured at high photon densities. 

We first report on a two-laser photoionization experiment of benzene where the stepwise excitation was accomplished by frequency doubled dye lasers. The first laser pumped the molecule to a selected vibronic level of its first excited siglet state from where it was ionized... 

The study of Rydberg spectra and ionization thresholds of ten lanthanides and actinides has been reported by PAISNER et al. [8.80]. In these experiments, high-lying states were accessed by time-resolved stepwise excitation using pulsed dye lasers tuned to resonant transitions. Atoms excited to levels within 1000 cm of the ionization limit were then photoionized by 10.6 ym radiation from a pulsed CO2 laser. The measurements allowed the accurate determination of ionization thresholds from Rydberg convergences to within 0.0005 eV. 

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