Laser-excited resonance ionization spectroscopy

Graphite furnace AAS Atomic fluorescence spectroscopy Inductively-coupled-plasma optical-emission spectroscopy Glow-discharge optical-emission spectroscopy Laser-excited resonance ionization spectroscopy Laser-excited atomic-fluorescence spectroscopy Laser-induced-breakdown spectroscopy Laser-induced photocoustic spectroscopy Resonance-ionization spectroscopy... 

Laser excited resonance ionization spectroscopy (LERIS)... 

Fig. 4 Right Principle of resonance ionization spectroscopy. Three tunable pulsed Dye lasers are used to stepwize excite and ionize the atom. The effective cross-sections are indicated for the different transitions in cm2. Left Layout of a laser ion source for on-line mass separators.

In atomic laser spectroscopy, the laser radiation, which is tuned to a strong dipole transition of the atoms under investigation, penetrates the volume of species evaporated from the sample. The presence of analyte atoms can be measmed by means of the specific interaction between atoms and laser photons, such as by absorption techniques (laser atomic absorption spectrometry, LAAS), by fluorescence detection (laser-induced fluorescence spectroscopy, LIFS), or by means of ionization products (electrons or ions) of the selectively excited analyte atoms after an appropriate ionization process (Figures lA and IB). Ionization can be achieved in different ways (1) by interaction with an additional photon of the exciting laser or of a second laser (resonance ionization spectroscopy, RIS, or resonance ionization mass spectrometry, RIMS, respectively, if combined with a mass detection system) (2) by an electric field applied to the atomization volume (field-ionization laser spectroscopy, FILS) or (3) by collisional ionization by surrounding atoms (laser-enhanced ionization spectroscopy, LEIS). 

Resonance ionization spectroscopy (RIS) is based on resonance multistep excitation of high-lying levels of free analyte atoms and their subsequent ionization. Ionization may be caused by the photons of the final laser step (photoioniza-... 

Figure lA. Resonance ionization spectroscopy is performed by monitoring molecular ions while tuning the laser wavelength. If the energy of one photon is in resonance with a neutral electronic excited state a second photon is able to be absorbed, giving rise to an ion current peak. Thus, the neutral UV-absorption spectrum is transferred to the ion current which can be recorded mass selectively (in opposition to the absorption). Thus UV spectroscopy and mass spectroscopy are combined as a two-dimensional technique. 

Figure ID. If a tunable laser is used for secondary excitation of molecular ions, resonance dissociation spectroscopy of molecular cations may be performed. Here excited cationic levels are subject to laser spectroscopy. They serve as intermediate states for the process of resonance enhanced multiphoton dissociation. This is quite similar to resonance ionization spectroscopy of neutrals. The difference is that a dissociation instead of an ionization continuum is finally reached by multiphoton excitation. The advantage of this technique is that it is independent of high ion numbers (as necessary for absorption spectroscopy), fluorescence [necessary for laser-induced fluorescence (LIF)] or predissociation and therefore is fairly general. In addition, mass selectivity is intrinsic and one may benefit from state selective ion formation if resonance multiphoton ionization is used as an ion source.

G. Gerber By applying two-photon ionization spectroscopy with tunable femtosecond laser pulses we recorded the absorption through intermediate resonances in cluster sizes Na with n = 3,. 21. The fragmentation channels and decay pattern vary not only for different cluster sizes but also for different resonances corresponding to a particular size n. This variation of r and the fragmentation channels cannot be explained by collective type processes (jellium model with surface plasmon excitation) but rather require molecular structure type calculations and considerations. 

We present the results of experimental studies of photon-negative ion interactions involving the dynamics of two electrons. Resonances associated with doubly excited states of Li and He" have been observed using laser photodetachment spectroscopy. Total and partial photodetachment cross sections have been investigated. In the former case, the residual atoms are detected irrespective of their excitation state, while in the latter case only those atoms in specific states are detected. This was achieved by the use of a state selective detection scheme based on the resonant ionization of the residual atoms. In addition, in the case of Li-photodetachment, the threshold behavior of the Li(2 P)+e-(ks) partial cross section has been used to accurately measure the electron affinity of Li. 

The following estimation illustrates the possible sensitivity of resonant two-photon ionization spectroscopy (Fig. 1.36a). Let Nk be the density of excited molecules in level Ek, Pki the probability per second that a molecule in level Ek is ionized by photons from laser L2 and /la = Nin aikiS.x (1.34) the number of photons absorbed per second on the transition Ei Ek.li Rk is the total relaxation rate of level Ek, besides the ionization rate (spontaneous transitions plus collision-induced deactivation) the signal rate in counts per second for the absorption path length Ax and for incident laser photons per second under steady state conditions is ... 

In the technique of multiphoton-ionization spectroscopy, two or more photons excite atoms or molecules from the ground state to an excited state which may be ionized by several methods (see Sect.8.2.5), e.g., field ionization, photoionization, collisional, or surface ionization. If the laser is tuned to multiphoton resonances, ionization signals are obtained if the upper level is ionized which can be, for instance, monitored with the setup shown in Fig.8.42. The ionization probe is a thin wire inserted into a pipe containing the atomic vapor. If the probe is negatively biased relative to the walls of the pipe, thermionic emission will lead to space-charge-limited current. Ions produced by the laser excitation partly neutralize the space charge, thereby allowing an increased electron current to flow (see Sect.8.2.4). 

Fundamental Physical Applications of Laser Spectroscopy. - Multiple Photon Dissociation. -New Sub-E)oppler Interaction Techniques. - Highly Excited States, Ionization, and High Intensity Interactions. - Optical Transients. - High Resolutionand Double Resonance. - Laser Spectroscopic Applications. - Laser Sources. - Laser Wavelength Measurements. - Postdeadline Papers. 

Also crucial to our experiment were recent advances in the techniques of optical spectroscopy[13]. We combine the pulsed Ps source with a high power narrowband dye laser[23] capable of partially saturating the highly forbidden two photon 1S-2S transition over a sizable volume of space, the use of two-photon techniques[24] that allow us to avoid the considerable first order Doppler width and to excite all the Ps when the laser is tuned to the atomic resonance, and a single atom, resonant ionization detector [25] with a low background counting rate and 40% quantum efficiency. 

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