Stepwise laser excitation

Fig. 16.5 (a) Schematic diagram of the experimental setup, (b) Energy diagram of the Na atom showing the levels populated by the stepwise laser excitation and the microwave transitions, (c) Sketch of the sequence of events experienced by the Na atoms (from... 

Stepwise laser excitation and ionization techinques can be used to determine lifetimes of excited levels in atoms. Oscillator strengths or transition... 

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) ... 

Figure 9. (a) Field ionization-rf experimental arrangement (b) energy levels showing stepwise laser excitation and microwave transition (c) the sequence of events in the experiment for two atoms excited by the laser excitation to level n. One of the atoms undergoes the microwave transition to n + 1 and ionizes at time The atom remaining in n ionizes at... 

Stepwise laser excitation. Optical double resonance. ... 

Ackerhalt, J. R., and Eberly, J. H. (1976). Coherence versus incoherence in stepwise laser excitation of atoms and molecules. Physical Review A, 14, 1705-1710. 

In this contribution we demonstrate vibrationally induced molecular dissociation in two polyatomic molecules, namely chromium hexacarbonyl and diazomethane (Fig. 1), employing fs mid-infrared laser sources [1,2]. As will be shown below, in both cases the initially excited mode does not lead directly to reaction but provides the doorway to access the right combination of modes. Thus both reactions have three steps (1) Stepwise multiphoton excitation of the doorway mode, (2) sharing of energy with other modes, and (3) formation of... 

The laser excitation prepares the atoms in the lowest energy Stark state with m =2 in a chosen n-manifold (n=24 for example) (see Fig. 1-b). This excitation is itself a stepwise process involving three pulsed dye laser beams in resonance with the 2S-2P, 2P-3D and 3D-n, m =2, nx=0 transitions... 

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... 

As described previously, (4-benzoylphenoxy)methylnaphthalene exhibited C—O bond dissociation via the T state. The multiple laser excitation technique allows us to study stepwise cleavage of two equivalent bonds in a molecule. To provide clear evidence for the stepwise photocleavage of two equivalent bonds in a molecule, we introduce here two C—O bond cleavages of l,8-bis[ (4-benzoylphenoxy)methyl]naphthalene (l,8-(BPO-CH2)2Np) to give acenaph-thene with the three-step excitation using three-color three-laser flash photolysis (Scheme 2.14) [147],... 

K. Teuchner, J. Ehlert, W. Freyer, D. Leupold, P. Altmeyer, M. Stilcker, K. Hoffmann, Fluorescence studies of melanin by stepwise two-photon femtosecond laser excitation, J. Fluoresc. 10, 275-282 (2000)... 

Quite often, monochromatic laser light is applied under conditions of high photon flux, to excite the species of interest efficiently. Under such circumstances, a process that utilizes -photon resonance in the first excitation step and requires additional m photons for final ionization (most frequently m = 1 is encountered) occurs with measurable probability and is of analytical relevance. The technique is called REMPI and uses, as noted, stepwise resonant excitation of an atom or molecule via stable intermediate energy levels. It is usually described as (n- -m)-REMPI spectroscopy the most frequently used (2 -b 1)-REMPI principle is depicted in Figure 5.4. 

The fluorescence induced by the second laser allows the accurate determination of the molecular parameters for those excited states on which the fluorescence transitions from are terminating. The LIF method can therefore be extended by stepwise excitation to the investigation of many molecular states which may not even have been found before. Of particular interest are dissociating excited states with repulsive potential curves below bound states E. These continuous states often cannot be studied by direct absorption from the ground state because the Franck-Condon factors for the transitions may be quite small. As an example of such investigations we mention the two-step excitation of the iodine molecule I2 (Fig.8.29). Selected (V, J ) levels in the B rig state are populated by optical pumping with a cw dye laser. Starting from these levels a krypton laser excites further levels in a... 

A large number of atoms and molecules have been meanwhile investigated by level-crossing spectroscopy using laser excitation. Because of the available high laser intensity highly excited states can be studied, which have been populated by stepwise excitation (see Sect.8.8). Often resonance lamps are used to excite the first resonance level and a dye laser pumps the next step. Many experiments have been performed with two different dye lasers, either in a pulsed or a cw mode [10.91]. These techniques allow measurement of... 

This also causes a reduction in the observed gain since the population of the lower laser level is increased by the combined effects of stepwise electron excitation and radiation trapping of the 2p Is neon transitions. However, this neon metastable bottleneck is certainly not the main effect limiting the output power of He-Ne lasers. 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

A) During the luultiphoton excitation of molecular vibrations witli IR lasers, many (typically 10-50) photons are absorbed in a quasi-resonant stepwise process until the absorbed energy is suflFicient to initiate a unimolecular reaction, dissociation, or isomerization, usually in the electronic ground state. 

The NIR femtosecond laser microscope realized higher order multi photon excitation for aromatic compounds interferometric autocorrelation detection of the fluorescence from the microcrystals of the aromatic molecules confirmed that their excited states were produced not via stepwise multiphoton absorption but by simultaneous absorption of several photons. The microscope enabled us to obtain three-dimensional multiphoton fluorescence images with higher spatial resolution than that limited by the diffraction theory for one-photon excitation. 

Discussion of Photoelectron and Photofragment Images. The simplest picture for photoexcitation of a molecular Rydberg state would be that of a vertical transition (Av = 0), producing only O2, X(2Ilg)(t = 2) (direct ionization) in the example case. Here electronic motion (ionization) is assumed to be much faster than nuclear motion (dissociation). 02 is much more complicated, of course, and some of the deviations from the simplistic picture could be due not only to the molecule but also to the unconventional three-photon preparation scheme. It is thus important to consider the differences in one-photon and stepwise (2 + 1) excitation. Even with direct one-photon excitation at the energy equivalent of three laser photons, it is known, [78] for example, that the quantum yield for ionization is only 0.5 the other half of the molecules do, in fact, dissociate. 

The vertical electron affinity (EA) of acetone is given as —1.51 eV by Jordan and Burrow386. Lifshitz, Wu and Tiernan387 determine—among other compounds—the excitation function and rate constants of the slow proton transfer reactions between acclone-Ih, acetone-Dg and other ketones. The acetone enolate anion has been produced in a CO2 laser induced alkane elimination from alkoxide anions by Brauman and collaborators388-390. These show, e.g. that the methane elimination from t-butoxide anion is a stepwise process ... 

Figure 1. Optogalvanic signal for stepwise excitation of sodium (3s - 3p — nd, ns) in an H,-air flame. Each transition is split into two components by the fast mixing of the fine structure states, 3p,/t — 3pi/t. The data are not normalized for the variation of laser power with wavelength. At this level of sensitivity the one-photon signal (3s —> 3p) is undetectable.

Figure 2. Comparison of the- stepwise excitation results (O) with the model calculation ( ). The enhancement (the two-photon signal divided by the one-photon signal) normalized for laser energy is plotted against the absorption coefficient for the 3p -> nd transitions. For visual clarity a curve is drawn through the points of the model calculation and a dashed line of unit slope is drawn through the data at high principal quantum number, n.

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