Laser selective

Zewail A H 1980 Laser selective chemistry—is it possible Phys. Today Nov, 27-33... 

Laser Selective Chemistry Is It Possible A. H. Zewail, Phys. Today 33, 27 (1980). 

R. D. Levine As emphasized by the speakers on femtosecond pumping schemes, an important point is that the initial excitation is localized within the Franck-Condon regime. The question is whether the sheer localization can be used to advantage to induce laser-selective chemistry (K. L. Kompa and R. D. Levine, Acc. Chem. Res. 27, 91 (1994)]. As we understand better the topography of potential-energy surfaces for polyatomic molecules, it may be possible to launch the system with such initial conditions that it will, of its own accord, proceed to cross a particular transition state and so exit toward a particular set of products. 

The TOF spectrum of the Cl or Br atom is measured as a function of wavelength, with the polarization of the laser selected so that only TTq is excited. The peaks that are observed in the spectrum can be identified with atoms in either the 3/2 Dr pl/2 state> so that the experimental data can be used to determine the probability, P, for the diabatic crossing. The Landau-Zener formula for this probability is given by... 

These equations have been analyzed in some detail by Lucht and Laurendeau (2) and Chan and Daily (3). When the laser is turned on, molecules in the state are pumped to the E state. Since the laser selectively pumps from one rotational sublevel to another, the other rotational sublevels in the E state can be populated only by rotational relaxation. If quenching, or electronic de-excitation, is fast compared to rotational relaxation, then only the laser coupled state will be populated. On the other hand, if rotational relaxation is fast then all the rotational states will be populated and in Boltzmann equilibrium. 

With this understanding, the spectra of Eu could probe the microstructure of nanomaterials, because the Dq to Fi transition was very sensitive to the site symmetry. Yan et al. used laser selective excitation technique to research the high-resolution spectra of f-YV04 Eu NPs (Yan et al., 2003a). When the Eu dopants approach the NPs surface, the D2d symmetry is broken. In emission spectra this is reflected by several points the increased number of emission lines the shortened luminescent lifetime, the enhanced splitting of energy levels, and the broadened emission peaks. 

Crystalline silicon is the most widely used semiconductor material today, with a maiket share of above 90%. Because of its indirect electronic band structure, however, the material is not able to emit light effectively and therefore carmot be used for key applications like light-emitting diodes or lasers. Selected one- or two-dimensional silicon compounds like linear or branched polysilylenes [1] or layered structures like siloxene [2], however, possess a direct band gap and therefore exhibit intense visible photoluminescence. Siloxene, a solid-state polymer with a sheet-like layered structure and an empirical formula Si H (OH) , in particular, is considered as an alternative material for Si-based liuninescent devices. Detailed studies of stmctural and photophysical properties of the material, however, are strraigly impeded by its insolubility in organic solvents. 

A pulse laser is another excitation source for generating various reactive intermediates efficiently. In the case of pulse lasers, selective excitation of the ground and excited states is easy because laser pulses with various wavelengths can be obtained by harmonic generation with nonlinear crystals. Recent development of a laser utilizing optical parametric oscillation, which emits a variable wavelength, enlarged the scope of study. Thus, two-color two-laser flash photolysis has been adapted to a wide variety of fields [3]. Furthermore, utilization of... 

Other hand, the potential success of laser selective chemistry depends on the existence of metastable vibrational energy distributions. Structural information about the nature and effects of the weak bonding interaction are obtained from spectral shifts, intensities and, in favorable cases, rotational structure in the photodissociation spectra. Rotational structure, vibrational frequencies and band intensities are all influenced by the intramolecular potential. 

A popular technique is that of light (or photon) assisted OMVPE. Typically, a mercury lamp may be used to provide light at 185 and 254 nm, or a laser tuned to a specific wavelength may be used. In the case of UV assist, either the UV is used to assist with reactant decomposition — at or above the deposition plane, or the light is focused onto the substrate to promote reactivity and surface mobility. A laser, selectively tuned to a specific molecular transition, may be employed to promote molecules to an excited... 

The laser-scanning device described above appeared to accurately measure the surface failure topography of a variety of failed block-shear specimens. The laser selected was sensitive to veiy small displacements and appeared to adequately resolve a variety of surface irregularities. Also, the incremental scan pattern or beam grid used to map the failure surface was sufficient to reconstruct a digital contour of that surface. 

Another key consideration in the design of trapping MS combined with optical spectroscopy experiments is the choice of light source. Here, we limit the discussion to that convenient modem light-source, the laser, which emits typically light that is highly directional and monochromatic (except for ultrashort pulses that can exhibit spectral widths of several dozens of nanometers). Laser selection criteria include... 

If the laser bandwidth is small enough to resolve the hyperfine structure, a specific hfs component can be selectively populated. This can result in a nuclear spin orientation as illustrated in Fig. 5.5 for the Na atom with a nuclear spin 7 = 3/2. The laser selectively populates the F = 2 component in the upper 2 P /2 state. The fluorescence from this state terminates at both the F" = 1 and F" = 2 components in the 3 5i/2 state. The F = 1 component is again excited into the 3 P f2 state. 

Here, the stepwise excitation is, in particular, very helpful since only those Rydberg levels are excited that are connected by allowed electric dipole transitions to the known intermediate level, populated by the pump. This shall be illustrated by the example of a relatively simple molecule, the lithium dimer Li2 [581, 582]. When the pump laser selectively populates a level in the B Flu state, all... 

Here, the stepwise excitation is, in particular, very helpful since only those Rydberg levels are excited that are connected by allowed electric dipole transitions to the known intermediate level, populated by the pump. This shall be illustrated by the example of a relatively simple molecule, the lithium dimer Li2 [10.71,10.72]. When the pump laser selectively populates a level Jj ) in the Ylu state, all levels (u, 7 = 7 dz 1 or 7 = 7p in the Rydberg states ns E with = 0 and nd Delta, nd Tl, or nd S with = 2 and X = 2, 1, 0 are accessible by probe laser transitions with A7 = +1 (R-lines), A7 = 0 (Q-lines), or A7 = — 1 (P-lines). This is shown in (Fig. 10.25), where all possible transitions from the two A components of the B Ylu state with parity — 1 and +1 into the different Rydberg states are compiled. 

Fig. 6 Single pulse of a mode-locked iodine laser selected by an optical switch time scale 1 ns/div, measured with the Thomson CSF-TSN 660 4 GHz oscilloscope.

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