Laser induced molecular formation

Time-dependent laser-induced molecular formation from repulsive surfaces [49, 52]... 

Th. Mercouris, l.D. Petsalakis, C.A. Nicolaides, Time-dependent laser-induced molecular formation from repulsive surfaces, Chem. Phys. Lett. 208 (1993) 197. 

The dependence of the dissociation constant kj) and the recombination constants kjt in the reaction (a) on the optical polarization of the atoms A was predicted by Bernheim [65] and Kastler [225] and later demonstrated experimentally [7, 363]. The dependence can be understood from the obvious fact that only hydrogen-like atoms with opposite electronic spins may recombine and form a molecule A2(X1E+). Hence we have kR = kft(l — S2), where S is the degree of electron spin polarization of the atoms. A convenient indicator of dimer formation is provided by the kinetics of the laser-induced molecular fluorescence after switching on magnetic resonance which destroys the polarization of the atoms, as performed by Huber and Weber [201] for a Na — Na2 mixture. The se-... 

Greenwood, RF, Strachan, M.G., Willett, G.D., Wilson, M.A. (1990) Laser-induced molecular ion formation and gas-phase ion formation from polycyclic aromatic hydrocarbons, coals and coal products detected by Fourier transform mass spectrometry. Organic Mass Spectrometry, 25, 353-362. 

Abstract—"TYm origins and the dynamics of optical nonlinearities in nematic liquid crystal films, namely, laser-induced molecular reorien-tational and thermal refractive index changes, are analyzed in the context of optical wave mixings. Theoretical expressions for the basic non-linearities, the rise and decay time, diffraction efficiencies, and other pertinent parameters involved in the dynamic grating formation are derived. Experimental results obtained with visible and infrared laser pulses are analyzed. Some newly observed novel nonlinear processes are also reported. 

Matsuzaki, A. (1994).Nonlinear Dynamics in Laser-Induced Aerosol Formation from Vaporized CS2 Period Fluctuation in Aerosol Burst. Journal of Molecular Structure (Theochem), Vol.310, (July 1994), pp. 83-93, ISSN 0166-1280... 

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... 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Fiorini, C., Prudhomme, N., de Veyrac, G., Maurin, I., Raimond, P., and Nunzi, J. M. Molecular migration mechanism for laser induced surface relief grating formation. Synthetic Met, 2000, 115, pp. 121-125. 

The technique based on laser-induced breakdown coupled to mass detection, which should thus be designated LIB-MS, is better known as laser plasma ionization mass spectrometry (LI-MS). The earliest uses of the laser-mass spectrometry couple were reported in the late 1960s. Early work included the vaporization of graphite and coal for classifying coals, elemental analyses in metals, isotope ratio measurements and pyrolysis [192]. Later work extended these methods to biological samples, the development of the laser microprobe mass spectrometer, the formation of molecular ions from non-voIatile organic salts and the many multi-photon techniques designed for (mainly) molecular analysis [192]. 

Other techniques utilize lasers for sample evaporation/pyrolysis and excitation such as laser induced desorption (LID) or laser microprobe mass analysis (LAMMA) (see e g. [1]). Some of the sample introduction procedures in Py-MS enhance the information obtained from Py-MS by the use of time-resolved, temperature-resolved, or modulated molecular beams techniques [10]. In time-resolved procedures, the signal of the MS is recorded in time, and the continuous formation of fragments can be recorded. Temperature-resolved Py-MS allows a separation and ionization of the sample from a platinum/rhodium filament inside the ionization chamber of the mass spectrometer based on a gradual temperature increase [11]. The technique can be used either for polymer or for additives analysis. Attempts to improve selectivity in Py-MS also were done by using a membrane interface between the pyrolyzer and MS [12]. 

As was already suggested in the introduction, once the mechanism as well as the experimental parameters of laser induced multiple photon ionization have been investigated these ions may be used in spectroscopic and ion-molecule interaction studies. For two cases such an application has now been demonstrated in this laboratory by Proch, Trickl and Sha /II,12/. For practical experimental reasons the two diatomic molecules N and CO were chosen to address questions of cold- eam formation and E to E energy transfer. Nitrogen molecular ions N X were made in selected vibrational states by going through the folfowii sequence of excitation steps. 

Laser-induced fluorescence polarization (LIFP) has been used as a detection technique in immunocapillary electrophoresis analysis of haptens. Polarization of fluorescence depends on the molecular size of the molecule. Small molecules exhibit low fluorescence polarization. LIFP was used to distinguish the peak of the fluorescent hapten-Ab complex from the peak of the excess of free fluorescent hapten in competitive assays [124]. Besides, the dependence of LIFP on molecular size has made it possible to perform quantitation without separating free from bound labeled hapten [125]. An attempt to use LIFP to quantitate staphylococcal enterotoxin A (28 kDa) did not show an increase of polarization upon formation of the Ag-Ab complex due to the high molecular weight of the Ag [126]. Formation of the complex could be identified by measuring the LIFP peak while varying the Ag/Ab ratio. 

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.

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