Laser-induced multiphoton excitation

Laser-induced multiphoton excitation, 160 Laser sputtering, 158 Li -bound diol complexes, 204 LIF spectroscopy... 

The decay of benzene from the S2 state under collision-free condition has also been studied. J. P. Reilly and co-worker studied the nanosecond UV laser induced multiphoton ionization/fragmentation processes. The rate equation model was used for the simulation and the lifetime of the second excited singlet state was estimated to be 20 ps.19 More recently the... 

Laser-induced multiphoton ionization spectroscopy has been applied to the parent, methyl-, and chloro-substituted pyrazines <85ANC29ll). The ions produced by laser photoionization in the supersonic jet are mass-analyzed using a time of flight mass spectrometer, in which the spectra obtained reflect the absorption of the n-n transition. Mass-resolved excitation spectroscopy by laser ionization in the infrared region has also been applied to the conformational analysis of a-alkyl substituted pyrazines <92JA5269). Dynamic and structural properties of electronically excited states of pyrazine have been elucidated from high resolution laser spectroscopy with MHz resolution <88JST(173)201). 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Fig. 1.14. Multiphoton excitation scheme in Hj showing the laser-induced fluorescence detection method...

Here we present the latest results from our group focused on the design of tailored femtosecond pulses to achieve control of nonlinear optical excitation in large molecules based on the concept of multiphoton intrapulse interference (Mil) [1-4]. Our goal is to elucidate well-defined and reproducible pulse shapes that can be used to enhance or suppress particular nonlinear optical transitions in large molecules such as laser dyes and proteins in solution. We demonstrate the use of Mil to probe the local and microscopic environment of molecules by selective two-photon laser induced fluorescence (LIF). 

We demonstrate the Mil method, which couples the sensitivity of multiphoton excitation on the spectral phase of the laser pulses to probe microscopic chemical environment-induced changes in the multiphoton excitation spectrum of sensitive reporter molecules. We carry out the optimization of the required phase functions in solution and provide theoretical simulations. We show experimental images whereby pH-selective two-photon microscopy is achieved and demonstrate how selective excitation can be used to enhance contrast and, consequently, to achieve functional imaging, using fluorescent probes sensitive to changes in their local environment. 

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 photolytic and probe pulses are colinear when they reach the sample. The photolytic pulse produces excited states and photofragments, and the probe pulse which follows closely behind must be used to analyse the concentration and/or the chemical nature of the transients. The major detection processes are known as laser-induced fluorescence (LIF) and multiphoton ionization (MPI). Transient absorptions can also be used in some cases, and this is similar to ps spectroscopy. 

The detection efficiency of C6H5X (X = F, Cl, Br and I) was investigated with the laser multiphoton ionization method152. The laser-induced ion yield depends mainly on the cross sections of the transitions available to the molecule ground state and on the lifetime of the intermediate electronic state that is initially excited. If a species has a radiative lifetime that is very short compared to the pulse duration, it may relax after excitation and will not be ionized. Molecular ions will therefore be obtained when laser pulses that are at least as short as the excited-state lifetimes are employed. The S excited states of halobenzenes are estimated to have subnanosecond lifetimes, with the exception of fluorobenzene for which a lifetime of the order of 9-10 ns has been calculated at 2ex = 266 nm. Picosecond laser pulses are therefore found effective in producing ionization of halobenzenes with short lifetimes, whereas nanosecond pulses are not152. 

The theory we used has been originally applied to the extensive fragmentation induced in polyatomic molecules upon laser multiphoton excitation and was more recently applied to the fragmentation of Ceo. 

The laser-induced breakdowns are caused by three consequent mechanisms [22] (i) tiie excitation of electrons in tiie conduction band by impact and multiphoton ionization (MPI), (ii) radiation-induced heating of tiie conduction-band electrons, and (iii) transfer of the plasma energy to the lattice. The key benefit of ultia-short femtosecond laser pulses lies in their ability to deposit energy in materials in a very short time interval. Heat diffusion is frozen during the interaction and the shock-like energy deposition leads to ablation for ultra-short pulses. This is because tiie pulse... 

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