Laser-induced photodissociation

Fig. 25. Laser-induced photodissociation of iodine molecules (10 2mol dm 3) in (a) carbon tetrachloride, and (b) hexadecane. The time dependence of the transmission of light at 530 nm is shown ( ) together with error bars. After Chuang et al. [266],...

It was later shown in two independent studies that the relative importance of NbmC met-cars and nanocrystals in the mass spectrum is critically dependent on the experimental conditions. It has been proved that the concentration of hydrocarbon in the carrier gas, the nature of the laser used as the vaporization source, the laser power selected, and, finally, the direet detection of cluster ions or the photoionization of neutral species drastically influence the relative proportion of met-cars to nanocrystals. This selectivity has been attributed to distinct mechanisms of cluster growth for met-cars and nanocrystals (Section 5.9.2.3). Laser-induced photodissociation of 3 X 3 X 3 nanocrystals M14C13+ (M = Ti, V) and larger clusters assumed to have a fee crystal structure has been reported by Pilgrim and Duncan. The titanium carbide cluster corresponding to the 1044-amu peak in the mass spectrum has been assigned to the 3 x 3 x 4 fee fragment in which one tita-... 

In the predissociation problem the coincidence can only be accidental since the potentials and the coupling cannot be modified. For a diatomic molecule submitted to an electromagnetic field, fhe wavelength and the intensity of the field are two external parameters which allow one to produce at will such coincidences. This explains the occurrence of ZWRs in laser-induced photodissociation. Such a flexibility can also be exploited to produce the EPs occurring in this context. 

The occurrence of ZWRs and of EPs in fhe confexf of laser-induced photodissociation can be exploited in various ways. We have shown how ZWRs can be beneficial in purification scenarios. We have also shown that EPs can be exploited in vibrational transfer scenarios. Such scenarios could ultimately be applied for vibrational cooling strategies by depleting all vibrational levels except the ground (v = 0) one. 

Evans and Fixman [274] suggested that the initial distribution of iodine atoms formed in the laser-induced photodissociation of iodine molecules by Chuang et al. [266] might not be unique. Instead, they have considered various combinations of iodine atom separations of 0.27 or... 

Figure 8 Schematic diagram of a triple-quadrupole laser-induced photodissociation mass spectrometer operated in a coaxial configuration. IS is the ion source Q1 is the precursor ion mass filter Q2 is the ion photodissociation region Q3 is the photofragmeni ion mass filter ID is the off-axis ion detector PD is a photon detector BS is a beam splitter and lA is an iodine absorption cell.

Bowers, W. D. Delbert, S. S. McIver, R. T. Consecutive laser-induced photodissociation as a probe of ion structure. Anal. Chem. 1986,58,969-972. 

The advancement of the application of lasers in combination with the molecular beam technique has made a great impact in the understanding of primary photodissociation processes. For state-specific detection of small fragments, laser-induced fluorescence, multiphoton ionization, and coherent laser scattering have provided extremely detailed information on the dynamics of photodissociation. Unfortunately, a large number of interesting... 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

The photolysis of methyl nitrite at low temperature in an argon matrix was studied157. The products include formaldehyde, and nitroxyl HNO which also reacts to form N2O and water. The 355-nm photodissociation of gaseous methyl nitrite has been studied by monitoring the nascent NO product using a two-photon laser-induced fluorescence... 

Figure 1. Photodissociation cross section of CH2CHO (top) and CD2CDO (bottom). Peaks 1—7 are the only features seen in the laser-induced fluorescence spectrum of CH2CHO (Ref. 8).

FIGURE 1. Polarization of laser induced fluorescence of 0H(X n) photodissociated from H2O in the 145-185 nm region. The absorption transition moment is perpendicular to the molecular plane corresponding to the transition A P-j-XlA]. The dissociated 0H(x2n) is also in the molecular plane, since the induced fluorescence intensity of OH is preferentially polarized along the Z axis (25) perpendicular to the molecular plane. The OH radical rotates on the H2O plane XY plane) after dissociation. The unpaired p-orbitals of excited H2O and of dissociated OH are perpendicular to the molecular plane. 

A much clearer picture evolves when one decomposes the total spectrum into the partial photodissociation cross sections a(, n,j) for absorbing a photon with wavelength A and producing NO in a particular vibrational-rotational state with quantum numbers (n,j). Experimentally this is accomplished by measuring so-called photofragment yield spectra. The idea is, in principle, simple the NO product is probed by laser-induced fluorescence (LIF). However, instead of scanning the wavelength Alif of the probe laser (in order to determine the final rotational state distribution) one fixes Alif to a particular transition NO(2n, nj) —>... 

All the techniques already developed for photodissociation dynamics can be used here (laser-induced fluorescence, time resolved spectroscopy, multi photon spectroscopy, etc). 

Supersonic expansions have been used to form small metal aggregates, (2 < n < 4). Emphasis is placed on the analysis of bound-free transitions in these small metal clusters. Discussion focuses on the characterization of variously produced sodium supersonic expansions and the analysis of laser Induced atomic fluorescence resulting from the photodissociation of triatomic sodium clusters. We will consider (1) the nature of observed "fluctuation" bands corresponding to bound-free transitions involving a repulsive excited state which dissociates to yield (Na-D line) sodium atoms and ground state,, ... 

Figure 8. Laser induced atomic fluorescence (LIAF) from the photodissociation of cold" Na, with an Ar ion pumped (coumarin 102) dye laser. Key a, Pi/, b,...

New fusion applications include the concept of production of intense negative ion beams ( ) (for neutral beam injection for heating and diagnostics in tokamaks or other magnetically confined plasmas (26, 28)) by using photodissociation to ion pairs (e.g. NaLi + hVyy Na + Li") in supersonic molecular beams. Another promising concept is the use of laser induced fluorescence to monitor very low tritium concentrations (as little as 10 Tj/cm ) under fusion reactor conditions (29). 

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