Ultraviolet laser pulse

The ozone is photolyzed by a 266-nm (near ultraviolet) laser pulse. The fixed pump frequency co for CARS detection of the nascent O2 molecules is provided... 

In Laser Ionization Mass Spectrometry (LIMS, also LAMMA, LAMMS, and LIMA), a vacuum-compatible solid sample is irradiated with short pulses ("10 ns) of ultraviolet laser light. The laser pulse vaporizes a microvolume of material, and a fraction of the vaporized species are ionized and accelerated into a time-of-flight mass spectrometer which measures the signal intensity of the mass-separated ions. The instrument acquires a complete mass spectrum, typically covering the range 0— 250 atomic mass units (amu), with each laser pulse. A survey analysis of the material is performed in this way. The relative intensities of the signals can be converted to concentrations with the use of appropriate standards, and quantitative or semi-quantitative analyses are possible with the use of such standards. 

MPI is especially valuable for elemental analyses with typical useful yield of 10 . Because SALI is laser-based, expected improvements over the next few years, in particular for vacuum-ultraviolet laser technology, should have a significant impact. High repetition rate Nd—YAG systems with sufficient pulse energy are already available to 50 Hz, and probably can be extended to a few hundred Hz. 

FIGURE 15.2 Common protein ionization methods used for MS-based proteomics. Two common ionization technologies are currently available for protein analysis. Top ESI volatilizes and ionizes peptides and proteins in solution. Bottom MALDI uses analytes that are co-crystallized in a matrix composed of organic acid on a solid support. A pulse of ultraviolet laser evaporates the matrix and analyte into gas phase, resulting in generation of single charge ions. 

The output from a ruby giant-pulse laser (2 Joule, 30 nsec half-width, = 6943A) passes a KH2PO4 crystal where, due to the nonlinear characteristics of this material, the second harmonic at X = 3471 A is generated with an efficiency of 3 %.The two wavelengths are separated by means of a water filled quartz prism. The ultraviolet light pulse serves as pump pulse. 

HOD. A 722.5 nm laser pulse (A,i) excites the third overtone stretch of OH. After a short delay, a pulse of ultraviolet radiation of frequency V2 (wavelength X2) dissociates the molecule, and a third pulse with a wavelength near 308 nm (X3) probes the OH or OD fragments by laser-induced fluorescence. It is observed that with a dissociation wavelength of 266 or 239.5 nm, the products are almost exclusively H + OD,... 

For the flash photolysis experiments, the ultraviolet pulse (20 ps FWHM) was delivered to the samples via four linear Xenon flashtubes surrounding the sample cell. The lamps were fired from the laser console through a variable delay to provide the desired time delay from the flash peak to the laser pulses. 

Finally we note that studies of control in solution [186, 187] indicate that control in the presence of collisional effects is indeed possible. For example, coherent control of the dynamics of I3 in ethanol and acetonitrile has been demonstrated. Specifically, I3 was excited with a 30-fs ultraviolet (UV) laser pulse to the first excited state, The resultant wave function was comprised of a localized wave function on the ground electronic state and a corresponding depletion of wave function density, that is, a hole, on the ground electronic state. In this instance the target of the control was the nature of the spectrum associated with the coherences associated with the symmetric stretch. By manipulating various attributes of the exciting pulse (intensity, frequency, and chirp of the excitation pulse), aspects of the spectrum were controlled, despite the decoherence associated with collision effects. 

Reaction selectivity is observed on laser-induced desorption and dissociation of NO and CO chemisorbed on Ni, Pd, and Pt surfaces via the electronic transition using visible and ultraviolet nanosecond-pulsed lasers, as listed in Table 6. The open circle shows that desorption and dissociation have been observed, while the cross mark means that they have not been observed. These metals are isoelectronic and the band structure is very similar, but the activity on laser-induced desorption and dissociation is remarkably different. The origin of the different desorption activity between Pt and other transition metals of Ni and Pd may be closely related with the nature of the antibonding 2-ira state in adsorbed NO and CO [11]. 

In a sample containing a mixture of compounds, individual species may be resonance enhanced at different wavelengths. In some cases it may be possible to measure resonance Raman spectra from individual components in a mixture by selective excitation of specific absorption bands. Moreover, the assignment of resonance-enhanced vibrations provides detailed information about the local symmetry of the species. RRS has been used widely to characterize biological samples in which electronic transitions occur at visible excitation wavelengths, and commercial continuous wave lasers are readily available. Resonance Raman spectroscopic characterization of solid catalysts and adsorbed species has seen limited application. Many catalytic materials are white, but their electronic transitions often occur at ultraviolet wavelengths. With the availability of continuous wave and tunable, pulsed ultraviolet laser sources, we anticipate the application of RRS to catalysts will increase substantially. This expectation has motivated the present review. 

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