Lasers molecular vapors

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

Laser molecular beam epitaxy, fabrication method for inorganic materials, 7 415t Laser photochemical vapor deposition (LPCVD), 19114-116 Laser pointers, 14 678 Laser-promoted dehydrohalogenation,... 

In 1985 chemists at Rice University in Texas used a high-powered laser to vaporize graphite in an effort to create unusual molecules believed to exist in interstellar space. Mass spectrometry revealed that one of the products was an unknown species with the formula Ceo- Because of its size and the fact that it is pure carbon, this molecule has an exotic shape, which the researchers worked out using paper, scissors, and tape. Subsequent spectroscopic and X-ray measurements confirmed that Ceo is shaped like a hollow sphere with a carbon atom at each of the 60 vertices. Geometrically, buckyball (short for buckminsterfullerene ) is the most symmetrical molecule known. In spite of its unique features, however, its bonding scheme is straightforward. Each carbon is xp -hybridized, and there are extensive delocalized molecular orbitals over the entire structure. 

Another approach to gas laser action is to use f-f transitions of optically-excited lanthanide molecular vapors. 

Gain was measured for the F3/2" Il]/2 transition from one molecular vapor, a NdCl3-A1Cl3 complex (16.). Intense excited state-excited state quenching and low vapor pressures limit the attractiveness of this lasing medium. The excited-state kinetics for Nd(thd)3 chelate vapors have also been investigated and the prospects for laser action discussed (62). 

For liquids and rare earth molecular vapors, high-frequency molecular vibrations are present. These lead to strong nonradiative deactivation of excited states and, as discussed in the sections on liquid and gas lasers, are a determining factor for achieving stimulated emission. 

While laser action in rare earth molecular vapors has not been achieved, Krupke (1976) has examined cases where fluorescence of high quantum... 

Spectral and kinetic studies of the Tb-Al-Cl complex by Hessler et al. (1977) and Krupke and Jacobs (1977) indicate that the fluorescence properties of the D4 state of Tb " " in this molecular vapor also satisfy the dynamical requirements for achieving optical gain. Rare-earth vapors are presently being considered as the amplifying medium in large lasers for inertial confinement fusion experiments. 

Hastie, J.W. Bonnell, D.W. Schenck, P.K. Molecular Basis for Laser-Induced Vaporization of Refractory Materials NBSIR 84-2983 National Technical Information Service Washington, DC., 1984. 

The fonnation of clusters in the gas phase involves condensation of the vapour of the constituents, with the exception of the electrospray source [6], where ion-solvent clusters are produced directly from a liquid solution. For rare gas or molecular clusters, supersonic beams are used to initiate cluster fonnation. For nonvolatile materials, the vapours can be produced in one of several ways including laser vaporization, thennal evaporation and sputtering. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

Quite apart from the fullerene cluster molecules, numerous other molecular allotropes of carbon, C , have been discovered in the gases formed by the laser vaporization/supersonic expansion of graphite. The products are detected by mass... 

The molecular weights and molecular weight distributions (MWD) of phenolic oligomers have been evaluated using gel permeation chromatography (GPC),23,24 NMR spectroscopy,25 vapor pressure osmometry (VPO),26 intrinsic viscosity,27 and more recently matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).28... 

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