Laser Argon-ionized

The principles of ion sources which use a primary ion beam for sputtering of solid material on sample surface in a high vacuum ion source of a secondary ion mass spectrometer or a sputtered neutral mass spectrometer are shown in Figure 2.30a and Figure 2.30b, respectively. Whereas in SIMS the positive or negative secondary ions formed after primary ion bombardment are analyzed, in SNMS the secondary sputtered ions are suppressed by a repeUer voltage and the sputtered neutrals which are post-ionized either in an argon plasma ( plasma SNMS ), by electron impact ionization ( e-beam SNMS ) or laser post-ionization are nsed for the surface analysis (for details of the ionization mechanisms see references 122-124). 

In the argon-ion laser, argon gas at about 1 torr is ionized by an electric discharge to produce Af and Ar, These ions can then iiiulcrgo laser transitions to lower... 

Laser ablation can be used to volatilize samples. An intense laser pulse is focused onto a solid at sufficient pulse power and energy (e.g., millijoule energy in a pulse of 10 nanosecond, or less, duration) to remove material from the surface. Typical conditions result in crater formation after one or a few laser pulses. If performed in flowing argon at atmospheric pressure, the ablated material can be fed into an ICP for ionization. Ions may also be formed by an ablation pulse in a vacuum and directly focused and extracted into a mass analyzer. Laser ablation/ionization is used primarily for analysis of solid compounds and materials. Virtually the entire periodic table can be analyzed with this method. 

Instead of measuring the fluorescence intensity /fi(Al), the excitation spectrum can also be monitored via resonant two-photon ionization (RTPI). This is illustrated by Fig. 9.7, which shows a RTPI spectrum of a band head of the Cs2 molecule, excited by a tunable cw dye laser and ionized by a cw argon laser [9.7] using the arrangement of Fig. 9.5. 

Krypton lasers are also ionized gas lasers and are very similar in general characteristics to argon lasers (27). Krypton lasers having total multiline output up to 16 W are available commercially. The strongest line at 0.6471 p.m is notable because it is in the red portion of the spectmm, and thus makes the krypton laser useful for appHcations such as display and entertainment. 

Resonance ionization methods (RIMS) have also been explored for improving Th ionization efficiency for mass spectrometric measurement (Johnson and Fearey 1993). As shown in Figure 3, two lasers are required, a continuous resonant dye laser for resonance of thorium atoms, and a continuous UV argon laser for transition from resonance to ionization. Consequently, sophisticated laser instrumentation is required for these methods. 

Gunther D, Heimich CA (1999) Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier. J Anal At Spectrom 14 1363-1368 Habfast K (1998) Fractionation correction and multiple collectors in thermal ionization isotope ratio mass spectrometry. Inti J Mass Spectrom 176 133-148... 

In order to record excitation spectra, the radical ions must first be thermalized to the electronic ground state, which happens automatically if they are created in condensed phase (e.g. in noble-gas matrices, see below). In the gas-phase experiments where ionization is effected by collision with excited argon atoms (Penning ionization), the unexcited argon atoms serve as a heat bath which may even be cooled to 77 K if desired. After thermalization, excitation spectra may be obtained by laser-induced fluorescence. 

Resonance ionization spectroscopy is a photophysical process in which one electron can be removed from each of the atoms of a selected type. Since the saturated RIS process can be carried out with a pulsed laser beam, the method has both time and space resolution along with excellent (spectroscopic) selectivity. In a recent article [2] we showed, for example, that all of the elements except helium, neon, argon, and fluorine can be detected with the RIS technique. However, with commercial lasers, improved in the last year, argon and fluorine can be added to the RIS periodic table (see figure 2). 

Argon (Ar), 17 343. See also ArF laser bulk quantities of, 17 363 commercial distribution of, 17 362-363 cryogenic shipping, 8 40 doubly ionized, 14 684—685 economic aspects of, 17 365-366 electrostatic properties of, 1 621t in ethylene oxidation, 10 651 gas bulk separation, l 618t high purity, 13 460, 468 in light sources, 17 371-372 liquefaction, 8 40... 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

In contrast, the LA-ICP-MS (in comparison to laser ionization mass spectrometry (LIMS) where the ion source operates under high vacuum conditions) at present, in spite of the disadvantage of a higher polyatomic ion formation rate, uses an argon plasma ionization at normal pressure - a promising inorganic mass spectrometric technique for trace, isotope and surface analysis which will... 

Calibration and quantification procedures are easier in LA-ICP-MS compared to other solid-state mass spectrometric techniques because the laser ablation and the ICP ion source operate at normal pressure and the laser ablation of solid samples and ionization of analytes are separated in space and time. Therefore the advantage of solution calibration in ICP-MS can be applied in this solid-state analytical technique. The introduction of solution based calibration, which is only possible in LA-ICP-MS, was an innovative step in the development of this sensitive mass spectrometric technique. A number of different calibration approaches using aqueous standard solutions in the dual gas flow technique have been discussed by various authors.74 75 In the dual gas flow injection technique , the nebulized standard solution and the laser ablated sample material are mixed in the -piece and the two gas flows from the nebulizer (e.g. ultrasonic nebulizer) and laser ablation chamber are added. Using solution based calibration with the addition of a standard solution, Leach et alP determined minor elements in steel reference materials with a relative accuracy of a few %. In comparison to the so-called dual gas flow technique proposed in the literature, where the argon flow rates through the nebulizer and ablation cell add up to 11 min-1 (e.g. 0.451 min-1 and... 

The laboratory layout consists of a molecular beam apparatus and a laser system. NaK clusters are created in an adiabatic coexpansion of mixed alkali vapour and argon carrier gas through a nozzle of 70 pm diameter into the vacuum. Directly after the nozzle the cluster beam passes a skimmer. Next, the laser beam coming from perpendicular direction irradiates the dimers and eventually excites and ionizes them. The emerging ions are extracted by ion optics, mass selected by QMS and recorded by a computer. 

Fig. 4.7. The two left-hand panels Comparison of the double-ionization correlation densities (4.20) without (left-hand column panels (a), (c), and (e)) and with (right-hand column panels (b), (d), and (/)) electron-electron repulsion in the final state. The interaction Vi2 is specified by the three-body contact interaction (4.14b). Parameters are for argon ( 01 = 0.58a.u., E02 = 1.015a.u.), the laser frequency is u = 0.057a.u. (Ti Sa). Panels (a) and (b) I = 2.5 x 1014Wcm 2 (Up = 0.54a.u.), Pij > 0.5a.u. [35] (c) and (d) as before, but with pXJJ < 0.5a.u. (e) and (/) I = 4.7 x 1014 Wcm 2 (Up = 1.0 a.u.), p1 or p2 < 0.1 a.u. [39]. The two right-hand panels same as the left-hand panels, but with V 2 specified by the Coulomb interaction (4.14a). From [18]...

Argon ion laser A CW or pulsed laser emitting hnes from 334 to 529 run from singly ionized argon. Principal emissions are at 488.0 and 514.5 nm. 

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