Ar and Kr Ion Lasers

The last line of Table 7.2 indicates the total optical output power for each ion laser, in terms of multiline visible power. However, this power is distributed among many wavelengths and is not useful directly for Raman spectroscopy. Multiline output is often used to pump dye lasers or titanium sapphire lasers, but these cases are fairly rare in analytical applications. Most often, a prism is added to the laser cavity to select one of the wavelengths listed in Table 7.2. As apparent in the table, Ar+ and Kr+ have a few strong lines that are popular for Raman (e.g., 488, 514.5, and 647.1 nm) plus several more at lower power. The mixed-gas Ar+/Kr+ laser provides less power but covers a wider range of visible wavelengths than Ar+ or Kr+ alone. 

The air-cooled Ar+ system avoids the need for water cooling but with a substantial reduction in power. Most air-cooled ion lasers operate on 110 V 

Ultraviolet output is available from both Ar and Kr+ lasers, sometimes with relatively minor modification. For a conventional large-frame Ar+ laser, the 

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A comparison between the efficiency of excitation with lasers and mercury lamps has been undertaken by Evans etal. and Brandmuller etal. Since that time, lasers have improved considerably and a later comparison would be even more in favor of laser applications. Since several commercial laser Raman spectrometers are now available 190 ), with He-Ne lasers, Ar" or Kr -ion lasers and neodymium lasers, most current investigations employ lasers as light sources, j)... 

Figure 2-3 Output powers and wavelengths obtainable from a Spectra Physics Model 375 dye laser pumped with Ar-ion and Kr-ion lasers. (Reproduced with permission.)...

Laser action occurs in the noble gas ions Ne, Ar Kr and Xe but that in Ar and Kr produces the most useful lasers. 

The mby fluorescence emission is induced by laser excitation and can be revealed through a monochromator and a CCD detector. The wavelength and the power of the laser excitation are not restrictive at low pressure, and even few milliwatts of the 647.1-nm excitation line of a Kr ion laser can induce an easily detectable fluorescence emission. Any lower wavelength can be used as well. Typical exciting laser fines used are the 488- and 514.5-nm emissions of an Ar ion laser. Things are more complicated at pressures of 100 GPa, where the mby signal decreases in intensity and the two components are unresolved [235, 248-251]. Recently, it has been demonstrated by means of x-ray diffraction that... 

These ion lasers are very inefficient, partly because energy is required first to ionize the atom and then to produce the population inversion. This inefficiency leads to a serious problem of heat dissipation, which is partly solved by using a plasma tube, in which a low-voltage high-current discharge is created in the Ar or Kr gas, made from beryllium oxide, BeO, which is an efficient heat conductor. Water cooling of the tube is also necessary. 

This works only for a dilute concentrations of the atoms, to prevent the absorption of the photons into the gas in the form of heat due to atom-atom collisions. Only certain atoms and ions have optical transitions amenable to laser cooling, since it is extremely difficult to generate the amounts of laser power needed at wavelengths much shorter than 300 nm. The following is a partial list of atoms that have been laser-cooled H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Cr, Er, Fe, Cd, Ag, Hg (plus metastable Al, Yb, He, Ne, Ar, Kr), and some ions. 

Gas Lasers. A variety of ga.s lasers is available commercially. These devices are of four types (1) neutral atom lasers such as He-Ne (2) ion lasers in which the aciive species is Ar or Kr (3) molecular lasers in which the lasing medium Is CO, or Nil and (4) excimer lasers. The helium-neon laser is the most widely encountered of all lasers because of its low initial and maintenance costs, its great reliability, and its Kivv H)wcr consumption. The nuist important of its output lines is at 6.32.8 nm. It is generally operated in a continuous mode rather than a pulsed mode. 

Applications of multiwavelength TCSPC to laser scanning microscopy have been demonstrated in [35, 60]. Spectrally resolved detection in diffuse optical tomography is described in [23]. A multianode MCP PMT and an SPC-330 TCSPC module were used to resolve the luminescence of alkali halides under N, Ar, Kr, and Xe ion irradiation [266]. 

At the opposite end of the spectrum, UV sources for CE-LIF are becoming increasingly popular. UV radiation is capable of inducing fluorescence in many intrinsic fluorophores, including a number of biologically relevant molecules. The frequency-doubled Ar-ion laser (257 nm) was one of the first examples reported of UV-excitation for CE-LIF, and yielded improvements in LODs for a number of substances, such as conalbumin (1.4 x 10 M). As another example, a frequency-doubled Kr laser operating at 284 nm has been used for the analysis of neuropeptides and small biomolecules, and exhibited LODs for tryptophan of 800 zmol. In addition to frequency-doubled ion lasers, a number of relatively inexpensive pulsed lasers such as frequency quadrupled YAG (266 nm), KrF (248 nm), and hollow-cathode metal vapor lasers have appeared, which provide deep-UV excitation (e.g., 224 and 248 nm). ... 

Techniques for the preparation of metal cluster/nanoparticles can be classified into three primary categories condensed phase, gas phase, and vacuum methods. In condensed phase synthesis, metal and semiconductor nanoparticles are prepared by means of chemical synthesis, which is also known as wet chemical preparation. In gas phase synthesis, metal is vaporized, and the vaporized atoms are condensed in the presence or absence of an inert gas. In vacuum methods, the metal of interest is vaporized with high-energy Ar, Kr ions, or laser beams in a vacuum, and thus generated metal vapor is deposited on a support. 

Argon (Ar) and krypton (Kr) rare-gas ion lasers have applications in many diverse fields, but in the context of laser chemistry their main importance is the use as an optical pumping source for other lasers. 

The complexity of some of these problems of interference, matrix and inter-element fractionation is exemplified by the analysis of Sr isotopes in both solution mode and laser ablation (LA-) MC-ICP-MS. Sr isotope studies have been traditionally carried out using TIMS although MC-ICP-MS can also be used but requires correction (or monitoring) for mass bias, interfering elements, doubly charged ions, argides and dimers, and unidentified interferences. Primarily, isobaric interferences from Rb and Kr (present in trace quantities within the Ar plasma gas) must be corrected but this is not wholly straightforward. 

These lasers are also called—incorrectly— excimer lasers. It will be clear that they could be called exciplex lasers. The active material is a gas mixture which contains a halogen (F2 or Cl2 in most cases) and a rare gas such as Kr, Ar or Xe. These cannot form any stable compounds in their ground states, but excited state species do exist and can fluoresce. These excited state species e.g. KrF) are formed through the recombination of ions, for instance... 

In swarm experiments, an important step forward was the development of a new method in which the SIFT techniques were combined with laser-induced-fluorescence (LIF) detection for monitoring the ion vibrational states (Kato et al., 1993). In this way, both the vibrational states of the reactant ion and the vibrational states of some reaction products could be detected, and the influence of vibrational energy on reaction rates of thermal ions (where the translational-to-vibrational energy transfer is negligible) could be studied. The method was primarily used to study reactions of Nj(t) = 0 to 4) with Ar (which will be discussed separately), N2 and O2 (Kato et al., 1993), Kr (Kato et al., 1996), H2 (de Grouw et al., 1995), and HCl (Krishnamurthy et al., 1997). In the reaction... 

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