Extreme ultraviolet pulse

Parra E, Alexeev I, Fan J, Kim KY, McNaught SJ, Milchberg HM (2000) X-ray and extreme ultraviolet emission induced by variable pulse-width irradiation of Ar and Kr clusters and droplets. Phys. Rev. E 62 R5931-R5934... 

Some of the applications of third- and higher-order frequency conversion are given in Table VII. The th harmonic generation is used to produce radiation at a frequency that is q times the incident frequency. The most commonly used interaction of this type is third-harmonic conversion. It has been used to produce radiation at wavelengths ranging from the infrared to the extreme ultraviolet. Third-harmonic conversion of radiation from high power pulsed lasers such as CO2, Ndiglass, Nd YAG, ruby, and various rare-gas halide and rare-gas excimer lasers has... 

As a fundamental study on field induced chemical reactions, Neidel and Vrakking et al. observed attosecond d3mamics of electrons in a series of small- and medium sized neutral molecules by monitoring time-dependent variation of the parent molecular ion 3delds [296]. The information on electron dynamics was extracted from experimental data on the basis of the relation between the time dependent dipole and ionization. This was performed in the two-color femtosecond near infrared (NIR) pump-attosecond extreme ultraviolet (XUV) probe experiment. They claim that the time-dependent dipole induced by the moderately strong NIR pulse field is monitored with attosecond time resolution. The oscillations are interpreted in terms of a time dependent screening induced by the polarization of the molecule, which alters the photoionization yield of the neutral molecule. This scheme can be considered as the first example of molecular attosecond Stark spectroscopy. 

Dorfman and collaborators have recently developped a very promising technique for the production of carbenium ions as transient species in halocarbon sdvents based on the dissociative ionisation of suitable precursors induced by pulse radiolysis of the solvent. While the extremely interesting kinetic results vdiich this group is obtaining will be discussed in Sect. II-G4, it is emphasised here that the fast time response of the apparatus used allows the characterisation of carbenium ions hitherto unobservable because of their excessive reactivity. The ultraviolet absorption spectrum and some reactions of the benzylium ion have been studied for the first time wdth this powerful tool. From the point of view of cationic pdymerisation, the information obtained in this type of work is particularly relevant, since it deals vrith the identification and reactivity of carbenium icais formed in very low concentration in the nght kind of medium. Cation radicals had already been prepared by pulse radiolysis involving nondissociative ionization (electron ejection or transfer), as will be discussed in Sect. II-K. 

Femtosecond laser applications, e.g., in electronic or medical technology require microstructuring on uneven substrates. This cannot be achieved by conventional photolithography which only functions on completely flat supporting materials. Ultraviolet lasers (excimer, fourth harmonic Nd YAG) have been widely used for such purposes. However, when high precision is required and substrates are extremely fragile and thermally sensitive, the very low heat effect by subpicosecond laser pulses can avoid this micromachining problem. 

All these reactions are rapid and a maximum concentration of O2 is reached within 5 xsec of the radiation pulse. The decay of O2 is followed spectrophotometrically at around 250 nm. The time resolution of pulse radiolysis is very high, and reaction times as short as 2 X 10 sec can be easily followed. In common with the direct assays that utilize the ultraviolet absorption of O2, the problem of sample absorption in this region arises. Also the maximum single pulse yield of O2 (ca. 200 xM) is less than that obtained from a solution of potassium superoxide. However, the technique has proved extremely useful for working with pure enzymes. The mechanisms decribed in Section I have all been obtained by this technique. 

In most experiments, ultraviolet or infrared absorption, resonance fluorescence, or laser-induced fluorescence (LIF) is used to follow how transient concentrations change after the photolysis pulse. These optical techniques vary considerably in their sensitivity and hence to the extent to which they isolate the primary reaction. LIF is extremely sensitive, enabling one to follow decays of concentrations from an initial value of 10 ° cm , but its use is restricted to species with a bound-bound electronic transition within the range of tunable dye lasers. LIF has been used to follow the kinetics of reactions of, inter alia, the radicals OH [12-14], CN [15] and CH3O [16,17]. It is more difficult to apply to radical atoms vihich usually have allowed electronic transitions only in the vacuum ultraviolet. Some LIF measurements utilising two-photon excitation of atoms have been reported [18]. 

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