Rare gas excimers

For the heavier noble gases, the core multiplicity of the metastable states give rise to eight potential curves (six for 3P2 and two for 2P0). The Ne2 potentials have been calculated by Schneider and Cohen,87 who have also performed scattering calculations for this system. Theoretical and experimental data exist for Ar2,88 whereas only qualitative estimates of the potentials are available for Kr2 and Xe2. These excited states play a prominent role in the rare-gas excimer lasers.89,90 ... 

The rare gas excimers readily transfer energy to various additives. Rates for transfer to nitrogen and hydrogen in krypton are known at 1 atm. Because excimer species have strong absorptions in the visible region, it is necessary to quench them when studying reactions of other intermediates by absorption spectroscopy. Ethane has been shown to be convenient for this purpose. The rate constant for excitation transfer from excimers to ethane in xenon was measured by the pulse-probe technique to be 3.4 x 10 ° molal s at pressures near 50 bar. Thus, addition of a small concentration of ethane can be used to reduce the absorption due to excimers to a small level at nanosecond times. 

Janssens H, Vanmarcke M, Desoppere E, Lenaerts J, Boucique R, Wieme W. (1987) A general consistent model for formation and decay of rare gas excimers in the 10 Vio+ mbar pressure range, with application to krypton. / Chem Phys 86 4925 934. 

The spectroscopic and kinetic behaviour of rare-gas excimer states have been studied in some detail. Rapid quenching of Hc2(a Z ) by several collision... 

In 1970 the first report of the molecular hydrogen laser opened up a decade of activity in VUV laser development, which included the appearance of rare gas excimer and exciplex lasers and the achievement of tunable coherent radiation in the Lyman-a region via harmonic generation. The surge of activity in the development of VUV lasers arose in part from the uniqueness of the VUV region, in part from the ultimate interest in X-ray lasers and, from our perspective, from the exciting prospects in spectroscopy and molecular dynamics promised by narrow linewidth, tunable, high-power VUV laser pulses for state-selective studies. Here we review the principles on which VUV lasers are based. 

The rare gas excimer lasers are based on bound-continuum transitions from an excited diatomic species to its dissociative ground state. The observed continuum emission is a superposition of the Franck-Condon factors from the vibrational levels of the upper state. Thus these molecular dissociation lasers display relatively broad fluorescence as a consequence of the steeply repulsive ground-state potential, and there is always a population inversion on such transitions. However, the net gain is significantly lower than that for a bound-bound transition because of the distribution of oscillator strength over the broad fluorescence band. Figure 1 illustrates schematic potential energy curves for such transitions in the excimer and exciplex lasers. 

Many of the photochemical advances that have occurred during the past two decades have followed from the development of microwave discharge vacuum ultraviolet light sources, emitting either intense monochromatic atomic resonance radiation at fixed wavelengths (determined by nature rather than for utility) or over broad continua produced by the fluorescent decay of rare gas excimers which provide tunable sources after passage through a vacuum monochromator (but at the cost of reduced intensity). 

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... 

A number of points are clear. First, in all cases the major expense of laser photons is the hardware, not the energy (even at Austin prices). Secondly, the intrinsically greater efficiency of the lower-energy lasers, especially the economic attractiveness of the CO2 laser, is evident. One can easily understand why laser chemistry schemes based upon multiphoton infrared absorption attract so much effort. Thirdly, on a per-unit-time basis the ion laser is more than twice as expensive to operate than even the rather exotic excimer laser. This is because of the inherent energetic inefficiency of the rare-gas plasma as a gain medium and because of the extrinsic, and hideous, expense of ion laser plasma tubes (and their poor reliability). 

Excimer formation in certain gases and vapor mixtures is known from modern UV lasers typical examples are excited complexes between rare gas... 

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... 

The most prominent feature in luminescence of Xe, Kr and Ar - the so-called M-band (Fig.la) - is formed by 1,3SU+— Xg+ transitions in (R2 ) excimer M-STE (R=rare gas atom). The negative electron affinity (Table 1) is a moving force of the cavity ("bubble") formation around A-STE in the bulk of crystal, and the desorption of atoms and excimers from the surface of solid Ne and Ar [11], Radiative "hot" transitions in desorbed excimers of Ar and Ne result in a W-band. 4-bands are emitted by A-STE (R ). 

Pulse-probe studies using the Laser Electron Accelerator Facility (LEAF) at Brookhaven National Laboratory have revealed changes in optical absorption occurring on the picosecond time scale in rare gas fluids. In xenon, excimers are formed which absorb in the visible and near infra-red as shown in Fig. la. The absorption grows in during the first 50 picoseconds [see Fig. 1(b)].This growth is concomitant with ion recombination that leads first to excited atoms, reaction 1(a), which immediately form excimers, Xe, because of the high density of xenon. 

These features were very similar to those of the well-known bound-free transitions of Rg2 and Rg2X excimers (X = halogen), which appear at high rare-gas pressures in the longer-wavelength region of Rg atomic line and the main RgX excimers. Therefore, the continuous bands were ascribed to the bound-free transitions of RgRg J ... 

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