**2.3. Kr2F active medium**

Emission band of Kr2F at 420 nm is spectrally shifted relative to emissions of XeF(C‐A) at 480 nm and Xe2Cl at 490 nm that enables twofold broadening of the amplification band (**Figure 1**) with the use of two different active media in an amplifier chain.

The Kr2F excitation mechanism relies on the KrF2 photodissociation absorption in the VUV spectral range around 164 nm [42] to produce KrF(B) excimers. The utilization of KrF(B) in secondary processes is different, depending on the composition and pressure of the working mixture. For example, in low‐concentration Xe admixtures, exchange processes take place, resulting in XeF(B) formation with a yield close to 100% and laser action at 353 nm upon pumping by radiation from exploding wire [42]. Being mixed with Kr at a pressure of ∼1 bar, KrF(B) forms Kr2F(4 2 Γ) excimers in three‐body recombination collisions [43].

It was found that the laser action in Kr2F\* also occurs if, instead of Kr, nitrogen is admixed to the working mixture. This observation was attributed to the formation of KrN2F\* four‐atomic excimers that produce Kr2F\* in exchange reactions with Kr atoms generated upon photochem‐ ical decomposition of KrF2 vapor by VUV pump radiation [43]. Note that, despite a complex Kr2F\* formation mechanism, which involves three stages of chemical transformations in mixtures with nitrogen (at one of the stages, products of photochemical processes react with each other), the Kr2F\* yield is rather high providing ∼70% of KrF(B) molecules to be trans‐ formed into Kr2F\* excimers [43]. Lasing in Kr2F at 450 nm was observed upon optical pumping of KrF2:N2 = 1:1500 Torr and KrF2:CF4:Kr = 1:300:1200 Torr gas mixtures by the VUV radiation from an open discharge initiated by an exploding wire [44].

The most detailed results of experimental investigations of above considered broadband active media and a complete bibliography on the works can be found in Refs. [3, 20, 45].
