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The direct collapse black hole (DCBH) scenario describes the isothermalcollapse of a pristine gas cloud directly into a massive, M_BH=10^4-10^6 M_sunblack hole. In this paper we show that large HI column densities of primordialgas at T~10^4 K with low molecular abundance - which represent key aspects ofthe DCBH scenario - provide optimal conditions for pumping of the 2p-level ofatomic hydrogen by trapped Lyman alpha (Lya) photons. This Lya pumpingmechanism gives rise to inverted level population of the 2s_1/2-2p_3/2transition, and therefore to stimulated fine structure emission at 3.04 cm(rest-frame). We show that simplified models of the DCBH scenario amplify theCMB by up to a factor of 10^5, above which the maser saturates. Hyperfinesplitting of the 3-cm transition gives rise to a characteristic broad (FWHM ~tens of MHz in the observers frame) asymmetric line profile. This signalsubtends an angular scale of ~ 1-10 mas, which translates to a flux of ~ 0.3-3microJy, which is detectable with ultra-deep surveys being planned withSKA1-MID. While challenging, as the signal is visible for a fraction of thecollapse time of the cloud, the matching required physical conditions implythat a detection of the redshifted 3-cm emission line would provide directevidence for the DCBH scenario.

3-cm Fine Structure Masers: A Unique Signature of Supermassive Black Hole Formation via Direct Collapse in the Early Universe