The photochemistry of jet-cooled CH₄, SiH₄ and GeH₄ molecules following excitation at the Lyman-α wavelength (121.6 nm) has been investigated by high resolution photofragment translational spectroscopy methods. Complementary ab initio calculations of selected portions of the potential energy surfaces for the various components of the ¹T₂ and ³T₂ excited states arising from the 3sa₁←1t₂ electron promotion are presented in the case of CH₄. The form of the H atom recoil velocity distribution arising in the 121.6 nm photolysis of CH₄ is rationalised in terms of initial excitation to both the 2¹A′ and 1¹A″ excited states (Jahn–Teller components of the degenerate ¹T₂ state), followed by a range of decay mechanisms. CH₄(2¹A′) molecules can decay adiabatically, ia sequential extension of first one, then a second, C–H bond with eventual formation of two H atoms and CH₂(ã¹A₁) products, or after internal conversion (IC) to the ground state. The H + CH₃() products resulting from the IC process display a recoil velocity distribution characterised by an anisotropy parameter β∽ + 2, implying that the fragmentation involves irreversible extension of the C–H bond along which the transition dipole points at the instant of photon absorption. Fragmentation of CH₄(1¹A″) molecules to H + CH₃() products proceeds ia intersystem crossing to the lowest ³A′ potential energy surface. The recoil anisotropy of these products (β∽ − 0.45) implies that this radiationless process also occurs on a timescale that is rapid compared to the parent rotational period. Both single H–C bond fission channels may yield CH₃() products with such high levels of internal excitation that they are unstable with respect to further unimolecular decay; any H atoms that result from this secondary decay must contribute to the observed yield of slow H atoms with β∽0. All H atoms resulting from Lyman-α photolysis of both SiH₄ and GeH₄ have (low) kinetic energies and little or no recoil anisotropy, compatible with their being formed ia three body fragmentation to, primarily, H + H + SiH₂/GeH₂(¹A₁) products. Faster H atoms are evident in the total kinetic energy release (TKER) spectra obtained following 157.6 nm photoexcitation of SiH₄, but the power dependence of this fast H atom signal implies that these arise as a result of a two photon process involving initial formation of SiH₂ + H₂ products and subsequent photolysis of the nascent silylene fragments.