Vacuum ultraviolet photochemistry of methane, silane and germane

The photochemistry of jet-cooled CH4, SiH4 and GeH4 molecules following excitation at the Lyman-alpha 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 T-1(2) and T-3(2) excited states arising from the 3sa(1)<-- 1t(2) 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 2(1)A' and 1(1)A excited states (Jahn-Teller components of the degenerate T-1(2) state), followed by a range of decay mechanisms. CH4(2(1)A') molecules can decay adiabatically, via sequential extension of first one, then a second, C-H bond with eventual formation of two H atoms and CH2((a) over tilde (1)A(1)) products, or after internal conversion (IC) to the ground state. The H+CH3((X) over tilde) products resulting from the IC process display a recoil velocity distribution characterised by an anisotropy parameter beta similar to +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(1(1)A) molecules to H+CH3((X) over tilde) products proceeds via intersystem crossing to the lowest (3)A' potential energy surface. The recoil anisotropy of these products (beta similar to -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((X) over tilde) 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 beta similar to0. All H atoms resulting from Lyman-alpha photolysis of both SiH4 and GeH4 have (low) kinetic energies and little or no recoil anisotropy, compatible with their being formed via three body fragmentation to, primarily, H+H+SiH2/GeH2((X) over tilde (1)A(1)) 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+H-2 products and subsequent photolysis of the nascent silylene fragments.