Vacuum ultraviolet photochemistry of methane, silane and germane

Abstract

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.