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 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
) 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)