Heavy hydrides: H2Te ultraviolet photochemistry.
The room-temperature ultraviolet absorption spectrum of H2Te has been recorded. Unlike other group-6 hydrides, it displays a long-wavelength tail that extends to 400 nm. Dissociation dynamics have been examined at photolysis wavelengths of 266 nm (which lies in the main absorption feature) and 355 nm (which lies in the long-wavelength tail) by using high-n Rydberg time-of-flight spectroscopy to obtain center-of-mass translational energy distributions for the channels that yield H atoms. Photodissociation at 355 nm yields TeH(2Pi1/2) selectively relative to the TeH(2Pi3/2) ground state. This is attributed to the role of the 3A' state, which has a shallow well at large R(H-TeH) and correlates to H+TeH(2Pi1/2). Note that the 2Pi1/2 state is analogous to the 2P1/2 spin-orbit excited state of atomic iodine, which is isoelectronic with TeH. The 3A' state is crossed at large R only by 2A", with which it does not interact. The character of 3A' at large R is influenced by a strong spin-orbit interaction in the TeH product. Namely, 2Pi1/2 has a higher degree of spherical symmetry than does 2Pi3/2 (recall that I(2P1/2) is spherically symmetric), and consequently 2Pi1/2 is not inclined to form either strongly bonding or antibonding orbitals with the H atom. The 3A'<--X transition dipole moment dominates in the long-wavelength region and increases with R. Structure observed in the absorption spectrum in the 380-400 nm region is attributed to vibrations on 3A'. The main absorption feature that is peaked at approximately 240 nm might arise from several excited surfaces. On the basis of the high degree of laboratory system spatial anisotropy of the fragments from 266 nm photolysis, as well as high-level theoretical studies, the main contribution is believed to be due to the 4A" surface. The 4A"<--X transition dipole moment dominates in the Franck-Condon region, and its polarization is in accord with the experimental observations. An extensive secondary photolysis (i.e., of nascent TeH) is observed at 266 and 355 nm, and the corresponding spectral features are assigned. Analyses of the c.m. translational energy distributions yield bond dissociation energies D0. For H2Te and TeH, these are 65.0+/-0.1 and 63.8+/-0.4 kcalmol, respectively, in good agreement with predictions that use high-level relativistic theory.