Ultraviolet photochemistry of the group-V hydrides

Abstract

This dissertation discusses the molecular properties and ultraviolet photochemistry of the group-V hydrides (NH₃, PH ₃, AsH₃, SbH₃, BiH₃). Relativistic effects become increasingly important for the heavier group-V hydrides and can be manifest in studies of the photodissociation dynamics.

High-n Rydberg time-of-flight (HRTOF) spectroscopy has been used to study the 193.3 nm photolysis of AsH₃. The center-of-mass (c.m.) translational energy distribution for the one-photon process, AsH ₃ + hv → AsH₂ + H, P(E_c.m.), indicates that AsH₂ internal excitation accounts for ∼ 64% of the available energy [i.e., hv – D₀(H ₂As-H)]. Secondary AsH₂ photodissociation also takes place. Analyses of superimposed structure atop the broad P(E_c.m.) distribution suggest that AsH₂ is formed with significant a-axis rotation as well as bending excitation. Comparison of the results obtained with AsH ₃ versus those of the lighter group-V hydrides (NH₃, PH ₃) lends support to the proposed mechanisms. Of the group-V hydrides, AsH₃ lies intermediate between the nonrelativistic and relativistic regimes, requiring high-level electronic structure theory.

The room temperature absorption spectrum of SbH₃ has been recorded. The absorption spectrum is a broad continuum with no discernible structure; however, a longwavelength tail is evident. The HRTOF technique has also been used to investigate the photodissociation dynamics of SbH ₃ following 193.3 nm photolysis. The overall shapes of the translational energy distributions were inconsistent, precluding confident analysis. In spite of this, it is apparent that SbH2 products are formed with substantial internal excitation and secondary photodissociation occurs. These general observations are consistent with the results obtained for AsH₃.