Photodissociation of jet-cooled isocyanic acid has been examined by photofragment ion imaging of H(D) from H(D)NCO and CO from HNCO, and by laser induced fluorescence (LIF) of NH(a 1Δ) from HNCO. Only modest recoil anisotropy is observed in the H+NCO channel at 243.1 nm (β=-0.13±0.05), while the D+NCO channel at approximately the same wavelength reveals no anisotropy (β=0.00±0.05), confirming that the dissociation of H(D)NCO from the opening of the H(D) channel proceeds via vibrational predissociation on the S0(1A′) surface. In contrast, substantial anisotropy (β=-0.66±0.08) is observed in the NH(a 1Δ)+CO channel at 230.1 nm, but this value can correspond to dissociation on either S0 or S1. The photolysis region between 243 and 230 nm thus appears important in providing clues to the dissociation mechanism and the competition between different potential energy surfaces. At 217.6 nm, product state distributions exhibit clear dynamical biases. CO is produced in both v=0 and v=1, while NH(a 1Δ) distributions correlated with different rovibrational levels of CO, although different in shape, are always cold, consistent with the global NH distribution measured by LIF. The NH distributions indicate dissociation on S1(1A″), and can be described by Franck-Condon mapping of transition state wave functions in the HNC bending coordinate without additional torque, implying little anisotropy in the potential along that coordinate. On the other hand, a larger torque is manifest in the CO rotational distribution. Although at 217.6 nm the dissociation is likely to be dominated by decomposition on S1, competition with radiationless decay is still manifest. From analysis .of the CO photofragment velocity distribution at 230.1 nm, the NH(a 1Δ)+CO dissociation threshold is determined at 42 765 ±25 cm-1.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry