540--900 nm photodissociation of 300 K NCNO: One- and two-photon processes

The laser photodissociation of 300 K NCNO throughout the region 540--900 nm is reported, and both 1- and 2-photon processes are discussed. By monitoring CN fragments produced via the 1-photon process, we show that with photolysis wavelengths >592 nm, dissociation occurs predominantly by exciting NCNO ''hot bands.'' At shorter photolysis wavelengths, dissociation from the ground vibrational state of NCNO is observed as well, but the contributions from hot bands are still manifest in high CN rotational levels which are energetically inaccessible from the ground state (D0 = 48.8 kcal mol-1). Energy distributions in the CN fragments were determined for excess energies up to 1800 cm-1, and are in agreement with phase space theory calculations and a vibrational predissociation mechanism. In addition, throughout the region 620--900 nm, stepwise two-photon photodissociation proceeds using the A 1A'' state as a gateway, and results in rotationally and vibrationally ''hot'' CN fragments. The hot CN fragment yield vs photolysis wavelength shows peaks which correspond exactly to peaks in the NCNO absorption spectrum, allowing us to obtain high resolution spectra of the A 1A''reverse arrow X 1A' absorption system. The one- and two-photon processes are in competition, and the latter disappears at wavelengths where one-photon photodissociation of NCNO via its ground vibrational level sets in. The nature of the electronic states involved in the one- and two-photon processes is also discussed