Thermal Decomposition of NCN3 as a High-Temperature NCN Radical Source: Singlet-Triplet Relaxation and Absorption Cross Section of NCN(3Σ)

The potential of the thermal decomposition of cyanogen azide (NCN3) as a high-temperature cyanonitrene (NCN) source has been investigated in shock tube experiments. Electronic ground-state NCN((3)Sigma) radicals have been detected by narrow-bandwidth laser absorption at overlapping transitions belonging to the Q(1) branch of the vibronic (3)Sigma(+)-(3)Pi subband of the vibrationally hot (A) over tilde (3)Pi(u)(010)-(X) over tilde (3)Sigma(-)(g)(010) system at (v) over tilde = 30383.11 cm(-1) (329.1302 nm). High-temperature absorption cross sections sigma have been directly measured at total pressures of 0.2-2.5 bar, log[sigma/(cm(2) mol(-1))] = 8.9-8.3 x 10(-4) x T/K (+/- 25%, 750 < T < 2250 K). At these high temperatures, NCN((3)Sigma) formation is limited by a. slow electronic relaxation of the initially formed excited NCN((1)Delta) radical rather than thermal decomposition of NCN3. Measured temperature-dependent collision-induced intersystem crossing (CIISC) rate constants are best represented by k(CIISC)/(cm(3) mol(-1) s(-1)) = (1.3 +/- 0.5) x 10(11) exp[-(21 +/- 4) kJ/mol/RT] (740 < T < 1260 K). Nevertheless, stable NCN concentration plateaus have been observed, showing that NCN3 is an ideal precursor for NCN kinetic experiments behind shock waves.