EVIDENCE FROM ESR OF NONLINEAR CHARACTER IN $C_{4}$ FORMED BY VUV PHOTOLYSIS IN AN ARGON MATRIX

$^{1}$ W. R. M. Graham, K. I. Dismuke, and W. Weltner, Jr., Ap. J. 204, 301 (1976). $^{2}$ R. J. Van Zee, R. F. Ferrante, K. J. Zeringue, and W. Weltner, Jr., J. Chem. Phys. 88, 3465 (1988).Author Institution: Department of Physics, Box 32916, Texas Christian UniversityThe $C_{4}$ molecule has been produced by trapping the products of the vacuum uv photolysis of diacetylene and butadiene in Ar at 12 K. Signals at the mag netic field positions of the $xy_{1}$ and $xy_{2}$ lines previously identified1,2 for triplet C4 formed from graphite vapor, exhibit splitting into separate x and y components in the well resolved ESR spectra obtained by the photolytic decomposition of the $C_{4}H_{2}$ and $C_{4}H_{5}$ precursors in dilute, mixtures of $\sim 1:20,000$ in argon. Derived zero-field splitting constants are $|D| = 0.2237(2)$ and $E = 2.52(1) \times 10 cm^{-1}$. The results of extensive annealing experiments, at a wide range of mole ratios, argue against multiple trapping sites being responsible for the observed splitting. The spectrum is unaffected by using either parent molecule fully substituted with the deuterium isotope $(I = 1)$, indicating that hyperfine splitting due to the presence of hydrogen stome $(I = 1/2)$ cannot account for the observed splittings. Further evidenc suggesting nonlinear character for $C_{4}$ trapped in solid argon by the photolytic decomposition of $C_{4}H_{2}$ and $C_{4}H_{5}$, is provided by the hyperfine splittings observed when butadiene, isotopically substituted with carbon-13, is used as the parent molecule