Fundamental, bending (nu(6), nu(7), nu(8), nu(9)), and CC-stretch (nu(2), nu(3)) hot band :Spectra in the antisymmetric CH stretch (nu(4)) regioxi near 3330 cm(-1) have been observed and analyzed for jet cooled diacetylene (HC equivalent to C-C equivalent to CH) under sub-Doppler conditions. Diacetylene is generated in situ in the throat of a pulsed supersonic slit expansion by discharge dissociation of acetylene to form ethynyl (C equivalent to CH) + H, followed: by radical attack (HC equivalent to CH + C equivalent to C-H) to form HC equivalent to C-C equivalent to CH + H. The combination of (i) sub-Doppler line widths and (ii) absence of spectral congestion permits rotational structure and Cori:ills interactions in the nu(4) CH stretch fundamental to be observed and analyzed with improved precision. Of particular dynamical interest, the spectra reveal diacteylene formation in highly excited internal vibrational states. Specifically, multiple Pi <- Pi and Delta <- Delta hot bands built on the nu(4) CH stretch fundamental are observed, due to doubly degenerate bending vibrations [cis C equivalent to C-H bend (nu(6)), trans C-C equivalent to C bend (nu(7)), trans C equivalent to C-H bend (nu(8)) and Cs C-C equivalent to C bend (nu(9))], as well as a heretofore unobserved Sigma <- Sigma band assigned to excitation of nu(2) or 2 nu(3) CC stretch. Boltzmann analysis yields populations consistent with univeisally cold rotations (T-rot approximate to 15 +/- 5 K) and yet superthermal vibrations (T-vib approximate to 85-430 K), the latter of which is quite anomalous for the high collision densities in a slit jet expansion. In order to elucidate the physical mechanism for this excess vibrational excitation, high level ab initio CCSD(T) calculations have been pursued with explicitly correlated basis sets (VnZ-f12; n = 2,3) and extrapolated to the complete basis set (CBS) limit using MOLPRO quantum chemistry software. The results suggest that the extensive hot band structure observed arises from (1) highly exothermic CCH + HCCH addition to yield a strongly bent HCCHCCH radical intermediate (Delta H = -62.6 kcal/mol), followed by (ii) rapid fragmentation over a submerged transition state barrier (Delta H = -18.9 kcal/mol) to form vibrationally hot diacetylene + H products (Delta H = -25.6 kcal/mol), and consistent with crossed molecular beam studies by Kaiser et al. [Phys. Chem. Chem. Phys. 2002, 4, 2950.] Finally, RRKM fragmentation rates for this complex are calculated, which exceed collision frequencies in the slit jet expansion and suggest near unity quantum efficiency for diacetylene formation.