We use gas-phase negative ion photoelectron spectroscopy to study the quasilinear carbene propargylene, HCCCH, and its isotopologue DCCCD. Photodetachment from HCCCH- affords the (X) over bar(B-3) ground state of HCCCH and its (a) over tilde((1)A), (b) over tilde (B-1), (d) over tilde((1)A(2)), and (B) over tilde((3)A(2)) excited states. Extended, negatively anharrnonic vibrational progressions in the (b) over tilde(B-3) ground state and the open-shell singlet (b) over tilde (B-1) state arise from the change in geometry between the anion and the neutral states and complicate the assignment of the origin peak. The geometry change arising from electron photodetachment results in excitation of the v(4) symmetric CCH bending mode, with a measured fundamental frequency of 363 +/- 57 cm(-1) in the X(B-3) state. Our calculated harmonic frequency for this mode is 359 cm(-1). The Franck-Condon envelope of this progression cannot be reproduced within the harmonic approximation. The spectra of the (a) over bar((1)A), (d) over bar((1)A2), and (B) over bar((3)A(2)) states are each characterized by a short vibrational progression and a prominent origin peak, establishing that the geometries of the anion and these neutral states are similar. Through comparison of the HCCCH- and DCCCD- photoelectron spectra, we measure the electron affinity of HCCCH to be 1.156 +/- (0.010)(0.095) eV, with a singlet triplet splitting between the (X) over tilde(B-3) and the a((1)A) states of Delta E-ST = 0.500 +/- (0.10)(0.1)eV (11.5 + +/- (2.3)(0.2) kcal/mol). Experimental term energies of the higher excited states are T-0 [(b) over tilde(B-1)] = 0.94 + (0.22)(0.20) eV, T-0 [(d) over tilde((1)A(2))] = 3.30 + R:geV, T-0 [(B) over tilde((3)A(2))] = 3.58 +/- (0.10)(0.02) eV. The photoelectron angular distributions show significant g character in all the frontier molecular orbitals, with additional a character in orbitals that create the (X) over tilde(B-3) and b(B-1) states upon electron detachment. These results are consistent with a quasilinear, nonplanar, doubly allylic structure of X(3B) HCCCH with both diradical and carbene character.