VOLUME 56, NUMBER 15 PHYSICAL REVIEW LETTERS 14 APRII. 1986 Structure of C3 as Measured by the Coulomb-Explosion Technique I. Plesser and Z. Vager Department of Nuclear Physics, Weizmann Institute oI Science, Rehovot, Israel and R. Naaman Department of Isotopes Research, Weizmann Institute of Sci'ence, Rehovot, Israel (Received 28 January 1986) By application of the Coulomb-explosion technique with a new type of detection system, the structure of the neutral C3 molecule has been studied. The detector allo~s for the first time simul- taneous determination of the velocity vectors of all nuclei involved, from which correlation between internal degrees of freedom in the molecules can be derived. PACS numbers: 35.20. — i Studies on the dissociation of fast (megaelectron- volt) molecular-ion beams in thin foils suggest' a novel alternative approach to the determination of molecular structure. In such experiments, individual molecular projectiles rapidly lose many electrons and the monatomic highly charged fragments repel each other via the Coulomb force (" Coulomb explosion" ). The final-state momentum distribution of each mol- ecule is measured and related to the initial molecular structure. In this particular investigation a new type of detection system has been utilized which allows, for the first time, simultaneous determination of the velo- city vectors of all nuclei involved without introduction of any bias depending on geometry or orientation. The main tool for the obtaining of structural infor- mation on isolated molecules is spectroscopy. The ob- served spectral lines are analyzed by use of a vibrational-rotational model, which assumes small dis- placements from the equilibrium position, and dif- ferent time scales between rotational and vibrational motions. Although this model was found to be very useful when combined with perturbation methods, there are many interesting systems for which it is not valid. Among those are "floppy" systems (some of the van der Waals complexes, for example), or molecules that are highly vibrationally excited. In these cases, because of the large amplitude vibrations, the interaction between rotation and vibration be- comes important, and the conventional spectroscopical analysis is not possible. For these systems an alterna- tive method for structural verificatio is very desir- able. In this work the structure of the neutral C3 molecule was studied as a first demonstration of the technique and its capability. The molecule C3 was chosen be- cause of both experimental convenience and its theoretical interest. Gausset and co-workers6 analyzed the 4050-A band, observed also in spectroscopic stud- ies of comets, as an electronic transition in the linear C3 molecule. The ground electronic state was assigned as a ('X) state, which has a very low-frequency bend- ing mode (63 cm ') and asymmetric and symmetric stretching vibrations with wave numbers of 1230 and 2040 cm ', respectively. Recent theoretical work' confirmed the linear structure, and concludes that the C — C — C bond angle can be changed by a large amount from its equilibrium value, with a very small change in either energy or bond length. The large- amplitude bending combined with the availability of existing experimental and theoretical works makes the C3 system an interesting test case for the new tech- nique. The Pelletron accelerator at the %eizmann Institute is equipped with a sputter ion source in which C3 is produced when a graphite target is bombarded with Cs atoms. The negative carbon clusters are then injected, after mass selection, into the accelerating region, in which they reach energies of 4 Mev per atom. At the center of the accelerator, in a field-free region, an electron is stripped by a N2 gas stripper, operated at a pressure corresponding to the single-collision regime. The neutral C3 molecules thus formed continue fur- ther with the same velocity for about 20 m, while the charged particles are swept away by a magnet. The neutral molecules impinge upon a thin (200-A) carbon foil„which strips many of the electrons, a process fol- lowed by dissociation of the system and the production of carbon ions in different charge states. A small mag- net deflects the carbon beams, according to their charges, into three detectors situated 2. 1 m below the carbon foil. The detector array was built from three parts, each of which is a 5&5-cm multiwire gas detector. A full description of the detector will be given elsewhere. 9 The spatial resolution of the detector was measured by insertion of a mask with 0.5-mm-diam holes in front of the detector, and was found to be better than 1 mm. The time resolution was found to be better than l Qc 1986 The American Physical Society