The C2H radical is an excellent prototype for examining important phenomena: electronic states; curve crossings and nonadiabatic transitions; intramolecular and dissociation dynamics; and so on. It is small enough to provide experimental parent and product state resolution, and it is tractable at a high level of theory: both electronic structure and quantum mechanical nuclear dynamics, including nonadiabatic couplings. It is central to a large body of chemistry, and a number of conical intersections need to be taken into account simultaneously. Its complexities are advantageous, as they derive from processes that are difficult to unravel in larger radicals. Because C2H is small, tractable at a high level of theory, and rife with crossings, nonadiabatic couplings, etc., it provides a unique platform.
Conical intersections lead to subtle effects. For example, consider the coupled system at energies near 32A’. Because 12A’ and 22A’ intersect conically and remain coupled X᷉/A᷉ above their intersection, the 32A’/ 22A’ intersection results in a manifold in which and characters are present in wave functions. This provides access to ground state products.
To characterize the C2H molecular beam, a TOF mass spectrometer samples expanded gas at intervals as short as 5 μs (see figure on the right) This diagnostic has also been used in other experiments in our laboratory with great success. For example, condensed H2O/CO2 is ablated using an IR pulse, and TOF mass spectra are recorded. The figure below shows one such TOF spectrum. The largest peaks are off-scale; the most intense has S/N of ~ 103.
Referring to the above figure, tunable UV will be directed to the region of the focused e-beam. It will overlap the e-beam and be synchronized to a given TOF spectrum. Yield spectra will be obtained by monitoring C2+ as the frequency is scanned. This signal arises from dissociation: C2H → C2 + H , which is known to dominate (i.e., over fluorescence decay of electronically excited C2H) throughout a large spectral range. Short residence times can be achieved by photolyzing samples at the end of the tube. With 193 nm radiation focused with a cylindrical lens such that it intercepts a region of 2 mm length at the end of the tube, C2H pulse durations of 10-20 μs are achieved.
Assume that ground state C2H is photodissociated at ~ 240 nm and the available energy is 2500 cm–1. The figure shows C2 vibrations and rotations for and . The 718 cm-1 separation between X1Σ+g and , a3Πμ, as well as vibrations, are easily resolved, and even rotational levels will be resolved for J greater than ~ 8. Because HRTOF records an entire spectrum with each laser firing, there is no distortion across the TOF range. Thus, relative populations are obtained with good accuracy.