Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Abstract

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.

Ultrafast isomerization initiated by X-ray core ionization

“…This relaxation begins before interruption by the Auger relaxation. The extent of the geometry change during the core excitation depends on the corehole lifetime and is typically neglected in considering core ionization 2,4,41 . This approximation is better for cases of core excitation of nuclei with fast Auger relaxation rates, as well as for heavier nuclei that experience unscreened fields.…”

“…In fact, it is necessary to account for both the geometry change and the acquired momentum during the core-hole lifetime when considering the Franck-Condon overlaps with the dicationic states. Hence, transition-state theory applied to the dication potential energy surface without accounting for the evolution during the core hole-lifetime is insufficient to describe the isomerization [2][3][4] , because the initial state excited by the core ionization is not static.…”

“…Measurements reported by Osipov et al 1,2 indicate that, when initiated by X-ray core ionization, this isomerization can occur on a timescale shorter than 60 fs. This ultrafast rate was unexpected based on the predicted barrier heights in the electronic states populated in relaxation on core ionization 3,4 . We investigated this process in a time-resolved X-ray pump/X-ray probe experiment at Linac Coherent Light Source (LCLS), a free electron laser (FEL) X-ray source at SLAC National Accelerator Laboratory.…”

“…At the same time, the vibrational excitation of the acetylene dication after Auger decay is expected to be smaller than the barrier height 2 . This would imply isomerization times on the picosecond timescale according to the transition-state theory 4 . The mechanism of the unexpectedly fast isomerization could not be probed by the measurements reported in refs 1,2, because they were not time resolved.…”

“…As suggested by Gadea et al 41 , when lighter nuclei are involved, molecular geometry can begin to evolve during the core-hole lifetime. This geometry change during the core-hole excitation, which is typically neglected in the analysis of core-ionization experiments, implies that transition-state theory applied to the potential energy surface of the dication alone is insufficient to account for the ultrafast proton migration 4 . This result will inform realistic models of radiation damage of biomaterials in single-molecule diffraction experiments at FEL X-ray sources 5,6,42,43 .…”