Branching ratios of C2 products in the photodissociation of C2H at 193 nm

Experimental and theoretical results are combined to show that vibrationally excited C2H radicals undergo photodissociation to produce C-2 radicals mainly in the B (1)Delta (g) state. Infrared (IR) emissions from the photolysis of acetylene with a focused and unfocused 193 nm excimer laser have been investigated using step-scan Fourier transform infrared (FTIR) emission spectroscopy at both low and high resolution. With an unfocused laser, the low-resolution infrared emission spectra from the C2H radicals show a few new vibrational bands in addition to those previously reported. When the laser is focused, the only emissions observed in the 2800-5400 cm(-1) region come from the electronic transitions of the C-2 radicals. Most of the emissions are the result of the B (1)Delta (g)-->A (1)Pi (u) transition of C-2 although there are some contributions from the Ballik-Ramsay bands C-2(b (3)Sigma (-)(g)-->a (3)Pi (u)). A ratio of [B (1)Delta (g)]/[b (3)Sigma (-)(g)]=6.6 has been calculated from these results. High quality theoretical calculations have been carried out to determine what kind of ratio could be expected if the photodissociation products are formed solely by adiabatic dissociation from the excited states of C2H. To accomplish this, the geometries of different electronic states of C2H (X (2)Sigma (+), A (2)Pi, 3-6 (2)A', and 2-5 (2)A') were optimized at the complete active space self consistent field [CASSCF(9,9)/6-311G**] level. The calculated normal modes and vibrational frequencies were then used to compute Franck-Condon factors for a variety of vibronic transitions. In order to estimate the oscillator strengths for transitions from different initial vibronic states of C2H, transition dipole moments were computed at different geometries. The overall Franck-Condon factor for a particular excited electronic state of C2H is defined as the sum of Franck-Condon factors originating from all the energetically accessible vibrational levels of C2H(X,A) states. The adiabatic excitation energies were calculated with the multi-reference configuration interaction/correlation-consistent polarized valence triple zeta [MRCI(9,9)/cc-PVTZ] method. The overall Franck-Condon factors were then multiplied by the corresponding oscillator strengths to obtain the total absorption intensities characterizing the probabilities for the formation of different excited states. Then, the excited states of C2H were adiabatically correlated to various electronic states of C-2 (B (1)Delta (g), A (1)Pi (u), B' (1)Sigma (+)(g), c (3)Sigma (+)(u), and b (3)Sigma (-)(g)) to predict the photodissociation branching ratios from the different states of C2H, such as X(0,nu (2),0), X(0,nu (2),1), A(0,0,0), and A(0,1,0). For C2H produced by 193 nm photodissociation of acetylene, the calculations gave the following B:A:B-':b:c branching ratios of 38:32:10:14:6. This means that the theoretical branching ratio for the [B (1)Delta (g)]/[b (3)Sigma (-)(g)] is 2.7, which is in excellent agreement with experiment. (C) 2001 American Institute of Physics.