Vibrational preexcitation of a state containing three quanta of C–H stretch (3ν1) results in C–H acetylenic and C–D methyl bond rupture in the ∼243.1 nm photolysis of CD3C≡CH, in contrast to previous observations of the almost isoenergetic 193 nm photodissociation of propynes. The C–D bond fission is the dominant pathway with a D/H branching ratio of 2.0±0.5 at a combined energy of ∼50 830 cm−1. The average translational energies of D and H atoms are nearly identical, although the C–H acetylenic and C–D methyl bond energies differ quite extensively, pointing to different dynamics on the involved potential energy surfaces.

Absorption and free-jet action spectroscopy of states in the region of four quanta of C-H stretching of acetylene (C 2 H 2 ) around 12 676 cm Ϫ1 were studied. Rovibrational excitation combined with 243.135 nm promotion of C 2 H 2 molecules to the upper electronic trans-bent states, Ã 1 A u /B 1 B u , and H photofragment ionization generated action spectra measuring the H yield as a function of the excitation wavelength. The findings show that the (1030 0 0 0 ) IR bright state of the third C-H stretch overtone has a smaller photodissociation cross section than the (1214 0 0 0 ) combination band containing trans-bend mode excitation and lying in its vicinity, due to a favorable Franck-Condon factor for the latter. Certain line pairs accessing similar JЈ levels of the (1030 0 0 0 ) state show anomalous intensities in the action spectrum, with P transitions excessively enhanced over R transitions. This implies enormous dissociation efficacy as a consequence of Coriolis-type local resonances with nearby states of another band and due to the overlap of the P transitions with another band.

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The photofragmentation dynamics of acetylene, C 2 H 2 , was explored via vibrationally mediated photodissociation. Direct near infrared (NIR) excitation efficiently prepared rovibrational states in the region of three C-H stretch quanta ($9640 cm À1 ), subsequently $243.1 nm UV 1 photons promoted the pre-excited C 2 H 2 molecules to the A ˜1A u state and dissociated them and finally the ensuing H atoms were probed by UV 2 photons via (2 + 1) resonantly enhanced multiphoton ionization. UV dependent action spectra, monitoring the H photofragment yield vs. the UV 1 dissociating laser wavelength displayed sharp peaks depending on the combined energy and the initially excited rovibrational state. These spectra indicate that the rovibrational transitions belonging to the (1112 0 0 0 ) combination band, containing trans-bend mode excitation, are excessively enhanced over those of the (0030 0 0 0 ) state with three C-H stretch quanta, due to favorable Franck-Condon (FC) factor. The UV absorption from these states sampled particular rovibronic levels of the potential well on the upper A ˜1A u state, disclosing transitions to FC active vibronic modes, involving couplings between torsion and cis-bend with C= =C stretch and trans-bend modes. These results suggest that the initial state preparation samples the bound rather than the purely repulsive region of the A ˜1A u state and that the rovibronic structure is the essential factor in affecting the absorption cross-section. They also indicate that the provided energy is insufficient to overcome the barrier on the A ˜1A u state, leading to the dominant non-adiabatic predissociative photofragmentation into C 2 H (X ~2S + ) + H.

The photofragmentation of propyne-d(3), D(3)C-C[Triple Bond]C-H, following approximately 243.1 nm photodissociation of rovibrationally excited molecules promoted to the second (3nu(1)) and third (4nu(1)) acetylenic C-H overtone and to the third (4nu(CD)) methyl overtone has been investigated. The resulting H and D photoproducts were detected via (2+1) resonantly enhanced multiphoton ionization. The measured room-temperature photoacoustic and jet-cooled action spectra allowed derivation of the molecular parameters of the C-H overtones and the Doppler profiles revealed the translational energies associated with the H(D) photofragments and the H to D branching ratios. Propensities toward the latter were encountered, while the translational energy disposal in both photofragments was essentially identical for a given preexcitation. This behavior agrees with that found for the almost isoenergetic 193.3 nm photolysis of propyne [Qadiri et al., J. Chem. Phys. 119, 12842 (2003)], but contradicts previous findings. The bond fission of C-H and C-D is preceded by internal conversion to, and isomerization on, the ground-state potential energy surface (PES), followed by extensive intramolecular vibrational redistribution. For molecules preexcited to 3nu(1) and 4nu(1) an additional minor channel opens, where elimination of H occurs directly on the accessed excited PES, while that of D on the ground state.