Differing reactivities in the predissociation of acetylene isotopomers pre-excited with three CH stretching quanta

Action Spectroscopy and Predissociation of Vibrationally Excited C2HD

H and D photofragments, produced via vibrationally mediated photodissociation of jet-cooled C2HD allowed measurement of vibrational and vibronic excitation action spectra of the parent as well as H and D Doppler profiles. The initial vibrational preparation at energies around 9700 cm−1 enhanced C—H and C—D bond cleavage for transitions from an initially excited bent state to the upper A ~ state, rather than from a close lying state containing three C—H stretch quanta, due to favorable Franck—Condon factors for the former. The bound region of the A ~ state was accessed, resulting in non-adiabatic dissociation and slight preferential C—H over C—D bond cleavage.

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene Viewed by IR−UV Double Resonance Spectroscopy. 2. Perturbed States with J = 17 and 18

Collision-induced state-to-state molecular energy transfer between rovibrational states in the 12,700 cm(-1) 4nu(CH) manifold of the electronic ground state X of acetylene (C(2)H(2)) is monitored by time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy. Rotational J-states associated with the (nu(1) + 3nu(3)) or (1 0 3 0 0)(0) vibrational combination level, initially prepared by an IR pulse, are probed at approximately 299, approximately 296, or approximately 323 nm with UV laser-induced fluorescence via the Alpha electronic state. The rovibrational J-states of interest belong to a congested manifold that is affected by anharmonic, l-resonance, and Coriolis couplings, yielding complex intramolecular dynamics. Consequently, collision-induced rovibrational satellites observed by IR-UV DR comprise not only regular even-DeltaJ features but also supposedly forbidden odd-DeltaJ features. A preceding paper (J. Phys. Chem. A 2003, 107, 10759) focused on low-J-value rovibrational levels of the 4nu(CH) manifold (particularly those with J = 0 and J = 1) whereas this paper examines locally perturbed states at higher values of J (particularly J = 17 and 18, which display anomalous doublet structure in IR-absorption spectra). Three complementary forms of IR-UV DR experiments (IR-scanned, UV-scanned, and kinetic) are used to address the extent to which intramolecular perturbations influence the efficiency of J-resolved collision-induced energy transfer with both even and odd DeltaJ.

H and D release in ∼243.1 nm photolysis of vibrationally excited 3ν1, 4ν1, and 4νCD overtones of propyne-d3

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.