Experimental and theoretical studies explore the reactivity of the symmetric and the antisymmetric stretching vibrations of monodeuterated methane (CH3D). Direct infrared absorption near 3000 cm−1 prepares CH3D molecules in three different vibrationally excited eigenstates that contain different amounts of symmetric C–H stretch (ν1), antisymmetric C–H stretch (ν4), and bending overtone (2ν5) excitation. The reaction of vibrationally excited CH3D with photolytic chlorine atoms (Cl, 2P3/2) yields CH2D products mostly in their vibrational ground state. Comparison of the vibrational action spectra with the simulated absorption spectra and further analysis using the calculated composition of the eigenstates show that the symmetric C–H stretching vibration (ν1) promotes the reaction seven times more efficiently than the antisymmetric C–H stretching vibration (ν4). Ab initio calculations of the vibrational energies and eigenvectors along the reaction coordinate demonstrate that this difference arises from changes in the initially excited stretching vibrations as the reactive Cl atom approaches. The ν1 vibration of CH3D becomes localized vibrational excitation of the C–H bond pointing toward the Cl atom, promoting the abstraction reaction, but the energy initially in the ν4 vibration flows into the C–H bonds pointing away from the approaching Cl atom and remains unperturbed during the reaction. A simple model using vibrational symmetries and vibrational adiabaticity predicts a general propensity for the greater efficiency of the symmetric stretch for accelerating the reaction in the vibrationally adiabatic limit.

Selective vibrational excitation permits control of the outcome of a reaction with two competing channels. The thermal reaction of CH3D with Cl (2P3/2) yields two reaction products: CH3 from the D-atom abstraction and CH2D from the H-atom abstraction. We prepare the first overtone of the C–D stretching vibration (2ν2) at ∼4300 cm−1 and react the vibrationally excited molecule with photolytic Cl atoms. The 2+1 resonance enhanced multiphoton ionization spectra for the products show that the 2ν2 vibrational excitation of CH3D exclusively increases the probability of breaking the C–D bond, yielding CH3 but no CH2D. By contrast, vibrational excitation of the combination of the antisymmetric C–H stretch and CH3 umbrella (ν4+ν3) vibrations, which has total energy similar to that of 2ν2, preferentially promotes the H-atom abstraction reaction to produce CH2D over CH3. The vibrational action spectra for the two products permit the separation of the two sets of interleaved transitions to give band origins and rotational constants of the 2ν2 state and the ν4+ν3 state of CH3D.

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.

Experiments explore the influence of different C-H stretching eigenstates of CH3D on the reaction of CH3D with Cl(2P3/2). We prepare the mid |110>|0>(A1,E), mid |200>|>0(E), and mid |100>|0> +nu3 +nu5 eigenstates by direct midinfrared absorption near 6000 cm(-1). The vibrationally excited molecules react with photolytic Cl atoms, and we monitor the vibrational states of the CH2D or CH3 radical products by 2+1 resonance enhanced multiphoton ionization. Initial excitation of the |200>|0>(E) state leads to a twofold increase in CH2D products in the vibrational ground state compared to|100>|0> +nu3 +nu5 excitation, indicating mode-selective chemistry in which the C-H stretch motion couples more effectively to the H-atom abstraction coordinate than bend motion. For two eigenstates that differ only in the symmetry of the vibrational wave function, |110>|0>(A1) and |110>|0>(E), the ratio of reaction cross sections is 1.00 +/- 0.05, showing that there is no difference in enhancement of the H-atom abstraction reaction. Molecules with excited local modes corresponding to one quantum of C-H stretch in each of two distinct oscillators react exclusively to form C-H stretch excited CH2D products. Conversely, eigenstates containing stretch excitation in a single C-H oscillator form predominantly ground vibrational state CH2D products. Analyzing the product state yields for reaction of the |110>|0>(A1) state of CH3D yields an enhancement of 20 +/- 4 over the thermal reaction. A local mode description of the vibrational motion along with a spectator model for the reactivity accounts for all of the observed dynamics.

Vibrationally mediated photodissociation combined with Doppler spectroscopy and time-of-flight detection of H-atoms provides information on the photofragmentation dynamics from selected rovibrational states of à 1 A 2 ′′-state ammonia. The competition between adiabatic dissociation forming excited-state NH 2 ( 2 A 1 ) + H and nonadiabatic dissociation leading to ground-state NH 2 ( 2 B 1 ) + H products changes drastically for dissociation from different parent levels prepared by double-resonance excitation. The H-atom speed distributions suggest that the nonadiabatic dissociation channel is the major pathway except for dissociation from the antisymmetric N-H stretching (3 1 ) parent level, which forms exclusively NH 2 ( 2 A 1 ) + H. The energy disposal depends strongly on the parent state with as little as 2% of the available energy channeled into translational energy for dissociation from the 3 1 state.

Vibrationally mediated photodissociation action spectroscopy provides rotation-vibration spectra of jet-cooled ammonia in the 2.3 μm and 3.0 μm regions by detecting the emission of electronically excited NH2(Ã 2A1) produced by the photodissociation of the vibrationally excited molecules. Vibrational excitation changes the relative photofragmentation yield of NH2(Ã 2A1) markedly. Isoenergetic photolysis of ammonia molecules with one quantum of antisymmetric N–H stretching excitation (ν3) or two quanta of bend (2ν4) yields three times more excited state NH2(Ã 2A1) than photolysis of NH3 with a quantum of symmetric N–H stretch excitation (ν1). By contrast, the relative yield is insensitive to initial vibrational excitation of the combination bands ν1+ν2 and ν2+ν3 that contain the umbrella (inversion) motion ν2. The vibrational mode dependence of the NH2(Ã 2A1) photofragment yield arises from either enhanced Franck–Condon factors for electronic excitation or from an increased probability for the competing nonadiabatic dissociation to form the ground state NH2(X̃ 2B1) product.

Vibrationally mediated photodissociation action spectroscopy provides vibronic spectra of the à state of jet-cooled ammonia by detecting the H-atoms produced by the photodissociation of vibrationally excited molecules. Initial vibrational excitation to selected rotation-inversion levels in the N–H stretching fundamental changes the Franck–Condon factors for the subsequent electronic transition markedly. Analysis of the vibronic structure in the à state reveals a progression in both the umbrella and the bending modes and provides fundamental frequencies for the symmetric and antisymmetric stretching motions. Additional state selectivity in infrared–ultraviolet optical double resonance excitation combined with photofragment detection allows rovibronic analysis of the rapidly predissociating levels in the à state of ammonia. The lifetime for NH3(Ã) excited to four quanta of bending motion is as short as 13±4 fs.

Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v5) detected with time-resolved Fourier transform spectroscopy