Robert J. Holiday

The relative reactivity of CH3D molecules with excited symmetric and antisymmetric stretching vibrations

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 change...

Control of bimolecular reactions: Bond-selected reaction of vibrationally excited CH3D with Cl(2P3/2)

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 rotat...

Mode- and bond-selective reaction of Cl(P3∕22) with CH3D: C–H stretch overtone excitation near 6000cm−1

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.

Vibrational spectroscopy and photodissociation of jet-cooled ammonia

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(A 2A1) produced by the photodissociation of the vibrationally excited molecules. Vibrational excitation changes the relative photofragmentation yield of NH2(A 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(A 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(A 2A1) photofragment yield arises from either enhanced Franck–Condon factors for electronic excitation or from an increased probability for the competing non...

Vibronic structure and photodissociation dynamics of the à state of jet-cooled ammonia

Vibrationally mediated photodissociation action spectroscopy provides vibronic spectra of the A 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 A 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 A state of ammonia. The lifetime for NH3(A) excited to four quanta of bending motion is as short as 13±4 fs.

Photodissociation of vibrationally excited ammonia: Rotational excitation in the NH2 product

Vibrationally mediated photodissociation combined with H Rydberg atom photofragment translational spectroscopy reveals the state-to-state photofragmentation dynamics of selected parent rovibronic levels of A 1A2″ state ammonia. Analysis of the time-of-flight spectra determines the population of quantum states in the NH2 partner fragment for dissociation from the excited state bending vibration (41). Dissociation from the bending state produces rotationally excited NH2 predominantly (≈75%) in its ground vibrational state, in agreement with theoretical predictions.