S. Rosenwaks

Mode‐selective bond fission: Comparison between the photodissociation of HOD (0,0,1) and HOD (1,0,0)

The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O–H or O–D stretching vibrational energy is described. Stimulated Raman excitation and coherent anti‐Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) (O–H stretch) or (1,0,0) (O–D stretch) vibrationally excited HOD. The OD and OH fragments are detected by laser‐induced fluorescence. In the photodissociation of HOD (0,0,1), the yield of both fragments is enhanced [relative to HOD (0,0,0)], but the yield of OD is increased 2.5±0.5 times more than that of OH. In the photodissociation of HOD (1,0,0), no enhancement of the yield of the fragments is obtained. Our results show that even the very lowest possible level of vibrational excitation can be ‘‘leveraged’’ to effect selective bond breaking. Also, these results demonstrate that bond cleavage does not necessarily occur on the weakened bond and they agree with theoretical calculations indicating that the yield of OD and O...

Standoff detection of trace amounts of solids by nonlinear Raman spectroscopy using shaped femtosecond pulses

We demonstrate a single-beam, standoff (>10m) detection and identification of various materials including minute amounts of explosives under ambient light conditions. This is obtained by multiplex coherent anti-Stokes Raman scattering spectroscopy (CARS) using a single femtosecond phase-shaped laser pulse. We exploit the strong nonresonant background for amplification of the backscattered resonant CARS signals by employing a homodyne detection scheme. The simple and highly sensitive spectroscopic technique has a potential for hazardous materials standoff detection applications.

Direct observation of preferential bond fission by excitation of a vibrational fundamental: Photodissociation of HOD (0,0,1)

The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O–H stretching vibrational energy is described. Stimulated Raman excitation and coherent anti‐Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) vibrationally excited HOD. The OD and OH fragments are detected by laser induced fluorescence. The photodissociation of the HOD (0,0,1) molecules yields at least three times more OD than OH.

Photodissociation of HOD (νOD=3): Demonstration of preferential O–D bond breaking

It has been predicted that photodissociation of vibrationally excited HOD may preferentially yield either OD+H or D+OH, depending on the vibrational mode and the dissociation wavelength. To date, only the former preference has experimentally been demonstrated. In the present work preferential O–D bond breaking has been achieved from the photolysis of HOD (νOD=3) at 193 nm. HOD was prepared in a specific rovibrational level of the second overtone of the O–D stretch via infrared excitation. The subsequent photolysis led to enhancement of both OH and OD production, the OH/OD branching ratio being 2.6±0.5. The results agree with the predictions of Imre and co‐workers [J. Phys. Chem. 93, 1840 (1989)] on the enhancement of νOD≥3 photodissociation but differ in the branching ratio obtained at the specific photolysis wavelength.

Controlling bond cleavage and probing intramolecular dynamics via photodissociation of rovibrationally excited molecules

Photodissociation studies of vibrationless ground state molecules pervade diverse areas of chemical physics, while those of rovibrationally excited molecules are expected to have even more impact due to the additional fascinating possibilities they offer and the new horizons they open. Photodissociation of rovibrationally excited species involves a double-resonance scheme in which a photodissociative transition is initiated from an excited rovibrational state that might substantially affect the intensity and wavelength dependence of the photoabsorption spectrum. In favourable cases, when the energy is disposed in vibrations that are strongly coupled to the reaction coordinate, this pre-excitation might influence photodissociation pathways and lead to selective bond cleavage. In other cases it might influence the branching ratio between different fragments by altering the photodissociation dynamics. Moreover, the photodissociation of rovibrationally excited species can serve as a sensitive means for detection of weak vibrational overtone transitions of jet-cooled molecules, and therefore a promising way for revealing specific couplings and time evolution of the prepared vibrational states. Experimental studies on different polyatomics are used to demonstrate the above aspects and to show how the mechanism of chemical transformations and the nature of rovibrationally excited states are highlighted by photolysis of these pre-excited molecules.

Absolute rate constants and reactive cross sections for the reactions of O(1D) with molecular hydrogen and deuterium

Abstract The dynamics of the reaction of O( 1 D) with molecular hydrogen and deuterium has been investigated using “superthermal” O( 1 D) atoms generated by laser photolysis of N 2 O at 193 nm. H/D atom products have been detected under single-collision conditions by vacuum ultraviolet laser-induced fluorescence at the Lyman-α transition. With a calibration method using HCl photolysis as a source of well-defined H atom concentrations, the following absolute rate constants k and absolute reactive cross sections σ R have been determined: k = (2.7±0.6) × 10 −10 cm 3 s −1 molecule −1 and σ R (0.12 eV) =7.6±1.5 A 2 for the reaction O( 1 D)+H 2 →OH+H, and k =(2.3±0.5)×10 −10 cm 3 s −1 molecule −1 and σ R (0.18 eV)=7.0±1.4 A 2 for the reaction O( 1 D)+D 2 →OD+D.

Scalar and vectorial properties in the photodissociation of tert‐butyl nitrite from the S1 and S2 states

A detailed comparison between the dynamics of photodissociation of (CH3)3CONO from its first two excited singlet states is presented. The fragmentation processes are studied by exciting the molecule at 365.8 and 351.8 nm [S1(nπ*)←S0 transition] and at 250 nm [S2(ππ*)←S0 transition] and probing the NO fragment by single photon laser induced fluorescence combined with polarization and sub‐Doppler spectroscopy. The μ, v, and J vector correlations, Λ‐doublet and spin–orbit populations, and the vibrational, rotational, and translational energy content of the NO fragment are determined. The scalar and vectorial properties point on different mechanisms of fragmentation from the S1 and S2 states, but both are highly selective. The findings of this study, especially those concerning the less studied S2 state, can be utilized to predict the behavior of other alkyl nitrites and demonstrate the power of the techniques mentioned above in characterizing the dynamics of photodissociation, even for large molecules.

Rotational-state dependent selectivity in the bond fission of C2HD (5ν1)

Abstract We report the first demonstration of rotational-state selectivity in bond fission. C 2 HD is prepared in the 5 ν 1 vibrational-state and photodissociated by ∼ 243.1 nm photons that also probe the H/D fragments. The production of both H and D is greatly enhanced upon rovibrational excitation and is rotational-state dependent. The H/D branching ratio for the photodissociation of C 2 HD (5 ν 1 ) is 1.5 ± 0.2 for J ′ = 2 and 4.9 ± 0.5 for J ′ = 4. Possible mechanisms for the rotational dependence are discussed.

Chemiluminescence of AlO

Ultraviolet, visible, and near ir emission has been observed from flames produced by reactions of Al atoms with various oxidants. Spectra of the flames and photon yields for the reactions were measured. Emission from the A, B, and C states to the ground state of AlO has been identified. A continuumlike emission between 280 and 750 nm also has been observed; it is suggested that this emission is not from diatomic aluminum oxide. Photon yields obtained are 3% for Al + microwave discharged O2, 2% for O3, 0.4% for N2O, and less than 0.005% for O2, CO2, NO2, NO, and CO.

Absolute rate constants, reactive cross-sections and isotopic branching ratio for the reaction of O(1D) with HD

Abstract Using the laser photolysis vacuum-UV laser-induced fluorescence ‘pump-and-probe’ technique, Doppler profiles of H and D atoms from the reaction O( 1 D) + HD were measured under single-collision conditions, O( 1 D) atoms were generated by laser photolysis of N 2 O at 193 nm. With a calibration method using HCl photolysis as a source of well defined H atom concentrations the following absolute rate constants k and absolute reactive cross-sections σ R have been determined: k = (1.3 ± 0.3) × 10 −10 cm 3 s −1 molec −1 and σ R (0.14 eV) = 4.0 ± 0.9 A 2 for the reaction channel O( 1 D) + HD → OD + H, and k = (1.0 ± 0.3) × 10 −10 cm 3 s −1 molec −1 and σ R (0.14 eV) = (3.0 ± 0.7) A 2 for the reaction channel O( 1 D) + HD → OH + D. The isotopic branching ratio for the reaction O( 1 D) + HD was measured directly to be Γ H/D = 1.35 ± 0.20. In addition, from the measured H and D atom Doppler profiles the fraction ƒ T of the available energy released as translational energy was determined for the individual product channels to be ƒ T (OD + H) = 0.41 ± 0.07 and ƒ T (OH + D) = 0.32 ± 0.05.