Ilana Bar

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...

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.

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.

Vibrational spectra of α-glucose, β-glucose, and sucrose: anharmonic calculations and experiment.

The anharmonic vibrational spectra of α-D-glucose, β-D-glucose, and sucrose are computed by the vibrational self-consistent field (VSCF) method, using potential energy surfaces from electronic structure theory, for the lowest energy conformers that correspond to the gas phase and to the crystalline phase, respectively. The results are compared with ultraviolet-infrared (UV-IR) spectra of phenyl β-D-glucopyranoside in a molecular beam, with literature results for sugars in matrices and with new experimental data for the crystalline state. Car-Parrinello dynamics simulations are also used to study temperature effects on the spectra of α-D-glucose and β-D-glucose and to predict their vibrational spectra at 50, 150, and 300 K. The effects of temperature on the spectral features are analyzed and compared with results of the VSCF calculations conducted at 0 K. The main results include: (i) new potential surfaces, constructed from Hartree-Fock, adjusted to fit harmonic frequencies from Møller-Plesset (MP2) calculations, that give very good agreement with gas phase, matrix, and solid state spectra; (ii) computed infrared spectra of the crystalline solid of α-glucose, which are substantially improved by including mimic groups that represent the effect of the solid environment on the sugar; and (iii) identification of a small number of combination-mode transitions, which are predicted to be strong enough for experimental observation. The results are used to assess the role of anharmonic effects in the spectra of the sugars in isolation and in the solid state and to discuss the spectroscopic accuracy of potentials from different electronic structure methods.

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.

Writing Droplets of Molecularly Imprinted Polymers by Nano Fountain Pen and Detecting Their Molecular Interactions by Surface-Enhanced Raman Scattering

Molecularly imprinted polymer (MIP) droplets were printed using a pipet or a nano fountain pen on surface-enhanced Raman scattering (SERS)-active surfaces, to directly monitor the uptake and release of a template molecule, the beta-blocking drug propranolol, by SERS. The monitored SERS bands can be related to the template, allowing for its detection but also identification in the MIP. This is an advantage if the technique is to be used during the development phase of MIPs as microstructures, but equally for the readout of MIP-based biochips.

Identification of organic compounds in ambient air via characteristic emission following laser ablation

The potential of laser-induced breakdown spectroscopy (LIBS) as a simple and rapid method for detection of organic compounds in ambient air was investigated. Ablation of samples of aromatic hydrocarbons and nitroaromatic compounds covering various surfaces was performed using the second (532nm) or fourth (266nm) harmonic of a nanosecond pulsed Nd:YAG laser. The plasma emission following the ablation consisted mainly of spectral features related to the CN (B 2 Σ + -X 2 Σ - ) violet system and C 2 (d 3 IIg -a 3 II u )Swan system, and to some C, H, O, and N atomic lines. The CN, O and N lines include some contribution from the interaction of the ensuing plasma with background atmospheric air. There is a correlation between the intensity ratios of the products and the molecular structure, providing the possibility of identifying these compounds by LIBS.

Rotational alignment and non-statistical a doublet population in no following (CH3)3CONO photodissociation

Abstract The rotational alignment and internal state distribution of the NO fragment ejected from the photodissociation of (CH 3 ) 3 CONO in its first absorption band are determined. The rotational distribution is non-Boltzmann and unaffected by the dissociating wavelength. The NO fragment is rotationally aligned and the Π − antisymmetric. Λ doublet component is preferentially populated, indicating planarity of the fragmentation process.

State‐to‐state photodissociation of the fundamental symmetric stretch vibration of water prepared by stimulated Raman excitation

The state‐to‐state photodissociation at 193 nm of the fundamental symmetric stretch vibration of water, H2O (1,0,0), is studied. Stimulated Raman excitation and coherent anti‐Stokes Raman scattering are used to prepare and detect, respectively, particular rotational states of H2O (1,0,0). Laser induced fluorescence is used for monitoring the OH species which are formed from particularly selected rotational states of the H2O (1,0,0) and also from photodissociation of all occupied rotational states of the ground vibrational state, H2O (0,0,0), at room temperature. The cross section for photodissociation from a particular rotation of H2O (1,0,0) at 193 nm is found to be ∼550 times greater than that for H2O (0,0,0). The formation of the OH product in different rotational, Λ‐doublet and spin–orbit states is analyzed for the photodissociation of H2O (0,0,0) and for the photodissociation of the 101, 110+111, 212+211, and 303 rotational states of H2O (1,0,0). The rotational distribution of the OH resulting from p...