Coherent phase control of the product branching ratio in the photodissociation of dimethylsulfide

Coherent phase control of the photodissociation reaction of the dimethylsulfide has been achieved by means of quantum-mechanical interference between one- and three-photon transitions. Dimethylsulfide was irradiated by fundamental and frequency-tripled outputs of a visible laser (600.5-602.5 nm), simultaneously to yield CH3S+ and CH3SCH2+ fragment ions. The branching ratio of the two product channels could be modulated with variation of the phase difference between the light fields. This accounted for the difference between the molecular phases of the two product channels. The phase lag was observed to have a maximum value of 8 degrees at 601.5 nm. This is the first result of a selective bond breaking in a polyatomic molecule by the coherent phase control.     

Determination of spin-orbit branching fractions in the photodissociation of halogenated hydrocarbons

Two methods based on vacuum ultraviolet (vuv) photoionization are presented for the determination of the spin-orbit branching fractions of the halogen atom produced in the photodissociation of halogenated hydrocarbons. Both methods make use of differences in the photoionization cross sections of the 2P(3/2) ground state and the 2P(1/2) excited-state of the neutral halogen atom. In the first approach, measurements of the total photoionization signal of the halogen atom are made at several vuv wavelengths, and the difference in the wavelength dependences for the 2P(3/2) and 2P(1/2) atoms allows the extraction of the branching fractions. In the second approach, the vuv wavelength is set close to the ionization threshold of the 2P(3/2) atom (well above that of the 2P(1/2) atom), and measurements are made at several electric field strengths, which shift the ionization threshold and thus vary the photoionization cross sections. In both methods, the relative cross sections of the ground- and excited-state atoms are determined by using the known branching fractions for the 266 nm photodissociation of methyl iodide. These methods are applied to the photodissociation of isopropyl iodide and allyl iodide, two systems for which standard ion-imaging techniques do not provide unique branching fractions.     

Effect of the Gouy phase on the coherent phase control of chemical reactions

We show how the spatial phase of a focused laser beam may be used as a tool for controlling the branching ratio of a chemical reaction. Guoy discovered [Acad. Sci., Paris, C. R. 110, 1250 (1890)] that when an electromagnetic wave passes through a focus its phase increases by pi. In a coherent control scheme involving the absorption of n photons of frequency omega(m) and m photons of frequency omega(n), the overall phase shift produced by the Gouy phase is (n-m)pi. At any given point in space, this phase shift is identical for all reaction products. Nevertheless, if the yields for different reaction channels have different intensity dependencies, the Gouy phase produces a net phase lag between the products that varies with the axial coordinate of the laser focus. We obtain here analytical and numerical values of this phase as the laser focus is scanned across the diameter of the molecular beam, taking into account the Rayleigh range and astigmatism of the laser beam and saturation of the transition. We also show that the modulation depth of the interference pattern may be increased by optimizing the relative intensities of the two fields.     

Orbital phase control of the preferential branching of chain molecules.

The orbital phase theory was applied to the stabilities of the branched isomers (1) of E(4)H(10) (E = C, Si, Ge, Sn) relative to the normal ones (2). The orbital phase prediction was confirmed by ab initio molecular orbital (MO) and density functional theory (DFT) calculations as well as by some experimental results. Further applications to the relative stabilities of other alkane and alkene isomers lead to the preference of the branched to the normal isomers, the neopentane-type to isobutane-type branching, the terminal to inner methyl branching, and the methyl to ethyl inner substitution in the longer alkanes, as well as the preference of isobutene to 2-butene moieties. The preferential stabilization of the branched isomers was shown to be general and controlled by the orbital phase.     

Hydrogen bond dynamics in the excited states: photodissociation of phenol in clusters

We have investigated the photodynamics of phenol molecules in clusters. Possible reaction pathways following the photoexcitation of hydrogen-bonded phenol clusters have been identified theoretically using ab initio calculations. Experimentally we have studied the phenol molecules and clusters of various size distributions in a molecular beam apparatus. In particular, we have measured the H-fragment kinetic energy distributions after the excitation with 243 nm and 193 nm laser radiation. At 243 nm the KED spectra did not show any significant difference between the photodissociation of isolated molecules and phenol in larger clusters, while at 193 nm the contribution of the fast H-fragments is significantly suppressed in clusters with respect to the bare phenol molecule. We have interpreted the experimental results within the framework of the suggested reaction pathways.     

Gas phase solvatochromic effects of phenol and naphthol photoacids

A quantum chemical study of spectral shifts by single molecule solvation of phenol, α-naphthol, and β-naphthol is presented. The methods employed include the equation-of-motion coupled cluster, the similarity transformed equation-of-motion coupled cluster, single excitation configuration-interaction, and time-dependent density functional theory. Based on the calculations, there is no evidence that there is significant charge-transfer between the solute and the solvent. Instead, it appears that the observed solvation redshift is due to the nature of the excited state on the solute molecule.     

Infrared predissociation of ternary cluster cations: the solvent effects on the branching ratio

Infrared (IR) predissociation of hydrogen-bonded ternary cluster ions such as aniline-water-ethanol (AWE(+)), aniline-water-isopropanol (AWP(+)), aniline-methanol-ethanol (AME(+)), aniline-water-pyrrole (AWPy(+)), and aniline-water-benzene (AWB(+)) was examined in the region of 2700-4000 cm(-1) to explore the key factors which determine the branching ratios in the concurrent unimolecular dissociation. The smaller solvent molecule was predominantly ejected when the binding energies of the two were not too different. On the other hand, when they were far off, the binding energy also acted significantly on the branching ratio. Besides, mode-selective IR predissociation was observed, while the selectivity was not quite distinct. The IR predissociation of ternary cluster ions bound via hydrogen bonding is considered to occur on a time scale much faster than intramolecular vibrational energy redistribution, which was proved by a statistical transition state theory.     

Complete quantum control of the population transfer branching ratio between two degenerate target states

A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic passage scheme is proposed to realize complete control of the population transfer branching ratio between two degenerate target states. The control is achieved via a three-node null eigenstate that can be correlated with an arbitrary superposition of the target states. Our results suggest that complete suppression of the yield of one of two degenerate product states, and therefore absolute selectivity in photochemistry, is achievable and predictable, even without studying the properties of the unwanted product state beforehand.     

H atom elimination from the pi sigma* state in the photodissociation of phenol

Photodissociation of phenol at 248 nm was studied using multimass ion imaging techniques. Photofragment translational energy distribution of H atom elimination was measured. The results demonstrate that H atom elimination occurs on the pi sigma(*) excited state which has repulsive potential-energy functions with respect to the stretching of OH bond. It supports the recent ab initio calculation.     

Site and isotopic effects on the angular anisotropy of products in the photodissociation of ethene at 157 nm

We measured angular-anisotropy parameters beta(E(t)) of fragments from photolysis of ethene and four isotopic variants at 157 nm using photo-fragment translational spectroscopy and selective photoionization. The averaged beta value of products ranges from -0.17 to 0.10, depending on dissociation pathways. Angular distributions of atomic hydrogen produced from C(2)H(4) and C(2)D(4) are isotropic. For dissociation into C(2)H(2) + H(2), beta has a small negative value whereas dissociation into C(2)D(2) + D(2) has an isotropic angular distribution. The photolysis of dideuterated ethene reveals site and isotopic effects on the angular distributions of products; products H(2), HD, and D(2) from photolysis of 1,1-CH(2)CD(2) have negative, nearly zero, and positive values of beta, respectively. Molecular hydrogen from photolysis of 1,2-cis-CHDCHD has a negative beta value and the anisotropy has a trend D(2) > H(2) > HD. Photolysis of 1,2-trans-CHDCDH produced a result similar to photolysis of 1,2-cis-CHDCHD for the angular anisotropy of molecular hydrogen except slightly more isotropic. A calculation of optimized geometries of ethene in the ground electronic state and pertinent transition structures enables a qualitative interpretation of the site and isotopic effects on the angular anisotropy of products. We deduce that the photoexcited state of ethene at 157 nm has a major character (1)B(1u) that produces a transition dipolar moment parallel to the C=C bond.     

Geometric effects control isothermal oxidation of graphite flakes

The oxidation, in a neat oxygen atmosphere, of high-purity and highly crystalline natural graphite and synthetic Kish graphite was investigated. The physico-geometric model function of the kinetic rate equation was experimentally determined by isothermal thermogravimetric analysis at 650?°C. Analytic solutions for basic flake shapes indicate that this function strictly decreases with conversion. However, for both samples the experimental data trend was a rapid initial increase followed by the expected decrease to zero. High resolution field emission scanning electron microscopy, of partially oxidized flakes, provided plausible explanations for this discrepancy. Rapid development of macroscopic surface roughness during the initial stages of oxidation was evident and could be attributed to the presence of catalytic impurities. Large fissures along the planes of the natural graphite and the initiation, growth and coalescence of internal cavities in the Kish graphite were observed. Flake models incorporating the latter two features are difficult to analyse analytically. However, a facile probabilistic approach showed that reasonably good agreement with experimental data was possible.     

Electronically excited products in the photodissociation of dianthracene and related compounds

The photodissociation of dianthracene and related compounds leads to appreciable fractions of excited state excimer or exciplex products, dependent on temperature and solvent viscosity. The experimental evidence strongly suggests the involvement of an intermediate with an electronic structure which correlates with that of the benzyl radical.     

Isotope effects in the photofragmentation of symmetric molecules: the branching ratio of OD/OH in water

With HOD initially in its vibrational ground state, we present a new detailed interpretation of the OD/OH branching ratio (approximately 3) in the photoinduced process D+OH<--HOD-->H+OD, in the first absorption band. Using semiclassical arguments, we show that the branching ratio has little to do with the initial distribution of configurations, but the initial momentum distribution plays a key role in determination of the branching ratio. The formation of D+OH arises from initial situations where OD is stretching, and it stretches faster than OH, whereas all other motions lead to H+OD. This picture is confirmed by quantum wave-packet calculations.     

Theoretical study of the UV photodissociation of Cl2: potentials, transition moments, extinction coefficients, and Cl*/Cl branching ratio

Potential energy curves for the X (1)Sigma(g) (+) ground state and Omega=0(u) (+), 1(u) valence states and dipole moments for the 0(u) (+), 1(u)-X transitions are obtained in an ab initio configuration interaction study of Cl(2) including spin-orbit coupling. In contrast to common assumptions, it is found that the B (3)Pi(0(+)u)-X transition moment strongly depends on internuclear distance, which has an important influence on the Cl(2) photodissociation. Computed energy curves and transition moments are employed to calculate the A, B, C<--X extinction coefficients, the total spectrum for the first absorption band, and the Cl(*)((2)P(1/2))/Cl((2)P(3/2)) branching ratio as a function of excitation wavelength. The calculated data are shown to be in good agreement with available experimental results.     

Structural effects on the photodissociation of alkoxyamines

The search for photosensitive alkoxyamines represents a huge challenge. The key parameters governing the cleavage process remain unknown. The dissociation process of light sensitive alkoxyamines is studied as a function of their chemical structures. The photochemical properties of 6 selected compounds are investigated by ESR and laser flash photolysis. It is found that (i) the selectivity of the cleavable N-O vs. C-O bond and (ii) the efficiency of the nitroxide formation are strongly related to the alkoxyamine structure. The distance between the chromophore and the aminoxy group is a key parameter for an efficient pathway of the radical generation as displayed by the photopolymerization ability of these alkoxyamines.     

Electronic polarization effects in the photodissociation of Cl2

Velocity mapped ion imaging and resonantly enhanced multiphoton ionization time-of-flight methods have been used to investigate the photodissociation dynamics of the diatomic molecule Cl(2) following excitation to the first UV absorption band. The experimental results presented here are compared with high level time dependent wavepacket calculations performed on a set of ab initio potential energy curves [D. B. Kokh, A. B. Alekseyev, and R. J. Buenker, J. Chem. Phys. 120, 11549 (2004)]. The theoretical calculations provide the first determination of all dynamical information regarding the dissociation of a system of this complexity, including angular momentum polarization. Both low rank K = 1, 2 and high rank K = 3 electronic polarization are predicted to be important for dissociation into both asymptotic product channels and, in general, good agreement is found between the recent theory and the measurements made here, which include the first experimental determination of high rank K = 3 orientation.     

The ultraviolet photodissociation of DCl: H/D isotope effects on the Cl(P) atom spin–orbit branching

The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength tail of the first UV absorption band (A¹Π₁←X¹Σ⁺) has been investigated at five wavelengths in the range 200–220 nm. Ground state Cl(²P_3/2) and spin–orbit excited Cl^*(²P_1/2) photofragments were monitored using (2+1) resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. The product branching fractions are reported and compared with previous experimental results and high-level quantum mechanical calculations for HCl and DCl. A significant H/D isotope effect in the branching fractions is found at all the studied wavelengths, in quantitative agreement with recent theoretical predictions.     

Dynamics at conical intersections: the influence of O-H stretching vibrations on the photodissociation of phenol

Comparing the recoil energy distributions of the fragments from one-photon dissociation of phenol-d(5) with those from vibrationally mediated photodissociation shows that initial vibrational excitation strongly influences the disposal of energy into relative translation. The measurements use velocity map ion imaging to detect the H-atom fragments and determine the distribution of recoil energies. Dissociation of phenol-d(5) molecules with an initially excited O-H stretching vibration produces significantly more fragments with low recoil energies than does one-photon dissociation at the same total energy. The difference appears to come from the increased probability of adiabatic dissociation in which a vibrationally excited molecule passes around the conical intersection between the dissociative state and the ground state to produce electronically excited phenoxyl-d(5) radicals. The additional energy deposited in electronic excitation of the radical reduces the energy available for relative translation.     

Communication: Conditions for one-photon coherent phase control in isolated and open quantum systems

Coherent control of observables using the phase properties of weak light that induces one-photon transitions is considered. Measurable properties are shown to be categorizable as either class A, where control is not possible, or class B, where control is possible. Using formal arguments, we show that phase control in open systems can be environmentally assisted.     

Gas and solution phase chloroiodomethane short-time photodissociation dynamics in the B-band absorption

Resonance Raman spectra were obtained within and to the red of the B-band absorption spectrum of gas phase chloroiodomethane and chloroiodomethane in cyclohexane solvent. The spectra show the fundamental and overtones of the nominal C---I stretch (nν₅) and combination bands of the CH₂ wag (ν₃), I---C---Cl bend (ν₆), and the CH₂ scissor (ν₂) fundamentals with the C---I stretch bands (nν₅). The chloroiodomethane B-band short-time photodissociation dynamics have significant substituent effects relative to the B-band of iodomethane due to the presence of the C---Cl chromophore n(X) → σ^* (C---X) transitions ≈170 nm that are close to the B-band absorption of chloroiodomethane but absent in iodomethane.