Using the Phase of Light as a Photochemical Tool

Contribution of the Gouy phase to two-pathway coherent control of the photoionization and photodissociation of vinyl chloride

The electric field of a light wave accumulates a pi phase shift as it passes through a focus. We show here how this effect, known as the Gouy phase, may be used to control the branching ratio of a unimolecular reaction when the products are formed with different numbers of photons. We demonstrate this control method for the ionization and dissociation of vinyl chloride, using absorption of 177 and 532 nm photons to induce a pair of interfering paths. Excellent agreement between the observed and calculated phase shift as a function of the axial coordinate of the laser focus indicates that fragmentation occurs via a ladder switching mechanism. The axial dependence of the modulation depth is evidence of loss of coherence at higher internal temperatures of the molecule.

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.

Role of the gouy phase in the coherent phase control of the photoionization and photodissociation of vinyl chloride

We demonstrate theoretically and experimentally that the Gouy phase of a focused laser beam may be used to control the photoinduced reactions of a polyatomic molecule. Quantum mechanical interference between one- and three-photon excitation of vinyl chloride produces a small phase lag between the dissociation and ionization channels on the axis of the molecular beam. Away from the axis, the Gouy phase introduces a much larger phase lag that agrees quantitatively with theory without any adjustable parameters.

Coherent spectroscopy in dissipative media: Time-domain studies of channel phase and signal interferometry

We extend a recently formulated coherence spectroscopy of dissipative media [J. Chem. Phys. 122, 084502 (2005)] from the stationary excitation limit to the time domain. Our results are based on analytical and numerical solutions of the quantum Liouville equation within the Bloch framework. It is shown that the short pulse introduces a new, controllable time scale that allows better insight into the relation between the coherence signal and the phase properties of the material system. We point to the relation between the time-domain coherence spectroscopy and the method of interferometric two-photon photoemission spectroscopy, and propose a variant of the latter method, where the two time-delayed excitation pathways are distinguishable, rather than identical. In particular, we show that distinguishability of the two excitation pathways introduces the new possibility of disentangling decoherence from population relaxation.

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.

Laser Control of Desorption through Selective Surface Excitation

We review recent developments in controlling photoinduced desorption processes of alkali halides. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. We then extend the exciton mechanism of desorption, developed for alkali halides, to metal oxide surfaces, in particular magnesium oxide. In addition, these results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.

Coherence spectroscopy in dissipative media: A Liouville space pathway approach

We address the possibility of using coherent control tools to extract useful information about the interaction of a system with a dissipative environment. To that end we extend previous work, which developed a coherence spectroscopy based on two-pathway excitation phase control, from the isolated molecule limit to dense media. Specifically, we explore the properties of the channel phase, an observable of energy-domain two-pathway excitation experiments that was shown in the isolated molecule limit to carry information about the phase properties of the material system. Our analysis is based on the combination of steady state and time-dependent analytical perturbative approaches within the density matrix formalism, complemented by nonperturbative numerical simulations. We find that the channel phase carries significantly richer information in the presence of decoherence mechanisms than in their absence. In particular, rescattering events in the structured continuum introduce new features in the channel phase spectrum, whose structure conveys information about both the molecular continuum and the system bath interaction.

Coherent control of photofragment separation in the dissociative ionization of IBr

We have investigated coherent control of the dissociative ionization of IBr using phase-controlled two-color omega+2omega laser pulses with an intensity of 1.0 x 10(12) W/cm and a pulse duration of 130 fs. The directional asymmetries of the photofragment angular distributions showed oscillation behavior dependent on the relative phase difference between the omega and 2omega pulses. The phase dependencies of the directional asymmetries observed for iodine ions and bromine ions were out of phase with each other. This result shows that a phase-controlled omega+2omega optical field can produce molecular orientation in which the optical field discriminates between parallel and antiparallel configuration of molecules that have a permanent dipole.

Laser control of product electronic state: Desorption from alkali halides

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin-orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P(1/2)), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.