Photodissociation of HI and DI: Polarization of atomic photofragments

Generation of Sub-nanosecond H Atom Pulses for Scattering from Single-Crystal Epitaxial Graphene

Pulsed molecular beams allow high-density gas samples to be cooled to low internal temperatures and to produce narrow speed distributions. They are particularly useful in combination with pulsed-laser-based detection schemes and have also been used as pump pulses in pump–probe experiments with neutral matter. The mechanical response of pulsed valves and chopper wheels limits the duration of these pulses typically to about 10–100 μs. Bunch compression photolysis has been proposed as a means to produce atomic pulses shorter than 1 ns—an experimental capability that would allow new measurements to be made on chemical systems. This technique employs a spatially chirped femtosecond duration photolysis pulse that produced an ensemble of H atom photoproducts that rebunches into a short pulse downstream. To date, this technique could not produce strong enough beams to allow new experiments to be carried out. In this paper, we report production of pulsed H atom beams consistent with a 700 ps pulse duration and with sufficient intensity to carry out differentially resolved inelastic H scattering experiments from a graphene surface. We observe surprisingly narrow angular distributions for H atoms incident normal to the surface. At low incidence energies quasi-elastic scattering dominates, and at high incidence energy we observe a strongly inelastic scattering channel. These results provide the basis for future experiments where the H atoms synchronously collide with a pulsed-laser-excited surface.

Ultrahigh-density spin-polarized hydrogen isotopes from the photodissociation of hydrogen halides: new applications for laser-ion acceleration, magnetometry, and polarized nuclear fusion

Recently, our group produced spin-polarized hydrogen (SPH) atoms at densities of at least 1019 cm-3 from the photodissociation of hydrogen halide molecules with circularly polarized UV light and measured them via magnetization-quantum beats with a pickup coil. These densities are approximately 7 orders of magnitude higher than those produced using conventional methods, opening up new fields of application, such as ultrafast magnetometry, the production of polarized MeV and GeV particle beams, such as electron beams with intensities approximately 104 higher than current sources, and the study of polarized nuclear fusion, for which the reaction cross sections of D-T and D-3He reactions are expected to increase by 50% for fully polarized nuclear spins. We review the production, detection, depolarization mechanisms, and potential applications of high-density SPH.

Photofragment spin-polarization measurements via magnetization quantum beats: dynamics of DI photodissociation

We report the electron-spin polarization of D atoms from the photodissociation of DI, at 213 nm and 266 nm, by measuring the magnetization quantum beats of the D atoms with a pick-up coil. We determine that the polarization P is large at both wavelengths (|P|∼ 1), however it is positive at 213 nm, and negative at 266 nm. These results, in both cases, are of opposite sign to calculations, which assume adiabatic dissociation along the A¹Π₁ or a³Π₁ states. We interpret these results as evidence that nonadiabatic coupling between these states needs to be included for the theoretical treatment of DI photodissociation.

Photon catalysis of deuterium iodide photodissociation

The photodissociation of deuterium iodide is catalyzed by the electric field supplied by nonresonant IR photons.

Photodissociation of HBr. 1. Electronic Structure, Photodissociation Dynamics, and Vector Correlation Coefficients

Ab initio potential energy curves, transition dipole moments, and spin-orbit coupling matrix elements are computed for HBr. These are then used, within the framework of time-dependent quantum-mechanical wave-packet calculations, to study the photodissociation dynamics of the molecule. Total and partial integral cross sections, the branching fraction for the formation of excited-state bromine atoms Br(2P(1/2)), and the lowest order anisotropy parameters, beta, for both ground and excited-state bromine are calculated as a function of photolysis energy and compared to experimental and theoretical data determined previously. Higher order anisotropy parameters are computed for the first time for HBr and compared to recent experimental measurements. A new expression for the Re[a1(3) (parallel, perpendicular)] parameter describing coherent parallel and perpendicular production of ground-state bromine in terms of the dynamical functions is given. Although good agreement is obtained between the theoretical predictions and the experimental measurements, the discrepancies are analyzed to establish how improvements might be achieved. Insight is obtained into the nonadiabatic dynamics by comparing the results of diabatic and fully adiabatic calculations.

Atomic polarization in the photodissociation of diatomic molecules

The angular momentum polarization of atomic photofragments provides a detailed insight into the dynamics of the photodissociation process. In this article, the origins of electronic angular momentum polarization are introduced and experimental and theoretical methods for the measurement or calculation of atomic orientation and alignment parameters described. Many diatomic photodissociation systems are surveyed, in order to provide an overview both of the historical development of the field and of the most state-of-the-art contemporary studies.

Photodissociation of HI and DI: Testing models for electronic structure via polarization of atomic photofragments

The photodissociation dynamics of HI and DI are examined using time-dependent wave-packet techniques. The orientation and alignment parameters aQ(K) (p) are determined as a function of photolysis energy for the resulting ground-state I(2P(3/2)) and excited-state I(2P(1/2)) atoms. The aQ(K) (p) parameters describe the coherent and incoherent contributions to the angular momentum distributions from the A 1pi(1), a 3pi(1), and t 3sigma(1) electronic states accessed by perpendicular excitation and the a 3pi(0+) state accessed by a parallel transition. The outcomes of the dynamics based on both shifted ab initio results and three empirical models for the potential-energy curves and transition dipole moments are compared and contrasted. It is demonstrated that experimental measurement of the aQ(K) (p) parameters for the excitation from the vibrational ground state (upsilon=0) would be able to distinguish between the available models for the HI potential-energy curves and transition dipole moments. The differences between the aQ(K) (p) parameters for the excitation from upsilon=0 stand in sharp contrast to the scalar properties, i.e., total cross section and I* branching fraction, which require experimental measurement of photodissociation from excited vibrational states (upsilon>0) to distinguish between the models.