Quantum simulator to emulate lower-dimensional molecular structure.

Bottom-up quantum simulators have been developed to quantify the role of various interactions, dimensionality, and structure in creating electronic states of matter. Here, we demonstrated a solid-state quantum simulator emulating molecular orbitals, based solely on positioning individual cesium atoms on an indium antimonide surface. Using scanning tunneling microscopy and spectroscopy, combined with ab initio calculations, we showed that artificial atoms could be made from localized states created from patterned cesium rings. These artificial atoms served as building blocks to realize artificial molecular structures with different orbital symmetries. These corresponding molecular orbitals allowed us to simulate two-dimensional structures reminiscent of well-known organic molecules. This platform could further be used to monitor the interplay between atomic structures and the resulting molecular orbital landscape with submolecular precision.

Cold collisions of OH(2Pi) molecules with He atoms in external fields.

We present rigorous quantum calculations for low-temperature collisions of OH((2)Pi) molecules with He atoms in the presence of external electric and magnetic fields. We show that electric fields of less than 15 kV/cm can be used to enhance the probability for Stark relaxation in collisions of OH (F(1), J = 3/2, M = 3/2, f) molecules by 3 orders of magnitude. The inelastic cross sections display a pronounced resonance structure as a function of the electric field strength. We find that collisions of rotationally excited OH molecules become less sensitive to electric fields with increasing rotational excitation. The calculated total cross sections for (4)He-OH are dominated by elastic scattering, increase monotonically with decreasing collision energy, and show no rapid variations near thresholds, at variance with recent experimental observations (Sawyer et al. Phys. Rev. Lett. 2008, 101, 203203).

Photodissociation dynamics of the A 2Sigma+ state of SH and SD radicals.

Atomic sulfur products from predissociation of the lowest rotational states of SH/SD A (2)Sigma(+) (v(')=0,1,2) are studied using velocity map imaging. The dissociation process, which is slow compared to rotation, is dominated by interference effects due to predissociation of states with low rotation quantum numbers prepared by photoexcitation using overlapping transitions of different parities. The measured product angular distributions can be modeled using the methods presented recently by Kim et al. [J. Chem. Phys. 125, 133316 (2006)]. The S((3)P(J)) (2+1) resonance enhanced multiphoton ionization scheme used in the detection step of the experiment is sensitive to the angular momentum polarization of the atomic fragments. S((3)P(J)), J=2,1,0, fine-structure yields, angular distributions, and atom polarization parameters are reported. Strong polarization of the S((3)P(2,1)) products was observed along with a weak sensitivity of the branching ratio to excess energy and a full insensitivity of the atomic product polarization to excess energy. None of the data fit the predictions of either adiabatic or diabatic photodissociation, emphasizing the need for a fully quantum treatment.

The photodissociation dynamics of ozone at 193 nm: an O(1D2) angular momentum polarization study.

Polarized laser photolysis, coupled with resonantly enhanced multiphoton ionization detection of O(1D2) and velocity-map ion imaging, has been used to investigate the photodissociation dynamics of ozone at 193 nm. The use of multiple pump and probe laser polarization geometries and probe transitions has enabled a comprehensive characterization of the angular momentum polarization of the O(1D2) photofragments, in addition to providing high-resolution information about their speed and angular distributions. Images obtained at the probe laser wavelength of around 205 nm indicate dissociation primarily via the Hartley band, involving absorption to, and diabatic dissociation on, the B 1B2(3 1A1) potential energy surface. Rather different O(1D2) speed and electronic angular momentum spatial distributions are observed at 193 nm, suggesting that the dominant excitation at these photon energies is to a state of different symmetry from that giving rise to the Hartley band and also indicating the participation of at least one other state in the dissociation process. Evidence for a contribution from absorption into the tail of the Hartley band at 193 nm is also presented. A particularly surprising result is the observation of nonzero, albeit small values for all three rank K = 1 orientation moments of the angular momentum distribution. The polarization results obtained at 193 and 205 nm, together with those observed previously at longer wavelengths, are interpreted using an analysis of the long range quadrupole-quadrupole interaction between the O(1D2) and O2(1Deltag) species.

Interaction of NH(X 3Sigma-) with He: potential energy surface, bound states, and collisional Zeeman relaxation.

A detailed analysis of the He-NH((3)Sigma(-)) van der Waals complex is presented. We discuss ab initio calculations of the potential energy surface and fitting procedures with relevance to cold collisions, and we present accurate calculations of bound energy levels of the triatomic complex as well as collisional properties of NH molecules in a buffer gas of (3)He. The influence of the external magnetic field used to trap the NH molecules and the effect of the atom-molecule interaction potential on the collisionally induced Zeeman relaxation are explored. It is shown that minute variations of the interaction potential due to different fitting procedures may alter the Zeeman relaxation rate at ultralow temperatures by as much as 50%.

Theoretical study of the He-HF+ complex. II. Rovibronic states from coupled diabatic potential energy surfaces.

The bound rovibronic levels of the He-HF+ complex were calculated for total angular momentum J=1/2, 3/2, 5/2, 7/2, and 9/2 with the use of ab initio diabatic intermolecular potentials presented in Paper I and the inclusion of spin-orbit coupling. The character of the rovibronic states was interpreted by a series of calculations with the intermolecular distance R fixed at values ranging from 1.5 to 8.5 A and by analysis of the wave functions. In this analysis we used approximate angular momentum quantum numbers defined with respect to a dimer body-fixed (BF) frame with its z axis parallel to the intermolecular vector R and with respect to a molecule-fixed (MF) frame with its z axis parallel to the HF+ bond. The linear equilibrium geometry makes the He-HF+ complex a Renner-Teller system. We found both sets of quantum numbers, BF and MF, useful to understand the characteristics of the Renner-Teller effect in this system. In addition to the properties of a "normal" semirigid molecule Renner-Teller system it shows typical features caused by large-amplitude internal (bending) motion. We also present spectroscopic data: stretch and bend frequencies, spin-orbit splittings, parity splittings, and rotational constants.

Vibrational predissociation in the HCl dimer.

We present results of a combined theoretical and experimental study on the vibrational predissociation of the HCl dimer. On the theoretical side, photodissociation linewidths and product-state distributions for monomer stretch excited states with total angular momentum J=0 were computed, using the Fermi golden rule approximation. The resonances investigated include excitation of the hydrogen bond donor and acceptor stretches, as well as combinations of one of these modes with the intermolecular stretch and geared bend modes, for both even and odd permutation symmetry. Line strengths for the transitions from the J=1, K=0 ground state to excited states with J=0 were computed using quasibound states. On the experimental side, the photofragment angular distribution method was employed to obtain complete final-state distributions for the monomer stretch excited states. Three different transitions were probed, all starting from the lower tunneling component of the ground state: the (R)Q(0)(1) transition for excitation of the acceptor stretch and the (Q)R(0)(0) transition and unresolved (R)Q(0) branch for the donor stretch excitation. We find that, in contrast to the HF dimer, the excited-state alignment of the HCl dimer, resulting from excitation using a polarized laser beam, is completely lost on the time scale of the dissociation. The agreement between theory and experiment for the product-state distributions and line strengths is reasonable. The computed lifetimes are 1-2 orders of magnitude too small, which is attributed to a deficiency in the potential energy surface.