Session K1: Focus Session: Recent Advances in Collision Studies

Two Electrons Far From Home

We have developed a computational method that allows us to solve the time dependent Schrodinger equation for two electrons where the number of coupled angular momentum is large. The ability to include a large number of coupled angular momentum is necessary when the two electrons strongly interact at large distances from the nucleus. We have performed test calculations that demonstrated the stability of the method and the convergence with increasing number of angular momenta; the method was stable through the largest number of coupled angular momenta we attempted (approximately 200). We have used this method to perform quantum calculations for the system described in A.L. Landers et al, PRL 102, 223001 (2009). In this experiment, a photon is absorbed by a Ne 1s electron leaving a core hole; the outgoing electron has low energy (order 1 eV). The Ne+ subsequently decays which usually gives an Auger electron of approximately 800 eV. The interaction between the Auger electron and photo-electron gives an interesting pattern in the distribution with respect to the angle between the electrons. A classical calculation gave qualitative agreement with experiment but substantive differences remain. We have also used this computational method to investigate the interaction between two Rydberg electrons. In recent experiments (private communication R.R. Jones), two electrons were placed in similar energy Rydberg states for an alkaline-earth atom. Using a pulsed laser, one electron is placed in a Rydberg wave packet with the other valence electron left in a tightly bound state. A subsequent laser excites the tightly bound electron to a similar radius orbit. This system can be partly controlled by varying the time between the laser pulses and by varying the energies of the two Rydberg electrons. The quantum calculation can study the ionization of this system as a function of time.   

Large Effects of Electric Fields on Atom-Molecule Collisions at Millikelvin Temperatures

Controlling interactions between cold molecules using external fields can elucidate the role of quantum mechanics in molecular collisions. In the millikelvin temperature regime, only a few partial waves contribute to the scattering, which allows us to understand in detail the role of quantum mechanics. We create a new experimental platform in which ultracold atoms and cold molecules are separately cooled and trapped by magnetic and electric fields and then brought together to study collisions. Using this new playground, we explore elastic and inelastic collisions between rubidium atoms and ammonia molecules in the ground ro-vibrational state. We use quantum-mechanical scattering calculations and experimental measurements to show that electric fields can have a major effect on the molecular collisions, even in the absence of dipole-dipole interactions.   

Dissipation in Parametrically Driven Few-Atom Systems

Few body systems provide an exciting platform for studying irreversible processes such as dissipation in isolated systems. They are sufficiently small such that all degrees of freedom can be modeled, and yet large enough that they may display irreversible behavior. Few interacting atoms in an optical micro-trap is an excellent testing ground in this field as the atoms are effectively shielded from interactions with the environment. In this talk we will describe our recent experiments. We trap a controlled number of atoms in an optical micro-trap and drive the trapping potential parametrically. Strong resonances in the energy absorbed by the atom are expected due to periodic motion of the atom in the trap. These provide a standard method of measuring trap frequencies. We perform the experiment and gradually change the initial number of atoms in the trap from 1 to 50. When only one atom is present in the trap we observe excellent agreement with the single particle model. When more atoms are introduced to the trap we observe a gradual transition to a strongly damped regime where the trap dependent resonances vanish.   

Astrochemistry in an Ion Storage Ring

Dissociative recombination (DR) of molecular ions plays a key role in controlling the charge density and composition of the cold interstellar medium (ISM). Experimental data on DR and reliable predictions based on a good knowledge on the underlying quantum mechanisms are required in order to understand the chemical network in the ISM and related processes such as star formation from molecular clouds. Needed data include not only total reaction cross sections, but also the chemical composition and excitation states of the neutral products. Utilizing the TSR storage ring in Heidelberg, Germany, we are carrying out DR measurements for astrophysically important molecular ions. TSR is unique for being the only storage ring in the world currently capable of such DR studies. We use a merged electron-ion beams technique to generate high-quality phase-space cooled, stored ion beams. This is combined with event-by-event fragment counting and fragment imaging. The count rate of detected neutral DR products yields the absolute DR rate coefficient. Imaging the distribution of fragment distances provides information on the kinetic energy released including the states of both the initial molecule and the final products. Additional details of the reaction dynamics are given by the pattern of the fragment positions. A recently introduced energy- and position-sensitive detector combines these two basic approaches and, in addition, allows for identification of fragmentation channels by fragment-mass combination within each dissociation event. Such combined information is essential for studies on DR of polyatomic ions with multi-channel multi-fragment breakup. We report recent DR results on D$_3$O$^+$, DCND$^+$, D$_2$Cl$^+$ and other systems.   

Study of the dissociative recombination of HCNH$^+$ and its isomers

The hydrogen isocyanide HNC is less stable than the hydrogen cyanide HCN so that there should be no abundance of HNC compared with HCN in a thermochemically equilibrated system at temperature around 100K. Surprisingly, astrophysical observations of the interstellar medium have reported rather different results for the ratio [HNC]/[HCN]. Therefore, much interest has been recently directed towards the main mechanism of production of these neutral elements, namely the reaction of dissociative recombination (DR) e$^-+$HCNH$^+\rightarrow$ HNC/HCN+H. There exist controversies in the literature on whether the DR reaction proceeds via a direct or an indirect mechanism. Our previous results indicate that the direct DR cross section is small. Therefore, we investigate the indirect mechanism for HCNH$^+$ by electron capture in excited Rydberg states. First, we use a simplified model considering the electronic capture via Renner-Teller non-adiabatic couplings as the decisive step of the DR reaction. This procedure, which already provided good results for other polyatomic ions as H$_3^+$, HCO$^+$ or H$_3$O$^+$, allows an estimation of the absolute cross section. Other possible DR reactions yielding HNC or HCN involve the metastable isomers H$_2$CN$^+$ and H$_2$NC$^+$. We also performed scattering calculations on these systems to estimate the direct DR cross sections.   

Dynamics of Dissociative Electron Attachment to CO$_2$ Investigated By 3D Momentum Measurements

Dissociation of a stable molecule by low-energy electrons via a resonant temporary negative ion species, in the dissociative electron attachment (DEA) process, plays an important role in natural phenomena such as atmospheric and interstellar chemistry and radiation damage in biological systems by low-energy secondary electrons. DEA has also been suggested as a new tool to control chemical reactions through enhancement of specific dissociation pathways. We present fully-differential cross sections for dissociative electron attachment to CO$_2$ measured using a 4$\pi$ momentum spectrometer. The range of electron energies of these measurements span three DEA resonances, leading to dissociation of O$^-$, from 3 to 15 eV. While the resonant states involved in each of these processes for CO$_2$ have long been considered to be accurately identified from several early measurements of the DEA energy dependence and kinetic energy release, we find interesting dynamics that shed new light on this fundamental system.   

Isotope effect in the high harmonics of water

We present evidence that bending motion launched by strong field ionization of the inner valence $3a_1$ orbital of H$_2$O strongly affects the high harmonic spectrum. The measured high harmonics of H$_2$O and D$_2$O are modeled consistently with solutions of the time-dependent Schr\"odinger equation for several different laser intensities. This result introduces a new method to characterize weights of different ionic states prepared by strong field ionization.