Daniel J. Haxton

Dissociative electron attachment to the H2O molecule II: nucleardynamics on coupled electronic surfaces within the local complexpotential model

Dissociative electron attachment to the H 2 O molecule II: nuclear dynamics on coupled electronic surfaces within the local complex potential model Daniel J. Haxton, 1, 2, ∗ T. N. Rescigno, 2 and C. W. McCurdy 2, 3 Department of Chemistry, University of California, Berkeley, California 94720 Lawrence Berkeley National Laboratory, Chemical Sciences, Berkeley, California 94720 Departments of Applied Science and Chemistry, University of California, Davis, California 95616 We report the results of a first-principles study of dissociative electron attachment (DEA) to H 2 O. The cross sections were obtained from nuclear dynamics calculations carried out in full dimen- sionality within the local complex potential model by using the multi-configuration time-dependent Hartree method. The calculations employ our previously obtained global, complex-valued, potential energy surfaces for the three ( 2 B 1 , 2 A 1 , and 2 B 2 ) electronic Feshbach resonances involved in this process. These three metastable states of H 2 O − undergo several degeneracies, and we incorporate both the Renner-Teller coupling between the 2 B 1 and 2 A 1 states, as well as the conical intersection between the 2 A 1 and 2 B 2 states, into our treatment. The nuclear dynamics are inherently multi- dimensional and involve branching between different final product arrangments as well as extensive excitation of the diatomic fragment. Our results successfully mirror the qualitative features of the major fragment channels observed, but are less successful in reproducing the available results for some of the minor channels. We comment on the applicability of the local complex potential model to such a complicated resonant system. I. INTRODUCTION In the preceeding paper [1], referred to hereafter as (I), we presented global representations of the three ( 2 B 1 , A 1 , and 2 B 2 ) complex-valued potential energy surfaces of the metastable states of H 2 O − which underlie disso- ciative electron attachment to water. This paper is con- cerned with the calculation of the cross sections for that physical process. Prior experimental and theoretical re- sults [2–20] have characterized the various breakup chan- nels and determined the spatial symmetries of the three metastable electronic states of H 2 O − , the 2 B 1 , 2 A 1 , and B 2 electronic Feshbach resonances, which are respon- sible for production of H − and O − . As explained in ref. [18] and (I), the energetically lowest H + OH − chan- nel does not directly correlate with any of the three Fesh- bach states. We therefore conclude that OH − production must be due to nonadiabatic effects. We pursue this problem theoretically using a coupled Born-Oppenheimer treatment of the nuclear motion. The first task, which was described in (I), is the construc- tion of three-dimensional, complex-valued potential en- ergy surfaces for these three states, which have a negative imaginary component due to the finite probability of elec- tron autodetachment back to H 2 O + e − . These complex- valued potential energy surfaces, which are functions of the nuclear geometry q, are defined as V (q) = E R (q) − i Γ(q) where E R is the resonance position and Γ is the width ∗ Present address: Department of Physics and JILA, University of Colorado, Boulder Colorado 80309 of the resonance, which is related to the lifetime by τ = 1/Γ. (We use atomic units throughout this paper.) The present article, which we label (II), is concerned with the use of these potential curves within the local complex potential (LCP) model [21–25] to calculate the nuclear dynamics leading to dissociation. The analysis of the dynamics yields the DEA cross section as a function of incident electron energy. We must account for two major nonadiabatic physical effects in calculating the quantum dynamics of the nuclei. As described in (I), the three potential energy surfaces have several degeneracies which lead to coupling among them. First, the 2 B 1 and 2 A 1 states become members of a degenerate 2 Π pair in linear geometry, and for this rea- son there will be Renner-Teller coupling between them. We expect this coupling to be relevant for DEA via the A 1 state, because the gradient of its potential energy surface will cause the system to move towards linear ge- ometry after the electron attaches. Second, there is a conical intersection [18] between the 2 B 2 and 2 A 1 states which leads to coupling between them. For this reason, as described in (I) we constructed a set of diabatic 2 B 2 and 2 A 1 surfaces, along with a coupling term, which we use in the calculations presented in this paper. In Fig. 1, we show the real parts E R of the constructed potential energy surfaces along a two-dimensional cut which includes the equilibrium geometry of the neutral (r 1 = r 2 = 1.81a 0 ; θ HOH = 104.5 ◦ ). The degenera- cies which lead to the nonadiabatic effects listed above can be seen in this figure. The two-dimensional cut depicted is that for which the two OH bond lengths are equal (r 1 = r 2 ), corresponding to C 2v symmetry. (In C 2v symmetry, the adiabatic and diabatic 2 A 1 and B 2 surfaces coincide.) The backside of this cut lies at r 1 = r 2 = 1.81a 0 , which is the equilibrium value of the bond lengths in neutral H 2 O, and is marked with

Femtosecond Photoelectron Imaging of Transient Electronic States and Rydberg Atom Emission from Electronically Excited He Droplets

Ultrafast relaxation of electronically excited pure He droplets is investigated by femtosecond time-resolved photoelectron imaging. Droplets are excited by extreme ultraviolet (EUV) pulses with photon energies below 24 eV. Excited states and relaxation products are probed by ionization with an infrared (IR) pulse with 1.6 eV photon energy. An initially excited droplet state decays on a time scale of 220 fs, leading predominantly to the emission of unaligned 1s3d Rydberg atoms. In a second relaxation channel, electronically aligned 1s4p Rydberg atoms are emitted from the droplet within less than 120 fs. The experimental results are described within a model that approximates electronically excited droplet states by localized, atomic Rydberg states perturbed by the local droplet environment in which the atom is embedded. The model suggests that, below 24 eV, EUV excitation preferentially leads to states that are localized in the surface region of the droplet. Electronically aligned 1s4p Rydberg atoms are expected to originate from excitations in the outermost surface regions, while nonaligned 1s3d Rydberg atoms emerge from a deeper surface region with higher local densities. The model is used to simulate the He droplet EUV absorption spectrum in good agreement with previously reported fluorescence excitation measurements.

Ultrafast probing of ejection dynamics of Rydberg atoms and molecular fragments from electronically excited helium nanodroplets

The ejection dynamics of Rydberg atoms and molecular fragments from electronically excited helium nanodroplets are studied with time-resolved extreme ultraviolet ion imaging spectroscopy. At excitation energies of 23.6 ± 0.2 eV, Rydberg atoms in n = 3 and n = 4 states are ejected on different time scales and with significantly different kinetic energy distributions. Specifically, n = 3 Rydberg atoms are ejected with kinetic energies as high as 0.85 eV, but their appearance is delayed by approximately 200 fs. In contrast, n = 4 Rydberg atoms appear within the time resolution of the experiment with considerably lower kinetic energies. Major features in the Rydberg atom kinetic energy distributions for both principal quantum numbers can be described within a simple elastic scattering model of localized perturbed atomic Rydberg atoms that are expelled from the droplet due to their repulsive interaction with the surrounding helium bath. Time-dependent kinetic energy distributions of He(2) (+) and He(3) (+) ions are presented that support the formation of molecular ions in an indirect droplet ionization process and the ejection of neutral Rydberg dimers on a similar time scale as the n = 3 Rydberg atoms.

Dissociative electron attachment to carbon dioxide via the 8.2 eV Feshbach resonance

Momentum imaging experiments on dissociative electron attachment (DEA) to CO2 are combined with the results of ab initio calculations to provide a detailed and consistent picture of the dissociation dynamics through the 8.2 eV resonance, which is the major channel for DEA in CO2. The present study resolves several puzzling misconceptions about this system.

Probing autoionizing states of molecular oxygen with XUV transient absorption: Electronic-symmetry-dependent line shapes and laser-induced modifications

U. S. Army Research Laboratory; U. S. Army Research Office [W911NF-14-1-0383]; U. S. Department of Energy Office of Basic Energy Sciences, Division of Chemical Sciences [DEAC02-05CH11231]; US DOE Basic Energy Sciences [DE-SC0012198]; Arizona TRIF Photonics Fellowship

Dissociative electron attachment to carbon dioxide via the 2

Author(s): Moradmand, A; Slaughter, DS; Haxton, DJ; Rescigno, TN; McCurdy, CW; Weber, T; Matsika, S; Landers, AL; Belkacem, A; Fogle, M | Abstract: Momentum imaging measurements from two experiments are presented and interpreted with the aid of ab initio theoretical calculations to describe the dissociative electron attachment (DEA) dynamics of CO2. The dynamics of the transient negative ions of CO2- involve several conical intersections taking part in mechanisms that have only recently been understood. We address the problem of how the 4-eV 2Πu shape resonance in CO2 proceeds to dissociate to CO(1Σ+) + O-(2P) by DEA.

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ABSTRACT The method of McCurdy, Baertschy, and Rescigno, J. Phys. B, 37, R137 (2004) [1] is generalised to obtain a straightforward, surprisingly accurate, and scalable numerical representation for calculating the electronic wave functions of molecules. It uses a basis set of product sinc functions arrayed on a Cartesian grid, and yields 1 kcal/mol precision for valence transition energies with a grid resolution of approximately 0.1 bohr. The Coulomb matrix elements are replaced with matrix elements obtained from the kinetic energy operator. A resolution-of-the-identity approximation renders the primitive one- and two-electron matrix elements diagonal; in other words, the Coulomb operator is local with respect to the grid indices. The calculation of contracted two-electron matrix elements among orbitals requires only O(Nlog (N)) multiplication operations, not O(N4), where N is the number of basis functions; N = n3 on cubic grids. The representation not only is numerically expedient, but also produces energies and properties superior to those calculated variationally. Absolute energies, absorption cross sections, transition energies, and ionisation potentials are reported for 1- (He+, H+2), 2- (H2, He), 10- (CH4), and 56-electron (C8H8) systems.

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© 2015 IOP Publishing Ltd. Attosecond transient absorption spectra near the energies of autoionizing states are analyzed in terms of the photon coupling mechanisms to other states. In a recent experiment, the autoionization lifetimes of highly excited states of xenon were determined and compared to a simple expression based on a model of how quantum coherence determines the decay of a metastable state in the transient absorption spectrum. Here it is shown that this procedure for extracting lifetimes is more general and can be used in cases involving either resonant or nonresonant coupling of the attosecond-probed autoionizing state to either continua or discrete states by a time-delayed near infrared (NIR) pulse. The fits of theoretically simulated absorption signals for the 6p resonance in xenon (lifetime = 21.1 fs) to this expression yield the correct decay constant for all the coupling mechanisms considered, properly recovering the time signature of twice the autoionization lifetime due to the coherent nature of the transient absorption experiment. To distinguish between these two coupling cases, the characteristic dependencies of the transient absorption signals on both the photon energy and time delay are investigated. Additional oscillations versus delay-time in the measured spectrum are shown and quantum beat analysis is used to pinpoint the major photon-coupling mechanism induced by the NIR pulse in the current xenon experiment: the NIR pulse resonantly couples the attosecond-probed state, 6p, to an intermediate 8s (at 22.563 eV), and this 8s state is also coupled to a neighboring state (at 20.808 eV).

An efficient basis set representation for calculating electrons in molecules

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Investigation of coupling mechanisms in attosecond transient absorption of autoionizing states: comparison of theory and experiment in xenon

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