Session N8: Atomic and Molecular Structure, Dynamics, and Properties

Space- and time-resolved absolute measurements of ionization in atomic and molecular gases

Field ionization of atoms and molecules is a fundamental aspect of light-matter interactions that plays a role in the propagation of ultrashort optical pulses and in high harmonic generation. We present absolute time-resolved and 2D in space-resolved measurements of the ionization of the noble gases He through Xe as well as O$_2$ and N$_2$ using ultra-broadband 2D spectral interferometry. The refractive index is measured with 5 fs time resolution and 3 $\mu$m spatial resolution, with absolute results ranging from the multiphoton through tunneling ionization regimes. These results, combined with our previous measurements at nonionizing intensities [1,2], constitute a measurement of the nonlinear response of the noble gases from the weak field limit up to full ionization of the first electron. For the molecular gases, we use prepulses to prepare an ensemble of partially aligned molecules and examine alignment-dependent ionization rates. Our measurements may serve as an experimental benchmark of calculations of strong field ionization.\\[4pt] [1] J. K. Wahlstrand, Y.-H. Cheng, and H. M. Milchberg, Phys. Rev. A 85, 043820 (2012).\\[0pt] [2] J. K. Wahlstrand, Y.-H. Cheng, and H. M. Milchberg, Phys. Rev. Lett 109, 113904 (2012).   

Double Resonance Spectroscopy of the $D {}^1 \Pi_u^+$ and $B''\bar{B}^1\Sigma^+_u$ States Near the Third Dissociation Threshold of H$_2$

Double-resonance laser spectroscopy via the $E,F {\,}^1 \Sigma_g^+, v'=6, J'$ state was used to probe the energy region below the third dissociation limit of molecular hydrogen. Resonantly-enhanced multi-photon ionization spectra were recorded by detecting ion production as a function of energy using a time of flight mass spectrometer. Energies and line widths for the $v = 14 - 17$ levels of the $D{}^1 \Pi_u^+$ state of H$_2$ are reported and compared to experimental data obtained by using VUV synchrotron light excitation and fully ab initio non-adiabatic calculations of $D {}^1 \Pi_u^+$ state energies and line widths. Several high vibrational levels of the $B''\bar{B\,}^1\Sigma^+_u$ state were also observed in this region. Term energies and rotational constants for the $ v=67-69$ vibrational levels are reported and compared to highly accurate ro-vibrational energy level predictions from fully ab initio non-adiabatic calculations of the first six ${}^1 \Sigma_u^+$ levels of H$_2$. While additional observed transitions can be assigned to other other states, several unassigned features in the spectra highlight the need for a fully integrated theoretical treatment of dissociation and ionization in this energy region.   

Photoionization of Ground and Excited States of Ti I

Photoionization of ground and many excited states (n$\leq$10) of Ti I is studied. Absorption lines of TiO are abundant in spectra of red supergiants (RSG). The amount of Ti I locked in TiO can have direct effect on the fluxes, spectral energy distributions and thus in determination of fundamental stellar parameters and abundances for RGS stars. Photoionization cross sections of Ti I are needed for for the spectral analysis. Calculations have been carried out in the ab initio R-matrix method in LS coupling. A close coupling wave function of 36 LS states of the core has been used. Photoionization of the ground state $3d^24s^2(^4F)$ shows low energy resonances indicating availability of Ti$^+$ to combine with oxygen. Various other features, particularly of Seaton resonances due to photo-excitation of core, in the photoioniation cross sections will be illustrated. Photoionization cross sections over 900 states will be presented.   

Spectroscopy of L-shell Xenon for Ion Temperature and Velocity Measurements on ITER

In the ITER tokamak, the ion temperature and bulk toroidal velocity will be measured as a function of plasma minor radius using an imaging crystal spectrometer. The diagnostic relies on measuring the Doppler broadening and shift of x-ray lines from embedded impurity ions. However, in line with current trends in magnetic fusion devices, the ITER plasma is designed to have few heavy impurity ions, limited to those of argon and tungsten. Neither element produces ions whose radiation can cover the broad range of temperatures that are expected for ITER plasmas between the core and a fractional minor radius of $r/a < 0.8$, throughout which the diagnostic is to function. While L-shell tungsten lines, in particular those from neonlike W$^{64+}$, can be employed to diagnose the hottest parts of the plasma, it has been suggested to inject iron in order to utilize its K-shell emission to diagnose the cooler regions. Here, we show that the L-shell x rays of neonlike Xe$^{44+}$ can provide the same information as iron. Moreover, we show that L-shell xenon ions will also persist in the hottest part of ITER plasmas and thus can be used in lieu of tungsten or krypton, whose injection had also been suggested. Moreover, because xenon is a noble gas, it can be readily removed from the plasma withou   

Signature of chaos in the $4f$-core-excited states for highly-charged tungsten ions

We evaluate radiative and autoionizing transition rates in highly charged W ions in search for the signature of chaos. In particularly, previously published results for Ag-like W$^{27+}$, Tm-like W$^{5+}$, and Yb-like W$^{4+}$ ions as well as newly obtained for I-like W$^{21+}$, Xe-like W$^{20+}$, Cs-like W$^{19+}$, and La-like W$^{17+}$ ions (with ground configuration [Kr]$4d^{10}4f^k$ with $k$ = 7, 8, 9, and 11, respectively) are considered that were calculated using the multiconfiguration relativistic Hebrew University Lawrence Livermore Atomic Code (HULLAC code) and the Hartree-Fock-Relativistic method (COWAN code). The main emphasis was on verification of Gaussian statistics of rates as a function of transition energy. There was no evidence of such statistics for above mentioned previously published results as well as for the transitions between the excited and autoionizing states for newly calculated results. However, we did find the Gaussian profile for the transitions between excited states such as the [Kr]$4d^{10}4f^k$ - [Kr]$4d^{10}4f^{k-1}5d$ transitions , for newly calculated W ions.   

Development of a Cartesian sinc DVR basis for single and double ionization

In this investigation, we explore properties of a grid-based representation designed to calculate phenomena involving single and double (auto)ionization. The method employs a powerful representation of the two-electron operator within a basis of sinc functions. It consists of a tensor decomposition of the operator such that two-electron matrix elements may be computed in O(N$^2$) operations, as opposed to the O(N$^4$) calculations required for the usual Gaussian basis sets used in quantum chemistry. The basis and methods are tested with the hydrogen atom and H$_2^+$. Results indicate that, in addition to being more scalable, the technique is more accurate than variational method.   

Electronic Structure Mediated Vibrational Coherence in Methyl Acetophenone Isomers

The role of ground and excited state electronic structures in influencing the vibrational coherences in gas phase polyatomic molecules has been a hot topic for quite some time. Here we explore the time resolved dynamics of acetophenone and its methyl substituted isomer when excited by intense 800nm femtosecond pump and probe pulses. The parent ion yield show 500 fs modulations that die down within 3ps. Similar modulations having the same timescales in the parent ion yield are also observed for the p-methyl isomer. The o-methyl isomer however shows longer 1ps modulations. Interestingly enough no oscillations are observed for the meta isomer. Quantum chemical calculations at the CASSCF/6-311G level of theory predicts that upon excitation the neutral ground state is planar and the energy spacing between the levels is very small. Preliminary calculations also predict torsional motion coupled to electronic modulations on the D$_{\mathrm{0}}$ state and further calculations are being performed to ascertain the involvement of the D$_{\mathrm{1}}$ and D$_{\mathrm{2}}$ states. This could help us better understand the electronic effect of substitution on a benzene ring.   

Broadband-To-Monochromatic X-ray conversion of Zr K$_{\alpha,\beta}$ Lines and High-Energy-Density (HED) Plasma Diagnostics

Experimental and theoretical studies of the formation of K$_{\alpha}$, K$_{\beta}$ X-ray complexes from high-Z elements are of interest in diverse areas such as HED plasma sources and biomedical applications. Conversion of bremsstrahlung-to-monochromatic (B2M) X-ray K$_{\alpha}$, K$_{\beta}$ lines is studied using a broadband 150 KV X-ray generator and a solid Zirconium target. We carried out Breit-Pauli atomic structure calculations for all possible fine-structure components of transition arrays leading to the formation of K$_{\alpha}$ and K$_{\alpha}$ complexes in H- to F-like Zr (Z=40), and other high-Z elements up to platinum and gold (Z=78,79) [1,2]. Fine structure averaged energies and cross sections for K$_{\alpha}$ and K$_{\beta}$ resonances in each ionization state of Zr are presented. Such K$_{\alpha}$ resonances [1,2] have been observed in X-ray laser produced warm dense matter at the LCLS-XFEL at SLAC [3]. Numerical simulations using the code FLYCHK indicate temperature and density sensitivities in laser-irradiated plasmas.\\[4pt] [1] Pradhan et al, J. Phys. Chem. A \textbf{113}, 12356 (2009)\\[0pt] [2] Nahar et al, Can. J. Phys. 89, 483 (2011); Nahar \& Pradhan (2014)\\[0pt] [3] S. M. Vinko et al, Nature \textbf{482}, 59 (2012).   

Quantum-mechanical definition of atoms and their interactions in molecules

Assignments of indistinguishable electrons to particular atomic nuclei in a molecule are generally regarded as meaningless, as are associated definitions of fragment atomic and atomic-interaction operators. As a consequence, a generally agreed upon quantum-mechanical definition of a ``chemical bond'' between atoms in a molecule is largely absent. In the present report, a computationally-viable quantum-mechanical definition of chemical bonding between atoms in a molecule is presented based on the Born-Oppenheimer approximation, the Couloumb Hamiltonian operator, and the conditional context afforded by representation theory. An orthonormal (Eisenschitz-London) outer product of atomic spectral eigenstates is employed to provide meaningful assignments of electrons to particular atomic nuclei in a molecule, as well as support of corresponding well-defined self-adjoint atomic and atomic-interaction fragment operators. Total molecular energies obtained in this representation are partitioned into a sum of atomic terms which describe distributions of atomic energy promotions for the individual atoms and a pairwise-atomic sum of distributions among universal interaction energies which describe chemical bonds among the constituent atoms. Illustrative clarifying applications are reported.   

Photodetachment and formation mechanisms of cosmic anions

The recent discovery of molecular anions outside the solar system has triggered a growing interest in the search of their formation and destruction mechanisms. In this talk, we present theoretical calculations of the photodetachment cross sections for the CN$^-$, C$_2$H$^-$, C$_3$N$^-$ and C$_4$H$^-$ molecular ions using the complex variational Kohn method. The results are compared with recent experimental measurements and we discuss threshold laws and the role of excited electronic open channels. We then focus on potential formation of these ions via radiative attachment, either by spontaneous emission of the continuum electron, or through initial electron capture into excited anion vibrational states followed by radiative relaxation. Finally, we treat the potential role of dipole states in the radiative attachment process.