We report an explicitly time-dependent approach to the generation of linear absorption spectra for molecular systems within the framework of equation-of-motion (EOM) coupled-cluster (CC) theory. While most time-dependent CC approaches consider the perturbation and time-evolution of a CC wave function, the present work considers the time-evolution of a CC dipole function. The dipole function formalism introduces no approximations and requires the evolution of only one time-dependent quantity, either the left or right dipole function. This time-dependent framework can be used to compute linear absorption spectra for molecules with a high density of states over a broad spectral range, a case for which conventional frequency-domain computations may become impractical. We validate the approach by comparing absorption spectra for small molecules computed at EOM second-order approximate CC (CC2) and time-dependent EOM-CC2 (TD-EOM-CC2) levels of theory. TD-EOM-CC2 computations are also used to predict extreme ult...

http://pubs.acs.org/doi/pdf/10.1021/acs.jctc.6b00796

Linear Absorption Spectra from Explicitly Time-Dependent Equation-of-Motion Coupled-Cluster Theory

The 4d absorption spectra of I, ${\mathrm{I}}^{+}$, and ${\mathrm{I}}^{2+}$ have been observed in the 45--300-eV region using the dual laser-produced plasma technique. At lower energies each spectrum is dominated by intense 4d\ensuremath{\rightarrow}5p transitions, which are identified with the aid of multiconfiguration Hartree-Fock calculations. The excited states decay by direct autoionization involving 5s or 5p electrons and rates for the different processes and resulting linewidths were calculated and compared, where possible, with experiment. In the case of neutral iodine 4d\ensuremath{\rightarrow}6p transitions are also observed in the region close to the $^{2}$${\mathit{D}}_{5/2}$ threshold. The dominant feature in each spectrum, however, arises from 4d-f excitation, which evolves from a 4d-\ensuremath{\varepsilon}f shape resonance in neutral iodine to a feature containing discrete 4d\ensuremath{\rightarrow}nf transitions in ${\mathrm{I}}^{2+}$. \textcopyright{} 1996 The American Physical Society.

/pdf/trends-in-4d-subshell-photoabsorption-along-the-iodine-189pmr6q93.pdf

Trends in 4d-subshell photoabsorption along the iodine isonuclear sequence: I, I+, and I2+

Recently, the spectrum of singly ionized chromium has come under theoretical scrutiny by Dolmatov (1996 J. Phys. B: At. Mol. Opt. Phys. 29 L673) and also by Donnelly et al (1997 J. Phys. B: At. Mol. Opt. Phys. 30 L285). Both of these theoretical investigations, employing the spin-polarized RPAE (SPRPAE) and R-matrix techniques, respectively, concerned the 3p photoabsorption cross section from the 3d5 6S5/2 ground state of Cr+. The two calculations are consistent with each other and agree with the previously reported experimental data for the 3p4s resonances and the onset of the 3p3d giant resonance (Costello 1991 Phys. Rev. A 43 1441). However, beyond the peak of the giant resonance, both calculations result in cross sections which differ considerably from the experimental data. From further dual-laser plasma experiments carried out under different spatio-temporal conditions and atomic structure calculations on metastable states this discrepancy can be ascribed to the presence of metastable excited singly ionized chromium within the plasma.

Metastable state contributions to the measured 3p photoabsorption spectrum of Cr+ ions in a laser-produced plasma

The 4d inner-shell absorption spectra of Sb, Sb+, Sb2+ and Sb3+ have been obtained with the dual-laser-produced plasma technique. Discrete structure has been observed in the 30-40 eV region and identified as originating from 4d-5p transitions by comparison with the results of Hartree-Fock calculations performed within the superposition of configurations method. For comparison of theoretical and experimental spectra, 4d photoabsorption cross sections were calculated which took non-radiative decay rates into account. It was found to be necessary to include absorption from excited states of the ground configurations, especially in the higher ionization stages. Synthetic spectra, which allowed for thermal lower state population distributions, were produced and these are in agreement with the experimental data.

http://iopscience.iop.org/article/10.1088/0953-4075/32/20/304/pdf

Discrete structure in the 4d photoabsorption spectrum of antimony and its ions

This paper demonstrates that time-integrated space-resolved laser-induced plasma spectroscopy (TISR-LIPS) is a useful technique in the vacuum ultraviolet (VUV) for the quantitative determination of the carbon content in steels. The standard reference samples used were carbon-iron alloys containing a relatively wide concentration range of carbon (0.041-1.32%). In the experiments the output of a Q-switched Nd:YAG (1064?nm) laser, with approximately a 1?J maximum output pulse energy and approximately a 12?ns temporal pulse width, was focused onto the surface of each sample (under vacuum) in order to produce the emitting plasma. A fore-slit mounted in the target chamber allowed spatially-resolved spectral measurements in the axial direction of the plasma and provided emission lines that were almost free of the background continuum. A 1?m normal incidence vacuum spectrometer, equipped with a 1200 grooves mm-1 concave grating and a micro-channel plate/photodiode array detector combination, was used as the detection system. A particularly interesting feature of this work is the demonstration that VUV spectroscopy allows ionic lines to be used and linear calibration curves were obtained for the five carbon spectral lines (from C+ and C2+) under investigation. The limits of detection for all lines were determined; the lowest detection limit (87?10?ppm) was obtained from the C2+ 97.70?nm line, which compares favourably with the only available value in the literature of 100?ppm.

Time-integrated laser-induced plasma spectroscopy in the vacuum ultraviolet for the quantitative elemental characterization of steel alloys

A new multilaser multichannel spectrometer system has been developed that allows a wide range of investigations of the interaction of extreme-ultraviolet (EUV) photons with matter in the form of free atoms or ions, e.g., in gases or plasmas or bound as in solids. The EUV photons are generated by a laser-produced plasma. Applications to the study of photoabsorption by thin foils, gases, and ground- and excited-state atoms and ions are described. The design and performance of a collimated, quasi-monochromatic, intense source of EUV radiation based on the combination of a laser-produced plasma with a EUV multilayer mirror is also reported.

Bernd F. Sonntag

Papers

Extreme-ultraviolet studies with laser-produced plasmas