Session U4: Atomic and Molecular Structure and Spectroscopy II

Calculation of Ionization in Direct Frequency Comb Spectroscopy

Direct Frequency Comb Spectroscopy (DFCS) is currently the most precise technique known for measuring the structure of atomic and molecular systems. Usually in DFCS one measures the fluorescence signal coming from the excited states of the target system as a function of the laser's repetition frequency (f$_{rep})$ or offset (f$_{off})$ frequency. In recent years this process also has been thoroughly modeled theoretically. Although subsequent ionization of the excited states by the comb laser is possible, it has not been considered, either in theory or in experiment. The goal for this work was to expand existing computer code to include photoionization. Our calculations for atomic Rb show that the ion yield is comparable to fluorescence. Furthermore, the ionization spectrum, as a function of f$_{rep}$ or f$_{off}$, replicates the structure of the corresponding fluorescence spectrum. Experimentally, this could be useful because ion detection efficiency is generally very high. We have constructed an apparatus to test the theoretical predictions. We show the results of our calculations and our measurements.   

Developments in Cavity-Enhanced Direct Frequency Comb Spectroscopy (CE-DFCS)

We achieve a quantum-noise-limited absorption sensitivity of $1.7 \times 10^{-12}$ cm$^{-1}$ per spectral element at 400 s of acquisition time using the cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) technique in the mid-infrared. A frequency comb is locked to a high-finesse optical cavity and spectra are recorded using a fast-scanning Fourier transform spectrometer with an ultralow-noise autobalancing detector. In this talk, we will discuss our recent technical achievements and detailed understandings of the cavity-enhanced technique and mid-infrared detection methods. In addition, we will present trace gas detection as an application of the technique.   

Hyperfine Interactions and Electric Dipole of Cs 11s by Using Electromagnetically Induced Transparency

Using electromagnetically induced transparency, the hyperfine structure of the 11s state of cesium was measured and analyzed. To improve the accuracy of frequency measurement, a reference probe beam produced from an acousto-optical modulator overlapped with the original probe beam was served as a frequency marker. The hyperfine magnetic dipole constant A of Cs 11s can be derived from the splitting intervals of the observed spectrum. The result is A\,=\,38.83$\pm$0.26\,MHz. A numerical simulation based on solving the steady state density matrix solution involving dressed-state atom-photon interaction picture, multi-intermediate levels and optical pumping can quantitatively fit the experimental data.   

Detecting the single photon recoil using trapped ions

I will report on our current work to measure the recoil due to a single photon scattering from a single ion. For this experiment two ions are loaded into a linear ion trap: one well characterized ``measurement'' ion and one ``spectroscopy ion'' on which the photon scattering event is to be detected. The photon recoil energy excites the common vibrational mode shared by both ions. In order to detect this extremely small vibration, we make use of a very sensitive highly non-classical motional state. Our technique could have interesting applications in performing spectroscopy of atoms or molecules at the single photon / single atom level.   

Subnatural spectroscopy by two-photon frequency modulation

We demonstrate a two-photon frequency-modulation subnatural spectroscopy technique. The two levels for the target resonance are both resonantly coupled to an auxiliary level by two phase coherent lasers under common phase modulation. Cross correlation in the converted amplitude modulation of the two lasers was measured versus detuning. Linewidth thirty times narrower than the measured natural width (zero laser power width) of the target resonance was experimentally achieved using Coherent-Population-Trapping as a proof-of-principle system. Dependence of linewidth on laser power, modulation depth and frequency was investigated. Experimental results agree well with theoretical model. This technique is generally applicable to many systems for precision spectroscopy, metrology and imaging.   

Sub-femto-Tesla Scalar Atomic Magnetometer with Spin-squeezing

Atomic shot noise sets the fundamental limit on precision of atomic frequency measurements. Spin-squeezing techniques can reduce long-term atomic shot noise for systems with non-linear spin-relaxation processes [1]. Magnetometers using dense hot alkali-metal vapors are naturally limited by such non-linear relaxation due to spin-exchange collisions.~ We have developed a scalar atomic magnetometer utilizing a multi-pass atomic cell [2]~ to interact with $10^{13}$ atoms with an optical density of 5000. By operating the magnetometer in a pulsed mode with high initial spin polarization and two probe measurement pulses, we have realized magnetic field sensitivity of 0.5 fT/$\sqrt{Hz}$, already more than an order of magnitude better than previous state-of-the-art for scalar atomic magnetometers. We are continuing to refine this system to realize an improvement in the long-term sensitivity from spin-squeezing measurements beyond the present magnetic field sensitivity records.\\[4pt] [1] G. Vasilakis, V. Shah, M. V.~ Romalis,~ Phys. Rev. Lett. 106, 143601 (2011). \newline [2] S. Li, P. Vachaspati, D. Sheng, N. Dural, M. V. Romalis, Phys. Rev. A 84,~061403R~(2011).   

Excitation energies, radiative and autoionization rates, dielectronic satellite lines, and dielectronic recombination rates for excited states of Yb-like W

Energy levels, radiative transition probabilities, and autoionization rates for [Cd]$4f^{14}5p^65l'nl$, [Cd]$4f^{14}5p^66l''nl$, [Cd]$4f^{14}5p^55d^2nl$, [Cd]$4f^{14}5p^55d6l''nl$, [Cd]$4f^{13}5p^65d^2nl$, and [Cd]$4f^{13}5p^65d6l''nl$ ($l'=d, f, g$ , $l''=s,p,d,f, g$, $n=5-7$) states of Yb-like tungsten (W$^{4+}$) are calculated using the RMBPT, HULLAC, and COWAN codes. Branching ratios relative to the [Cd]$4f^{14}5p^65d$, [Cd]$4f^{14}5p^66s$, and [Cd]$4f^{14}5p^66p$ thresholds in Tm-like tungsten and intensity factors are calculated for satellite lines, and dielectronic recombination (DR) rate coefficients are determined for the singly excited, as well as non-autoionizing core-excited states in Yb-like tungsten. Contributions from the autoionizing doubly excited states and core-excited states (with n up to 100), which are particulary important for calculating total DR rates, are estimated. Synthetic dielectronic satellite spectra from Yb-like W are simulated in a broad spectral range from 200 to 1400 \AA. These calculations provide recommended values critically evaluated for their accuracy for a number of W$^{4+}$ properties useful for a variety of applications including for fusion applications.   

Testing the accuracy of the coupled-cluster method for trivalent atoms

We use coupled-cluster method for computation of electron correlation energies, hyperfine constants and dipole matrix elements of some of the low-lying states of atomic boron. The calculations are done with converging techniques and include up to triple excitation terms. The goal is to establish the accuracy of the couple-cluster method for a system of three valence electrons so that it can be extended to compute the structure of heavier atoms in the same atomic group such as thallium. High precision computations on heavy atoms will in turn lead to more stringent constraints on new physics beyond the Standard Model, derived from atomic parity violation.   

Relativistic Pseudopotential Followed by Restoration Method for Studying Heavy-Atom Systems

Precise all-electron four-component treatment of molecules with heavy elements is yet rather consuming. In turn, the relativistic pseudopotential (RPP) method is the most straightforward way now to study efficiently ``valence'' (optic, electric, chemical etc.) properties of rather complicated systems. However, the valence molecular spinors are usually smoothed in atomic cores. Therefore, direct calculation of electronic densities near heavy nuclei within the RPP approach is impossible. In the report, an approach based on the RPP method and one-center core-restoration technique [1] developed by the authors for such studies is discussed. It efficiency is illustrated in benchmark to-date calculations of magnetic$-$dipole and electric quadrupole hyperfine$-$structure constants, as well as the space parity (P) and time-reversal symmetry (T) nonconservation effects in polar heavy-atom molecules, including HfF$^+$, WC, PbF$^+$, PbO, YbF, ThO and some other candidates which are studied now as promising molecules for the experimental search of the electron electric dipole moment (eEDM). \\[4pt] [1] A.V.Titov, N.S.Mosyagin, A.N.Petrov, T.A.Isaev, D.DeMille, Progr.\ Theor.\ Chem.\ Phys., {\bf 15B}, 253 (2006).