Two-step Photoionization Research Articles

Autoionizing even Rydberg series of Ne: a comparison of many-electron theory and experiment

The 2p51/2np′ J = 0, 1, 2 and 2p51/2nf′ J = 2 autoionizing Rydberg states (ARS) of Ne have been studied by two-step two-colour photoionization of ground-state neon atoms via the intermediate states 2p53/23s J = 1 and 2p51/23s′ J = 1, followed by time-of-flight ion detection. Strong interference between the direct and virtual intershell photoexcitation channels in ARS production, especially for the np′ J = 0 series, is revealed. The computed reduced width Γr = 6431cm−1 and the quantum defect μp = 0.7634 of the 2p51/213p′ J = 0 resonance agree well with the experimental results Γ = 5334cm−1 and μp = 0.7662 determined with a Fano-type lineshape analysis. The measured lineshapes of the np′ J = 0, 1, 2 resonances are also in good agreement with the many-electron computation.

Deflecting, focusing, and funnelling atoms by near-field light

We describe atom deflection with a slit-type deflector as an elemental technique of precise atom control using near-field light. In the demonstration experiments, cold Rb atoms deflected by repulsive near-field light induced in a 200?nm-wide slit are detected by means of two-step photoionization. Then, we consider a near-field-light lens of atomic de Broglie waves by developing the atom deflector. The feasibility of nanofocusing is shown by numerical analyses with Fourier atom optics. Finally, we describe atom funnels with evanescent light to generate a cold atomic beam required for atom manipulation using nanometric near-field light. The flux intensity of cold Rb atoms outputted from a 240??m outlet is estimated to be 7.5 ? 107?atom?cm?2?s?1 at a blue detuning of 1.2?GHz. In order to increase the flux intensity to 1012?atom?cm?2?s?1, we develop a Si-made funnel with a micron-sized outlet.

Alternate technique for simultaneous measurement of photoionization cross-section of isotopes by TOF mass spectrometer

New measurements of photoionization cross-sections of the lithium isotopes are reported employing a Time of Flight (TOF) mass spectrometer in conjunction with an atomic beam apparatus. Using a two-step selective photoionization and saturation technique, we have simultaneously measured the photoionization cross-section of the 2p excited state of both the isotopes Li6 and Li7 as 15±2.5 Mb and 18 ±2.5 Mb where as the corresponding number densities have been determined as N0≈5.3×1010 atoms/cm3 and N0≈6.2×1011 atoms/cm3 respectively.

Isotope-selective trapping of rare calcium ions using high-power incoherent light sources for the second step of photo-ionization

Rare calcium isotope 48Ca+ (0.187%) has been selectively loaded in a linear Paul trap using two ultraviolet light emitting diodes with the output power of 85 mW for the second excitation in a two-step photo-ionization process. Isotope selectivity has been achieved by utilizing the isotope shifts for the 4s21S0–4s4p1P1 transition of neutral calcium atom. Sympathetic cooling of 48Ca+ ions has been demonstrated using 40Ca+ ions as refrigerant ions. Purification of rare isotope 42Ca+ ions (0.647%) from a mixture of 40Ca+ (96.9%) and 42Ca+ ions has been performed by adjusting the detuning of the cooling laser frequency, which overcomes the imperfect selectivity for some rare isotopes having close resonance frequencies to that of 40Ca in the 4s21S0–4s4p1P1 transition. The methods can be applied to 43Ca+ ion (0.135%) that has been considered as one of the attractive candidates for quantum information processing as well as for an optical frequency standard.

Generation of low-density plasma by two-step photoionization of barium and its evolution in electric field

Two-color, two-step resonant photoionization has been used to produce low-density (∼106 – 107cm−3) barium photoplasma in an atomic beam. The two-photon induced specific absorption coefficient at 355nm for the photoionization process has been measured. The motion of the finite size photoplasma bounded by vacuum is experimentally studied in a static electric field in parallel-plate electrode configuration. It is observed that at sufficiently high electric fields all electrons leave the plasma and the evolution of the left over ion bunch is understood on the basis of independent particle motion.

Low temperature microwave study of photoconductivity in rare-earth-doped dielectric crystals

Transient photoconductivity induced by UV (355 nm) laser pulses in doped dielectric crystals CaF 2 :Sm 2 + , CaF 2 :Eu 2 + and LSO:Ce 3 + was studied by 8-mm microwave technique in a wide range of temperatures, from 300 K down to 5 K. It was possible to study the dynamics of free and bound photoexcited electrons. In LSO:Ce 3 + , the thermally-activated photoionization process at room temperature changes to a two-step photoionization at liquid helium temperature. In CaF 2 :Eu 2 + , two-step photoionization was detected at any temperature and electronic traps cause broadening of the photoconductivity signal at low temperature. In CaF 2 :Sm 2 + , the one-step photoionization process was confirmed at any temperature.

Spectral hole burning and energy level structure of Tb 3+:LiYF 4

To investigate the potential for photon-gated hole burning through two-step photoionization in Tb 3+:LiYF 4, energies of the 4f 8 and 4f 75d levels relative to the host band states were determined. From X-ray photoemission spectroscopy, the 4f 8 ground-state binding energy was found to be in the band gap at 2.9 eV above the valence band maximum. The effects of electron–lattice coupling on ionization processes were considered, and it was determined that the level at 54950 cm −1 is the lowest 4f 75d state that will produce ionization. Those results were extended to other rare-earth ions in LiYF 4. High-resolution spectral hole burning on the 7F 6– 5D 4 transition displayed hole/anti-hole structure attributed to population redistribution among the Tb 3+ hyperfine levels with additional sidebands due to 7Li and 19F nuclear spin flips.

Oscillations of low-density barium photoplasma in electric field

A low-density barium photoplasma has been generated by two-step photoionization in an atomic beam. The transition from independent particle nature to collective effects has been observed in a V-shaped potential well. The potential is produced by a plate–grid–plate geometry in which the grid is negatively biased while the plates are grounded. Analytical expressions for the ion currents on the plates and grid have been deduced for a charge particle moving in the potential well and compared with experiments. The results are in good agreement and gave a method to estimate the ion transmission factor for the grid.

Investigation of new odd-parity autoionization Rydberg levels of neutral europium

Transitions to the odd-parity autoionizing energy levels of europium atom from excited states are investigated by the method of two-step laser photoionization. Four new odd-parity autoionizing Rydberg series converging to the first excited ionic level 4f 76s 7S 3 have been discovered in the energy region 45735–47404 cm −1. Sixty-seven new autoionizing levels, which could not be assigned to any of these series, have also been reported. The quantum defects of the Rydberg series show small deviation for lower Rydberg levels indicating inter series interactions or perturbations.

Angular correlation theory for double photoionization in a rare-gas atom: Ionization by polarized photons

This is a sequel to an earlier article on the theory of angular correlation for double photoionization. Here we consider the two-step double photoionization of a rare gas atom under the influence of a polarized photon beam described by appropriate Stokes parameters. Cylindrical mirror analysers (CMA) are used to detect the outgoing electrons. Theoretical values of the correlation functions are obtained for linearly polarized light. Two different situations is handled. Once, the value of the correlation function is obtained keeping the photo-electron in a fixed direction. The other case the direction of the Auger electron is kept fixed. Comparison with experiments on xenon shows excellent agreement for the case of (4d_{5/2}) photoionization followed by a subsequent (N_{5}-O_{23}O_{23} ^{1}S_{0}) Auger decay for a polarized incident photon of energy (94.5eV) {[}J. Phys.B,

Two-step photoionisation of palladium

The efficiency of two-step ionisation of palladium involving the 276.4-nm 4d10 1S0 — 4d95p3P1 transition is measured as a function of the laser radiation intensity in experiments aimed at laser separation of isotopes. It is shown that to obtain the efficient (over 50%) two-step ionisation of palladium by 10-ns laser pulses with a pulse repetition rate of 10 kHz, an average radiation intensity should exceed 100 W cm-2. The results of comparison of theoretical and experimental data on the two-step ionisation efficiency make it possible to estimate the photoionisation cross section of the 4d95p3P1 state (σi ≈ 10-16 cm2).

High Spatial Resolution Atom Detector with Two-Color Optical Near Fields

We describe a slit-type detector for ground-state atoms with nanometer optical near fields. The detector with a slit width of 70 nm and a slit length of 100 µm is fabricated by processing a silicon-on-insulator wafer with photolithography and anisotropic etching. Measurement with a fiber probe of the intensity distribution on the slit reveals that the spatial resolution is 96 nm. Rubidium atoms can be selectively detected by means of two-step photoionization. The detection efficiency is estimated at 25% for Rb atoms with a mean velocity of 10 m/s. The atom detector can be used for atom deflection experiments with a fiber probe.

Novel technique for the extraction of ionization profiles from spatial density measurements

A method has been devised to extract the two-dimensional ionization profiles from spatially resolved ion density measurements made in a capacitively coupled argon rf plasma. This technique is valid if the production process is ionization and the loss process is ambipolar diffusion. As this procedure calculates the total production (ionization) term, processes such as two-step ionization and photoionization are included. The main benefit of this new technique is that it extracts ionization rates that pertain to the total ionization in the discharge. It follows that this technique could be used to determine when other ionization processes (e.g., photo- and two-step ionization) are important. On testing with experimental data, the inversion process was observed to reproduce the original source term, thereby verifying that the iteration process converges to a real solution. The calculated ionization rate using the integral method is in good agreement with the ionization profiles obtained from the density measurements. Hence, there is now a method of determining the ionization rate that is independent of electron energy probability function measurement and that includes all ionization processes, not just ground state ionization by electron–neutral collisions.

Simple and efficient photo-ionization loading of ions for precision ion-trapping experiments

We report a simple and efficient method to load a Paul trap with Ca+ ions. A beam of neutral atomic calcium is ionized in a two-step photo-ionization process using uv-diode lasers near 423 nm and 390 nm. Photo-ionization of a calcium beam for loading a Paul trap has first been demonstrated by Kjaergaard et al. The advantages of our method are the use of cheap and easily handled diode-laser systems and the large cross section for field ionization when exciting high-lying Rydberg states. Finally, we discuss the advantages of photo-ionization for ion generation compared to loading by electron bombardment.

Energy broadening due to photoion space charge in a high resolution laser photoelectron source

The results of experimental and theoretical studies, aiming at a quantitative characterization of photoion-induced energy broadening effects in a laser photoelectron source, are reported. The electron source is based on two-step cw laser photoionization of potassium atoms in a collimated beam. In the experimental studies, the attachment spectra for the formation of (N2O)9O− cluster ions through a narrow vibrational Feshbach resonance (full width at half maximum 2.3 meV) were measured as a function of the photocurrent. The theoretical studies involved Monte Carlo simulations of the broadening effects and were based on potential distributions caused by realistic spatial distributions of the photoions. Using the corresponding electric field distribution, trajectories were calculated for a representative ensemble of electrons, and effective electron energy distributions were obtained from averages over the electron trajectories in the volume relevant for electron attachment. Furthermore, the effects of additional weak electric fields, applied along the atomic beam direction, have been simulated. For our geometry (ionization volume about 2 mm3) the effective space charge related energy width is found to be about 16 μeV/pA.

Alexandrite laser pumped LiF:F 2− laser

For the first time to our knowledge a direct alexandrite laser pumping of LiF:F 2 − crystals was suggested and realized by means of energy transfer from the F 3 − to the F 2 − color centers (CCs). Tunable laser oscillation of LiF:F 2 − in the 1000–1300 nm spectral range with a maximum efficiency of 10% was achieved. The process of two-step photo-ionization of the F 3 − CCs under 740 nm excitation was demonstrated experimentally on the basis of the measured decay rate of F 3 − centers. The photo-ionization cross-section from the first excited level of F 3 − centers was calculated to be 2×10 −22 cm 2. Stable output lasing of the LiF:F 2 − laser at room temperature under 793 nm excitation was predicted and realized.

State-selective optical near-field resonant ionization spectroscopy of atoms near a dielectric surface.

Spin-sensitive optical near-field microscopy and spectroscopy are proposed based on the study on the conserved quantities in optical near-field interactions of atoms with dielectric surfaces. A two-step photoionization spectra of Cs atoms resolving hyperfine structures are demonstrated near a planar dielectric surface by using evanescent waves. These techniques of state/spin-selective excitation and highly sensitive detection, combined with the techniques of optical pumping, will open up possibilities of space- and polarization-sensitive detection of optical near-fields using atomic probes. This novel method provides us with a useful technique for the observation of polarization nature of the optical near-field and controlling the spin states of mesoscopic electronic systems.

Investigations of new high-lying even-parity energy levels of the samarium atom below its first ionization limit

Systematic mapping of high-lying even-parity levels (low-lying configurations) of the samarium atom below the first ionization potential was carried out with a two-step photoexcitation and photoionization method. Five hundred seventy-one new energy levels were discovered in the region 34 850–45 300 cm-1, and an unambiguous assignment of angular momenta was given to most of the energy levels in accordance with the electric dipole selection rules.

Rotational channel interactions of vibrationally excitednpRydberg states of the triatomic hydrogen molecule

We have carried out a precise measurement of the wavelength dependence of the two-step photoionization cross section of triatomic hydrogen via the $3s{}^{2}{A}_{1}^{\ensuremath{'}}$ Rydberg state at energies beyond the ionization limits that belong to the first symmetric-stretch excited vibrational state ${{1,0}^{0}}$ of the ion core. In this energy region, rotational and vibrational channel couplings due to the interaction between the outer p electron and the ${\mathrm{H}}_{3}^{+}$ core are expected. We find strong coupling between the two p series with total angular momentum $N=2$ which converge to the rotational levels ${N}^{+}=1$ and 3 of the underlying ${\mathrm{H}}_{3}^{+}{{1,0}^{0}}$ core. Surprisingly, regular perturbations are also detected in the symmetric-stretch-excited $N=1$ p series. Rydberg levels of the ${\mathrm{ndE}}^{\ensuremath{'}}$ ${{1,0}^{0}}$ series are identified as the perturbing channel. This implies a change in the orbital angular momentum of the Rydberg electron during the short-range interaction. The experimental spectra are analyzed in two-channel quantum-defect models. Eigenchannel quantum defects, coupling constants, and transition moment ratios are determined and compared to the results of recent ab initio calculations.

Dissociative attachment of low-energy electrons to vibrationally excited $\mathsf{Na_2}$ molecules using a photoelectron source

The process of dissociative attachment (DA) of low-energy electrons ) to vibrationally excited sodium dimer molecules is studied with high electron energy resolution ( ) in a supersonic molecular beam. A novel photoelectron source, based on two-step photoionization of the sodium atoms in the beam, may deliver a current of up to 1 nA and has been used with a current of typically 0.2 nA in this experiment. The energy dependence of the rate of sodium anion formation is determined by ion detection based on a time-of-flight analysis. The molecules are selectively excited to levels using the technique of coherent population transfer by delayed pulses (STIRAP). The comparison of the experimental data with recent resonance model calculations based on improved potential curves reveals generally good agreement for levels v ”>12. For some distinct differences between theoretical and experimental results persist.