Distortion of spectral line profiles of laser-cooled ions in a radio-frequency ion trap

We report on the first evidence of electron capture dissociation (ECD) in a radio frequency (rf) ion trap. Peptide ions, [substance P]2+, trapped in a two-dimensional, linear rf ion trap were cleaved by electrons injected along the central axis of the trap. Along the axis, the rf field component was zero and a magnetic field of 50 mT was applied. This electron injection scheme keeps the energy of the electrons below 1 eV, preventing them from heating by the rf field. The present ECD efficiency is approximately 4% by irradiation of electron current of 0.2 microA for 80 ms. ECD in rf traps may open high-throughput and low-cost ECD applications to obtain molecular structure information complementary to collision-induced dissociation.

The ion emission properties of laser-produced plasmas as a function of laser intensities between 4–50 GW cm−2 and varying angles with respect to the target normal were investigated. The plasmas were produced by focusing 1064 nm, 6 ns pulses from an Nd:YAG laser on various metal targets. The targets used for this study include Ti, Mo, and Gd (Z=22,42,64). It is noted that all ion profiles are composed of multiple peaks—a prompt emission peak trailed by three ion peaks (ultrafast, fast, and thermal). Experimentally, it is shown that each of these ion peaks follows a unique trend as a function of laser intensity, angle, and distance away from the target. Theoretically, it is shown that simple analytical models can be used to explain the properties of the ions. The variations in the ion velocity and density as a function of laser intensity are found to be in good agreement with theoretical models of sheath acceleration, isothermal self-similar expansion, and ablative plasma flow for various ion peaks.

The charge transfer process between a laser-cooled magnesium ion and a thermal barium atom in a radio-frequency ion trap in extremely low energy regime is observed. The merit of using the trap is that the generated ions are also confined in the trap. The occurrence of the charge transfer process is confirmed by the detection of the laser-induced fluorescence from the generated ions. The cross section of the process is estimated to be 10−3 A2∼10−1 A2.

Using the dark-spot Zeeman-tuned slowing technique, we have measured the capture velocity of a magneto-optical trap as a function of the trap laser intensity. The values obtained are in reasonable agreement with calculations based on a simple model. Nevertheless, the dependence of capture velocity in the low-intensity regime seems to be slightly different from the predictions. The measurement of this type of trap parameter is of interest in the analysis of exoergic cold collisions, where the knowledge of such values is important for the interpretation of experimental results.

The possibility of injecting ions from an initially fast moving beam into a multipole radio-frequency (RF) ion trap without the use of buffer gas is described. The chosen trap geometry gives rise to an oscillating electric field along the direction of the incoming ions, and through an analytical model as well as numerical simulations it is demonstrated that the energy exchange between the injected ions and this oscillating field governs the trapping dynamics. Most notably, if ions arrive at the trap during specific phases of the RF field, they can be effectively decelerated and stored with low kinetic energy even if their kinetic energy initially exceeds the depth of the trapping potential well. An experimental apparatus for trapping ions from a fast beam is described, and experimental investigations demonstrating the described trapping dynamics are presented.

The relaxation of trapped Cl${{}_{2}}^{\ensuremath{-}}$ ions and their resulting column density in a multipole radio-frequency (RF) ion trap have been investigated after loading the trap from an initial fast-moving beam exploiting a mechanism described recently [A. Svendsen et al., Phys. Rev. A 87, 043410 (2013)] where the injection is mediated through the exchange of energy between ions and the oscillating RF field. The temporal relaxation of the energy distribution of the trapped ion cloud was probed by observing the evolution of the resulting time-of-flight distribution of ions after extraction and fragment mass analysis in a quadrupole mass filter. The ion energy distribution was found to be essentially stationary after $\ensuremath{\sim}20$ ms. The resulting column density of trapped ions after relaxation was probed by two-dimensional position-resolved photodissociation of the trapped Cl${{}_{2}}^{\ensuremath{-}}$ ions. A detailed statistical analysis of the ion column density in the ring-electrode trap is given, and by comparison to the experimental data, a value of the maximum adiabaticity parameter of ${\ensuremath{\eta}}_{\mathrm{max}}\ensuremath{\simeq}0.28$ is inferred. It is further demonstrated how the present experimental system allows for time-resolved mass spectrometry by probing explicitly the populations of both parent (Cl${{}_{2}}^{\ensuremath{-}}$) and daughter (Cl${}^{\ensuremath{-}}$) ions as a function of time after closing the trap and after laser irradiation. Finally, it is discussed how the setup can be used to obtain absolute photodissociation cross sections via a tomographic method without assumptions on the decay law for the trapped ions.

A radio-frequency (RF) ion trap has been constructed for high resolution laser spectroscopy of metallic ions. Ions in externally generated laser plasma have been directly introduced into the RF ion trap. An Nd:YAG laser is used to vaporize and ionize sample metals placed behind a ring electrode. Both hyperbolic and cylindrical electrodes are successfully used for confinement of the ions. Trapped ions are detected either with a quadrupole mass spectrometer or with a photomultiplier for the measurement of laser-induced fluorescence. Metallic ions such as Ca+, Ba+, La+, Nd+, Tm+, Lu+, and Ta+ have been confined for the time range of several to 20 minutes in the presence of He buffer gas, and a doubly charged ion Ba2+ for several seconds. Some ions like Nd+, Lu+, Hf+, and Ta+ are found to be highly reactive with background gaseous molecules.

This study explores optical characteristics in quantum dots. CdSe quantum dots samples have been prepared and an optical photoluminescence experimental setup has been created to measure the light emission from the quantum dots as a function of time and laser intensity. Initial baseline measurements and photoluminescence spectrum has been measured. This preliminary work sets up future studies of quantum dot photobrightening, which is when the emitted light from a CdSe quantum dots gradually increases with time while under constant laser illumination. Future work will investigate photobrightening as a function of laser intensity and with the presence of plasmonic nanoparticles to give insight into plasmonic enhancement and light interaction between plasmonic particles, quantum dots, and photobrightening effects. Results of this study can add value to future quantum dot technologies.

Above-threshold detachment (ATD) rates for ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{F}}^{\ensuremath{-}}$ ions in the high-energy plateau region are calculated as functions of photon number and laser intensity by solving the time-dependent Schr\"odinger equation within the Sturmian-Floquet approach. Pronounced enhancements of the ATD rates are found (up to an order of magnitude) as the laser-field intensity passes across ponderomotive-potential-induced channel closings. We confirm the zero-range potential model results of Borca et al. [Phys. Rev. Lett. 88, 193001 (2002)] for negative ions whose initial states have $s$ symmetry, and we investigate here the case of initial states having $p$ symmetry. Depending on the symmetry of the initial state, the enhancement is found to be most pronounced for even- or odd-channel closures, which is consistent with threshold laws applicable at the closing of particular multiphoton channels. Variations of ATD electron angular distributions as functions of laser intensity near channel closings are also investigated and found to be sensitive to initial-state symmetry.

Quantitative assessment and suppression of anharmonic potential of quadrupole linear radiofrequency ion traps with round electrodes

The determination of the rotational quadrupole alignment of diatomic molecules via REMPI detection is investigated. In this process a high focal intensity usually increases the detection probability. At high intensities the AC Stark effect may cause a splitting of the normally degenerate mJ sublevels of a rotational state J beyond the spectral width of the exciting radiation. This leads to a selective detection of only certain mJ states with the consequence that deduced alignment factors can be misleading. From the theoretical considerations line profiles are explicitly calculated for dynamic polarizabilities which represent the B 1Σ+ u←X 1Σ+ g transition of H2, in order to fit an experimental (3+1) REMPI spectrum and to predict (1+1') line shapes as a function of laser intensity. It is further shown that the deduced quadrupole alignment factor A 0 (2) is significantly changed by the second order AC Stark effect when the intensities are chosen high enough to observe asymmetric broadened line profiles. Different combinations of relative linear polarizations of the exciting and ionizing laser beams are discussed.

In a linear radio-frequency (rf) ion trap, the rf null is the point of zero electric field in the dynamic trapping potential where the ion motion is approximately harmonic. When displaced from the rf null, the ion is superimposed by fast oscillations known as micromotion, which can be probed through motion-sensitive light-atom interactions. In this work, we report on the emergence of the rf pseudonull, a locus of points where the ion responds to light as if it were at the true rf null, despite being displaced from it. The phenomenon is fully explained by accounting for the general two-dimensional structure of micromotion and is experimentally verified under various potential configurations, with observations in great agreement with numerical simulations. The rf pseudonull manifests as a line in a two-dimensional parameter space, determined by the geometry of the incident light and its overlap with the motional structure of the ion. The true rf null occurs uniquely at the concurrent point of the pseudonull lines induced by different light sources.

Control over the charge of a nanoparticle (NP) in a radiofrequency ion trap is crucial for mass spectrometric and charge dependent investigations of single NPs. We show how this is achieved for positively charged silica NPs (nominal diameter 100 nm, 350-1400 e) with a simple experimental realization using a standard cold cathode gauge. The change of the NP charge is the result of processes, where electrons and cations interact with the trapped NP. We investigated how NP charging depends on pressure and gas type as well as on the ion trap amplitude and waveform, which can be used for charging and discharging in a wide NP charge range. The measurement of average charging rates as a function of the NPs' charge for widely varied experimental parameters allows us to capture essential relationships between gas pressure, NP charge, trap potential and net charging rates. The acceleration of gas cations by the trap potential is shown to be the driver of electron abstraction from the NP by gas cations and thereby makes high NP charges accessible.

The anharmonicity of the trap potential influences the motion of trapped and laser-cooled ions. The magnitudes of higher-order components of the trap potential depend on the electrode geometry. In this paper we provide a numerical study of the anharmonicity of linear Paul trap potentials with different electorode geometies. We have also experimentally observed nonlinear resonances of laser-cooled ions, and have studied the effect of anharmonicity in three types of liner Paul trap that have different distances between electrodes.

We report studies of gray-track damage formation in flux-grown potassium titanyl phosphate exposed to 532 nm, picosecond pulses. We investigate the time evolution of damage formation and the evolution of gray-track susceptibility as a function of laser intensity. We demonstrate that the energy loss in the crystal is for the most part due to the formation of the centers of absorption. For 26 ps pulses, the intensity damage threshold was found to be much higher than for nanosecond pulses. Evidence is found that the mechanism of gray-track formation is nonlinear as a function of intensity and as a function of pulse duration.