Resonant Excitation Process Research Articles

Mass-selective instability and resonance ejection modes for DIT with rectangular asymmetric wave shape.

We consider the operation of a digital linear ion trap with resonance radial ejection and mass selective instability modes. Periodic wave shape has a positive part with amplitude and duration and negative part with amplitude and duration , where T is the period. The mapping of the stability diagram, calculations of the well's depth and ion oscillations spectra are presented. The process of resonant excitation of ion oscillations by a dipole sinusoidal signal is studied, as well as ion ejection at the stability boundary. The trajectory method is used for this purpose. It is shown that the mass selectivity of dipole excitation is twice as large for rectangular wave shape compared to sinusoidal wave shape. Increasing the diameter of the round rods of the linear trap gives an increase in the resolving power. The possibility of DIT operation in mass-selective instability mode at the boundary point is discussed.

Resonance Excitation Process of Lithium Atoms in Elliptically Polarized Laser Fields (Invited)

Resonance Excitation Process of Lithium Atoms in Elliptically Polarized Laser Fields (Invited)

Crossover from Reverse Saturable to Saturable Absorptions in Two-Dimensional Tungsten Disulfide

The Tungsten disulfide atomic layer has enormous scientific merits that play a key role in the nonlinear absorption process. Four distinct excitonic peaks in the absorption spectra are crucial for identifying the polarity of nonlinear absorption by virtue of the resonant or non-resonant excitation process. The excitation source of 532 nm is enough for the one-photon optical transition at both low-energy excitonic peaks and two-photon transition at both higher-energy excitonic peaks. The Tungsten disulfide atomic layer exhibited the reverse saturable (or positive nonlinear) absorption through the two-step excitation process which could be possible due to the optical transition at the higher energy excitonic peaks. In addition, the polarity of the nonlinear absorption coefficient and its interchange have been analytically studied with the aid of a magnitude of excited-state absorption cross-section and Ground-state absorption cross-section. Besides, the various possible factors affecting the Z-scan results are discussed.

Study on the mechanism of blue light generation by ultra-broadband absorption in Rb–He mixture

In the ultra-broadband range, by continuously tuning the wavelength of the pump laser to excite the rubidium-helium (Rb–He) mixture, an obvious blue fluorescence signal can always be detected. Through the experimental study of the difference between resonant excitation and non-resonant excitation to produce blue fluorescence, it is verified that Rb–He excimer plays an important role in ultra-broadband absorption, and five mechanisms for producing blue fluorescence are summarized: (1) excimer + one-photon resonant excitation process from 5P; (2) two-photon resonant excitation process from 5S; (3) one-photon resonant excitation process from 5S + energy pooling process; (4) excimer + one-photon resonant excitation process from 4D; (5) excimer + energy pooling process. The influence of ionization on the generation of blue fluorescence is discussed. Under several typical pump wavelengths (based on different mechanisms of generating 420 nm fluorescence), the variation of 420 nm fluorescence with the Rb vapor atom concentration, the characteristics of 420 nm collimated light, and its competition with the fluorescence were discussed. The ultra-broadband absorption properties of the alkali metal-noble gas system can be applied to the detection of infrared signals.

The Resonant Excitation Process in Commercial Quadrupole Ion Traps Revisited.

The collision-induced resonant excitation process in real quadrupole ion traps is revisited theoretically and experimentally by explicitly including in the discussion the influence of higher order potential impurities. This includes mainly the dependence of the secular oscillation frequency fion on the ion's oscillation amplitude zmax. Due to frequency calibration, commercial ion traps use excitation frequencies fexc that are higher than the theoretical secular oscillation frequency fion. This may lead to switching in frequency order between fexc and fion that can allow ions to stay longer in on-resonance. It is also found that there is a most efficient but also a harshest excitation frequency, which are not identical. These phenomena are explained and described with a simple harmonic oscillator model and precise numerical calculations, using the trajectory simulation program ITSIM 5.0. Experimental MS2 have been performed with the thermometer ion leucine-enkephalin, which are then in line with expectations from the trajectory calculations. The important difference to the existing literature is that, here, overexcitation is characterized by the observed a4/b4 fragment-ion ratio, while the fragmentation efficiency was kept constant. By slightly increasing the excitation frequency one can obtain drastically different effective collisional temperatures. This knowledge gives even commercial ion traps, without instrument adjustments, the possibility of producing energetically versatile fragment ion spectra. It is also shown that the damped driven harmonic oscillator cannot be used as a simplified model of the motion during the resonant excitation process in real ion traps.

Observation of rotational coherence in an excited state of CO.

The vacuum ultraviolet (VUV) radiation is generated in the strong-field-ionized CO molecules through 2+1 resonance excitation with two-color femtosecond laser pulses. When scanning the relative delay between two pump pulses, the rotational-resolved VUV radiations show periodic oscillations lasting as long as 500 ps. Fourier analysis reveals that these oscillations correspond to rotational beat frequencies of the A2Πi state of CO+, which is the result of multi-channel interference during the resonant excitation process. High resolution of Fourier transform spectra up to 0.067cm-1 allows us to obtain the fine energy levels of the A2Πi state. The theoretical calculation is in good agreement with the experimental observation. This work reveals the rotational coherence of the ionic excited state and shows the prospect of rotational coherence spectroscopy in measuring fine structures of molecular ions.

A simulation study of excitation contour of a linear trap with spatial harmonics.

The process of nonlinear resonant excitation of ion oscillations in a linear trap is studied. There is still no detailed simulation of the resonance peak in the literature. We propose to use the excitation contour to describe the collective ion resonance. The excitation contour is a resonant mass peak obtained by the trajectory method with the Gaussian distribution of the initial coordinates and velocities. The following factors are considered: excitation time, low order hexapole and octopole harmonics with amplitudes A3 and A4, the depth of the initial ion cloud position. These multipoles are used for selective ion ejection from linear ion trap. All these factors affect the ion yield and the shape of the contours. Obtained data can be useful for control of such processes as ion fragmentation, ion isolation, ion activation, and ion ejection. Simulated resonance peaks are important for the theoretical description of the ion collective nonlinear resonances.

X-ray spectra of the Fe-L complex

The Hitomi results for the Perseus cluster have shown that accurate atomic models are essential to the success of X-ray spectroscopic missions and just as important as the store of knowledge on instrumental calibration and astrophysical modeling. Preparing the models requires a multifaceted approach, including theoretical calculations, laboratory measurements, and calibration using real observations. In a previous paper, we presented a calculation of the electron impact cross sections on the transitions forming the Fe-L complex. In the present work, we systematically tested the calculation against cross-sections of ions measured in an electron beam ion trap experiment. A two-dimensional analysis in the electron beam energies and X-ray photon energies was utilized to disentangle radiative channels following dielectronic recombination, direct electron-impact excitation, and resonant excitation processes in the experimental data. The data calibrated through laboratory measurements were further fed into a global modeling of the Chandra grating spectrum of Capella. We investigated and compared the fit quality, as well as the sensitivity of the derived physical parameters to the underlying atomic data and the astrophysical plasma modeling. We further list the potential areas of disagreement between the observations and the present calculations, which, in turn, calls for renewed efforts with regard to theoretical calculations and targeted laboratory measurements.

Strong-field dissociative Rydberg excitation of oxygen molecules: Electron-nuclear correlation

We experimentally investigate strong-field dissociative Rydberg excitation (DRE) of ${\mathrm{O}}_{2}$ molecules by using the photoelectron photoion coincidence spectrum, in which the outgoing neutral Rydberg fragments created by the strong laser fields are postionized by the weak static dc field of the spectrometer and afterwards detected as charged particles. The peak positions of the sum-kinetic energy release (KER) spectrum of the ejected nuclear fragments and electrons of the dissociative single ionization (DSI) channel are measured to be similar to that of the DRE channel, although their nuclear KER spectra are distinct. It indicates that the same number of photons is absorbed by the molecule in accessing the DRE and DSI channels, which are partitioned by the electron and nuclear fragments with various proportions. Different pathways towards the DRE and DSI channels are discussed based on the correlated dynamics of the nuclear fragments and electrons. Our results show that the DRE channel observed here is mostly accessed by the multiphoton resonance excitation process.

Revisiting the Fe xvii Line Emission Problem: Laboratory Measurements of the 3s–2p and 3d–2p Line-formation Channels

Abstract We determined relative X-ray photon emission cross sections in Fe xvii ions that were mono-energetically excited in an electron beam ion trap. Line formation for the 3s (3s−2p) and 3d (3d−2p) transitions of interest proceeds through dielectronic recombination (DR), direct electron-impact excitation (DE), resonant excitation (RE), and radiative cascades. By reducing the electron-energy spread to a sixth of that of previous works and increasing counting statistics by three orders of magnitude, we account for hitherto unresolved contributions from DR and the little-studied RE process to the 3d transitions, and also for cascade population of the 3s line manifold through forbidden states. We found good agreement with state-of-the-art many-body perturbation theory (MBPT) and the distorted-wave (DW) method for the 3s transition, while in the 3d transitions known discrepancies were confirmed. Our results show that DW calculations overestimate the 3d line emission due to DE by ∼20%. Inclusion of electron-electron correlation effects through the MBPT method in the DE cross-section calculations reduces this disagreement by ∼11%. The remaining ∼9% in 3d and ∼11% in 3s/3d discrepancies are consistent with those found in previous laboratory measurements, solar, and astrophysical observations. Meanwhile, spectral models of opacity, temperature, and turbulence velocity should be adjusted to these experimental cross sections to optimize the accuracy of plasma diagnostics based on these bright soft X-ray lines of Fe xvii.

Silicon Nitride based Medium Contrast Gratings for Doubly Resonant Fluorescence Enhancement

We present the design, fabrication, and experimental characterization of silicon nitride based medium-index contrast gratings on glass substrate for fluorescence enhancement in the yellow to red spectral range with resonances for both incident excitation and fluorescence emission wavelengths under surface normal incidence. A comparison of the design space to realize resonant field enhancement in high-index contrast silicon and medium-index contrast silicon nitride grating structures is presented. The one-dimensional sub-wavelength grating structures studied here are designed with large duty cycle (∼80%) to account for the medium refractive index contrast (Δn ∼ 0.5) between silicon nitride and the glass substrate to ensure that the device operates in the two-mode regime. The resonant enhancement of fluorescence is experimentally verified using rhodamine-B isothiocyanate dye as the fluorophore of interest. A resonant enhancement of 10.8 times is demonstrated in this sample when compared to un-patterned film for transverse electric-transverse magnetic (TE–TM) polarization combination. We have also performed simulation study with plane wave excitation and incoherent dipole array emission to model the resonant excitation and emission processes, respectively. The simulations corroborate well with the best observed experimental results for the doubly resonant fluorescence configuration. Silicon nitride based medium contrast gratings are a promising platform to fabricate scalable structures for resonant enhancement of light–matter interaction with potential applications in high-sensitivity biological fluorescence assays and as a platform for polarization selective interrogation of light emission from nanoscale emitters attached to the grating.

Spatiotemporal evolution of ultrafast magnetic-field generation in molecules with intense bichromatic circularly polarized UV laser pulses

We present ultrafast magnetic-field pulse generation in aligned molecules from numerical solutions of time-dependent Schr\odinger equations. The one-electron molecular ion ${{\mathrm{H}}_{2}}^{+}$ as a benchmark model is used to describe the ultrafast photophysics process. Schemes with bichromatic high-frequency corotating and counterrotating $(\ensuremath{\omega},2\ensuremath{\omega})$ and ($\ensuremath{\omega},3\ensuremath{\omega})$ circularly polarized UV laser pulses are presented to produce the spatial and temporal evolution of the generated magnetic field. We discuss how interference effects between multiple resonant excitations modulate the evolution of the generated magnetic field. It is found that the modulation of generated magnetic fields is dependent on the pulse frequency and helicity combination and the molecular alignment, which is attributed to the different resonant excitation processes with various molecular orbitals that vary the intramolecular coherent electron currents. The spatiotemporal evolution of generated magnetic fields is shown to be related and strongly different for molecular around-axis and in-plane electron currents. The scheme allows one to control the induced magnetic-field-pulse generation as a tool for ultrafast optical magnetism.

Plasmonic Fields Focused to Molecular Size

AbstractEfficient use of light energy is regarded as a key factor in solving energy challenges to create a sustainable society. The highly concentrated photon energy generated by localized surface plasmon resonance excitation in the vicinity of metal nanostructures can enhance light‐matter interaction. Optimization of the interactions between plasmons and electrons in materials can lead to novel light energy applications. To overcome the current limitations for these interactions, the plasmon field must be focused to an extremely small size close to the molecular scale. Formation of the plasmonic field at the quantum limit may cause interesting phenomena with unique photoresponses. Recently, following the development of nanofabrication techniques, detailed investigations have been undertaken to understand these processes. In this focus review, we describe recent advances in the strong interactions between highly localized photons and electrons in nanomaterials, including molecules, nanocarbons, and quantized nanoparticles. First, we outline the plasmonic properties that depend on the metal nanostructures. In addition, we describe surface‐enhanced Raman scattering (SERS), which is used to detect interactions between plasmons and materials. The importance of the resonant electronic excitation process, which is a chemical effect based on the charge transfer contribution, is discussed while considering the unique molecular selectivity in SERS. We then highlight the unique photoresponse properties that are used for ultra‐sensitive detection of single molecules by the localized plasmon field. These properties are major advantages of the plasmon field. Next, we introduce strong coupling between plasmons and excitons. This coupling state is promising because of its ability to modify the intrinsic optical properties of materials via creation of a novel absorption wavelength region to accumulate the light energy. Finally, we discuss the use of plasmon excitation for effective chemical reactions accompanied by electron transfer. We conclude that reduction of light to the molecular scale would open novel routes for energy manipulation required by the next generation.

Ultrafast molecular photoionization by two-color orthogonally polarized ultraviolet laser pulses: Effects of relative pulse phases

We present molecular photoionization by two-color 2ω1=ω2 orthogonally polarized ultraviolet laser pulses. Simulations are performed on aligned H2+ by numerically solving time-dependent Schrödinger equations. Two ionization processes with one ω2 photon interfering with two ω1 photon absorption are studied at different molecular alignments. Molecular frame photoelectron momentum and angular distributions exhibit asymmetries which are functions of the relative pulse phase. For resonant excitation processes by the ω1 pulse, symmetric distributions are obtained. An attosecond ionization model is adopted to describe the ultrafast ionization dynamics. The dependence of the ionization asymmetry on the molecular alignment allows to further monitor interference effects on orbital symmetry.

Theoretical resonant electron-impact vibrational excitation, dissociative recombination and dissociative excitation cross sections of ro-vibrationally excited BeH+ ion

A theoretical study of resonant vibrational excitation, dissociative recombination and dissociative excitation processes of the beryllium monohydride cation, BeH+, induced by electron impact, is reported. Full sets of ro-vibrationally-resolved cross sections and of the corresponding Maxwellian rate coefficients are presented for the three processes. Particular emphasis is given to the high-energy behaviour. Potential curves of , and symmetries and the corresponding resonance widths, obtained from R-matrix calculations, provide the input for calculations which use a local complex-potential model for resonant collisions in each of the three symmetries. Rotational motion of nuclei and isotopic effects are also discussed. The relevant results are compared with those obtained using a multichannel quantum defect theory method. Full results are available from the Phys4Entry database.

Interband Heating Processes in a Periodically Driven Optical Lattice

Abstract We investigate multi-“photon” interband excitation processes in an optical lattice that is driven periodically in time by a modulation of the lattice depth. Assuming the system to be prepared in the lowest band, we compute the excitation spectrum numerically. Moreover, we estimate the effective coupling parameters for resonant interband excitation processes analytically, employing degenerate perturbation theory in Floquet space. We find that below a threshold driving strength, interband excitations are suppressed exponentially with respect to the inverse driving frequency. For sufficiently low frequencies, this leads to a rather sudden onset of interband heating, once the driving strength reaches the threshold. We argue that this behavior is rather generic and should also be found in lattice systems that are driven by other forms of periodic forcing. Our results are relevant for Floquet engineering, where a lattice system is driven periodically in time in order to endow it with novel properties like the emergence of a strong artificial magnetic field or a topological band structure. In this context, interband excitation processes correspond to detrimental heating.

Interference asymmetry of molecular frame photoelectron angular distributions in bichromatic UV ionization processes

We investigate molecular photoionization by ultrafast bichromatic linearly polarized UV laser pulses at frequencies perpendicular to the internuclear axis R involving π orbital excitation. Results from numerical solutions of time dependent Schrödinger equations for aligned show that molecular frame photoelectron angular distributions (MFPADs) exhibit signatures of asymmetry perpendicular to the molecular symmetry axis, arising from interference of coherent electron wave packets created by respectively one and two-photon absorption. A resonant excitation process between the ground state and the excited state is triggered by the pulse. The asymmetry of MFPADs varies periodically with pulse intensity I0 and duration T, which we attribute to coherent resonant Rabi oscillations in electronic state population. A perturbative model is adopted to qualitatively describe and analyze these effects in both resonant and nonresonant photoionization processes.

Study of optical nonlinearity of CdSe and CdSe@ZnO core–shell quantum dots in nanosecond regime

Thioglycolic acid capped cadmium selenide (CdSe) and CdSe@ZnO core–shell quantum dots have been synthesized in aqueous phase. The sample was characterized by UV-vis spectrophotometer, TEM and Z-scan technique. The nonlinear optical parameters viz. nonlinear absorption coefficient [Formula: see text], nonlinear refractive index [Formula: see text] and third-order nonlinear susceptibilities [Formula: see text] of quantum dots have been estimated using second harmonic of Nd:YAG laser. The study predicts that CdSe@ZnO quantum dots exhibits strong nonlinearity as compared to core CdSe quantum dots. The nonlinearity in quantum dots is attributed to the presence of resonant excitation and free optical processes. The presence of RSA in these nanoparticles makes them a potential material for the development of optical limiter.

Electron-impact resonant vibrational excitation and dissociation processes involving vibrationally excited N2 molecules

Resonant vibrational excitation cross sections and the corresponding rate coefficients for electron–N2 collisions occurring through the resonant state are reviewed. New calculations are performed using accurate potential energy curves for the N2 electronic ground state, taken from the literature, and for the resonant state, obtained from R-matrix calculations. The calculations are extended to resonant excitation processes involving the N2 ground state vibrational continuum, leading to dissociation. Electron-impact dissociation is found to be significant from higher vibrational levels. Accurate analytical fits for the complete set of the rate coefficients are provided. The behavior of the dissociative cross sections is investigated for rotationally excited N2 molecules, with J = 50, 100 and 150, and for different vibrational levels.

Simultaneous resonant excitation of low-frequency eccentric wave and tilt wave on tidally deformed disks

Abstract Simultaneous excitation of low-frequency eccentric precessing mode (one-armed p-mode) and tilt mode on tidally deformed disks is considered. If the orbit of the secondary star is eccentric and its orbital plane is misaligned with the disk plane of the primary, the above-mentioned two low-frequency oscillation modes are simultaneously excited on the primary disk, the former having prograte precession and the latter having retrograde precession. This excitation of disk oscillations is due to a wave–wave resonant excitation process considered by Kato (2013, PASJ, 65, 75). If parameter values relevant to Be/X-ray binary systems are adopted, the periods of these excited oscillations are around 10 times the orbital period of the secondary, which may be comparable with the timescale of giant outbursts observed in Be/X-ray systems.