Two-photon Ionization Research Articles

Two-photon sequential double ionization of argon in the region of Rydberg autoionizing states of Ar+

A theoretical study of two-photon sequential double ionization of Ar is presented for the photon energies in the region of Rydberg autoionizing states 3p4 (1D) nl of the ion Ar+ overlapping with the particle-hole autoionizing states 3s3p6np of neutral Ar. The atomic and ionic autoionizing states lead to sharp variations of the angular correlation function between the two outgoing electrons, as well as in the angular distributions of the first and the second emitted electrons. A strong influence of the second step ionization on the first ionization step is demonstrated.

Determination of nerve agent metabolites in human urine by femtosecond laser ionization mass spectrometry using 2-(bromomethyl)naphthalene as a derivatizing reagent

Nerve agent metabolites (NAMs) derived from alkyl methyl phosphonic acids, such as ethyl methylphosphonic acid (EMPA), isopropyl methylphosphonic acid (IMPA), and pinacolyl methylphosphonic acid (PMPA), were extracted from human urine using diethyl ether as an extractant. After exchanging the diethyl ether solvent to acetonitrile, the analytes were derivatized with 2-(bromomethyl)naphthalene (BMN). The reaction products of the BMN and NAMs, i.e., MN-EMPA, MN-IMPA, and MN-PMPA, were separated by gas chromatography (GC) and measured by mass spectrometry (MS) using a femtosecond laser emitting at 267 nm as the ionization source for resonance-enhanced two-photon ionization (RE2PI). The limits of detection (LOD) were <1 ng/mL for these analytes. The use of BMN increased the volatility of the analytes for separation by GC and also increased the ionization efficiency via the RE2PI process as the result of presence of a naphthalene functional group. A two-dimensional GC-MS display can be used for comprehensive analysis of NAMs, by-products, and impurities in the sample. Then, this approach could be used to confirm the use of chemical weapons and for forensic identification.

C-H/π and C-H-O Interactions in Concert: A Study of the Anisole-Methane Complex using Resonant Ionization and Velocity Mapped Ion Imaging.

Noncovalent forces such as hydrogen bonding, halogen bonding, π-π stacking, and C-H/π and C-H/O interactions hold the key to such chemical processes as protein folding, molecular self-assembly, and drug-substrate interactions. Invaluable insight into the nature and strength of these forces continues to come from the study of isolated molecular clusters. In this work, we report on a study of the isolated anisole-methane complex, where both C-H/π and C-H/O interactions are possible, using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI). Using 2CAP and VMI, we derive the binding energies of the complex in ground, excited, and cation radical states. The experimental values from the two methods are in excellent agreement, and they are compared with selected theoretical values calculated using density functional theory and ab initio methods. The optimized ground-state cluster geometry, which is consistent with the experimental observations, shows methane sitting above the ring, interacting with anisole via both C-H/π and C-H/O interactions, and this dual mode of interaction is reflected in a larger ground-state binding energy as compared with the prototypical benzene-methane system.

Two- and three-photon double ionization of helium by soft x-ray free-electron laser pulses

The multiphoton double ionization of He was investigated using a soft x-ray free-electron laser (SX-FEL), in the perturbative intensity regime (<1013 W cm−2). The photon energy was tuned to the transition between the 1s and 2p levels of He+ ions. At this resonance energy of 40.8 eV, both sequential three-photon double ionization (S3PDI) and two-photon double ionization (2PDI) were measured by electron spectroscopy and by ion time-of-flight mass spectroscopy. We distinguished S3PDI from 2PDI by electron spectroscopy. Based on these experimental data, SX-FEL pulse-energy dependences of electron and ion yields for both S3PDI and 2PDI were discussed. The angular distribution of 2p photoelectrons emitted from He+(2p) ions in the S3PDI process was examined as an example of the simplest photoemission dynamics.

Angularly resolved two-photon above-threshold ionization of helium

Two-photon single ionization of helium is studied for photon energies from the two-photon ionization threshold up to 70 eV by numerically solving the full-dimensional time-dependent Schr\odinger equation. Analysis of the photoelectron angular distributions reveals the dominant two-photon mechanisms and the relevance of radial and electron correlation terms as a function of energy and pulse parameters, as well as the breakdown of a single-active-electron picture, qualitatively valid at low photon energies, at energies above ca. 50 eV.

Polarization tagging of two-photon double ionization by elliptically polarized XUV pulses

We explore the influence of elliptical polarization on the (non)sequential two-photon double ionization of atomic helium with ultrashort extreme ultraviolet (XUV) light fields using time-dependent full ab-initio simulations. The energy and angular distributions of photoelectrons are found to be strongly dependent on the ellipticity. The correlation minimum in the joint angular distribution becomes more prominently visible with increasing ellipticity. In a pump-probe sequence of two subsequent XUV pulses with varying ellipticities, polarization tagging allows to discriminate between sequential and nonsequential photoionization. This clear separation demonstrates the potential of elliptically polarized XUV fields for improved control of electronic emission processes.

Time-dependent perturbation theory beyond the dipole approximation for two-photon ionization of atoms

We develop a time-dependent perturbation theory (PT) beyond the dipole approximation to analyze the two-photon ionization dynamics of atoms exposed to short laser pulses with an arbitrary polarization. In a wide range of laser parameters, the good performance of the PT method is validated by comparing the results of a number of physical quantities with those calculated by numerically solving the time-dependent Schr\odinger equation beyond the dipole approximation. Subsequent applications of the PT method in the nonresonant regime allow us to unveil the important role of the interferences between the dipole and nondipole transition pathways to the photoelectron momentum shift along the laser-propagation direction. In particular, we find that the ratio of the probability of each dipole transition channel to the total ionization probability oscillates with the increase of the photon energy. The interferences among different pathways and the oscillatory behavior of the ratios jointly lead to a series of minima in the linear momentum transfer for the case of the linear polarization.

Imaging the dissociation dynamics of Si2+ via two-photon excitation at 193 nm

In the one-color experiment at 193 nm, we studied the photodissociation of Si2+ ions prepared by two-photon ionization using the time-sliced ion velocity map imaging method. The Si+ imaging study shows that Si2+ dissociation results in two distinct channels: Si(3Pg)+Si+(2Pu) and Si(1D2)+Si+(2Pu). The main channel Si(3Pg)+Si+(2Pu) is produced by the dissociation of the Si2+ ions in more than one energetically available excited electronic state, which are from the ionization of Si2(v=0−5). Particularly, the dissociation from the vibrationally excited Si2(v=1) shows the strongest signal. In contrast, the minor Si(1D2)+Si+(2Pu) channel is due to an avoided crossing between the two 2Πg states in the same symmetry. It has also been observed the one-photon dissociation of Si2+(X4Σg−) into Si(1D2)+Si+(2Pu) products with a large kinetic energy release.

Many-electron character of two-photon above-threshold ionization of Ar

The absolute generalized cross sections and angular distribution parameters of photoelectrons for the two-photon above threshold $3p$-ionization of Ar were calculated in the exciting photon energy range from 15.76 to 36 eV. The correlation function technique developed earlier was extended for the case when an intermediate-state function is of a continuum-type. We show that two-photon ionization of Ar near the $3p^{4}$ threshold to a large extent is determined by the $(3p\dashrightarrow\varepsilon d)^{2}$ two-photon absorption via the giant resonance. This many-electron correlation causes (i) an increase of the photoionization cross sections by more than a factor of 3; (ii) the appearance of resonances in the exciting-photon energy range of the doubly-excited states. The predictions are supported by a good agreement between length and velocity results obtained after taking into account of the higher-order perturbation theory corrections.

Sub-Doppler electronic spectrum of the benzene-D2 complex.

Excitation spectrum of the benzene-D2 van der Waals complex in the vicinity of the S 1 ← S 0 60 1 vibronic transition of the monomer was recorded with sub-Doppler resolution by utilizing mass-selective two-color resonance-enhanced two-photon ionization. Contrary to the previous report on the benzene-H2 complex [M. Hayashi and Y. Ohshima, J. Phys. Chem. A 117, 9819 (2013)], both spin isomers correlating to para and ortho D2 (with rotational angular momentum j = 1 and 0, respectively) are identified by using a gas sample of normal D2. Three and two vibronic bands involving vdW-mode excitation were observed for the para and ortho species, respectively, in addition to their origin bands. Comparison of the results for the two spin isomers has allowed us to make unambiguous band assignments, and vibrational frequencies of all the three vdW modes have been determined for benzene-H2 and -D2. Among the three modes, the two-dimensional vdW twist is correlated to the hindered internal rotation of H2/D2 and the barrier for the internal rotation has been evaluated: 72 and 66 cm-1 for benzene-H2 and -D2, respectively. Vibronic-state dependence of the intermolecular distance between benzene and H2/D2 is discussed on the basis of precisely determined rotational constants. Homogenous line broadening has been identified for all the observed vibronic bands, and the corresponding upper-state lifetimes are determined to be in the range of 0.3-0.7 ns.

用双光子电离探测技术研究奇宇称的Eu原子光谱

用双光子电离探测技术研究奇宇称的Eu原子光谱

Signal and pointing accuracy of ultraviolet laser in micro-pattern gaseous detector

In the study of the gas detectors, it is an important calibration method to use the ultraviolet (UV) laser with two-photon ionization mechanism for producing ionized signal. In the last decades, micro pattern gas detector, especially gaseous electron multiplier and micromesh gaseous detector, has been widely used in high energy experiments. These kinds of gaseous detectors have the advantages of higher ion backflow suppression ability, smaller &lt;b&gt;&lt;i&gt;E&lt;/i&gt;&lt;/b&gt; × &lt;b&gt;&lt;i&gt;B&lt;/i&gt;&lt;/b&gt; effect and good radiation resistance under the relatively higher count rate environment. To obtain a higher spatial resolution with a UV laser calibration system in gaseous electron multiplier detector, two critical technical issues remain to be resolved: the measurability of the laser signal and the accuracy of the laser beam position. In this paper, the studies in simulation and experiment are conducted to discuss these two critical questions. In the simulation section, the simulation results provide an estimation of signal in the gaseous electron multiplier detector with UV laser of 266 nm wavelength in the mixture working gases of Ar/CO&lt;sub&gt;2&lt;/sub&gt; (70/30), and give an evaluation of the laser pointing accuracy and the possible relative error of the electron drift velocity. In the experiment section, a UV laser calibration prototype is designed and developed. A pulsed laser of 266 nm wavelength is used as a signal source, which has a Gaussian-like cross section with a frequency of 10 Hz. The experimental results indicate that the signal of the UV laser in a triple gaseous electron multiplier detector reaches 400 mV for a readout strip width of 6 mm, a gain of detector of 5000, and a gain of amplifier of 10 mV/fC. For the calibration laser, the angle accuracy is discussed and tested. The angle uncertainty of the laser can be kept under 5′, and the accuracy of the drift velocity can reach 6.4 × 10&lt;sup&gt;−4&lt;/sup&gt; with a shift of 0.33 mm in the &lt;i&gt;z&lt;/i&gt; direction when the laser beam transmits a distance of 400 mm in the gas chamber. All of these results show that the laser beam specific parameters are the main reference for designing the prototype detector. According to the optimal parameters, a gaseous prototype detector will be tested in the next study.

A pump–probe scheme with a single chirped pulse to image electron and nuclear dynamics in molecules

A single chirped few-femtosecond pulse can be used to control and image coupled electron-nuclear dynamics. Using full ab initio simulations of the simplest molecule, as a prototype target, we show that for intermediate values of the chirp, interference between sequential and direct contributions enables significant control over ionization yields, even when taking into account the effective decoherence introduced by nuclear motion and the presence of an electronic continuum. For larger values of the chirp, the single chirped pulse reproduces a classical pump–probe setup, with the chirp parameter mapping an effective time delay between the pumping and probing frequencies of the pulse. After demonstrating this numerically, we present a full analytical solution for the two-photon ionization amplitudes that provides an intuitive analogy between the molecular dynamics induced by a single chirped pulse and a traditional pump–probe setup.

R -matrix-with-time-dependence theory for ultrafast atomic processes in arbitrary light fields

We describe an ab initio and non-perturbative $R$-matrix with time-dependence theory for ultrafast atomic processes in light fields of arbitrary polarization. The theory is applicable to complex, multielectron atoms and atomic ions subject to ultrashort (particularly few-femtosecond and attosecond) laser pulses with any given ellipticity, and generalizes previous time-dependent $R$-matrix techniques restricted to linearly polarized fields. We discuss both the fundamental equations, required to propagate the multielectron wavefunction in time, as well as the computational developments necessary for their efficient numerical solution. To verify the accuracy of our approach, we investigate the two-photon ionization of He, irradiated by a pair of time-delayed, circularly polarized, femtosecond laser pulses, and compare photoelectron momentum distributions, in the polarization plane, with those obtained from recent time-dependent close-coupling calculations. The predictive capabilities of our approach are further demonstrated through a study of single-photon detachment from F$^{-}$ in a circularly polarized, femtosecond laser pulse, where the relative contribution of the co- and counter-rotating $2p$ electrons is quantified.

Intermolecular dissociation energies of hydrogen-bonded 1-naphthol complexes.

We have measured the intermolecular dissociation energies D 0 of supersonically cooled 1-naphthol (1NpOH) complexes with solvents S = furan, thiophene, 2,5-dimethylfuran, and tetrahydrofuran. The naphthol OH forms non-classical H-bonds with the aromatic π-electrons of furan, thiophene, and 2,5-dimethylfuran and a classical H-bond with the tetrahydrofuran O atom. Using the stimulated-emission pumping resonant two-photon ionization method, the ground-state D 0(S 0) values were bracketed as 21.8 ± 0.3 kJ/mol for furan, 26.6 ± 0.6 kJ/mol for thiophene, 36.5 ± 2.3 kJ/mol for 2,5-dimethylfuran, and 37.6 ± 1.3 kJ/mol for tetrahydrofuran. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3 (using the def2-TZVPP basis set), and ωB97X-D [using the 6-311++G(d,p) basis set] predict that the H-bonded (edge) isomers are more stable than the face isomers bound by dispersion; experimentally, we only observe edge isomers. We compare the calculated and experimental D 0 values and extend the comparison to the previously measured 1NpOH complexes with cyclopropane, benzene, water, alcohols, and cyclic ethers. The dissociation energies of the nonclassically H-bonded complexes increase roughly linearly with the average polarizability of the solvent, (S). By contrast, the D 0 values of the classically H-bonded complexes are larger, increase more rapidly at low (S), but saturate for large (S). The calculated D 0(S 0) values for the cyclopropane, benzene, furan, and tetrahydrofuran complexes agree with experiment to within 1 kJ/mol and those of thiophene and 2,5-dimethylfuran are ∼3 kJ/mol smaller than experiment. The B3LYP-D3 calculated D 0 values exhibit the lowest mean absolute deviation (MAD) relative to experiment (MAD = 1.7 kJ/mol), and the B97-D3 and ωB97X-D MADs are 2.2 and 2.6 kJ/mol, respectively.

Mass-analyzed threshold ionization spectroscopy of trans-o-methylanisole

In this study, we recorded the vibronic and cation spectra of trans-o-methylanisole by using resonant two-photon ionization and mass-analyzed threshold ionization techniques. The excitation energy of the S1 ← S0 transition and the adiabatic ionization energy were measured as 36,364 ± 2 cm−1 and 64,755 ± 5 cm−1, respectively. The red shifts of these values with respect to those of anisole and toluene indicated that the interactions of CH3 and OCH3 substituents with the ring are stronger in the electronically excited S1 and cationic ground D0 states than that in the electronically ground S0 state. Most of the observed bands in the S1 and D0 states involve in-plane ring deformation and methyl internal rotation. The vibrational frequencies of modes 6b1, 6a1, and 11 were lower in the S1 state than in the D0 state. This suggested that trans-o-methylanisole is more rigid in the D0 state than in the S1 state.

Complete reconstruction of bound and unbound electronic wavefunctions in two-photon double ionization

To be complete, the characterization of the photoionization process of atoms and molecules requires the extraction of all quantum-mechanical phases and amplitudes. So far, complete experiments have accessed only the ionization process of neutral atoms and molecules. Here we report the quantum-mechanically complete characterization of the single and double ionization of neon to yield doubly charged ions. The first ionization step by intense, polarized extreme ultraviolet light from a free-electron laser leaves the ion in a polarized state (that is, one in which the angular momentum of the ion is aligned in space). By controlling the polarization of the light, we determine the bound and continuum components of the system in the first and second ionization steps leading to the formation of doubly charged neon ions. We test the validity of our approach by characterizing the influence of autoionizing ionic states on the two-photon double-ionization mechanism. Our results are important for understanding the physics of the interaction of extreme ultraviolet radiation with ions. Photoionization is one of the most important photophysical events. This process can now be characterized in a quantum-mechanically complete manner by use of polarization-controlled extreme-ultraviolet light derived from a free-electron laser

Analysis of the S1 ← S0 and D0 ← S1 spectra in m-bromofluorobenzene via resonance-enhanced multiphoton ionization and slow electron velocity-map imaging spectroscopy

Vibrations of m-bromofluorobenzene (m-BrFPh) in the first excited state (S1) and the cationic ground state (D0) are investigated by resonance-enhanced multiphoton ionization (REMPI) spectroscopy and slow electron velocity-map imaging (SEVI) spectroscopy, respectively. Quantum chemical calculations have been adopted to assign the observed vibrational bands, as well. The band origin of the S1 ← S0 electronic transition defines the adiabatic excitation energy to be 36,987 ± 4 cm−1 based on the resonance-enhanced one-color two-photon ionization spectrum of m-BrFPh. And adiabatic ionization potential (IP) of 73,903 ± 8 cm−1 is determined from band origin in the SEVI spectra. Meanwhile, the computed frequencies are in excellent agreement with the experimental observations and Franck-Condon simulations are performed to aid us to confidently assign major vibrational modes in S1 and D0 states. And the changes in the wavenumber of the vibrational modes upon S1 ← S0 and D0 ← S1 transitions are examined. Furthermore, the mixing of vibrational modes both between S0 & S1 and between S1 & D0 has been discussed.

Transition from Water Wires to Bifurcated H-Bond Networks in 2-Pyridone·(H2O) n, n = 1-4 Clusters.

Mass-selective two-color resonant two-photon ionization (2C-R2PI), UV/UV hole-burning, and infrared (IR) depletion spectra of supersonic jet-cooled 2-pyridone·(H2O) n clusters with n = 1-4 have been measured to investigate the local hydration patterns around 2-pyridone (2PY) as a function of cluster size. As shown by others, the IR frequencies of the OH and NH stretches of the n = 1, 2 clusters are characteristic of water wires stretching from the NH to the C═O group of 2PY. We identify two isomers (3A and 3B) of the n = 3 cluster in the 2C-R2PI spectrum and separate them by IR/UV and UV/UV hole-burning techniques. Isomer 3A exhibits a three-membered water wire, extending the n = 1, 2 structural motif. Isomer 3B exhibits bifurcated water wires with the first H2O donating to two waters that form H-bonds to the C═O group. This increases the H-bond strength between the NH group of 2PY and the proximal H2O molecule, lowering the NH stretch to ∼2800 cm-1. The n = 4 cluster is also bifurcated with two water wires between the bifurcating H2O and the C═O group. The cluster-selective IR spectra are complemented with density-functional calculations using the PW91, B3LYP, B97-D, and M06-2X functionals, where the latter two include long-range dispersive interactions, and with the ab initio correlated SCS-CC2 method. The calculated IR spectra provide firm assignments of the structures of the n = 1-4 cluster structures and allow us to understand the evolution of individual H-bond strengths with increasing cluster size.

Bond dissociation energies of FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi.

Resonant two-photon ionization spectroscopy has been used to investigate the spectra of the diatomic late transition metal silicides, MSi, M = Fe, Ru, Os, Co, Rh, Ir, Ni, and Pt, in the vicinity of the bond dissociation energy. In these molecules, the density of vibronic states is so large that the spectra appear quasicontinuous in this energy range. When the excitation energy exceeds the ground separated atom limit, however, a new decay process becomes available-molecular dissociation. This occurs so rapidly that the molecule falls apart before it can absorb another photon and be ionized. The result is a sharp drop to the baseline in the ion signal, which we identify as occurring at the thermochemical 0 K bond dissociation energy, D0. On this basis, the measured predissociation thresholds provide D0 = 2.402(3), 4.132(3), 4.516(3), 2.862(3), 4.169(3), 4.952(3), 3.324(3), and 5.325(9) eV for FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi, respectively. Using thermochemical cycles, the enthalpies of formation of the gaseous MSi molecules are derived as 627(8), 700(10), 799(10), 595(8), 599(8), 636(10), 553(12), and 497(8) kJ/mol for FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi, respectively. Likewise, combining these results with other data provides the ionization energies of CoSi and NiSi as 7.49(7) and 7.62(7) eV, respectively. Chemical bonding trends among the diatomic transition metal silicides are discussed.