Multiple Photoionization Research Articles

Multiple photoionization of Ar in the K-edge region: New results

Single and multiple photoionization branching ratios of Ar have been measured for photon energies around the K-edge region between 3198 and 3212 eV using synchrotron radiation and time-of-flight mass spectrometry. The present data is normalized to the absolute values using the accurately measured data of Zheng et al. [L. Zheng, M. Cui, Y. Zhao, K. Chen, J. Electron. Spectrosc. Rel. Phenom. 152 (2006) 143–147]. A comparison with other experimental and theoretical data is presented and discrepancies are discussed.

Double Ionization of Quaterrylene (C40H20) in Water-Ice at 20 K with Lyα (121.6 nm) Radiation

Polycyclic aromatic hydrocarbon (PAH) molecules undergo facile ionization in cryogenic water-ices resulting in near quantitative conversions of neutral molecules to the corresponding singly charged radical cations. Here we report, for the first time, the production and stabilization of a doubly ionized, closed shell PAH in water-ice. The large PAH quaterrylene (QTR, C40H20) is readily photoionized and stabilized as QTR 2+ in a water-ice matrix at 20 K. The kinetic analysis of photolysis shows that the QTR 2+ is formed at the expense of QTR +, not directly from QTR. The long-axis polarized S1-S0 (1(1)B(3u) <-- 1(1)Ag) transition for QTR 2+ falls at 1.59 eV (782 nm). TD-DFT calculations at the B3LYP level predict that this transition falls at 1.85 eV (670 nm) for free gas-phase QTR 2+, within the 0.3 eV uncertainty associated with these calculations. This red shift of 0.26 eV is quite similar to the 0.24 eV red shift between the TD-DFT computational prediction for the lowest energy transition for QTR + (1.68 eV) and its value in a water matrix (1.44 eV). These results suggest that multiple photoionization of such large PAHs in water-ice can be an efficient process in general.

Multiple ionization of the fullerene by a single photon

We consider the single and multiple photoionization processes in a fullerene molecule. The quantum states of C60 are calculated within the Hartree–Fock and the frozen-core approximations. For the description of multiple photoionization probabilities, statistical energy deposition (SED) model was improved and used in addition. The advantage of the SED model is that it is simple enough to be applied to polyatomic systems, where more elaborate calculations are hardly feasible. We account for the electron–electron interaction responsible for the multiple ionization within the random phase approximation with exchange that describes the dynamical polarization of the electronic cloud of the target. As a result, different multi-electron ionization cross-sections of C60 in the photon energy range ∼10–250eV are compared, analyzed, and their orders of magnitude are estimated. We also discuss the diffraction of the photoelectron wave on the fullerene shell which results in oscillations in the photoionization cross-section as a function of the photon energy.

Multiple photoionization following 3d5/2-shell threshold ionization of Xe

Multiple photoionization of Xe near the 3d5/2-shell threshold photoionization region is studied by threshold electron–ion coincidence spectroscopy. The coincidence spectra of Xe3+ to Xe7+ ions exhibit characteristic profiles associated with multi-step post-collision interactions in Auger cascades following 3d5/2-shell threshold photoionization. The Auger cascade decay channels leading to the formation of multiply charged ions are deduced from the energies of the profile peaks, which increase gradually with increasing charge state. The formation of Xe3+ to Xe5+ ions is found to arise from cascades of normal Auger decays, whereas the formation of Xe6+ and Xe7+ ions involves double Auger decays. The branching ratio of double to normal Auger decays is estimated to be 0.25 (±0.1) for the decays following the creation of 3d5/2-hole states in Xe.

Time-dependent studies of single and multiple photoionization ofH2+

A time-dependent method is used to study the photoionization of the simplest one-electron molecule, ${\mathrm{H}}_{2}^{+}.$ We use the variational principle to solve the time-dependent Schr\odinger equation for ${\mathrm{H}}_{2}^{+}$ in spherical coordinates $(r,\ensuremath{\theta})$ centered on the center of mass of the ${\mathrm{H}}_{2}^{+}$ system in a time-varying electromagnetic field, in the fixed-nuclei approximation. Bound and continuum states of ${\mathrm{H}}_{2}^{+}$ are obtained by diagonalizing the two-dimensional Hamiltonian for ${\mathrm{H}}_{2}^{+}$ on a uniform lattice. Two different algorithms for the time propagation of the Schr\odinger equation are described, the first an explicit time propagator involving matrix multiplication and the second an implicit time propagator involving matrix inversion. Single-photoionization cross sections for ${\mathrm{H}}_{2}^{+}$ are presented for the cases where the laser field is oriented both parallel and perpendicular to the internuclear axis. Excellent agreement is found between the present calculations and previous work. Two- and three-photon ionization cross sections are also presented for the cases where the laser field is oriented both parallel and perpendicular to the internuclear axis. Comparison with previous work is available only for the parallel orientation case where good agreement is found with a previous time-independent calculation.

Ionic pathway studies in fragmentation of the core-excited (CH 3) 3SiBr molecule involving the Br(3d) and Si(2p) inner-shells in the range of 60–133 eV

Dissociative multiple photoionization of bromotrimethylsilane [(CH 3) 3SiBr] has been probed in the valence, the Br(3d), and the Si(2p) core-level photoexcitation/photoionization regions by time-of-flight (TOF) mass spectrometry and photoion–photoion coincidence (PIPICO) method together with linearly polarized synchrotron radiation. The inner-shell photoionization spectra are dominated by a huge resonance attributed to the diffusion of the outgoing photoelectrons by the centrifugal barrier for the Br(3d 9 ϵf ) outgoing channel. In addition to the giant resonance, discrete photoabsorption bands below the Br(3d 5/2) and Si(2p 3/2) thresholds have been observed in both the total photoion and PIPICO yield spectra. In the valence excitation/ionization region, single ionization leading to the formation of C n H p SiBr +( n=0–3), C n H p Si +( n=0–3), and CH 3 + prevails. Above the Br(3d) threshold including the Si(2p) region, however, destruction of the three or more bonds could account for the dominant PIPICO channels leading to the formation of H +–CH p +( p=1–3) and CH n +–H p Si +( p=0,1) ion pairs. The exclusive formation of only the singly charged ions over the entire energy range indicates that charge redistribution is much faster than the bond cleavages.

Site-specific dissociation of (CH 3) 3SiI involving I(4d), Si(2p), C(1s) and I(3d) inner-shell excitations in the range of 88–1000 eV

Dissociative multiple photoionization of iodotrimethylsilane [(CH 3) 3SiI] has been investigated in the I(4d), Si(2p), I(4p), I(4s), C(1s), I(3p), and I(3d) core level excitation/ionization regions in the range from 88 to 1000 eV by time-of-flight (TOF) mass spectrometry and the photoion–photoion coincidence (PIPICO) method coupled to synchrotron radiation. The inner-shell spectra below 160 eV are dominated by a powerful giant resonance which has been identified as a resonance owing to the diffusion of the outgoing photoelectrons by the centrifugal barrier in the I(4d 9ϵf) outgoing channel. In addition to the huge resonance, discrete photoabsorption bands around the Si(2p) (106.77 eV) and the C(1s) (289.95 eV) edges are observed in both the total photoion and PIPICO yield spectra. Site-selective dissociation patterns were elucidated by the coincident measurements of the fragment ions formed via a single photon absorption. At low energies below the Si(2p) threshold region (88–107 eV), dissociation pathways leading to the formation of H +–I 2+ and H +–I + are dominant. Above the energy, however, near total destruction giving rise to CH 3 +–SiH +, H +–CH p + ( p=0–3), H +–SiH p + ( p=0–3), SiCH n +–I +, and H +–SiCH p + ( p=0–5) are dominant.

Dissociative multiple photoionization of SiBr 4 and GeBr 4 in the VUV and X-ray regions: a comparative study of inner-shell processes involving Si(2p, 2s), Ge(3d, 3p, 3s), and Br(3d, 3p, 3s)

Dissociative multiple photoionization of MBr 4 (M=Si, Ge) in the Si(2p, 2s), Ge(3d, 3s, 3p), and Br(3d, 3p, 3s) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 50∼944 eV for SiBr 4 and 50∼467 eV for GeBr 4. Total photoion and photoion–photoion coincidence (PIPICO) yields have been measured as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the 3d shells owing to the Br(3d 10)→Br(3d 9 ϵf) excitation, showing the similar patterns for both of the systems. The ranges and the intensities of the shape resonances are found to be tremendously broad and enhanced, respectively, by the tetrahedral arrangement of the bromine ligands. In addition to the giant resonances, we have observed discrete features corresponding to the Br(3d), Si(2p), and Si(2s) in SiBr 4and to the Br(3d), Ge(3p), and Ge(3s) in GeBr 4. The dissociation processes of multiply charged parent ions have also been evaluated from the variations of photoelectron–photoion coincidence (PEPICO) and PIPICO yields with the photon energy. Over the entire energies examined, most efficient PIPICO channels involve Br +–Br +, Br +–MBr +, and M +–Br + (M=Si, Ge), the formation of which indicates that the total destruction of the molecules is a dominant process in the dissociative photoionization of the molecules.

Dissociative multiple photoionization of Br 2, IBr, and I 2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p)

Dissociative multiple photoionization of the bromine, the iodine monobromide, and the iodine molecules in the Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 90∼978 eV for Br 2, 60∼133 eV for IBr, and 86∼998 eV for I 2. Total photoion and photoion–photoion coincidence (PIPICO) yields have been recorded as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the inner-shells owing to the Br(3d 10)→Br(3d 9 ϵf), I(4d 10)→I(4d 9 ϵf), and I(3d 10)→I(3d 9 ϵf) transitions. The dissociation processes of the multiply charged parent ions have also been evaluated from variations of photoelectron–photoion coincidence (PEPICO) and PIPICO spectra with the photon energy. From each Br(3p 3/2) (189.9 eV) and I(4p 3/2) threshold (129.9 eV), quintuple ionization of the molecules begins to play important roles in the photoionization, subsequently yielding ion pairs of X 3+–X 2+ (X=Br, I). From the I(3d 5/2) threshold (627.3 eV), loss of six electrons from iodine molecule additionally begins to play a minor role in the multiple photoionization, giving rise to the formation of ion pairs of either I 3+–I 3+ or I 4+–I 2+. A direct comparison of the strengths and the ranges of the I(4d) and Br(3d) giant resonances was successfully made from dissociative photoionization of IBr. Over the entire energy range examined, 60< E<133 eV, biased charge spread relevant to the specific core-hole states of IBr is observed, presumably reflecting the fact that charge localizes mostly in the excited atoms, which can be accounted for mainly by a two step decay via a fast dissociation followed by autoionization upon the VUV absorption.

Dissociative Multiple Photoionization of GeCl4: The Ge(3d,3p,3s) and Cl(2p,2s) Inner-Shell Excitation in the Range 88∼1006 eV

The dissociative multiple photoionization of germanium tetrachloride (GeCl 4 ) in the Ge(3d,3p,3s) and Cl-(2p,2s) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the range 88∼1006 eV. Total and individual photoion yields have been recorded as a function of the incident photon energy. Discrete resonances owing to the Ge(3p) → (9a 1 ) * and the Ge(3s) → (11t 2 ) * transitions are observed as doublet and singlet features, respectively. The inner-shell excitation spectrum is dominated by overlapped numerous discrete resonances of the Cl(2p) inner-shell electron, arising from the large overlap of the valence state wave function with the initial inner-shell state one. Prominent ion intensities of Cl + , Ge + , and GeCl + are detected along with low-intensity Cl 2 + . GeCl 2 + , and GeCl n + (n = 2-4) ions in the whole energy range examined. Dissociation processes have also been investigated by photoelectron-photoion coincidence (PEPICO) and photoion-photoion coincidence (PIPICO) methods. The triple and quadruple photoionization of GeCl 4 is found to dominantly occur above the Cl(2p) edge, leading to the exclusive formation of the GeCl n + (n = 0, 1) + nCl + (n = 2-3) channels.

Dissociation processes of core-excited CBr 4 involving the Br(3d, 3p, 3s) and C(1s) inner-shells in the range 50–460 eV

The dissociative multiple photoionization of CBr 4 in the valence, and in the Br(3d, 3p, 3s) and C(1s) inner shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the range 50–460 eV. Total and individual photoion yield curves have been recorded as a function of the photon energy. Several discrete resonances are observed below the Br(3d) threshold region. Also a giant shape resonance in the total ion yield curve is observed beyond the Br(3d) region, which can be identified as a shape resonance owing to the diffusion of the outgoing photoelectron by the angular momentum barrier in the 3d 9ϵf outgoing channel. Various monocations of Br + and CBr n + ( n=0–3) are detected along with dications of Br 2+ and CBr 3 2+ in the whole energy range examined. Dissociation processes of multiply charged CBr 4 parent ions have also been investigated by photoelectron–photoion coincidence (PEPICO) and photoion–photoion coincidence (PIPICO) methods. The dominant dissociation pattern is found to involve C +–Br + and Br +–Br + in the whole energy range examined. Distinct energy dependence of dissociation processes is observed in the Br(3p) and Br(3s) regions. With the help of ab initio HF/6-311++G(d) calculation, we estimated the photoabsorption positions and symmetries for the discrete core-excited states using the Z+1 equivalent ionic core model. The specific excitation and dissociation of molecules provides information on energy dissipation processes in conjunction with the relevant core hole states.

Pathway studies in the fragmentation of Ge(CH3)4 involving Ge(3d,3p,3s) and C(1s) inner-shell photoexcitation in the range 49.5–450 eV

The dissociative multiple photoionization of tetramethylgermane (Ge(CH3)4) in the valence, and in the Ge(3d,3p,3s) and C(1s) inner-shell regions has been studied by using time-of-flight mass spectrometry coupled to synchrotron radiation in the range 49.5–450eV. Total and individual photoion yields have been recorded as a function of the incident photon energy. Several discrete resonances over a structureless giant resonance are observed below the Ge(3p), Ge(3s) and C(1s) threshold regions. The structureless giant resonance corresponding to the Ge(3d) presumably arises from the continuum enhancement caused by the 3d→ϵf transition. Various monocations of H+, H2+, CHn+ (n=0–4), C2Hn+ (n=0–5), GeHn+, GeCHn+, GeC2Hn+, and GeC3Hn+ are detected in the whole energy range. Dissociation processes have also been investigated by photoelectron–photoion and photoion–photoion coincidence methods. The dominant dissociation channel is found to be CHn+–GeCHn+ in the whole energy examined. Specific energy dependence of dissociation processes is observed in the Ge(3p) and Ge(3s) regions. With the help of ab initio HF/6-311++G(2df,p) calculation, we roughly estimated the photoabsorption positions and symmetries for the discrete core hole states.

Photon W value for krypton in the M-shell transition region.

Absolute W values for krypton have been measured for incident X rays with energies in the range of 85 to 1000 eV, using monochromatic synchrotron radiation and a multiple-electrode ion chamber technique that yields the absolute intensity of the X-ray beam and the photoabsorption cross section. To improve the purity of the incident X rays, the electron storage ring was operated at an energy lower than the normal mode, and thin filters were used. The W values are derived from the measured photon intensity and photoabsorption cross section, using the mean charges of the residual ions obtained in previous work. A considerable oscillation of the W values with the photon energy was found in the region near the krypton 3d electron ionization edge. The results are discussed and compared with data in the literature for low-energy electrons and with the calculations from a model that includes multiple photoionization effects related to inner-shell ionization.

Photon W-value for Xe in the soft X-ray region

Abstract The W -value for Xe has been measured in the soft X-ray region using monochromatized synchrotron radiation and a multi-electrode ion chamber. For improving the purity of the incident photons the electron storage ring was operated with an energy lower than the normal mode. The photon W -value was derived on an absolute scale through evaluation of low and high density limits in the gas-density dependence of photoion currents. The W -value changes at the 4p ionization threshold slightly and increases steeply at the 3d threshold. This behavior is compared with the experimental W -value for electrons and that calculated on the basis of a model which takes into account multiple photoionization effects connected with inner-shell ionization.

Photon W-value for Ar in the sub-keV X-ray region

Absolute measurements of the W-value for Ar have been performed using monochromatized synchrotron soft X-rays and a multiple-electrode ion chamber. For improving the purity of soft X-rays, the electron storage ring was operated with an energy lower than the normal mode and thin filters were utilized. Using the absolute intensity of the soft X-ray beam provided with the ion chamber technique, the photon W-value was derived, which shows a considerable variation with the photon energy near the Ar 2p electron ionization edge. The experimental W-value has been reproduced using a model proposed here, which considers multiple photoionization effects related to inner-shell ionization.

Pathway Studies in Si(2p) Inner-Shell Processes of H2Si(CH3)2 by Mass Spectrometry and the Photoion−Photoion Coincidence Method in the Range 24−133 eV

Dissociative multiple photoionization processes of dimethylsilane (H2Si(CH3)2) have been investigated in the valence and Si(2p) core level photoexcitation/photoionization regions by time-of-flight ...

DIAGNOSING PLASMA CONDITIONS IN TARGETS IRRADIATED BY INTENSE LIGHT ION BEAMS USING K α SATELLITE LINE INTENSITY RATIOS

The characteristics of K α satellite line emission generated in intense light ion beam experiments is investigated. Inner-shell X-ray lines are emitted from moderate-temperature ( T∼10 1–10 2 eV) plasmas as 2p electrons of the target ions fill in K-shell vacancies created by beam ion-impact ionization. We focus, in particular, on the utilization of line intensity ratios from He-like and Li-like ions to diagnose target plasma temperatures and densities. Although the K α transitions for the He-like and Li-like atomic systems are the same as those in laser-produced and Z-pinch plasmas, the populating mechanisms for their upper states are completely different. That is, the upper states are populated from lower ionization stages by ion-impact ionization. This enables diagnosis of plasmas in the 10–100 eV temperature regime using the extensive instrumentation developed for X-ray crystal spectroscopy. Target plasma characteristics are simulated using a one-dimensional collisional-radiative equilibrium model in which atomic level populations are computed by solving multilevel statistical equilibrium equations self-consistently with the radiation field and ion beam properties. We utilize atomic models which include for all ionization stages a detailed treatment of autoionization states which have K-shell vacancies. Results are presented for Li beam-heated Al and Cl plasmas and proton beam-heated Al plasmas. We find that, as in the case of laser-produced plasmas, the He-like intercombination-to-resonance ratio provides a good density diagnostic. However, because the He α line is produced in ion beam-heated plasmas with a relatively low degree of ionization, it is less susceptible to resonant self-absorption effects as compared to laser plasmas. In addition, the Li-like satellite line structure is significantly different for plasmas heated by ion beams. Unlike laser-produced plasmas, the Li(dabc) and Li(qr) lines tend to be more intense than the Li(kj) lines. Because of this, we find the Li(dabc)-to-He α ratio to be a good temperature diagnostic for intense light ion beam experiments. We also discuss the effects of beam-induced multiple ionization, photoionization, and resonant self-absorption on the line ratio diagnotics.

Final ion-charge resolving electron spectroscopy: Photoionization studies on Sm and Eu.

A new type of electron spectroscopy for the investigation of gas-phase targets has been developed. Based on electron-ion coincidence measurements using synchrotron radiation the method for the first time allows an electron spectrum to be split into the different final charge states. These spectra give deep insight into different, in part spin-dependent, multiple photoionization mechanisms, which is demonstrated for atomic samarium and europium in the photon energy range of the 4d giant resonances.

Ponderomotive acceleration in the auroral region: A kinetic simulation

Alfvén waves with frequencies of the order of several Hertz have been observed in the auroral region. These waves are thought to be responsible for the acceleration of ionospheric particles out of the ionosphere and into the magnetosphere. As an Alfvén wave propagates into an inhomogeneous ionosphere, the amplitude of the wave decreases resulting in a gradient in the electric field and a ponderomotive force. The nature of the ponderomotive force is to accelerate ions out of the ionosphere, while accelerating electrons into the ionosphere decreasing the ambipolar electric field. Ponderomotive acceleration of ions can penetrate to low enough altitudes affecting the production regions for O+ and H+ ions. To understand the influences of ponderomotive acceleration on the lower ionosphere, a one‐dimensional hybrid particle code is used. The simulation model allows for multiple species, atmospheric chemistry and photoionization, tilted dipolar coordinate system, corotation, and convection. The electrons are treated as a charge neutralizing fluid where the parallel and perpendicular temperatures and heat flows are given by the 16‐moment transport equations. Coulomb, ion‐neutral, and electron‐neutral collisions are included in the hybrid simulation using newly developed collision techniques applicable to kinetic simulations. The altitude dependence of an Alfvén wave propagating from 6000 to 200 km in an inhomogeneous ionosphere is determined by solving Maxwell's equations. The parallel and perpendicular envelopes are determined and the resulting nonlinear, non‐resonant ponderomotive acceleration of ionospheric ions is calculated. The ponderomotive force is included in the kinetic simulation in the same fashion as gravity or the self‐consistent electric field. The acceleration event lasts for 10 min, by which time the density and flow speed profiles have reached equilibrium. The density and flow speed at 2500 km after 10 min of acceleration increase from 4.3 cm−3 and 1.6 km/s to 34 cm−3 and 6.0 km/s for H+, while the same quantities for O+ increase from 1670 cm−3 and −0.0009 km/s to 4795 cm−3 and 7.0 km/s. Collisional coupling between the H+ and O+ contributes to enhancing the outflow of H+. The net response of the ionosphere to the nonlinear ponderomotive force is an enhancement in ionospheric outflow of ions into the magnetosphere.

Shake-off processes in photoionization and Auger transition for rare gases irradiated by soft X-rays

Branching ratios of multiply charged Ne, Ar, Kr and Xe ions produced following soft X-ray absorption by the rare gases have been measured using a time-of-flight mass spectrometer and monochromatized synchrotron radiation. Ratios of double, triple and quadruple valence photoionization to single valence photoionization, of multiple Auger transitions to the normal Auger transition from a core hole or core-valence two-hole state, and of the multiple photoionization with a core hole to single core photoionization have been determined using the measured branching ratios of the multiply charged ions.