Photoionization Yield Research Articles

Giant plasmon excitation in free C60 and C70 molecules studied by photoionization.

Single-photon ionization of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ has been studied using synchrotron radiation in an energy range from the ionization potential (7.54\ifmmode\pm\else\textpm\fi{}0.04 eV for ${\mathrm{C}}_{60}$ 7.3\ifmmode\pm\else\textpm\fi{}0.2 eV for ${\mathrm{C}}_{70}$) up to 35 eV. The photoion yield is dominated by a strong resonance at about 20 eV with a FWHM of \ensuremath{\sim}10 eV. We interpret this as being due to autoionization via the giant plasmon resonance recently predicted in the RPA approximation. ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ offer the unique possibility for a quantitative comparison of this collective excitation in the free molecule with that observed in the solid phase by electron-energy-loss spectroscopy at about 28 eV.

Vacuum ultraviolet photoionization mass spectrometric study of C6

Gaseous C60 has been studied by photoionization mass spectrometry between the ionization threshold and 40.8 eV. An adiabatic threshold of 7.57±0.01 eV is observed, which may be slightly low due to hot bands. The energy derivative of the photoion yield curve is in rough agreement with the He i photoelectron spectrum on the positions of some peaks, but others are weak or absent. The discrepancy is not attributed to autoionization, but rather to selection rules governing the ejection of low energy electrons into high angular momentum waves. C++60 is observed at higher energies, and becomes ∼0.6 as intense as C+60 at 40.8 eV. The photoion yield curve of C++60, approximately linear well above threshold, appears to exhibit curvature near threshold, thwarting an attempt to make a distinction between two alternative values of the second ionization potential. Fragmentation to form C+58 is only observed at the highest energy, 40.8 eV. The unimolecular decay is modelled by quasiequilibrium theory. In this model, the kinetic shift is of the order of 30 eV, and the minimum energy for dissociation into C+58+C2 seems to be 6.0–6.5 eV.

Photoinduced electron transfer between styrylanthracenes and electron donors and acceptors in acetonitrile

The decay pathways of the lowest singlet and triplet excited states (1trans* and 3trans*) of trans-n-styrylanthracenes (n-StA, where n= 1, 2 or 9 on the anthracene) have been studied in acetonitrile at room temperature. Fluorescence lifetimes (τF) and quantum yields (ϕF), as well as the yield and spectral and kinetic properties of the lowest triplet state of the three StAs, were determined by steady-state and transient techniques. The formation and the decay of the respective StA radical cations (trans˙+) were observed by laser flash photolysis; the yield of photoionization is ca. 0.07 on 353 nm excitation and is enhanced by fluorescence quenching with 1,4-dicyanobenzene. The formation and decay of the StA radical anions (trans˙–) in the presence of diethylaniline (DEA) is concluded from transient conductivity (Δκ) and optical results. DEA significantly enhances the yield of trans˙– and the initial amplitude of Δκ and correspondingly quenches ϕF. The bimolecular interaction between 1trans* and 4-bromodimethylaniline enhances the triplet population.

Single-photon ionization of C 60- and C 70-fullerene with synchrotron radiation: determination of the ionization potential of C 60

The photoionization threshold region of C 60 has been studied using single-photon ionization with synchrotron radiation. Mass selection is realized with a time-of-flight mass spectrometer. The ionization potential of C 60 has been determined to 7.58 +0.04 −0.02 eV.Significant structure in the photoionization yield is seen in the energy range up to 1 eV above threshold.

Low Energy Auger Electrons - a Probe of Electron Correlations in Atoms and Molecules

Until very recently Auger decay was exclusively regarded as an inner-shell phenomenon. However, recent studies of the decay of certain valence states in atoms and molecules excited by electron correlation accompanying photoinization have shown the importance of non-radiative decay processes, so-called particular Auger transitions. The fact that such states decay non-radiatively was previously ignored, and is of critical importance for the interpretation of double-photoinization data, for both, photoion yield measurments, particularly near threshold, and photoelectron spectra, especially at low kinetic energies. Hence the spectroscopy of these participator Auger transitions is of considerable interest for our understanding of electron correlations. Two modes of such transitions were identified; the inner-valence type in which the inner vs hole is filled in the decay process, and the valence-multiplet type in which the valence shell remains unchanged in occupancy during the transition, but experiences a change of LS multiplet symmetry. The analysis of these processes shows that in the sudden limit up to 30-40% of the double-ionization rate in neon and argon is due to particular Auger decay, which was previously attributed to simultaneous double ionization.

Decay Channels of Resonantly Excited 2p-1ns,md States of Ar and 3d-1np States of Kr

The resonantly excited Ar 2p5ns, md and Kr 3d9np states have been studied experimentally by recording the electron spectra over a wide kinetic energy region and at several resonance energies. Experimental intensity distributions between different line groups have been compared with the results of multiconfiguration Dirac-Fock calculations. Satisfactory agreement has been found between experimental and theoretical branching ratios. The relative production efficiencies of multiply-ionized atoms have been extracted from the calculated transition rates and compared with recent experimental photoion yield results.

One-Color and Two-Color Resonance-Enhanced Two-Photon Ionization Studies of Benzene: Sudden Drop of Photoionization Yield at the Onset of the Third Channel Region

Abstract Benzene molecules in a supersonic jet were photoionized using a tunable (233–262 nm) pulsed dye laser, and the resonance-enhanced two-photons ionization (RE2PI) processes of the S1 state were investigated by time-of flight (TOF) mass-spectrometric technique. Without further fragmentation of C6H6+ ions under low laser power conditions (< 20 μJ/pulse), high-resolution excitation spectra of the parent ion have been measured for the 611n progression (n = 0,1,2,3). The relative photoionization intensity suddenly drops at the onset of the third channel band, 6113. Excitations of the rotationally resolved 6113 and 6112 bands under carefully controlled low laser powers have revealed the fact that the relative intensity of the 6113 band (above 3rd channel) to that of the 6112 band (below 3rd channel) decreases with decreasing the excitation laser power. This has confirmed the idea that the ionizing process is competing with nonradiative relaxation processes at the resonant state. Two-color RE2PI excitation with the pumping (ω1) and electrically delayed probing (ω2 = 272.0 nm) lasers has given population decay rates of vibronic state longer than 20 ns. Excitation spectra of the parent ion with scanning ω1 (241–248 nm) and those with the optically delayed (2.8–8.0 ns) fixed wavelength of ω2 have been observed. Relative population yields for the 71 and 6113 vibronic states drastically decreased in a similar fashion to the relative fluorescence yield for the longer delayed excitation. The present RE2PI measurement of the relative population yields for selective vibronic band excitations revealed that the ionization method can detect the non-fluorescent transient species produced the optical excitation above the 3rd channel of the S1 state of benzene.

Photoionization cross sections in resonance ionization mass spectrometry of sputtered metal atoms: Applications to material analysis

(1+1) resonant multiphoton ionization of several metal atoms was investigated in the 285–301-nm-wavelength range. Fe, Mg, Cr, Ti, and Ni free atoms were produced using Ar+-ion sputtering of different metal samples in ultrahigh-vacuum conditions. The photoionization yields, measured as function of the laser fluence, exhibit saturation or near saturation at the highest laser energy density available in the experiment (100 mJ/cm2). From these measurements the ionization cross sections of the intermediate excited states are deduced, assuming saturation of the excitation step. In this way values ranging between 0.7 and 4×10−17 cm2 are obtained. One can take advantage of the relatively large cross sections to detect these sputtered metal atoms by resonance ionization mass spectroscopy with great sensitivity. Some examples are given, demonstrating, on a single-laser-shot basis, the real analyses of Mg and Ti atoms in materials at the ppm level.

Core level photoionization mass spectroscopy of the fluoromethylsilanes, Si(CH 3) xF 4- x ( x = 0–4), around the Si 2p ionization edges

Total ion yield spectra, total electron yield spectra, photoionized mass spectra and mass resolved partial photoion yield spectra of the five fluoromethylsilane compounds, Si(CH 3) x F 4- x ( x = 0–4), measured around the Si 2p core ionization edges using monoc synchrotron radiation are reported. Photoionized mass spectra of the five compounds were also measured around the Si 2s and F 1s ionization edges. For each of the fluoromethylsilane compounds, total ion yield and total electron yield spectra are very similar to each other and to the corresponding photoabsorption spectra with the exception of the decreased intensities of the discrete resonances. The photoion mass spectra show that almost all possible molecular fragments are generated by excitation and ionization of the Si 2p, Si 2s and F 1s core electrons. Branching ratios of the photoion mass spectra peak areas indicate that the methyl groups are much more labile than the fluorine atoms at all photon energies, both below and above the core ionization edges, in the mixed fluoromethylsilane compounds. Relative areas of the methyl peaks in the photoion mass spectra are enhanced at the discrete resonances below the ionization threshold with more pronounced enhancements above the ionization threshold. An explanation for the greater lability of the methyl ligands is proposed based on the effect of the electron withdrawing and donating properties of the fluorine atoms and methyl groups which also affect the relative Auger transition rates and core hole lifetimes. Partial ion yields of the SiF + and SiMe + fragment ions also exhibit some specific enhancements at Si 2p→σ* resonances below the ionization edge.

Alkyl chain length effects on the photoionization of N-alkylphenothiazines and sulfonated alkylphenothiazines in anionic alkyl sulfate and cationic alkyltrimethylammonium bromide micelles

The photoionization yields of N-alkylphenothiazines solubilized in either cationic or anionic micelles at 77 K were studied as a function of both alkyl and surfactant chain length. The results are compared to similar work on sulfonated N-alkyl-phenothiazines. The location of the phenothiazine moiety with respect to the micelle-water interface together with cation-water interactions is a major factor controlling the photoionization efficiency. The position of the N-alkylphenothiazine chromophore within a micelle can be altered by changing the alkyl chain length. The photoefficiency is also affected by the surface charge of the micelle.

Photoionization mass spectrometric study of N2H2 and N2H3: N–H, N=N bond energies and proton affinity of N2

The photoion yield curves of N2H+2, N2H+, and H+2 from N2H2 have been studied by photoionization mass spectrometry. The adiabatic ionization potential of N2H2 determined (9.589 ± 0.007 eV) is in excellent agreement with that obtained by photoelectron spectroscopy (PES). The appearance potential of N2H+ (10.954 ± 0.019 eV at 0 K) is in good agreement with earlier electron impact measurements. The appearance potential of H+2 (<13.526 ±0.03; probably ≤13.403 ±0.036 eV, at 0 K) leads to ΔH0f0 (N2H2) >43.8 ±0.7; probably ≥46.6 ±0.8 kcal/mol. Consequently, proton affinity (PA) (N2) <121.8 ±0.8; probably ≤119.0 ±0.9 kcal/mol, which is in good agreement with results from proton transfer experiments and with most ab initio calculations. The N■N ‘‘double bond’’ energy determined from N2H2→2NH is <126.6 ±1.0; probably ≤123.8 ±1.1 kcal/mol. The adiabatic ionization potential of N2H3 is found to be 7.61±0.01 eV. This observation leads to D0 (H2NNH–H)=80.8±0.3 kcal/mol, and D0(HNNH–H)≂43.8±1.1 kcal/mol.

Photoionization mass spectrometric study of Si2H6

The adiabatic I.P. of Si2H6 obtained by a photoionization mass spectrometric study at two temperatures is 9.74±0.02 eV. The first fragment, Si2H+4, initially appears with a shallow slope at ≤10.04±0.02 eV, and with a much steeper slope at ≤10.81±0.02 eV. It is argued that the initial onset corresponds to formation of H2SiSiH+2, while the steeper onset is attributed to formation of H3SiSiH+. The second fragment, Si2H5, has an appearance potential of ≤11.59±0.02 eV (11.41±0.03 is a probable value). Successive decomposition leads to Si2H+2 (from Si2H+4 ) and Si2H+3 (from Si2H+5 ). The photoion yield curve for Si2H+3 also displays shallow and steep onsets. Upper limits for the appearance potentials can be readily extracted, but the true thermochemical onsets are less well defined. Heats of formation (or upper limits) are presented for each of these species. For Si2H+6, Si2H+5, and Si2H+4, the experimental values are in good agreement with recent ab initio calculations. For the daughter species, the experimental values exceed the calculated ones, as expected.

The effect of the antioxidant spermine on the photophysical processes and photodegradation of tryptophan in aqueous solution at 293 K

The influence of the antioxidant spermine on the photophysical processes and photodegradation of tryptophan at neutral pH was studied by fluorescence and absorption spectroscopy. Spermine, which is a polyamine, binds cooperatively to tryptophan with a binding constant of 1.2 × 10 −3 mM −3.7 at 20 °C. It is suggested that, when the charged spermine molecules bind to tryptophan, the rotation of the zwitterion around the C αC β bond is blocked and only one rotamer form of tryptophan is present. The hindered rotation gives rise to the following spectroscopic changes. (i) Minor changes are observed in the absorption spectrum, since the two most frequent rotamers have different spectra. (ii) The quantum yield of the S 1 band increases by a factor of 2.6, as is also the case when the amino group is deprotonated. ( iii) The yield of photodegradation increases from zero to 0.08 for excitation at 280 nm, which is equal to the yield of photoionization under similar conditions.

Experimental study of autoionizing states of AuI

This paper describes the investigation of the autoionizing states of AuI by exciting the states of 6d 2 D 3/2 and 6d D 5/2 with tunable dye lasers by means of a resonance ionization time-of-flight mass spectrometer (RI-TOFMS). Many new autoionizing states were found, a few of which have narrow-linewidths of about 0.1 cm−1. The photoionization efficiencies for different channels were compared. Among the autoionizing states found so far, the highest photoionization yield is given by two newly obtained states from the state 6d 2 D 5/2, 3683.6 cm−1 and 4636.5 cm−1 above the first ionization limit. In addition, the Shore-Fano profile parameters of several autoionizing resonances were determined by a nonlinear fitting program.

Photoionization mass spectrometry of kinetic energy-selected ions. An ion retarding potential difference method applied to NO + and O + from N 2O

A new method is described to obtain photoionization efficiency curves of kinetic energy-selected fragment ions. The method is tested and discussed in the case of the dissociative photoionization yields of NO + and O + from N 2O. The attention is focussed on the dissociative autoionization processes in the threshold region, in the Franck-Condon region of the N 2O (à 2Σ +) state and in the Franck-Condon gap between the à 2Σ + and B̃ 2π states.

The reaction of biphenyls in the upper triplet states in micellar systems

Reactions of the upper triplet states of biphenyl and its halogenated derivatives were investigated in aqueous micellar systems at room temperature with a two-step excitation method. Irradiation of the triplet states of biphenyl and its chloro and bromo derivatives produced hydrated electrons (e aq −) and a decrease in the triplet states. No evidence of carbonhalogen bond homolysis, with a rate competing with e aq − formation (τ<1 ps), was obtained for the compounds investigated. The yield of photo-ionization was markedly reduced in a cationic micelle, i.e cetyltrimethylammonium bromide, compared with that in an anionic micelle, i.e. sodium dodecylsulfate. This is becase of the enhanced geminate recombination of electron—cation pairs caused by the positive micellar surface charge which prevents the escape of electrons from the cationic micelle.

Photoionization of HBr and DBr near threshold

Photoionization is observed in HBr (at 300 K) below the adiabatic threshold. The photoion yield curve has structure, and is independent of both pressure and electric field over a large range. The peaks can be simulated rather well by a model which assumes rotational autoionization, with ΔN≂−4. This model does not exclude concomitant processes with ΔN=−1,−2,−3. A formal theory is also presented, which describes ΔN=−4 as occurring through successive quadrupole transitions, in second-order perturbation theory. A tentative conclusion is drawn, based on preliminary studies of other molecules, that a type of rotational autoionization can occur in heteronuclear diatomic molecules without an electric field, and in homonuclear diatomic molecules with such a field. The photoionization of DBr has been studied with similar conditions. A corresponding simulation is in good agreement with the observed structure below the adiabatic threshold. In addition, one peak in a triad observed in HBr above threshold, and predicted by an MQDT calculation to be absent in DBr, is still observed.

Photoionization Cross Section of Hydrogen Cyanide

Abstract The cross section has been calculated for photoionization of HCN. The unbounded orbital is approximated by a plane-wave, orthogonalized with respect to 820 Å. The yield of photoionization at the threshold is obviously triggered by the interaction of Rydberg states with the ionization continuum and the deviation by the factor 2.7 of the calculated cross section compared with experimental data occurs due to the uncertainties of data in the half-widths.

On the electronic structure of liquid water: Conduction-band tail revealed by photoionization data

The photoionization yield curves ▪ as a function of photon energy E ph of pure water or dilute aqueous solutions display an exponential behavior in the ionization threshold region. This suggests that these curves reflect the existence of an Urbach tail lying below the conduction-band edge of water. A quantitative treatment of various reported photoionization data provides an experimental basis for the determination of the energy distribution of electron trap states in the conduction-band tail of water and presents a picture of the electronic structure of liquid water which is coherent with previous results of photoelectron emission spectroscopy.

Photoion yield spectra of the lanthanides in the region of the 5p excitation: The elements Gd, Tb, Dy, Ho, Er, Tm, and Yb.

The photoion yield spectra of singly and doubly charged lanthanides Gd, Tb, Dy, Ho, Er, Tm, and Yb were measured in the region of the 5{ital p} excitation between 20 and 40 eV. Monochromatized synchrotron radiation was focused on an atomic beam and a time-of-flight spectrometer was used for the charge-selective detection of the photoions. The interpretation of the experimental results is partly based on time-dependent local-density approximation calculations of the partial photoionization cross section of the subshells 4{ital f}, 5{ital p}, 5{ital d}, and 6{ital s}.