Ion Yield Ratio Research Articles

Few-photon single ionization of cold rubidium in the over-the-barrier regime

Photoionization of the rubidium (Rb) atoms cooled in a magneto-optical trap, characterized by the coexistence of the ground 5$S_{1/2}$ and the excited 5$P_{3/2}$ states, is investigated experimentally and theoretically with the 400 nm femtosecond laser pulses at intensities of $I=3\times10^9$ W/cm$^2$ - $4.5\times10^{12}$ W/cm$^2$. Recoil-ion momentum distribution (RIMD) of Rb$^+$ exhibits rich ring-like structures and their energies correspond to one-photon ionization of the 5$P_{3/2}$ state, two-photon and three-photon ionizations of the 5$S_{1/2}$ state, respectively. With the increasing of $I$, we find that experimental signals near zero-momentum (NZM) in RIMDs resulted from the 5$P_{3/2}$ state enhance dramatically and its peaked Rb$^+$ momenta dwindle obviously while that from the 5$S_{1/2}$ state is maintained. Meanwhile, the ion-yield ratio of the 5$S_{1/2}$ over the 5$P_{3/2}$ states varies from $I$ to $I^{1.5}$ as $I$ increases. These features indicate a transition from perturbative ionization to strong-perturbative ionization for the 5$P_{3/2}$ state. Numerical simulations by solving the time-dependent Schr\"odinger equation (TDSE) can qualitatively explain the measurements of RIMD, photoion angular distributions, as well as ion-yield ratio. However, some discrepancies still exist, especially for the NZM dip, which could stem from the electron-electron correlation that is neglected in the present TDSE simulations since we have adopted the single-active-electron approximation.

Influence of orbital sets on the Ar+(2s−1) multiple Auger decay

The single, double, triple, and quadruple Auger decays of ${\mathrm{Ar}}^{+}(2{s}^{\ensuremath{-}1})$ are studied, based on many-body perturbation theory. The level-to-level investigation indicates that the double Auger primarily comes from the cascade process involving the Coster-Kronig decay. Moreover, the complex triple Auger and quadruple Auger decay are expressed by the multistep approach, i.e., the combination of cascade and direct processes. On the other hand, by employing the separate orbital sets for optimizing the single-electron wave functions, our ion yield ratios (${\mathrm{Ar}}^{2+}:{\mathrm{Ar}}^{3+}:{\mathrm{Ar}}^{4+}:{\mathrm{Ar}}^{5+}=3.5:88.7:7.5:0.26$) agree quite well with the recent experimental data (3:89:8:0.3) [Lablanquie et al., Phys. Chem. Chem. Phys. 13, 18355 (2011)]. Moreover, the single, double, and triple Auger spectra, as well as lifetime, are all in accord with the measurements of Lablanquie et al.

Multiple Auger Decay Following Xe+ (4) IonizationSupported by the National Key Research and Development Program of China (Grant No. 2017YFA0402300), and the National Natural Science Foundation of China (Grant Nos. 11774344 and 11474033).

The Auger decay for the many-electron Xe+ (4) state is studied in detail, using multistep approaches. It is found that the single Auger decay channels are primarily Coster–Kronig processes, which is in accord with other theoretical and experimental results. The double and triple Auger decays result primarily from cascade processes, i.e., the sequential two-step and three-step Auger decay, and as such, the contributions from direct processes can be neglected. Level-to-level rates for single, double, and triple decays are obtained, based on which comprehensive Auger electron spectra and ion yields are obtained. Our decay paths and Auger electron spectra are in agreement with the experimental analysis [Hikosaka et al., Phys. Rev. A 76 (2007) 032708], and our ion yield ratios (Xe2+: Xe3+: Xe4+ = 4.6 : 87.0 : 8.4) are also in line with their values (5.0 : 86.0 : 9.0). However, with respect to the ion yield ratios, a discrepancy still remains among the experimental and theoretical results. Taking into account the complexity of Xe’s electronic structure, further, more detailed experiments are still required.

Sequential single-photon and direct two-photon absorption processes for Xe interacting with attosecond XUV pulses

We investigate the interaction of Xe with isolated attosecond XUV pulses. Specifically, we calculate the ion yields and determine the pathways leading to the formation of ionic charged states up to Xe$^{5+}$. To do so, in our formulation we account for single-photon absorption, sequential multi-photon absorption, direct two-photon absorption, single and double Auger decays, and shake-off. We compare our results for the ion yields and for ion yield ratios with recent experimental results obtained for 93 eV and 115 eV attosecond XUV pulses. In particular, we investigate the role that a sequence of two single-photon ionization processes plays in the formation of Xe$^{4+}$. We find that each one of these two processes ionizes a core electron and thus leads to the formation of a double core-hole state. Remarkably, we find that the formation of Xe$^{5+}$ involves a direct two-photon absorption process and the absorption of a total of three photons.

Bromine-isotope Selective Ionization Using Field-free Alignment of IBr Isotopologues with a Switched Nanosecond Laser Pulse

We demonstrate isotope selective ionization of I79Br and I81Br isotopologues using field-free alignment induced by a linearly-polarized nanosecond laser pulse with rapid turn off at the maximum. Following the “switched” nanosecond pulse irradiation, a 60-fs laser pulse ionizes one of the isotopologues preferentially. The ion yield ratio I(I81Br+)/I(I79Br+) varies in the range of 0.93–1.06, depending on time delay between the pulses. This is the first demonstration of laser isotope separation of heavy elements using field-free alignment and angular dependent ionization.

Isotope-selective ionization utilizing field-free alignment of isotopologues with a train of femtosecond laser pulses

We propose a strategy of isotope-selective ionization for a binary mixture of isotopologues of homonuclear diatomic molecules, utilizing field-free alignment with a train of femtosecond laser pulses. Field-free alignment can be achieved simultaneously for two isotopologues consisting of two atoms with the same atomic mass number \ensuremath{\alpha} or \ensuremath{\beta} $({}^{\ensuremath{\alpha}}{X}_{2}\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\beta}}{X}_{2})$, utilizing a pulse train with their time interval of ${T}_{\mathrm{com}}^{\mathrm{rev}}=\ensuremath{\beta}{T}_{\ensuremath{\alpha}}^{\mathrm{rev}}=\ensuremath{\alpha}{T}_{\ensuremath{\beta}}^{\mathrm{rev}}$, where ${T}_{\ensuremath{\alpha}}^{\mathrm{rev}}$ and ${T}_{\ensuremath{\beta}}^{\mathrm{rev}}$ are the rotational revival times of the isotopologues. We demonstrate experimentally that a train of four alignment pulses with their interval of ${T}_{\mathrm{com}}^{\mathrm{rev}}(\ensuremath{\alpha}=14,\phantom{\rule{0.16em}{0ex}}\ensuremath{\beta}=15)$ creates transiently aligned ${}^{14}{\mathrm{N}}_{2}$ and antialigned ${}^{15}{\mathrm{N}}_{2}$ just before ${T}_{\mathrm{com}}^{\mathrm{rev}}/2$ after the last alignment pulse and vice versa just after ${T}_{\mathrm{com}}^{\mathrm{rev}}/2$. Highly isotope-selective ${\mathrm{N}}_{2}$ ionization is achieved at these timings with another femtosecond laser pulse, which induces the nonresonant multiphoton ionization with the cross section remarkably depending on the angle between the molecular axis and the laser-electric-field direction. The ion yield ratio $I({}^{15}{N}_{2}^{+})/I({}^{14}{N}_{2}^{+})$ ranges from 0.49 to 2.00, which is wider than the range obtained with a single alignment pulse [H. Akagi et al., Appl. Phys. B 109, 75 (2012)].

Control of the photoionization/photodissociation processes of cyclopentanone with trains of femtosecond laser pulses

A train of three equally spaced femtosecond laser pulses is employed to control the photoionization/photodissociation processes of cyclopentanone. With the increase of pulse separation, a strong modulation of product ion yield is observed. More than ten-fold changes of ion yield ratio between different products can be realized. The experimental observations further explain the compositions and formation pathways of peaks in the mass spectra. The controlling mechanisms are also discussed.

A rate equation method for the sequential double ionisation, including autoionising state excitation, of a noble gas

AbstractA set of rate equations have been tested against a more robust set of Time-Dependent Density Matrix (TDDM) equations [D. P. W. Middleton, L. A. A. Nikolopoulos, J. Mod. Opti. 59, 1650 (2012)] by using them to determine the populations of ion species and autoionising states (AIS) in noble gas atoms when interacting with a strong external field. Two field shapes were tested here — sinusoidal and square — and a variety of pulse characteristics were examined, i.e. intensity, duration and photon energy, for the neon atomic system. It was found that the rate equations were sufficiently accurate only when the external field is way off-resonant with the AIS. Moreover, analytical solutions of the rate equations in the square pulse case agree with the numerical solutions for a time-dependent pulse containing many cycles. An attempt to model a stochastic field was also made and it was found that the use of such a field diminished and broadened the ion yield ratio due to the presence of an added bandwidth.

Isotope-selective ionization utilizing molecular alignment and non-resonant multiphoton ionization

We demonstrate laser nitrogen isotope separation, which is based on field-free alignment and angular-dependent ionization of 14N2 and 15N2 isotopologues. A linearly polarized short laser pulse (λ ~ 795 nm, Δτ ~ 60 fs) creates rotational wave packets in the isotopologues, which periodically revive with different revival times as a result of different moments of inertia. Another linearly polarized short laser pulse (λ ~ 795 nm, Δτ ~ 60 fs) ionizes one of the isotopologues selectively as a result of their different angular distributions. In the present experiments, the ion yield ratio R [=I(15N2 +)/I(14N2 +)] can be changed in the range from 0.85 to 1.22, depending on the time delay between the two laser pulses.

Multiphoton dissociation and vibrational mediated dissociation of chlorinated methanes, CH 4− nCl n ( n = 2, 3, 4) at 355 nm

In this paper, the experimental results for the photodissociation, PD, and photoionization, PI, of chlorinated methanes, CH 2Cl 2, CHCl 3, and CCl 4, studied by cooled molecular jets and Time of Fly Mass Spectrometry detection, ToF-MS, are presented. Photodissociation processes outcome from multiphoton absorption at 10 9–10 10 W cm −2 intensities at 355 nm laser radiation. The results indicated that photodissociation dominates over photoionization processes as some detected ions are originated from the absorption of up to five photons. The main dissociative channels lead to the formation of H +, C +, CH +, CH 2 +, 35Cl +, 37Cl +, C 35Cl +, and C 37Cl +. The analysis of the ratio of ion yield from different isotopologues for each molecule, as a function of the energy per pulse, showed that the formation of the more abundant ions: Cl + and CCl +, is strongly influenced by changes in the vibrational structure of the excited states as a consequence of the two different stable chlorine atoms. As a first approach to get some insight of the experimental results, the ground state vibrational energy levels for chlorinated methanes, were calculated using Density Functional Theory. From the abovementioned calculations was possible to think that the changes in energy of the vibrational levels drive the ion yield of observed ions due to different isotopic composition of chlorinated methanes.

Quantitative analysis of the surface composition of mixed self‐assembled monolayers using ToF‐SIMS and FT‐IR analyses

AbstractMixed self‐assembled monolayers (SAMs) consisting of different molecules have been used to modify surface wettability, functionalize end groups for chemical binding, and control adsorption of proteins, DNA, or cells. In these applications, control over the quantification of the surface composition of a mixed SAM is especially important when designing biochips and biosensors. Previously, we presented a method by which ToF‐SIMS can be used to quantify the composition of two adsorbates with acetylene and propene functional groups from their secondary ion yield ratio in mixed SAMs on gold substrates. We concluded that although the ion yield ratio of the two adsorbates in mixed SAM was not equal to the mole fraction of Diyne (‐acetylene) and Diene (‐propene) in a solution, this was due to a different secondary ion formation, and not to the difference in the amount of adsorbates on the surface. In this study, we revisit the same sample system, this time applying a FT‐IR technique to test the accuracy of our previous findings. We found that the absorbances of the acetylene and propene functional groups neatly and linearly correlated with the mole fraction in the solution, which were consistent with our previous conclusions. We also used this methodology to quantify mixed silane SAMs on a silica surface. Our study shows that the ToF‐SIMS and FT‐IR techniques can complement one another and be useful to quantify mixed SAMs on various substrates. Copyright © 2010 John Wiley & Sons, Ltd.

SIMS quantification of SiGe composition with low‐energy ion beams

AbstractSystematic SIMS analyses with low‐energy (250 eV ∼1 keV) oxygen, cesium and krypton primary beams have been carried out on a set of fully strained uniform epitaxial Si1−xGex films (x = 5 ∼ 60%), as well as a germanium ion‐implanted silicon standard to investigate the matrix effect under various conditions. It is shown that preferential ion yield enhancement of one matrix component over the other can occur as the result of primary ion incorporation. Through defining a matrix yield factor, this work demonstrated that constant secondary ion yield ratios between Si ion and Ge ion over a large concentration range are only valid under some very specific analysis conditions. Emphases were placed on oxygen beam analyses with regard to steady‐state ion yields and surface transients. Both show some unique features only accessible under low‐energy conditions. Copyright © 2010 John Wiley & Sons, Ltd.

Ion yields and erosion rates for Si1−xGex(≤x≤1) ultralow energy O2+ secondary ion mass spectrometry in the energy range of 0.25–1 keV

We report the SIMS parameters required for the quantitative analysis of Si1−xGex across the range of 0≤x≤1 when using low energy O2+ primary ions at normal incidence. These include the silicon and germanium secondary ion yield [i.e., the measured ion signal (ions/s)] and erosion rate [i.e., the speed at which the material sputters (nm/min)] as a function of x. We show that the ratio Rx of erosion rates, Si1−xGex/Si, at a given x is almost independent of beam energy, implying that the properties of the altered layer are dominated by the interaction of oxygen with silicon. Rx shows an exponential dependence on x. Unsurprisingly, the silicon and germanium secondary ion yields are found to depart somewhat from proportionality to (1−x) and x, respectively, although an approximate linear relationship could be used for quantification across around 30% of the range of x (i.e., a reference material containing Ge fraction x would give reasonably accurate quantification across the range of ±0.15x). Direct comparison of the useful (ion) yields [i.e., the ratio of ion yield to the total number of atoms sputtered for a particular species (ions/atom)] and the sputter yields [i.e., the total number of atoms sputtered per incident primary ion (atoms/ions)] reveals a moderate matrix effect where the former decrease monotonically with increasing x except at the lowest beam energy investigated (250 eV). Here, the useful yield of Ge is found to be invariant with x. At 250 eV, the germanium ion and sputter yields are proportional to x for all x.

Quantitative analysis of mixed self-assembled monolayers using ToF-SIMS

The spacing of chemical functional groups on self-assembled monolayers (SAMs) plays an important role in controlling the density of biomolecules in biochips and biosensors. In this sense, a mixed SAM made of two different terminal groups is a useful organic surface since spacing can be easily controlled by changing a relative mole fraction in a mixture solution. In this study, an acetylene–OCH 2O(EG) 3(CH 2) 11S–S(CH 2) 11(EG) 3OCH 2O–propene (Eneyne) SAM and mixed SAMs made by a mixture of (S(CH 2) 11(EG) 3OCH 2O–acetylene) 2 (Diyne) and (S(CH 2) 11(EG) 3OCH 2O–propene) 2 (Diene) were produced on gold substrates and measured by using ToF-SIMS. The secondary ion yield ratio of [Au·S(CH 2) 11(EG) 3OCH 2O–acetylene] − to [Au·S(CH 2) 11(EG) 3OCH 2O–propene] − was measured for each mixed SAM and plotted as a function of the mole fraction of Diyne to Diene in a SAM solution. The ion yield ratio of a mixed SAM produced from a solution with a mole fraction of 0.5 (i.e., 1:1 mixture) was 0.3, which corresponded well to the ion yield ratio measured from an Eneyne SAM. A time-dependent experiment of Eneyne SAM formation and immersion experiment of Eneyne SAM into Diyne solution or into Diene solution were performed. The relative ion yield ratio of 0.3 was due to a different secondary ion formation and not due to the difference in the amount of adsorbates on the surface, nor to the different binding strengths onto the gold surface. Our study shows that a mixed SAM with well-controlled spacing can be produced and quantified by using the ToF-SIMS technique.

Positive and negative analyte ion yield in matrix-assisted laser desorption/ionization

The ratios of positive to negative analyte ion yields for matrix-assisted laser desorption ionization were studied for fibrinopeptide A, angiotensin I and bradykinin in combination with six matrices (CHCA, DHB, 4-NA, ATT, ANP, 5-AQ). The selection of these particular compounds was based on their acid/base properties. The measurements were carried out on two different time-of-flight instruments, one of which was equipped with a charge collection detector. The findings are: (i) for a desorption from a non-metallic substrate the total positive/negative ion yield ratio is ≈1. (ii) The analyte positive/negative ion yield is strongly dependent on many factors, and generally scatters a lot. More acidic matrices were found to produce a positive/negative analyte ion yield ratio >1, whereas for basic matrices, the ratio was ≤1. (iii) For selected matrices, the positive/negative analyte ion yield ratio follows the acid/base properties of the analyte. (iv) Clear indications for a significant contribution of clusters as carriers of the total charge in MALDI were obtained from the data.

Secondary ion emission from polymer layers by atomic and molecular ion bombardment: Data evaluation based on linear-cascade sputtering theory

This paper constitutes an attempt to rationalise impact-energy dependent yields of molecular secondary ions emitted from polymer samples under bombardment with atomic and molecular primary ions. The evaluation was based upon a comparison with sputtering yields calculated from linear-cascade sputtering theory, including threshold effects. To explore general trends, sputtering yields for carbon, silicon and silver were calculated under impact of normally incident C, F, S, Ga, Xe, Au, SF 5, C 11, C 60 and Au 5. The yields of carbon, for example, bombarded with C 60 are larger than for Ga by factors of ∼5 and ∼10 at 10 and 100 keV, respectively. However, owing to the fact that the effective threshold energy for sputtering increases with the number of constituents of the projectile, the yields for molecular ion impact start to exceed the yields for atomic ions only at energies between 0.5 and 5 keV. The analysed experimental results relate to molecular ion emission from one monolayer (1 ML) and 9 ML films of polymethacrylate on silver bombarded with Xe and SF 5 ions at energies E between 0.5 and 10 keV. Comparison of (initial) secondary ion yields S + ( m/ z 143) with calculated sputtering yields suggests that S + constitutes the sum of two contributions. The first, labelled S lo + , prevails at low energies and appears to reflect molecule ejection due to the mean effect of nuclear energy deposition (“ordinary” linear-cascade sputtering). The second contribution, S hi + , dominates at E > 1.5 keV, increases with a high power of E, but does not correlate with the calculated sputtering yield. It is suggested that S hi + is a measure of those impact events which occasionally generate a very high energy density at the surface, thus providing optimum conditions for very efficient ejection of molecules that are located at the rim of the agitated area. The SF 5/Xe secondary ion yield ratios are distinctly different for the two contributions, only about 0.3 for S lo + but ∼4 (1 ML) and ∼20 (9 ML) for S hi + . The pronounced secondary ion “yield enhancement” frequently reported in the literature for molecular versus atomic ion bombardment appears to be due to an enlargement of the S hi + contribution, more so the higher the impact energy and the more massive the projectile. The total (integrated) secondary ion yields, estimated by making use of the reported damage cross sections, were found to be the same for SF 5 bombardment of the 1-ML and the 9-ML samples. This finding calls for more attention towards obtaining high secondary ion yields at minimum sample consumption.

Molecular hydrogen ion elimination from alkyl iodides under strong laser beam irradiation

The elimination of H 2 + from alkyl iodides under strong (up to 5 × 10 15 W cm −2) laser irradiation is studied by means of time-of-flight mass spectrometry. The study has been performed by using 60 fs ( λ = 800 nm) and 35 ps ( λ = 1064, 532, 355 and 266 nm) laser pulses. It is concluded that the H 2 + ions are ejected from ionic states via Coulomb explosion processes. The molecular rearrangement leading to H 2 + formation is attributed to a tunneling process through a H transfer barrier. For the case of methyl iodide, about 10% of the doubly charged parent ions undergo molecular rearrangement. From a comparison of the H 2 +/H + ion yield ratio of the studied molecules, it turns out that the H 2 + formation from H atoms bonded to a terminal carbon atom is more efficient than that arising from H atoms bonded to central C atoms of the molecular chain.

Secondary ion yield improvements for phosphated and sulfated molecules using substrate-enhanced time-of-flight secondary ion mass spectrometry

Aminoethanethiol (AET), at an appropriate pH, forms a charged self-assembled monolayer (SAM) on Au surfaces. We have shown that an AET-SAM can be used to adsorb organic and inorganic anions from aqueous and ethanol solutions. The adsorbed anions can then be sputtered with high efficiency for characterization by time-of-flight secondary ion mass spectrometry. We present here the application of the AET monolayer for the characterization of more complex and biologically relevant organic sulfates and phosphates up to 1150 Daltons, each featuring a charged anionic site to facilitate the adsorption to the monolayer by ion-pair formation. We monitored the mass spectra and secondary ion emission following 20 keV (CsI) n Cs + ( n=0–2) projectile impacts on targets composed of thin layers deposited on Si, Au, and AET/Au substrates; and of thick targets (thick with respect to the 10–100 nm range of projectile). For each molecule used in this study, the secondary ion yield increased when the anion was deposited as a thin layer, relative to a thick layer, and is greatest for AET/Au substrates compared to bare Au and polished Si substrates. Additionally, the intact-to-fragment ion yield ratio is lower when sputtering from the solid salt target than from the AET monolayer.

The ‘infinite velocity method’: a means of concentration calibration in secondary ion mass spectrometry?

The so-called infinite velocity (iv) method, recently suggested by P. van der Heide as a means of quantifying, without standards, the signals measured by secondary ion mass spectrometry (SIMS), has been analysed critically. The iv method is essentially based upon two assumptions. (1) Secondary ion energy spectra can be described as a product of the isotopic concentration of the analysed species, the flux of sputtered neutrals, the instrument transmission, the detector efficiency, and the ionization probability, an ansatz that had already been used repeatedly in earlier work. (2) The ionization probability increases exponentially with decreasing inverse ion velocity, and in the limit of infinite velocity, all sputtered atoms are fully ionized. Hence, the matrix effect in SIMS is removed. Secondary ion yields at infinite velocity can be derived by extrapolation of measured energy spectra after eliminating the energy dependence of the sputtered atom flux and the instrument transmission, both considered to be the same for all elements and isotopes. It is shown that, if the energy-dependent neutral flux and the instrument transmission were in fact universal correction functions, concentration calibration could be achieved by extrapolation of ion yield ratios, without even knowing the analytical form of these functions. Literature results as well as (re)analysed data of van der Heide provide clear evidence that the matrix effect is not removed at infinite velocity. Many of the previously ignored problems originate from the fact that the formalism used for data processing has never been outlined in a quantitatively correct manner. As a result, the importance of normalization factors associated with the use of the unknown input functions remained unnoticed (e.g. the surface binding energy and the shape of the energy and angular distributions). Element-specific differences in these parameters give rise to corresponding errors in concentration calibration. The frequently derived non-exponential inverse-velocity dependence of the ‘corrected’ ion yields can be reproduced if one considers rather small deviations from the arbitrary assumptions made in the data processing. It is concluded that, even if all errors in the basic formalism are removed, the iv method does not provide reliable data, neither on elemental concentrations nor on ionization or neutralization phenomena, the reason being that (1) none of the parameters and functions required for extracting ion fractions from measured energy spectra is known with the required accuracy, and (2) the matrix effect cannot be removed by extrapolation procedures.

Quantification of Sulfur in Copper by Secondary Ion Mass Spectrometry (SIMS)

Secondary Ion Mass Spectrometry (SIMS) measurements were performed for oxygen-free copper-base alloys with various sulfur concentrations from 0.02 to 14.0 mass ppm, and the influence of molecular ions 16 O 2 - on the analysis of sulfur was studied. The secondary ion yield ratios of 34 S - to 32 S - were nearly the same as their isotopic abundance ratio in the analyzed sulfur concentration range. The secondary ion yield ratio of 16 O 2 to 16 O - decided by the high mass resolution analyses was 1.3 ± 0.5 x 10- 3 , which suggested that only 3% of the secondary ions of the 32m /e signal, where m is the mass number and e is the electric charge, would come from 16 O 3 - ions in the most dilute alloy case. From these facts, the interference by 16O 2 - ions can be neglected as far as oxygen-free copper concerns. The determination limit on the measurement of 32 S - ions was less than 0.02 mass ppm, and that on the measurement of 34 S - ions was 0.02 mass ppm with the normal resolution analyses.