Absolute Emission Cross Sections Research Articles

Laboratory Study of the Cameron Bands and UV Doublet in the Middle Ultraviolet 180–300 nm by Electron Impact upon CO2 with Application to Mars

We have observed electron impact fluorescence from CO2 to excite the Cameron bands (CBs), CO (a 3Π → X 1Σ+; 180–280 nm), the first-negative group (1NG) bands, CO+ (B 2Σ+ → X 2Σ+; 180–320 nm), the fourth-positive group (4PG) bands, CO (A 1Π → X 1Σ+; 111–280 nm), and the UV doublet, CO2 + ( 288.3 and 289.6 nm) in the ultraviolet (UV). This wavelength range matches the spectral region of past and present spacecraft equipped to observe UV dayglow and aurora emissions from the thermospheres (100–300 km) of Mars and Venus. Our large vacuum system apparatus is able to measure the emission cross sections of the strongest optically forbidden UV transitions found in planetary spectra. Based on our cross-sectional measurements, previous CB emission cross-sectional errors exceed a factor of 3. The UV doublet lifetime is perturbed through spin–orbit coupling. Forward modeling codes of the Mars dayglow have not been accurate in the mid-UV due to systematic errors in these two emission cross sections. We furnish absolute emission cross sections for several band systems over electron energies 20–100 eV for CO2. We present a CB lifetime, which together with emission cross sections, furnish a set of fundamental physical constants for electron transport codes such as AURIC (Atmospheric Ultraviolet Radiance Integrated Code). AURIC and Trans-Mars are used in the analysis of UV spectra from the Martian dayglow and aurora.

A velocity map imaging spectrometer for measuring absolute differential cross sections for ion-induced electron emission from molecules.

In this paper, we present a newly developed crossed beam experimental setup that utilizes the velocity map imaging (VMI) technique to simultaneously measure both the kinetic energy and emission angle of electrons emitted from atoms or molecules upon ion collision. The projectile ion beam with keV to MeV kinetic energy orthogonally crosses the neutral target beam produced by an effusion cell. The emitted electrons are extracted and analyzed by a multi-electrode VMI spectrometer. By monitoring the target density, the projectile ion beam intensity, and the beams' overlap, we are able to measure absolute differential cross sections for collision-induced electron emission from molecules. The characterization of the setup and the methodology will be presented as well as first results for electron emission from uracil upon 0.98 MeV/u 12C4+ collision.

Differential electron emission from polycyclic aromatic hydrocarbon molecules under fast ion impact

Interaction between polycyclic aromatic hydrocarbon (PAH) molecule and energetic ion is a subject of interest in different areas of modern physics. Here, we present measurements of energy and angular distributions of absolute double differential electron emission cross section for coronene (C24H12) and fluorene (C13H10) molecules under fast bare oxygen ion impact. For coronene, the angular distributions of the low energy electrons are quite different from that of simpler targets like Ne or CH4, which is not the case for fluorene. The behaviour of the higher electron energy distributions for both the targets are similar to that for simple targets. In case of coronene, a clear signature of plasmon resonance is observed in the analysis of forward-backward angular asymmetry of low energy electron emission. For fluorene, such signature is not identified probably due to lower oscillator strength of plasmon compared to the coronene. The theoretical calculation based on the first-order Born approximation with correct boundary conditions (CB1), in general, reproduced the experimental observations qualitatively, for both the molecules, except in the low energy region for coronene, which again indicates the role of collective excitation. Single differential and total cross sections are also deduced. An overall comparative study is presented.

Lyman-series emission after valence and core excitation of water vapor

We report Lyman-series emission cross sections of neutral hydrogen dissociation fragments after valence (15--34 eV) and inner-shell (533--542 eV) excitation of water vapor with monochromatic synchrotron radiation as functions of the exciting-photon energy. In the valence excitation energy region, the thermodynamical limits of the production of the differently excited hydrogen fragments are directly observed and absolute emission cross sections are determined. For resonant inner-shell excitations, the fluorescing excited hydrogen state is found to be strongly dependent on the molecular or Rydberg-like character of the excitation.

Double differential electron emission in ionization of Coronene by fast ions

SynopsisWe present the absolute differential electron emission cross section of coronene under fast bare O-ion impact. The results show some features which cannot be explained by simple ion-atom type collision result. The difference of the energy and angular distribution data with that for CH4 and Ne may indirectly indicate the influence of collective excitation. We have performed the CB1 calculation for comparison.

Induced ionization of small molecules: comparison of experiment with quantum and classical calculations

SynopsisThe ion-impact induced ionization of small molecules was investigated both experimentally and theoretically. Electron energy spectra were taken by an energy dispersive electrostatic spectrometer at different observation angles. The obtained absolute electron emission cross sections were compared with the results of CDW-EIS and CTMC calculations. Both theories are in a reasonable agreement with the experimental data.

Energy and angular distribution of electrons in ionization of He and Ne by 6-MeV/u bare carbon ions: Comparison with continuum-distorted-wave eikonal-initial-state calculations in prior and post forms

We have measured the absolute double-differential cross sections (DDCS) for electron emission in ionization of He and Ne atoms under the impact of 6-MeV/u ${\mathrm{C}}^{6+}$ ions. Data were collected between 1 and 500 eV for He, while for Ne this range was extended up to 1000 eV. The angular ranges covered in the experiment are ${30}^{\ensuremath{\circ}}$ to ${150}^{\ensuremath{\circ}}$ and ${20}^{\ensuremath{\circ}}$ to ${160}^{\ensuremath{\circ}}$ for He and Ne, respectively. The DDCS spectra are compared with the prior and the post forms of the state-of-the-art continuum-distorted-wave eikonal-initial-state model. Both the theoretical models show very good agreement with the energy and angular distributions of the DDCS in the case of He. For Ne, at low energies both are going together and matching very well with the data. In the high-energy region, at extreme forward and backward angles, although both the forms are underestimating the experimental data to some extent, the prior form shows much better agreement compared to the post form. This post-prior discrepancy is attributed to the influence of dynamic screening, on the ionized one, produced by the electrons remaining in the target. The single differential cross sections (SDCS) in emission angle $(\frac{d\ensuremath{\sigma}}{d{\ensuremath{\Omega}}_{e}})$ and electron energy $(\frac{d\ensuremath{\sigma}}{d{\ensuremath{\epsilon}}_{e}})$ are deduced by integrating the electron DDCS spectra. While excellent agreement is obtained for the $\frac{d\ensuremath{\sigma}}{d{\ensuremath{\epsilon}}_{e}}$ spectrum, the $\frac{d\ensuremath{\sigma}}{d{\ensuremath{\Omega}}_{e}}$ provides a further sensitive test to the adequacy of the theoretical model employed. The total cross section obtained from the SDCS spectra is about 11% higher than the prior model for He and about 6% lower for Ne. To get the quantitative picture of the two-center effect, the forward-backward angular asymmetry parameter has been deduced as a function of velocity of the ejected electrons. For both the targets, it is very well reproduced by both the forms of the theory. For the Ne target, $K$-LL Auger angular distribution has also been studied, which shows small asymmetry caused by multiple vacancies in the $L$ shell along with the $K$-shell vacancy.

ELECTRONIC EXCITATION AND CHARGE TRANSFER PROCESSES IN COLLISIONS OF H+, H$_2^+$, AND H$_3^+$ IONS WITH CARBON MONOXIDE AT TYPICAL SOLAR-WIND VELOCITIES

Luminescence in the 200-580 nm spectral region was observed in the collisions of H+, H, and H with CO in the 50-1000 eV projectile energy range. Using computer simulations, we have identified emission of the following products in the observed spectra: the CO+(A-X) comet-tail system, CO+(B-X) first negative system, CO+(B-A) Baldet-Johnson system, and CO(b-a) third positive system. Also, an emission from atomic hydrogen (Hβ line at 486nm) has been observed. From the analysis of the experimental spectra, we have determined the absolute emission cross-sections for the formation of the observed products. Computer simulations gave the excited-product population distributions over vibrational and rotational energy levels. The vibrational level distribution from the CO+(A-X) comet-tail system is compared with the data for CO excited by 100 eV electrons and extreme ultraviolet radiation (XUV) photons. We have used these data to analyze the excitation conditions in the comet Humason (1961e). From the vibrational population distributions observed in the comet, we found that this distribution can be reproduced if electrons produce 25%, protons 70%, and XUV photons produce 5% of the emitting molecules. We find that the ratio of the CO+(B-X) emission to the sum of two main emissions (CO+(A-X)+CO+(B-X)) is velocity dependent and does not depend on the projectile ion type. For small velocities (below 100 km s–1) the ratio is about 5%, while for higher velocities it increases to 30%. For these data, we have found an empirical formula that satisfactorily describes the experimental data: R = R max(1 – v th/v), (where R max = 33%, v th = 87 km s–1). This could be used to infer the velocity of ions producing the observed emission of CO+ products.

6.0 MeV u−1 carbon ion (C6+ and C4+)-induced secondary electron emission from water vapor

We have precisely measured absolute double differential cross sections (DDCSs) for secondary electron emission produced in collisions of 6.0 MeV u−1 C6+ and C4+ ions with water vapor. Theoretical calculations of the DDCSs were made for C6+ ions using the continuum distorted wave-eikonal initial state model (CDW-EIS) in its straight-line version of the impact parameter approximation, showing general good agreement with experimental data, except in the intermediate- and high-energy region (> 50 eV), particularly at the backward angles (> 110°). On the other hand, the single differential cross section (SDCS), which was obtained by integrating the measured DDCSs over the solid angle, showed fairly good agreement with the CDW-EIS in the low-energy region (< 200 eV), while a significant discrepancy between the observed SDCS and the Rudd-model scaling (× 36) can be seen, suggesting that a simple Z2-scaling law (i.e. first Born approximation) is not applicable for high-Z bare projectiles such as C6+ ions. The SDCS of C4+ ions was observed to be smaller than that of C6+ ions by ∼ 50% in the low-energy region (< 200 eV) due to the screening effect of its bound electrons in C4+ ions, which could be explained quantitatively by taking account of an effective projectile charge.

Interaction of slow HCI with gaseous targets: absolute x-ray emission cross sections and contribution of multi-capture processes

Collisions of 15 keV/q Ar17+ ions with a gaseous jet have been studied by high-resolution x-ray spectroscopy techniques. Absolute x-ray emission cross sections and initial population mechanisms on singly and multiple excited states are extracted.

Impact ionization of molecular oxygen by 3.5-MeV/u bare carbon ions

We have measured the absolute double-differential cross sections (DDCSs) for electron emission in ionization of O${}_{2}$ molecules under the impact of 3.5-MeV/u C${}^{6+}$ ions. The data were collected between 10 and 600 eV, in an angular range of ${30}^{\ensuremath{\circ}}$ to ${150}^{\ensuremath{\circ}}$. The single-differential cross sections (SDCSs) in emission angle and electron energy are deduced from the electron DDCS spectra. Also, the total cross section has been obtained from the SDCS spectra. The DDCS spectra as well as the SDCS spectra are compared with continuum distorted-wave eikonal initial-state calculations which employ molecular wave functions built as linear combinations of atomic orbitals. The DDCS ratio i.e. $\ensuremath{\sigma}{{}_{\mathrm{O}}}_{2}/2{\ensuremath{\sigma}}_{\mathrm{O}}$, derived by dividing the experimental DDCS for molecular oxygen with the theoretical DDCS for atomic oxygen, does not show any primary or secondary oscillations arising from Young-type interference, which is apparently in contrast to what has been observed earlier for H${}_{2}$ and in agreement with the model calculation. Similarly, the forward-backward angular asymmetry increases monotonically with the velocity of the emitted electrons. However, the results on the DDCSs, SDCSs, the asymmetry parameter, and the nonexistence of oscillations are in qualitative agreement with the predictions of the model used.

Charge-transfer excitation in collisions of He+ ions with CO2 molecules under beam conditions

The charge-transfer excitation of CO2+ in the collisions of He+ ions with CO2 was investigated in the (5/1000)eV laboratory energy range of the He+ projectile. The observed luminescence is due mostly to the electronically excited CO2+(A, B) molecules. The cross-section for the reactions leading to the CO2+(A+B) products varies only slightly in the (200/1000)eV laboratory energy range. The observed decrease of the excitation function for energies up to ∼200eV is probably associated with the formation of long-lived collision complex. A normalization procedure has been used to obtain the absolute emission cross sections for the charge transfer excitation processes studied.

Investigation of slow collisions for (quasi) symmetric heavy systems: what can be extracted from high resolution x-ray spectra

We present a new experiment on (quasi) symmetric collision systems at low velocity, namely Ar17 + ions (v = 0.53 au) on gaseous Ar and N2 targets, using low- and high-resolution x-ray spectroscopy. Thanks to an accurate efficiency calibration of the spectrometers, we extract absolute x-ray emission cross sections combining low-resolution x-ray spectroscopy and a complete determination of the ion beam–gas jet target overlap. Values with improved uncertainty are found in agreement with previous results (Tawara et al 2001 Phys. Rev. A 64 042712). Resolving the whole He-like Ar16 + Lyman series from n = 2–10 with our crystal spectrometer enables us to determine precisely the distribution of the electron capture probability and the preferential npref level of the selective single-electron capture. Evaluation of cross sections for this process as well as for the contribution of multiple-capture is carried out. Their sensitivity to the ℓ-distribution of n levels populated by single-electron capture is clearly demonstrated, providing a stringent benchmark for theories. In addition, the hardness ratio is extracted and the influence of the decay of the metastable 1s2s 3S1 state on this ratio is discussed.

Absolute and Relative Measurements of Optical Emission Cross Sections for the N2+1N(v',v'') Bands by Electron Impact

As a confirmative study on the electron impact optical emission of the first negative [B 2 Σ u + →X 2 Σ g + , 1N( v ', v '')] bands of the nitrogen molecule, measurements have been performed on the absolute emission cross section for the 1N(0, 0) band (λ= 391.2 nm) in the electron energy range from 18.8 to 300 eV and on the cross section ratios 1N( v ', v '')/1N(0, 0) for v ' = 0 and 1 at 100 eV, around the cross-section maximum. From these results, the maximum cross sections for the excitation of the N 2 + B 2 Σ u + ( v ' = 0, 1) vibrational levels have been determined. The absolute measurement was made directly using a photon counting system calibrated with a standard light source and an ionization gauge calibrated with a capacitance diaphragm gauge. The absolute maximum cross section measured for emission of the 1N(0, 0) band is (15.7 ±1.7)×10 -18 cm 2 . This value is about 10% lower than the previous Borst–Zipf value (17.4 ±1.7)×10 -18 cm 2 , which has been referred to most frequently. The maximum cro...

Fragmentation of isoxazole molecules by electron impact in the energy range 10–85 eV

Fragmentation of isoxazole molecules by electron impact excitation, that produces excited atomic and molecular fragments, has been studied using the optical excitation technique. Excited hydrogen atoms H( n), n = 4–7, have been detected by observation of the H β to H ε lines of the Balmer series. The diatomic CH(A 2Δ) and CN(B 2Σ +) fragments have been identified by their A 2Δ → X 2Π r and B 2Σ + → X 2Σ + emission bands, respectively. The appearance energies for the H( n = 4), CH(A 2Δ) and CN(B 2Σ +) have been measured to be 21.9, 14.0 and 10.6 eV, respectively. Absolute emission cross sections have been also obtained for the fragments in electron energy ranges from their respective appearance thresholds up to 85 eV. Possible fragmentation processes are discussed.

Photoionization of Xe near 5s threshold: II. 5s- main line and satellites

Photon-induced fluorescence spectroscopy was applied to measure absolute fluorescence emission cross sections from the main 5s- and first satellite levels of XeII upon photoionization from XeI in a broad exciting-photon energy range (23–28 eV) with 20 meV bandwidth of the exciting radiation and in a narrow energy range (23.25–23.50 eV) with a bandwidth of 1.8 meV. The average fluorescence resolution amounted to 0.07 nm. Fluorescence emission cross sections were computed with taking into account many-electron correlations and doubly excited states. Computed cross sections, especially in integrated form, agree well with the results of experiment. Many features observed in the high resolution experiment are identified to originate from nf-Rydberg series.

Excitations from dissociative fragments produced inH++H2Ocollisions

We report on photon emissions in the $200--800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ region resulting from collisions of $200\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ protons with ${\mathrm{H}}_{2}\mathrm{O}$. The most prominent features observed in the spectrum are the Balmer series of hydrogen and two OH molecular bands. Several less intense ${\mathrm{O}}^{+}$ as well as neutral O lines are also observed. The absolute photon emission cross sections of the major lines and bands were measured. The results indicate that a primary dissociation pathway involves the formation of ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ by removing a $1{b}_{2}$ electron. The unstable ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ ion further dissociates into ${\mathrm{H}}^{+}+\mathrm{O}\mathrm{H}$ or $\mathrm{O}{\mathrm{H}}^{+}+\mathrm{H}$. The dominant presence of neutral hydrogen lines and ${\mathrm{O}}^{+}$ lines leads to the conclusion that the subsequent dissociation of $\mathrm{O}{\mathrm{H}}^{+}$ into $\mathrm{H}+{\mathrm{O}}^{+}$ prevails over the other possible dissociation pathway leading to ${\mathrm{H}}^{+}$ and neutral oxygen fragments.

Dissociation and fragmentation of furan by electron impact

Dissociation and fragmentation processes that produce electronically excited atomic and molecular fragments, following electron impact excitation, have been studied in furan. The optical excitation technique has been employed over the electron incident energy range 15–95eV. Formation of excited hydrogen atoms H(n) in the n=4–10 states has been detected by observation of the Balmer series. The diatomic CH fragments are formed in the A2Δ, B2Σ− and C2Σ− electronic states and the C2 fragments in the d3Πg excited state. The appearance energies for the H(n=4) (Hβ line) and the CH(A2Δ) (A2Δ→X2Πr transition) fragments have been measured to be 20.7±0.4 and 19.0±0.3eV, respectively. Absolute emission cross sections have been also determined for the above two transitions in the electron energy range from the appearance energy thresholds up to 95eV. Possible dissociation and fragmentation processes are discussed.

Emission cross sections for electron impact onCl2

We have measured the absolute emission cross section for the Cl{sub 2}{sup +} A {sup 2}{pi}{sub u}-X {sup 2}{pi}{sub g} system for 10-750 eV electrons incident on chlorine molecules. The cross section for the system possesses a broad peak at 100 eV of 2.0x10{sup -16} cm{sup 2}, typical of such ionization processes. At this energy, the A-X cross section is responsible for two-thirds of the total Cl{sub 2}{sup +} production and one-third of the total ionization cross section. We also measured the emission cross section from 7 to 740 eV for the 307 nm system of Cl{sub 2}. The energy dependence of the cross section indicates that the excitation is to a Cl{sub 2} {sup 1}{sigma}{sub u}{sup +} or {sup 1}{pi}{sub u} state with a decay to a repulsive potential.

Cometary x-rays: line emission cross sections for multiply charged solar wind ion charge exchange

Absolute line emission cross sections are presented for 1 keV amu−1 charge-exchange collisions of multiply charged solar wind ions with H2O. These cross sections can be used to model charge-exchange processes with cometary targets with similar binding energies such as H, O, CO2 and CO. A parameter-free model is used to successfully predict the recently observed x-ray spectra of comet Linear C/1999 S4 and McNaught-Hartley C/1999 T1. We show that the resulting spectrum is extremely sensitive to the time variations of the solar wind composition.