Deexcitation Cross Sections Research Articles

Low-energy inelastic electron scattering from carbon monoxide: Excitation and de-excitation of the X1Σ+, a3Π, a′3Σ+, A1Π, d3Δ, e3Σ−, I1Σ− and D1Δ electronic states

Cross-sections for electronic excitation and de-excitation among the ground state and lowest-lying seven electronic excited states of carbon monoxide (CO) by low-energy electron impact are computed using the R-matrix method. The excitation cross-sections from the ground state to the electronic states a3Π, a′3Σ+ and A1Π agree with previous experimental and theoretical results. In addition, the cross-sections for the I1Σ− and D1Δ states of CO, which will cascade to CO a′3Σ+ and A1Π states, are calculated. Furthermore, in contrast to the typical increase in electronic excitation cross-sections with collision energy, the de-excitation cross-sections show a negative trend with increasing energy.

An improved artificial neural network fit of the ab initio potential energy surface points for HeH+ + H2 and its ensuing rigid rotors quantum dynamics

Artificial neural networks (ANN) have been shown for the last several years to be a versatile tool for fitting ab initio potential energy surfaces. We have demonstrated recently how a 60-neuron ANN could successfully fit a four-dimensional ab initio potential energy surface for the rigid rotor HeH+ - rigid rotor H2 system with a root-mean-squared deviation (RMSD) of 35 cm−1. We show in the present study how a (40, 40) neural network with two hidden layers could achieve a better fit with an RMSD of 5 cm−1. Through a follow-up quantum dynamical study of HeH+(j1)-H2(j2) collisions, it is shown that the two fits lead to slightly different rotational excitation and de-excitation cross sections but are comparable to each other in terms of magnitude and dependence on the relative translational energy of the collision partners. When averaged over relative translational energy, the two sets of results lead to rate coefficients that are nearly indistinguishable at higher temperatures thus demonstrating the reliability of the ANN method for fitting ab initio potential energy surfaces. On the other hand, we also find that the de-excitation rate coefficients obtained using the two different ANN fits differ significantly from each other at low temperatures. The consequences of these findings are discussed in our conclusions.

New potential energy surface and rotational deexcitation cross-sections of CNNC by para-H2 (jp = 0).

The objective of this study is to enhance our understanding of the existence of molecules in interstellar space by determining the collisional rate coefficients with the most prevalent species. The study examines the impact of para-H2 collisions, specifically when it is in its ground vibrational state with a nuclear spin of para-H2, i.e., jp = 0, on causing the rotational deexcitation of the diisocyanogen (CNNC) molecule. These scattering data are obtained as a result of spherically averaging a four-dimensional potential energy surface (4DPES) over the H2 orientations. Using the CCSD(T)-F12a approach and aug-cc-pVTZ basis sets, the ab initio 4DPES for the CNNC-H2 van der Waals system is calculated. The CNNC-para-H2 4DPES attains a global minimum of 221.38 cm-1 at the CNNC and H2 center of mass distance (R) of 3.1 Å. The method of close coupling calculations is employed for the purpose of calculating the cross-sectional data of CNNC with para-H2 (jp = 0), for total energies up to 1000 cm-1. Rate coefficients are computed over the temperature range of 1 K to 100 K. Propensity suggests that even Δj transitions are strongly preferred. The rate coefficients for CNNC-H2 are determined to be 0.90-2.95 times those of CNNC-He, which implies it is not reliable to estimate the H2 rate coefficients by multiplying the rate coefficients for CNNC-He collision with a scaling factor of 1.38.

Spectral investigations of Nd3+:Ba(PO3)2+La2O3 glasses for infrared laser gain media applications

The investigation of the concentration dependent spectroscopic characteristics of Nd3+ ion in Ba(PO3)2+La2O3 (PBaLa) glasses have been carried out. Based on prominent Judd-Ofelt (JO) formalism, the three JO phenomenological quantitative intensity parameters (Ωλ = 2,4,6) were determined by using absorption levels. The very high Ω6 intensity parameter value reveals greater degree of structural distortion of the Nd3+ sites in PBaLa glass. From Ωλ, required radiative characteristics such as radiative emission probability, lifetime (τr), branching ratio (βr) and emission cross-section (σ(λ)) for excited fluorescent levels have been predicted to assess the potentiality of the glasses. The de-excitation (radiative emission) spectra observed for these glasses yield three prominent levels of 4F3/2 → 4I13/2,11/2,9/2 and their related effective bandwidths and de-excitation cross-sections have been quantified. The lifetimes of the 4F3/2 fluorescent state in Nd3+:PBLa for different concentrations (0.1–2.0 mol %) of Nd3+ ion have been determined by exciting with diode laser (808 nm). The quenching of lifetime could be attributed to the enhanced energy transfer phenomenon via cross-relaxation between pairs of the Nd3+ ions. The crucial spectroscopic characteristics that include Ωλ parameters, radiative properties, effective bandwidths, stimulated emission cross-section, gain bandwidth, figure of merit and optical gain of 4F3/2 → 4I11/2 transitions of PBaLaNd1.0 glass are determined which are also compared with other reported 85 Nd3+: glasses which include 63 Nd3+:phosphate glasses. These systematic analysis and critical comparison with similar results reveal that Nd3+:PBaLa glasses with a wide bandwidth of fluorescence can also be useful for optical amplifiers or laser gain media applications.

Projectile electron loss, excitation and de-excitation cross section database in a collision between two hydrogen atoms in the impact energy range between 50 keV–5.0 MeV, Part I: Target is in ground state

We present projectile electron loss, excitation, and de-excitation cross-sections database for H(nl)+H(1s) collision system. A four-body classical trajectory Monte Carlo (CTMC) and a quasi-classical trajectory Monte Carlo (QCTMC) methods were employed in the projectile energy range between 50 keV to 5.0 MeV.

R-Matrix Calculation of Electron Collisions with Molecular Oxygen in Its Electronically Excited States.

Low-energy electron collisions with the X3Σg- ground state and a1Δg and b1Σg+, the Herzberg states (c1Σu-, A'3Δu, and A3Σu+), and B3Σu- excited states of the O2 molecules are studied using the fixed-nucleus R-matrix method. Integral elastic scattering and electronic excitation cross sections from the X3Σg- ground state overall agree well with the available experimental and theoretical results. The electronic (de-)excitation cross sections for the electron impact with the Herzberg states and the B3Σu- state are reported. The value of elastic cross sections for the six excited states decreases with the increment of electron energy, except for the resonance peaks. As the case of excitation from the X3Σg- ground state, the O2- 2Πu resonance makes a dominant contribution to the (de-)excitation cross sections from the a1Δg, b1Σg+, and the Herzberg states. The magnitude of the de-excitation cross sections at the location of the 2Πu resonance from the Herzberg states to the ground state is about 2 to 8 times those of the excitation cross sections for the corresponding excitation transitions. These results should be significant for models of oxygen plasma and the dynamics of the Herzberg states of molecular oxygen in the earth's atmosphere.

Electronic structure calculations and quantum dynamics of rotational deexcitation of CNNC by He.

The quantum dynamics of rotational transitions of the diisocyanogen (CNNC) molecule undergoing collision with the helium (He) atom occurring in the interstellar medium (ISM) has been studied. The rotational deexcitation cross sections are extracted by first computing an ab initio potential energy surface of CNNC-He using the coupled-cluster with single and double and perturbative triple excitations with the F12a method (CCSD(T)-F12a) employing the aug-cc-pVTZ basis set. Utilizing the multipole expansions, collisional cross sections are determined for total energies of up to 1000 cm-1 by the close coupling equations. The discussion on propensity rules suggests that the transitions have even Δj values, while odd Δj valued transitions are forbidden due to C and N nuclei spin statistics. Quasi-bound states present in the CNNC-He van der Waals complex resulted in the resonances coming from the rapid oscillations in the values of the cross sections in the region of low energy. Rotational deexcitation rate coefficients are further worked out by averaging the calculated cross sections at temperatures below 200 K. The new findings of the study will be beneficial in modeling the abundance of diisocyanogen in the ISM.

Inelastic rate coefficients for collisions of C4H− with H2

ABSTRACT Carbon-chain anions were recently detected in the interstellar medium. These very reactive species are used as tracers of the physical and chemical conditions in a variety of astrophysical environments. However, the local thermodynamic equilibrium conditions are generally not fulfilled in these environments. Therefore, collisional as well as radiative rates are needed to accurately model the observed emission lines. We determine in this work the state-to-state rate coefficients of C4H− in collision with both ortho- and para-H2. A new ab initio 4D potential energy surface was computed using explicitly correlated coupled-cluster procedures. This surface was then employed to determine rotational excitation and de-excitation cross-sections and rate coefficients for the first 21 rotational levels (up to rotational level j1 = 20) using the close-coupling method, while the coupled-state approximation was used to extend the calculations up to j1 = 30. State-to-state rate coefficients were obtained for the temperature range 2–100 K. The differences between the ortho- and para-H2 rate coefficients are found to be small.

Electron collisions with molecular nitrogen in its ground and electronically excited states using the R-matrix method

A comprehensive study of electron collisions with the X 1Σg + ground state as well as the metastable A 3Σu + and a 1Πg excited states of the N2 molecule is reported using the fixed-nucleus R-matrix method. Integral elastic scattering and electronic excitation cross sections from the X 1Σg + ground state to the eight lowest electronic states, A 3Σu +, B 3Πg, W 3Δu, B′ 3Σu −, a 1Πg, a′ 1Σu −, w 1Δu and C 3Πu, overall agree well with the available experimental and theoretical results although updates of some recommended values are suggested. Accurate electron impact electronic transition cross sections starting from the A 3Σu + and a 1Πg metastable excited states are reported. The total summed electronic transition cross sections from the a 1Πg state is dominant: an order of magnitude higher than those of the X 1Σg + ground state. The de-excitation cross sections generally show a downward trend with increasing incident electron energy, which is different from the elastic and electronic excitation cross sections which generally increase with collision energy. There is a prominent 2Πu symmetry resonance peak at 2.8 eV for electronic de-excitation scattering of a 1Πg → B 3Πg, which significantly contributes to the total summed cross sections from the a 1Πg excited state. The present results provide a new insight which will aid understanding of electron spectra in the atmosphere of the Earth and Titan.

Collisional (de-)excitation of protonated cyanoacetylene (HC3NH+) by helium at low and moderate temperatures

ABSTRACT Protonated cyanoacetylene, HC3NH+, is detected in astrophysical media, where it plays a key role as an intermediate in the chemistries of HCN/HNC and of cyanopolyynes. We first generated a potential energy surface (PES) describing the intermonomer interaction between HC3NH+ and He in Jacobi coordinates using the highly correlated CCSD(T)-F12/aug-cc-pVTZ ab initio methodology. Then, scattering calculations based on an exact close-coupling quantum-scattering technique were done to obtain pure rotational cross-sections for the rotational (de-)excitation of HC3NH+ after collision with He for total energies up to 2500 cm−1. These cross-sections are used to deduce the collision rates in the 5–350 K temperature range for the low-lying rotational levels of HC3NH+ (up to $j\,\, = \,\,15$). In addition, we generated an average PES for the HC3NH+–H2 system. The preliminary results show that the H2($j_{\mathrm{H_2}} = 0$) and He state-to-state de-excitation cross-sections have similar magnitudes, even though the H2 cross-sections are larger by a factor of 2–2.5. This work should help with the accurate derivation of protonated cyanoacetylene abundances in non-local thermodynamical equilibrium astrophysical media. These will put more constraints on the chemical pathways involving the formation and destruction of HC3NH+ while going back to the cyanopolyyne family and more generally those parts of nitrogen-containing molecular chemistry.

New H2O–H2O collisional rate coefficients for cometary applications

ABSTRACT We re-introduce a semiclassical methodology based on theories developed for the determination of broadening coefficients. We show that this simple and extremely fast methodology provides results that are in good agreement with results obtained using the more sophisticate MQCT approach. This semiclassical methodology could be an alternative approach which allows to provide large sets of collisional data for very complex molecular systems. It saves time both on the determination of potential energy surfaces and on the collisional dynamical calculations. In addition, this paper provides more complete sets of rotational de-excitation cross-sections and rate coefficients of H2O perturbed by a thermal average of water molecules. Those data can be used in the radiative transfer modelling of cometary atmospheres.

Rotational relaxation of HCO+ and DCO+ by collision with H2

ABSTRACT The HCO+ and DCO+ molecules are commonly used as tracers in the interstellar medium. Therefore, accurate rotational rate coefficients of these systems with He and H2 are crucial in non-local thermal equilibrium models. We determine in this work the rotational de-excitation rate coefficients of HCO+ in collision with both para- and ortho-H2, and also analyse the isotopic effects by studying the case of DCO+. A new four-dimensional potential energy surface from ab initio calculations was developed for the HCO+–H2 system, and adapted to the DCO+–H2 case. These surfaces are then employed in close-coupling calculations to determine the rotational de-excitation cross-sections and rate coefficients for the lower rotational states of HCO+ and DCO+. The new rate coefficients for HCO+ + para-H2 were compared with the available data, and a set of rate coefficients for HCO+ + ortho-H2 is also reported. The difference between the collision rates with ortho- and para-H2 is found to be small. These calculations confirm that the use of the rate coefficients for HCO+ + para-H2 for estimating those for HCO+ + ortho-H2 as well as for DCO+ + para-H2 is a good approximation.

Exploratory study of the 180mTa(γ,γ′)180Ta reaction with mono-energetic low-energy photon beams at HIγS

Abstract For the first time the de-excitation cross section of the isomeric-state nuclide 180 m Ta has been measured with mono-energetic photon beams in the 1.56 MeV to 3.70 MeV energy range. The measurement of this inelastic scattering cross section on nature’s rarest isotope was performed at eight energies at the High-Intensity Gamma-Ray Source Facility with natural tantalum targets. After irradiation for typically 20 h with photon fluxes ranging from 0 . 3 × 1 0 7 γ /( cm 2 × s) to 3 . 2 × 1 0 7 γ /( cm 2 × s), the 180 m Ta de-excitation cross section was obtained from the yield of the K α 1 and K α 2 X rays of 180 m Hf at 55.79 keV and 54.61 keV, respectively, produced after electron capture on the unstable 180 m Ta ground state. The cross-section values found vary between ( 2 . 0 ± 0 . 7 ) mb and ( 4 . 7 ± 1 . 5 ) mb, providing a very large cross section for the de-excitation of 180 m Ta. Extensive tests were performed to check on the reliability of the present data. The effect of bremsstrahlung photons of higher energy than the mono-energetic beam was carefully studied and corrected for. It was found that even a very small contamination of the mono-energetic photon beam by bremsstrahlung photons with E γ > 7 . 6 MeV can provide a sizable yield of 180 m Hf X rays via the reaction 181 Ta ( γ , n ) 180 Ta, if a natural tantalum target is used, which contain four orders of magnitude more 181 Ta nuclei than 180 m Ta nuclei available for the 180 m Ta ( γ , γ ′ ) 180 Ta reaction of interest.

Cross Sections and Rate Coefficients for Rotational Excitation of HeH+ Molecule by Electron Impact

Cross sections for rotational excitation and de-excitation of the HeH+ ion by an electron impact are computed using a theoretical approach that combines the UK R-matrix code and the multi-channel quantum defect theory. The thermally-averaged rate coefficients derived from the obtained cross sections are fitted to an analytical formula valid for a wide range of temperatures.

Analysis of Data on the Cross Sections for Electron-Impact Ionization and Excitation of Electronic States of Atomic Hydrogen (Review)

A review is given of experimental and theoretical data on the cross sections for ionization, excitation, and deexcitation of atomic hydrogen. The set of the cross sections required to calculate the electron energy distribution function and find the level-to-level rate coefficients needed to solve balance equations for the densities of neutral and charged particles in hydrogen plasma is determined.

Ultracold rotational deexcitation of CO (1Σ+) collision with proton

Ultracold rotational deexcitation cross sections and rate coefficients have been computed for CO collision with proton using close-coupling method on the two-dimensional ground state potential energy surface computed at the MRCI/cc-pVTZ level of theory. State-to-state rate coefficients have been obtained for temperature range from 10−5K to 200K. The system displays higher propensity with j′=0 cross section magnitude closer to j′=1 state, and j′=2 cross section closer to j′=3 state, from initial j=4 state. In ultracold region, the magnitude of cross section increases with the decrease in collision energy following Wigner’s threshold law.

Probing Scattering Resonances in (Ultra)Cold Inelastic NO-He Collisions.

We theoretically study inelastic collisions between NO radicals and He atoms at low collision energies, focusing on the occurrence of scattering resonances. We specifically investigate de-excitation of rotationally excited NO radicals (X (2)Π1/2, v = 0, j = 3/2, f) at collision energies ranging from 10(-3) to 20 cm(-1) and compute integral and differential cross sections using quantum mechanical close-coupling calculations. Although unconventional, we show that the measurement of rotational de-excitation cross sections brings several advantages to experiments that aim to study rotational energy transfer at temperatures approaching zero kelvin. We analyze the nature and partial wave composition of the quasi-bound states associated with each individual resonance and compute the scattering wave functions. The differential cross sections contain the partial wave fingerprints of the scattering process and are found to change drastically as the collision energy is varied over the resonances. The prospects for measuring these differential cross sections in inelastic de-excitation collisions at low energies are discussed.

Pulse Radiolysis Studies of Collisional Deexcitation of Ne(3P1) by H2

The cross sections for the deexcitation of Ne(3P1) by H2 have been measured as a function of the mean collisional energy in the range of 17.3-37.9 meV or in the temperature range from 134 K to 293 K using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. The deexcitation cross sections are in the range of 2.1- 5.6 Å2 for Ne(3P1) and nearly constant or increase slightly with increasing the collisional energy.Journal of Institute of Science and Technology, 2014, 19(1): 75-78

Rotationally inelastic dynamics of LiH (X(1)Σ(+), v = 0) in collisions with Ar: State-to-state inelastic rotational rate coefficients.

A theoretical study of rotational collision of LiH(X1Σ+,v = 0, J) with Ar has been carried out. The ab initio potential energy surface (PES) describing the interaction between the Ar atom and the rotating LiH molecule has been calculated very accurately and already discussed in our previous work [Computational and Theoretical Chemistry 993 (2012) 20–25]. This PES is employed to evaluate the de-excitation cross sections. The ab initio PES for the LiH(X1Σ+)-Ar(1S) Van der waals system is calculated at the coupled-cluster [CCSD(T)] approximation for a LiH length fixed to an experimental value of 3.0139 bohrs. The basis set superposition error (BSSE) is corrected and the bond functions are placed at mid-distance between the center of mass of LiH and the Ar atom. The cross sections are then derived in the close coupling (CC) approach and rate coefficients are inferred by averaging these cross sections over a Maxwell-Boltzmann distribution of kinetic energies. The 11 first rotational levels of rate coefficients are evaluated for temperatures ranging from 10 to 300 K. We notice that the de-excitation rate coefficients appear large in the order 10−10 cm−3 s−1 and show very low temperature dependence. The rate coefficients magnify significantly the propensity toward ∆ J = −1 transitions. These results confirm the same propensity already noted for the cross sections.

Correction of the near threshold behavior of electron collisional excitation cross-sections in the plane-wave Born approximation

The modeling of NLTE plasmas requires the solution of population rate equations to determine the populations of the various atomic levels relevant to a particular problem. The equations require many cross sections for excitation, de-excitation, ionization and recombination. A simple and computational fast way to calculate electron collisional excitation cross-sections for ions is by using the plane-wave Born approximation. This is essentially a high-energy approximation and the cross section suffers from the unphysical problem of going to zero near threshold. Various remedies for this problem have been employed with varying degrees of success. We present a correction procedure for the Born cross-sections that employs the Elwert–Sommerfeld factor to correct for the use of plane waves instead of Coulomb waves in an attempt to produce a cross-section similar to that from using the more time consuming Coulomb Born approximation. We compare this new approximation with other, often employed correction procedures. We also look at some further modifications to our Born Elwert procedure and its combination with Y.K. Kim's correction of the Coulomb Born approximation for singly charged ions that more accurately approximate convergent close coupling calculations.