Intersystem Transitions Research Articles

Molecular mechanisms of influence of slow electrons on biological structures

Background: The article reviews investigations carried out by biophysicists of the Transcarpathian Branch of the Ukrainian Biophysical Society, dedicated to the memory of the prominent Ukrainian biophysicist, Professor Yuri Pavlovich Blagoi. Objectives: Study of the peculiarities of physical processes and structural changes in nucleic acid molecules initiated by low-energy electrons (10-1-102 eV). Materials and methods: Molecular components of nucleic acids – nucleobases, nucleosides – were chosen as objects of research. A complex of methods is applied: the spectral method for obtaining the luminescence spectra of molecules excited by an electron impact; the electric method in determining the complete cross-sections of the formation of positive and negative ions; the method of gas-phase mass spectrometry with electron impact ionization. A new approach is proposed, which is based on the development of the methods for obtaining biomolecules in an isolated (gas) state, the formation of molecular beams, and the implementation of a crossed electron and molecular beams method. Results: The processes of excitation, ionization and fragmentation of molecules of nucleic acid bases under electron impact are investigated. The radiation spectra of biomolecules in the wavelength region from 200 nm to 500 nm for different energies of incident electrons are obtained. For identification of spectral bands, the excitation functions of biomolecules are investigated, mass spectrometry analysis of the bases is performed, semi-empirical quantum-chemical calculations of structural parameters are carried out. It is established that the complex superpositional nature of the spectra reflects the simultaneous occurrence of various physical processes, including, in addition to the direct excitation of the singlet and triplet states of the base molecules, the dissociation excitation and excitation of ionized biomolecules and their fragments. The analysis of the excitation functions shows the presence of intersystem transitions with the formation of triplet metastable states of the nucleic acid bases molecules. It is shown that the processes of the appearance of negative ions are of resonant nature and are accompanied by the dissociation of molecules even at energies smaller than the thresholds for excitation and ionization.

Accurate configuration-interaction calculation of transitions in Sn II

A large-scale Breit–Pauli configuration-interaction calculation has been undertaken of all possible 85 E1 optically allowed and intersystem transitions in Sn II among the 22 energetically lowest-lying levels. We have presented a discussion of a sequence of increasingly complex calculations, performed in the LS coupling scheme, in which we have satisfied the necessary criterion in order that we could be confident that a satisfactory convergence of all important valence and core–valence correlation effects had been attained. Judging by the extensive checks we have subjected our results to, we estimate realistic uncertainties in the region of 5% for the stronger transitions studied, achieving an accuracy required for the proposed applications in astrophysical and fusion research. It is encouraging that we can propose possible deficiencies in the theoretical models of previous calculations in an attempt to account for disparities with present data. Although the observed consistency between our results and the greater part of experimental measurements is a comforting indication of reliability, the analysis is not without its discrepancies. We would argue, however, that the present work, representing the most accurate data available to date, warrants further elaborate experimental study. Our calculated fine-tuned oscillator strength for the astrophysically important transition at 1400.5 Å fl = 1.205 ± 0.060 puts into question the conclusions of Sofia et al (Sofia U J, Meyer D M and Cardelli J A 1999 Astrophys. J. 522 L137) that they measured gas-phase ratios of Sn/H in the interstellar medium that appear to be supersolar.

Resonance and intersystem transition lines of astrophysical importance in neutral sulfur

A systematic study of S I levels belonging to 3s23p4, 3s3p5, 3s23p3(4So, 2Do, 2Po)nℓ, where nℓ = 4s, 5s, 6s, 4p, 5p, 6p, 3d, 4d, 5d, 4f and 5f configurations and optically allowed and intersystem transitions among them are presented. A very large set of basis functions are generated using up to three electron promotions from the ground configurations for each of the 24 symmetries considered belonging to two parities. Many of the levels are reported for the first time. Relativistic effects are included through the Breit–Pauli Hamiltonian. The resulting fine-structure levels and oscillator strengths for some astrophysically important transitions are then presented and compared with the available measured and calculated values. Excellent agreement between the present lifetimes and the measurement of Berzinsh et al (1997 Phys. Rev. A 55 1836) and Beideck et al (1994 Astrophys. J. 428 393) are achieved for the 3s23p3(4So)ns(3S1o), 4 ⩽ n ⩽ 6, 3s23p3(4So)4p(3P0,1,2o) and 3s23p3(2Po)4s(3P0,1,2o) levels, respectively.

Line strength measurements for near-infrared intersystem transitions of NI

Intensities of 15 intersystem transitions and 5 LS allowed multiplets of neutral nitrogen (NI) from infrared part of the spectrum were measured by emission spectroscopy method. All allowed as well as LS-forbidden spectral lines belong to the 3p-3d transition array. The nitrogen spectra were excited in a wall-stabilized high-current arc ran in helium with small admixture of nitrogen. On the basis of these line (multiplet) intensity measurements, transition probabilities of 15 intersystem lines were determined relative to transition probabilities of appropriately chosen allowed multiplets. Finally the relative data for intersystem transitions were normalized to an absolute scale by applying reference multiplet strength values recommended by the Atomic Spectroscopy Group of NIST. The determined line strengths are compared with available calculated and — in the case of 5 transitions — also with experimental data.

J-File Sum Rule Tests for the 3s-3p Transition Array in Singly Ionized Nitrogen - Comparison of Measured and Calculated Data

1. IntroductionIn recent years quite a large number of transition probabilities of spectrallines of light elements have been determined including data for singly ionizednitrogen [1{7]. Some of these experimental as well as theoretical studies providealso data for a few triplet{singlet intersystem transitions, being very interestinge.g. from astrophysical point of view. The energy level system of NII has adistinguished feature compared to the neighboring two members of the carbonisoelectronic sequence (CI, OIII) | the energy difierence between the excitedterms 3

Relative Transition Probability Measurements for Prominent Infrared Spectral Lines of NI

Applying a high-current wall-stabilized arc operated either in helium or in argon always with some admixtures of nitrogen, relative transition probabilities for more than 100 individual spectral lines (fine structure components) have been measured. The studied lines belong mainly to the 3p–3d and 3p–4s transition arrays. Ten lines of the measured set are intersystem transitions. Our data are compared with other experimental results (for about one half of the studied set there are available), with evaluated on the basis of the LS coupling scheme, with semiempirical data, and with recent CIV3 calculations. Some of our results are compared also with experimental data for the next member of the isoelectronic sequence (OII). Our new data on relative line strengths of NI may be very useful for interpretation of stellar spectra and helpful for the improvement of atomic structure codes, used in theoretical approaches devoted to oscillator strengths calculations.

Atomic transition probabilities of F I spectral lines from3s−3pand3p−3dtransition arrays

We have measured the relative transition probabilities of about $100 3s\ensuremath{-}3p$ and $3p\ensuremath{-}3d$ lines of neutral fluorine in the visible and near-infrared spectrum with a wall-stabilized high-current arc, which is operated under conditions very close to partial local thermodynamic equilibrium. The set of measured lines includes about 40 intersystem transitions. Our data have been placed on an absolute scale by normalizing several strong transitions to the results of the OPACITY Project calculations, which are expected to be quite accurate for such transitions. We estimate that the uncertainties of our absolute transition probability values are in the \ifmmode\pm\else\textpm\fi{}15% to \ifmmode\pm\else\textpm\fi{}20% range, while the uncertainties of the relative values do not exceed 6%, except for very weak transitions. Our results indicate that especially for $3p\ensuremath{-}3d$ transitions appreciable departures from LS coupling are encountered. Comparisons with other results indicate that earlier experimental data agree well with ours on a relative scale, but need to be renormalized.

Transition rates for the doublet-quadruplet intersystem lines in C II and N III

Radiative decay rates for the 2s22p 2P0-2s2p2 4P intersystem transitions at 233 nm in C+ and at 175 nm in N2+ have been calculated taking configuration interaction (CI) and relativistic effects into account with the help of the computer code SUPERSTRUCTURE. The present physical model yields satisfactory agreement between the calculated probabilities and recent measurements obtained by recording the time dependence of the photon intensities emitted from the metastable levels in the quartet system 2s2p2 4P. In particular, the large discrepancies between the new experimental data and previous theoretical results have been considerably reduced in the case of N III.

Intersystem transitions of interstellar carbon monoxide toward zeta Ophiuchi

Absorption from seven intersystem (triplet-singlet) transitions of interstellar (12)CO were detected in ultraviolet spectra of zeta Oph. The observed equivalent widths are approximately consistent with the transitions' predicted f-values and the (12) CO column density derived from the weakest of the observed A-X bands. These unsaturated intersystem transitions provide the opportunity to measure the (12)CO column density for heavily reddened (dense) sight lines. Laboratory measurements of oscillator strengths more precise than available ones will be needed to derive accurate column densities.

Measurements of radiative-decay rates of the 2s22p(2P degrees)-2s2p2(4P) intersystem transitions of C+

The radiative-decay rates of the 2[ital s][sup 2]2[ital p]([sup 2][ital P0])-2[ital s]2[ital p][sup 2]([sup 4][ital P]) intersystem transitions of C[sup +] ions have been measured by recording the time dependence of the [similar to]233-nm emission. A cylindrical radio-frequency ion trap was used to store electron-impact-produced C[sup +] ions. The time-dependent signals were analyzed by multiexponential least-squares fits to the data. The measured radiative-decay rates to the ground term are 146.4(+8.3,[minus]9.2) s[sup [minus]1] for [sup 4]P[sub 1/2], 11.6(+0.8,[minus]1.7) s[sup [minus]1] for [sup 4][ital P][sub 3/2], and 51.2(+2.6,[minus]3.5) s[sup [minus]1] for [sup 4][ital P][sub 5/2]. Comparison of the measured values with theoretical values is presented.

The Lyman-Birge-Hopfield bands in aurora

A numerical model of the luminosity of the Lyman-Birge-Hopfield bands in aurora is presented. A new method of determining the production rates of atmospheric species excited by auroral electrons has been applied in this model. A special feature of the suggested approach is the direct relationship between the initial electron flux and the production rate at the arbitrary altitude, which allows the solution of the electron transport equation to be avoided. The contribution of the intersystem transitions between the a 1∏ g , ′ 1 σ − u , w 1 Δ u states into the population of the vibrational levels of the a 1∏ g state is discussed. Good agreement between the theoretical results and experimental data is demonstrated.

Measurements of Transition Probabilities for Intersystems Lines of Atomic Ions Used in Diagnosis of Astrophysical Plasmas

In many low-charge ions of astrophysically abundant light elements, the spins of the ground and first-excited terms are different. Because of spin-orbit interactions, the states of these terms are mixtures of LS basis states, and as a consequence, transitions with ∆S = 1, i.e., ‘intersystem transitions’, can occur. The_transition probabilities (A-values) for such lines in low-Z ions are about 104 sec -1 to 102 sec-1. These values are of the same order of magnitude as the collisional de-excitation rates in many low-density astronomical objects. Consequently, intersystem lines, which are not readily seen in the laboratory, are significant features in many astronomical spectra. IUE spectra that show such lines in novae, hot stars, cool stars, symbiotic stars, binary stars, variable stars, Herbig-Haro objects, H II regions, planetary nebulae, quasars, and galaxies are discussed in Kondo et al. (1982). Moreover, because of the commensurate radiative and collisional de-excitation rates for the upper levels, the ratios of intersystem-line intensities to allowed-line intensities are density sensitive in many objects.

Intercombination lines of AlVIII, AlIX, and AlX ions

Several aluminum lines observed in the Princeton Large Torus tokamak discharges have been identified as intersystem transitions, establishing the energies of the Al VIII 2s2p/sup 3/ /sup 5/S, Al IX 2s2p/sup 2/ /sup 4/P, and Al X 2s2p /sup 3/P terms. Some observations of isoelectronic transitions in scandium and titanium ions are also reported.

The microwave spectrum of gauche-ethylamine

The microwave spectrum of the ground state of the normal species of gauche-ethylamine CH 3CH 2NH 2 and that of -NHD, -NDH, as well as -ND 2 isotopic species were measured and assigned. The ground state splits into four substates due to two internal large amplitude motions: inversion ( s and a) and internal rotation ( o and e) about the CN axis. Intersystem transitions due to tunneling as well as vibrational-rotational perturbations affect not only the absorption frequencies but also the Stark effect and NQHFS. The rotational constants for the two symmetrical inversion states ( s) were fitted for the normal species as (all values in MHz) A se = 32 423.470 ± 0.184, B se = 8 942.086 ± 0.039, and C se = 7 825.520 ± 0.048, and A so = 32 378.733 ± 0.182, B so = 8 940.906 ± 0.052, and C so = 7 825.551 ± 0.042 with the interaction constants Q a s = 151.12 ± 0.52 and Q b s = 44.4 ± 7.0. The antisymmetrical inversion states ( a) were fitted as A ae = 32 423.347 ± 0.142, B ae = 8 942.027 ± 0.029, and C ae = 7 825.525 ± 0.031, and A ao = 32 378.720 ± 0.142, B ao = 8 940.984 ± 0.029, and C ao = 7 825.573 ± 0.031 with the interaction constants Q a a = 167.10 ± 0.31, Q b a = 48.1 ± 5.4. The energy splitting due to intersion was determined (in MHz) as Δ ν inv = 1 391.39 ± 0.19 and that due to internal rotation as Δ ν tors = 1 170.58 ± 0.18. The cis barrier separating the two equivalent torsional states was calculated as 690 cm −1, and the inversion barrier between the inversion states was calculated as 1400 cm −1, both using the Dennison-Uhlenbeck model. A simple model explaining the inversion splittings of the monodeuterated species is proposed. Comparing the relative intensities for several temperatures the gauche form was observed to be energetically higher than the trans form by 110 ± 50 cm −1. The dipole moment could only be fitted by taking into account the internal motions yielding (in Debye) μ a eff = 0.11 ± 0.01, μ b eff = 0.65 ± 0.01, and μ c eff = 1.014 ± 0.015. The quadrupole coupling constants (in MHz) were found as χ aa = − χ + = 2.268 ± 0.043 and χ bb − χ cc = χ − = 3.120 ± 0.035.

B I Isoelectronic Sequence: Observation of 2s2p2-2p3 Intersystem Lines in Sc XVII and Improved Measurements for CI XIII, K XV, Ca XVI, Sc XVII, and Ti XVIII

Spectra of Cl through Ti (excluding Ar) were produced with 1 GW 15 ns) pulses from a Nd-glass laser impinging on solid targets, and observed with a 10.7 m grazing incidence spectrograph. Transition arrays 2sm2pn-2sm-12pn+1 in the B I isoelectric sequence were recorded. Two intersystem transitions 2s2p2 4P-2p3 2D of Sc XVII were identified, and improved wavelength measurements of allowed lines were made for each spectrum. From a least squares fit of radial energy integrals to Sc XVII energy levels, percentage compositions of the levels were obtained.

C I Isoelectronic Sequence: Observations of 2sm2pn-2sm-1 2pn+1 Intersystem Transitions and Improved Measurements for CI XII, K XIV, Ca XV, Sc XVI, Ti XVII, and V XVIII

Spectra of C1 through V (excluding Ar) were produced with a 1 GW (15 ns) pulse from a Nd-glass laser impinging on solid targets and observed with a 10.7-m grazing incidence spectrograph. Strong 2sm2pn-2sm-1 2pn+1 transition arrays in the C I isoelectronic sequence were recorded, from which intersystem lines were identified and improved wavelength measurements of allowed lines were made. Hartree-Fock calculations of the radial integrals were compared with those obtained from least squares fits of the energy levels of this sequence.

N I Isoelectronic Sequence: Observations of 2sm2pn-2sm-1 2pn+1 Intersystem Transitions and Improved Measurements for CI XI, K XIII, Ca XIV, Sc XV, Ti XVI, and V XVII

Spectra of Cl through V (excluding Ar) were produced with 1 GW (15 ns) pulses from a Nd-glass laser impinging on solid targets and observed with a 10.7-m grazing incidence spectrograph. Strong 2sm2pn-2sm-1 2pn+1 transition arrays in the N I isoelectronic sequence were recorded, from which intersystem lines were identified and improved wavelength measurements of allowed lines were made. Hartree-Fock calculations of the radial integrals were compared with values obtained from least squares fits of the energy levels of this sequence.

F I and O I Isoelectronic Sequences: Observations of 2sm2pn -2sm-12pn+1 Intersystem Transitions and Improved Measurements for CI, K, Ca, Sc, Ti, and V

Spectra of Cl through V (excluding Ar) were produced with a GW (15 ns) pulse from a Nd-glass laser impinging on solid targets and observed with a 10.7 m grazing incidence spectrograph. Strong 2sm2pn -2sm-12pn+1 transition arrays in the F I and O I isoelectronic sequences were recorded, from which intersystem lines were identified and improved wavelength measurements of allowed lines were made. Hartree-Fock calculations of the radial integrals were compared with those obtained from least squares fits of the newly derived energy levels to obtain improved predictions in the oxygen sequence by means of scaled Hartree-Fock integrals.

Microwave spectrum of acetaldehyde with asymmetric CH2D and CHD2 internal rotors

The microwave spectra of asym-CH2DCHO and asym-CHD2CHO have been investigated in detail in the range 8–40 GHz. Tunnelling of the methyl group between the two equivalent conformations causes splitting of the torsional ground state into a symmetric (0+) and antisymmetric level (0–). The rotational levels based on these torsional substates are perturbed by coupling between internal and overall rotation, causing deviations from rigid-rotor behaviour and the appearance of intersystem (0+↔ 0–) transitions. The spectral assignment was helped considerably by microwave–microwave and radiofrequency–microwave double-resonance techniques. A coupled-level Hamiltonian augmented by centrifugal distortion terms has been used to analyse the spectra. The pure torsional splittings of asym-CH2DCHO and asym-CHD2CHO have been determined to be 804.5(1) and 183.6(1) MHz, respectively.All five quartic centrifugal distortion constants have been determined for each torsional substate for both molecules. These constants agree well with values calculated from the published harmonic force field. Anomalies have been observed in the Q-branch, J1,J(∓)–J0,J(±) series of asym-CH2DCHO which are almost certainly due to near degeneracies with J= 16 and 17, Ka= 3 levels of sym-CH2DCHO. These accidental resonances indicate a lowering of the zero-point energy of sym-CH2DCHO relative to asym-CH2DCHO of 15.55 cm–1.

XUV spectra of the 1973 June 15 solar flare observed from Skylab. II - Intersystem and forbidden transitions in transition zone and coronal ions

An analysis is given of the intersystem lines of transition zone ions and high-temperature forbidden lines for the 1973 June 15 flare. The electron density in the erupting part of the plasma is at least approx.10/sup 13/ cm/sup -3/ at temperatures of approx.1.3 x 10/sup 5/ K. The density of the stationary component of the flare plasma at similar temperatures varies between approx.1.5 x 10/sup 11/ cm/sup -3/ and approx.10/sup 12/ cm/sup -3/. A density decrease observed in the stationary component occurs nearly simultaneously with an intensity enhancement of forbidden coronal lines such as Fe XII. These lines are believed to originate in arch-type structures. The electron density in the temperature regions approx.1.5 x 10/sup 6/ K is between 2 x 10/sup 10/ cm/sup -3/ and 2 x 10/sup 11/ cm/sup -3/. These regions appear to be stationary. Their motion relative to the observer is less than 4 km s/sup -1/. A continuous energy input is necessary to account for the transition zone emission. The transition zone emission arises in small regions, perhaps filaments.