Spin forbidden transitions in the Ka = 0 subband of NO2 in the λ = 592.5 nm region

In a high resolution laser excitation spectrum of NO₂, lines were recorded which do not follow the selection rule ΔN = ΔJ = ΔF of “spin allowed” transitions. Line positions and intensities of these “spin forbidden” lines were investigated for all rotational lines up to N″ = 12 of the K_a = 0 subband around λ = 592.5 nm. While the observed line intensities of “spin allowed” transitions can be well described by the J-coupling scheme, neither the J- nor the G-coupling scheme sufficiently describes the “spin forbidden” transitions. The observations can be fitted satisfactorily by perturbation theory, in which the Fermi interaction in ²A₁ is treated as the perturber. This looks similar to a superposition of J and G scheme in the ²A₁ ground state.     

The energy of thermal electrons in electron beam created helium discharges

We have measured the electron energy of the thermal group of electrons in both longitudinal and transverse electron beam created helium glow discharges. The measurement technique employs the ratio of intensities of spectral lines in the 2s³S?np³P He I series. Values of kT_e between 0.07 and 0.11 eV were obtained. These energies are typical of the beam-generated electric field free plasmas. The competitive loss of helium ions by recombination and by charge transfer in a He?Hg electron beam created plasma is calculated. The results are applied to the Hg⁺ laser pumping scheme using a electron beam created He?Hg plasma.     

Spectroscopic diagnostics of carbon and aluminum laser-produced plasmas

VUV emission spectra of plasmas produced by focusing laser radiation with intensity of 10¹⁰–10¹¹ W/cm² on carbon and aluminum targets were studied. Using the partial local thermodynamic equilibrium model for an electron density exceeding 10¹⁷ cm^?3, the spectroscopic diagnostics and the analysis of ion composition of plasmas were carried out. The electron temperatures determined for carbon and aluminum plasmas from the ratio of intensities of ionic lines were found to be 8±3 eV and 11±4 eV, respectively. Stark broadening of aluminum lines was measured and parameters of electron broadening were determined. Using the spatially resolved measurement of Stark line broadening, the spatial density distribution and the law of electron gas expansion were found. The electron gas in the hot region of size 5 mm with an average density of (5±2) 10₁₇cm ^?3 experienced one-dimensional expansion according to the law 1/z ^1.1 with increasing distance z from the target.     

A line list of allowed and forbidden rotational transition intensities for water

Pure rotational lines are important for monitoring water concentrations in many environments both in space and on earth. A list of line intensities of rotational transitions for H₂¹⁶O is calculated using variational nuclear-motion wave functions and an ab initio dipole moment surface. This methodology should be equally reliable for both allowed and forbidden rotational transitions. Extensive comparisons are made with available intensity data for these transitions including the HITRAN and JPL databases. Problems are identified with some of these data. A list of 555 allowed and 846 forbidden rotational transition lines within the ground vibrational state is made available.     

Spectroscopic study of microwave discharge as a radiation source for transition probability measurements

The optical properties of a discharge in a mixture of helium (200 torr) and hydrogen (0.1 torr) were investigated. Line intensities were measured photoelectrically. Partial LTE is demonstrated by the linearity of plots of log(Iλ/gA) vs E. The temperatures T_He = 3300 and T_H = 3400 K are obtained from the slopes of the plots for He and H, respectively. Relative A-values for the hydrogen Balmer lines derived from the helium temperature agree with theoretical values within ±20 per cent. The uncertainties (≤ 7 per cent) estimated from the discrepancy between the temperatures are smaller than those (±10 per cent) due to errors in the observed line intensities.     

Characterization of compact bright soft X-ray source based on picosecond laser plasma

Radiation emission of silicon and aluminum plasmas produced by 40-ps laser pulses with peak intensity above 10¹⁴ W/cm² was studied. High-resolution soft X-rayspectra of H-like and He-like ions were analyzed to determine plasma parameters. We compared the line shape of resonance transitions and their intensity ratios to corresponding dielectronic satellites and the intensities of the intercombination lines of He-like ions with the results of model calculations. Such comparisons gave average values of the electron number density N_e=(1-1.9)×10²¹ cm^-3 and the electron temperature T_e=460–560 eV for Si plasmas and about 560 eV for Al plasmas produced by the first and the second laser harmonics. The plasma size is about 100 μm. According to our estimations, more than 10¹² photons were produced within the resonance line spectral width and in the solid angle 2π during the total decay period. PACS 41.50.+h; 52.25.Os; 52.50.Jm     

2-D electron spin transient nutation spectroscopy of Lanthanoid ion Eu2+(8S7/2) in a CaF2 single crystal on the basis of FT-pulsed electron spin resonance spectroscopy: Transition moment spectroscopy

This work demonstrates the usefulness of pulsed electron spin resonance (ESR)-based two-dimensional electron spin transient nutation (2-D ESN) spectroscopy for complete assignments of complicated fine-structure hyperfine ESR spectra including hyperfine forbidden transitions from electronic and nuclear high-spin systems. The 2-D ESN spectroscopy is termed transition moment spectroscopy as spectra are acquired as a function of transition moment instead of transition energy used in conventional spectroscopy. We have applied the novel spectroscopic technique to Eu²⁺ ion (S=7/2,I=5/2), which has two isotopes (¹⁵¹Eu [47.9%] and¹⁵³Eu [52.1%]), in a CaF₂ single crystal as a model system. We have completely identified the complicated fine-structure hyperfine ESR spectra by invoking the spectral simulation of the 2-D ESN spectra on the basis of transition moment analyses. The analyses are based on exact numerical calculations of the transition moments as well as a perturbation-based analytical approach combined with reduced rotation matrices for the nuclear part of the transition moment. This is the first example of the spectral simulation for 2-D ESN spectra including the hyperfine allowed and forbidden transitions in high-spin systems. In addition, we have made simulation of the fine-structure forbidden transitions, which reproduces the angular variations of the observed spectra at liquid helium temperatures.     

Inelastic electron scattering in nickel

Using a 200 keV electron spectrometer, with an energy resolution of ～0.25 eV and a momentum resolution of ～0.2 A^-1, we have measured the energy loss spectra for transmission of electrons through thin (～600 Å) Ni films. These results address the general question of the validity of momentum transfer estimates in electron loss scattering.Using low-energy electron backscattering, we have observed the dipole forbidden M₁ transition at 112 eV. For high-energy scattering, we have observed this transition only at high momentum transfer (q? 2 A^-1). These results indicates sizable contributions from high momentum transfer collisions in the low-energy experiments.     

CO2-laser gas discharges in narrow gaps

We have studied RF discharges as excitation mechanisms for distributed feedback (DFB) CO₂ lasers. For CO₂ laser plasmas the reduced electric fieldE/N has to be in a well-defined range. The reduced electric fieldsE/N of gas discharges in the narrow gaps with widths of the order of 100 m required for DFB are considerably above this range. In order to study the feasibility of these RF-excited discharges for DFB CO₂ lasers we have measured the electron temperatureT _e in their plasmas. From helium-line-intensity ratios we have deduced a lower limit of the electron temperatureT _e of 4eV. The observed high intensities of bands of singly ionized nitrogen indicate an even higher electron temperature, but an efficient pumping of the upper laser level is not possible with an electron temperature above 2.5 eV.We have estimated the electron densityn _e and the current densityj _e from ratios of the intesities of forbidden and allowed helium lines. The high current densityj _e is in the range of abnormal glow discharges.In the gas discharges between narrow gaps the electron oscillation amplitudex _e is large than the electrode separationd. In order to replace the resulting high electron losses a high electron temperatureT _e is necessary to sustain the gas discharge. Because of this high electron temperatureT _e an efficient pumping of the upper laser level is not possible.     

Determination of transition probability for the 655-nm Tl line

Studies of high-presure Hg-TlI a.c. (50 Hz) arc plasmas have been used to verify the validity of Boltzmann statistics at the moment of maximum electron density (5 ms) by applying LTE criteria. For a known plasma temperature, the transition probability of the optically-thin 655-nm line of Tl was derived from emission measurements by using the self-reversed 535-nm line of Tl as reference [A₆₅₅ = (3.74±0.37) × 10⁶ s^-1].     

Kinetics of pulsed cathodoluminescence

The kinetics of pulsed cathodoluminescence, which appears upon irradiation of condensed media with high-power electron beams of nanosecond duration, is analyzed. Four types of emission are considered: recombination emission, exciton emission, intracenter emission, and emission of electron and hole centers. It is found that the maximum difference in the kinetics of the intensities of emission of these types occurs in the time interval t≤t ₁≈A ^?1, and, in the far afterglow, an exponential decay occurs with the characteristic time (2A)^?1 for the first and second types of luminescence and A ^?1 for the fourth type. For the intracenter luminescence, the decay time is equal to (2A)^?1 and A _i ^?1 for short-lived and long-lived radiative levels, respectively (A and A _i are the probabilities of linear recombination of electron-hole pairs and decay of a radiative level, respectively). In the case of long-lived radiative levels, a strong peak is observed in the spectrum of intracenter luminescence in the far afterglow.     

Observation of hydrogen and helium satellites in a turbulent plasma

Time-resolved measurements of satellites about the (1s2s?1s3d) forbidden transition in helium, satellites both of the H_δ line of hydrogen and of its splittings and, finally, satellites of He II 4868 Å have been observed. The behavior of these satellites is correlated relative to each other and to previously reported satellites both on H_β and near the 6632 Å (1s2p?1s3p) forbidden transition in helium. The appearance of the satellites is attributed to a strong field interaction between orthogonal components of the dynamic and quasistatic electric fields generated in a plasma by an instability.     

Observations of MeV ions emitted from a laser produced plasma

Fast-ion emission from tungsten plasmas has been studied with neodymium laser intensities from 5x10¹² to 2x10¹⁵ W/cm². Ion energies into the MeV region have been observed. The variation of ion energy (E_I) with laser intensity (I) was found to be of the form E_I∞I^0.65.     

Third-order theory of the line intensities in the allowed and forbidden vibrational-rotational bands of C3v molecules

A third-order theory of the intensities of the allowed and “forbidden” (perturbation-allowed) transitions to the fundamental vibrational levels of C_3v semirigid molecules has been worked out by using the method of contact transformations applied to the electric dipole moment operator. Explicit expressions have been obtained for the linestrengths of the allowed (Δk = 0, ±1) as well as forbidden (Δk = ±2, ±3, ±4) transitions from the ground vibronic state to the fundamental vibrational levels of C_3v molecules. The treatment takes into account all the important Coriolis and anharmonic interactions in a C_3v molecule, including the effect of the “2, 2” and “2, −1” l-type interactions and the Δk = ±3 interactions on the intensities of the allowed and forbidden vibrational-rotational transitions. The expressions for the linestrengths of the allowed and forbidden transitions are given here in a form suitable to fit the experimental data on the intensities in the vibrational-rotational spectra of C_3v molecules.     

Electron temperature measurements in low-density plasmas by helium spectroscopy

Methods of measuring the electron temperature in low-density plasmas by He spectroscopy are examined. These utilize either the relative intensities of singlet and triplet lines or the absolute intensities of single lines. Calculations from measured and theoretical data show that both methods are seriously influenced by secondary processes, the most important of which are excitation from the metastable levels 2¹S and 2³S, and excitation transfer in electron-atom collisions combined with imprisonment of resonance radiation. The calculations give parameter limits for the validity of different methods and combinations of lines. Due to the secondary processes, the determination of T_e from relative line intensities is restricted to low-density, short-duration plasmas (typically n_e < 2 × 10¹⁶ m^-3, t_ex < 5 × 10^-6 s) or to even lower densities that depend on the apparatus dimensions (typically n_e < 3 × 10¹⁵ m^-3, L ≈ 0.1 m). For the determination of T_e from absolute line intensities, the situation is more favourable and, with a suitable choice of lines, typical restrictions on n_e and t_ex are n_e < 5 × 10¹⁷ m^-3, t_ex < 10^-5 s or n_e < 10¹⁷ m^-3, L ≈ 0.1 m for electron temperatures above 10 eV. For temperatures below 10 eV and degrees of imprisonment below 7% measurements are possible for electron densities up to 10¹⁹–10²⁰ m^-3, without any limits on t_ex or L.     

Photothermal ionization via excited states of sulfur donor in silicon

Structures in the photoionization cross-section spectra below the extrinsic edge of the doubly charged sulfur donor (613 meV) are attributed to the two-step photothermal excitation process in which the bound electron at the ground state first makes an optical transition to an excited state and it is then thermally released from the excited state to the conduction band. A weak peak (cross-section 7 × 10⁻¹⁹ cm²)at 425 meV is attributed to the intervalley optical transition 1s(A₁)→1s(T₂). Peak observed at 570 meV (10⁻¹⁷ cm²) is attributed to the 1s(A₁→2p₀ intervalley optical transition and the peak at 591 meV (3 × 10⁻¹⁷ cm²) to the 1s(A₁)→2p± intravalley optical transition. Data for electron bound at the neutral gold center has no structures which is consistent with the lack of excited states of a neutral impurity potential.     

Negative ions Ar<Superscript>−</Superscript>, Kr<Superscript>−</Superscript>, and Xe<Superscript>−</Superscript> in superfluid helium

The catalogue of negative ions in superfluid helium has been extended using the example of Ar⁻, Kr⁻, and Xe⁻. Such objects cannot exist in vacuum, since the polarization attraction of an electron to the inert A atom is insufficient for the formation of the bound state A⁻. However, these objects exist in helium as stable or metastable with a very long lifetime. The effect is due to the electron localization in liquid helium. If a mixture of excited A* atoms and electrons is prepared in the gas phase above liquid helium, the reaction A* + e = A*⁻ becomes possible for all atoms of the periodic table. Such charges can be immersed into liquid helium by the electric field. In this case, the radiative decay A*⁻ = A + e allowed in vacuum can be forbidden in liquid. This leads to the formation of the new unique objects A⁻, which can exist in liquid helium but are absent in nature. The size of such charged formations has been determined and is close the radius of a usual electron bubble in helium.     

Resonance Raman scattering in InAs near the E1 gap

Measurements of resonance Raman scattering in InAs at 77°K near the E₁ gap have been extended to 2.73 eV. The peak in the resonance curve appears at about 2.66 eV, 70 meV above the optical gap, and gives a larger temperature shift of the resonance than previously reported. Resonance lineshapes are obtained for allowed TO and LO and for forbidden LO phonon scattering. The forbidden scattering intensities are consistent with selection rules predicted for linear q-dependent and/or surface electric field induced scattering mechanisms.     

Hydrogen maser atomic beam resonances

Au^? ions at anionic places are formed in gold doped crystals by a reducing treatment withF centers. The ultraviolet absorption consists of 4 bands, which are namedA, B, C, andD in analogy to the isoelectronic centers of the s² type, like Tl⁺. TheB band oszillator strength strongly increases with temperature in accordance with a phonon allowed transition. The ratio of the dipole strength of theC band to that of theA band as a function of the relative position of theB band is compared with Suganos prediction. Zero phonon lines are found at helium temperatures for theA band in NaCl (2,985 Å), KCl (3,068 Å), and KBr (3,145 Å) and for theC band in KCl (2,329 Å). In KCl the Huang-Rhys factor isg=3.4 for theA band. The vibronic structure comes from the relatively large radius 6s ² state of the negative ion. Uniaxial stress splits the zero phonon line. The results definitely agree with the stress splitting behaviour of a degenerateΓ ₁→Γ ₄ transition. Inversion symmetry of the center is confirmed by the absence of a linear Stark effect.     

Variation of the ion dynamics parameter in stark-broadened helium lines

Stark broadening of quasidegenerate He lines at λ = 447 nm and 492 nm hasbeen investigated at low electron densities (N_e = 10²¹ and 3 x 10²¹ m^-3). The perturber mass was varied by using H⁺, He⁺ and Ar⁺ ions. Variation of the parameter (T₀/μ)^1/2 by a factor of 3 is accompanied by marked ion-dynamical effects in the forbidden component. The results are compared with calculations according to the unified theory and the model microfield method. Characteristic deviations are observed and critically discussed.