Laser excited population redistribution in the 2p3p multiplet in neon

Using neon hollow cathode lamp and employing laser optogalvanic technique, we have studied population redistribution in the 2p⁵3p multiplet. The spectra recorded in the laser energy region of 23300 cm^− 1 -23600 cm^− 1 show transitions originating from both the laser excited levels as well as from an adjacent level whose population builds up as a result of collisional deactivation. Employing the optical delay technique, we have been able to extract decay rates associated with the collisional population mixing of the p levels.     

Anomalous behavior of optogalvanic signal in a miniature neon discharge lamp

A strong optogalvanic effect has been observed in a negative glow of a miniature neon discharge lamp using tunable pulse dye laser pumped by a copper vapor laser. A comparative study on temporal evolution of optogalvanic signal in a positive and negative dynamic resistance region of the discharge is described. Dye laser beam was tuned to various neon transitions 1s_i → 2p_j (Paschen notations) within 570-617 nm wavelength range. Anomalous behavior of optogalvanic signal was observed at 588.2 nm for (1s₅ → 2p₂) neon transition at low discharge current (<220 μA). This anomalous behavior is the attributes of damped oscillations of optogalvanic signal that correlate with negative dynamic resistance (dV/di < 0) of the discharge. Penning ionization at low discharge current and small energy mismatch is assumed to be the main cause of the negative dynamic resistance. Penning ionization process has been explained by resonantly ionizing energy transfer via collisions between neon buffer gas atoms in the lowest metastable state (1s₅) and electrode sputtered atoms in ground state using their partial energy level diagram.     

Rydberg transitions in neon observed by optogalvanic effect in the 830-870 and 380-420 nm regions

New optogalvanic (OG) Rydberg-Rydberg transitions of neon have been observed in the near-infrared region (830-870 nm), using a commercial Fe-Ne hollow cathode. They involve transitions from the 3d[3/2] J=1 and 3d[7/2] J=3,4 levels to high-lying nf levels. In addition, other OG transitions, observed in the blue range, have been completely assigned to ns, nd, ns′ and nd′ Rydberg series excited from the 3p[1/2] J=1 and 3p[5/2] J=2,3 levels of neon. These transitions and assignments allowed us to extend the range of tunable laser calibration on the two edges of the visible range, where there is a lack of available calibration lines, i.e. the near-infrared and the far-blue range, with a 0.01 nm absolute accuracy.     

Optogalvanic detection of velocity-selective optical pumping in an open, cascade atomic medium

We performed two-color spectroscopy of the (4s²) ¹S₀ → (4s4p) ¹P₁ → (4p²) ¹D₂ calcium atomic transition and observed velocity-selective optical pumping in a calcium hollow cathode lamp by means of optogalvanic detection. The optical pumping signature in optogalvanic detection is compared to that of fluorescence and transmission detections. The optogalvanic technique is found to be a very sensitive method of detecting optical pumping and could be used in distinguishing optical pumping from electromagnetically induced transparency.     

Measurements of electron density, temperature and photoionization cross sections of the excited states of neon in a discharge plasma

In the present work emission and absorption spectroscopy have been used to determine the plasma parameters of neon in a hollow cathode discharge lamp. The excitation temperature is determined using the intensity ratio method and Boltzmann's plot method whereas the electron density is determined from the Stark broadening of the spectral lines. The behavior of the optogalvanic signal as a function of laser energy has been studied for three transitions from the 2p⁵3s [1/2]₂ metastable state following ΔJ=ΔK=0, ±1 dipole selection rules. The saturation technique has been used to measure the photoionization cross section from three intermediate states 2p⁵3p′ [1/2]_1, 2p⁵3p′ [3/2]₂ and 2p⁵3p [5/2]₃ up to the 2p^{5 2}P_1/2 ionization threshold.     

Relationship of the laser-induced population perturbation of excited levels with the dynamical optogalvanic signal in a discharge plasma

For a pulsed-laser excitation of various neon transitions (1s_j → 2p_k) in a glow discharge the population perturbations in the upper and lower levels are measured by emission and absorption spectroscopy, and the dynamical optogalvanic signals are observed. We propose that the population perturbation in the lower levels (1s₂–1s₅) as a whole is responsible for the optogalvanic signal, and that metastable-level populations determine its decay characteristics. The sign reversal of the optogalvanic signal that depends on the excitation condition is interpreted in this context.     

One- and two-photon laser optogalvanic spectroscopy of neon in the 570-626 nm region

Eleven two-photon transitions originating from the 2p⁵3s[3/2]₂, 2p⁵3s′[1/2]_o, 2p⁵3s[3/2]₁, and 2p⁵3s′[1/2]₁ states to the 2p⁵4d configuration states have been investigated in the optogalvanic spectrum of neon in the visible region (570-626 nm) for the first time. The two-photon assignments are confirmed by evaluating the temporal evolution, power dependency, and line widths of the optogalvanic signals. The time evolution of the optogalvanic signals for the two-photon transition originating from the metastable 2p⁵3s[3/2]₂ state to the 2p⁵4d′[3/2]₂ state has also been studied at different discharge currents.     

Spectroscopic characterization of the potential energy functions of Ne2 Rydberg states in the vicinity of the Ne(S0) + Ne(4p′) dissociation limits

The gerade autoionizing Rydberg states of Ne₂ have been studied in the range 162 000-172 000 cm⁻¹ by 1 + 1′ resonant two-photon excitation from the Ne₂ X ground state via different vibrational levels of the Ne₂ C state. A rotationally resolved part of the spectrum allowed the determination of the potential energy functions of two states of 1_g and characters in the vicinity of the Ne(2p⁶¹S₀) + Ne (2p⁵4p′) dissociation limit. The presence of maxima in these potential energy functions is interpreted as originating from a repulsive interaction between the Rydberg electron and the neutral atom.     

On the Time‐Resolved Optogalvanic Spectra of Neon and Krypton

In this work time resolved optogalvanic signals associated with transitions excited from the first metastable state of neon and krypton have been studied. These gases have similar energy state configurations and it is of significant interest to study their time resolved optogalvanic waveforms resulting from transitions belonging to the states of same quantum numbers. The experimentally observed optogalvanic signals recorded for different discharge currents have been fitted to a theoretical model to obtain parameters that determine amplitudes, instrumental time constants and decay rates of the 1s levels. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Laser optogalvanic spectroscopy of argon in the wavelength region 605–740 nm

Two-photon optogalvanic transitions in Ar glow discharge with Nd: YAG laser pumped dye laser excitation in the frequency range 13520–16520 cm⁻¹ has been studied using linear and circular polarization. The intensities of two-photon optogalvanic transitions are very sensitive to changes in the incident laser power which is not the case with one-photon transitions. Intensity ratio for circular and linear polarized light for two photon transitions 6s′[1/2]°1←4s[3/2]°2, 6s′[1/2]°0←4s[3/2]°2, and 5d[1/2]°0←4s[3/2]°2, 5d[1/2]°1←4s[3/2]°2 are quite different from the other two-photon transitions. This has been explained as due to near one photon resonance of 4p′[3/2]1 level for the first pair and 4p′[1/2]1 for the second pair of transitions. The ratio of optogalvanic intensity for circular to linear polarized light has been theoretically estimated and compared with the observed results.     

Modification of spectral characteristics and optogalvanic response in neon hollow cathode under laser excitation

The changes in emission characteristics of a neon hollow cathode discharge by resonant laser excitation of 1s ₅→2p ₂ and 1s ₅→2p ₄ transition have been studied by simultaneously monitoring the optogalvanic effect and the laser induced fluorescence. It has been observed that resonant excitation causes substantial variation in the relative intensities of lines in the emission spectrum of neon discharge.     

Specific mass shift of potassium 5P1/2 state

We have measured the isotope shift between ⁴¹K and ³⁹K in the 4s_1/2 → 5p_1/2 transition at 405 nm using saturation spectroscopy. Our measured isotope shift is 456.1 ± 0.8 MHz, implying a residual isotope shift (sum of specific mass shift and field shift) of −52.7 ± 0.8 MHz. We deduce a specific mass shift of −40 ± 5 MHz, which would imply that the 5p_1/2 state has a considerably larger specific mass shift than the 4p_1/2 state. We have in addition measured the 5p_1/2 hyperfine splitting for ⁴¹K.     

The study of dominant physical processes in the time-resolved optogalvanic spectra of neon

We present the dominant physical processes responsible for the production of the optogalvanic signal in the spectra of neon. We have investigated the effects on the optogalvanic signal by scanning a dye laser across the neon transitions in the DC discharge plasma. Time-resolved spectra are obtained at a fixed wavelength of the dye laser resonantly tuned to an optically allowed transition. The temporal evolutions of the signals are registered on a storage oscilloscope. Three transitions from the 3s[1/2]₂ metastable state corresponding to the ΔJ = ΔK = 0, ±1 dipole selection rules have been selected to investigate the dominant physical processes responsible for the optogalvanic signals. The change in the signal amplitude as a function of the discharge current has been registered. In addition the electron collisional ionization rate parameter ratios have been determined for the transitions corresponding to dipole selection rules ΔJ = ΔK = -1, ΔJ = ΔK = 1 and ΔJ = ΔK = 0, as 1.63, 1.75 and 1.0 respectively. The effective lifetimes of the upper levels involved in the aforementioned transitions are also calculated as 62.5 μs, 31.25 μs and 12.85 μs respectively.     

Symmetry and structure of N–O shallow donor complexes in silicon

Shallow donors in silicon related to nitrogen–oxygen complexes have been investigated by piezospectroscopy of their hydrogenic transitions in the far infrared. Complete stress dependences up to 0.25 GPa were obtained for the 1s→2p₀ and 1s→2p_± transitions of the most prominent members of the (N, O)-family, N–O-3 and N–O-5. Very unusual for shallow donors in silicon, the symmetry of the ground state wave function is T₂-like. The lifting of orientational degeneracy for stress in the 〈1 0 0〉, 〈1 1 1〉, and 〈1 1 0〉 directions is compatible with a C_2v defect symmetry. Data from the other species of the (N, O)-family are indicative for the same symmetry. The microscopic structure of these centers, in part contradictory to present theoretical models, is discussed.     

Two-photon enhanced optogalvanic signal and population inversion

Two-photon enhancement of the optogalvanic signal is shown to be possible, both theoretically and experimentally, even in the case of population inversion of the first transition. This is shown, in particular, for the neon 1s₂ → 2p_i transitions.     

Electronic energy levels of lithium gadolinium tetraphosphate (LiGdP4O12)

The positions of 15 electronic energy levels above the ground level ⁸S_7/2 of Gd³⁺ in flux-grown single crystals of LiGdP₄O₁₂ have been determined from 4f–4f transitions observed in absorption spectra in the range 190–340 nm at T = 293 K. The experimental energy levels have been compared with theoretical ones obtained by two parameters, F₂ = 392 cm⁻¹ and ζ_4f = 1525 cm⁻¹, in the intermediate coupling approximation.     

N2O emission in the 4.5 μM region: high excitation of the bending mode transitions v1v2v3→v1v2(v3−1) with (2v1+v2)=5

Eight emission spectra of pure N₂O and N₂O + N₂ + He mixtures excited by a radio frequency discharge were recorded by Fourier Transform Spectroscopy at a resolution of 0.005 and 0.004 cm⁻¹ in the 4.5 μm region. Results (wavenumbers, band centers, and spectroscopic constants) concerning nine new vibrational transitions which have not been observed before, and which occur between highly excited levels of the bending mode are reported. The derived spectroscopic parameters allow us to reproduce the experimental wavenumbers with an RMS error lower than 4.5 × 10⁻⁴ cm⁻¹.     

Improved infrared and visible emission spectra of the H3 and D3 molecules

Better-resolved Rydberg-Rydberg emission spectra of the neutral H₃ and D₃ molecules in the infrared and visible regions, with less interference from H₂ and D₂ emission, have been obtained by using a Droege-Engelking type of corona discharge source. Using nl_λ notation, the lower electronic states are 3p₁ in the infrared and 2p₀ in the visible, and the upper electronic states are mixed (3s,3p₀,3d₀,3d₁,3d₂) states. In particular, a line near 16 842 cm⁻¹ in H₃, previously obscured by an H₂ line, reveals a (3s,3d) interaction that is confirmed by other lines. The spectra are analysed including this interaction. However, fits to effective Hamiltonians still have relatively large standard deviations, probably partly due to poor convergence of the rotational expansions and partly due to many small perturbations of the levels by background states.     

Oxidation of a cesium-covered Ni (1 1 0) surface studied by metastable-induced electron spectroscopy

An initial stage of oxidation of a cesium-covered Ni (1 1 0) surface has been studied by metastable-induced electron spectroscopy (MIES) and low-energy electron diffraction (LEED). The MIES brought spectra with Cs 6s induced peak (P_6s), Cs 5p (P_5p), O 2p induced peak (P_ox) and a structure related to the substrate Ni 3d states (P_3d). The work function change Δφ showed an oscillatory behavior in the progress of surface oxidation. The process is divided into three stages: (i) at low O₂ exposures, Δφ > 0 with unchanging P_5p and P_6s; (ii) at moderate exposures, Δφ < 0 with a drastic decrease in the P_6s intensity; (iii) at higher exposures, Δφ > 0 with shifts of peaks P_5p and P_ox to higher energies, together with an appearance of peak P_3d. A three-step model of initial oxidation of alkali-covered Ni (1 1 0) surfaces is presented.