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.     

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.     

Experimental evidence of non-inverted population in a neon hollow-cathode

By the use of two step excitation from the (2p⁵ 3s)¹P₁ state of neon very large enhancements of the optogalvanic signals were obtained. This proves that no inverted population between the levels 2p_1,2,3,4,5 and ¹P₁ occurs as it was proposed in a recent paper. A new explanation of the inverted optogalvanic signals obtained for transitions originating from this ¹P₁ state is proposed.     

The study of the 1s4-2pj optogalvanic transients in a neon discharge plasma

Time dependent optogalvanic signals induced by the 1s₄ → 2p_j laser excitations have been studied in neon DC plasma. The decay rates related to all the four 1s_i levels have been derived by fitting the waveforms with a mathematical rate equation model. The temporal signatures of three transitions namely 638.3, 650.7 and 724.5 nm related to the 2p₇, 2p₈ and 2p₁₀ upper levels, respectively, have been found to be different from the rest of the transitions. We relate these effects to the population redistribution of decaying channels and to the processes responsible for the optogalvanic effect.     

Chopped CW laser-induced optogalvanic effect in a neon hollow cathode discharge

A detailed model for the optogalvanic effect in a neon hollow cathode discharge irradiated by a chopped CW dye laser is presented. A rate equation formalism is used to calculate the evolution of the first and second electronic configuration populations coupled by the laser and of the electric charges number density. Processes as ambipolar-like electrons loss, electronic collisional coupling of level populations and electron emission by the cathode due to VUV radiation from the 1s ₂ resonant level are taken into account and further discussed.The transients and steady-state magnitude of the optogalvanic signal are calculated, compared with experimental data and related to population changes. We predict sign changes of the optogalvanic signal when the laser is tuned over transitions originating from the resonant level with respect to transitions involving the metastables states. The optogalvanic signal is shown to be basically determined by the laser-induced variations of the excited-state populations while changes in the electron temperature, due to laser energy transfer by collisions between electrons and excited atoms, play a negligible role.     

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.     

Population of 2p 55s levels of neon in He-Ne plasma: I. The evolution of mechanisms in the discharge and the afterglow

The spectroscopic study of population processes of neon 2p ⁵5s states was carried out in helium afterglow with small admixture of neon at PHe = 38.1 torr, [He]/[Ne]=10^?5 with pulsed discharge afterglow in helium with small admixture of neon (pressure equal to 38.1 mm Hg; ). It is established that the main mechanism of population of 3s ₂ level (in Paschen’s notation) in the discharge and the initial after-glow is the excitation transfer from metastable atoms of He(2¹ S ₀). The other three levels—3s ₃, 3s ₄, and 3s ₅—corresponding to 2p ⁵5s configuration are populated in the afterglow as a result of the dissociative recombination HeNe⁺ of ions with electrons. The same process is also the main channel of population of 3s ₂ level in the late afterglow phase, when the concentration of He(2¹ S ₀) atoms is small. The hypothesis of recombination mechanism is confirmed by observation of the response of line intensities to pulsed electron heating. The partial coefficients of dissociative recombination into 2p ⁵5s states are estimated.     

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.     

Absolute cross sections for the excitation of the 1s 2- and 1s 4-levels of neon and argon by proton impact (100 keV to 1150 keV)

The excitation of the 1s ₂ and 1s ₄ levels in neon and argon has been studied by proton impact in the energy range of 100 to 1150 keV. The results are compared with other experimental and theoretical data.     

Excitation of the 1s 2- and 1s 4-levels of neon and argon by He+-impact at energies between 100 keV and 1000 keV

The excitation of the 1s ₂- and 1s ₄-levels of neon and argon by He⁺-impact has been studied in the energy range of 100 keV to 1000 keV. The total emission cross sections for the 1s ₂ levels show pronounced maxima at about 350 keV impact energy, whereas the cross sections for the 1s ₄ levels increase with decreasing energy.     

Inner-shell excitation and EXAFS-type phenomena in the chloromethanes

Small angle inelastic scattering of 2.5 keV electrons was used to study the inner-shell excitation of CH₄, CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄ and C₂H₅Cl in the regions of carbon 1s, chlorine 2p and chlorine 2s excitation. Structure observed below the carbon 1s ionization threshold in each molecule is assigned to promotions of a carbon 1s electron to unoccupied valence and Rydberg orbitals. Trends in the distribution of spectral intensities through the series of chloromethane carbon 1s spectra are discussed in terms of the growth of a potential barrier. Broad features are observed in the chlorine 2p continua of CH₂Cl₂, CHCl₃ and CCl₄ and the carbon 1s continuum of CCl₄ which are assigned as the energy loss equivalent of extended X-ray absorption fine structure (EXAFS).     

Relaxation of the Ne(2p1) level and populations of Ne(1s2) and Ne(1s4) in an afterglow

Radiation trapping of the transition 2p₁-1s₂ of neon has been demonstrated during laser-induced fluorescence in a pulsed microwave discharge through neon. The population of Ne(1s₂) atoms is deduced. Selective excitation is useful for the determination of lifetimes and cross sections of collisions with neutral atoms. The results may be utilized in processes for the creation of metastable atoms.     

On the possibility of laser generation without inversion on the fine-structure levels of a helium atom

It is shown that laser generation is possible in principle upon coherent excitation according to the V-scheme of the (1s2p)³ P _j levels with j = 1, 2 of a helium atom from the (1s2s)³ S ₁ level of the same atom when the latter level is, in turn, populated by electron or proton impact.     

Contribution of hexadecapole alignment to the polarization of quadrupole emission under laser excitation conditions

The linear polarization of quadrupole emission by the J=2-J ₀=0 transition under conditions of laser excitation in a gas medium is studied. Cases of excitation through dipole and quadrupole absorption of monochromatic laser radiation are considered. Taking into account the anisotropy of collisional relaxation, the contributions of polarization moments of the second and fourth ranks, i.e., those with usual and hexadecapole alignments, to the signal of linear polarization of quadrupole emission are calculated. The dependence of this signal on the laser frequency, the density of the gas medium, and the angles determining the orientation of the system of axes of observation of light polarization relative to a laser ray is studied. The numerical calculations of contributions of usual and hexadecapole alignment to the signal of linear polarization are made for the magnetic quadrupole transition J=2-J ₀=0 between the states 2p ⁵(² P ₁ ^2/0 )3p′[3/2]₂ and 2p ⁵(² P ₁ ^2/0 )3s 3s′[1/2]⁰ of neon atoms in the xenon atmosphere.     

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.     

Vibronic structure of the 3s Rydberg state of the 2-methylallyl radical

Resonance-enhanced multiphoton ionization combined with electronic ground state depletion spectroscopy of jet-cooled 2-methylallyl (C₄H₇) radicals provides vibronic spectra of the 3s and 3p Rydberg states. Analysis of the vibronic structure following one-photon and two-photon excitation of rovibronically cold 2-methylallyl radicals and its isotopologues C₄H₄D₃ and C₄D₇ reveals transitions to more than 30 vibrational levels in the 3s Rydberg state that are identified and reassigned on the basis of predictions from ab initio calculations and results from pulsed-field-ionization zero-kinetic-energy photoelectron spectra obtained with resonant multiphoton excitation via selected intermediate states. Depletion spectroscopy reveals transitions to short-lived 3p Rydberg states that have a large oscillator strength.     

Inner shell excitation and ionization of the monohalobenzenes

Energy loss (excitation) spectra of the gaseous monohalobenzenes, C ₆H ₅X (X = F, Cl, Br and I), were obtained by fast electron impact in the regions of the respective carbon 1 s, chlorine 2 p and 2 s, bromine 3 d and iodine 4 d edges. Gas phase X-ray PES measurements of the binding energies of these levels are also reported. Structure observed below the ionization limits has been interpreted with the aid of term values derived from the two sets of measurements.     

Excitation of low-lying even levels of the holmium atom in the electron-atom collisions

In an experimental study of excitation of the holmium atom by electrons with an energy of 50 eV, the excitation cross sections are measured for 102 transitions from even levels of Ho I. Radiative transitions to the 4f ¹¹6s ^{2 4} I°_{13/2,11/2,9/2} levels of the ground state term are considered. Twenty-nine optical excitation functions are recorded in the exciting electron energy range of 0–250 eV. An classification of a number of Ho I lines located in the UV and visible spectral regions is proposed using the known energy levels.     

Optogalvanic signals generated in a neon positive column discharge by resonant chopped CW radiation at 588.2 nm

The optogalvanic line profile for the 1s₅?2p₂ transition (Paschen notation) in a neon positive column discharge has shown a change in sign as a function of dye laser power for a pressure of 1.5 Torr and small currents (2–20 mA). Rate equation analysis suggests this may be due to the increase in the 1s₃ metastable density.     

The role of the configuration interaction in excitation of the Cd(II) principal series at electron-atom collisions

The excitation functions of Cd(II) spectral lines, namely, the lines of the 4d ¹⁰5s ² S _1/2?4d ¹⁰ np ² P ₃ ^2/0 (n≤10) principal series and the line corresponding to a transition from the Beitler level 4d ⁹(5s5p ¹ P ⁰)² P ₃ ^2/0 , are analyzed under the conditions of electron-cadmium atom collisions in the electron energy range from excitation thresholds to 400 eV. It is found that the excitation functions of the spectral lines attributed to transitions from the Beitler level are similar to those of the principal series lines corresponding to transitions from the 4d ¹⁰ np ² P ₃ ^2/0 (n=9, 10) levels nearest to the Beitler level. This similarity is explained by the significant admixing of the Beitler level to the initial levels of the principal series lines. It is shown that the admixing substantially affects the excitation functions of this series for the states more distant from the Beitler level (up to n=6).