Thermionic Diode Research Articles

Spectroscopic Investigation of the Odd-Parity 3d 2 D → nf 2 F Transitions of Neutral Sodium

We report new experimental data on term energies and effective quantum numbers of the odd parity Rydberg states of sodium in the 40687–41408 cm–1 energy range. The experiment was performed using a two-color scheme of three-photon laser excitation in conjunction with a thermionic diode ion detector. The new observation includes much extended nf 2 F (12 ≤ n ≤ 51) series excited from the 3d 2 D intermediate state. In addition, oscillator strengths of the 3d 2 D → nf 2 F (16 ≤ n ≤ 45) Rydberg transitions have been determined and a complete picture is presented from n = 4–45 incorporating the present work and earlier computed results.

Measurements of photoionization cross section of the 4p levels and oscillator strength of the 4p→nd 2D3/2,5/2 transitions of potassium

We have carried out measurements of absolute photoionization cross sections of the 4p excited levels and oscillator strengths of the 4p→nd Rydberg transitions in potassium using a two-step photo-excitation and ionization technique in conjunction with a thermionic diode ion detector. The measurements were conducted using the linearly polarized laser light and the absolute values of the cross sections from the 4p 2P3/2 and 2P1/2 excited levels have been determined at the ionization threshold as (6.3±0.9)Mb and (5.4±0.8)Mb respectively. In addition, photoionization cross sections have been determined at various ionizing wavelengths above the first ionization threshold to explore different energy regions of the continuum. The oscillator strengths for the 4p 2P1/2→nd 2D3/2 and 4p 2P3/2→nd 2D3/2,5/2 Rydberg transitions have been deduced by using the measured cross sections of the 4p 2P1/2 and 2P3/2 levels at the ionization threshold. The new results are in good agreement with the available theoretical and experimental data.

Femtosecond laser enhanced current in a thermionic diode with barium vapor

We studied the signal from a thermionic diode when a femtosecond laser beam was spatially overlapped by an excimer pumped dye laser beam. The nanosecond dye laser was scanned from 435 to 438nm in order to excite the autoionizing levels of barium by two photon absorption. The broadband ultrashort laser light was centered at 427nm, which is also above the first ionization limit of barium. The bias voltage between the cell body and the tungsten rod (set at either 9 or 0V) was used to collect electrons after the barium ions had been created by multiphoton (auto) ionization. The overall background of the thermionic signal was appreciably elevated due to the two photon ionization by the broadband femtosecond laser. We measured the thermionic signal with and without femtosecond laser overlap, and with a biasless and biased thermionic diode. The effect of the femtosecond laser was appreciable enhancement of the background ionization continuum. This was especially visible in the presence of noble gases at pressure of 50mbar. Argon produced the largest and helium produced the smallest enhancement in the background continuum. In addition, we observed a few broad spectral features of a presumably collision induced nature.

Three-step laser excitation of the odd-parity 5s5d 3D → 5snf 3F states of cadmium

We report new experimental data on the term energies and effective quantum numbers of the highly excited odd parity states of cadmium in the 71 773–72 500 cm−1 energy range. The experiment was performed using three dye lasers simultaneously pumped by the second harmonic (532 nm) of the Nd;YAG laser. The vapor containment and detection system was a thermionic diode ion detector working in a space charge limited mode. The new observations include the 5snf 3F3 (12 ⩽ n ⩽ 52), 5snf 3F4 (13 ⩽ n ⩽ 33) and 5snf 3F2 (12 ⩽ n ⩽ 22) Rydberg series excited from the 5s5d 3D multiplet. A two parameter fit to the transitions energies of the 5snf 3F3 series yields the binding energy of the 5snd 2D2 level as 13 042.178 ± 0.02 cm−1 and consequently the first ionization of cadmium is determined as 72 540.05 ± 0.13 cm−1, which is in good agreement with the previously reported value.

The effect of samarium doping on structure and enhanced thermionic emission properties of lanthanum hexaboride fabricated by spark plasma sintering

Single‐phase polycrystalline solid solutions (La1−xSmx)B6 (x = 0, 0.2, 0.4, 0.8, 1) are fabricated by spark plasma sintering (SPS). This study demonstrates a systematic investigation of structure–property relationships in Sm‐doped LaB6 ternary rare‐earth hexaborides. The microstructure, crystallographic orientation, electrical resistivity, and thermionic emission performance of these compounds are investigated. Analysis of the results indicates that samarium (Sm) doping has a noticeable effect on the structure and performance of lanthanum hexaboride (LaB6). The analytical investigation of the electron backscatter diffraction confirms that (La0.6Sm0.4)B6 exhibits a clear (001) texture that results in a low work function. Work functions are determined by pulsed thermionic diode measurements at 1500–1873 K. The (La0.6Sm0.4)B6 possesses improved thermionic emission properties compared to LaB6. The current density of (La0.6Sm0.4)B6 is 42.4 A cm−2 at 1873 K, which is 17.5% larger than that of LaB6. The values of ΦR for (La0.6Sm0.4)B6 and LaB6 are 1.98 ± 0.03 and 1.67 ± 0.03 eV, respectively. Furthermore, the Sm substitution of lanthanum (La) effectively increases the electrical resistivity. These results reveal that Sm doping lead to significantly enhanced thermionic emission properties of LaB6. The compound (La0.6Sm0.4)B6 appears most promising as a future emitter material.

Experimental investigation of photoionization cross section for the 3d 2D excited states of lithium and sodium

We report experimentally measured photoionization cross sections for the 3d 2D excited states of lithium and sodium at first ionization threshold. The experiments were performed using two dye lasers simultaneously pumped by the second harmonic of a Nd:YAG laser. The vapor contentment and the detection system was a thermionic diode ion detector operating in a space charge limited mode. Photoionization cross sections of the excited states were deduced from the dependence of ion signal intensity on the ionizing laser energies as 19 ± 3 Mb and 21.5 ± 3.5 Mb for lithium and sodium respectively, which are in good agreement with the previously computed theoretical results.

THE EFFECT OF ETCHING TIME ON RECTIFYING CHARACTERISTIC IN SnO2/p-Si AND SnO2/p-PoSi HETEROJUNCTION SCHOTTKY DIODES

We have fabricated SnO 2/p- Si and SnO 2/p- PoSi heterojunction diodes by spray pyrolysis method. To prepare porous Si substrates, the etching time was varied from 10 to 20 and 30 mins. In these samples, the SEM micrographs showed a distributed pore areas surrounded by columnar walls with various height. The data analysis of the rectified I–V characteristic, using thermionic emission Schottky diode theory, showed that although the barrier height is about 0.5–0.6 eV in all samples other two important diode parameters, i.e. the ideality factor n and the series resistance rs, are strongly etching time-dependant and are increased with increasing the etching time.

Photoionization studies from the 3p P2 excited state of neutral lithium

In the present work, the experimentally determined oscillator strengths of the 3p P2→nd D2 (16≤n≤43) Rydberg transitions of lithium have been reported for the first time. The experiments were performed using two dye lasers simultaneously pumped by the second harmonic (532 nm) of an Nd:YAG laser, and the vapor containment and detection system was a thermionic diode ion detector. The first frequency-doubled dye laser excites the ground-state atoms to the 3p P2 excited state at 325.35 nm, and the second dye laser was scanned up to the first ionization threshold. The f-values of the aforementioned transitions have been determined using the threshold value of the photoionization cross section (30±4.8 Mb) from the 3p P2 excited state. A complete picture of the oscillator strengths from n=3 to 43 has been presented, including the previously known oscillator strength and determined in this work.

Photoionization from the5p 2P3/2state of rubidium

We report two-step photoionization studies from the 5$p$ ${}^{2}$${P}_{3/2}$ excited state of rubidium using two dye lasers simultaneously pumped by a common Nd:YAG laser in conjunction with a thermionic diode ion detector. The photoionization cross section at the first ionization threshold is measured as 18.8 \ifmmode\pm\else\textpm\fi{} 3 Mb and at excess energies of 0.013, 0.106, 0.229, and 0.329 eV is measured as 15, 13.6, 12.6, and 12.5 Mb, respectively. The measured value of the photoionization cross section at the threshold is used to calibrate the oscillator strengths of the 5$p$ ${}^{2}$${P}_{3/2}$ \ensuremath{\rightarrow} $n$d ${}^{2}$${D}_{5/2}$ (22 \ensuremath{\leqslant} $n$ \ensuremath{\leqslant} 52) Rydberg transitions.

Two-color three-photon excitation studies of thallium Rydberg states

We present new data on the even-parity Rydberg states of atomic thallium using two-step three-photon laser excitation technique in conjunction with a thermionic diode ion detector. Atoms are excited from the 6p 2P 1/2 ground state to the 7p 2P 1/2 intermediate state via two-photon excitation and subsequently promoted to the high lying ns 2 S 1/2 and nd 2D 3/2 Rydberg states. The first ionization potential of thallium is determined as 49,266.66(1) cm –1 using data for the ns 2 S 1/2 (25 ≤ n ≤ 54) and nd 2D 3/2 (24 ≤ n ≤ 65) Rydberg series. This value is believed to be more accurate because the contribution due to the hyperfine structure splitting of the 7p 2P 1/2 state (0.07185 cm –1) is much smaller as compared to that of the 6p 2P 1/2 ground state (0.711 cm –1).

The Roots of Automation Before Mechatronics [Historical

The inventions of Kenotron (the thermionic valve diode, 1904) by John A. Fleming and Audion (the thermionic valve triode, 1906) by Lee De Forest started the revolution of electronics, that merged with mechanics, automation, and computer science around the mid-1950, generating present-day industrial robots and eventually mechatronics. A brief look at the history of automation before mechatronics came is given in this paper.

Two-step laser excitation of the highly excited even-parity states of atomic mercury

We report term energies and quantum defects of highly excited even-parity states of mercury in the 83 876–84 140 cm-1 energy range, employing a two-step laser excitation scheme via the $6s^2 ~^1$ S0↦6s6p3P1 inter-combination transition. Two dye lasers, pumped by a common Nd:YAG laser, were frequency doubled by BBO crystals and used to record the spectra in conjunction with a thermionic diode ion detector. Our new observations include the much extended $6snd ~^3$ D2 (22 ≤n ≤52) series and a few members of the $6sns ~^3$ S1 (24 ≤n ≤30) Rydberg series. Members of the $6snd ~^3$ D2 Rydberg series with such a high n value are reported for the first time. The relative intensities of the $msnd ~^3$ D2 and $msns ~^3$ S1 transitions (m = 4, 5 and 6) of group II-B elements excited from the $msmp ~^3$ P1 inter-combination states are also discussed.

A Micro-Insulation Concept for MEMS Applications

Small scale, thermally driven power sources will require appropriate insulation to achieve sufficiently high thermal conversion efficiencies. This paper presents a micro-insulation design, which was developed for a thermionic microbattery, which converts the decay heat from radioactive isotopes directly to electricity using a vacuum thermionic diode. The insulation concept, which is suitable for any small scale application, separates two planar surfaces with thin, semicircular posts, thus reducing conduction heat transfer and increasing the relative radiation heat transfer. In this case, the surfaces are silicon wafers and the columns are SU-8, a photoresist material. The experimental results indicate that this design is adequate for a practical power source concept, and they are supported by a numerical model for the effective thermal conductivity of the structure. The results show that a typical design of 20 columns/cm2 with a 200 μm diameter and a 10 μm wall thickness has an apparent thermal conductivity on the order of 10−4 W/m K at a pressure of 1 Pa. System models of a thermionic power source indicate that this is sufficiently low to provide practical efficiency.

Oscillator strengths of the 4s4p $~^{{\mathsf{1,3}}}$ P1↦ 4snd $~^{\mathsf {1,3}}$ D2 transitions of neutral calcium

We report measurements of f-values of the $4s4p~^{1}$ P1↦4snd 1D2 ( $30 \leqslant n \leqslant 70$ ) and the $4s4p~^{3}$ P1↦4snd 3D2 ( $20 \leqslant n \leqslant 50$ ) Rydberg transitions in calcium using a thermionic diode ion detector in conjunction with a Nd:YAG pumped dye laser system. Absolute values of the photoionization cross-sections have been determined for transitions from the $4s4p~^{1}$ P1 and $4s4p~^{3}$ P1 excited states to the 4s ionization threshold as 140(20) Mb and 117(20) Mb respectively. The measured threshold photoionization cross-sections have been used to calibrate the f-values of the above-mentioned Rydberg transitions.

Measurement of Parameters of X-Band High-Power Microwave Superradiative Pulses

This paper describes the waveguide antenna, the detector based on the thermionic microwave diode, and the alcohol-filled calorimeter developed at the Institute of High Current Electronics. The techniques were used to measure parameters of X-band microwave pulses with a peak power of approximately 1 GW and a pulse length from 0.7 to 1 ns. The error of the peak power measurement was about 15%.

Photoexcitation study of the 4s 2S1/2 state of atomic sodium

We report the first measurements of oscillator strengths for 4s 2S1/2 → np 2P1/2,3/2 (19 ⩽ n ⩽ 57) Rydberg transitions in atomic sodium using a thermionic diode ion detector in conjunction with an Nd:YAG pumped dye laser system. The f-values have been calibrated with the measured photoionization cross-section from the 4s 2S1/2 excited state at the first ionization threshold as 0.65(0.10) Mb. The binding energy of the 4s 2S1/2 level is determined as 15 709.444(8) cm−1 by employing the Rydberg relation to the observed np 2P1/2,3/2 Rydberg series. Addition of the binding energy to the known energy of the 4s 2S1/2 level yields the first ionization potential of sodium as 41 449.44(1) cm−1, which is in excellent agreement with the recognized value.

Atomic jet with ionization detection for laser spectroscopy of Rydberg atoms under collisions and fields

An efficient atomic jet setup offering many unprecedented advantages over a conventional heat pipe setup used in multi-photon spectroscopy, mainly of alkaline-earth metals, has been constructed by a scheme in which the sample material is encapsulated in a disposable cartridge oven located inside a thermally stabilised heat-pipe and is made to effuse in to a row of atomic beams merging to form a jet target. This novel scheme combines the advantages of both high density atomic beam with convenient geometry for orthogonal excitation and high sensitive ionisation detection capabilities of thermionic diodes, besides eliminating several problems inherent in the usual heat-pipe operation. Out of various designs, typical results are presented for a linear heat-pipe with vertical atomic jet used in two-photon spectroscopy of highly excited states of Sr I. Controlled excitations of both Rydberg and non-Rydberg states, which cannot otherwise be accessed from the ground state due to parity and spectroscopic selection rules, have been achieved by employing a weak electric field complimented by collisions. The atomic jet setup is also found very useful for the study of collisional broadening and shift of excited states and time evolution of Rydberg atoms.

Experimental studies of the oscillator strengths of the 6s6p 1,3P1 → 6snd 1,3D2 Rydberg transitions in barium

We report new measurements for the oscillator strengths of the 6s6p 1P1 → 6snd 1D2 and 6s6p 3P1 → 6snd 3D2 Rydberg transitions of barium using a thermionic diode ion detector in conjunction with a Nd:YAG pumped dye laser system. The f-values have been calibrated with the absolute photoionization cross-section measured at the first ionization threshold from the 6s6p 1P1 and 6s6p 3P1 excited states employing the saturation technique. An excellent agreement is found for the 6s6p 1P1 → 6snd 1D2 Rydberg transitions with the earlier reported work. The data on the oscillator strength of the 6s6p 3P1 → 6snd 3D2 Rydberg transitions are reported for the first time. The oscillator strength densities in the continuum corresponding to the 6s6p 3P1 excited state have also been determined and a smooth merging of the discrete f-values into the oscillator strength densities is observed across the ionization threshold.

Measurements of photoionization cross sections from the 4p, 5d and 7s excited states of potassium

New measurements of the photoionization cross sections from the 4p 2P 1/2,3/2, 5d 2D 5/2,3/2 and 7s 2S 1/2 excited states of potassium are presented. The cross sections have been measured by two-step excitation and ionization using a Nd:YAG laser in conjunction with a thermionic diode ion detector. By applying the saturation technique, the absolute values of the cross sections from the 4p 2P 3/2 and 4p 2P 1/2 states at 355 nm are determined as 7.2±1.1 and 5.6±0.8 Mb, respectively. The photoionization cross section from the 5d 2D 5/2,3/2 excited state has been measured using two excitation paths, two-step excitation and two-photon excitation from the ground state. The measured values of the cross sections from the 5d 2D 5/2 state by two-photon excitation from the ground state is 28.9±4.3 Mb, whereas in the two-step excitation, the cross section from the 5d 2D 3/2 state via the 4p 2P 1/2 state and from the 5d 2D 5/2,3/2 states via the 4p 2P 3/2 state are determined as 25.1±3.8 and 30.2±4.5 Mb, respectively. Besides, we have measured the photoionization cross sections from the 7s 2S 1/2 excited state using the two-photon excitation from the ground state as 0.61±0.09 Mb.

Multi-photon excitation spectra of the 3snℓ (ℓ = 0, 1, 2 and 3) Rydberg states of magnesium

New experimental data on the highly excited ℓ = 0, 1, 2 and 3 Rydberg states of magnesium have been acquired using two-photon and two-step laser excitation technique in conjunction with a thermionic diode ion detector. The new observations include even parity 3sns 1S0 (8 ⩽ n ⩽ 24) and 3snd 1D2 (7 ⩽ n ⩽ 62) Rydberg states approached directly from the 3s2 1S0 ground state via two-photon excitation, and the odd parity 3snp 1P1 (20 ⩽ n ⩽ 61) and 3snf 1F3 (14 ⩽ n ⩽ 66) Rydberg states accessed by the two-step excitation process via 3s4s 1S0 and 3s3d 1D2 intermediate states. The Rydberg relation fit to the new data of the np 1P1 and nf 1F3 series yields the binding energies of the 3s4s 1S0 and 3s3d 1D2 levels as 18 167.702 cm−1 and 15 267.972 cm−1, respectively. By adding the binding energies to the corresponding energies of the aforementioned levels, a precise value of the first ionization potential of magnesium is determined as 61 671.04 ± 0.04 cm−1. Using this ionization potential value, the quantum defects for the ns 1S0, np 1P1, nd 1D2 and nf 1F3 Rydberg series have been determined as 1.526(2), 1.046(2), 0.602(2) and 0.049(2) cm−1 respectively.