Mohamed M. Moselhy

Microhollow cathode discharge excimer lamps

Microhollow cathode discharges are high-pressure, nonequilibrium gas discharges between a hollow cathode and a planar or hollow anode with electrode dimensions in the 100 μm range. The large concentration of high-energy electrons, in combination with the high-gas density favors excimer formation. Excimer emission was observed in xenon and argon, at wavelengths of 128 and 172 nm, respectively, and in argon fluoride and xenon chloride, at 193 and 308 nm. The radiant emittance of the excimer radiation was found to increase monotonically with pressure. However, due to the decrease in source size with pressure, the efficiency (ratio of excimer radiant power to input electrical power), has for xenon and argon fluoride a maximum at ∼400 Torr. The maximum efficiency is between 6% and 9% for xenon, and ∼2% for argon fluoride.

Excimer emission from microhollow cathode argon discharges

Microhollow cathode discharges (MHCDs) operated in rare gases are sources of intense excimer emission. Of particular interest is argon, because of its relatively low cost and the short wavelength (128 nm) of its excimer emission. The measured internal efficiency, obtained in static argon at atmospheric pressure, was found to be on the order of 1%. Flowing argon through a direct current (DC) MHCD at atmospheric pressure caused the argon excimer internal efficiency to increase to 6%, indicating that the low efficiency in static argon is mainly due to impurities. Applying 10 ns pulses to the DC plasma resulted in an increase in excimer power from 30 mW DC to 180 mW peak power, at an efficiency of 5–6%. The increase in excimer power correlates with an increase in the electron density. For DC operation, electron densities of 1015 cm−3 were measured in atmospheric pressure argon micro-plasmas, which increased to values beyond 1016 cm−3 for nanosecond pulsed operation. This increase in electron density and excimer power is due to pulsed electron heating, an effect that has allowed us to raise the mean electron energy from 1 eV, for DC operation, to 2.25 eV in the pulsed mode.

Series operation of direct current xenon chloride excimer sources

Stable, direct current microhollow cathode discharges in mixtures of hydrochloric acid, hydrogen, xenon, and neon have been generated in a pressure range of 200–1150 Torr. The cathode hole diameter was 250 μm. Sustaining voltages range from 180 to 250 V at current levels of up to 5 mA. The discharges are strong sources of xenon chloride excimer emission at a wavelength of 308 nm. Internal efficiencies of approximately 3% have been reached at a pressure of 1050 Torr. The spectral radiant power at this pressure was measured as 5 mW/nm at 308 nm for a 3 mA discharge. By using a sandwich electrode configuration, consisting of five perforated, alternate layers of metal and dielectric, a tandem discharge—two discharges in series—could be generated. For an anode–cathode–anode configuration the excimer irradiance, recorded on the axis of the discharge, was twice as large as that of a single discharge. The extension of this basic tandem electrode structure to a multiple electrode configuration allows the generatio...

Resonant energy transfer from argon dimers to atomic oxygen in microhollow cathode discharges

The emission of atomic oxygen lines at 130.2 and 130.5 nm from a microhollow cathode discharge in argon with oxygen added indicates resonant energy transfer from argon dimers to oxygen atoms. The internal efficiency of the vacuum-ultraviolet (VUV) radiation was measured as 0.7% for a discharge in 1100 Torr argon with 0.1% oxygen added. The direct current VUV point source operates at voltages below 300 V and at current levels of milliamperes.

Xenon excimer emission from pulsed microhollow cathode discharges

By applying electrical pulses of 20 ns duration to xenon microplasmas, generated by direct current microhollow cathode discharges, we were able to increase the xenon excimer emission by more than an order of magnitude over direct current discharge excimer emission. For pulsed voltages in excess of 500 V, the optical power at 172 nm was found to increase exponentially with voltage. Largest values obtained were 2.75 W of vacuum-ultraviolet optical power emitted from a single microhollow cathode discharge in 400 Torr xenon with a 750 V pulse applied to a discharge. Highest radiative emittance was 15.2 W/cm2. The efficiency for excimer emission was found to increase linearly with pulsed voltages above 500 V reaching values of 20% at 750 V.

Argon excimer emission from high-pressure microdischarges in metal capillaries

We report on argon excimer emission from high-pressure microdischarges formed inside metal capillaries with or without gas flow. Excimer emission intensity from a single tube increases linearly with gas pressure between 400 and 1000 Torr. Higher discharge current also results in initial intensity gains until gas heating causes saturation or intensity drop. Argon flow through the discharge intensifies emission perhaps by gas cooling. Emission intensity was found to be additive in prealigned dual microdischarges, suggesting that an array of microdischarges could produce a high-intensity excimer source.

A flat glow discharge excimer radiation source

Increasing the current of a microhollow cathode discharge in high-pressure xenon allowed us to generate disc-shaped 100- to 150-/spl mu/m-thick plasma layers on the planar cathode of a microhollow electrode system, with diameters approaching 1 cm. The plasma layer has been found to emit intense excimer radiation at 172 nm. The maximum of the excimer emission shifts with increased current toward the perimeter of the plasma disc, and the spectral distribution in the area close to the cathode hole changes from vacuum ultraviolet to visible. Plasma filaments, radiating in the visible, are formed which extend from the center to the perimeter of the disc shaped plasma. They correspond to areas of reduced excimer emission, indicating a transition from nonthermal to thermal plasma.

Excimer emission from cathode boundary layer discharges

The excimer emission from direct current glow discharges between a planar cathode and a ring-shaped anode of 0.75 and 1.5 mm diameter, respectively, separated by a gap of 250 μm, was studied in xenon and argon in a pressure range from 75 to 760 Torr. The thickness of the “cathode boundary layer” plasma, in the 100 μm range, and a discharge sustaining voltage of approximately 200 V, indicates that the discharge is restricted to the cathode fall and the negative glow. The radiant excimer emittance at 172 nm increases with pressure and reaches a value of 4 W/cm2 for atmospheric pressure operation in xenon. The maximum internal efficiency, however, decreases with pressure having highest values of 5% for 75 Torr operation. When the discharge current is reduced below a critical value, the discharge in xenon changes from an abnormal glow into a mode showing self-organization of the plasma. Also, the excimer spectrum changes from one with about equal contributions from the first and second continuum to one that i...

Vacuum ultraviolet spectroscopy of microhollow cathode discharge plasmas

Hollow cathode discharge devices with hole dimensions in the range from 0. 1 —0.5 mm (microhollow cathode discharges or MHCDs) can be operated at high pressure (up to and exceeding atmospheric pressure). MHCDs are known to be efficient sources of non-coherent ultraviolet (UV) and vacuum ultraviolet (VUV) radiation when operated in rare gases, rare gas — halide mixtures, and gas mixtures containing rare gases and trace amounts of gases such as H2, 02, and N2. Highest internal efficiencies in direct current MHCD excimer sources of close to 10% were obtained in xenon at a pressure of 400 Ton. By applying nanosecond electrical pulses to the dc discharge the efficiency could be increased to approximately 20%. The radiative emittance which for dc discharges in xenon was measured as 1 .4 W/cm2 could be increased to over 15 W/cm2 through pulsed operation. In addition to rare gas and rare-gas halide excimer emission, intense, monochromatic atomic line emissions have been reported from high-pressure MHCD plasmas in pure rare gases and in rare gases admixed with trace amounts (less than 1 %) of H2, O2, and N2. . The atomic line emission is the result of a near-resonant energy transfer process involving the excimers and the diatomic molecules. For instance, Ne2* excimers in the bound 3?u state have enough energy to dissociate H2 and excite one of the H atoms to the n —2 state. The subsequent decay of the excited H atom results in the emission ofthe 121.6 nm H Lyman-? line. We discuss the results of dc and time-resolved emission spectroscopy in the UV and VUV to elucidate the microscopic mechanisms of the rare gas excimer formation and emission processes, the properties of the MHCD plasma, and microscopic details of the near-resonant energy transfer processes that lead to the emission of the intense atomic line radiation in the range 100 — 1 30 nm.

Excimer emission from microhollow cathode discharges

Summary form only given. Microhollow cathode discharges (MHCDs) combine the possibility for direct current, high-pressure operation with non-equilibrium plasma conditions necessary for efficient excimer formation. When operated in rare gases (Xe, Ar, Ne) or rare gas halides (ArF, XeCl) these discharges were found to be intense sources of excimer radiation. Conversion efficiencies (from input electrical power to output optical power) of several percent were achieved. Although modeling results predict a monotonous increase of radiant power with pressure, in MHCDs it has a maximum at 400 Torr. The observed maximum of the radiant power at constant current was found to be due to the nonlinear reduction of the excimer source area with increasing pressure. The excimer source is located in the cathode opening only at high pressures and low currents. Otherwise, the source extends over the cathode surface outside of the hole. The emitting area decreases by a factor of four over the pressure range from 200 Torr to 760 Torr, whereas the radiant emittance increases monotonically with pressure up to 10 W/cm/sup 2/ at atmospheric pressure. For DC operation, the current was limited to 8 mA to avoid thermal damage.