Self-focussing of CO2 laser radiation in SF6

Self-focussing of high-power TEA CO₂ laser pulses for a number of 10 m P-band lines is reported in SF₆ molecular gas. Application of this effect to estimating the intensity-dependent refractive index of the gas is demonstrated.     

Minimization of the photolysis-driven density gradients by slow pumping in the iodine laser amplifier with regard to the beam quality

The UV photolytical pumping of perfluoralkyl iodides C₃F₇I in the large atomic iodine laser amplifier gives rise to density waves in the gas mixture. The increasing gas temperature during photolysis contributes to the increase of the sound wave velocities. Our calculations of the time interval during which the influence of the oscillating part of the gradient of the refractive index on the focusability was at a minimum was made at a constant gas temperature, which was regarded as a parameter. The results are given in a nomographic chart for various parts of He and SF₆ in gas mixture and for various laser tube diameters. The applicability of a heavier buffer gas-argon and a lighter iodine CF₃I considering the sound waves velocities was investigated.The authors wish to thank dr. K. Rohlena for many useful discussions.     

The influence of vibrational excitation on the burning voltage of a pulsed electric discharge in HF laser working media

The burning voltages of an intermediate pressure self-sustained volume discharge (SSVD) in SF₆ and SF₆-C₂H₆ mixtures irradiated by a 10.6 μm pulse TEA CO₂ laser, have been measured on varying the laser fluences over a wide range. The delay between the voltage application and the laser pulse onset is 4 μs, and the laser pulse lasts ∼3 μs. The considerable rise observed in the discharge voltages with increasing absorbed specific laser radiation energy, is due to electron attachment to vibrationally excited molecules of SF₆. Different processes of relaxation of the vibrational energy stored in SF₆ molecules are analyzed and the relevant characteristic times are numerically assessed. The gas heating process owing to vibration-translation energy exchange is qualitatively described in terms of the “thermal explosion”. The relation between the “explosion” and delay times determines the peculiarities of electron attachment to vibrationally excited SF₆ molecules. The burning voltages of a submicrosecond non-irradiated SSVD in the above-mentioned media versus the specific electric energy deposited are also measured. They are compared to those of a laser-illuminated SSVD at commensurable specific laser energy depositions. It is concluded that electron attachment to the discharge-produced vibrationally excited SF₆ molecules is not capable of noticeably affecting the discharge voltages of a submicrosecond non-irradiated SSVD. PACS 42.55 Ks; 52.80     

Effect of collisions on the resonance vibrational polarizability of gaseous SF<Subscript>6</Subscript> and CF<Subscript>4</Subscript> molecules

The density dependences of the absorption cross sections and refractivity are experimentally studied for the SF₆ and CF₄ molecules in pure gases in the region of their ν₃ infrared vibrational-rotational antisymmetric modes. The dispersions of the refractive index are determined for both compounds by the Kramers-Kronig transformation of the spectral data obtained, and, for the SF₆ isotopomers, they are also measured by the method of two-color interferometry. Strong nonlinear dependences of optical parameters and their dispersions on the gas density are observed. The values of second optical virial coefficient B _R (ν) obtained for pure SF₆ are more than an order of magnitude greater than the values found earlier for mixtures of SF₆ with buffer rare gases. The results of calculations of the second virial coefficients of the absorption cross section and refractivity in terms of the DID model of interacting dipoles are in agreement with the experimental data in the band wings. Correlations between the behavior of the spectral dependence of functions B _R (ν) and the parameters of model intermolecular potentials used in the calculations are found.     

Sulfur isotope enrichment in SF6 by high intensity CO2 laser radiation

Enrichment of ³⁴SF₆ following irradiation of SF₆?H₂ mixtures by the focused output of a pulsed TEA CO₂ laser has been studied as a function of the number of laser pulses, excitation wavelength, total pressure, and laser energy.     

Importance of pre-ionisation for the non-chain discharge-pumped HF laser

The importance of pre-ionisation for the non-chain discharge-pumped HF laser is studied through experiments on an X-ray photo-triggered laser using mixtures of Ne, SF₆, and ethane. The discharge dynamic in Ne/SF₆ mixtures or pure SF₆, as well as the stabilisation effect induced by C₂H₆ and consequences for the laser performance, are investigated for pre-ionisation electron density values, n_eo, ranging from 10⁶ cm^-3 up to 10⁹ cm^-3, as well as for the so-called discharge self-breakdown mode. Without ethane, the minimum n_eo value which is needed to complete 100% homogeneous charge deposition in the plasma is a very sharply increasing function of the SF₆ pressure. This hinders performance optimisation when the molecule used to react with F-atoms, for instance H₂, has no effect on the discharge dynamic. The minimum ethane partial pressure that is needed to stabilise the discharge depends on n_eo, the pumping pulse duration, the deposited electric charge, and the SF₆ pressure. Discharges in Ne/SF₆ can be much more efficiently stabilised by addition of a small amount of ethane than by an increase of n_eo. A pre-ionisation density as low as 10⁶ cm^-3 is sufficient to achieve the maximum laser energy value, but total suppression of the pre-ionisation has a detrimental effect on the active medium homogeneity. Received: 30 May 2000 / Revised version: 9 October 2000 / Published online: 9 February 2001     

Improved separation of the rare sulfur isotopes by infrared multiphoton dissociation of SF6

The dissociation probabilities of³²SF₆ and some of³⁴SF₆ have been measured at a large number of CO₂ laser lines both at room temperature and at 140 K. The longwavelength wing of this dissociation spectrum is exponential in the wavenumber. Its logarithmic slope is proportional to the inverse temperature. Selectivities are high enough at 140 K, that the photons are consumed only for the rare isotope in the case of³⁴SF₆ and nearly so for³⁶SF₆. For³³SF₆ further improvement of the selectivity would be desirable.     

Effect of mode-locking on the multiple-photon reaction of SF6

The effect of mode locking on the CO₂ laser-induced reaction of SF₆ is reported. A mode-locked laser pulse enhances the SF₆ reaction yield by a small (8%) but significant amount. The mode-locked pulse also decreases the isotopic selectivity of the reaction. This small effect is greater for enrichment of ³³SF₆ than for ³⁴SF₆. These results suggest that absorption mechanisms that have high power dependence play a minor role in the multiple-photon reaction.     

Absorption of CO2 laser pulses at different wavelengths by ground-state and vibrationally heated SF6

The absorption of CO₂ laser pulses by low pressure SF₆ gas has been investigated over a wide range of energy fluxes. For laser energy fluxes of 0.01–1 J cm^-2 the effective absorption cross section varies between 0.2 and 2 × 10^-18 cm². For each laser line an individual dependence on the energy is found and in some cases minor changes in the absorption behaviour seem to occur around 0.1 J cm^-2. SF₆ excited with an average vibrational energy content of up to 20 photons/molecule does not absorb measurable amounts of 9.4 μm laser light. The influence of various SF₆ and Ar pressures on the temporal shape of the transmitted pulses has been investigated.     

Diode laser spectrum of the ν3 band of 34SF6

A diode laser was used to measure the absorption spectrum of the ν₃ band of ³⁴SF₆. This isotopic species, which is present in the natural sample (4.2%), was cooled in a molecular beam of pure SF₆. Subbranches up to J = 22 were recorded and identified. The molecular parameters, determined with a simple fitting procedure, are compared with those known of ³²SF₆ and ³³SF₆.     

Spectral characteristics of IR-multiphoton excitation in supersonic molecular beams

The multiphoton absorption of SF₆ was investigated in supersonic molecular beams in dependence on the fluence and the wavelength of the CO₂ laser. The temperatures of the SF₆ molecules have been reduced using seeded beams of different concentrations. The experimental results are discussed on the basis of the known spectroscopic data of SF₆ and provide some novel information about the spectral characteristics of the ir-multiphoton excitation of strongly cooled molecules in the collision-free case.     

Some aspects of CO2 laser induced fluorescence in SF6−H2 mixtures

Measurements in SF₆?H₂ mixtures of HF¹ fluorescence at 2.8 μm induced by pulsed CO₂ laser radiation are reported. The dependence of fluorescence intensity on laser fluence is found to be strongly affected by the laser beam geometry in the interaction region. Our results show that the technique of HF¹ fluorescence intensity detection can be a sensitive and reliable single-shot measure of multiple-photon dissociation of SF₆ in a collisionless regime on condition that the laser fluence is uniform along the interaction region which is monitored.     

IR absorption of highly vibrationally excited SF6

In a double-resonance experiment, the absorption of various CO₂ laser lines by sulfur hexafluoride was measured, before and after the SF₆ was pumped by a fixed frequency CO₂ laser to a level of 5 quanta/molecule. The absorption is substantially shifted to longer wavelengths. But the short wavelength wing of the absorption band is not completely bleached. Instead a shoulder of several cm⁻¹ width is left. This shoulder is probably important for the explanation of the infrared laser induced dissociation of SF₆.     

Collisionless dissociation and isotopic enrichment of SF6 using high-powered CO2 laser radiation

Dissociation of ³²SF₆ and the resultant isotopic enrichment of ³⁴SF₆ using high-powered CO₂ laser radiation has been studied with higher experimental sensitivity than previously reported. Enrichment factors have been measured as a function of laser pulse number, wavelength, energy and time duration. A geometry-independent dissociation cross section is introduced and measured values are presented. Threshold energy densities, below which no dissociation was observed, were also determined.     

AlF3 film deposited by IAD with end-Hall ion source using SF6 as working gas

A novel and effective process to fabricate high quality fluoride thin films was presented. Aluminum fluoride films deposited by a conventional thermal evaporation with an ion-assisted deposition (IAD) using SF₆ as a working gas at around room temperature were investigated. In this study, the optimal voltage and current, 50 V and 0.25 A, were found according to the optical properties of the films: high refractive index (1.489 at 193 nm), low optical absorption and extinction coefficient (<10⁻⁴ at 193 nm) in the UV range. The physical properties of the film are high packing density and amorphous without columnar structure. It was proved that using SF₆ working gas in IAD process is a good choice and significantly improves the quality of AlF₃ films.     

Transmission electron microscopy investigation of crystalline silicon surface irradiated by femtosecond laser pulses in different background atmospheres

This article aims to obtain structural and compositional characteristics of a crystalline silicon surface irradiated by femtosecond laser pulses in SF₆, N₂, air, and vacuum background atmospheres by performing transmission electron microscopy observation of ??110?? cross-sectional specimens. Conical microstructures covered with defective outer layers were formed in SF₆ gas. The elemental sulfur dopants in the surface microstructure, which located in close proximity to defects, were mainly concentrated at the tip region of the microcones, and about several hundred nanometers thick. In N₂ atmosphere, the defects produced regularly on the silicon surface were of the same types with those formed in SF₆ gas and confirmed to be stacking faults and overlapped twins. Furthermore, silicon crystalline grains with different orientations were observed on the silicon surface irradiated in N₂, air, and vacuum atmospheres. Especially, ??-Si₃N₄ crystalline grains were found to be formed in N₂ and air as chemical products when elemental nitrogen exists, and the SiO₂ amorphous phase was formed in air by the oxidation effect. Based on these experimental results, the relevant interaction mechanisms between pulsed laser and crystalline silicon were suggested to be mainly attributed to laser-assisted chemical etching and laser ablation, i.e., if volatile silicon compounds can be produced in a reactive gas atmosphere (e.g., SF₆), the strong laser-assisted chemical etching dominates over the laser irradiation process. Otherwise, laser ablation is the dominant mechanism such as in N₂, air, and vacuum.     

Generation of high-energy secondary pulsed molecular beams

A method is suggested for generating high-intensity secondary pulsed molecular beams in which the kinetic energy of molecules can be controlled by an intense laser IR radiation through the vibrational excitation of molecules in the source. High-intensity [≥10²⁰ molecule/(sr s)] SF₆ molecular beams with a kinetic energy of ?1.0 eV without carrier gas and of ?1.9 and ?2.4 eV with carrier He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) gases, respectively, were obtained.     

Dynamics and correlated performance of a photo-triggered discharge-pumped HF laser using SF6 with hydrogen or ethane

3 , has been performed in Ne/SF₆/H₂ and Ne/SF₆/C₂H₆ mixtures. Parameters involved have been the storage line capacitance and the circuit inductance, the capacitors charging voltage, the RH-molecule type and partial pressure, and the X-ray dose for the preionization. High laser performance has been achieved with C₂H₆: an output energy up to 3 J corresponding to a specific energy of 9.6 J/l at an efficiency of 4.7%, which strengthens the advantage of the photo-triggering technique to energize high-power HF lasers. However the optimum performance achieved with H₂, 5.75 J/l and 3.5%, are lower. It is shown, through a time-resolved study of the electrical discharge and spatial dynamics correlated to laser power and energy measurements, that discharge instabilities are responsible for the poor laser performance of the mixture with H₂. These instabilities, which lead to arc development, are characteristics of the discharge in Ne/SF₆. It is demonstrated for the first time that addition of a heavy hydrocarbon, such as C₂H₆, to that mixture induces the discharge stabilization so that the laser emission arises in a homogeneous active medium. This effect allows us to achieve better laser performance than with H₂. Received: 17 March 1998/Revised version: 13 July 1998     

IR multiphoton absorption of SF6 in flow with Ar at moderate energy fluences

6 in flow with Ar (SF₆: Ar=1:100) in conditions of a large vibrational/rotational temperature difference (T_V≃230 K, T_R≃60 K) was studied at moderate energy fluences from ≃0.1 to ≃100 mJ/cm², which are of interest for isotope selective two-step dissociation of molecules. A 50 cm Laval-type slit nozzle for the flow cooling, and a TEA CO₂-laser for excitation of molecules were used in the experiments. The laser energy fluence dependences of the SF₆ MPA were studied for several CO₂-laser lines which are in a good resonance with the linear absorption spectrum of the ν₃ vibration of SF₆ at low temperature. The effect of the laser pulse duration (intensity) on MPA of flow cooled SF₆ with Ar was also studied. The results are compared with those obtained in earlier studies. Received: 4 September 1995/Revised version: 15 February 1996     

Energy and pressure dependence of the CO2 laser induced dissociation of sulfur hexafluoride

Low pressure SF₆ with its isotopes in natural abundance was irradiated by a pulsed CO₂ laser operated on theP20 line (10.6 μm band). Dissociation yields of³²SF₆ and³⁴SF₆ were measured separately. If the radiation is focussed into the cell, the dissociation yield is proportional to the 3/2 power of the laser energy, as was derived under general conditions and confirmed experimentally. The reaction probabilityP(Φ), the fraction of molecules dissociated by an energy flux Φ, was measured using parallel light. For both isotopes,P(Φ) saturates at high energy flux close toP=1. At a lower flux (2 J cm⁻²), the dissociation probability of³²SF₆ displays a threshold, whereas the dissociation probability of³⁴SF₆ is a very steep function of Φ over the whole range of fluxes.P(Φ) at the higher energy flux was measured in a cavity absorption cell, in which up to 80% of the molecules were dissociated by a single pulse. Below 0.2 mbar SF₆ the dissociation yields for both isotopes are pressure independent. Above 2 mbar the isotopic selectivity is completely lost. Addition of hydrogen always decreases the dissociation yields.