Strong dependence of ir multiphoton absorption and dissociation yield of SF6 molecule on intensity (duration) of exciting laser pulse

The role of played by the intensity of the exciting TEA CO₂ laser pulses in the processes of ir multiphoton absorption (MPA) of SF₆ molecules cooled toT _R?40 K andT _v?160 K in a pulsed supersonic jet and the dissociation of SF₆ in a bulk atT?300 K under essentially collisionless conditions have been investigated. A strong dependence of MPA and the dissociation yield on pulse intensity were observed. The frequency dependences of the intensity effects were studied.     

Diode laser study of IR multiphoton-induced depletion of rotational sublevels of the ground vibrational state of SF6 molecules cooled in a pulsed free jet

A pulsed frequency tunable diode laser was used to investigate the IR multiphoton-induced depletion of rotational sublevels of the ground vibrational state of SF₆ molecules cooled in a pulsed free jet at exciting energy densities between ≈10^-2 and 2.3 J cm^-2. The depletion of all rotational sublevels was effective at considerable (≈5–11 cm^-1) pumping frequency detunings from the linear absorption spectrum (LAS) of the molecule the width of which under the conditions of experiment (T_rot ≈ 18 K) was ≈2–3 cm^-1. The fraction of molecules excited by a pumping pulse from individual rotational sublevels was measured and its dependence on the exciting pulse frequency and energy density investigated. The effect of collisions on the depletion of the rotational sublevels was studied.     

Direct measurement of multiphoton molecular absorption of IR laser radiation by pyroelectric detector

The applicability of a pyroelectric detector (PED) to multiphoton absorption measurements (MPA) under ir laser excitation of molecules was examined. The possibility of MPA measurement of SF₆ molecules in the pressure range of 10⁻³÷10⁻¹ Torr with a time resolution of ∼ 7 μs has been shown. It has been demonstrated that the degree of vibrational relaxation of highly vibrationally excited molecules at PED surface does not depend on excitation level under the studied conditions.     

Molecular dissociation of SF6 by ultra-short CO2 laser pulses

We have measured the absolute dissociation probability of SF₆ gas irradiated by CO₂ laser pulses varying in duration from 0.5 nsec to 100 nsec. We find a threshold for dissociation of ? 1.4 J/cm², independent of pulse duration. For fixed energy density, the dissociation probability increases as the pulse duration decreases, but not nearly to the degree expected from theory. We have also determined the dissociation probability in the ν₂ + ν₆ combination band and find it to be 10³ times less than in the ν₃ band, in contradiction to some conclusions recently reported by Ambartzumian et al. The dissociation mechanism takes place without collisions, and if collisions occur, they tend to be detrimental to the dissociation rate.     

Electron-beam excitation of vacuum ultraviolet hydrogen laser

Stimulated emission of the Lyman-Band transitions of H₂ near 1600 Å has been achieved using a commercial electron-beam generator as an energy source. A 10⁴ A, 3 ns pulse of 400 keV electrons propagated 1.5m, through H₂ at pressures of 10–100 torr. Electron-molecule collisions produced inversion and superradiance at the kW power level.     

Ne(I) spectral lines from a Ne-O2 d.c. glow discharge

Measurements have been made of intensities of the spectral lines emitted from an Ne-O₂ d.c. discharge with small discharge current (1–4 mA) under the following conditions: gas pressures of 2 and 3 torr and oxygen partial pressures (P₀₂) up to 0.1 torr. All of the Ne(I) line intensities observed decrease when O₂ is added. The Ne(I) λ5852 line (1s₂-2p₁) has been studied in detail as a representative example. The population density of the 2p₁ level of neon has been obtained from the intensity measurements as a function of P₀₂. The energy-distribution function of electrons has been determined using Druyvesteyn's method in order to calculate the population density for a corona model. The high-energy tail of the measured distribution function is markedly reduced when O₂ is added. It is shown that inelastic collisions of electrons with O₂ produce large energy losses for the electrons. These cause a decrease in population density of the 2p₁ level when O₂ is added. The population density of the 2p₁ level at a gas pressure of 2 torr is 1.2×10⁴ cm^-3 in pure neon and 5.2×10² cm^-3 in an Ne-O₂ mixture (P₀₂ = 0.01 torr). The electron densities and average electron energies are 3.5×10⁸ cm^-3 and 8.7 eV and 1.7×10⁸ cm^-3 and 5.3 eV, respectively, for the specified two cases.     

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     

Effective generation of characteristic <Emphasis Type="Italic">K</Emphasis>-rays from large laser-excited SF<Subscript>6</Subscript> clusters in the presence of an Ar carrier gas

The parameters of characteristic sulfur K _α-rays generated from SF₆ clusters that are surrounded by argon atoms and are irradiated by intense laser radiation have been analyzed. It has been found that the formation of large SF₆ clusters under the optimum experimental parameters is accompanied by the high-efficiency generation of the characteristic X rays, and the flux density of the characteristic X-ray photons is 100 photons/(mrad² pulse) at a laser-pulse energy of 5 mJ. It has been shown that the third-harmonic generation process can be used to characterize the spatial sizes of the gas-cluster jet and the region of the cluster plasma.     

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.     

Calculation of the pressure shift of the positronium hyperfine interval

We calculate the fractional positronium hyperfine pressure shift Δ(T) using a quantum mechanical thermal averaging and obtain for Δ(293 K)(±30%) in He, Ne, Ar, Kr, Xe, N₂ and SF₆ respectively; (-0.15, -0.34, -0.96, -1.41, -1.85, -1.12, -1.1) × 10^-7/torr (0°C).     

Collisionless dissociation of SF6 using two resonant frequency CO2 laser fields

The collisionless dissociation of SF₆ has been studied using simultaneous irradiation by two frequencies from a CO₂ laser which are both nearly resonant with the SF_6v3 absorption band. It was found that the dissociation was enhanced, and occurred over a wider frequency range, than for single frequency dissociation. No threshold effect was observed for a weak resonant and a much higher energy field pumping slightly off-resonance. For such two frequency irradiation, the peak in the dissociation curve was found to be shifted to lower frequencies with respect to that for single frequency dissociation.     

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.     

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     

Pulse duration and collision effects in the multiphoton dissociation of HDCO

Measurements of multiphoton dissociation (MPD) of HDCO gas irradiated by intense CO₂ laser pulses varying in shape and having a duration from 150 ns to ～ 1 μs are reported. It is found that for the same laser pulse energy the short pulses dissociated about an order of magnitude more molecules than the long pulses. Taking into consideration the collision-induced MPD of HDCO previously found, it is shown that to a reasonably good approximation, all the present MPD data for the different pulse durations can be represented on a single, smooth and sharply rising curve of yield versus intensity. This finding is in contrast to the flat intensity response at constant pulse energy found recently for collisionless MPD in SF₆.     

The influence of reorientational collisions on coherent optical processes

Numerical calculations of pulse transmission in SF₆ are performed to appraise the role of reorientational collisions. No support is found for the recent postutate calling for thermally equilibrated magnetic sublevels in self-induced transparency.     

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.     

High resolution study of anion formation in low-energy electron attachment to SF<Subscript>6</Subscript> molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF₆ molecules in helium carrier gas, nozzle temperatures T₀= 300–600 K, stagnation pressures p₀= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF₆ molecules at high energy resolution. Using the standard LPA method, the yield for SF₆^- as well as SF₅^- and F^- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF₆^- and SF₅^- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF₆^- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF₅^- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T₀= 300 K and shifting to lower energies with rising T₀) and a monotonical decrease towards higher energies. SF₅^- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF₆ molecules with rising pressure. The SF₅^-/SF₆^- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF₅^- spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at p₀= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F^- formation is similar to that for SF₆^-, but the F^- signals are three to four orders of magnitude lower than those for SF₆^-; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.     

Pure rotational spectra of 32SF535Cl, 32SF537Cl,and 34SF535Cl up to 300 GHz

Pure rotational spectra of the three molecules ³²SF₅³⁵Cl, ³²SF₅³⁷Cl, and ³⁴SF₅³⁵Cl in their ground vibrational states have been observed up to 300 GHz (8 < J < 80). Molecular parameters have been computed with good accuracy for the three isotopic species. The “K-type” splitting characteristic of molecules belonging to the C_4v symmetry group has been clearly seen and measured.     

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.     

Detection of SF<Subscript>6</Subscript> molecules sublimating from the surface of (CO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> nanoparticles in a cluster beam by the infrared multiphoton excitation method

It has been found that SF₆ molecules captured by large van der Waals clusters (CO₂) _N (where N ≥ 10² is the number of monomers in a cluster) in intersecting molecular and cluster beams sublimate from the surface of clusters after a certain time and carry information on the velocity and temperature (internal energy) of clusters. Experiments have been carried out for detecting these molecules by means of a pyroelectric detector and the infrared multiphoton excitation method. The multiphoton absorption spectra of molecules sublimating from the surface of clusters have been obtained. The temperature of the (CO₂) _N nanoparticles in the cluster beam has been estimated using these spectra and comparing them with the infrared multiphoton absorption spectra of SF₆ in the initial molecular beam.