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.     

Pulsed optoacoustic detection of multiple photon excitation in molecules

Multiple photon excitation, saturation, and linear absorption of SF₆-argon mixtures when irradiated by a high power CO₂ TEA laser is investigated using a pulsed optoacoustic technique. At low intensities the expected linear dependence of the absorption on laser intensity is observed. At intermediate intensities the absorption exhibits a square root dependence on the incident laser intensity, a dependence which is typical for saturation of an inhomogeneously broadened absorption. At even higher intensities, the absorption shows an intensity dependence typical of multiple photon excitation. The laser intensity was varied between 0.016 kW/cm² and 5 MW/cm², values lower than that needed to produce multiple photon dissociation of SF₆. Increasing the collision frequency of the absorbing molecules with an inert buffer gas is observed to quench the multiple photon excitation.     

Optically pumped 15.90 μm SF6 laser

Optically pumped vibrational transition lasing has been achieved for the first time in a nonlinear molecule. Laser radiation at 628.74 ± 0.02 cm^?1 was generated from SF₆ using CO₂ TEA laser excitation. The SF₆ pumping is shown to be via absorption of two photons.     

Continuous-wave CO2 laser spectroscopy of SF6, WF6, and UF6

The linear absorption of CO₂ laser radiation in SF₆, WF₆, and UF₆ has been measured by using optoacoustic detection techniques. Absolute absorption coefficients per Torr as low as 1 × 10^?7 cm^?1 Torr^?1 in a 2-cm active path length could be measured by taking advantage of calibration measurements performed with SF₆.     

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.     

Strengths of the SF6 transitions pumped by a CO2 laser

The absolute intensities of the SF₆ transitions from the ground state to ν₃=1 that fall within ±1.5 GHz of the CO₂ P(14), P(18) and P(20) laser lines have been calculated. Good agreement has been found between these theoretical results and the available experimental values, especially for the firm assignments of R(28) A⁰₂ and P(33) A¹₂ for SF₆ absorption nearest the CO₂ P(14) and P(18) laser emissions, respectively.     

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.     

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.     

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     

Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

A temperature measurement technique using SF₆ molecules as tiny probe thermometers is described, and results are presented, for large (CO₂) _N van der Waals clusters (with N ≥ 10²) in a cluster beam. The SF₆ molecules captured by (CO₂) _N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF₆ in the incoming beam. As a result, the nanoparticle temperature in the (CO₂) _N cluster beam is estimated as T _cl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO₂ laser are performed to determine the velocity (kinetic energy) of SF₆ molecules sublimating from clusters, and the cluster temperature is found to be T _cl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.     

High precision spectroscopy of SF6 with cw high pressure tunable CO2 laser

Investigations are reported on the frequency characteristics of the cw high pressure CO₂ laser and its use for high precision spectroscopy. The problem of using a cw tunable CO₂ laser with a mixture of a few isotopes (¹²C¹⁶O₂, ¹²C¹⁸O₂, ¹²C¹⁶O¹⁸O and others) in a cw high pressure tunable CO₂ laser is discussed. Broadening of the tuning range has given us the opportunity to find seven new absorption lines of SF₆ near the transition P(20) of the 00⁰1–10⁰0 band of the CO₂ laser.     

Measurement of the nonlinear refractive index of SF6 with a hollow prism

We measure the nonlinear refractive index of SF₆ at three different CO₂ laser lines. Higher terms than the cubic nonlinearity are found and the behavior of the refractive index as a function of the laser intensity depends strongly on the frequency used, reflecting the characteristic SF₆ absorption dynamics.     

Low-temperature absorption contour of the ν3 band of SF6

A band contour analysis is carried out for the ν₃ absorption in SF₆. Values of ΔB = ? (1.0 ? 1.5) × 10^?4cm^?1, ζ₃ = 0.701, and ν₀ = 948.2cm^?1 are found. Tentative assignments are given for the SF₆ rotational states which are pumped by the P(14) through P(22) lines of the CO₂ laser.     

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.     

Multiphoton absorption of broad-band CO2 laser radiation by SF6

Increase of the emission bandwidth of a high-pressure CO₂ laser up to 1.5 cm^–1 increases the multiphoton absorption cross-section of SF₆. Comparison with the previously found [9] increased absorption for shorter pulses suggests that this is also a bandwidth effect. Spectral structures as narrow as 1 cm^–1 above the 10^th absorption step are invoked to explain the observations. The temperature effect, which disappears in the broad-band case, confirms this view.     

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.     

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.     

Measurement of collisional multiphoton energy deposition by a new optothermal detector

The infrared multiphoton absorption of SF₆ under collisional condition has been measured by a new optothermal detector which is based on measuring the change of resistance of a platinum filament. The optothermal signal, i.e., the absorbance of CO₂ laser energy by SF₆, was enhanced by the addition of Ar or NH₃. But the increase of optothermal signal caused by adding NH₃ is much greater than that caused by adding Ar, mainly due to the energy siphoning from excited SF₆ molecules to NH₃ molecules.     

Observation of rotational population transfer effect for an HF chemical laser

When a CO₂ gas absorption cell was placed within the optical cavity of a low pressure pulsed HF laser, the specific vibration- rotation line of the HF laser disappeared, which influenced both the intensity and pulse duration of the neighbouring lines. This behavior strongly depends upon the rotational population transfer in HF. By measuring the time response of this behavior, it was found that the rate constant for the rotational population transfer was in the range of 10⁷ s^-1 torr^-1 for the SF₆/H₂ lasing mixture.     

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.