A high-resolution cw dye laser spectrometer

The construction and utilization of a high-resolution cw dye laser spectrometer is described. The laser incorporates pressure tuned intra-cavity etalons enabling the single mode output laser frequency to be tuned more than one nm in a continuous scan. In the scan the laser jumps from one cavity mode to the next in ～ 50 MHz steps. In comparison to molecular Doppler widths this mode jumping is negligible. As an example of the use of the instrument, a high-resolution absorption spectrum of molecular iodine between the sodium D lines is presented. A unique frequency counter provides the calibration of the absorption spectrum.     

Spectral considerations in the laser isotope separation of Uranium Hexafluoride

Key spectral features important in the laser isotope separation of Uranium Hexafluoride are reviewed. Specifically the (v ₃+v ₄+v ₆) band lying in the frequency range of the CO₂ laser, the 3v ₃ band which is covered by the CO laser, and the fundamentalv ₃ band of UF₆ are considered. Computer-calculated spectra show that the ternary combination bands (v ₃+v ₄+v ₆) and 3v ₃ are completely dominated by the solid Q-branches of some 300 to 1000 hot bands, but that the fundamentalv ₃ band has structured P-and R-branches with peaks and holes, provided the pressure is below a few torr. Laser isotope separation can be achieved either on the isotope-shifted slopes of the Q-branches envelopes (“Q-slope method”) or on the coincidence of a peak and a hole in the P-or R-branches regions (“peak-hole method”). Room temperature experiments were carried out in 1972/1973, which used a CO₂ laser with an internal U²³⁸F₆ filter tube that forced lasing on what is believed to have been the P-16 line falling on the right-hand Q-slope of the (v ₃+v ₄+v ₆) band of UF₆. With a chemical reaction for final isotope separation, the measurements yielded a U-235 enrichment factor of about 1.1, in agreement with the maximum possible value of 1.5 for Q-slope operation and the fact that a high cut was used.     

New ultra-high resolution dye laser spectrometer utilizing a non-tunable reference resonator

A new dye laser spectrometer utilizing a non-tunable reference resonator is described. The resonator consists of two Zerodur mirrors optically contacted to a Zerodur spacer. Frequency scanning of the laser is provided by acoustooptic modulation. Residual drifts of the resonator frequency — measured on line — are compensated automatically by corresponding corrections of the modulation frequency. The stability during several hours and the resettability of the dye laser frequency are±2.5 kHz and±10 kHz, respectively.     

A dye ring-laser spectrometer for precision spectroscopy

A dye ring-laser spectrometer is described, which is frequency-stabilized to a cavity. The potential short-term stability is on the Hz level. This is proved by analyzing the Allan standard deviation and the spectral density of the error signal used for stabilization. Precision frequency tuning is achieved by electrooptic generation of a single sideband to the laser frequency which is used for the stabilization, and by varying the modulation frequency. Thus, continuous tunability is achieved across a 3 GHz frequency span, at a about 1 Hz laser stability relative to the cavity, for time periods between 10^–5 and 1 s.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Synthesis of ethylamine with a cw tunable CO2 laser

The successful synthesis of ethylamine with a cw tunable CO₂ laser is reported. This action occurs at normal pressure (5.32×10⁴ Pa) and temperature (<100° C). No catalyst is used. The experiment shows a high directionality of this reaction. No other product except ethylamine is yielded. A possible mechanism for this reaction is discussed.     

A 1 MWP-terphenyl dye laser

P-terphenyl in cyclohexane, has been pumped by a KrF laser in a longitudinal configuration. Dye laser pulses of 11 mJ, 1 MW were obtained with over 10% conversion efficiency. N₂ purge of the dye solution is shown to increase the conversion efficiency. The tuning curve between 321.8 nm and 365.3 nm is presented.     

Efficient laser isotope separation of 13C using novel linear multi-pass cavity

¹³C isotope has been separated in the form of enriched product C₂F₄ by selective multi-photon dissociation (MPD) of Freon-22 (CHClF₂) using the 9P(26) laser line of a transversely excited atmospheric CO₂ laser. The non-linearity factor, γ, that determines the dependence of the yield of ¹³C isotope on the fluence (J/cm²) has been determined for various laser rotational lines (9P(20)–9P(26)) and the advantage of a lower γ in the case of 9P(26) is highlighted for macroscopic production of ¹³C isotope. It is also shown that a higher value of the optimum fluence at 9P(26) not only results in a higher enrichment efficiency but in a relatively lower value of γ also. The laser pulse energy is efficiently utilized for selective MPD of Freon-22 by focusing the pulse energy repeatedly with the help of a novel linear multi-pass cavity (LMPC). The novelty of this optical arrangement lies in its ability to maintain the laser fluence around an optimum value for a desired enrichment of ¹³C in the product. This also ensures a higher quantity of enriched product because of the higher reaction volume. The advantage of the LMPC over the conventionally used Herriott multi-pass cell has also been presented. The gain in reaction volume in the present optical cavity having 20 passes with a constant fluence in each pass is as high as 12. Isotope-selective MPD of Freon in a LMPC with constant fluence in each pass showed a distinct advantage in energy utilization to separate ¹³C isotope over the gradually reducing fluence case.This revised version was published online in August 2005 with a corrected cover date.     

Experimental selection of molecules for isotope separation by multiple-photon dissociation in an intense IR field

Consideration is given to selectivity estimation methods during polyatomic molecules dissociation by multiple photon absorption. Taking the CH₃NO₂ molecule as an example, the possibilities of selectivity estimations made on the electron-excited dissociation products (luminescence) are shown, as well as on the measurements of the energy absorbed in a strong IR field being transformed into heat. Some advantages of the last method of the selectivity estimation are discussed. The experiments on nitrogen-isotopes separation in the mixture of CH₃ ¹⁴NO₂ and CH₃ ¹⁵NO₂ molecules are carried out when exciting thev ₇ vibration with the isotope shift of about 7 cm⁻¹ and thev ₁₃ vibration with no isotope shift in the linear absorption spectrum. The contribution of secondary chemical reactions to the separation process is discussed.     

Effect of laser dye deterioration on performance

Loss of gain in a dye medium as a result of decomposition by pump photons is considered in terms of disappearance of dye molecules and the appearance of a new, single chemical product having absorption in the fluorescence band. The cases of small signal gain and saturated gain are applied to a single pass through a transversely excited amplifier. Loss of output, defined as “quantum yield of laser deterioration”,Q _L , is related to the true quantum yield of molecular destruction of the dye,Q _M , and other known parameters. For the experimentally common, saturated gain condition, the smallestQ _L can be isQ _M , which requires a photochemically bleachable dye with high gain, preferably at high concentration, in a long cavity, operating at high injection and pumping fluxes. Reversing these conditions, the highest valueQ _L can be, compared withQ _M , is unrestricted. This work was performed under the auspices of the U.S. Energy Research and Development Administration. Contract No. W 7405 Eng 48.     

Mechanism of isotope scrambling for CO2 laser photolysis of trifluoremethane-h and-d mixtures

The mechanism of isotope scrambling was studied for infrared multiphoton dissociation of trifluoromethane-h (CHF₃) and-d (CDF₃) mixtures by a CO₂ TEA laser. The CO₂ laser was tuned to the R(14) line of the 001–100 transition, where CDF₃ absorbs the light selectively. Although the highly selective decomposition of CDF₃ occurred at low partial pressures of CDF₃, significant decomposition of CHF₃ was also observed with increasing pressure of CDF₃. The selectivity (s) was found to depend only on the pressure of CDF₃, a resonant molecule, but not on that of CHF₃, an off-resonant molecule. The mechanism involving the formation of two kinds of vibrationally excited molecules, unimolecular decomposition (rate constantk ₂ ),V-V energy transfer (k _3b), and collisional deactivation leads to the simple relationship,s=1+(k ₂ /k _3b)/[CDF₃], which can explain qualitatively the observed results. The ratiok ₂ /k _3b has values of 20–200 Torr, probably depending on experimental conditions.     

Shorter dye laser wavelengths from substitutedp-terphenyls

The spectral and laser characteristics of 14 CH₃-, CF₃-, and F-substitutedp-terphenyls have been investigated. 2,2″-dimethyl-p-terphenyl in cyclohexane solution exhibits a shortest tuned laser wavelength of 311.2 nm, thus extending the range of dye laser emission by 1000 cm⁻¹ in the uv. Peak output powers of 1.5 MW and 14% conversion efficiency at 332 nm were obtained under KrF-laser pumping in an untuned cavity.     

A grating tuned cw dye laser

We report on the operation of a rhodamine 6G cw dye laser involving a simple tuning mechanism capable of achieving high output powers (～500 mW) and resolving power (0.004 nm) close to the Doppler limit for a range of light gases. The system employs a 600 line/mm diffraction grating external to, but coupled with a high Q laser cavity; continuous tuning over 50 nm of the fluorescence spectrum of the dye was obtained.     

Laser isotope separation of 13C: A comparative study

IR laser chemistry of (CF₃Br/Cl₂) mixture and neat CF₂HCl are examined in the context of ¹³C enrichment. Decomposition extent, enrichment factor and energy absorbed are measured for both systems at their respective optimum conditions. A direct comparison is obtained by keeping extraneous factors such as laser, its pulse duration, cell, irradiation geometry etc. the same. The halogen scavenged CF₃Br MPD requires lower fluence compared to neat CF₂HCl irradiation. Overall throughput for a product with 60–65% ¹³C content in a single stage is the same for both systems requiring a similar amount of energy. However, at lower enrichment levels, CF₂HCl MPD is better than (CF₃Br/Cl₂) photolysis in terms of both product yield and energy absorption.     

The application of a high pulse repetition rate CO2 laser with high average power for isotope separation by molecular dissociation in a strong IR field

Considering a SF₆ molecule we demonstrate feasibility of using high pulse repetition rate CO₂ laser for isotope separation by selective molecular dissociation in a strong IR field. Dependences of dissociation efficiency as well as separation selectivity on pulse repetition rate up to 150 Hz are investigated. The inherent thermal effects are discussed.     

A high-power single-mode cw dye ring laser

Due to spatial hole burning, standing-wave dye lasers require a large amount of selectivity inside the cavity for single-mode operation. The output power of these lasers is limited by losses caused by the frequency selecting elements. In a travelling-wave laser, on the other hand, spatial hole burning does not exist, thereby eliminating the need for high selectivity. A travelling-wave cw dye laser was realized by unidirectional operation of a ring laser, yielding single mode output powers of 1.2 W at 595 nm and of 55 mW in the UV-region with intracavity frequency doubling.     

Tunable distributed-feedback laser based on dye doped microporous quartz glass

The first distributed feedback (DFB) dye laser on the basis of microporous quartz glass (MQG) is reported. MQG consists of a specially processed quartz sample doped with a dye. As a pumping source a second harmonic YAG: Nd³⁺ laser with 12.5 pps repetition rate was used. Stable generation of the narrow line tuned within a spectral range of 562–584 nm and 633–663 nm with a linewidth of 0.04 nm was obtained. A high photochemical stability of the dye in the quartz matrix and reliability of MQG as an active medium for the tunable DFB laser were experimentally demonstrated.     

Second-stage enrichment in the laser separation of carbon isotopes

There is a general agreement that efficient infrafed laser induced separation of carbon isotopes requires a two-stage process. An efficient first stage 1%50%¹³C enrichment was shown by Gauthier et al. [1] to be feasible and competitive with conventional technology. In this work, second-stage CO₂ laser enrichment of equimolar mixtures of¹²CHClF₂ and¹³CHClF₂ has been demonstrated yielding tetrafluoroethylene containing 95% or 99%¹³C. Forward enrichment by selective decomposition of the¹³CHClF₂ fraction was very efficient, absorbing only 6 and 16 eV, respectively, per carbon atom produced at 95% and 99%¹³C content.On leave from the Institute of Physical and Chemical Research, Hirosawa, Wako-shi, Saitama, 351, Japan, Summer 1983Issued as N.R.C.C. No. 23638NRC/Acadia University Summer Student, 1983     

Tritium isotope separation by CO2-laser irradiation at low temperatures

Tritium isotope separation by CO₂-laser induced multiphoton dissociation of CTF₃ is investigated. For the optimization of the performance of this working substance, trifluoromethane, the conditions to yield high-selectivity at high-operating pressure and low-critical fluence for complete dissociation are studied using our deconvolution procedure. The irradiation conditions are varied over the following ranges; wavenumber: 1052–1087 cm^–1, gas temperature: 25°C to –78°C, CHF₃ pressure: 5–205 Torr. The selectivities exceeding 10⁴ are observed for 85–205 Torr CHF₃ at –78°C by the irradiation at 1057 cm^–1.     

High resolution dye laser spectrometer for measurements of isotope and isomer shifts and hyperfine structure

A high resolution cw dye laser spectrometer with a frequency calibration system of 100 kHz resolution is described. Two examples of measured isotope shifts will be discussed.     

Astigmatically compensated cw dye laser resonators using lenses

The compensation of the astigmatism of Brewster-angled dye cells can be performed with lenses as well as with spherical mirrors. The advantages of cw dye laser resonators using lenses lie in the linear configuration of the optics and the convenient adjustability of astigmatism compensation. An output efficiency of 30% already at 1.6W pumping power has been observed. In addition, ring laser resonators with lenses have been investigated and they delivered up to 400 mW single mode power in travelling wave operation at less than 2 W pumping power.