Simultaneous phase-lock of five CO2 lasers to a primary Cs frequency standard

The output of a CO₂ laser, operating on theP _I(18) transition of¹³C¹⁶O₂ at 26941 GHz (11.128 m) was phase-locked to a 5 MHz signal from a primary Cs frequency standard by means of a frequency chain having only CO₂ lasers as infrared sources. Simultaneously, four other CO₂ lasers in the chain were phase-locked to the 26941 GHz output. This provided CO₂ laser frequencies at 26 450 305, 26 940 815, 28 694 625, 29 442 480, and 33 185 715 MHz having zero long-term-average frequency error relative to the Cs standard, and the ±10^–13 (3 Hz) long-term absolute uncertainty of the standard.     

Assignment of the HCOOH CW-submillimeter laser

Twenty one laser lines of the 250–1000 GHz range have been assigned in the v₆ and v₈ excited states of the H¹²COOH molecule. The microwave study of these two states has allowed us to determine the molecular constants and to calculate the energy levels up to J = 50. The values of the energy levels of the ground state are well known and allow the frequency calculation of infrared rovibrational transitions near the 9.6–10.6 μm region and the comparison with the frequencies of the CO₂ laser lines. A microwave infrared double-resonance experiment has also been performed to confirm assignment based on the calculation of the energy levels. The value of the two band centers has been determined.     

Heterodyne frequency measurements on carbonyl sulfide near 1050 cm

Heterodyne techniques have been used to measure the frequency differences between carbonyl sulfide (OCS) absorption lines and CO₂ laser transitions. A tunable diode laser was used both to scan the OCS absorption spectrum and to provide a beat signal against a CO₂ laser. Frequency differences as great as 8.6 GHz were measured. Many different OCS hot-band transitions were measured near 1050 cm⁻¹, and the measurements on the 02⁰0-00⁰0 band have been extended to such high J levels (J′ = 86) that higher-order centrifugal distortion terms are needed to fit the data.     

Evaluation of downstream-mixing scheme for 9.4-µm CO2 gasdynamic laser

A theoretical analysis of a downstream-mixing 16-μm CO₂ gasdynamic laser revealed the possibility of utilizing the downstream-mixing scheme for the generation of 9.4-μm radiation using a CO₂ gasdynamic laser. The flow-field has been analyzed using complete two-dimensional, unsteady laminar form of Navier-Stokes equations coupled with the finite rate vibrational kinetic equations. The analysis showed that integrated small-signal gain of 11.5m⁻¹ for Lorentzian broadening and 4.8m⁻¹ considering Voigt function can be obtained for N₂ reservoir temperature of 2000°K and velocity ratio 1:1 between the CO₂ and N₂ mixing streams. These results (presented in graphs) clearly highlight the large potential of downstream-mixing CO₂ gasdynamic laser for 9.4-μm laser generation.     

Infrared-microwave two-photon spectra of the ν3 bands of 12CH3F and 13CH3F

Infrared-microwave two-photon spectra have been obtained for the ν₃ bands of ¹²CH₃F and ¹³CH₃F with a two-photon spectrometer employing a CO₂ laser and a computer-coupled microwave source operating in the 8–18 GHz region. Even though the intensities of the spectra for the double parity levels in these molecules are inversely proportional to the square of the microwave frequency, transitions have been observed with microwave frequencies of up to 16 GHz. Comparison of these observed two-photon frequencies to frequencies predicted from infrared laser Stark spectroscopy, and to frequencies calculated from vibration-rotation parameters obtained by fitting these and other frequencies, shows agreement to within a few MHz. Spectroscopic parameters for the ground and ν₃ excited states of the two species are reported.     

The submillimeter wave spectrum of 32S16O2, 32S16O2(ν2), and 34S16O2

Extensive new measurements in the region 400–1000 GHz have been made on ³²S¹⁶O₂, ³²S¹⁶O₂(ν₂), and ³⁴S¹⁶O₂. These measurements represent almost a threefold extension in the frequency region for which high-resolution microwave data are available. These data have been combined with the previously available microwave data for this analysis. The results, when extrapolated into the far infrared, compare favorably with recent results obtained from high-resolution FIR spectroscopy.     

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.     

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.     

Microwave spectrum of NF2

About 350 lines in the microwave spectrum of NF₂ have been measured in various ranges of frequency between 13.0 and 65.2 GHz by using two types of Zeeman effect spectrometers. Complete assignment of all lines has been achieved and, via the general microwave computer program SPINRO, the rotational constants, centrifugal distortion constants, dipole moment, electronic spin-rotation coupling constants, the constants for the coupling of the several nuclear spins with the electron spin and the nitrogen quadrupole coupling constants have all been obtained.By drawing upon the observed vibrational frequencies the average geometry of NF₂ has been evaluated. Force constants and Coriolis coupling constants have also been derived.The values of the spin coupling constants for N and for F indicate that NF₂ is a π-radical with the spin density mainly located on nitrogen. The multiplet patterns indicate that the ground electronic state wavefunction is antisymmetric to rotation about the molecular symmetry axis and so, for a π-radical, identifies the ground state as ²B₁ as has previously been assumed for this molecule.     

A superior microwave absorption material: Ni2+-Zr4+ Co-Doped barium ferrite ceramics with large reflection loss and broad bandwidth

Large reflection loss and wide bandwidth are significant targets, determining the microwave absorption ability. However, it is still a challenge to simultaneously satisfy the two conditions. As a multifunctional material, BaFe₁₂O₁₉ possess excellent electromagnetic properties in the microwave frequency band. Due to the natural resonance phenomenon of the material, BaFe₁₂O₁₉ can produce a large magnetic loss which correlates with Fe³⁺ content, and the microwave absorption characteristics of barium ferrite can be modulated by ion doping. As a typical magnetic metal, Ni coupled with high-valence state Zr⁴⁺ doping helps to produce double resonance peaks. In this work, Ni²⁺-Zr⁴⁺ co-doping M-type barium ferrites (BaFe_12-2xNi_xZr_xO₁₉, BNZFO-x, x = 0–0.8) were prepared conveniently by solid-state reaction method. Several necessary measurements to characterize its microwave absorption property have been operated such as morphology, magnetic performance and electromagnetic parameters. The results show that reflection loss and bandwidth can be simply tuned by tailoring Ni²⁺-Zr⁴⁺ content. The reflection loss peak drifts from 18 GHz to 9.76 GHz, which involves a half of the studied frequency range. The maximum reflection loss achieves −60.6 dB and the corresponding bandwidth over −10 dB is 7.68 GHz for BNZFO-0.6 ceramic with only 2.1 mm thickness. Thus, the doping of Ni²⁺-Zr⁴⁺ ion pairs is beneficial to improve the absorbing properties of the material, and the superior microwave absorption property may originate from its inner double natural resonance in micro-scale. The excellent microwave absorption properties suggest that BNZFO-x is a promising candidate applied for designing electromagnetic shielding devices.     

Investigations of self-, H2- and He-broadening for rotational transitions of HCCC15N,CF3D and CF3CCH by the microwave transient emmission technique

The pressure dependence of coherent dephasing time T₂ has been determined by investigating transient emission signals of molecular gas samples, following pulsed microwave excitation. Experimental results on cyanoacetylene (HCCC¹⁵N), deuterated fluoroform (CF₃D) and 3,3,3-trifluoropropyne (CF₃CCH) are reported for the pure gas and also for mixtures with H₂ and He for rotational transitions in the frequency range from 5.8 GHz to 19.6 GHz.     

Spin-orbit coupling and dissociation of CO2 molecules

Potential surfaces of the CO₂ molecule for the ground and excited ³ B ₂, ¹ B ₂ electronic states are calculated by quantum chemistry methods. The calculation of the spin-orbit coupling in the molecule shows a large the matrix element, which removes the prohibition for the dissociation-recombination process CO₂(X ¹Σ) + M ? CO(X ¹Σ) + O(³ P) + M. The barrier on the potential curve for ³ B ₂, the energy of which exceeds the limit of dissociation into components in the ground states, explains the data on the dissociation and recombination energies measured in experiments with shock tubes. The absorption cross section of CO₂ molecules in the UV spectral region measured at high temperatures allowed us to plot branches of potential curves near their minima for two upper singlet states assigned to the ¹ B ₂ and ¹ A ₂ symmetry.     

Heterodyne frequency measurements of carbonyl sulfide transitions at 26 and 51 THz. Improved OCS,O13CS,and OC34S molecular constants

Heterodyne frequency measurements were made on selected absorption features of carbonyl sulfide (OCS) near 26 THz (860 cm^?1) and 51 THz (1700 cm^?1). Frequency differences were measured between a tunable diode laser (TDL) locked to carbonyl sulfide absorption lines and either a stabilized ¹³CO₂ laser or a CO laser which was referred to stabilized CO₂ lasers. These measurements are combined with conventional TDL measurements and published microwave measurements to obtain new, more reliable molecular constants for OCS, O¹³CS, and OC³⁴S. New frequency measurements are given for nine CO laser transitions between 1686 and 1726 cm^?1.     

The microwave spectrum and centrifugal distortion constants of difluoroborane,HBF2

The microwave spectra of two isotopic species of the unstable molecule difluoroborane have been measured between 8 and 36 GHz. Transitions have been measured up to J = 56 and K_a = 9 for H¹⁰BF₂, and up to J = 66 and K_a = 11 for H¹¹BF₂. Improved values for the rotational constants, boron nuclear quadrupole coupling constants, and quartic and sextic centrifugal distortion constants have been evaluated for both species.     

High frequency study of electropolymerised conducting polypyrrole thin film

The polypyrrole (PPY) thin films were synthesized by electropolymerisation in potassium nitrate solution. The substrate used was stainless steel. DC conductivity, microwave reflection, microwave conductivity and microwave dielectric constant of the conducting PPY thin films are reported. DC conductivity was between 1.6 × 10⁻² S/cm and 42.3 × 10⁻² S/cm. Microwave conductivity was between 10 S/cm and 160 S/cm. The ?′ generally decreases as frequency increases similarly ?″ also decreases with increases in frequency. The measurements have been carried over the frequency range 8.2-12 GHz. These polypyrrole thin films were characterized by FTIR. The polypyrrole thin film increases the reflectivity of the stainless steel.     

Simple frequency measurement chain to 30 THz

We report harmonic mixing experiments between a 72 GHz mm-wave and 3.7 THz NH₃-laser radiation, as well as between 3.7 THz and the 10 m R(30) CO₂-laser line. For the former mixing a semiconductor diode is used, while for the higher frequencies an MIM tunnel diode served as the nonlinear element. Beat signals are of sufficient signal to noise ratio for precise optical frequency measurement.Visiting from National Institute of Metrology, Beijing, P. R. China     

Absolute frequency measurement of CO2 laser lines with a femtosecond laser comb and new determination of the CO2 molecular constants and frequency grid

Absolute frequency measurements of a CO₂ laser stabilized on saturated absorption resonances of CO₂ laser lines are reported. They were performed using a femtosecond-laser frequency comb generator and two laser diodes at 852 and 782 nm as intermediate oscillators, with their frequency difference phase-locked to the CO₂ laser. Twenty ¹²C¹⁶O₂ laser lines in the P and R bands at 9 μm were measured with a relative uncertainty of a few 10⁻¹² limited by the CO₂ frequency reproducibility. A new determination of the CO₂ molecular constants was obtained from these data and previous measurements in the 10 μm band. The CO₂ frequency grid was also calculated, with an improvement of two orders of magnitude compared to the previous grid of Maki et al. [J. Mol. Spectrosc. 167 (1994) 211].     

Infrared-microwave two-photon spectroscopy of the ν2 band of 14NH3

Transitions in the ν₂ band of ¹⁴NH₃ were recorded by means of an infrared laser microwave two-photon spectrometer. The spectrometer, which uses a minicomputer to step the microwave frequency and record the spectrum, is described. With sample cells outside the laser cavity good lineshapes are obtained, so that the accuracy of frequency measurement was limited by the resettability of the CO₂ or N₂O lasers employed, ～0.0002 cm^?1. The present data are compared to previously obtained results based on CO₂ or N₂O laser frequencies and to recently reported calculations. Rotational constants derived for the hypothetical inversion-free ground and v₂ = 1 states are reported.     

Single-crystal EPR studies of transition-metal ions in inorganic crystals at very high frequency

Electron paramagnetic resonance (EPR) single-crystal rotation studies at very high frequency (249.9 GHz) of transition metal ions with electron spins greater than one-half are reported. At 249.9 GHz, the spectra are in the high-field limit despite large zero-field splittings. This leads to a considerable simplification of the spectra, and aids in their interpretation. Single-crystal 249.9 GHz EPR spectra of Ni²⁺ in Ni₂CdCl₆· 12H₂O, Mn²⁺ (0.2%) in ZnV₂O₇, and Fe³⁺ (2%) in CaYA10₄ were recorded at 253 K in an external magnetic field of up to 9.2 T, along with those at X-band and Q-band frequencies at 295 K and lower temperatures. The goniometer used at 249.9 GHz for single-crystal rotation is based on a quasi-optical design and is an integral part of a special Fabry-Pérot resonator. The values of the spin-Hamiltonian parameters were estimated from a simultaneous fitting of all of the observed line positions at several microwave frequencies recorded at various orientations of each crystal with respect to the external magnetic field with least-squares fitting in conjunction with matrix diagonalization. Estimates of zero-field splitting parameterD at room temperature are: for Ni²⁺, about ?31 GHz (site I) and about ?7 GHz (site II); for Mn²⁺, about 6 GHz; and for Fe³⁺, about 29 GHz.     

Simultaneous detection of CO and CO2 at elevated temperatures using tunable diode laser absorption spectroscopy near 1570 nm

Simultaneous measurements of CO and CO₂ at elevated temperatures are demonstrated using a single semiconductor distributed-feedback (DFB) laser near 1570 nm. Wavelength modulation spectroscopy with second-harmonic detection is used to improve the detection sensitivity and accuracy. A proper line pair near 6368.086 and 6368.330 cm^?1 is selected using some line-selection criterions for the target temperature range of 300–1000 K. Normalization of the 2f signal with the 1f signal magnitude is used to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling. The CO and CO₂ concentrations measurements are within 2.21% and 2.55% of the expected values over the tested temperature range of 300–1000 K. The minimum detectable concentrations of CO and CO₂ at 1000 K are 80 ppm m and 153 ppm m, respectively.