Dissociation of NH3 to electronic ground-state fragments by high-intensity CO2 laser radiation

The dissociation of NH₃ into electronic ground-state fragments NH₂(²B₁) + H by high-intensity CO₂ laser radiation has been observed under essentially collision free conditions and as a function of NH₃ pressure, using laser fluorescence excitation for the NH₂ detection. Luminescence from excited NH₂ (²A₁) produced directly by the CO₂ laser has also been observed. The relative yield of NH₂ (²A₁) is estimated to be smaller than that of NH₂ (²B₁) by several orders of magnitude.     

采用圆偏振啁啾飞秒激光脉冲操控CS2分子的相干拉曼散射过程

采用两束圆偏振啁啾飞秒激光脉冲,非共线相干激发三原子分子CS₂液体. 在相位匹配的方向上,探测到由CS₂频率为397 cm^-1的振动模式产生的强度对称分布的相干反斯托克斯拉曼散射(CARS)信号和相干斯托克斯拉曼散射(CSRS)信号. 当调整两束激发光的圆偏振状态时,CARS,CSRS信号的强度、偏振、波长均发生规律性的改变:CARS,CSRS信号的强度分布反映了CS₂ 在不同极化状态下的受激拉曼散射截面大小;信号光的 关键词： 啁啾脉冲 相干反斯托克斯拉曼散射(CARS) 相干斯托克斯拉曼散射(CSRS) 2')" href="#">CS₂     

New FIR laser lines from an optically pumped far-infrared laser with isotopic 13C16O2 pumping

A ¹³C¹⁶O₂ laser optically pumping a FIR laser has resulted in 17 new FIR cw emissions from 78.5 μm to 917 μm. The FIR media were: CD₃OD, CH₃OD, CD₃OH, NH₃ and ¹⁵NH₃. Interesting effects have been observed with a combination of NH₃ and CD₃OD resulting in a new FIR emission. Two new FIR emissions at 181.5 μm and 355.5 μm have been observed with a ¹²C¹⁶O₂ laser optically pumping CD₃OD.     

Single‐beam coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO2 via phase and polarization shaping of a broadband continuum

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO₂ is demonstrated using a single femtosecond (fs) laser beam. A shaped ultrashort laser pulse with a transform‐limited temporal width of ∼7 fs and spectral bandwidth of ∼225 nm (∼3500 cm⁻¹) is employed for simultaneous excitation of the CO₂ Fermi dyads at ∼1285 and ∼1388 cm⁻¹. CARS signal intensities for the two Raman transitions and their ratio as a function of pressure are presented. The signal‐to‐noise ratio of the single beam–generated CO₂ CARS signal is sufficient to perform concentration measurements at a rate of 1 kHz. The implications of these experiments for measuring CO₂ concentrations and rapid pressure fluctuations in hypersonic and detonation‐based chemically reacting flows are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

CARS diagnostics of a CO2-laser pyrolytic experiment for the production of nanosized ceramic powders

Spatial distributions of rotational temperatures and molecular number densities of C₂H₂ and H₂ were measured with CARS during the production of ultrafine SiC powders in a laser pyrolytic process flame. By means of a CO₂ laser, the reaction gases SiH₄ and C₂H₂ (or alternatively C₂H₄) are converted into SiC and H₂. From the CARS measurements temperature gradients are determined between 8.8 × 10⁵ K/m and 1.6 × 10⁶ K/m with corresponding heating rates of 1.8 × 106 K/s and 1.3 × 10⁶ K/s. The CARS data also allow an estimation of the gas expansion behaviour in the reaction zone. Moreover, they show that diffusive velocity components of the hydrogen in the hot reaction zone do not exceed 0.4 m/s.     

High-power,line-tunable14NH3 and15NH3 lasers

A high-power, line-tunable NH₃ laser pumped by a TEA CO₂ laser has been developed for application to laser isotope separation of tritium. Laser lines from¹⁵NH₃ are newly observed in a range from 11.24 to 13.23 m. The performance of an¹⁵NH₃ laser is compared to that of an¹⁴NH₃ laser.     

High resolution diode laser measurements on the ν2 bands of 14NH3 and 15NH31

More than 500 lines in the ν₂ bands of ¹⁴NH₃ and ¹⁵NH₃ have been measured in the region 740–1200 cm^?1 with a diode laser spectrometer, with an accuracy better than 0.0005 cm^?1 for most of the lines. Wavenumbers (in cm^?1) were determined using a 3-in. Ge etalon for calibration and OCS, N₂O, NH₃, and CO₂ lines as references. The diode laser data were combined with pure inversion and inversion rotation frequencies and sets of rotational constants were obtained by the method of merged least squares. Perturbations between the Δk = ±3 levels have been taken into account in these calculations.     

CARS investigations of quenching and photochemical reactions in the Na+H2 collision system

Summary We have carried out parallel studies of the quenching process in Na(3p)+H₂ collisions and the possible reactive process in Na(3p)+H₂ (v ^″=1,2,3) collisions. Rich CARS spectra which were obtained at H₂ pressure of 100 mbar and oven temperature of 600 K indicate the presence of vibrationally excited H₂ and photochemically produced NaH molecules. Temporal resolution of NaH CARS lines was employed in order to rule out competing collisional processes. We make use of resonantly enhanced CARS methods which enabled us to achieve very high sensitivity for NaH detection. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

Temperature measurement by single-shot dual-line CARS in low-pressure flows

A CARS method, adapted for diagnostics in low-pressure unsteady flows is described. The technique employs two narrow-band dye lasers and a single-frequency pump laser. Single-shot rotational temperature is obtained from the ratio of the intensities of two isolated Q lines. Temperature-measurement accuracy is discussed. The advantage of referencing the CARS signals from the flow in a cell filled with the same gas is shown. Demonstrative experiments are performed using N₂ at pressures of 10²–10³ Pa.     

Infrared laser stark spectroscopy of ammonia

Refined values for molecular constants of¹⁴NH₃ and less precise values for those of¹⁵NH₃ and¹⁴NH₂D were obtained with laser Stark spectroscopy. Paper was prepared for presentation at FICOLS     

Self focussing of CO2 laser radiation in NH3 gas

Self focussing of CO₂ laser radiation in ammonia gas is reported for a number of laser emissions in near coincidence with NH₃ absorption lines. Detailed results for the aR(11) NH₃ transition are in excellent accord with theory accounting for intensity dependent saturation of both absorption and the associated anomalous dispersion responsible for nonlinear refraction n₂, giving a value for n₂ of 2.375 x 10^-11 esu torr^-1.     

Optically pumped submillimetre laser action in formaldoxime and ammonia

Strong submillimetre laser action has been obtained on five lines of a new laser gas, formaldoxime. Several new emission lines have also been produced by isotopic CO₂ laser pumping of¹⁴NH₃ and¹⁵NH₃. One of these lines at 102.9 m is a rotation-inversion transition in the 2v ₂ state, and is the first example of cw laser action in such a highly excited state.     

Sensitive detection of H2 molecules by two-photon excited laser-induced fluorescence

The sensitive detection of H₂ molecules was demonstrated by means of twophoton excited laser-induces fluorescence spectroscopy with a narrow-band ArF excimer laser. A detection limit of 2×10¹⁴ cm^–3 was obtained with an excitation power of 150 kW. This is already comparable with that obtained by the coherent anti-Stokes Raman scattering (CARS). This technique was successfully applied to measure a spatial distribution of H₂ in a town-gas burner.     

Narrow-band BoxCARS applied to CO2 laser discharges

Summary CARS (Coherent Anti-Stokes Raman Scattering) has developed into a powerful tool for studying molecular systems. One of its possibilities is to derive vibrational and rotational temperatures as well as concentrations of molecules from measurements of the energy level population differences. A very good spatial resolution of CARS technique is one of its important advantages. This feature has been utilized for making spatially resolved measurements of the vibrational and rotational temperatures of N₂ in a d.c.-excited transverse-flow CO₂ laser discharge. Apart from that also spectra of CO₂, CO and O₂ in the discharge have been taken, which allowed us to evaluate the spatial distributions of those components in the discharge. Additionally first investigations of a microwave-excited CO₂ laser module have been performed for comparison. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

Two-photon spectroscopy of 15NH3 with isotopic CO2 lasers and double resonance studies with a new coincidence

Eleven transitions of the ν₂ band of ¹⁵NH₃ were measured by the technique of infrared-microwave two-photon spectroscopy using an isotopic CO₂ laser and 20-mW microwave power. By combining these results with those of previous two-photon measurements and with Stark laser spectroscopy results, the spectroscopic constants of this band were calculated. Using the previously unknown coincidence between the R(18) ¹³CO₂ laser line and the _asQ(5,4) infrared transition, velocity tuned multiple-photon Lamb-dip signals were observed. Very intense double resonance signals were observed on the (5,4), (6,4), and (4,4) microwave transitions and the magnitude of these effects was compared to those observed previously in ¹⁴NH₃.     

Voltage-current characteristics of superatmospheric pressure CO2 laser discharges

The IR NH₃ laser radiation was applied for selective dissociation of CCl₄ molecules. The dissociation yield, its pressure dependence and isotopic selectivity were measured. Two-frequency dissociation of CCl₄ by the NH₃ laser and the CO₂ pump laser radiations was carried out. It was shown that the NH₃ laser is very effective for CCl₄ dissociation.     

Diagrammatic representation of the third-order polarization in transient CARS

Summary A diagrammatic perturbative approach is adopted to derive the expression for the third-order polarization,P ⁽³⁾ which originates the anti-Stokes emission in a time-resolved CARS experiment. The various contributions toP ⁽³⁾ are calculated assuming that laser fields are off-resonance with respect to any electronic level of material system. The resonant part of the polarization consists of two terms, in which the roles of the pump and probe pulses at frequency ω₁ are exchanged. The global expression allows the direct calculation of the signal time profile in a transient CARS experiment once a model is assumed for the laser pulse shape.     

New cw two-photon pumped and Raman fir laser lines in 14NH3 and 15NH3

Application of infrared-microwave two-photon pumping on ¹⁴NH₃ and ¹⁵NH₃ yields 17 new cw laser lines in the far infrared. In addition a new Raman laser line in ¹⁵NH₃ is found.     

Experimental investigation of the MIR optically pumped ammonia bidirectional ring laser

We have studied the Doppler-broadened 11.76 m ¹⁵NH₃ emission line optically pumped in a ring resonator by a cw CO₂ laser operating on the 10R(42) line. In the pure ammonia, even if the pumping occurs inside the Doppler width, the two-photon Raman process seems to be responsible for the laser emission. Both in the pure ¹⁵NH₃ and in the ¹⁵NH₃, N₂, and He mixture we demonstrate the possibility of obtaining a unidirectional laser amplifier.This work has been supported by Direction des Recherches Etudes et Techniques under contract No. 87/222     

Strong selective excitation of Raman-active vibrations and energy transfer between low-lying vibrational states of a CO2 molecule studied by PARS and CARS

We experimentally demonstrate the novel technique of inducing the highly nonequilibrium distribution of molecules over vibrational states by two-frequency coherent Raman excitation. The technique can be used for selective excitation of totally symmetric and other Raman-active molecular transitions. Two counter propagating focused laser beams (second harmonic of Nd: YAG and dye laser) were used to induce population difference changes at the 00⁰0–10⁰0 transition in CO₂ molecules. The excitation and relaxation kinetics of this and neighbours vibrational modes of CO₂ were studied both by CARS and PARS. Vibrational excitation of up to 20% of the total number of irradiated molecules is measured; previously unknown desactivation constant of CO₂ (10⁰0) and CO₂ (02⁰0) states via CO₂ (01¹0) state is estimated to be K = (3±1) × 1 ⁵s^-1torr^-1.