Superradiance effects on the infrared absorption and vibrational temperature of optically pumped molecules

Using controlled feedback and coincident far infrared absorption, we have shown that the superradiant emission process in NH₃ pumped by the 9R16 line of a CO₂ TEA laser has a marked effect on the absorption of laser energy and vibrational heating of the gas. Since many polyatomic molecules pumped by IR laser are efficient superradiant far infrared sources, the effect is important for laser induced fluorescence studies in these polyatomics.     

Single- and multi-photon absorption of carbon dioxide laser lines by SO2 and CO2 molecules

Summary We have used 60 rovibrational lines of a Lumonics TEA CO₂ laser to record the spectra of coincidence of SO₂ IR absorption with CO₂ laser emission. We extended the same type of investigation to CO₂ molecules and observed an absorption at practically all the rovibrational lines of the laser. We have also recorded the increase of absorption with pressure of the studied molecule and with its temperature. Our work reveals very accurately the fine structure of these absorptions that shows a clear discreteness in the intensities of the lines. Some implication of this discreteness in multiphoton absorption is also presented, through data that indicate very strong vibrational coupling between ground and excited electronic states of SO₂. The authors of this paper have agreed to not receive the proofs for correction.     

Methanol laser lines from torsionally excited co stretch states, and from OH-bend, CH3-rock, and CH3-deformation states

Using a quasi-CW CO₂ oscillator-amplifier combination with peak power 300 Watt, we have generated FIR laser emission in weak absorption bands of CH₃OH. 40 new lines are reported, and their wavelengths are measured with a relative accuracy of 5×10^–5. A total of 72 lines are assigned. 34 of these involve torsional n=1, 2, and 3 states of the CO stretch and the vibrational ground state. The remaining lines are associated with the CH₃-rock, OH-bend, and CH₃-deformation modes. The latter are located 1460 cm^–1 above the ground state, and are pumped by simultaneous vibrational excitation and torsional deexcitation.     

Laser optogalvanic spectrum of pure bromine discharge

Doppler limited laser optogalvanic (LOG) spectra are obtained by irradiating a bromine discharge with a cw dye laser. The discrete bands of the Br₂ B-X system appear superimposed on a strong continuum. The LOG spectrum is closely identical with the absorption/emission spectrum of Br₂. Some extra bands and assigned in theB-X system are also observed and their vibrational quantum number assignment is given.     

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].     

Optoacoustic laser spectroscopy of excited vibrational molecular states

A new detection method for absorption from excited vibrational states is suggested, based on optoacoustic detection of weak absorption in a heated gas. Using this method CO₂ laser radiation (λ=9.6 μm) absorption was investigated from excited vibrational states of CO₂, BCl₃, and BF₃ molecules.     

Emission Spectra Inside the Ammonia s-Multiplets Pumped by a Line-Narrowed Tunable CO2 Laser

By using a line narrowed high power tunable laser we have analysed the FIR emission spectrum inside some , compact sQ, sR k-multiplets. Jumps between adjacent k-systems are observed , although tunable emissions of dominant lines are produced. The more favourable gain overlap of the K Raman lines in the fundamental vibrational level induces an off-resonance dominance of this process respect to the standard Raman scattering in the excited vibrational level. The AC Stark splitting has been also observed when the CO₂ laser is posed in resonance with the absorption lines.     

Laser induced infrared fluorescence of Cs2 and Rb2

Laser induced cross fluorescence of Cs₂ and Rb₂ molecules in the infrared region (1.0–1.6 μm) has been observed using several exciting wavelengths from an argon-ion laser and a tunable cw dye laser. New emission bands have been observed. The lower states of these bands have gerade symmetry and therefore have heretofore escaped observation by conventional absorption spectroscopy.     

Evaluation of ammonia absorption coefficients by photoacoustic spectroscopy for detection of ammonia levels in human breath

Photoacoustic spectroscopy represents a powerful technique for measuring extremely low absorptions independent of the path length and offers a degree of parameter control that cannot be attained by other methods. We report precise measurements of the ammonia absorption coefficients at the CO₂ laser wavelengths by using a photoacoustic (PA) cell in an extracavity configuration and we compare our results with other values reported in the literature. Ammonia presents a clear fingerprint spectrum and high absorption strengths in the CO₂ wavelengths region. Because more than 250 molecular gases of environmental concern for atmospheric, industrial, medical, military, and scientific spheres exhibit strong absorption bands in the region 9.2–10.8 μm, we have chosen a frequency tunable CO₂ laser. In the present work, ammonia absorption coefficients were measured at both branches of the CO₂ laser lines by using a calibrated mixture of 10 ppm NH₃ in N₂. We found the maximum absorption in the 9 μm region, at 9R(30) line of the CO₂ laser. One of the applications based on the ammonia absorption coefficients is used to measure the ammonia levels in exhaled human breath. This can be used to determine the exact time necessary at every session for an optimal degree of dialysis at patients with end-stage renal disease.     

Study of Mn laser ablation in methane atmosphere

Laser ablation of Mn target in vacuum and in the presence of CH₄ was studied under 308 nm laser irradiation. Time-resolved emission using gated detection and scanning monochromator and absorption using the cavity ring-down spectroscopy were used to study vaporized plume. In the CH₄ atmosphere we observed transitions identified as C₂ and MnH bands, while these spectral features were not detected in emission spectra. This is a clear evidence of importance in combining both spectroscopic techniques in laser vaporized plume study.     

Quenching of infrared emission from CO2 by near-resonant vibrational interactions with SF6, BCl3, and PF5

The transfer of substantial amounts of vibrational energy from CO₂ to specific complex molecules has been observed in flow-tube experiments by monitoring the spectra from the 4·3 μm fundamental bands and the 2·7 μm mixed-mode Fermi-resonance bands of CO₂. Small amounts of SF₆, BCl₃, and PF₅ were found to reduce the intensities of these emissions significantly. Quenching cross-sections were calculated using a non-equilibrium analysis.     

Absorption of CO 2 laser emission by freon-12

The infrared (IR) absorption of freon-12 (CF₂Cl₂) was studied in the emission range of a 3-W tunable CW CO₂ laser by using a brass cell with KBr windows that was located outside the laser resonator. The results show that CF₂Cl₂ absorbs all CO₂ laser emission lines in the ranges of 1073–1083 cm^-1 and 937–943 cm^-1. The most strongly absorbed laser line was 10P (28) ( 937.21 cm^-1). Absorption coefficient values were obtained for all available wavelengths of the CO₂ laser as the CF₂Cl₂ pressure was varied from 5 to 1000 mbar. By using the HITRAN database for freon-12, the absorption coefficients were calculated at the 10P (28) and 9R (28) lines as functions of the gas pressure and compared with the experimental values. The calculated results are in reasonable agreement with the experiment. PACS 33.20.Ea; 42.55.-f; 42.55.Lt     

ICLAS of C2H2 between 10 140 and 10 600 cm

The absorption spectrum of ¹²C₂H₂ has been recorded by intracavity laser absorption spectroscopy (ICLAS) in the spectral region 10 140-10 600 cm⁻¹, where three absorption bands were previously observed by Fourier transform spectroscopy. Thirteen bands starting from the vibrational ground state could be detected and rotationally analyzed. All corresponding excited vibrational levels could be assigned using the polyad model (M. I. El Idrissi, J. Liévin, A. Campargue, and M. Herman, J. Chem. Phys.110, 2074-2086 (1999)). The assignment procedure is detailed and relative intensity features are discussed.     

The absorption spectrum of 12C2H2 IV. The regions 7600–9200 cm−1 and 10600–11500 cm−1

The absorption spectrum of ¹²C₂H₂ has been recorded by intracavity laser absorption spectroscopy (ICLAS) in the 10600–11 500 cm^?1 spectral region, where no absorption bands were previously reported. Fifteen bands starting from the vibrational ground state are observed and rotationally analysed. All corresponding excited vibrational levels were assigned using the polyad model, the so-called cluster model (El Idrissi, M.I., Liévin, J., Campargue, A., and Herman, M., 1999, J. chem. Phys., 110, 2074) which allows vibrational energies, rotational B_v constants and, to some respect, relative band intensities to be predicted. Additional data and constants are also provided in the range 7600–9200cm^?1, whenever improving the literature results, from spectra recorded previously at ULB using Fourier transform spectroscopy. The assignment procedure in the range recorded by ICLAS is detailed, leading to a deeper understanding of vibration-rotation and intensity features of the absorption bands within the frame of the cluster model.     

Absorption and emission spectra of yttria-stabilized zirconia and magnesium oxide

We have investigated the luminescence and absorption spectra of doped and undoped ZrO₂-Y₂O₃ and MgO crystals at room- and low temperatures. The crystals used are partly doped with the transition metals Ni, Co, Cr and the rare earth Pr. The emission spectra were obtained under laser excitation at different wavelengths. The observed optical emission and absorption bands of the MgO crystals doped with Ni, Co and Cr correspond to transitions between spin-orbit split crystal field levels of the transition metals. Luminescence and absorption bands of undoped yttria-stabilized zirconia (YSZ) crystals are due to color centers, absorption bands of the doped YSZ correspond to the well known transitions of the Ni²⁺, Co²⁺ and Pr³⁺ ions, respectively. The emission spectra of the doped YSZ obtained under various laser excitations can be explained by an energy transfer process between the color center and the doping materials. The influence of annealing on the absorption and emission of Pr³⁺/Pr⁴⁺ is investigated.     

IR absorption measurements and new FIR emission lines in CH3OH by use of a frequency-tunable CW waveguide CO2 laser

The IR absorption in CH₃OH in the vicinity of CO₂ laser lines has been measured quantitatively by use of a 300 MHz tunable waveguide CO₂ laser with output powers of about 3 W. Information on frequency offsets from the CO₂ line centers, small signal and saturated absorption coefficients of FIR laser pump transitions is obtained. Some stronger pump transitions with frequency offsets larger than 50 MHz gave rise to the observation of 8 new FIR emission lines with wavelengths from 43 to 125 μm.     

Interferometric determination of the dispersion of the vibrational polarizability and the integral intensities of fundamental bands of trifluorochloromethane molecules CF<Subscript>3</Subscript>Cl

Using two-color interferometry, the dispersion of the refraction and the vibrational polarizability of freon molecules CF₃Cl of natural isotopic composition was studied in the lasing range of an IR laser on the basis of the main isotopomer of carbon dioxide (¹²C¹⁶O₂). Simultaneous processing of the data of the absolute refractometric and relative absorption measurements using the Kramers-Kronig formalism made it possible to refine the integral intensities of the fundamental vibrational bands ν₁ and ν₄ and to determine the dispersion of the vibrational polarizability in the range from 1050 to 1250 cm^?1.     

Effects of field broadening and pump line selection on multi-level absorption in molecular systems

The problem of multi-level absorption of i.r. laser radiation in polyatomic gases is examined using the density matrix formalism. The general condition under consideration is that field broadening effects are significant, but the detailed and discrete features of the absorption spectra remain very important. Collisional effects are taken into account through macroscopic relaxation terms, but the cumulative effects of collisional relaxation are assumed to cause only minor perturbations to the overall vibrational-rotational populations over the optical pumping time scale of interest. Various approximate formulae are then derived for locating the optimum frequency for optical pumping of three consecutive anharmonic vibrational levels within a given normal mode and for predicting the intensity- and frequency-dependence of the resultant uppermost level population density during a quasi-steady state balance among the absorption, stimulated emission, dephasing collision and rotational relaxation rates. Numerical examples are illustrated for pumping of the three lowest vibrational levels within the non-degenerate asymmetric stretching mode of the linear triatomic molecule CO₂ for which the absorption spectra are relatively simple and well known. Some possible applications of these results to the problem of i.r. laser frequency conversion and to the interpretation of wavelength-selective i.r. photodissociation recently observed in many symmetric top molecules, such as BCl₃, SiF₄ and SF₆ for which the absorption spectra are much more complex and the spectroscopic constants much less certain, are also discussed.     

Detailed vibrational population distributions in a CO2 laser discharge as measured with a tunable diode laser

A tunable diode laser (TDL) operating in the 2150–2350 cm^–1 wavenumber region is used to probe a conventional cw CO₂ laser discharge. Absorption lines in more than 25 different vibrational bands are observed, enabling us to determine absolute vibrational populations inall levels of concern to the dynamics of the 10 m CO₂ laser. Levels in thev ₃ mode of CO₂ as high as 00°9 are monitored, and it is found that anharmonic effects play a significant role in the populations of such levels. Thev ₁ andv ₂ mode populations are also investigated in detail, and it is found that these modes are strongly coupled and maintain a common vibrational temperature under all discharge conditions. The use of a TDL is shown to be a powerful technique for investigating the dynamics of infrared molecular lasers.This work was supported in part by the National Science and Engineering Research Council, Canada     

An experimental and computational study of methyl ester decomposition pathways using shock tubes

The high-temperature decomposition of three simple methyl esters: methyl acetate, methyl propionate and methyl butanoate, were studied behind reflected shock waves using tunable diode laser absorption of CO₂ near 2.7 μm. CO₂ yield measurements were made over the range of temperatures 1260-1653 K, pressures of 1.4-1.7 atm and reactant concentrations of 2-3%, with the balance Ar. The CO₂ absorption strengths near 2.7 μm are approximately 50 to 1000 times stronger than the bands near 2.0 and 1.55 μm, respectively, and offer opportunities for significantly more sensitive and accurate combustion measurements than previous absorption work using CO₂ bands at shorter wavelength. The experiments provide the first laser-based time-history measurements of the CO₂ yields during pyrolysis of these bio-diesel surrogate fuels in a shock tube. Model predictions for CO₂ yields during methyl butanoate pyrolysis at high temperatures, using the detailed reaction mechanisms of [E. M. Fisher, W. J. Pitz, H. J. Curran, C. K. Westbrook, Proc. Combust. Inst. 28 (2000) 1579-1586.] and others, are significantly lower than those measured in this study. However, an improved methyl butanoate model which extends the recent theoretical work of [L.K. Huynh, A. Violi, J. Org. Chem. 73 (2008) 94-101.] provides substantially improved predictions of CO₂ yields during methyl butanoate pyrolysis. As earlier mechanisms predicted low yields of CO₂ from methyl butanoate decomposition, these new findings imply that existing bio-diesel fuel models, which rely on the rapid formation of two oxygenate radicals from methyl esters (rather than a single non-reactive CO₂ molecule) to account for the tendency for soot reduction, may have to be revisited.