Alexander A. Kachanov

High Resolution Absorption Spectroscopy of the ν1=2-6 Acetylenic Overtone Bands of Propyne: Spectroscopy and Dynamics

The rotationally resolved nν1 (n=2–6) overtone transitions of the CH acetylenic stretching of propyne (CH3–C≡C–H) have been recorded by using Fourier transform spectroscopy (n=2), various intracavity laser absorption spectrometers (n=3, 4, and 6) and cavity ring down spectroscopy (CRDS) (n=5). The 2ν1, 3ν1, and 6ν1 bands exhibit a well-resolved and mostly unperturbed J-rotational structure, whose analysis is reported. The 5ν1 band recorded by pulsed CRDS shows an unresolved rotational envelope. In the region of 12 700 cm−1, an anharmonic interaction is confirmed between 4ν1 and 3ν1+ν3+ν5. The band at a higher wave number in this dyad exhibits a partly resolved K-structure, whose analysis is reported. The mixing coefficient of the two interacting states is determined consistently using different procedures. The 1/35 anharmonic resonance evidenced in the 4ν1 manifold induces weaker intensity borrowing from the 2ν1 and 3ν1 levels to the ν1+ν3+ν5 and 2ν1+ν3+ν5 level, respectively, which have been predicted an...

Rotationally resolved absorption spectrum of the O2 dimer in the visible range

Abstract The rotationally resolved absorption spectrum of two bands of the oxygen dimer near 630 and 578 nm have been recorded by intracavity laser absorption spectroscopy both in a supersonic slit expansion of pure O 2 and in a cell cooled at 77 K. These bands correspond to the transitions [ O 2 ( 1 Δ g ) v=0 ] 2 ←[ O 2 ( 3 Σ g − ) v=0 ] 2 and [ O 2 ( 1 Δ g ) v=0 – O 2 ( 1 Δ g ) v=1 ]←[ O 2 ( 3 Σ g − ) v=0 ] 2 . From the extension of the highly congested rotational structure, the dissociation energy of the ground and excited states are estimated to be 80 and 40 cm −1 , respectively. These values agree reasonably well with the results of ab initio calculations of the potential energy surface.

Intracavity laser absorption spectroscopy of N2O with a vertical external cavity surface emitting laser

Abstract The Intracavity Laser Absorption Spectrum (ICLAS) of nitrous oxide, 12 N 2 16 O , has been recorded between 9560 and 9980 cm −1 with a Vertical External Cavity Surface Emitting Laser (VECSEL). Thirteen bands were observed, 11 of which are new. The rovibrational analysis has revealed the occurrence of a number of local perturbations due to anharmonic or Coriolis couplings, which could be analysed on the basis of the polyad model of effective Hamiltonian.

Ultraviolet cavity ring-down spectroscopy of free radicals in etching plasmas

Abstract Many reactive species of interest in technological plasmas absorb light in the UV spectral region (200–300 nm). Measurement of these weak absorbances (typically 10 −2 –10 −4 for a single pass) allows us to determine their absolute concentration. Low-resolution absorption spectra of these systems have previously been obtained by broad-band absorption spectroscopy. Here we present spectra obtained using laser cavity ring-down spectroscopy, which has much higher spectral resolution, and potentially higher sensitivity. Spectra were obtained for CF, CF 2 , AlF and SiF 2 radicals in capacitively-coupled radio-frequency plasmas in fluorocarbon gases. This technique offers the possibility of real-time (1 s) absolute concentration measurements during wafer processing.

Spectroscopic and Ab Initio Investigation of the νOH Overtone Excitation in Trans-Formic Acid

Fourier transform spectroscopy and intracavity laser absorption spectroscopy are used to record the absorption spectrum of formic acid at high spectral resolution, in the ranges of the 2νOH (6968.258 cm−1) and 4νOH (13 284.075 cm−1) vibrational bands of the trans-rotamer, respectively. Numerous perturbations combined with a large line density limit the extent to which the vibration–rotation analysis is performed. Some 689 lines are assigned in the first overtone band and related vibration–rotation constants are determined. Only the band origin and upper state principal A-rotational constant are determined for the n=4 overtone band, because of much higher spectral density. Interpolation, helped by literature data, provides all missing principal rotational constants in the nOH series, for n=1 to 4. All major vibration–rotation parameters appear to evolve very smoothly along the series. This trend is fully supported by ab initio calculations performed at the MP2/cc-pVTZ level of theory and based on an effect...

Intracavity laser absorption spectroscopy of HOCl. II. High overtones, perturbations, and intramolecular dynamics

High resolution spectra of the 4ν1 and 5ν1 OH-stretch overtone and the weak 4ν1+ν2 combination band of transient HO35(37)Cl in the energy range 12 500–16 500 cm−1 have been recorded using ultrasensitive intracavity laser absorption spectroscopy (ICLAS). For this investigation, two different spectrometers, a titanium:sapphire and a dye-jet ICLA spectrometer have been employed. We report line assignments for both isotopomers, refined Dunham parameters, and the spectroscopic constants for the excited rovibrational states. Strong and weak Fermi-type resonances as well as Coriolis interactions at ≈65%–85% of the dissociation energy E0 have been found and the role of dark perturbers is discussed. The dark 2ν1+3ν2+3ν3 state has been analyzed and identified to be the perturber of 3ν1+2ν2. From the detailed analysis of the experimental data the anharmonicity parameters of the asymmetric stretch ν3 are refined. The 5ν1 band at about 85% of E0 has been found to be strongly mixed with the 4ν1+2ν2+ν3 zero order vibrat...

INTRACAVITY LASER ABSORPTION SPECTROSCOPY DETECTION OF SINGLET CH2 RADICALS IN HYDROCARBON FLAMES

Abstract The spectrum of methylene, the CH 2 radical, in the singlet electronic state was detected with a high signal/noise ratio in methane-oxygen-nitrogen flames using intracavity laser absorption spectroscopy. The spectra were recorded at different pressures including first measurements at atmospheric pressure. The maximum concentration of the singlet CH 2 radical in a 30 Torr flat flame was estimated based on literature data from laser photolysis experiments done at room temperature. The value of 2 × 10 13 cm −3 seems to be higher than that predicted by the model calculations. The dependence of the calculated maximum concentration of singlet CH 2 on the equivalence ratio is in qualitative agreement with measured values.

ABSOLUTE HCO CONCENTRATION MEASUREMENTS IN METHANE/AIR FLAME USING INTRACAVITY LASER SPECTROSCOPY

Intracavity laser absorption spectroscopy was used to measure the absorption spectra of a premixed, flat methane/air flame at a total pressure of 30 Torr. The spectra were measured in the spectral range of 16 000–16 300 cm−1. A flat flame burner was placed inside the cavity of a broadband dye laser pumped by a cw argon-ion laser. The spectrum of the laser output was measured by a high resolution spectrograph (with a spectral resolution of 0.003 nm). The spectrum of HCO radicals (A 2A′′←X 2A transition) was measured with a high signal-to-noise ratio at different positions above the burner, providing the first quantitative measurement of the absolute concentrations of the HCO radical in flames. The linewidths of the individual rotational lines in the spectrum can be closely fitted by the equation Γ=X+ZN2(N+1)2, where X=0.37±0.03 cm−1 and Z=(8±0.5)10−6. The rotational temperature of the HCO radicals was evaluated from the spectra, but the error and the data scatter are relatively high since the lines with a...

Measurement of trace gases by diode laser cavity ringdown spectroscopy

Optical spectroscopic methods based on direct absorption offer a quantitative measurement of the absorbance, which is the product of the concentration, the molar absorption coefficient of the transition being observed and the length of the absorption path. An absorption sensitivity adequate for trace detection may be achieved by increasing the path length. One solution is offered by cavity ringdown spectroscopy (CRDS), attractive for its simplicity. We recently demonstrated that an external cavity diode laser (ECDL) can be conveniently employed for CRDS instead of a pulsed laser, contrary to previous applications. Here we extend this result to distributed feed-back (DFB) diode lasers. Paying special attention to the coupling of the laser source to the cavity, we developed an extremely simplified CRDS scheme with a sensitivity of about 10-8/cm/(root)Hz. We then built detectors for methane and HF, working close to the optical wavelengths 1.65 and 1.31 micrometer, respectively With an optical assembly of about 50 cm length and a response time of about 1 s, these devices accurately measure atmospheric methane concentrations in the range 0.5 to 200 ppmv, and HF concentrations from 0.1 to 50 ppmv.

High sensitivity intracavity absorption spectroscopy of CHD3 in the near infrared with a titanium:sapphire laser

Abstract By using a Ti: sapphire crystal as an amplifying medium the intracavity laser absorption spectroscopy technique (ICLAS) has been extended to the near-infrared range (10000–12000 cm −1 ). This range of energy is particularly interesting for the overtone spectroscopy of XH stretching vibrations (XC, N, O, Si, Ge). As an illustration of the possible spectroscopic applications of this spectrometer we present here the absorption spectrum of CHD 3 in the range of the Δν CH = 4 overtone transitions. Due to the Fermi interaction between the ν 1 stretching mode and the ν 5 bending mode, two bands have been observed at 11266.95 and 11062.6 cm −1 . Their rotational analysis is reported and discussed.