M. Scotoni

The 3←0 CH stretch overtone of benzene: An optothermal study

The second CH stretch overtone of benzene at the rotational temperature of about 20 K has been measured in a supersonic molecular beam. Infrared absorption has been detected by means of the optothermal method. At least seven vibrational bands have been resolved in the wave numbers range from 8740 to 8870 cm−1. The main absorption peak is located around 8827 cm−1, but a significant absorption is observed also in ‘‘red’’ region of the spectrum, around 8770 cm−1. Experimental spectra have been compared with a theoretical calculation carried out following the method of Sibert et al. [J. Chem. Phys. 81, 1115 (1984)]. No free parameters have been included in the calculation in order to improve the fit with experimental data. Calculations do not provide a detailed quantitative description of experimental data. However the two spectra are qualitatively similar: (a) both are characterized by a main absorption structure located in the 8820–8840 cm−1 region and (b) calculations confirm the experimental observation t...

Collisional orientation of the benzene molecular plane in supersonic seeded expansions, probed by infrared polarized laser absorption spectroscopy and by molecular beam scattering

The large number of elastic and inelastic collisions which take place during supersonic gaseous expansions produce not only acceleration and internal cooling of molecules, but also their alignment or orientation. The collisional alignment of the rotational angular momentum, corresponding to the orientation of the benzene molecular plane, in supersonic seeded expansions with lighter carrier gases is demonstrated via two complementary experiments: one interrogating benzene via polarized laser light IR absorption the other one probing its orientation via molecular beam scattering on rare gas targets. Typical seeding gases are helium, neon and their mixtures, and molecular hydrogen. Total stagnation pressures are of the order of ∼1 bar and ∼0.1 mm nozzle. A propensity is demonstrated for benzene molecules in seeded molecular beams to fly with the molecular plane preferentially oriented parallel to the molecular beam propagation direction. The analysis of the experimental results has been carried out using a p...

Optical enhancement of diode laser-photoacoustic trace gas detection by means of external Fabry-Perot cavity

An optical enhancement method applied to a diode laser photoacoustic trace gas detector is presented. In order to improve the detection sensitivity, the light intensity inside the acoustic resonator is amplified using a Fabry-Perot cavity. A feedback signal stabilizes the laser frequency to the optical cavity length, in order to maintain the light amplification constant during the probe frequency scan. The usefulness of the optical amplifier is demonstrated by showing two ethylene spectra obtained at 1.624μm with and without the optical enhancement. Tens to hundreds ppb (part per billion) sensibility for molecules absorbing in the region between 1.5–1.7μm can be obtained.An optical enhancement method applied to a diode laser photoacoustic trace gas detector is presented. In order to improve the detection sensitivity, the light intensity inside the acoustic resonator is amplified using a Fabry-Perot cavity. A feedback signal stabilizes the laser frequency to the optical cavity length, in order to maintain the light amplification constant during the probe frequency scan. The usefulness of the optical amplifier is demonstrated by showing two ethylene spectra obtained at 1.624μm with and without the optical enhancement. Tens to hundreds ppb (part per billion) sensibility for molecules absorbing in the region between 1.5–1.7μm can be obtained.

Opto‐thermal spectroscopy of the benzene 4←0 C–H stretch overtone

The 4←0 C–H stretching overtone of benzene has been measured in a supersonic molecular beam by means of the opto‐thermal technique. Several vibrational transitions contribute to the absorption spectrum in the 11 400–11 550 cm−1 region. The upper limit for the homogeneous linewidth is found ≊17 cm−1.

Thick and homogeneous surface layers obtained by reactive ion-beam-enhanced deposition☆

Abstract A recently developed method, reactive ion-beam-enhanced deposition (RIBED), which consists of simultaneous or sequential deposition and implantation steps, has proved to be very effective in the production of surface compounds with interesting surface properties. This hybrid technique allows the depth of the treated region to be nearly independent of the projected range of the implanted ions and thereby thicker layers to be obtained than by conventional ion implantation. Moreover, with respect to unimplanted deposited layers, the ion beam increases the uniformity of the films and their adhesion to the substrate. In this work, we consider surface nitrides of chromium and boron, which are known to be hard and corrosion resistant. The surface compounds were characterized using various different techniques: Auger electron spectroscopy, secondary ion mass spectrometry, X-ray diffractometry and scanning electron microscopy. Electrochemical measurements were carried out on the chromium and boron nitride layers, obtained by RIBED, on iron.

Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br

Abstract An opto-thermal molecular beam study has been carried out to investigate the multiple-photon laser excitation of SF 6 and CF 3 Br. The molecular beam was produced by means of a supersonic expansion through a nozzle at variable temperature. The opto-thermal signal was measured by means of a high-sensitivity superconducting bolometer. The multiple-photon excitation of SF 6 has been measured as a function of the initial ro-vibrational population of the molecule. The experimental results have been compared with both previously published data of molecular beam and gas cell experiments and theoretical calculations. A satisfactory agreement has been found between some of our experimental results and the theoretical spectra obtained by means of the heat-bath feed-back model.

Infrared multiple photon excitation of sulfur hexafluoride in a molecular beam

Abstract The infrared multiple photon excitation of SF 6 in a supersonic molecular beam has been investigated by means of the laser-bolometric technique. In this experiment molecules are irradiated below dissociation threshold in a true collisionless regime. The absorbed energy is detected by means of a fast superconducting bolometer. The molecular beam is obtained by expanding pure SF 6 and SF 6 seeded in helium or argon at different pressures. The multiple photon excitation spectra are very sensitive to the molecular beam conditions. Spectra become narrower as the pressure is increased. This effect has been attributed mainly to the rotational cooling which occurs during beam formation.

Infrared spectroscopy of the CH stretching modes of partially F‐substituted benzenes. II. The fundamental spectrum of fluorobenzene

This work is part of a series aimed to investigate the fundamental and overtone transitions of the CH stretching modes in partially F‐substituted benzenes. In this article we report the study of the fundamental transitions of fluorobenzene (C6H5F) in the wave numbers range from about 3000 to 3150 cm−1. The absorption spectrum has been measured through both a low‐temperature opto‐acoustic cell and a supersonic‐beam opto‐thermal spectrometer. The opto‐thermal spectrometer enabled us to measure the detailed vibrational structure, providing the basis for an algebraic analysis of the fundamental CH transitions. We show, in particular, how benzene algebraic procedures must be modified for taking into account the effects induced by the substitution of one hydrogen atom with fluorine, i.e., (i) the reduction of symmetry and (ii) the frequency (chemical) shift due to the change of electron distribution. Moreover we include the contribution of anharmonic resonances between unperturbed CH stretching modes and overto...

The 3 ← 0 CH stretch overtone of benzene

Abstract The second CH stretch overtone of benzene has been studied both experimentally and theoretically. A recent high-resolution opto-thermal experiment showed that the overtone spectrum consists of several absorption bands located in the region between 8730 and 8875 cm−1. In this work, we investigate possible contributions of strong hot bands to the absorption around 8770 cm−1. Our results show that this region of the opto-thermal spectrum is not significantly affected by hot bands. The spectrum has been described by a theoretical calculation based on the algebraic theory. We find good agreement with experimental results. The low-frequency region and minor satellite peaks are reproduced reasonably well. Quantum labels are assigned to some of the observed peaks.

Molecular Beam Diagnostics by Means of Fast Superconducting Bolometer and Pulsed Infrared Laser

The laser-bolometric infrared spectroscopy is an efficient method for measuring the internal energy distributions of molecular beams. Additional informations about the kinetic energy distribution of molecules in a selected internal state can be obtained from time resolved experiments. A fast superconducting bolometer and a pulsed infrared CO2 laser have been used for testing the use of this technique as a universal tool for molecular beam diagnostics. Experimental results are presented and analyzed for pure SF6 and helium seeded with 5% SF6 beams. The efficiency of fast superconducting bolometers, used for molecular beam time-of-flight measurements, is discussed. A comparison is made between time resolved laser-bolometric technique and alternative molecular beam diagnostic methods.