A. Boschetti

State resolved rotational relaxation of CO in the free jet expansion of He–CO mixtures

We describe an infrared molecular beam spectrometer which uses a semiconductor diode laser and has been used to study the rotational relaxation of CO molecules in the free jet expansion of pure CO and a 90% He–10% CO mixture. The data for the mixture are analyzed by means of a kinematical method based on the use of the master equation and empirically scaled state to state rate constants. Information of the He–CO rotationally inelastic rate constants are obtained. The possibility of using free jet expansion to study rotational inelastic collisions is discussed.

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...

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.

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.

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.

An opto-thermal study of NH3 rotational relaxation in NH3-He supersonic expansions

Abstract We have investigated the rotational relaxation of NH 3 in NH 3 -He supersonic expansions by means of opto-thermal spectroscopy. Molecules are excited in the ν 3 +ν 4 band, around 5100 cm −1 , using the third Stokes line of a Raman-shifted pulsed laser. Rotational distributions are measured for different values of the source stagnation pressure and NH 3 concentration. At low rotational temperatures, populations deviate from a Boltzmann distribution. This is attributed to the reduction of para-to-ortho conversion during the expansion.

On Raman-induced photodissociation of neutral van der Waals clusters with visible laser light

Abstract The photodissociation of (CO2)n and Arn clusters has been investigated in a crossed-beams experiment, by means of the time-resolved opto-thermal technique. Clusters have been irradiated using both non-resonant visible and resonant infrared radiation (for (CO2)n only). The cross section for visible photodissociation of neutral (CO2)n clusters has been estimated to be at least four orders of magnitude smaller than that reported by Stamatovic, Howorka, Scheier and Mark.

Opto-Thermal Spectroscopy

About a decade ago, the first opto-thermal spectrometer was developed by Gough, Miller and Scoles [1] at the Waterloo University. The basic idea of this technique is to use a cryogenic thermal detector (bolometer) for measuring the internal energy of a supersonic molecular beam. When the molecular beam is illuminated by means of resonant radiation, the internal state of molecules may change and the corresponding energy variation is detected by means of the bolometer. Up to now opto-thermal methods have found application in the following fields: molecular beam diagnostics and internal state preparation; sub-Doppler infrared spectroscopy; photodissociation spectroscopy of dimers and clusters; spectroscopy of highly excited vibrational states (multiphoton excitations, overtone and combination bands).