Connecticut College, New London, Connecticut usa general Physics Institute, Russian Academy of Sciences, Moscow, Russia
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Lecture 2.NEAR-INFRARED DIODE LASER SPECTROSCOPY ON FREE RADICALS Nobukimi Ohashi Department of Physics, Faculty of Science, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan The near-infrared diode laser spectroscopy is one of powerful tools for studying spectra from molecular free radicals. The high-sensitivity, high resolution and tunability of the near-infrared diode laser system are of great advantage to investigation on energy levels of short-lived radicals which are complicated because of various intra-molecular interactions. In our laboratory, using 0.8 m, 1.3m and 1.5m tunable diode lasers, absorption spectra of several radicals have been studied in their electronic transitions. Results on HCSi , CCO (1) and FeC (2) obtained by studying in detail energy level structures with the use of 0.8-m diode laser system will be reported mainly in the present time. Of these radicals, CCO was investigated mainly with the use of several sets of laser diodes oscillating with inconvenient mode gaps in the early stage of our near-infrared diode laser spectroscopic study on radicals, and, on the other hand, FeC and HCSi were studied using an external cavity diode laser. For FeC, being an interesting radical composed of a 3d transition metal atom Fe, information on spin-orbit interaction between the triplet electronic ground state and a low-lying singlet electronic excited state will be reported somewhat in detail. For HCSi and CCO radicals, spectral varieties produced by Renner-Teller interaction, which is an interesting vibronic interaction, will be mentioned in a viewpoint of high-resolution spectroscopic interest. It can be said that details of spectral complication of these two radicals caused by combination of Renner-Teller effect and a spin-orbit interaction were made successfully clear of by using diode lasers which oscillate stably and are of high quality in tunability and resolution. Examples of solving the spectral complications will be shown. (1) M. Fujitake, R. Kiryu, and N. Ohashi, J. Mol. Spectrosc. 154, 169(1992). N. Ohashi, R. Kiryu, S. Okino, and M. Fujitake, J. Mol. Spectrosc. 157, 50(1993). H. Abe, T. Kikuchi, K. Takahashi. M. Fujitake, and N. Ohashi, J. Mol. Spectrosc. 167, 353(1994). H. Abe, T. Kawamoto, M. Fujitake, N. Ohashi, T. Momose, T. Shida, , J. Mol. Spectrosc. 180, 277(1996). H. Abe, M. Mukai, M. Fujitake, and N. Ohashi, J. Mol. Spectrosc. 195, 317(1999). (2) M. Fujitake, A. Toba, M. Mori, F. Miyazawa, N. Ohashi, K. Aiuchi, and K. Shibuya, J. Mol. Spectrosc.
The quantum cascade laser demonstrated in 1994 by J. Faist et al. [3] at mid-infrared wavelengths has experienced a rapid development of its performance and eventually cw-operation at room-temperature was demonstrated. Since it is based on intersubband rather than interband transitions the wavelength can be tuned over very wide ranges by properly adjusting layer thickness and electric field using technologically mature materials like InGaAs/AlInAs/InP or AlGaAs/GaAs. We have demonstrated AlGaAs/GaAs quantum cascade lasers [4] that emit at 4.5 THz, at 3.5 THz [5], and very recently at 2.8 THz and that have the potential for device-like implementation. The active region consists of a hundred repetitions of a chirped superlattice, specially engineered to achieve population inversion at energies below the optical phonon resonance. This core is embedded into a novel type of partially metallic waveguide to confine the very long wavelength radiation without concomitant high optical losses. The concept is loosely based on the surface plasmon configuration but makes use of a thin, highly doped layer with appropriate dielectric constant between the low-doped active core and the semi-insulating substrate to guide THz radiation with low optical losses of ~5-10 cm-1. Our devices currently operate in continuous-wave mode with output powers of 4 mW and up to 45 K heat sink temperature. Under pulsed excitation, output powers of 4.5 mW at low temperatures and still 1 mW at 65 K are measured [6]. [1] D. Mittleman (Ed.), Sensing with Terahertz Radiation, Springer, Berlin, 2003. [2] R. E. Miles, P. Harrison and D. Lippens (Eds.), Terahertz Sources and Systems, NATO Science Series II Vol. 27, Kluwer, Dordrecht, 2001. [3] J. Faist et al., Quantum Cascade Laser, Science 264, 553 (1994). [4] R. Köhler et al., Terahertz Semiconductor-Heterostructure Laser, Nature 417, 156 (2002). [5] R. Köhler et al., Low-threshold quantum cascade lasers at 3.5 THz ( = 85 µm), Optics Letters, in press (2003). [6] R. Köhler et al., High-performance continuous-wave operation of superlattice terahertz quantum-cascade lasers, Appl. Phys. Lett. 82, 1518 (2003). Lecture 4.LINE PROFILE STUDY WITH TUNABLE DIODE-LASER SPECTROMETERS M. Lepère Postdoctoral Researcher with F.N.R.S., Belgium Laboratoire de Spectroscopie Moléculaire, FUNDP, 61, rue de Bruxelles, B-5000 Namur, Belgium Diode-laser spectrometers are well adapted to the study of lineshapes for molecules in diluted phase. They permit to show the modifications induced by intermolecular forces on spectral line profile and give very precise line parameters for lineshape modelisation. The different line profile models take into account several effects. The first effect results from random motion of the active molecules which leads to a broadening of the line described by a Doppler profile when the sample is at thermal equilibrium. This is valid only if there are no significant interactions between molecules (very low pressure). At pressures below 120 mbar, the Doppler and collisional broadenings are concurrent and the profile is usually described by a Voigt profile. However, the Doppler line is narrowed by the confinement of the active molecules in the buffer gas. This effect is generally referred to as Dicke narrowing (or confinement narrowing), then the line profile is well described by either the Rautian or Galatry models. As the pressure increases, the collisional broadening is progressively the main effect and depends on the relative speed of the collision partners for which it may be necessary to take into account the different classes of speed from the Maxwell-Boltzmann distribution for the absorber. Precise determinations of spectroscopic line parameters such as collisional broadening and narrowing are very important for infrared remote sensing of the atmospheres. The temperature dependence of these parameters is also required for precise atmospheric sounding. For atmospheric temperatures (200-300K), it is important to determine precisely line broadenings and their temperature dependence. We will show examples of such studies (CH4, CH3D…) that we have realised using an absorption cell operating at selected temperatures (between room temperature and 77 K) with a temperature stabilization better than 0.5 K. Preliminary calculations show that the intermolecular potential variations, that have no important effect at room temperature, can produce differences of several order of magnitude at very low temperature (below 20 K). Thus, it seems very interesting to make measurements up to these temperatures using collisional cooling technique. This technique allows to obtain a gas mixture in thermodynamic equilibrium, and thus to know the pressure and the temperature of the gas sample. It is an advantage for the study of line profile and parameters. In collaboration with Professor Mantz, we have realized first measurements of collisional broadenings of CH4 diluted in He down to 15 K.
The sensitivity of the technique can be strongly improved using a resonant configuration, in which the measurement cell is carefully designed to be an acoustic resonator. When the laser modulation corresponds to an acoustic resonance of the cavity, an acoustic standing wave is built in the resonator. This standing wave can accumulate energy to an extend much larger than the energy input per cycle, leading to an increase of the wave amplitude in comparison to the non-resonant case. The acoustic signal is thus enhanced by the quality factor Q of the resonance, which can reach several hundreds for well-designed photoacoustic cells. The basic principles of resonant photoacoustic spectroscopy will be described and the different types of resonances (longitudinal, radial, azimuthal) will be discussed. Then, an optimal design of a photoacoustic cell coupled to a CO2-laser will be presented. This system has lead to the realization of a commercial instrument for extremely low NH3-concentrations measurement. Applications of this instrument to the control of the atmosphere in clean rooms in the semiconductor industry and to environmental monitoring will be demonstrated. Different experimental results obtained in these applications and showing a sub-ppb detection limit will be presented. Finally, applications of photoacoustic spectroscopy using near-infrared semiconductor laser diodes will be presented. Different cell configurations will be discussed as a function of the laser specifications. Directory: 2003 2003 -> Tr-41. 4-03-05-024 Telecommunications 2003 -> Review of some tsunamis in canada 2003 -> The skill of multi-model seasonal forecasts of the wintertime North Atlantic Oscillation 2003 -> 2930 Kerry Forest Parkway 2003 -> Canadian Interuniversity Sport (cis) Athletes of the Week 2003 -> Senior Airman Jason Plite Hometown 2003 -> Technical announcements: My apologies for the comparison pages; they were 2003 -> Pool Play Round 1 9: 20: 00 am 2003 -> 2004 Division II men's Golf Championships Final Round Results deland, fla 2003 -> Go Figure Edited by Kostya Kennedy and Mark Bechtel 645 Passing yards, an ncaa division II record, by University of Indianapolis quarterback Matt Kohn, in a 59-52 victory over Michigan Tech Download 1.65 Mb. Share with your friends:
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