THz commensurate echoes: Periodic rephasing of molecular transitions in free-induction decay

We report the first study of coherent transients excited by ultrafast pulses of THz radiation. Using a newly developed optoelectronic source of well-collimated beams of subpicosecond pulses of THz radiation to excite N2O vapor, we have observed the subsequent emission from the vapor of coherent THz pulse trains extending to as long as 1 nsec. The origin of these subpicosecond THz pulses (echoes) is a periodic rephasing, during the free-induction decay, of the more than fifty coherently excited rotational lines with commensurate transition frequencies. From the decay and reshaping of the echoes the coherent relaxation time T2 and the anharmonicity factor for the N2O molecule are evaluated.Peer reviewedElectrical and Computer Engineerin

Étude de la répartition des charges électroniques dans l'alcool méthylique

The absorption coefficients of some gaseous methyl alcohol vibration bands have been measured. From these measurements and from our previous study of the force field, we have determined the electronic charge distribution in this molecule and its variation during vibration.Nous avons mesuré les coefficients d'absorption d'un certain nombre de bandes de vibration du méthanol gazeux. En utilisant ces données et le calcul du champ de forces que nous avons précédemment effectué, nous avons étudié la répartition des charges électroniques dans cette molécule et la variation de cette répartition au cours de la vibration

HIGH PRECISION TUNABLE DIODE LASER CONTROL: APPLICATION TO THE STUDY OF DICKE NARROWING

Author Institution: Laboratorie de Physique Mol\'{e}culaire et Applications, C.N.R.S., Universit\'{e} Pierre et Marie CurieTo stabilize the frequency, otherwise very unstable, of a T.D.L., its beam is sent through a Michelson interferometer controlled by a stabilized He-Ne laser. The T.D.L. is actively forced to maintain its emission at a wavelength value $\lambda_{d}$ corresponding to an interference fringe of fixed order $K_{d}$. When $D_{1}$ the path difference of the interferometer, is changing, an optical device makes the vibration plane of the polarized He-Ne beam to rotate. When the azimuth plane is kept constant, $D$ is also maintained constant, the fluctuation of $k_{d}$ is reduced by nearly two orders of magnitude (i.e: $10^{3}$ to $10^{5}$ in the wavenumber units). Conversley, changing this azimuth by steps changes the value of $D$ by increments $D$. A change of $\lambda_{d}$ is then induced in order to preserve the relation $D = k_{d}\lambda_{d}$. In addition, the usual intensity fluctuations are completeley removed by a convenient double beam set-up. Finally, the signal-to-noise ratio obtained is better than 2000, allowing the measurement of gas traces. Application: At low pressure, the line profile narrowing resulting from the confinement of CO molecules by buffer gases (He, Ne, Ar, Xe and $N_{2}$) is studied to check the usual different collision models. The $\beta^{0}$ coefficient associated to this Dicke narrowing is compared to the dynamic friction coefficient $\beta^{0}_{ei}$ deduced from the diffusion constant, as a test for the validity of these different models. When the pressure broadening becomes preponderant, the line profile must be determined carefully, taking into account the absorber speed dependent effect, in order to preserve the signification of the $\beta^{0}$ parameter-mainly when the perturber mass is greater than the active molecule one. A line asymmetry has been observed for the CO-Xe mixture, its magnitude beeing in agreement with the measured pressure shift

ENERGY TRANSFER AND THERMALIZATION OF THE VIBRATIONAL ENERGY IN GASES AS A FUNCTION OF THE TEMPERATURE

Author Institution: Laboratorie de Spectroscopie Mol\'{e}culaire II, Universite de ParisWe have measured, using a laser-excited vibrational fluorescence method, between 300 K and 1000 K: The rate constants associated with the deactivation by collision of $CO_{2}$ and $N_{2}O$ excited on the $00^{0} 1$ vibrational level. The rate constants for nearly resonant vibrational energy exchange between $N_{2}O$ and $N_{2}$ or CO; the probabilities per collision of vibrational energy transfer were computed by a theory involving long-range intermolecular forces and compared to the experimental results

TEMPERATURE DEPENDENCE OF NEAR RESONANT VIBRATIONAL ENERGY TRANSFERS IN N2ON_{2}O ISOTOPIC MIXTURES

Author Institution: Laboratoire de Spectronomic Moleculaire, Universit\'e de Paris VIThe rate constants for near-resonant energy transfer from $^{14}N_{2} ^{16}O(001)$ to $^{15}N_{2} ^{16}O, ^{14}N- ^{15}N- ^{16}O, ^{15}N- ^{14}N- {16}O, ^{13}C^{16}O_{2}$ and $^{15}N_{2}$ have been measured as a function of the temperature between 300 and $800^{\circ} K$ . Very closely resonant transfers are obtained with such isotopic species. The experimental results are compared with rate constants calculated by a theory involving long-range intermolecular forces