1,721,099 research outputs found
Vibrational-translational relaxation time spectroscopy of polyatomic molecules
No full textWe report experimental studies of the frequency dependence of the vibrational-translational relaxation time of polyatomic molecules strongly excited by resonant laser radiation. In particular, we have examined the behavior of SF6 and CF2Cl2, two molecules of great practical and theoretical interest. Our analysis has highlighted a strong dependence of the relaxation time on the exciting laser wavelength. The laser ‘spectra’ of the relaxation times are very similar to the optical absorption spectra for both molecules, thus suggesting the possibility of using the study of the relaxation times to obtain useful spectroscopic information of polyatomic molecules
FAILURE OF THE LINEAR MIXTURE RULE IN THE VIBRATIONAL-RELAXATION OF CF2HCL IN ARGON
No full textThe vibration to translation (VT) relaxation rates of laser excited CF2HCl + argon mixtures have been measured by means of an interferometric method. Approximately exponential signals have been observed for neat CF2HCl and CF2HCl-rich mixtures, while for Ar-rich mixtures the interferometric signals are clearly non-exponential. For a given Ar fraction the VT rates have been found not to be additive. Deviations from the linear mixture rule are more evident for higher initial vibrational excitations of the relaxing molecules
Time resolved fluorescence polarization anisotropy of carbonaceous particles produced in combustion systems
No full textThe size of nanometric carbonaceous particles produced in various combustion systems is determined by means of time resolved fluorescence polarization anisotropy (TRFPA). We also compare the performances of two different experimental implementations of the technique, which are complementary in terms of cost, simplicity and resolution. Both methods are first employed on standard molecules to demonstrate the reliability of the results. A study of the sizes of nanometric particles collected at the exhaust of diesel and gasoline vehicle engine, as well as from controlled laminar flames is presented. The high sensitivity (0.04 nm) achieved with the use of a streak camera as detector makes the TRFPA technique particularly suitable for characterizing nanometric particles
KINETIC DESCRIPTION OF PLASMAS PRODUCED IN MULTIPHOTON-IONIZATION PROCESSES
No full textA kinetic description of inhomogeneous plasmas produced by irradiating an atomic gas by a well-focused ultrashort high-intensity laser pulse is presented. The Vlasov-Maxwell equation is integrated by representing the distribution function f (r, v; t) in the form f0(Dr - Bv, -Cr + Av), with f0 the initial distribution and the A, B, C, D functions depending on r, v, and t. On the basis of this representation the evolution of some charge distributions is examined, by focusing the attention on the transport of ion packets, the effect of the ponderomotive potential, and the capture of electrons by the ion potential well
Characterization of ultrafast fluorescence from nanometric carbon particles
No full textWe present two distinct implementations of the time resolved fluorescence polarization anisotropy (TRFPA) technique for the analysis of carbon nanoparticles collected from laminar laboratory flames and from the exhaust of a gasoline vehicle engine. By exploiting the high time-resolution and the spectral resolution of our TRFPA setups, we could identify two groups of particles, with diameter of 1.4 and 2.2 nm, respectively, within the laminar flame sample. On the other hand, the high time-resolution TRFPA analysis of the gasoline sample led to the identification of two distinct decay channels. The slower one was consistent with a single kind of small particle with diameter of 1.3 nm. Moreover, an analysis of the fast decay versus the temperature allowed us to rule out that it was due to the presence of lighter particles within the sample. Most likely, it is related to the relaxation of internal degrees of freedom
Diffusivity in water and fluorescence properties of organicnanoparticles produced in flames
No full textRotational and translational diffusion properties
in water of nano organic carbon (NOC) particles collected
from premixed laminar ethylene/air flames have been investigated
using both time resolved fluorescence polarisation
anisotropy (TRFPA) and fluorescence correlation spectroscopy
(FCS). Insight into the NOC sizes, structures and
rigidity has been gained through diffusion properties exploiting
their fluorescence in different spectral ranges. The
TRFPA measurements revealed the presence of two classes
of particles. The first composed of particles with a mean
size of 1.5 nm which absorb and fluoresce in the UV, and
a second class composed of slightly larger particles, about
2.2 nm, which absorb and fluoresce in the visible and were
also detected by FCS. From FCS measurements particle
concentration and fluorescence quantum yield have been
evaluated
Modeling the vibrational relaxation of polyatomic molecules. The methylfluoride case-study
No full textWe present in this paper a theoretical analysis of the vibrational translational (V-T) relaxation process in CH3F, carried out by using a numerical model based on rate equations. In particular, we have analysed the dependence of the V-T relaxation time on the average vibrational energy absorbed per molecule. We have also investigated the influence of the dependence of the rate constants used in the model, on the gas translational temperature. The results of the model clearly outline the strongly nonlinear character of the V-T relaxation process in CH3F, a situation commonly observed in other important polyatomic molecules of intermediate size each as SF6, freons, and related methylhalides
- …
