Espectroscopia láser en nitrógeno molecular

En este trabajo se intenta resumir la labor desarrollada en láseres gaseosos de excitación pulsada, particularmente en lo que respecta a láseres de nitrógeno molecular de geometría axial. De esta manera, se realiza un detallado análisis espectroscopias de la salida láser U.V. e I.R. a temperatura ambiente y de aire líquido, estudiando las distintas bandas de emisión obtenidas con distintas longitudes del canal de descarga y diferentes condiciones de excitación. Se estudia la estructura rotacional de 9 bandas I.R. de las 12 detectadas, encontrándose una lista bastante amplia de líneas nuevas. Al mismo tiempo se informa una nueva banda láser I.R. a 1,43 μm. Respecto a la emisión U.V. se analiza la estructura rotacional de la banda láser 0-1 (357,6 nm) a baja temperatura y de la banda 0-0 (337,1 nm) tanto en emisión láser como superradiante. En ambos casos se han encontrado nuevas líneas láser. Respecto a la banda 0-0 es menester destacar la observación de las 2 componentes del doblete A en casi todos los números cuánticos rotacionales de la rama Pi y algunas componentes de las ramas y P3. Por otro lado se discute la interacción entre los sistemas láser I.R. y U.V. Al respecto, en 1967, L.Alien et al publican un trabajo de cuyos resultados se puede inferir que la banda 0-0 debería crecer en intensidad cuando lo hiciera la 0-0 U.V., debido a que un proceso de cascada del estado $latexC^{3}$ $latex¶_{u}$ al $latexB^{3}$ $latex¶_{g}$ (U.V.) permitiría invertir la población en- U 9 + tre los estados $latexB^{3}$ $latex¶_{u}$ y $latexA^{3}$ $latex\Sigma^ {+}_{u}$ originando así la transición I.R. Posteriormente M.Garavaglía et al muestran resultados que no concuerdan con las predicciones de Alien et al. Con el objeto de echar más luz sobre este problema se estudió el comportamiento de las bandas 0-1 y 0-0 I.R. en distintas condiciones experimentales. De las mismas experiencias se obtienen conclusiones interesantes acerca de los mecanismos que intervienen en la inversión de población. Finalmente se trabaja con excitación pulsada de alta frecuencia para estudiar la emisión láser total y de cada banda. La radiación estimulada del N2 disminuye con el aumento de la frecuencia de excitación debido, según Sviridov y Tropikhin, a una disminución en la ganancia del medio. En este trabajo se muestra la variación de la salida láser en función de la frecuencia de excitación.Doctor en Físic

Espectroscopia láser en nitrógeno molecular

En este trabajo se intenta resumir la labor desarrollada en láseres gaseosos de excitación pulsada, particularmente en lo que respecta a láseres de nitrógeno molecular de geometría axial. De esta manera, se realiza un detallado análisis espectroscopias de la salida láser U.V. e I.R. a temperatura ambiente y de aire líquido, estudiando las distintas bandas de emisión obtenidas con distintas longitudes del canal de descarga y diferentes condiciones de excitación. Se estudia la estructura rotacional de 9 bandas I.R. de las 12 detectadas, encontrándose una lista bastante amplia de líneas nuevas. Al mismo tiempo se informa una nueva banda láser I.R. a 1,43 µm. Respecto a la emisión U.V. se analiza la estructura rotacional de la banda láser 0-1 (357,6 nm) a baja temperatura y de la banda 0-0 (337,1 nm) tanto en emisión láser como superradiante. En ambos casos se han encontrado nuevas líneas láser. Respecto a la banda 0-0 es menester destacar la observación de las 2 componentes del doblete A en casi todos los números cuánticos rotacionales de la rama P1 y algunas componentes de las ramas P2 y P3. Por otro lado se discute la interacción entre los sistemas láser I.R. y U.V. Al respecto, en 1967, L. Alien et al publican un trabajo de cuyos resultados se puede inferir que la banda 0-0 debería crecer en intensidad cuando lo hiciera la 0-0 U.V., debido a que un proceso de cascada del estado C3¶u al B3¶g (U.V.) permitiría invertir la población entre los estados B3¶g y A 3 Σ + u originando así la transición I.R. Posteriormente M. Garavaglía et al muestran resultados que no concuerdan con las predicciones de Alien et al. Con el objeto de echar más luz sobre este problema se estudió el comportamiento de las bandas 0-1 y 0-0 I.R. en distintas condiciones experimentales. De las mismas experiencias se obtienen conclusiones interesantes acerca de los mecanismos que intervienen en la inversión de población. Finalmente se trabaja con excitación pulsada de alta frecuencia para estudiar la emisión láser total y de cada banda. La radiación estimulada del N2 disminuye con el aumento de la frecuencia de excitación debido, según Sviridov y Tropikhin , a una disminución en la ganancia del medio. En este trabajo se muestra la variación de la salida láser en función de la frecuencia de excitación.Facultad de Ciencias Exacta

Size dependence of refractive index of goldnanoparticles

The extinction spectra of spherical gold nanoparticles suspended in a homogeneous media were measured and the results were adjusted with Mie's theory together with an appropriate modification of the optical properties of bulk material considering the limitation that introduces the size of nanoparticles on the dielectric function. Usually, the contribution of free electrons to the dielectric function is modified for particle size, while the contribution of bound electrons is assumed to be independent of size. This work discusses the separated contribution of free and bound electrons on the optical properties of particles and their variation with size for gold nanoparticles. The effects of dielectric function and its changes with size on extinction spectra near plasmon resonance are considered. The damping constant for free electrons was changed with size as usual and a scattering constant of C = 0.8 was used. For the bound electron contribution, two different models were analysed to fit the extinction spectra: on the one hand, the damping constant for interband transitions and the gap energy were used as fitting parameters and on the other, the electronic density of states in the conduction band was made size-dependent. For the first model, extinction spectra corresponding to particles with radius R = 0.7 nm were fitted using two sets of values of the energy gap and damping constant: $latex Eg = 2.3 eV \ and \ \gamma_{\mathrm {b}}=158~\mathrm {meV}/\hbar \ or \ Eg = 2.1 eV \ and \ \gamma_{\mathrm {b}}=200~\mathrm {meV}/ \hbar$. For the second model, a simple assumption for the electronic density of states and its contribution to the dielectric function in terms of size allowed to adjust extinction spectra for all samples explored (from 0.3 to 1.6 nm radius). This last model uses only one parameter, a scale factor R0 = 0.35 nm, that controls the contribution of the bound electrons in nanoparticles. Contrast between the maximum and the minimum in the extinction spectra near the resonance at 520 nm or alternatively the broadening of the plasmon band can be used to determine the size of gold nanoparticles with radius smaller than 2 nm

Size dependence of refractive index of gold nanoparticles

The extinction spectra of spherical gold nanoparticles suspended in a homogeneous media were measured and the results were adjusted with Mie’s theory together with an appropriate modification of the optical properties of bulk material considering the limitation that introduces the size of nanoparticles on the dielectric function. Usually, the contribution of free electrons to the dielectric function is modified for particle size, while the contribution of bound electrons is assumed to be independent of size. This work discusses the separated contribution of free and bound electrons on the optical properties of particles and their variation with size for gold nanoparticles. The effects of dielectric function and its changes with size on extinction spectra near plasmon resonance are considered. The damping constant for free electrons was changed with size as usual and a scattering constant of C = 0.8 was used. For the bound electron contribution, two different models were analysed to fit the extinction spectra: on the one hand, the damping constant for interband transitions and the gap energy were used as fitting parameters and on the other, the electronic density of states in the conduction band was made size dependent. For the first model, extinction spectra corresponding to particles with radius R = 0.7 nm were fitted using two sets of values of the energy gap and damping constant: Eg = 2.3 eV and γb = 158 meV/h or Eg =2 .1 eV and γb = 200 meV/ h . For the second model, a simple assumption for the electronic density of states and its contribution to the dielectric function in terms of size allowed to adjust extinction spectra for all samples explored (from 0.3 to 1.6 nm radius). This last model uses only one parameter, a scale factor R0 0.35 nm, that controls the contribution of the bound electrons in nanoparticles. Contrast between the maximum and the minimum in the extinction spectra near the resonance at 520 nm or alternatively the broadening of the plasmon band can be used to determine the size of gold nanoparticles with radius smaller than 2 nm.Facultad de IngenieríaCentro de Investigaciones Óptica

Visible and near-infrared backscattering spectroscopy for sizing spherical microparticles

Scattering is a useful tool for the determination of particle size in solution. In particular, spectroscopic analysis of backscattering renders the possibility of a simplified experimental setup and direct data processing using Mie theory. We show that a simple technique based on near-infrared (NIR) backscattering spectroscopy together with the development of the corresponding algorithm based on Fourier transform (FT) and Mie theory are a powerful tool for sizing microparticles in the range from 8 to 60 microm diameter. There are three wavelength intervals in the NIR, within which different diameter ranges were analyzed. In each one, the FT yields a coarse diameter value with an uncertainty dependent on the wavelength range. A more accurate value is obtained by further applying cross correlation between experimental and theoretical spectra. This latter step reduces the uncertainty in diameter determination between 30% and 40%, depending on wavelength interval and particle diameter. These results extend previous information on visible backscattering spectroscopy applied to sizing microparticles in the range between 1 and 24 mum diameter. This technique could be the basis for the construction of a portable and practical instrument

Population mechanisms in visible carbon monoxide pulsed lasers

The population mechanism in the Ångstrom system of a CO pulsed laser are studied through analysis of the rotational intensities distribution of the emitted bands. The observed spectra were simulated by using a simple excitation model. The results suggest that, apart from electron impact, there is a strong radiative contribution of the VUV 4+system to the gain of the visible emission through a selective depletion of the lower laser level

Size dependence of refractive index of gold nanoparticles

The extinction spectra of spherical gold nanoparticles suspended in a homogeneous media were measured and the results were adjusted with Mie’s theory together with an appropriate modification of the optical properties of bulk material considering the limitation that introduces the size of nanoparticles on the dielectric function. Usually, the contribution of free electrons to the dielectric function is modified for particle size, while the contribution of bound electrons is assumed to be independent of size. This work discusses the separated contribution of free and bound electrons on the optical properties of particles and their variation with size for gold nanoparticles. The effects of dielectric function and its changes with size on extinction spectra near plasmon resonance are considered. The damping constant for free electrons was changed with size as usual and a scattering constant of C = 0.8 was used. For the bound electron contribution, two different models were analysed to fit the extinction spectra: on the one hand, the damping constant for interband transitions and the gap energy were used as fitting parameters and on the other, the electronic density of states in the conduction band was made size dependent. For the first model, extinction spectra corresponding to particles with radius R = 0.7 nm were fitted using two sets of values of the energy gap and damping constant: Eg = 2.3 eV and γb = 158 meV/h or Eg =2 .1 eV and γb = 200 meV/ h . For the second model, a simple assumption for the electronic density of states and its contribution to the dielectric function in terms of size allowed to adjust extinction spectra for all samples explored (from 0.3 to 1.6 nm radius). This last model uses only one parameter, a scale factor R0 0.35 nm, that controls the contribution of the bound electrons in nanoparticles. Contrast between the maximum and the minimum in the extinction spectra near the resonance at 520 nm or alternatively the broadening of the plasmon band can be used to determine the size of gold nanoparticles with radius smaller than 2 nm.Facultad de IngenieríaCentro de Investigaciones Óptica

Visible and near-infrared backscattering spectroscopy for sizing spherical microparticles

Scattering is a useful tool for the determination of particle size in solution. In particular, spectroscopic analysis of backscattering renders the possibility of a simplified experimental setup and direct data processing using Mie theory. We show that a simple technique based on near-infrared (NIR) backscattering spectroscopy together with the development of the corresponding algorithm based on Fourier transform (FT) and Mie theory are a powerful tool for sizing microparticles in the range from 8 to 60 microm diameter. There are three wavelength intervals in the NIR, within which different diameter ranges were analyzed. In each one, the FT yields a coarse diameter value with an uncertainty dependent on the wavelength range. A more accurate value is obtained by further applying cross correlation between experimental and theoretical spectra. This latter step reduces the uncertainty in diameter determination between 30% and 40%, depending on wavelength interval and particle diameter. These results extend previous information on visible backscattering spectroscopy applied to sizing microparticles in the range between 1 and 24 mum diameter. This technique could be the basis for the construction of a portable and practical instrument.Facultad de Ingenierí

High spectral field enhancement and tunability in core-double shell metal-dielectric-metal spherical nanoparticles

In this work we have developed full Mie theory for spherical core-double shell NPs, considering the boundary conditions of the electric and magnetic fields at each interface, to study plasmon resonances, optical extinction and spectral field enhancement of a generalized metal-dielectric-metal NPs. Keeping gold as the outer metal shell, calculations were carried out for different core metals (gold, silver, copper and aluminium) and different intermetallic dielectrics (silica, titanium dioxide, aluminium oxide and water). The metal dielectric function includes free and bound electron size corrections. Theoretical results show that the structures Al-SiO2-Au, Ag-SiO2-Au and Cu-SiO2-Au have field enhancement maxima factors of 33, 30 and 20 respectively in the outer region, all larger than that for Au-SiO2-Au (18). For the intermetallic region, field enhancement factors between 459, 960 and 841 are obtained for the first three structures mentioned before, respectively, also larger than that for Au-SiO2-Au (770). Spatial field enhancement was also calculated for both regions, for different core metals and fixed core-double shell size. A hybridization model specially developed for a core-double shell system allows an insightful interpretation of its plasmon resonances, field enhancement maxima and lack of symmetry in the outer region field distribution.Centro de Investigaciones Óptica

Sizing gold nanoparticles by optical extinction spectroscopy

The measurement of optical extinction is used to determine the size of nearly spherical gold nanoparticles suspended in solution, produced by a 'reverse micelles' process. The contrast between the maximum and the minimum in the extinction spectra around 450 and 520 nm shows a linear dependence with the mean radius of the gold particles less than 3 nm; however, the method can be used to size particles up to 7 nm. Experimental results for extinction spectra can be fitted by Mie's theory if the optical constants from bulk material values are modified by introducing the limitation of the mean free path due to collisions of conduction electrons with the boundary of the nanoparticles.Facultad de Ciencias ExactasCentro de Investigaciones Óptica