بررسي رفتار زماني سيگنالهاي اپتوگالوانيك نئون در ناحيه 630-600 نانومتر

چكيده انگليسي :

Temporal Evolution of Optogalvanic Signals of Neon in the Range of 600 630 nm Region Fatemah Abyar F abyar firouzabady@ch iut ac ir March 06 2011 Department of Chemistry Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Prof M tabrizchi m tabriz @cc iut ac ir supervisor Dr H Farrokhpour farrokhpossion@gmail com supervisor Abstract Optogalvanic spectroscopy is a very useful method for observation of optical transitions in plasma media The optogalvanic effect occurs due to the variation of impedance in a gas discharge when illuminated by a laser beam resonant to an atomic or molecular transition of the gas within the discharge In fact the illumination of a gas discharge disturbs the steady state condition of the plasma Consequently the population of the two states involved in the transition will change hence the ionization balance and the impedance of the discharge varies The change is then fed to an A D convertor via a capacitor and the result is observed on a monitor In practice the laser frequency is scanned while the discharge current is recorded Any fluctuation in the discharge due to the laser shut is attributed to an optogalvanic transition In resonance condition a sudden increase in the discharge current is observed upon the laser shut Then the current fluctuates and slowly approach its initial value The temporal behavior of the discharge current depends on the decay rates of the states involved in the transition In this work the time evolution of the optogalvanic signal for six transitions of neon in the range of 600 630 nm were studied A dye laser TDL 90 Quantel France pumped by an Nd YAG YAG980 Quantel France was used as the laser source Rhodamine 101 was used to produce the laser light in the range of 600 to 630 nm The line width of dye laser was about 0 08 cm 1 corresponding to 3 pm at 600 nm A commercial Ag Ne hollow cathode lamp Narva Germany along with a home made adjustable dc power supply 200 600 V was used to create the discharge in neon Transitions from the states 2p53s 3 2 1 and 2p53s 1 2 0 to all configurations of 2p54d and 2p55s were studied These are all two photon transitions previously observed in the laser spectroscopy lab of Isfahan University of Technology The time evolution for all transitions was the same including a rapid increase in the current followed by a decrease to bellow the baseline and a slow increase to the baseline All temporal signals were best fitted to a three exponential sentences function corresponding to the mathematical expression proposed by Han et al This suggests that three states are involved in construction of the optogalvanic signal The first state is the lower state 3s that was depopulated due to the laser shut and gains its population again The upper states 4d and 5s although are populated by the laser shut they are strongly coupled to the 3p and 4p states so that their population are quickly loaded to the 3p and 4p via radiative decay Hence the newly populated states 3p and 4p make the optogalvanic signal via relaxation to the steady state condition of the plasma The experiment was repeated at different discharge current for all transitions The decay rates for the low state 3s and the two upper states 3p 4p were extracted as a function of different discharge currents with an appropriate instrumental time constant Finally the collisional and radiation decay rates for all levels involved in the transmissions were calculated The results are in good agreement with those previously reported data Key word Optogalvanic Effect optogalvanic spectroscopy Laser Spectroscopy Two photon Transition Radiative decay rate Plasma Neon

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بررسي رفتار زماني سيگنالهاي اپتوگالوانيك نئون در ناحيه 630-600 نانومتر

طيف سنجي اپتوگالوانيك , طيف سنجي ليزري , انتقالات دوفوتوني , سرعت , فروافت تابش , پلاسما