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p la tin u m electrodes t h a t a re n o t a s reactiv e in th e oxygen p la s m a , b u t in th e p r e s e n t w ork, th is w as n o t p o ssib le. F u r th e r m o r e , i t w as co n sid ered t h a t such a la m p m ay c o n trib u te co n sid erab ly to th e lin e sp e c tru m , a s gold, for ex am p le, h a s a m u ltitu d e of tr a n s itio n s a n d s p u t t e r i n g m a y h a v e b eco m e a p ro b le m . B e c a u s e o f th is , a “h o m e—m a d e ” d is c h a rg e la m p w a s c o n s tru c te d a n d filled w ith a ir. T he oxygen tra n s itio n s of in te r e s t w ere clearly visible in th is d isch arg e la m p u s in g a d if f e r e n t OM A u n i t (OM A II P r in c e to n A p p lie d R e s e a rc h ) c o u p le d w ith th e s p e c tr o m e te r ( q u ite o b v io u sly , t h is d isch arg e lam p re g u la rly b u r n t out). T he diode la s e r w as tu n e d u n til its profile w as coincident w ith th e tra n s itio n of in te re s t. T h e sp e c tru m w as d ig ita lly reco rd ed a n d i t w as d e te rm in e d w h ere u n d e r th e oxygen profile th e la s e r w as s ittin g d u rin g th e e x p e rim e n ta l ru n . G r e a te r a c c u ra c y in tu n in g could be a c h ie v e d u s in g th e r e s u lts fro m th e e m issio n e x p e rim e n ts, as th e lin e pro file a n d s p e c tra l p o s itio n w ere well kn o w n for th e tra n s itio n . T he diode la s e r p o ssessed sev eral m odes u n d e r its g a in curve. I t w as po ssib le to tim e th e la s e r to one of its m odes w ith in a n a rro w tu n in g ra n g e . H ow ever, in g e n e ra l th e la s e r w as n o t co n tin u o u sly tu n a b le . In fact, d u rin g th e tu n in g process, th e la s e r w as p ro n e to ju m p to a n a d ja c e n t m ode le a v in g sp e c tra l reg io n s t h a t w ere n o t accessible. T h e diode la s e r w ill be d isc u sse d in m ore d e ta il in th e follow ing sectio n . T he tr a n s itio n of i n te r e s t w as n o t perfectly coincident w ith a sta b le m ode of th e diode la s e r a n d as su ch it w as ex trem ely difficult to lock th e diode la s e r a t th e d esired w av elen g th for a long period of tim e. T he u n c e rta in ty in th e sp e ctra l p osition of th e diode la s e r o u tp u t w ith in th e oxygen profile, w as by fa r th e g r e a te s t c o n trib u to r to e rro r in th e s e e x p erim e n ts. I t is n o t a lto g e th e r c le a r as to how a m ore a c c u ra te w av ele n g th c alib ra tio n can be ach iev ed w h en
Chapter 4 : The experiments 128
oxygen tra n s itio n s a re b ein g in v e stig a te d by la s e r spectroscopy. I t w as only th ro u g h a know ledge of th e s p e c tra l p o sitio n of th e 777.196 n m tra n s itio n o b tain ed from th e em ission spectroscopic w ork, coupled w ith th e c alib ra tio n o b tain ed w ith th e hom e-m ade a ir d isc h arg e la m p ( th a t only su rv iv ed for a b rie f tim e) t h a t re s u lts w ere o b tain ed a t all. T h is p ro b lem w ill be of c o n sid era b le im p o rta n c e to an y o n e w ish in g to do sim ila r w ork in atom ic oxygen a t th e se w av elen g th s.
4.8.2 The infrared CW diode laser
T he u se of diode la s e rs in atom ic spectroscopy h a s b een d isc u sse d a t le n g th by C am p aro (1985). T h e diode la s e r is a sem ico n d u cto r diode. W hen electrically excited, th e electro n s a n d holes recom bine w ith in th e ju n c tio n a n d e m it th e rec o m b in a tio n energy. Above th re s h o ld , w hich is d e p e n d e n t on th e se m ic o n d u c to r ty p e , th e e m itte d r a d ia tio n is in te n s e e n o u g h to m a k e th e in d u c e d e m is s io n r a t e ex ceed th e s p o n ta n e o u s r a te . T h is e m issio n m a y be a m p lifie d , v ia m u ltip le reflectio n s, w ith in th e c ry stal a ssu m in g o th e r d e -e x citatio n p ro cesses a re slow in com parison.
T he diode la s e r em ployed in th e p r e s e n t e x p e rim e n ts w as a Liconix D iolite-800 diode la s e r sy stem d riv in g a sp lit cathode la s e r diode. T he system o p erated over th e sp ectral ran g e of 775-780 n m u sin g M itsu b ish i 780 n m diodes. T he sp e c tra l o u tp u t of th e diode la s e r is a L o re n tz ia n s h a p e d lin e of w id th le s s th a n 50 M H z. T he w a v e le n g th o u tp u t is v a rie d by a d ju s tin g th e te m p e ra tu re of th e device, th u s c h an g in g th e cav ity d im en sio n s.
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