Chapter 4 : The experiments 122
d e tu n in g s from lin e cen tre. T he p h o to m u ltip lier em ployed for th is p a r t of th e e x p erim e n t w as a n RCA 1P28. T h is p h o to m u ltip lie r w as p e ak e d for 400 n m w ith a n ab so lu te se n sitiv ity of 50 mA/W a t th is w av elen g th . I t h a d a n anode p u lse rise tim e of < 2 ns. T he RCA 1P28 re q u ire d 50 Q t e r m in a tio n to e lim in a te c a b le re fle c tio n s a t th e e x p e r im e n ta l freq u en cy .
4.5.5 Triggering and timing
T h e p re v io u s se ctio n d e sc rib e d th e p ro c e d u re w ith w h ich th e tim e h is to ry of th e Hß lin e w as m e a s u re d a t s e v e ra l d e tu n in g s . T h ese e x p e rim e n ts d e m o n s tr a te d t h a t th e e v o lu tio n a c ro ss th e lin e w a s e s s e n tia lly c o n s ta n t (from lin e c e n tre to ± 40 n m from lin e c e n tre ) w ith in th e firs t 3 [is. I t w as n e c e ssa ry th e re fo re to g a te th e OMA for th is p erio d of tim e for w hich th e tim e ev o lu tio n w as a c o n s ta n t. As d escribed in section 4.5.3, i t w as possible to g a te th e OMA dow n to a n in te rv a l of 0.5 [is. A p roblem aro se how ever in th e tim in g of th is g a te pu lse. O n receiv in g a trig g e r, th e OMA g a te p u lse g e n e ra to r re q u ire d
l[is to open. T h is w as due to electronic p ro p ag a tio n delays. H ence th e p u lse g e n e ra to r h a d to be trig g e re d l p s before th e a rriv a l of th e shock fro n t. I t w as n o t po ssib le to u se som e trig g e r u p s tr e a m of th e te s t section delayed by a n a p p ro p ria te tim e. A t shock velocities of 10 k m s '1, th e closest tim in g s ta tio n from w hich a trig g e r could be ta k e n w as, in tim e , 3 0ps aw ay. To d e la y th is trig g e r for th e a p p ro p ria te p erio d , re q u ire d know ing th e shock velocity to b e tte r th a n 1 p a r t in 30 from sh o t to shot. T his w as n o t possible in th e p re s e n t in v estig atio n .
To allow trig g e rin g l p s p rio r to th e shock a rriv in g , a special asse m b ly w as d esig n ed t h a t allow ed a p in photodiode to be flu sh m o u n te d to th e shock tu b e in n e r su rface, 10 m m (1 ps in tim e a t 10 k m s4 ) from th e
Chapter 4 : The experiments 123
tu b e exit. T he d esig n of th is photodiode trig g e r is p re s e n te d in figure 4.9. C are w as ta k e n to e n su re t h a t th e photodiode did n o t d is tu rb th e flow. T he diode w as in sid e th e d u m p ta n k d u rin g th e e x p e rim e n t. S ig n a ls com ing from i t w ere receiv ed , v ia a cable le a d in g th ro u g h a v acu u m tig h t seal, to its am plifier.
4.6 E m ission from m olecu lar n itro g en
4.6.1 The experimental arrangement
E m iss io n fro m m o le c u la r n itr o g e n w a s m o n ito re d a s a m e a n s of d e te rm in in g th e ro ta tio n a l te m p e r a tu re . S ectio n 5.4.3 o u tlin e s th e m e th o d by w h ich th e r o ta tio n a l te m p e r a tu r e m ay b e d e d u ce d by m e a s u r in g th e tim e re so lv e d e m iss io n of tw o d iffe re n t ro ta tio n a l t r a n s i t i o n s in a g iv e n v i b r a t io n a l b a n d . T h e e x p e r im e n ta l a r r a n g e m e n t fo r th e te c h n iq u e is p r e s e n te d in fig u re 4.7. T he e x p erim e n tal a rra n g e m e n t for th e se ex p erim e n ts w as th e sam e as th a t u se d for th e S ta r k b ro a d e n in g e x p erim e n ts. O ne p a rtic u la r ro ta tio n a l tra n s itio n w as chosen a n d th e sp e c tro m e te r w as p laced onto t h a t lin e (a c c u ra te w a v e le n g th c a lib ra tio n is d isc u sse d in sectio n 6.4). T h is te c h n iq u e gave one te m p o ra lly reso lv ed r e s u lt p e r firin g of th e shock tu b e. A n o th er ro ta tio n a l tra n s itio n (w ith in th e sam e v ib ra tio n a l band) w as c h o sen a n d th e s p e c tro m e te r w as p lac ed onto t h a t lin e . T h is p ro ced u re p ro d u ced th e two re q u ire d te m p o ra lly reso lv ed ro-vibronic tra n s itio n s , a n d relied on good shot-to-shot re p e ata b ility .
T he sp e c tro m e te r could n o t sim ply be “p lac ed ” a t th e co rrect sp e c tra l p o sitio n in o rd e r to m o n ito r th e e m issio n from a specific r o ta tio n a l level. I t w as n o t considered a c c u ra te enough to u se th e o re tic a l v a lu e s
Pin photodiode mounted 1cm from
tube exit
DDT Shock tube exit
Shock tube inner diameter
= 5 cm
Amplifier
Trigger for digital recorders
F igure 4.9