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... Chemical Instrumentation
Ediled by S. 2.
LEWIN, N e w York University, N e w York
I
3, N. Y.
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These art&, most of which are lo be coniribufed by p m t authors, are intended lo serve the reader8 of this J O ~ N AbyLcalling attentim, lo nnu developments i n the theory, design, w auailability of clmical laboratmy instrumalatia, o i by presenting useful insights and explanations of topics that are of practical imporlance lo those who use, or teach the use of, modern inslmmmlalion and i n s t m a t a l techniques.
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XX. Flash Photolysis-A
Technique for Studying Fast Reactions David N. Bailey and David M. Hercules, Deportment o f Chemistry and Laboratory for Nuclear Science, Mossochusettr Institute o f Technology, Cambridge, Mass. 02739 One of the most interesting developments in chemical instrumentstion hes been the use of flash photolysis for studying excited-state processes in organic molecules. Flash phntolysis produces a high power of radiant energy, as eompsrod with steady state sources, by dissipating a. moderate nmount of energy during a short period of time. Powers of 50 X lo6 W for a few microseconds are not uncommon with flash equipment. Such high powers are eapahle of prodocing high concentrations of short-lived intermediates in chemical reactions. Some published uses of flash equipment include flame studies (I), kinetie studies (2-51, obtaining absorption spectra. of intermediates (fi, 7) and triplets (8), determining radiative lifetimes of exeit,ed states ( g ) , stimulating laser emission ( l o ) , and studyingreactim merhanisms (11 ).
Theory Because the electronic processes that occur within a molecule are independent, of the mode of excitation, flash photolysis can be used to study the mechanisms of phatoc.hemical reactions. Figure 1 shows the electronic energy-level diagram of a typical organic molecule containing a r-electron system.' & is the ground stittc (3r singlet state), S* is the first excited singlet state, and TIand T2are the first and second triplet states, respectively. Photon ahsorption (process 1) raises the molecule to an excited singlet state, in which state it can undergo one of several processes. The most rapid process (ca. lo-" to 10-la sec) is the ~adialionless lnsa of excess vibrational energy called internal c a u e ~ s i o n(process 2 ) . This puts the molecule in the lowest vibrational level of the first excited singlet state where it remains for the lifetime of the state (ca. 1 0 F sec). A molecule in an
Figure I. The energy ievel diagrom of a typi c d organic molecule. Singlet ,tabs-So and S*; Triplet rtoter-TI and Tz; Absorption and 8; lnternol converrionprocesses-l 2, 3, and 6; Fluorercence-4; Intersystem crosing-5; Phosphorescence-7.
excited singlet state may undergo radialionless dei~ctivationt,o the ground st,ate by internal conversion (process 3); it may undergo a radiat,ive transition to the growtd state called ,fluoreseenee (process 4); or it may cross to a t,riplet state via intersystem crossing (process 5 ) , finally reaching the lowest vibrational level of the first triplet stat,e. Because spertroscopic selection rules formally forbid transitions between states of unlike multiplicity, a. triplet state has B long intrinsic lifetime (ca. seo or longer) relative to that of a singlet state (ca. soc). Ueact,ivittion of the triplet state may be accomplished hy internal conversion (proeese 6) or by the emission of a photon giving a longlived luminescence called phosphorescence (process 7). Because of the long lifetime oi the triplet state, it has been possible to observe LripleGlriplet absorption (process 8 ) for a number of molecules by flash photolysis. Besides losing its excitation energy by the processes described above, a mole-
I For a review of some essential features of organic phot,nchemistry, see ~ E R M A K E R ~ , P. A. slld VESLEY,G . F. THISJOI:RNAL 41, 535 (1964).
( C a l i n u e d on page A 8 4 )
Volume 42, Number 2, February 7 965
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Chemical Instrumentation oule in sn excited state may enter into a photochemical reaction from either singlet or triplet levels. A wluahle feature of flash photolysis is that it can be used to detect and monitor tripleta and/or free radicals as intermediates in photochemical reactions. Often 8wh observations are impossible by other technique3 because generally the concentrations of intermediste specie8 are to" low to dlow deteation under even high-intensity steadystate iJ1umination.
Experimenlal Design Apparatus fur fiash work ranges from simple designs, such as setting up s. flash tube and reflector next to a reaction vessel, to highly complex, well baffled systems, such as those used for obtaining spectra of intermediates. Porter has described the latter in considerable detail (18). 0,
Figure 2. An experimental design useful for spectroscopic monitoring of Rorh p h o ~ l y d r experiment. A. Sample Cell, 8. Water jacket andlor flller, C. Flash lubes, D. RcRector, E. Spectrowopls sourse, F. Monochromator, G. Monochmrnmtor, H. Detector.
Figure 2 shows s. generalized design of the apparatus used for spectroscopic flash studies. A is the sample cell; B is a water jacket for thermostatting the cell and/or filtering the output of the Bash tube(e); C is the flash tube(s); D is a reflector; E is a spectroscopic source, either a flash tub? or a steady-state lamp; P is a. monochromator for selecting radiation of a particular wavelength from the spectruscopic source; G is another monochromator for analyzing the light emerging from the cell; and H is a detector, the emtct nature of which depends on the experiment. By rearranging the various components and choosing the proper detector, this basic design csn be modified for use in a wide variety of experiments. To determine the complete absorption spectrum of an intermediate, one would use a. flash tube rtt E,no monochromatur at F, and a spectrograph with a photographic plate as a detector (combining G and H). A suitable period of time (ca. 10 to 1000 met) after the intermediate is produced by the main flash (C),a delay circuit would fire a flash tube a t E and the spectrum would be recorded. This may be done as many times as necessary to obtain the proper density on the photographic plate. Figure 3 shows spectra of transie~~ts produced in the flash pbutolysis of duroquinane (14). The change in the absorption spectrum can be seen as a difference in darkening of the plate as a function of the time delay between the photolysis flash and the analyzing flash. For spectrophotumetricdly monitoring
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Chemical lnstrumentation those shown in Figure 3 except the ordinstc would record phosphorescence or fluorescence intensity rather than light absorption.
Circuitry Figure 5 shows a block diagram of a flwh type circuit. The high voltage power supply must provide lnnn 200 to ca. 20 kv de depending on the particular application. The sturage section is usually a capacitor bank but may also contain indurt,nrs to control the flash dmatiun. The energy and duration a t n flash are given by R = '1%CVa aud 1 z 417 where V is t,lw voltage, aud C sod I, are the total capacitance and indrutance, respee(,ively, of the discharge circuit. This indicates that the condition uf high voltnge and low capacitance and iuduetance prvduces the highest power. The charging resistor isdates the storage section imm the power supply in order tc, prevent the cappacitor bank from discharging through the power supply when the flash is triggered, and to prevent the flesh tube from continuing t o fire after the capacitor bank hss been discharged. The flslih tube is triggered by a. pulse of ca. 5 to 25 kv from the trigger section. This pulse is applied either to a trigger wire wrapped around the flash tube or t o a spark gap connected in series with the Hash tube.
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Figure 4. Orcillotcopic trocer of duroquinone trmrientr in viscous poraffin solvent. 7, triplet; R duroremiquinone radical; F, Ra5h profile; S and S , scattered light from photolyris Ra.h Adopted from Bridge and Porter 11 31.
A typical circuit for the operation of
(Continued on page A88)
Chemical lnstrumentution
Figure 5. A block diagram of o boric Rarh lamp circuit.
fire t x ~ o500 j Hash tithes produring n Hash of 20 psec duration ( 1 6 ) . Tho high voltage dc supply charges eeah or the 10 p5 10 kv capacitors l o 10 kv, a voltagt: a t which t,he irtmps would fire spnntaneousl,y without, the spnrk gap in the rirruit. The spark gap is a type of high voltage, high ntrrent switch which is activated by a triggering pulse ( e n . 5 to 25 kv) ohtailled hy discharging s srnsll mpnuitor t h n n ~ g hthe primary of the t,rigger tmimforrner. JVheu the spnrk gnp is triggered, the frdl 1 0 kv or t h e RELAYS
Figure : .6
A detailed circuit for t h e operation of two Rorh tuber
Chemical lnstrumentertion the lamps and they fire. Then the storage capacitors mtomatically begin to charge for the next flash. The high frequency choke coils in the circuit permit the use of one power supply to charge both capacitors while isolating them from one another during the discharge (7). Also included in the circuit of Figure 6 are safety features which are strongly recommended dne to the combination of high voltages and large capacitances encountered in flash circuits. When the power is off, bath the main capacitor bank and the trigger capacitor are shorted through appropriate resistances, by use of relays. Another relay may be inserted in the primary of the trigger transformer to prevent accidental firing of the lamps. All safety features should he of the type which will aut,omrttically return to, and remain in, the safety position when the power is shut of. Should a flash of even shorter duration be desired, special components and circuits most he used. For example, a very short flash has been reported, producing only ca. 40 pj/flash and having a full width a t half-height flash duration of less than 10 nanoseconds (16). A special flash tnbe was made hy opening a. General Electrio NE-2 neon bulb and bending the electrodes until they were pi~rallel and within '/u in. of oneanother. Thebnlb was then connected to a diffusion pump and gently flamed. Pure hydrogen gas was admitted to a pressure of ca. 100 mm and the bulb was resealed. Figure 7 shows the cirruit used with this flash tube. Only the distributed capacitance (C') of the circuit was used, unless higher energy per flash was desired, in which ease a special low inductance ceramic capacitor (C) was used. In place of a: spark gap, a 2D21 thyratron tube was used to trigger the flash. The extremely high plate voltage used required a. high variable bias to prevent the thyratron from firing because of the plate voltage alone. Although these conditions exceeded the recommended operating conditions far the ZD21, the authors reported that most 2D21 thyratron tubes performed satisfactorily. The 20 Meg resistor between the power supply and the flash tube was large enough to cut off both the flash tube and the thyratron after firing. The 500°K resistor across the lamp prevented the flash tube from breaking down prematurely under the high supply valtsge
Figure 7. A circuit designed to produce Rmsher in the nonosecond region.
(Calinueda page A9d) A90 / Journal of Chemical Educofion
1:uvuntly a n ,llt,raviolet lamp fur gcner,,,i,,, i,,, Rashes, of constant shape, in the s ~ h ~ ~ s r ~ o s e cregiwl o n d has heen reported ( 1 3 ) . This lamp was made fn,m a commercially availnhle mermry contact relay, using the spark emissiol~ from the culttact as the sowee of radiation. The rise time uf the ptllse was 0.4 p s w and t,he hslf-nidth of the pr~lse
Chemical Instrumentation
,,,,,
and preve~ltcdit f ~ . r ~firing m a t all uldess it fired within it few rni~nrsecondsof lhe t h y a t r a n . X w h a eirrnit mnst be wired carcfdly to eliminate all stray inductmtre which would p r d o ~ r g the dwalinr~(of the flash. Table 1.
Summary of Operating Characteristics o f Some Typical Commercially Available Flash Tuber Energy "el.
Flash dura-
Arc
Strsight Strsighl
400 1.25
250 2
1 per Ill aec 8llO/sec
Straighl Straight Straight, Straight Slraight Straight
5 400 200 1011 600 111,0l10 GOO
G 10011 4011 80 600 2200 0011 1.511
G000/ser 1 per 20 sec 1 per Ill sec 1 per 5 sec 1 per 10 see 1 per 4 rnin 1 per 10 sec 1 per 10 ser XI/min
IT U
lull
Straight Helis Helix
. . .. . . .. .... .... .... . . ..
125 125 200
2.5/sec
. . ... . .. . .
G/rnin 2/min rnin
Adrlressrs for I'ohle 1: Edgerton, Ger!ncsliausen k Grier. Ioc. 100 Brookline A r e . Boston. Mass. 02114
EPP
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GE
General Electric I'hoto Lamp Uepartnlent "181 Nela Park Clerelaod 12. Ohio
Eleetro-PorerPaoa, Inc. 5 lladlcs St. Chrnhridge, 31ass. 02140
w,w fl.5 psec. An interesting feature is that the lamp c m be operated hctween 3 and 5 X 104pulsesper second. Cnrrently there are several comrnercixlly nvailahle complete Hash units fur Hash photolysis studies. Some of these units were developed for high speed photographic (17) nse and provide co. 1.5 j/fl:tsh with 8. duratioti of m. O..5 sec. Some units have high repeatability rates (up to a.6000/sec) while others may he Hashed only moe in 5 see. Other ~mitswere developed for exciting Inser artiou in eryatals. Because t,hey gxwide powers np t,o 20,000 j/flash (18), t,hese units should have applicatirm in Hash photolysis studies reqlliring moderate t,o high energies. The eomtructian of Hash lamps hiw heen desr:ribed in detail by Porter (It). 13let.trorles, spnecd apprupriately, are sexled into each end oi a quartz tnhc of the proper size and shape. The tube is Hsmed g e ~ t l ywhile under vacuum m d l.llen filled to .z pre,ess~lreof cn. 100 mm. with :%n inert gas a r XeIlon, or kryptrm are nsually used) and fired several times to oatg;ts the elertrocies. It is evnrnnt,ed again and refilled, then sealed off. Such s t h e will have a sxt,isfartory life if the outgassing of the electndes has been complete and the qxrating conditions are moderate. I t (.amot l h emphiwized too strongly t,hat the q~tnrtzto metal seals mmt. be conslrurted to withstand the violent electrim1 nod thermal shock of firing the Inmp. It is rerommended that stmdy safot,y shields be used around any Hash i~ppnri~t,ns,especially if high energies xrc heing dissipat,etl in a short time, hewuse lamp f:dures are oft,enqtlite violent. A large variety of flash ttlhes is :tvnilahle fmm several rnaimfsctnrels a t the present time. Normally the tubes are made of quartz, althnngh some are made of Pyrex. Xenon seems tu be preferred although on special order the Hash t \ h e may be filled wibh another gas w a . There are t,wo basic designs in use, the straight tube and the helix. T d d e 1 snmmsrizes repreaeutat,ive flash tiches m d their operating chnmcteristim under "l~orrnal' conditions. Spark raps ctmist of two heavy tttngst,en electrodes separated hy a few n~illirnpterswith .z sharply pointed trigger wire termilmted a short distauce from one of them. The eleetrr,des are enclosed in a container to reduce noise. The trigger probe provides the first spiwk which I~reaks dowu the gap to slluw the main current surge to pass. Tshle 2 gives the opernting characteristics of a large number of triggered spark gaps currently available. For pnwiding the triggering pidse, urdiosry mtomobile spark coils or tesla mils have been used, h ~ now t trigger tra~lsformersare common. A wide range of trigger transformers is available fur most applications. Except for flash circuits designed for very short dnrat,ion flashes, any commerrinlly nvaila1)le capacitor having the :~ppropriutecapacitance and voltage rati n g tray he used in the energy storage sectht. For Hashes of short dimtion it may he icecessnry to use specid low in(Continued on page A94)
Volume 42, Number 2, February 1965
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Chemical Instrumentation ductanco capacitors. If i t is necessary t o prolong the flash dwation to prevent failure of the flmh tnbe, s p e d inductors may be used in the circuit. There inductors most be capable of handling high currents and must he mounted securely and away from metal objerts which may be attracted to the inductor by the high magnetic field generated dnriog the discharge. The high voltage power supply needs t o provide only the appropriate voltage and current necessary to charge the storage capacitors in a resonable time. I t need not be filtered sinre it is effectively disconnected from the rircuit dnring discharge.
Acknowledgment The authors wish t o thank D. K. Roe and V. It. Landi for their helpful commenk and discussions. This work is supported in part through funds provided by the U. S. Atomic Energy Commission under contract AT(30-1)-905 and by USPHS grant G M 11766-01 from the National Institutes of Health. We are indebted to the Nstionsl Science Found* tion for a graduate fellowship awarded t o David N. Bailey.
Literature Cited ( I ) NORRISK, I
