Luminescence Kinetics of Microcrystalline Adenine Following Pulse

31 Luminescence Kinetics of Microcrystalline Adenine Following Pulse Irradiation

Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

E. M. FIELDEN and S. C. LILLICRAP Physics Department, Institute of Cancer Research, Clifton Avenue, Sutton, Surrey, England

The spectra and decay kinetics of the luminescence of microcrystalline adenine following irradiation by 4.3 Mev. elec trons are reported over a temperature range from 93° to 540°K. At the lowest temperatures the emission is followed from 5 µsec. to 5 min. after a 1.6 µsec. electron pulse, and over this time scale the emission intensity decreases by a factor of 10 . A first order phosphorescence decay com ponent is observed below 130°K. between 5 msec. and 8 sec. The residual non-exponential decay components can be explained by a trapping model possessing a uniform distri bution of traps from 0-0.22 e.v. deep with frequency factors ~10 sec. . An alternative explanation of the initial decay in terms of a cluster ("spur" type ) model that is consistent with the data is also discussed. 7

9

-1

b s o r p t i o n of energy f r o m i o n i z i n g r a d i a t i o n leads to direct excitation as w e l l as i o n i z a t i o n of t h e a b s o r b i n g m e d i u m . A d d i t i o n a l excited states m a y b e p r o d u c e d b y i o n r e c o m b i n a t i o n f o l l o w i n g the i n i t i a l i o n i z a t i o n events. T h e resultant e x c i t e d molecules m a y lose t h e i r excess energy b y c o l l i s i o n a l d e a c t i v a t i o n o r b y fission of t h e m o l e c u l e to g i v e r a d i c a l s either

spontaneously o r b y interactions

with

other

molecules,

or b y

emission of a p h o t o n . T h e fission processes w i l l b e r a p i d , o c c u r r i n g over a t i m e scale of t h e order of the m o l e c u l a r r e l a x a t i o n time, a l t h o u g h i t m a y r e q u i r e crossover f r o m the o r i g i n a l to a dissociative e x c i t e d state. I n a d d i t i o n to t h e e x c i t a t i o n of single molecules F a n o ( 9 ) has p r o p o s e d a c o l l e c t i v e e x c i t a t i o n effect to e x p l a i n t h e energy loss spectra of electrons i n s o l i d films. T h i s process i n v o l v e s the d e p o s i t i o n of 10 to 20 e.v. i n a v o l u m e of a b o u t ( 1 0 0 A . ) . T h i s energy m a y b e s u b s e q u e n t l y l o c a l i z e d 3

444 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

31.

F I E L D E N A N D LILLICRAP

Luminescence

445

Kinetics

b y r u p t u r e of a c h e m i c a l b o n d o r m a y b e d e l o c a l i z e d b y d i f f u s i o n o r transfer of e x c i t a t i o n energy. T h e r a d i a t i v e l i f e t i m e o f a n excited state d e p e n d s o n t h e s p i n selec t i o n rules a n d is t y p i c a l l y of t h e o r d e r of 10" sec. f o r a s p i n a l l o w e d 8

t r a n s i t i o n a n d u p to 1 0 times longer f o r a s p i n f o r b i d d e n t r a n s i t i o n . 9

O p t i c a l excitation g e n e r a l l y p r o d u c e s excited singlet states f o l l o w e d b y extremely r a p i d radiationless energy loss, l e a v i n g t h e e x c i t e d m o l e c u l e s i n t h e first singlet state Si. T h e r a p i d fluorescence process Si - » S is i n G

c o m p e t i t i o n w i t h t h e intersystem crossing process g i v i n g triplets Si -> T V E x c i t a t i o n b y s l o w electrons, h o w e v e r , does n o t obey a l l t h e o p t i c a l s p i n selection rules a n d n o r m a l l y f o r b i d d e n transitions s u c h as S —» T Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

G

a h i g h e r p r o b a b i l i t y of o c c u r r i n g d i r e c t l y (8, 14, 17).

t

have

T h u s , t h e absorp

t i o n of i o n i z i n g r a d i a t i o n m a y l e a d to t h e p r o d u c t i o n of triplets as a p r i m a r y step g i v i n g a h i g h e r y i e l d of triplets t h a n w o u l d have

been

p r o d u c e d i f t h e p r i m a r y e x c i t a t i o n was to singlet states o n l y . It has b e e n suggested (20)

w i t h some e x p e r i m e n t a l e v i d e n c e (2)

that e x c i t a t i o n to

h i g h singlet levels b y t h e faster electrons leads to e n h a n c e d intersystem crossing t o t r i p l e t states. E m i s s i o n spectroscopy

is a c o n v e n i e n t

method

of f o l l o w i n g t h e

b e h a v i o r of excited states, possessing m a n y advantages over a b s o r p t i o n spectroscopy, e s p e c i a l l y i n t h e s o l i d state. T h e l o w t e m p e r a t u r e t h e r m o l u m i n e s c e n c e spectra of o r g a n i c mate rials h a v e also b e e n i n v e s t i g a t e d (3, 15, 22, 25), a n d i t is f o u n d that most of t h e g l o w peaks l i e b e t w e e n 1 0 0 ° a n d 170 ° K .

Lehman and Wallace

(15) h a v e l i s t e d t h e r m o l u m i n e s c e n c e spectra, g l o w curves, a n d e m i s s i o n spectra f o r a large n u m b e r of b i o l o g i c a l l y i m p o r t a n t molecules.

The

l u m i n e s c e n c e w a s r e c o r d e d d u r i n g , a n d several m i n u t e s after, i r r a d i a t i o n b y sources of a w i d e range of l i n e a r energy transfer

(LET).

These

authors also r e p o r t e d a n effect of gas pressure o n t h e l u m i n e s c e n c e y i e l d at l o w temperatures that w a s i n t e r p r e t e d as t h e effect of gas p e n e t r a t i o n i n t o t h e c r y s t a l lattice. O t h e r w o r k e r s ( 7 ) , h o w e v e r , h a v e d e m o n s t r a t e d s i m i l a r gas effects a n d s h o w n t h e m to b e o w i n g to t h e t e m p e r a t u r e differences c a u s e d b y restricted c o n v e c t i o n c o o l i n g of t h e p o w d e r s b y the s u r r o u n d i n g gas. T h e kinetics of t h e r a d i o l u m i n e s c e n c e of organic c o m p o u n d s h a v e not b e e n w i d e l y p u b l i s h e d .

Bollinger a n d Thomas

(6)

reported the

r o o m temperature d e c a y kinetics of t h e l o n g - l i v e d s c i n t i l l a t i o n c o m p o n e n t of trans-stilbene.

T h e d e c a y profile w a s n o n - e x p o n e n t i a l over t h e 100

jusec. t i m e scale c o v e r e d a n d , apart f r o m i n t e n s i t y differences, t h e d e c a y profile w a s i d e n t i c a l f o r y - r a y s , neutrons a n d a-particles.

However, the

d e c a y kinetics of several i n o r g a n i c p h o s p h o r s e x c i t e d b y l o w energy electrons—e.g., c a t h o d e (21).

r a y tube

T h e t h e o r e t i c a l treatment

phosphors—have

been

investigated

of t h e k i n e t i c s o f t h e emission f r o m

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

446

RADIATION CHEMISTRY

1

these i o n i c lattices has b e e n o n t h e basis of e l e c t r o n t r a p p i n g , a n d one of t h e earlier papers (18) points o u t h o w a p p a r e n t l y s i m p l e d e c a y k i n e t i c s c a n arise f r o m v a r i o u s d i s t r i b u t i o n s of t r a p depths. t r a p p i n g i n o r g a n i c crystals is c o n f i r m e d b y t h e i r

T h e existence

of

thermoluminescent

b e h a v i o r , a n d i t is l i k e l y that some of t h e spontaneous e m i s s i o n i n v o l v e s t r a p p i n g processes. Several k i n e t i c schemes h a v e b e e n p u t f o r w a r d ( 4 , 1 3 , 24) to e x p l a i n the kinetics of e x c i t e d states i n s o l i d solutions a n d p u r e a n d m i x e d crystals f o l l o w i n g o p t i c a l a n d i o n i z i n g - p a r t i c l e excitation. T h e s e schemes, as w e l l as t h e d i r e c t q u e n c h i n g reactions f o r singlets a n d triplets, i n c l u d e t r i p l e t Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

t r i p l e t a n n i h i l a t i o n reactions

a n d b i m o l e c u l a r t r i p l e t q u e n c h i n g reactions T

1

+ T,

T, + S

0

T h e first r e a c t i o n , b y p r o d u c i n g a singlet state, c a n l e a d to a singletg r o u n d state emission a n d is responsible f o r " d e l a y e d

fluorescence."

T h e present p r o g r a m is a i m e d at filling t h e g a p b e t w e e n t h e e m i s s i o n d u r i n g r a d i a t i o n a n d the e m i s s i o n f o u n d several m i n u t e s after i r r a d i a t i o n . B y a s t u d y of t h e kinetics a n d spectra of t h e e m i s s i o n it is h o p e d to g a i n m o r e k n o w l e d g e of t h e processes i n v o l v e d . Experimental Radiation Source. T h e source of r a d i a t i o n is a M u l l a r d 4.3 M e v . e l e c t r o n accelerator, M o d e l S L 46, w h i c h p r o d u c e s 1.6 /xsec. d u r a t i o n pulses at u p to 250 m A , w i t h r e p e t i t i o n rates v a r y i n g f r o m a single p u l s e to 400 pulses p e r second. Irradiation Assembly. T h e i r r a d i a t i o n assembly is s h o w n i n F i g u r e 1. It consists of a r e c t a n g u l a r P e r s p e x ( L u c i t e , P l e x i g l a s ) b o x w i t h t w o c o m p a r t m e n t s . T h e rear section contains a p h o s p h o r u s p e n t o x i d e t r a y to p r e v e n t c o n d e n s a t i o n o n t h e c o o l e d s a m p l e . T h e f r o n t section contains the s a m p l e a n d is l i n e d w i t h b l a c k P V C tape to obscure t h e l u m i n e s c e n c e f r o m t h e i r r a d i a t e d Perspex. A t the f r o n t e n d of t h e box, i n l i n e w i t h t h e sample, is a 1 m m . t h i c k Perspex w i n d o w t h r o u g h w h i c h the electron i r r a d i a t i o n b e a m enters h o r i z o n t a l l y . D i r e c t l y above t h e s a m p l e is a S p e c t r o s i l q u a r t z w i n d o w 0.25 m m . t h i c k t h r o u g h w h i c h t h e l u m i n e s cence is o b s e r v e d . L i g h t e m i t t e d f r o m t h e s a m p l e passes t h r o u g h t h e w i n d o w a n d is t h e n reflected h o r i z o n t a l l y b y a f r o n t s u r f a c e d m i r r o r i n c l i n e d at 4 5 ° (see F i g u r e 1 ) . T h e p o w d e r e d s a m p l e is h e l d i n a 6 m m . d i a m e t e r c u p t u r n e d i n t h e e n d of a short a l u m i n u m r o d . A t h e r m o c o u p l e is fixed i n this r o d just b e l o w the sample c u p . T h i s c u p assembly makes g o o d t h e r m a l contact w i t h a 9 m m . d i a m e t e r c o p p e r r o d w h i c h passes h o r i z o n t a l l y i n t o t h e rear c o m p a r t m e n t . A s m a l l h e a t i n g c o i l is w o u n d o n t h e c o p p e r r o d close to t h e a l u m i n u m c u p . I n t h e rear c o m p a r t m e n t the c o p p e r r o d is s o l d e r e d to a c o p p e r t u b e of the same external d i a m e t e r


31.


Luminescence

447

Kinetics

w h i c h p r o t r u d e s f r o m the b a c k of t h e Perspex b o x a n d is b e n t u p w a r d s . A c y l i n d r i c a l brass c u p , 5 c m . diameter, is s o l d e r e d v e r t i c a l l y onto t h e c o p p e r tube. T h e brass c u p a n d t h e c o p p e r t u b e outside t h e b o x are i n s u l a t e d w i t h 4 c m . of e x p a n d e d p o l y s t y r e n e . T h e s a m p l e is c o o l e d b y f i l l i n g t h e brass c u p w i t h l i q u i d n i t r o g e n . A f t e r 10 m i n u t e s the s a m p l e has c o o l e d to 93 ° K . a n d , b y c o n t r o l l e d use of the h e a t i n g c o i l , i n t e r m e d i a t e temperatures c a n b e o b t a i n e d . A b o v e 200 °K., s o l i d C 0 - a c e t o n e is a m o r e suitable coolant. T h e i n t e r i o r of t h e b o x c a n b e flushed w i t h a n y a v a i l a b l e gas b y means of a n entrance t u b e i n the rear section of the b o x a n d a n exit t u b e i n t h e f r o n t section.


2

Mirror

Quartz Window

Thermocouple Beam Entrance Window Sample Cup

\

Polystyrene Insulation

10 cm Figure 1. Schematic diagrams of the irradiation assembly. For clarity the liquid nitrogen reservoir has been omitted from the front view and the inclined mirror from the side view I r a d i a t i o n s a b o v e r o o m t e m p e r a t u r e are c a r r i e d o u t b y p r e s s i n g t h e p o w d e r i n t o a 6 X 1 m m . slot m i l l e d i n a 5 m m . d i a m e t e r a l u m i n u m r o d . T h e r o d replaces the n o r m a l b i t i n a 2 5 W s o l d e r i n g i r o n w h i c h acts as a heat source. T h e assembly is m o u n t e d v e r t i c a l l y w i t h a t h e r m o c o u p l e fixed i n the s a m p l e h o l d e r . T e m p e r a t u r e s c a n be c o n t r o l l e d to w i t h i n 5 ° C . b y r e g u l a t i n g t h e current t h r o u g h the h e a t i n g element w i t h a V a r i a c transformer.

American Chemical feisty library 1155 16th St., HM Hart; Radiation Chemistry Washington, 0,C* 2 0038 DC, 1968. Advances in Chemistry; American Chemical Society: Washington,

448

RADIATION CHEMISTRY

1


T h e l i g h t e m i t t e d b y the i r r a d i a t e d p o w d e r s is r e l a y e d b y means of t w o q u a r t z lenses a n d t w o m i r r o r s to the d e t e c t i n g system i n the acceler ator c o n t r o l r o o m . T h e o p t i c a l a n d p h o t o m e t e r system are to be d e s c r i b e d i n d e t a i l elsewhere (10). B r i e f l y , the o p t i c a l system i m a g e d the p o w d e r sample o n the m o n o c h r o m a t o r slits w i t h a 1:1 object-to-image ratio a n d a l i g h t acceptance angle of f 3.5. T h e m o n o c h r o m a t o r is a B a u s c h a n d L o m b H i g h Intensity t y p e w i t h t w o gratings c o v e r i n g the range 180 to 400 n . m . a n d 350 to 800 n . m . T h e l i g h t e m e r g i n g f r o m the m o n o c h r o m a t o r is m o n i t o r e d b y a p h o t o m u l t i p l i e r , E M I t y p e 9 5 5 8 B Q , w h o s e o u t p u t w a s f e d d i r e c t l y to a n oscilloscope ( T e k t r o n i x t y p e 547 or 5 5 5 ) . The resultant d i s p l a y of l i g h t intensity vs. t i m e was p h o t o g r a p h e d . F i g u r e 2 gives a n e x a m p l e of t w o s u c h p h o t o g r a p h s ; the p h o t o g r a p h s are e n l a r g e d

Figure 2. Oscilloscope traces of luminescent decays from adenine following a 10 Krad pulse of 4.3 Mev. electrons of 1.6 jusec. duration. Curve A, 2 ^sec./large horizontal division, measured at 295°K. Curve B, 0.5 sec./large horizontal division, measured at 93°K. The vertical sensitivity of Curve B is a factor of 400 greater than that of Curve A


31.


Luminescence

Kinetics

449


to t w i c e t h e o r i g i n a l size f o r t h e p u r p o s e of analysis. T h e o p t i c a l system is a l i g n e d b y i l l u m i n a t i n g t h e p o w d e r w i t h a tungsten l a m p a n d t h e n f o c u s s i n g t h e l i g h t scattered f r o m t h e p o w d e r surface onto t h e entrance slit of the m o n o c h r o m a t o r . Calibration of the Photometer System. I n order to p r o d u c e m e a n i n g f u l emission spectra t h e r e l a t i v e responses at different w a v e l e n g t h s of the entire o p t i c a l a n d d e t e c t i n g system h a d to b e k n o w n . T h e o v e r a l l response is a f u n c t i o n of t h e response of t h e p h o t o - m u l t i p l i e r , t h e m o n o c h r o m a t o r efficiency a n d t r a n s m i s s i o n factors, t h e reflectivity of m i r r o r s a n d t h e transmission a n d d i s p e r s i o n of t h e lenses. T h e relative response of t h e t o t a l system w a s o b t a i n e d b y c o m p a r i s o n w i t h a k n o w n emission s p e c t r u m ; that of C e r e n k o v r a d i a t i o n . T h e s p e c t r u m of t h e C e r e n k o v l i g h t e m i t t e d w h e n h i g h energy particles pass t h r o u g h a m e d i u m has b e e n c a l c u l a t e d a n d m e a s u r e d e x p e r i m e n t a l l y (11, 19). T h e s p e c t r a l intensity is g i v e n b y : _ /

(

X

)

_

Const " A T '

/I -

V

1 \

W) 2

w h e r e p is t h e electron v e l o c i t y relative to l i g h t i n v a c u o a n d n is t h e refractive i n d e x of the m e d i u m . F o r t h e p u r p o s e of c a l i b r a t i o n t h e sample c u p was filled w i t h finely p o w d e r e d S p e c t r o s i l h i g h p u r i t y q u a r t z , w h i c h w a s a s s u m e d to e x h i b i t n e g l i g i b l e fluorescence c o m p a r e d w i t h t h e intensity of t h e C e r e n k o v light. W i t h the p h o t o m u l t i p l i e r o p e r a t i n g at a fixed voltage t h e intensity of C e r e n k o v l i g h t p r o d u c e d b y a single p u l s e of 4.3 M e v . electrons w a s r e c o r d e d at various w a v e l e n g t h s . A s the u l t r a v i o l e t g r a t i n g has h a l f t h e d i s p e r s i o n of t h e v i s i b l e range g r a t i n g , the entrance a n d exit slits w e r e d o u b l e d w h e n t h e f o r m e r was i n use to m a i n t a i n a constant b a n d - w i d t h . S u p p l e m e n t a r y C h a n c e - P i l k i n g t o n color filters w e r e u s e d over some w a v e l e n g t h ranges t o r e m o v e scattered l i g h t f r o m t h e m o n o c h r o m a t o r . T h e resultant s p e c t r u m consisted of t h e C e r e n k o v s p e c t r u m m o d i f i e d b y t h e system response. F r o m a c o m p a r i s o n of the m e a s u r e d s p e c t r u m w i t h t h e k n o w n s p e c t r u m t h e relative system response w a s d e d u c e d . F i g u r e 3 shows the response of the apparatus together w i t h the q u a r t z C e r e n k o v s p e c t r u m c a l c u l a t e d f r o m the expression above. C a r e w a s t a k e n to operate t h e p h o t o m u l t i p l i e r o n l y i n t h e linear r e g i o n of its characteristics. U n f o r t u n a t e l y t h e l u m i n e s c e n c e i n t e n s i t y of i r r a d i a t e d a d e n i n e i m m e d i a t e l y after t h e electron p u l s e w a s less t h a n 2 % of t h e C e r e n k o v p l u s fluorescence emission d u r i n g t h e pulse. T h u s , i n o r d e r to fill t h e oscilloscope d i s p l a y w i t h t h e l u m i n e s c e n c e signal, t h e oscilloscope amplifiers w e r e necessarily o v e r l o a d e d d u r i n g t h e r a d i a t i o n pulse. T h e t i m e t a k e n for t h e system to recover was m e a s u r e d b y observ i n g t h e o v e r l o a d r e c o v e r y of t h e amplifiers f o l l o w i n g a s i m i l a r p u l s e of C e r e n k o v l i g h t f r o m i r r a d i a t e d q u a r t z p o w d e r ( w h i c h has n o l o n g - l i v e d l u m i n e s c e n c e ) . U n d e r these c o n d i t i o n s i t w a s f o u n d that there w a s n o spurious s i g n a l 3.4 jusec. after t h e e n d of t h e pulse. S i m i l a r tests w e r e m a d e f o r a l l w o r k i n g c o n d i t i o n s of t h e p h o t o m u l t i p l i e r . Temperature Measurements. A C h r o m e l / A l u m e l t h e r m o c o u p l e m a d e from 0.25 m m . d i a m e t e r w i r e s w a s u s e d f o r a l l temperature measure ments. A b o v e 0 ° C . t h e t e m p e r a t u r e / E M F c u r v e f o l l o w e d t h e p u b l i s h e d


450

RADIATION CHEMISTRY

1


values (12) b u t b e l o w 0 ° C . there w a s a n a p p r o x i m a t e l y l i n e a r d e v i a t i o n w i t h t e m p e r a t u r e a m o u n t i n g to a n i n c r e a s e d E M F of 5 % at l i q u i d n i t r o g e n t e m p e r a t u r e . T h i s l o w t e m p e r a t u r e d e v i a t i o n is c a u s e d b y i m p u r i t i e s i n t h e w i r e s a n d is c o m m o n to several t h e r m o c o u p l e systems. T h e t h e r m o couple E M F was measured b y a Solartron d i g i t a l voltmeter, m o d e l LM1420.

Figure 3. Spectral response of photometer system. Curve A is the emission spectrum of Cerenkov light from irradiated quartz. Curves B, C, and D show the photometer response using Bausch and Lomb gratings. Curve B, ultraviolet grating type 33-86-01, no filter. Curve C, visible grating type 33-86-02, no filter. Curve D, visible grating type 33-86-02, using the Chance filters indicated above the curve Materials. T h e a d e n i n e w a s o b t a i n e d f r o m C a l b i o c h e m as A g r a d e microcrystalline powder a n d was used without further purification. This is a synthetic p r o d u c t , free f r o m c o n t a m i n a t i o n w i t h r e l a t e d n a t u r a l p r o d u c t s . I t s h o w e d o n l y one spot w h e n c h e c k e d b y t h i n l a y e r c h r o m a t o g r a p h y . M o s t of t h e p o w d e r passed t h r o u g h a 90JU sieve. A s a m p l e w a s also r e c r y s t a l l i z e d f r o m t r i p l y - d i s t i l l e d w a t e r . Dosimetry. F o r dose measurements t h e a d e n i n e p o w d e r i n the s a m p l e c u p w a s r e p l a c e d b y a s i m i l a r q u a n t i t y of L i F p o w d e r ( T L D 1 0 0 ) . T h e t h e r m o l u m i n e s c e n c e of t h e L i F w a s m e a s u r e d 24 h o u r s after a single p u l s e i r r a d i a t i o n o n a c o m m e r c i a l reader, t h e M a d i s o n R e s e a r c h S - 2 L . A t y p i c a l dose w a s 10 to 15 K r a d i n w a t e r f r o m a 1.6 /xsec. p u l s e . Experimental

Results

Emission Spectra. S p e c t r a of the l i g h t e m i s s i o n f r o m adenine p o w d e r , after r e c e i v i n g a 15 K r a d p u l s e of 4.3 M e v . electrons, w e r e m e a s u r e d at


31.

F I E L D E N AND LILLICRAP

Luminescence

b o t h r o o m t e m p e r a t u r e a n d 93 ° K .

451

Kinetics

F i g u r e 4 shows t h e e m i s s i o n spectra

of a d e n i n e at 5 /xsec. a n d 500 /xsec. after i r r a d i a t i o n at r o o m t e m p e r a t u r e , a n d at 5 /xsec, 500 ^sec., 600 m s e c , a n d 15 sec. after i r r a d i a t i o n at 9 3 ° K . T h e b a n d p a s s of the m o n o c h r o m a t o r f o r these measurements w a s 8.5 n . m . , a n d the spectra w e r e c o r r e c t e d f o r t h e s p e c t r a l response of t h e w h o l e system. T h e r o o m t e m p e r a t u r e spectra are s i m i l a r i n p o s i t i o n a n d shape to t h e c o r r e s p o n d i n g l o w t e m p e r a t u r e spectra, a l t h o u g h there are differences at the l o n g e r w a v e l e n g t h s . T h e r e is also a progressive n a r r o w i n g of t h e s p e c t r u m a n d a slight shift to shorter w a v e l e n g t h s w i t h increase i n t i m e . T h e l o w t e m p e r a t u r e spectra m e a s u r e d at 600 msec, a n d 15 sec. Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

after i r r a d i a t i o n are t h e same, w i t h i n e x p e r i m e n t a l error, as t h e u l t r a v i o l e t e x c i t e d l o w t e m p e r a t u r e p h o s p h o r e s c e n c e s p e c t r u m of s o l i d a d e n i n e d e s c r i b e d b y S i n g h a n d C h a r l e s b y (22)

a n d agree w e l l w i t h that f o u n d f o r

the i n t e g r a t e d e m i s s i o n at l o w temperatures pulses

following

.01 r a d x-ray

I t is n o t u n l i k e l y , therefore, that a c o m m o n process is

(16).

r e s p o n s i b l e f o r t h e f o u r s i m i l a r spectra a n d p o s s i b l y also f o r t h e b r o a d e r 5 /xsec. a n d 500 fxsec. spectra. T h e 15 sec. l o w t e m p e r a t u r e s p e c t r u m is i n agreement w i t h t h e l i m i t e d s p e c t r a l d a t a p u b l i s h e d b y L e h m a n a n d W a l l a c e (15)

0-6

for irradiated adenine powder.

0-6

r

300

400

500

600

700 300

400

500

600

700

Wavelength (am)

Figure 4. Emission spectra from adenine at 294°K. and 93°K., at different times after irradiation. — # — • — 5 \xsec, — O — O — 500 fxsec, — • — • — 600 msec, — • — • — 15 sec. The intensity scales have been expanded by different factors to allow comparison. At 294°K., expansion factors are X 1 (5 ^sec. spectra), X 100 (500 ^sec. spectra). At 93°K. expansion factors are X 2 (5 fxsec. spectra), X 100 (500 ^sec. spectra), X 200 (600 msec, spectra), X 4.10 (15 sec. spectra) 3


452

RADIATION CHEMISTRY

1

D e c a y P r o f i l e s . T h e d e c a y of a d e n i n e e m i s s i o n w a s r e c o r d e d over a range of temperatures.

F i g u r e 5 shows t h e d e c a y profiles at 475 n . m .

p l o t t e d f r o m 5 ^sec. to 1 msec, after i r r a d i a t i o n b y a n 11 K r a d electron p u l s e at 296 ° K . a n d 93 ° K .

I n a l l experiments zero t i m e is t a k e n to b e

the b e g i n n i n g of t h e p u l s e . T h e i n t e r e s t i n g features of this p l o t are that the r o o m t e m p e r a t u r e d e c a y is i n i t i a l l y s l o w e r t h a n t h e l o w t e m p e r a t u r e d e c a y a n d that t h e d e c a y curves cross at 230 /xsec. T h e l u m i n e s c e n c e is w e a k e r f o r a b o u t 200 /xsec. at 93 ° K . t h a n at r o o m t e m p e r a t u r e , w h e r e a s the i n t e g r a t e d e m i s s i o n w a s f o u n d to increase o n l o w e r i n g t h e t e m p e r a ture.

N e i t h e r c u r v e is e x p o n e n t i a l over this range.

I n addition to the


measurements at 475 n . m . , d e c a y profiles w e r e also r e c o r d e d at 400 n . m . a n d 520 n . m . If the 5 /xsec. spectra of F i g u r e 4 are t h e s u m of c o m p o n e n t spectra, t h e n t h e d e c a y at 400 n . m . i n p a r t i c u l a r m i g h t b e e x p e c t e d t o be c a u s e d b y a single c o m p o n e n t a n d h e n c e h a v e s i m p l e r k i n e t i c s t h a n i n a n o v e r l a p r e g i o n . H o w e v e r , n e i t h e r of t h e d e c a y curves at 400 n . m . or 520 n . m . c o u l d b e fitted to a s i m p l e d e c a y scheme. A b o v e r o o m t e m p e r a ture t h e decays are a l l s i m i l a r a n d n o n e w features appear.

200

400

600

800

1 Msec.

Time (usee)

Figure 5, Decay curves of adenine luminescence from 5 /msec, to 1 msec, after irradiation by an 11 Krad. electron pulse at 93°K. and 296°K. T h e d e c a y of a d e n i n e l u m i n e s c e n c e at 93 ° K . is p l o t t e d i n F i g u r e 6 f r o m 5 /xsec. to 5 m i n . after i r r a d i a t i o n . T h e d e c a y curves o u t to 30 sec. w e r e r e c o r d e d f o l l o w i n g single pulses of electrons a n d those f r o m 10 sec.


31.


Luminescence

453

Kinetics

to 5 m i n . f o l l o w i n g 140 electron pulses g i v e n i n 1.4 sec. T h e t w o curves w e r e i d e n t i c a l i n shape i n t h e r e g i o n of o v e r l a p f r o m 10 sec. to 30 s e c , a n d t h e p a r t o f the c u r v e t a k e n w i t h 140 pulses w a s therefore

matched

to t h e rest o f the c u r v e i n t h e r e g i o n of o v e r l a p since absolute c a l i b r a t i o n of dose w a s n o t accurate i n t h e m u l t i p l e p u l s e r u n .


io-y

•

lO"

6

I

10'

• 5

•

I

10

I -4

10'

I

I

I

1

10"

3

2

I

I

I

10"

1

1

1

10°

1

1

1

10

1 1

1

1

10

I 2

1

1

10

3

Time (sec)

Figure 6. Decay curves of adenine luminescence at 93°K. From 5 jxsec. to 30 sec. the decay was recorded following a single electron pulse of 11 Krad, and from 10 sec. to 5 min. following 140 electron pulses in 1.4 sec. The broken line is the continuation of the exponential portion of the curve (5 msec.-8 sec). The dotted line is the result when the exponential portion is subtracted from the total decay curve B e t w e e n 5 msec, a n d 8 sec. after i r r a d i a t i o n t h e d e c a y c u r v e is e x p o n e n t i a l w i t h a h a l f - l i f e o f 800 msec. obeys t h e s i m p l e l a w I oc f (15).

1

B e y o n d 10 sec. t h e e m i s s i o n

i n agreement w i t h L e h m a n a n d W a l l a c e

I f i t is a s s u m e d that t h e e x p o n e n t i a l d e c a y c a n b e e x t r a p o l a t e d

b a c k t o 5 /xsec. after i r r a d i a t i o n , t h e n t h e e x p o n e n t i a l p o r t i o n m a y b e s u b t r a c t e d f r o m t h e t o t a l d e c a y profile. T h e d e c a y c u r v e d e r i v e d i n this w a y is s h o w n i n F i g u r e 6 b y the d o t t e d l i n e . T h i s c u r v e also f o l l o w s a


454

RADIATION CHEMISTRY

1

r e c i p r o c a l p o w e r l a w d e c a y a l t h o u g h it does not extrapolate to meet the p o r t i o n of the c u r v e b e y o n d 10 sec. after i r r a d i a t i o n . Dose Dependence. I n o r d e r to investigate the effect of dose o n the l i g h t e m i s s i o n f r o m a d e n i n e the i n t e n s i t y of the e l e c t r o n b e a m w a s re d u c e d b y scattering a n d the c o m p l e t e d e c a y curves w e r e r e - r e c o r d e d . T h e thickest scattering plate u s e d r e d u c e d the dose to the p o w d e r b y a f a c t o r of 4.6. O v e r this l i m i t e d dose r a n g e it w a s f o u n d that the e m i s s i o n i n t e n s i t y at a n y t i m e after i r r a d i a t i o n w a s p r o p o r t i o n a l to the dose at b o t h r o o m t e m p e r a t u r e a n d at 93 °K., except at p o s t - i r r a d i a t i o n times less t h a n 50 ju,sec at r o o m t e m p e r a t u r e . A t these short times the l i g h t e m i s s i o n Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

p e r r a d s l i g h t l y i n c r e a s e d w i t h i n c r e a s i n g dose.

The deviation from a

l i n e a r dose d e p e n d e n c e w a s greater at the shortest t i m e a n d c o u l d b e a p p r o x i m a t e d to a ( D o s e ) 1

2

d e p e n d e n c e at 5 /xsec. T h e r e is a n i n d i c a t i o n

of a s i m i l a r s m a l l e r dose d e p e n d e n c e at short times at 9 3 ° K . , b u t this departure

is at

present

within

the

e x p e r i m e n t a l error

of

a

linear

dependence. Temperature Dependence. T h e t e m p e r a t u r e d e p e n d e n c e of the i n tensity at 475 n . m . at v a r i o u s times after i r r a d i a t i o n out to 4 msec, are p r e s e n t e d i n F i g u r e 7. A t temperatures a b o v e 3 0 0 ° K . there is a decrease i n the r e s i d u a l l u m i n e s c e n c e w i t h i n c r e a s i n g t e m p e r a t u r e . B e l o w 300 °K., h o w e v e r , the r e s i d u a l e m i s s i o n at a g i v e n t i m e after i r r a d i a t i o n has a m a x i m u m v a l u e at a p a r t i c u l a r t e m p e r a t u r e .

T h i s m a x i m u m occurs at

l o w e r temperatures as the o b s e r v a t i o n t i m e after i r r a d i a t i o n is i n c r e a s e d . A l l the d e c a y curves, w h e t h e r d i r e c t l y o b s e r v e d at constant t e m p e r a t u r e , or p l o t t e d at i n t e r m e d i a t e temperatures f r o m the s m o o t h e d curves of F i g u r e 7, are n o n - e x p o n e n t i a l i n f o r m a n d change g r a d u a l l y f r o m

the

shape at h i g h t e m p e r a t u r e to that at 93 ° K . B e c a u s e of the l o w i n t e n s i t y of the e m i s s i o n at temperatures a b o v e 1 6 0 ° K . for times greater t h a n 100 m s e c , it was not possible to f o l l o w the v a r i a t i o n of l u m i n e s c e n c e efficiency w i t h t e m p e r a t u r e over the c o m plete t e m p e r a t u r e range s h o w n i n F i g u r e 7.

It w a s , h o w e v e r , possible

to investigate the effect of t e m p e r a t u r e o n the e x p o n e n t i a l p o r t i o n of the l o w t e m p e r a t u r e d e c a y c u r v e over a l i m i t e d range near l i q u i d n i t r o g e n t e m p e r a t u r e . A n A r r h e n i u s p l o t of the e x p o n e n t i a l d e c a y at temperatures b e t w e e n 9 3 ° a n d 1 3 0 ° K . y i e l d e d a n a c t i v a t i o n energy of 0.007 e.v./molecule. T h i s v a l u e is i n agreement w i t h the a c t i v a t i o n energy f o u n d p r e v i o u s l y (16)

f o r the effect of t e m p e r a t u r e over the same range o n the

i n t e g r a t e d e m i s s i o n f o l l o w i n g 0.01 r a d x-ray pulses. A s the d o m i n a n t p a r t of the l o n g - l i v e d e m i s s i o n at the l o w t e m p e r a t u r e is the e x p o n e n t i a l , this is f u r t h e r e v i d e n c e , i n agreement w i t h the s p e c t r a l results, that the same process is r e s p o n s i b l e f o r e m i s s i o n at these extremes of dose. Radiation Damage. It w a s f o u n d that after d e l i v e r i n g a b o u t 1 M r a d to the p o w d e r at 9 3 ° K . , the slope of the e x p o n e n t i a l p o r t i o n of the d e c a y


31.


Luminescence

455

Kinetics

decreased f r o m a h a l f - l i f e of 800 msec, to 600 m s e c , a n d also t h e i n t e n s i t y of e m i s s i o n at 1 s e c after i r r a d i a t i o n d r o p p e d b y a factor of t w o . A f t e r w a r m i n g t h e p o w d e r to r o o m t e m p e r a t u r e a n d r e c o o l i n g , t h e d e c a y c u r v e r e t u r n e d to n o r m a l .


10-

1

10 _

03 C

c

ior

I —^14 m sea

10'

3.7msecT^*^

10"

100

200

300 400 Temperature (°KJ

500

600

Figure 7. A composite plot of the temperature dependence of the luminescence intensity at various times after irradiation out to 4 msec. The data on this plot are taken from decay curves measured at various temperatures between 93°K. and 540°K. P h y s i c a l E f f e c t s . T o investigate the effect of a different atmosphere on the light emission from adenine, d r y argon was introduced into the s a m p l e c o m p a r t m e n t a n d the d e c a y curves at r o o m t e m p e r a t u r e a n d 93 ° K . m e a s u r e d .

N o difference e x c e e d i n g e x p e r i m e n t a l error c o u l d b e

d e t e c t e d b e t w e e n these curves a n d those r e c o r d e d i n a n atmosphere of air. W e t samples also gave the same d e c a y c u r v e at r o o m t e m p e r a t u r e . F i n a l l y , to test w h e t h e r the e m i s s i o n d e p e n d e d o n t h e size of t h e crystals, as i t m i g h t d o i f t h e e m i s s i o n w e r e l i m i t e d b y d i f f u s i o n of some entities to the c r y s t a l surface b u t n o t i f i t arose f r o m p h y s i c a l defects


456

RADIATION CHEMISTRY

1

t h r o u g h o u t the crystal, a n attempt was m a d e to r e d u c e the c r y s t a l size b y g r i n d i n g it w i t h a n agate pestle a n d m o r t a r a n d to increase the size by

r e c r y s t a l l i z a t i o n . N e i t h e r of these processes c a u s e d any significant

change, b u t it is not c e r t a i n that they s u c c e e d e d i n c h a n g i n g the size of the crystals. Discussion T h e m a i n features of this experiment w h i c h r e q u i r e a n e x p l a n a t i o n are the c o m p l e x shape of the l u m i n e s c e n c e d e c a y curves, especially at Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

93 °K., a n d the t e m p e r a t u r e d e p e n d e n c e of the emission.

O n a simple

e x c i t a t i o n t h e o r y w h i c h w i l l b e c o n s i d e r e d i n i t i a l l y , o n l y t r i p l e t states r e m a i n 1 /xsec. or longer after i r r a d i a t i o n as a l l the singlets p r o d u c e d w i l l h a v e r e t u r n e d to the g r o u n d state or u n d e r g o n e intersystem crossing to t r i p l e t states.

I n a d d i t i o n to the d i r e c t phosphorescence,

T

x

-»

S, a 0

n u m b e r of different p a t h w a y s are a v a i l a b l e for the r e m o v a l of the re m a i n i n g triplets. A c o m p l e t e d e c a y scheme m u s t i n c l u d e the f o l l o w i n g processes. 7\ —> S

x

T —> S + hp T T

t

t

2

+ T —> Tj + S t

+ T —> Sj + S r

0

(II)

fc

Bimolecular triplet quenching

(III)

k

Triplet-triplet annihilation

(IV)

3

0

(I)

Phosphorescence

k

0

x

Non-radiative quenching

k

0

4

T h e singlet p r o d u c e d i n Process I V m a y l e a d to d e l a y e d b y the r a d i a t i v e process S i - » S

0

+

fluorescence

hv, or to the p r o d u c t i o n of a f u r t h e r

t r i p l e t b y intersystem crossing. A s the singlet S i has a v e r y short l i f e t i m e , Process I V is n o r m a l l y the rate d e t e r m i n i n g step i n d e l a y e d fluorescence. F r o m these reactions the rate of d i s a p p e a r a n c e of triplets is g i v e n b y : ~ - ^ - - ( ^ i + ^ ) [ T i ] + (*3 + 2 - a * ) I T i ] i

where a =

4

p r o b a b i l i t y of intersystem crossing, S i - » T

2

d)

x

T h e l u m i n e s c e n c e intensity ( I ) of a l l q u a n t a is g i v e n b y the e q u a t i o n : I = ^ = f c [ T ] +fc [T ] / 2

where / =

p r o b a b i l i t y that S w i l l x

1

4

1

2

(2)

fluoresce.

T h e i n i t i a l c o n c e n t r a t i o n of triplets f o l l o w i n g i r r a d i a t i o n m a y be assumed to be p r o p o r t i o n a l to the dose as no second or h i g h e r order process has b e e n p r o p o s e d for t h e i r p r o d u c t i o n (5, 24). T h e relative i m p o r t a n c e of the first a n d second order d e c a y processes d e p e n d s o n the i n i t i a l c o n c e n t r a t i o n of triplets a n d also o n the t i m e of o b s e r v a t i o n after


31.


Luminescence

457

Kinetics

i r r a d i a t i o n . T h i s is because, i n general, t h e rate of f o r m a t i o n of p r o d u c t s ( p h o t o n s i n this e x p e r i m e n t )

at a n y t i m e after i r r a d i a t i o n is d i r e c t l y

p r o p o r t i o n a l to dose i n first order reactions, whereas i n a s e c o n d o r d e r r e a c t i o n t h e d e p e n d e n c e of t h e p r o d u c t f o r m a t i o n rate o n dose varies from (dose)

2

at zero t i m e to ( d o s e ) ° at infinite t i m e .

A s t h e present

apparatus o n l y a l l o w s o b s e r v a t i o n f r o m 5 fisec. after i r r a d i a t i o n , i t w a s not possible to m a k e measurements at zero t i m e . D e c a y C u r v e a t 9 3 ° K . T h e d e c a y c u r v e at 9 3 ° K . ( F i g u r e 6 ) is r e a d i l y r e s o l v e d into three c o m p o n e n t s w h i c h m a y b e c o n s i d e r e d separately.


T h e t i m e intervals after i r r a d i a t i o n i n w h i c h these c o m p o n e n t s

occur

are 5 /xsec. t o 1 m s e c , 5 m s e c to 8 s e c , a n d 10 s e c to 300 sec. 5 M S E C , T O 8 SEC. A F T E R IRRADIATION.

F r o m 5 msec, t o 8 s e c after

i r r a d i a t i o n the c u r v e is e x p o n e n t i a l over n i n e h a l f - l i v e s a n d t h e intensity is d i r e c t l y p r o p o r t i o n a l to dose.

T h i s b e h a v i o r is that expected f r o m

phosphorescence (Process I I ) w i t h n e g l i g i b l e c o m p e t i t i o n f r o m b i m o l e c u lar Processes

I I I a n d I V . A s f u r t h e r e v i d e n c e that this is t h e process

observed, the s p e c t r u m taken at 600 msec, is i d e n t i c a l w i t h t h e u l t r a v i o l e t i n d u c e d phosphorescence of a d e n i n e (22).

H o w e v e r , as t h e exponent of

the e x p o n e n t i a l varies w i t h t e m p e r a t u r e i t w o u l d a p p e a r that t h e triplets are also b e i n g q u e n c h e d b y a radiationless process w i t h rate k

lt

If t h e

phosphorescence d e c a y (Process I I ) has a true rate constant m u c h s m a l l e r t h a n w o u l d give t h e o b s e r v e d 800 msec, h a l f - l i f e , t h e n t h e q u e n c h i n g Process I m u s t b e t h e rate d e t e r m i n i n g step i n t h e o b s e r v e d decay. this is so t h e n t h e a c t i v a t i o n energy of 0.007 e.v. (0.16 k c a l . )

If

obtained

earlier f r o m t h e t e m p e r a t u r e d e p e n d e n c e of t h e e x p o n e n t i a l d e c a y relates to t h e q u e n c h i n g process. H o w e v e r , i f k a n d k are a p p r o x i m a t e l y e q u a l , ±

2

the use of a s i m p l e A r r h e n i u s p l o t is not justified e v e n i f phosphorescence is a s s u m e d to h a v e zero a c t i v a t i o n energy. T h e h a l f - l i f e of the e x p o n e n t i a l gives t h e s u m k - f k = 0.875 s e c " at 9 3 ° K . w i t h t h e p o s s i b i l i t y that t

1

2

this is t h e v a l u e o f ki o n l y . 5 /XSEC. T O 1 M S E C , A F T E R IRRADIATION.

The

sections of t h e

decay

curves at times less t h a n 1 msec, a n d greater t h a n 10 sec. after i r r a d i a t i o n b o t h s h o w a n intensity d e p e n d e n c e w h i c h is i n v e r s e l y p r o p o r t i o n a l to the t i m e after i r r a d i a t i o n — i . e . , I cc r . I n a d e c a y scheme a p p l i c a b l e t o 1

short times after i r r a d i a t i o n , Processes I a n d I I h a v e to b e r u l e d o u t as b e i n g rate d e t e r m i n i n g since t h e s u m of t h e i r first o r d e r rate cannot g i v e a t i m e constant shorter t h a n 0.875 s e c " at 9 3 ° K . 1

constants As bi-

m o l e c u l a r processes h a v e a greater r e l a t i v e i m p o r t a n c e at short times w h e n t h e c o n c e n t r a t i o n of species is highest, Processes I I I a n d I V m a y be m a i n l y c o n t r i b u t i n g to t h e e a r l y p a r t of t h e decay. a n n i h i l a t i o n l e a d i n g to d e l a y e d

fluorescence

Triplet-triplet

has b e e n s h o w n b y a n u m

b e r of authors to f o l l o w u l t r a v i o l e t - a n d i o n i z i n g - i r r a d i a t i o n a n d has b e e n


458

RADIATION CHEMISTRY

1

suggested as a n e x p l a n a t i o n of the d e l a y e d c o m p o n e n t i n s c i n t i l l a t i o n counting ( 5 ) . If b i m o l e c u l a r Processes I I I a n d I V are t h e o n l y ones of i m p o r t a n c e at short times, t h e n E q u a t i o n s 1 a n d 2 m a y b e r e d u c e d t o : - d m

=(h

d t

(3)

+ 2-ah){T y i

(4)

I = h[TiVf S o l v i n g E q u a t i o n s 3 a n d 4 f o r the t i m e d e p e n d e n c e of J g i v e s :


I — where K =

+

(k

s

2 — a k ),

h (KTJ+ I

4

0

=

l )

(5) 2

v

[T ] fc f and T 2

0

4

0

=

;

initial concen

t r a t i o n of triplets. A p l o t of l o g I vs. l o g t w i t h these kinetics w o u l d s h o w a negative slope w h o s e m a g n i t u d e w o u l d increase w i t h t i m e a n d a p p r o a c h a v a l u e of —2 after several h a l f - l i v e s . A s t h e e x p e r i m e n t a l d a t a are a v e r y close fit to a r e c i p r o c a l plot, d e l a y e d

fluorescence

cannot b e c o n

t r i b u t i n g m u c h to t h e i n i t i a l decay. A l t e r n a t i v e l y , a l t h o u g h Process I I cannot b e t h e rate d e t e r m i n i n g process it m a y s t i l l b e t h e emission process p r o v i d e d that the q u e n c h i n g of emission determines t h e rate of decay.

If t h e b i m o l e c u l a r processes

(Processes I I I a n d I V ) are t h e i m p o r t a n t q u e n c h i n g processes, E q u a t i o n 3 remains a p p l i c a b l e .

A l s o , as d e l a y e d

fluorescence

has b e e n s h o w n to

b e u n i m p o r t a n t , t h e n either fc or the factor / is s m a l l a n d E q u a t i o n 2 4

reduces t o : J = *2lTi]

() 6

S o l v i n g E q u a t i o n s 3 a n d 6 f o r t h e t i m e d e p e n d e n c e of I g i v e s : (7)

(KTJ+1) where I

0

=

k [T ']. 2

phosphorescence

0

T h i s expression describes b i m o l e c u l a r q u e n c h i n g of

a n d a p p r o x i m a t e s to I a f

1

after 7 has decreased b y 0

a f e w h a l f - l i v e s . It thus fits t h e d a t a of F i g u r e 6 f r o m about 5 /xsec. after irradiation. I n the region where the reciprocal approximation holds, E q u a t i o n 7 also p r e d i c t s that t h e intensity is i n d e p e n d e n t of dose. ever, that t h e i n t e n s i t y is p r o p o r t i o n a l to dose.

It is f o u n d , h o w

T h i s dose

dependence

c o u l d arise i n this k i n e t i c scheme i f t h e species c a u s i n g l u m i n e s c e n c e are i n n o n - o v e r l a p p i n g clusters o r " s p u r s " a n d a n increase

i n dose s i m p l y

results i n a p r o p o r t i o n a t e increase i n t h e n u m b e r of s u c h clusters.

How

ever, this "cluster m o d e l " w o u l d i m p l y that v i r t u a l l y a l l triplets m u s t b e f o r m e d i n t h e clusters, a n d there is as y e t n o other e v i d e n c e f o r this.


31.


Luminescence

459

Kinetics

10 SEC. TO 300 SEC. A F T E R IRRADIATION. T h e r e c i p r o c a l d e c a y at times longer t h a n 10 sec. cannot b e c a u s e d b y b i m o l e c u l a r t r i p l e t reactions since, i f present, these b e c a m e n e g l i g i b l e c o m p a r e d w i t h t h e s i m p l e first o r d e r t r i p l e t d e c a y f r o m 10 msec, o n w a r d s . Processes I t o I V alone are therefore n o t sufficient to a c c o u n t f o r this l o n g - l i v e d e m i s s i o n a n d a n alternative e x p l a n a t i o n m u s t b e c o n s i d e r e d . T h e p r o p e r t y of t h e r m o l u m i n e s c e n c e demonstrates t h e existence of t r a p p i n g sites i n a d e n i n e p o w d e r ( 1 5 , 16, 22).

T h e luminescence decay

at times greater t h a n 10 sec. after i r r a d i a t i o n c a n b e a d e q u a t e l y e x p l a i n e d b y t h e existence of s u c h traps. A s t h e e m i s s i o n s p e c t r u m at 15 sec. is Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

i d e n t i c a l w i t h that of the phosphorescence ( F i g u r e 4 ) , i t is reasonable t o assume, i n s u c h a t r a p p i n g m o d e l , that t h e emission after 10 sec. is f r o m the same energy t r a n s i t i o n as t h e phosphorescence b u t that t h e release of energy f r o m t h e traps is t h e rate d e t e r m i n i n g step. T h e kinetics of d e l a y e d e m i s s i o n a r i s i n g f r o m electron t r a p p i n g i n i n o r g a n i c p h o s p h o r s has b e e n c o n s i d e r e d b y R a n d a l l a n d W i l k i n s ( 1 8 ) w h o s h o w e d h o w the f o r m of the d e c a y a n d its d e p e n d e n c e o n t e m p e r a ture d e p e n d e d o n the d i s t r i b u t i o n of t r a p depths. F o r a u n i f o r m d i s t r i b u t i o n of t r a p levels they s h o w e d that the i n t e n s i t y is i n v e r s e l y p r o p o r t i o n a l to t i m e after i r r a d i a t i o n for times greater t h a n one m i c r o s e c o n d . P r o v i d e d there is n o s a t u r a t i o n the i n t e n s i t y w i l l b e p r o p o r t i o n a l to dose.

This

r e l a t i o n a n d p r e d i c t e d dose d e p e n d e n c e agree w i t h the e x p e r i m e n t a l d a t a f r o m 10 sec. after i r r a d i a t i o n . I f t h e f r e q u e n c y factors (18) of t h e traps i n a d e n i n e are a l l of t h e same m a g n i t u d e , t h e n traps responsible f o r t h e l o n g t e r m d e c a y w i l l b e s h a l l o w e r t h a n t h e — 0 . 2 e.v. t r a p responsible for

t h e 1 2 0 ° K . g l o w peak (15, 16).

Thus, a trapping model with an

a p p r o x i m a t e l y u n i f o r m d i s t r i b u t i o n of t r a p depths w i l l a d e q u a t e l y ac c o u n t f o r t h e d e c a y at times greater t h a n 10 sec.

S u c h a m o d e l w i l l also

e x p l a i n t h e r e c i p r o c a l d e c a y f r o m 5 fisec. t o 1 msec, a n d its dose d e p e n d e n c e , p r o v i d e d t h e l i b e r a t i o n of energy f r o m t h e traps is also t h e rate d e t e r m i n i n g process i n this r e g i o n . Temperature Dependence.

A t room temperature

the exponential

phosphorescence d e c a y is absent, p r e s u m a b l y because of t h e r e m o v a l of t r i p l e t states b y t h e t e m p e r a t u r e sensitive q u e n c h i n g process f o u n d at l o w temperatures.

T h e d e c a y f r o m 5 /xsec. to 5 msec, d i d n o t fit a n y

s i m p l e d e c a y scheme a l t h o u g h t h e m e a n slope of the d e c a y o n a l o g - l o g p l o t w a s — 1 . I n the first 200 psec. after i r r a d i a t i o n the r o o m t e m p e r a t u r e e m i s s i o n is m o r e intense t h a n at 93 ° K . A s i m i l a r t e m p e r a t u r e d e p e n d e n c e of t h e l u m i n e s c e n c e of anthracene crystals has b e e n o b s e r v e d f o l l o w i n g u l t r a v i o l e t e x c i t a t i o n (1, 23).

T h i s b e h a v i o r w a s i n t e r p r e t e d as b e i n g

c a u s e d b y t h e e n h a n c e d intersystem crossing to the t r i p l e t states at t h e h i g h e r temperatures.

This model, however, w o u l d not explain w h y the

l u m i n e s c e n c e intensity of hot a d e n i n e p o w d e r i n F i g u r e 7 w a s l o w e r t h a n


460

RADIATION CHEMISTRY

that at r o o m temperature.

1

I n the t r a p p i n g m o d e l , h o w e v e r , the h i g h e r

l u m i n e s c e n c e intensity at r o o m t e m p e r a t u r e w o u l d result if the t i m e spent i n traps decreased w i t h i n c r e a s i n g t e m p e r a t u r e so that the energy nor m a l l y t r a p p e d at l o w t e m p e r a t u r e

was i m m e d i a t e l y a v a i l a b l e for re-

emission at the h i g h e r temperatures.

T h i s w o u l d also account f o r the

peaks i n the l u m i n e s c e n c e intensity vs. temperature plots of F i g u r e 7 if the subsequent decrease i n i n t e n s i t y at temperatures above the p e a k t e m p e r a t u r e w a s the result of a c o m p e t i n g temperature-sensitive q u e n c h i n g process w h i c h d o m i n a t e d after the deepest traps w e r e e m p t i e d .

The

t e m p e r a t u r e at the peaks of F i g u r e 7 changes w i t h o b s e r v a t i o n t i m e , a n d Downloaded by TUFTS UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch031

this t i m e c a n be t a k e n as a representative v a l u e of the l i f e t i m e , T , of the t r a p p e d species.

T h e r e l a t i o n s h i p b e t w e e n r a n d the energy of the t r a p ,

E , is g i v e n b y : I = se~

(8)

E/kT

T

A p l o t of l o g T vs. 1/T

should,

therefore, b e l i n e a r w i t h a slope of

F i g u r e 8 shows the result of this p l o t for o b s e r v a t i o n times be

—E/k.

t w e e n 7.5 /xsec. a n d 375 /xsec, a n d temperatures b e t w e e n 2 0 0 ° a n d 3 0 0 ° K . A t l o w e r temperatures the p e a k is not seen because of the a p p e a r a n c e of the T

t

-» S

of 0.22 e.v.

0

phosphorescence.

T h e slope of this p l o t gives a t r a p d e p t h

S u b s t i t u t i n g this t r a p d e p t h i n t o E q u a t i o n 8 y i e l d s a fre

q u e n c y factor, s, of 9 X 1 0 l o w e r l i m i t of 1 0

8

8

sec." . T h i s v a l u e m a y be c o m p a r e d w i t h a 1

sec." estimated b y L e h m a n a n d W a l l a c e ( 1 5 )

from

1

w h i c h they d e r i v e d a m i n i m u m v a l u e of 0.17 e.v. for the t r a p d e p t h re sponsible for t h e r m o l u m i n e s c e n c e .

U s i n g the h i g h e r v a l u e of s d e r i v e d

above together w i t h the g l o w p e a k d a t a of L e h m a n a n d W a l l a c e

(15)

y i e l d s a " t h e r m o l u m i n e s c e n c e " t r a p d e p t h of 0.19 e.v. i n closer agreement w i t h the present v a l u e of 0.22 e.v. A b o v e the p e a k temperatures

the e m i s s i o n q u e n c h i n g curves

are

a p p r o x i m a t e l y p a r a l l e l a n d a n A r r h e n i u s p l o t of these portions of the curves gives a n a c t i v a t i o n energy for the q u e n c h i n g of l u m i n e s c e n c e of a b o u t 0.15 e.v./molecule. T h i s v a l u e is m u c h larger t h a n 0.007 e.v./molec u l e o b t a i n e d for q u e n c h i n g of the phosphorescence at l o w temperatures. H o w e v e r , i t does agree a p p r o x i m a t e l y w i t h the l o w - d o s e x-ray d a t a

(16)

w h e r e it was f o u n d that there are t w o t e m p e r a t u r e sensitive q u e n c h i n g processes, one d o m i n a t i n g a b o v e a n d one b e l o w a t r a n s i t i o n t e m p e r a t u r e of 130 ° K . T h e a c t i v a t i o n energies i n R e f e r e n c e 16 for these t w o processes w e r e f o u n d to be 0.07 e.v./molecule a n d 0.007 e.v./molecule for the h i g h a n d l o w t e m p e r a t u r e regions, r e s p e c t i v e l y . Differences in Emission Spectra.

T h e cluster m o d e l , w h i c h

arose

f r o m a c o n s i d e r a t i o n of t r i p l e t state Processes I to I V o n l y , does not sug gest a n o b v i o u s e x p l a n a t i o n f o r the b r o a d e r spectra at short times


(see

31.


Luminescence

461

Kinetics

F i g u r e 4 ) , unless i m p u r i t i e s are responsible, as the t r a n s i t i o n T - » S t

0

+

hv gives t h e o b s e r v e d l u m i n e s c e n c e at a l l times after i r r a d i a t i o n . O n t h e t r a p p i n g m o d e l , w h e r e t h e l i b e r a t i o n of energy f r o m traps is t h e rate d e t e r m i n i n g process, t h e e m i s s i o n s p e c t r u m at short times is e x p e c t e d to be different f r o m t h e phosphorescence s p e c t r u m as this t r a n s i t i o n , 2\ —» S

0

+ hv, is m u c h s l o w e r t h a n t h e i n i t i a l d e c a y . H e n c e , a faster t r a n s i t i o n

m u s t b e responsible f o r this e a r l y emission. O n e p o s s i b l e t r a n s i t i o n w h i c h m a y b e c o n t r i b u t i n g here is the

fluorescence

transition S - » S x

the i n i t i a l release of energy f r o m traps leads to e x c i t e d singlet states. A s all immediate

fluorescence

is m a s k e d i n this e x p e r i m e n t b y t h e a c c o m


p a n y i n g C e r e n k o v l i g h t , i t w o u l d b e necessary

to operate

b e l o w the

C e r e n k o v energy l i m i t to test this p o i n t . ,-3 10"

3.0

5.0

4.0 T

x

10'»-3

(V ) 1

Figure 8. Dependence of the lifetime, T , of the trapped species on temperature. The lifetimes and corresponding temperatures are derived from Figure 7 (see text) Conclusions T h e e v i d e n c e r e v i e w e d a b o v e suggests that a l t h o u g h a c l u s t e r - m o d e l f o r the i n i t i a l f o r m a t i o n of t r i p l e t states w i t h subsequent

bimolecular

t r i p l e t q u e n c h i n g cannot b e r u l e d out, a m o r e p r o b a b l e e x p l a n a t i o n of


462

RADIATION CHEMISTRY

1

the e a r l y r e c i p r o c a l d e c a y c u r v e at b o t h r o o m t e m p e r a t u r e a n d 93 ° K . is the p r o p o s e d t r a p p i n g m o d e l , w h i c h has a n a p p r o x i m a t e l y u n i f o r m dis t r i b u t i o n of t r a p depths i n a range 0 to 0.22 e.v. T h i s t r a p p i n g m o d e l is also t h e most l i k e l y e x p l a n a t i o n of t h e l o w t e m p e r a t u r e e m i s s i o n at times longer t h a n 10 sec. after i r r a d i a t i o n . A t these l o n g e r times, e m i s s i o n is t h e result of a t r i p l e t to singlet g r o u n d state t r a n s i t i o n w h i c h is also responsible f o r t h e l o w t e m p e r a t u r e phosphorescence

f r o m 5 msec, to

8 sec. after i r r a d i a t i o n .


Acknowledgments T h e authors w o u l d l i k e t o t h a n k J . W . B o a g f o r h i s e n c o u r a g e m e n t a n d f o r m a n y h e l p f u l discussions a n d L . T . L o v e r o c k f o r h i s t e c h n i c a l assistance.

W e also w i s h to thank J . C u r r a n t a n d R . H a l e f o r their

o p e r a t i o n of t h e l i n e a r accelerator f o r these studies.

Literature Cited (1) Adolph, J., Williams, D. F.,J.Chem. Phys. 46, 4248 (1967). (2) Augenstein, L., Carter, J., Nag-Chaudhuri, Y., Nelson, D., Yeargers, E., "Symposium on Physical Mechanisms in Radiation Biology," L. Augen stein, R. Mason, B. Rosenberg, eds., p. 73, Academic Press, New York, 1964. (3) Augenstein, L. G., Carter, J. G., Nelson, D. R., Yockey, H. P., Radiation Res.Suppl.2,19 (1960). (4) Azumi, T., McGlynn, S. P.,J.Chem. Phys. 39, 1186 (1963). (5) Birks, J. B., "The Theory and Practice of Scintillation Counting," Chapter 6, Pergamon Press, London, England, 1964. (6) Bollinger, L. M., Thomas, G. E., Rev. Sci. Instrum. 32, 1044 (1961). (7) Carter, J. G., Birkhoff, R. D., Nelson, D. R., Health Physics 10, 539 (1964). (8) Collinson, E., Swallow, A.J.,Chem. Rev. 56, 471 (1956). (9) Fano, U., "Comparative Effects of Radiation," M. Burton, J. S. KirbySmith, J. L. Magee, eds., p. 14, J. Wiley and Sons, New York, Ν. Y., 1960. (10) Fielden, Ε. M. (to be published). (11) Frank, I. M., Tamm,I.,Dokl.Akad.Nauk,S.S.S.R. 14, 109 (1937). (12) Hodgman, C. D., Weast, R. C., Selby, S. M., eds., "Handbook of Chem istry and Physics," 45th ed., Chemical Rubber Publishing Co., 1964. (13) King, Τ. Α., Voltz,R.,Proc. Roy. Soc. A289, 424 (1966). (14) Kupperman, Α., Raff, L. M., "Symposium on Physical Mechanisms in Radiation Biology," L. Augenstein, R. Mason, B. Rosenberg, eds., p. 161, Academic Press, New York, 1964. (15) Lehman, R. L., Wallace, R., "Electronic Aspects of Biochemistry," B. Pull man, ed., p. 43, Academic Press, New York, 1964. (16) Lillicrap, S. C . , "Radiation Effects on Subcellular Systems and Model Compounds," p. 175, K. Lohs, A. Rakow, eds., Studia Biophysica 7, East Berlin, 1968. (17) Massey, H. S. W., Burhop, E. H. S., "Electronic and Ionic Impact Phe nomena," 1st ed., p. 141, Oxford University Press, London, 1952. (18) Randall, J. T., Wilkins, M. H. F., Proc. Roy. Soc. A184, 390 (1945).


31.


Luminescence

Kinetics

463

(19)

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Luminescence Kinetics of Microcrystalline Adenine Following Pulse

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