Concentration dependence of the lifetime of the first excited singlet level of stilbene dissolved in chloroform

Volume 32. numbgr 3

Received3 Jan&y

CHEMICAL

1 May 1975

PHYSICS LETTERS

1975

The S1 Lifetime~1 of dissolved stilbcne lx& been measnred.in the’concentr;itio’~ Age

from 5 ~~10”

&l/Z t6 2 x

10-l moVPusinga probe beam Nevada The time dependence of the p~puiation of the i$l%vel excite;l by ~~o-ph~~on&sorption has been observed by s~~~~photon absorption S1 + S,. A linear dependence of the recipmz.I I.&time on stiIbcne concentmtionc,+

has been found: l/~~ = 6 2 10’ s-l ‘cd.7 x log s-l P mol-1 CA.

i .

.

1.Introduction The applica&n.&f la& has led to appreciable’advantages in the measurement of intramolecular and

inte&nolecular relaxation times, especially in the shoit time range. Firstly, it is possible to excite molecular levels selectively and in short times by single- or multi-

prdcess consisting of a two-photon and a singe-photon abso~t~on has been observed under the action of lasers [3,4] .-In this process a two-photon absorption So 4 S, is followed by a single-photon absorption from.S, to a higher excited singlet ievel S,. The

populations nl and n, of the levels S1, Sk respectWy, and’the photon flux density. of the Iaser pulse Fare described by the fol~o~ng system &equations:-

photon absorption of intense laser pulses. Secondly a .: weaker laser pulse can be used as a p&e beam to measure the time dependence of the population of a special molecular Ievel with a high time resolutipn ‘, [I,ZJ. This probe, beam technique can be applied independently of the exister+of a fluorescence of the’ molecule. The employment of picosecond-pulse lasers’. permits the measurerrient of shorter ~elaxation~~times or lifetimes than conventional methods. In t&is paper a probe beam method has bken appiie? to investigate the concentration Pependence of.t.he S,

ai,(z,i)/at= $noc&~)F~(z,~) - ~I~(.z,~)/T~: - n&

I t)o(%qz,r)

an&,r)lai=

z1-n;

~fet~e. of stilbane di~o~ved in c~orofo~..Meas~~re-

fnx(z;r)ir,I’

nl(z,t)dwz,f)

&j&

)

: (0

- n,(z;r)~7,,

2) I “I ;f+o,

(2)

,

f) *

(3)

qents 6f the dependence on concentration are suitable for distinguishing b&ween.the,rcJa%ation of the excited, molecules by interact& with nonexcited mole-

(~(~1 is &e cr&ss section- of the two-photon transition s, + s, , u(1) & e cross section oFthe single-photon ‘trarisitiop S, *S,, ~~~ the relaxation titpe of the

cules of the substance, from thoseby interaction with’ rkj~cules of the solvent, and by s$qntaneowemission. : ::

Snd rr’the vdlocjty of the light wave propagating in the

‘_

transition Si._t Sl, 71 the liftitime of the level Sl, 2.erections)

:

‘.

’

.:

:. /.

i.,PrinGple’of ‘.

_’

_’

ihe pr$ebeam

m&hod

‘.‘:. ”

In some organic’ritole&les a tk&tep absdrption‘ ,, ,. ._‘, .. .,,, :. : . ‘_.,~., ._ : .: .. . .. ;.:. .,:,... ..,. .: -_ ,. . .. ., “._:,,, ,, ‘.,_ ‘...,.I ,. .’ :.,..‘. ,, I ; : _“. .,.-,. ._. ., _, :; “_ -. .,,, .,:, ‘._,.

Assuniing,a vejrshort relaxation time 7X.r one gets the following sqlution. for the population of the S 1 level

‘,‘,

-..

_

..

.:

,,

,.. ‘.. ,‘._

-’ -,

‘.

: ,, -:,_ ” ; ._

.. ‘. .

‘.

:

~. ;

::: ; ~,,.. ‘. .’

‘,

._ ‘_‘_‘.: .., (’ 2’ ..,_ .’

:,.

:.589’ ”

‘,:

-

‘.

-,

. _

$(z,t)=

;,

-..

1:

.,

inod2)exp(+Ti)

).

.,,.

.,.,_,

:

_,:

-,

,,

. .

:.:

: ; .,

‘.

.‘.

.‘.- : ,, (4) ”:. ‘X SF2(~,r’)exp(-t:‘?;jdtl:. -: . ..’ ,, 0 ‘.., ,’ ,. : ‘, -. .’ E+‘(A) &JWS ‘an‘exponential decrease of the pop&~: ~.‘~.~oII‘~& w-i&hlifetiny ?I after ‘hritc&g off’ the I&er pulse: ,The sake-proton absoription S, + S, of .‘a delayed probe beam v&h frequency.wg.measures ‘, ‘the population ?.I, The intensity of the probe beam is chosen to be small to avoid two-photon absorption The absorption of the probe beam depends on the time delay between the puk~p pukand the probe. pulse accord& t,o tI-&vari&on df nl during this, timel measuring the probe beam in~ensi~ fI- at the exit of the cell ,for two different delay times t and t + ’ $;‘the lifetime C&Ibe calculated from the following’ ,. equation’ .’

_~(~i~~)i~(r)= exp (-t&d

‘.

-’

(6)

. f&w3 fkr eq;.(5). A fast rearrangement of the nuc!ear frame (e.g., a rotation of 90” about the central bond in stilbene) is possible after the excitation to the S1 level (see, e.g.;. ref. [s]). if the time delay between pump and probe bFy@ is longer than’tie

,.

:

time of the rearsange&ent,

the measured parameter Ti.is the lifetime of the re- .. arranged species (stilbene relaxes to the rearranged’ state,after.about lo-‘” s a,t room temperature [SJ)!

‘;.; The pump beam (drawn line) enters the cuvette after

reflection at the mirrors M,, M, and produces the . . popuIation of the S1 level. A probe beam (dotted line) is achieved,by reflection at a glass plate G and weaken-’ ing by a filter F. This beam enters the cuvette (&ITOIS Ma, M4) and passesthrough the cuvette in the opposite direction to the pump beam. The intensities of the probe pulse are registrared by beam splitting before :&lass plate G,) and after the kvette (gf&s plate GCJ) by a fast photodiode, The change of the time delay tD between the pump and the probe pulse is realized by a variation in the distance of the mirror combinatiorr, MI--M, from the cuvette in the range of some nanoseconds., The intensities 1, and I, of the probe pulse are equalize9 in the absence of the pump pufse~by a 1.

neutra.I gray wedge before the measurements.

4. Results and

:

@s&s&n.

.’

-

:.:

,,3.‘E~pperime&l arraxigernent.

”

..

~e~s~~e~~en~~of the li~etime-rl of stilbene solu~ : cMoroforrn are’reaked with. the arrangement described above for concentrations~betkee,n 5 X ’ 10W2 mol/5? and 2,X !O:l mol/Q. The results are &ven in table 1. The accurac); of the measurements is .. estimated to be about iO%: A simple mode1 was used to di&ss the ‘dependence .. ., of lifetime r-! on concentration. yolecules of the dis- ‘. solved subs&e in the ground state and ,j.nthe excited “. state are ‘de.n&d by A and A*, respectively, those of. &tins in

expedmental’set-up of fig:1 has been’used to, measure the absorption of the probe beam. A ruby

:: ,.‘fie

laser ~&king in the TEMoo mode’emits a train of : picosecond pulses’wi~ a time distance, 9, of 10’ns

be&ken neighbouring putses,_a pulse duration of.20 ps and a power oFabout 100 MW corresfionding to a ; photon flux density F of about 5 X IO,*’ photoni ‘&+ ’ s-l at a 1ase.rbeam cross section of ,7 nk2;: 2. :,

.’ -‘. .

.:-

.-_

..

Volume 32, number 3

,_. _.

.) _. Table 1

.-

CHEMICAL PHYSICS LETTERS

,’ ._

Concentration

+ rtiy-‘l

sl 04

(mol k!-‘)

the solvent molecules by B. Assuming the number, of excited mdlecuies to be small the interaction between them’can be neglected: T’he deactivation of molecules A*‘is possible

by interaction

with niolccules

Iwq Fig. 2. Dependence

in the

of i-7’ dn stilbene concen~atiorr

ground state A, with t&e solvent molecules B and by sponttieous

emission (fluorescence). We describe

”

these interactions by the simple equation

solvent and deactivation by Euorescence. The follow‘ing values of.these parameters have been determined for stilbene dissolved in chlorofork

(kA

and kB rate constants, ?R fluorescence lifetime

of molecules A*.) (kAcASkBcB+ l/rR) = .I/T! characterizes the tiine after which the concentration of excited molecules is decreased to the value l/e. In a diluted solution the variation of cB by chanhg cA can

(K+ l/Q

=

(6 2 4) X 107 s-l I

k, =.(6;7 + 0.4) X 10’ P moI_’ s-l .

be neglected, i.e., K = cB kg is COnSlant and the

lifetime ?1 depends on CA in the fotiowing way iI’ ‘1 (C‘J = @M/r&

k, CA -

References (8)

The dependence of lifetime rI .on the stilbene concen-

2. The points represent the measured data from table 1. The straight line deter‘mined by the least squares method is consistent with bq. @I and albws a c~c~.ation of the parameters ‘. tration

is shown

in fig.

(Kt l/~) and kA4 The extrapolated value T:‘(O) is gitien by (Kt l/TR) and characterizes the reciprocal lifetime of the excited species by interaction with the

[l] E.J.,Scarlet, 3. FQueira’andH. Mahr.Appi. Phys. Letters

13 (1968) 71: [Z]

hi.?& Ma&y and P.M. Renbe&

Chem. Phys Letters 3

(1969) 534. [3]

3. KleInschmidt,

S.,Rentsch,

W. Tottkbe~

and B. Wilhelmi,

Chem. Phys, L+xs 24 (1974) 133. [4] E. Heurnmn, G. Slobin and W:TriebeI, Exp. Tech% 1 physik, to be publkkd[S] J. Saftiel, in: Photochemidry. Vol. 3, ed 0.L. Chapman

mew York, 1973).