Nanoseconds UV laser study of radiationless deactivation from upper electronic excited states in solution: Tb3+

1 August 1975

CHEMICAL PHYSICS LETTERS

Voluny j4, number 3

NANOSECONDS UV LASER STUDY OF RADIATIONLESS

DEACTWATION

FROM UPPER ELEC-I-RONIC EXCITED STATES IN SOLUTION: Tb3+ Ch.R.

GOLDSCHMIDT,

Deparrtnent

of Physical

Gabriel

Chemisrry.

STEIN and Elhanan Hebrew

Chiwrsit.v.

\wRZBERG

Jcmtxalerrz, Israel

Received 28 April 1975

in D20 solution, or in Nd laser at 165 nm Tb3’ perchlorate on transition from the upper 5 D3 and lower ‘D+ exThe appropriate energy _gp spacing of :his rare earth ion results in lifctimcs which enabled us to cbserve the decay of 5D~ as well as the growing in and decay of ‘D4. Radiationwith previous theoretic4 and esperiless processes from ‘D3 direct to ‘1; or cascading to ‘Da are discussed and correlated

Using 25 ns half width pulses of a frequencyquadrupled

b&ate glass, was excited to hip+ electronic s!ates and the fluorescence cited states IO various sub-levels of the lowest ‘F state were measured.

mental resulls on related systems involving the intern&on pared with relaied phenomena in glassy solids.

1. Introduction

which states, for aromatic qUantUIII efficiency

molecules:

“The

qFxI is

independent of the excitation wavelength (for non-ionizing radiation)“. For the other major group of subjects for the inkstigatign of radiative and radiationless transitions in sdlutions, the rare earth ions, ihe first alternative WBS @aint+ined by Ermolaev and Sveshnikova [2J and by flUOreSCeliCe

Biron et al. [3] , particularly

the non-complercd

in levels

Do upper electronic excited states in solution !ose relatively small quantities of energy so rapidly that their orzly pathway of radiationless relaxation leads to the, relatively long lived, often fluorescent, lowest excited electronic level? Or, alternatively, are there in -suitable cases parallel pathways, some of which lead directly from upper levels to radiationless processes bypassing the lowest .electronic excited state? in Vavilov’s The first alternative is incorporated law [I]

of

for the case of Eu3+ in

solution. -tiat Vavilov’s law is not a ;;eneral one for aromatKS, but often merely expresses experimental findings

._

~@kr apfiropriate conditions 1~s shown [ l] in a numgenerally

ber of cases,,specifica.lly azulene and more ‘benzene and some of its dsrivz.tives, where ,:q’d/tir radiationless processes weie shown _‘. ‘_ .qj8 ;; ::-. ‘. .. .-., ;-

radiative

to originate -1

above

solvated

the lowest

ion with

the medium,

fluorescent

one.

and com-

Recently,

for benzene, the detailed mechanism of some of these processes was indicated [4]. In the series of rare earth ions, a number of workers howed that fluorescence quantum efficiency from the lowest fluorescent level is in fact dependent on the e% citation wavelength [S-3]. The causes for some of the

contrary statements were pointed out [lo]. With the development of a more general theory

of the mechanism of radiative and radiationless processes in solutions of rare earth ions [7-9,l l] a clearer quan-

titative correlation between the energy gap between the lowest fluorescent and highest non-fluorescent electronic

processes

levels and the time-scale in solution was obtained.

of radiationless These ideas may

be extended to processes between levels above the lowest fluorescent one [7,8] _ In the specific case of Tb3+, of which the partial term scheme is shown in fig. 1, the ener_q gap between

the upper electronic excited Ievel 5D~ and the lcwer 5D4 is sufficiently large to make not only static experiments or! the excitation wavelength dependence of fluorescence yield from “D, possible, but also to permit kith high time resoIution, the observation of the time scale of events between 5D3 and 5D4 in solutions of the non_coinplextid ion as could be done pre.viously on some rare earths in chelates and in solids -,

.,

.,

Volume 34, number

3

CHEMICAL

PHYSICS

LETTERS

1 August 1975

tions, including glassy solids. The correlation between the phonon formalism in solids and the specific radiationless mechanisms in liquid solvents is discussed.

2. Experimental To investigate the emission characteristics we employed the apparatus shown schematically in fig. 2. TIxe experimental lifetime in D,O of the emission from the SD, level of Tb3+ is =S 3.8 ms. Due to the long lifetime of Tb3+ the light intensity is very low, in spite OF the high quantum yield of fluorescence. Tb3’ solutions were excited by an Nd glass laser (Laser Associates, Z.L.). 30 mJ low order mode pulses of 30 ns full width at half maximum (fwhm) were drawn at 1060 nm from an Owen-Illinois ED-2 rod. The 2 nun diameter beam was telescopically expanded and amplified by two successive amplifiers. The result-

2000’

IO 001

ing I.5 I output was first converted to 300 mJ of light at 530 nm by a phase matched (Q = 41° cut) KDP crysta!. FinzUy an ADP crysta! (0 = 90”) was used to obtain 265 nm ighl:, yielding after proper filtration of

all other wavelengths (Chance HA-3; Corning 7-X) 10 rn.J pulses of 25 ns fwhm. In vlcw of the low intensity of the fluorescence and relativeIy high intensity of stray !ight during the laser pulse (the time integrated stray LigJ-ttbeing several orders of magnitude lower than the time integrated fluorescence, but very short lived) it was of great importance to image the Tb3+ fluorescence carer’ully onto

Fig 1. Putia! term scheme of T’b3+. The values of the rate constants as derived in the text are: k’ = 1.5 X lo5 s-l, k” = 3 x lo5 s-‘, “;I = 3.2 x 10’ hi-’ sa , k;;= 8 X lo6 hl-’ s-l,

[12-141. In the present work we report our results on this system and correlate them with other related observa-

the photomultipher (after a Cctiing 3-75 filter to eliminate first the 265 run radiation and a Bausch and Lomb 16 A/mm 500 mm focal length monochromator)

KDP

.

Nd+3glass

laser

I

Fig. 2. Scheme of appaMus. A = amplifier, F = filter, C = ilubrescent solution cell, L = lens, hl = monochrom&r, plier as3mbly.

P=

phoromulti-

409

Volume 34. number a

3

CHEXlIC4i

PHYSICS

b

t.ETTERS

I Auyst

197.5

solid was 4% w/w Tb3+ in borax glass, as prepared by Reisfeld

et al. [ I.51 _ We thank

Professor

R. Reisfeld

for

a sample of her preparation.

3. Results The absorption

Fig. 3. Oscilloscope traces. (a) and (a) shape of the laser pulse at 1.06 N, (c) ‘l%‘+ in borate giass. Descending trace: fluorescence emission from upper DJ level. Ascending trace: emission from lower D4 level. (d) and (e) emission by Tb3+in solution at 1.2 arid 0.15 hl concentration, respectively.

to collect the greatest possible amount of fluorescence. To minimize mixing the integrated stray light with the relatively slow rising fluorescence emission we matched the 1P28A photomultipiier anode onto a 30 ohm impedance. The stray light signal was always kept well below 1 volt (20 mA) to assure lirrearity of the photomultiplier output during the fluorescence rise time. Considering the long fluorescence decay as a dc signal, the output after 100 ns never exceeded 50 mV (1 mA). In figs. 3a and 3b we show the shape of the laser pulse a~ given by the measurement of stray ligbht using the aqueous solution without Tb3+. Less than 100 ns after firing the laser the contribution of stray light is negligjble. The width of the oscilloscope traces in fig. 3 is due to statistical fluctuation of the emitted light, time re‘solved by ‘Lhefast detection sysl:em. For solutions Tb,$ (Fluka, 99.9%) was dissolved in aqueous HClO, and the solution in D,G prepared from this as described previously [7,8] _Tl,3* in the 410

..

.:,

. .

spectrum

of Tb3+ in the presence

of Cl- [16] shows that the 265 nm laser pulse in the present work is absorbed into a broad band with pez!(s) at = 26.5 nm. We remeasured the spectrum for Tb3+ in the presence of CIO; and found some minor differences from that in the presence of Cl-. These do not affect the present considerations. In fig. 3c we observe the relatively slow time scale of the growing in of the fluorescence measured at 5400 A From the lower 5Dq level in the solid matrix. This growing in gives a time constant of 7 z 10 ps for the lifetime of the upper SD, level. This is confirmed by the lifetime of fluorescence (measured at 4180 A) emitted from the upper 5D3 !evel. In D,O solution to make similar measurements one has to contend with greatly decreased fluorescence intensity because of additional quenching processes. To increase intensity we used first 1.2 M solutions of Tb3+ (fig. 3d). However we obtained then great!y shortened lifetimes of the upper 5D3 level and of the growing in of the 5D4 level, while the lifetime of the fluorescence from the lower 5 D4 level was relatively less affected. One possibility was that this specific effect is due to efficient self-quenching of the upper 5D3 level selectively, while the self-quenching of the lower 5De level is less efficient_ To investigate this, we obtained results in more dilute solutions, at 0.3 M and at 0.15 M concentration. The results are shown in figs. 3e and fig. 4. While in fig. 3d at 1.2 M concentration the lifetime of the upper state and the growing in of the lower was shortened ta the order of the lifetime of the laser flash itself, i.e., 25 ns, in fig. 3e, at the lowest 0.15 M concentration the growing in time is = 150 ns and thus easily measured. In fig. 4 we plot l/r = k, as a function of Tb3+ concentration where r is the lifetime and k, rate constant of all quenching processes for diiute solutions. The value of T = 21-23 ns for 1.2 M solutions by extrapolation of the

the total the two is obtained

straight line in fig. .$, in agreement with the estimate from fig. 3d. We nay: calculate the quantitative data from the equa-

CHEMICALPHYSICSLE’TTERS

Volume 34, number 3 1.5

iti _7” l.O-

s

rr

I

1

i

os-

0.07 0.0

1

1

02

a.4

03

CM Fig. 4. Inverse dependence solution

of fluorescence

radiationless& from ‘D; , bypassing ‘Da. We showed previousty [I I] that the absolute quantum yield of fluorescence from the SD4 level on direct excitation to this level is 8.4%. We measured the absolute yield from the same level on excitation to the upper 5D3 ievel, and found 5.6%. Thus we fiid in Hz0 TP = 0.67, close to the value ir ref. [17]. To assist in interpreting the high self-quenching efficiency of Tb3+ m _ quenching the upper 5DJ state, compared with the much smaller efficiency in selfquenching the lower 5D4 state we investigated the possibility that transitions of the type ‘F, + ‘F. (of fig. I), from the ‘F, ground state may play a specific role. We found in the absorption spectrum of Tb3+ in solution a broad weak bank around 1.75 u in the near IR. For these maasurements we used a Gary 14R instrtiment modified for work in the near IR. Solution spectra of Tb3+ in the presence of Cl- were previously given in detail [16], but did not extend this far into the IR.

1

:I 0.1

1 Aueust 1975

lifetime

of Tb3’

in

on Tb3+ concentration.

4. Discussion

tion [7,8] l/7. = k, = k, f

ka + kq [Tb3’],

where I+ is the natural rate constant of radiative, fluorescent processes, k, is the rate constant of quenching by environmental factors due to the solvent medium and k, is the self-quenching rate constant. For the upper SD4 Ievel we obtain k, from the slope of fig. 4, k, = 4.1 X 107 MB1 s-l. The intercept in fig. 4 gives (IQ f ka). We obtain ky separately from the experiments in the borax glass, where k, -%k,. Hence the lower limit of k, = IO5 s-l. We then calculate k, = 1 A X 106 s-l for !I,0 solution. We thus see that the lifetime of the upper state 5D, obtained from the build-up of the lower state in D,O solution, extrapolated to zero Tb3+ concentration 7 = 530 nr. Durir.g this lifetime energy losses may occur due to k,, SO that a smaller number of electronic excited iorls reach the lower 5D4 level, than reached the upper SD,. We measured this transition probability (TP) by measuring the fluorescence yield from the lower 5D4 level once when excitation was directly at 480 nm to this level, and then when excitation was at 377 nm to the upper level. Previously Dawson et al. [17] did so in D20 and Hz0 solutions of Tb3’ containing Cl-. They obtained in D,O TP = 0.8 and 0.59 from 5 D3 to 5 D4 for $0. Thus in D20 T 20% of excited ions decay

Self-quenching strongly affects the upper ‘D3 level, while the lower fluorescent level s D4 is much less affected by it. Previously, Pearson et al. [ 181 found similar though much less pronounced effects for Tb3+ in borate glass and accepted the interpretation first proposed by Varsanyi and Dieke [I91 that the enerB gap spanned by the ‘F ground state multiplet (fig. 1) matches closely that between the 5D, and 5D, levels, leading

to efficient

radiationless

decay

from

the upper

DJ to the lower excited state, De. This mechanism should not decrease the transition probability to D4 from upper levels. Another type of self-quenching mechanism, leading directly from the 5D, or 5D, level to the ‘F !eveI may also operate, affecting both fluorescent levels (fig. i). We may derive some approximate rate constants as follows. From the concentration dependence of F the rate of growing irk of the SD4 level (equal to the lifetime of the 5D, level) (fig. 4) we obtaGled l/r = k, f e radiative rate constant of the +Ic$b].F;,,th SD, upper level = 1 X I!+ s-l,oti;rjnec! from the results in borate glass. This constant w!: assume to be little affected by changini into the liquid phase. In borate glass, k,, the rate constant of quenching by modes specific to the liquid environment = 0. Self411

Volume 34, number 3

CHEMICAL PIlYSICS LETTERS

k, [Tb] we assume to be small in glass compared to its effect in liquid solution, in view of the results of Pearson et al. [ 181 comparzl with our present k,, the rate constant of overresults. In D,O sohtion, all radiationless deactivation of DJ due to the @and environment = 1.8 X IO6 s-l and k9 (the rate of overall selfquenching) = k’ (leading to De) + ki (leading quenching,

1

AE=lO-’ cm-’

25

to ‘F) = 4.1 X IO7 M-7 S.-I. Further, from the results of ref. [17] of the value of TP (the transition probability from 5Dx to 5D4) = 0.8 = (kh + kk [Tb] )/(X.f + ?ch

+ kz + kb [Tb] + ki[Tb]). Thus, the overail rate constants k of solvent quenching; a, and of self-quenching, q, are divided into their components k’, leading from Dj to D4 and k”, leading from D3 to F, 2s shown in fig. 1. Hence we obtain the approximate values of kh E 3.2 X 107 M-1 ~-1 and ,G”= 8 X I 06 ~-1 s-1 _ We x 105 s-l. alsoobtaink;= 1.5X 10 6 s‘-1;ke’=3 These approximate values allow us to argue on the order of magnitude of the various effects. First we shall discuss the mechanism of the reaction with rate constant k:, leading from D, to D,, the energy difference being dissipated by the solvent. In our series of preaeding investigations on radiative and radiationless transitions in solutions of rare earth ions we showed [7-l 11, following the work of Kropp and Windsor [20] and Heller [Zl] , that electronic energy rr!!y be dissipated in one step by transfer to harmonics of single energetic vibrations of one solvent molecule. The probability that this mechanism is the dominant ’ radiationless one depends on the magnitude of the enerw gap. We showed [I I] h ow- q uantitative correlation of rate constants and isotope effects with the quantum number (harmonic) of the lribration may be derived. In the present case, comparison with the data of ref. [l 11 leads to the proposal related to the energy gap correiation [7-l l] shown in fig. 5. Here Tb3+ (2) denotes the upper energy gap 6000 cm-l, between D, and D4. 7’his gap is similar $0 that for Dy3+ and Nd3+ between their lowest fluorescent and highest IIO~-fluorescect X IO6

levels. The absolute value of kh(Tb3+) = 1.5 may be compared witi k(Dy3’) = 2.4 X I@ 5-l in D20. Tb3+ requires v = 3, Dy3’ u = 4, and

s-:,

the ratio of the rzte constants should relate to the probability ofexciting the third and fourth harmonic, respectively. The ratio of 6O.k of the order obtained by us previotisly

foi other rare earth ions assuming

the

mechanism of electronic energy transfer to single vi-. 412

Au~usl 1975

O-H O-D (34501 I25001

-6 4-5

3-

-4 -3

2-2 l-1

0

1

Tb”((l)

?b’%?)

Or’=

HdU

Fig. 5. Energy gaps related to vibrational

quantum

numbers.

brations of the solvent. For I@ in H,O Y = 3. Jc~(D~~~) in H,O = 4.2 X lo5 s-l, within a factor of 3.5 of kl(Tb3’) in D20, where v also = 3. In solids it was occasionally suggested that the dissipation of such energy differences occurs by simultaneous excitation of several separate phoilons. In solution we showed experi!mentally that harmonics of single vibrations are likely to be involved [22] . It is possible

that in some solids a similar mechanism operates. There are two pathways (fig. 1) leading directly from the upper D3 level to the ‘F manifold, thus affecting the transition probability D; --F D4. One of these is the self-quenching mechanism. The other radi-

Volume

atioriess

34, number

dissipation

3

CHEMICAL

pathway

mediated

PHYSICS LETTERS

by the solvent

environment, characterized by $’ z= 3 X lo5 s-l, is observable in the case of Tb3+ since the energy gap of 6000 cm-l between D, and D4 yields k; 2 l-5 X IO6 S-I with which kl

can

appreciably

compete.

Previous-

I;-, in the case of Eu3’ (with energy gap of 12 250 cm -I between the top of the ground manifold and first fluorescent level and 1700 cm-l from the latter to the higher fluorescent level) we estimated [7,8,23] k, (from the upper level) =Z lo5 s-l compared to X-= IO4 s-l (HzO) and 50 s -I (D,C, from the lower level.

!. August 1975

References

[l] J.B. Birks, Photophysics

(Wiley-

Interscience, [ 21 V. Crmolaev 98.

26 (197G)

of aromatic molecules New York, 1970) p. 142. and E. Sveshnikova, Opt. Spectry.

and S.de laegere, Chem. Phys. Letters 20 (1973) 581. 141hf. Lurin. bi. Ofran and G. Stein, J. Phys. Chem. 78 (1974) [3]

H. Biron, C. Giirller-Walrr?nd

For Tb3+ now we End * 3 X iOs for the upper level

1904; H. Lutz and G. Stein, J. Phys. Chem. 78 (1974) 1909; Y. Ilan and G. Stein, Chem. Phys. Letters 31 (1975) 441. [51W.R. Dawsonand J.L. Kropp, J. Am. Opt. Sot. 55 (1965) 822.

and 2.3

!61

X IO3

s-1

(HzO) and 47 s-1 (D20), respectively, for the lower level. The aim of the present work was mainIy to establish experimentally that there are pathways leading to loss of electronic excited states before the lowest fluorescent level is reached, so that quantum efficiency of fiuorescence does depend on the exciting wavelength. We

refer only briefly therefore to the detailed discussions presented in the cases of Eu3’ [7,23] and Gd3+ [8] on the role of the upper !evel, close above the lowest fluorescent one, as a stage in radiationless energy loss. The present work on Tb3+ (where the gap is ==600@ cm-l, compared to = 1700 cm-l for Eu3+ and z 600 cm -l for Gd3+) exhibits two salient points in this respect: (1) the larger gap increases the lifetime of the upper state, thus making experimental studies easier; (2) the.magnitude of ki, from the upper state, makes it likely that as in the case of Eu3+ a mechanism involving collective vibrational modes of the solvation layer, leading to transfer of alectronic energy of the ion into translational ener,T of solvent molecules, is operative. This mechanism, which operates for energy gaps too great to be bridged by the mechanism of quenching by single vibrational modes, has been discussed in detail previously [7,8,23] . The interaction with the solvent environment and the coupling to solvent modes leading to radiatio,nless deactivation to the ground state increases the higher the energy level.

R. Reisfeld, J. Chem.

E. Greenberg,

R. Velapoldi

and R. Barnett,

Phys. 56 (1972) 16?8.

[71 Y. Hans and G. Stein,

J. Phys. 8 (1971)

Chem.

75 (1971)

3668;

366. [Sl Y. fiaas, G. Stein and E. Wiirzberg, J. Chem. Phys. 58 (1973) 2777. 191 Y. Haas, G. Stein and E. W’kzberg, J. Chem, Phys. 60 (1974) 25s. (101 G. Stein and E. Wirzbeig, Chem. Phys. Letters 29 (1974) 21. ill1 G. Stein and E. Wiirzberg, I. C%em. Phys. 62 (1975) 208. I121 E. Nardi and S. Yatsiv, J. Chem. Phys. 37 (1962) 2333. 1131M.L. Bhaumik and L.I. Nugent, J. Chem. Phys. 43 (19651 Chcm.

1141

Phys.

Letters

1680. h1.J.Weber and R.F. Schaefele, J. C’hem.Phys. 43 (1965) 1702.

L. Boehm, Ch.R. Goldschmidt and K. Reisfeld, to be published. DC. Stewart and D. Karo, Anal. Chem. 30 (19.58) 164. ;:;I W.R. Dawson, J.L. Kropp and b1.W.Wndsor, J. Chem. Phys. 45 (1966) 2410. [W A.D. Pearson, G.E. Pzterson and W.R. Northover, J. Appl. Phys. 37 (1966) 729. [19J F. Varsanyi and G.H. Dick, Phys. Rev. Letters 7 (1961) [ljl

._

442.

1201 J-L. Kropp and M.W. Windsor, J. Chem. Phys. 39 (1963) 2769. 1211 A. Heller, J. Am. Chem. Sot. 88 (1966) 2058. [22] Y. Haas and G. Stein, J. Phys. Chem. 75 (197 1) 3677. [23] Y. Haas and G. Stein, J. Phys. Chem. 76 (1972) 1093.

413