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Transient excited singlet state absorption in POPOP and dimethyl POPOP
Specrrochmzico
Acto. Vol .44/Y
No. 2. pp. 131-140. 198R.
Printed m Great Bntain.
Transient excited singlet state absorption in POPOP POPOP
and dimethyl
YERNENI V. RAO,* PUTCHA VENKATESWARLU,t M. C. GEORGE, H. JAGANNATH and G. CHAKRAPANI$ Department of Physics, Alabama A & M University, Normal, (Recbpiued 3 March
Alabama
1987; in,final form Y June 1987; accepted
35762, U.S.A.
18 June 1987)
Abstract-Transient excited singlet state absorption (ESSA) has been measured in POPOP solution in ethanol and dimethyl POPOP solution in toluene at room temperature in the region 458%6880 A using a nitrogen laser and nitrogen laser pumped dye laser. Extensive absorption with several submaxima and shoulders, which represent the vibrational structure, has been observed in both molecules in the region covered in the present study. Energy level schemes of the two molecules have been obtained with the help of the ground state absorption and fluorescence spectra recorded for the purpose. The observed structure in the ESSA has been tentatively interpreted to be due to transitions between the different vibrational levels of the lowest excited state S, and two other upper singlet electronic states S3 and S4 or S,. The occurrence of transitions from the higher vibrational levels of S1 in addition to those from its lowest vibrational level could be understood on the basis of the fact that the pump and the probe beams are made to overlap for a period of - 6-7 ns during which they interact simultaneously with the dye molecules. A plausible explanation for the relative variation in the strength of different transitions from S, as well as the observed difference (or otherwise) in the strength of these transitions relative to those of S, in each molecule is given on the basis of parity/symmetry selection rules. The experimental results obtained and the transition assignments made are being reported for the first time.
INTRODUCTION Laser dyes such as scintillators, coumarines, xanthenes, oxazenes and polymethines constitute a class of organic molecules that can lase under appropriate conditions of stimulation. It is well known that the lasing transition occurs from the first excited singlet electronic state S, to the ground state S, of the molecule in most of the laser dyes [l]. However, in addition to the lasing transitions, the molecules in the S, state might either undergo absorptive transitions to the other upper singlet electronic states S,, S,, S, etc. absorbing either the pump radiation or the stimulated emission or both, or relax to the lowest triplet electronic state T, through perturbative processes thereby becoming unavailable for laser action. Both these processes thus cause loss of laser efficiency. Of the two processes, the first is of greater significance in the context of the pulsed mode and the second in the context of the cw mode of operation of the dye lasers. Because of this the study of these two processes, especially the former, has attracted the attention of several groups of workers in recent times [2-161. In spite of such wide ranging interest and activity the problem of scarcity of data on the excited singlet state absorption (ESSA) in laser dyes still remains, indicating the need for further work. Transient excited singlet state absorption in rhodamine 6G solution in ethanol has been studied in detail in this laboratory [17a]. Preliminary studies on
*On leave from A. N. R. College, Gudivada, 521 301, A.P., India. tAuthor to whom correspondence should be addressed. :On leave from Indian Institute of Technology, Kanpur 208 016, India.
POPOP, dimethyl POPOP and a-NPO have also been carried out earlier [17b]. In view of the interesting results obtained in rhodamine 6G, a detailed investigation of the transient ESSA in these three molecules has recently been undertaken and the results obtained in POPOP solution in ethanol and dimethyl POPOP solution in toluene are presented in this paper. SAHAR and
WIEDER [S] reported
the cross-sections
for excited singlet state absorption at the nitrogen laser wavelength of 3371 A in some dyes including POPOP. MAGDE et al. [16] subsequently studied the excited singlet state absorption (ESSA) of POPOP solution in toluene in the region 35W6500 A and observed two broad absorption maxima at 3500 A and 4200 A. But the transitions responsible for the excited state absorption have yet to be identified. No such study has ever been undertaken in the case of dimethyl POPOP as far as the authors’ knowledge goes. The transient excited singlet state absorption spectra of both POPOP in ethanol and dimethyl POPOP in toluene obtained in the present study reveal extensive absorption with a structure consisting of several submaxima and shoulders in the region 458@6880 A. In order to understand the origin of these spectral features as well as the maxima observed by MAGDE et al. [16] one needs the energy level diagrams of these two molecules. So ground state absorption and fluorescence spectra of these two molecules, which yield the necessary data for constructing the energy level diagrams, have also been recorded. EXPERIMENTAL Laser grade dyes obtained from Kodak and the solvents of reagent grade obtained from Baker Chemical Co. were used 131
Y ERNENI V. RAO et
132
without further purification. Experimental details for recording the ground state absorption and the fluorescence spectra and for measuring the transient ESSA of the two dyes in solution at room temperature are essentially the same as those given previously [17] and only a brief description will be presented here. The ground state absorption spectrum in the region 2@%7000 A and the fluorescence spectrum excited by a IMW Molectron model UV24 pulsed nitrogen laser were recorded using a Perkin-Elmer Model 323 u.v.-via near i.r. spectrophotometer. In the experiment for measuring absorption from the excited singlet state, the probe and the pump beams were obtained, as shown in Fig. 1, from a dye laser (DL) and the IMW nitrogen laser (NL) respectively. To maximize pumping efficiency, the pump beam was focussed into the dye solution close to the front wall of the absorption cell (AC) using a cylindrical quartz lens (L) and to ensure maximum interaction with the excited state population the probe beam was made to pass through the focussed region in the dye solution. Furthermore it was seen that the pump and probe pulses of duration 10 ns and 6-8 ns respectively overlapped for a period of _ 6-7 ns by introducing an optimum delay of -3 ns between the two by means of an optical delay line (ODL) so that they simultaneously interact with the dye molecules during the period of overlap. This was done with the twin objective of (i) seeing that the probe pulse starts interacting with the excited state population just when the latter attains its peak value and (ii) investigating transitions from higher vibrational levels of S, in addition to those from its lowest vibrational level since such transitions should occur as the probe pulse samples the molecules in these higher levels while they are continuously produced by the pump pulse but before they can thermally relax into the lowest level. The prohe beam after emerging from the absorption cell (AC) was detected by a photodetector (PD) and the output signal A after being amplified (AP) and averaged by a boxcar integrator (BXR) was normalized with respect to a similarly processed signal B from a second photo detector (PD) used for monitoring the input intensity of the probe beam. The normalized signal A/B which takes into account any fluctuations in the intensity of the probe beam was recorded on a strip chart recorder by scanning the dye laser through the region of interest. The normalized signals A/B, and Al/B and A”lB obtained when the probe beam passed through the cell containing the pure solvent, and when it passed through the dye solution with the pump beam turned off and the pump beam turned on respectively were recorded by scanning the dye laser through the tuning ranges of six different laser dyes (coumarin 440,460
al.
and 503 and rhodamine 6G and B and RhB plus cresyl violet) to cover the region of the ESSA spectrum from 4580 to 6880A. However, there are two gaps in the region covered, one from 4760 to 4820 A and the other from 6260 to 6520 A which could not be spanned for want of suitable laser dyes in the laboratory. The reproducibility of the recordings was checked after each scan. ANALYSISAND RESULTS Analysis of the data on normalized transmitted probe beam intensities at different wavelengths is based on the following model. The pump beam excites
the solute molecules in the ground state S, to one or more higher singlet electronic states S, and the molecules so excited decay back, by non-radiative interactions such as internal conversion and solvent environmental relaxation, to the lowest excited singlet state S, in a very short time of the order of l-10 ps. Intersystem crossing from S, into the lowest triplet state 7’i is assumed to be negligible in view of the fact that the intersystem crossing rate (- 10’ s-l) K, 6 l/T,, TP being the duration of the pump pulse (N 10 ns). Of the normalized signals recorded as described above (A/B) represents absorption due to the solvent, while (N/B) and (N/E) represent respectively the combined absorption due to the solvent and the solute molecules in the ground state S,, and that due to solvent and the solute molecules in the excited singlet state S, as well as the ground state S,, at the probe wavelength. It therefore follows that: A, = B, exp( - a)L, Ai = B, exp [ - (u + (a,) N)L]
and
A;’ = &w[
-(a + (a,) (N - 4 + WWI,
where L is the absorption path length, a is the linear absorption coefficient of the solvent, (a,) and (a,) are respectively the cross-sections for absorption out of the ground and the excited states S, and S, of the solute molecule at the probe wavelength and N and n are respectively the total number of solute molecules per
0s NL
=’
DL
/_ / BS
M
-
[]
ODL
!I AC l-r
I
A
l-l
81 BXR I I
Fig. 1. Schematic diagram of the experimental arrangement to measure the excited singlet state absorption (ESSA). AC Absorption cell, AP Amplifier, BS Beam splitter, BXR Boxcar integrator, DL Dye laser, M Mirror, NL Nitrogen laser, ODL Optical delay line, PD Photo-diode detector, R strip chart recorder.
Transient
volume and the number of those lifted to the S, state by the pump beam which are encountered by the probe beam. In the expression for A” the effect of stimulated emission on n at the probe wavelength is neglected since there is no evidence of such emission in the region covered by the present study in either molecule. The ground state absorption parameter is given by unit
N((T~)~L = In(A/B),
--ln(A’/B),
and the ground state absorption to its maximum value by
Similarly the differential absorption parameter between the first excited singlet state S, and the ground state S, is given by ~(a, -u,)~
L = In(A’/B), -In(A”/B),
and the relative differential (a, -us h/(0,
absorption
= In(A’/A”),, cross-section
relative
(as)/(a, h,,,, = In(,4/A’),/ln(A/A’),.,
-us ),,, = In(A’/A”),/ln(A’/A”),,,~X.
(a) Absorption and fluorescence. The ground state absorption spectrum of POPOP solution in ethanol shows two broad absorption bands, one with a maximum at 27850cm-’ and two subsidiary maxima or
l%?230
=, '49470. 49435 48980 48110' 4mj *469x)/: 462co/, :4"g/ l43m-
39230 36130
37080
POPOP 2,2’-p-Phenylenebis
Mel form
:
by
(i) POPOP
= ln(A/A’),,
cross-section
133
excited singlet state in POPOP
(5-phenyloxarole)
C,,H,,N,O,
2990 1830 700 0 Intensity
Fig. 2. Energy level diagram of POPOP. Levels marked with an asterisk are newly assigned from the ESSA spectrum while the others are assigned from the ground state absorption or fluorescence spectra. The pairs of energy levels at 46 170 and 46 200 cm-‘, and 49 430 and 49 470 cm-‘, though recognized as representing single levels. are shown separately in the figure to indicate the difference in their origins.
134
Y ERNENI V. RAO et al.
shoulders at 26480 and 29060 cm-‘, and the other with two maxima at 48 110 and 48 980 cm- ’ and three subsidiary maxima or shoulders at 46200, 47 230, 49 430 cm-‘, besides a group of three close lying maxima at 37080, 38 130 and 39230 cm-‘. The first band extending over the region 25000-32OOOcm-’ very probably represents the transition from the lowest vibrational level of the ground state S, to three different vibrational levels of the first excited singlet electronic state S, while the group of three maxima, and the second absorption band extending over the region 4100&50 000 cm- ’ represent transitions to different vibrational levels of two other upper singlet electronic states labeled S, and Ss respectively. All the energy levels together with the approximate intensity profiles of the absorption bands are shown in Fig. 2. The fluorescence spectrum of POPOP solution in ethanol extends over the region 26 450-28 000 cm- ’ and shows a peak at 23 840 cm-’ accompanied by two shoulders at 22680 and 24970 cm-‘. As expected, the fluorescence spectrum mirrors the absorption spectrum in the region 2500&32000 cm-‘. The normalized fluorescence Q(n)/Q,, and absorption ~@)/s,,,~,,, spectral wings in the overlapping region of these spectra have been found to rise steeply and intersect each other at 25 670 cm- ‘.* The energy corresponding to the point of intersection represents the energy separation between the lowest vibrational level of the first excited singlet state S, and that of the ground state S, [l] and is shown accordingly in Fig. 2. Assuming that transitions from the lowest vibrational level of S, to three different vibrational levels of the ground state S, are responsible for the fluorescence peak and the two accompanying shoulders, the levels at 700, 1830 and 299Ocm-’ in the ground state have been identified. (b) Excited singlet state absorption. The normalized transmitted probe beam intensities through POPOP solution in ethanol at a concentration of 1 x 10m4 M/l were recorded in the tuning ranges of six different laser dyes covering the region 4580-6880A as described earlier. The middle four tuning ranges span the region 482&6260 A without any break as their adjacent ends overlap while the first and the last tuning ranges are separated by gaps from this region. The ground state absorption parameter N(os)L and the differential absorption parameter n(u, -a&L at different wave-
*It may he noted here that the point of intersection of the absorption and fluorescence spectral wings of POPOP in solid solution of the tetrahydro-2-methyl furan at 77 K as determined from the spectra reported by PAVLOPOLJLOSand HAMMOND [18] is about 25 500 cm-’ which represents the position of the lowest vibrational level of S, above that of S, in the solid solution. tThis is primarily a consequence of the inevitable variations in the pulse to pulse intensity of the probing dye laser when it was operating at the extremes of its tuning range. But the reproducibility of the data over the rest of the dye laser tuning range was checked and found to be very good.
lengths were calculated using the relations derived above. As the calculated values of the parameters in the overlapping portions of the adjacent tuning ranges are found to vary?, a consistent set of values of these quantities over the mid region has been obtained by multiplying the calculated values in the different tuning ranges by appropriate matching factors; no such exercise was attempted in the extreme two tuning ranges. The values of the relative differential absorption cross-section (c~ -a,)J(o, -as),,, are plotted against wavelengths in Fig. 3. In order to assess the effect of the ground state absorption on the relative differential absorption at different probe wavelengths, the relative ground state absorption cross-sections (c,)J(es)maX after being multiplied by a factor N(c, )&n(c, - 0s L are also plotted against wavelength. In the extreme case when all the molecules are lifted to the excited singlet state S, by the pump beam, i.e. when n = N, the sum of the two ordinates corresponding to any wavelength gives (~,)~/(a, - ~7s)~~~ at that wavelength. But in any practical situation, n will be some small fraction of N and the ordinate of the second plot will have to be reduced by the same fraction before so adding to get (a,),/(~, - u,),,,~~ Thus it is obvious from the figure that the plot of (a, -u,)J(u, -as),,,,, versus 1 essentially represents the excited singlet state absorption spectrum. The spectrum shows extensive absorption throughout the region studied and consists of five submaxima and six shoulders listed in Table 1. All these submaxima and shoulders can be tentatively interpreted to be due to transitions between the different vibrational levels of the first excited singlet state S, and another upper singlet electronic state S,. Thus the two shoulders at 21270 and 20440cm-‘, the submaximum at 19080cm-’ and another shoulder at 17850cm-’ probably arise as a result of transitions respectively from four different vibrational levels (at 25 670,26 480, 27 850 and 29 060 cm- ‘) belonging to S,, to a common upper level at 46 920 + 20 cm-‘, while the shoulder at 19680cm-‘, the submaximum at 18310cm-’ and another shoulder at 17 120 cm-’ may be attributed to transitions respectively from the second, third and fourth vibrational levels of S, to another common upper level at 46 170+ 10 cm- ‘. Similarly the shoulder at 19910 cm-’ and the two submaximumat 19080and’ 17 720 cm- ’ probably represent transitions from the first, second and third vibrational levels of S, respectively to yet another common upper level at 45 570 + lOcm_‘, while the remaining two submaxima at 16610and1524Ocm-’ may be due to transitions from the second and the third vibrational levels of S, respectively to a fourth common upper level at 43090cm-‘. Of these four upper levels the one at 46 170+ 10 cm-’ almost coincides with the level at 46 200 cm- ’ and the other at 46 920 f 20 cm-’ is close to the level at 47 230 cm-’ obtained from the ground state absorption and shown in Fig. 2, while the remaining two lie in the region of strong ground state absorption although no specific maxima or shoulders
Transient
excited singlet state in POPOP
Y (in cm-‘) 72
21
20
19
I8
17
16
15
145)
I
I
I
I
I
I
I
I
I
Fig. 3. Excited singlet state absorption (ESSA) spectrum of POPOP solution (1 x lOmA M/l) in ethanol in the region 4500-6880 4 at room temperature. Different submaxima and shoulders in the spectrum, indicated
by arrows, are designated by numbered letters, Al, A2, A3, Bl etc; the number in each designation represents the number of the vibrational level in the S, state, from which the transition responsible for the corresponding submaximum submaxima and/or shoulders
or shoulder originates, while each letter marks out a specific group of resulting from transitions terminating in a common vibrational level in the upper singlet electronic state S, or S,.
are located at the corresponding positions. It is therefore reasonable to assume that these four levels are four different vibrational levels of the S, state. That the same state responsible for the strongest and the most extensive absorption from the ground state S, is involved in the excited singlet state absorption is what one would ordinarily expect for such a complex molecule in which the parity selection rules do not operate as it has no center of symmetry. As can be seen from Fig. 3 the excited singlet state absorption in the present study falls off to zero at 4580 A (21830 cm-‘) and no search was made for the maxima observed by MADGE et al. [16] at 42OO.A (23 800 cm- ‘) and 3500 A (28 560 cm- ‘). Of these the one at 23 800 cm- ’ may be attributed to a transition from the lowest vibrational level of S, to an upper level at 49 470 cm _ ’ which almost coincides with the level at 49 430 cm-’ belonging to the S, state while the other at 28 560 cm-’ is probably due to a transition from S, to a state higher than S, which falls outside the region of the present study of the ground state absorption. The transitions assigned to the submaxima and shoulders observed in the present study as well as the maxima
reported by MAGDE shown in Fig. 2.
et al.
are given in Table
1 and
(ii) DimethyI POPOP (a) Absorption and fluorescence. The ground state absorption spectrum of dimethyl POPOP solution in toluene consists of a prominent band in the region 23 60@-32 300 cm-’ and three weak bands with maxima at 34 950,41280 and 44 290 cm-‘. The strong band having a peak at 26 970 cm ’ and three shoulders at 25 590,26 040 and 28 200 cm- ’ probably arise as a result of transitions from the lowest vibrational level of the ground state S, to three different vibrational levels of the lowest excited singlet electronic state while the three weak bands represent transitions from the S, state to three other upper singlet electronic states labeled S,, S, and S,. The energy levels and the approximate intensity profiles of the absorption bands are shown in Fig. 4. The fluorescence spectrum of dimethyl POPOP solution extending from 25 770 to 17 670 cm-’ showsa peak at 23 170 cm- ’ and two shoulders at 22 000 and 24320cm-’ and appears as a mirror image of
136
YERNENI
I
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V. RAO et al.
I
Dimethyl popop ?,2’-p-Phenylenebis (4-methyl-5-phenytoxozole) H3C
\
P3
Kc00
Mel form.
:
CP6H2,N,0,
2810 1640 490 0 Intensity
Fig. 4. Energy level diagram of dimethyl POPOP. Levels marked with an asterisk are newly assigned from the ESSA spectrum, while the others are assigned from the ground state absorption or fluorescence spectra. The pairs of energy levels at 41280 and 413OOcm-‘, and 44280 and 44290cm-‘, though recognized as representing single levels, are shown separately in the figure to indicate the difference in their origins.
region spectrum in the the absorption The normalized fluorescence 23600-323OOcm-t. and absorption s(&s_ spectral wings rise Q(MQL steeply in the overlapping region 25 770-23 600 cm- ’ and intersect each other at 24 810 cm-‘. The energy corresponding to the point of intersection represents the height of the lowest vibrational level of the first excited singlet state S, above that of the ground state and the position of the lowest vibrational level of St state is shown accordingly in Fig. 4. Using this value for the lowest vibrational level of St, it is then possible to use the fluorescence spectrum to assign three vibrational levels of S, at 490, 1640, 2810 cm-‘.
(b) Excited singlet state absorption. The excited singlet state absorption of dimethyl POPOP solution in toluene at a concentration of 1 x 10m4M/l was measured in the region 4580-6880A following the same procedure as that used in the case of POPOP. The values of the relative differential absorption crosssection (a, -a,)J(u, -or),_ as well as the quantity W,),/n(c, -QlaX are plotted against wavelength in Fig. 5. The plots clearly show that the effect of the ground state absorption on the differential absorption between the ground and the first excited singlet states is negligibly small. It is, therefore, reasonable to treat the (a, - a,),/(~, -crg)max versus I plot as essentially the
Transient
Table
1. Transition
ESSA maximum submaximum or shoulder (vcm-‘)
assignments
137
excited singlet state in POPOP
to the observed features in the ESSA spectrum of POPOP solution (1 x 10m4 M/l) in ethanol
Designation
Transition assignment Between vibrational levels Electronic Te, u (cm-‘) state Lower Upper* Upper
21270 20 440 19080 17 850
Al A2 A3 A4
25 670 26480 21850 29 060
46 46 46 46
20 20 20 20
S3 S, S, S,
47 47 47 47
19 680 18310 17 120
B2 83 84
26 480 27 850 29 060
46170+ 10 46 170 f 10 461705 10
S3 S, S,
46 200 46 200 46 200
19910 19080 17720
Cl c2 c,
25 670 26 480 27 850
45 570 f 10 45 570 + 10 45 570 f 10
S, S, S,
16610 15240
02 03
26 480 27 850
43 090 43 090
S, S,
23 800::
El F1
25 670
49 470
S,
25 670
54 230
S”
28 560::
920 920 920 920
+ k f +
Ground state absorption maximum submaximumt or shoulder (cm I) 230 230 230 230
49 430
*These are the mean positions obtained by adding the wave numbers of the submaxima or shoulders in the ESSA to the vibrational levels of the state S, from which the corresponding transitions take place. t Blank space in this column indicates that the upper vibrational level Te, t’ lies in the region of strong S,, + S, absorption but no specific maximum, submaximum or shoulder is located at the corresponding position. : Magde et al.
spectrum as is done excited singlet state absorption here. The spectrum shows extensive absorption in the whole region covered in the present study. There are as many as five submaxima and seven shoulders in the spectrum, all of which are listed in Table 2. As in the case of POPOP it is possible to tentatively attribute all
Table 2. Transition
ESSA maximum submaximum or shoulder (vcm- ‘)
assignments
Designation
but one submaxima and the seven shoulders to transitions from the different vibrational levels of the first excited singlet state S, to those of another upper singlet electronic state S,. The transition assignments made are presented in Table 2 and shown in Fig. 4. Of the four new vibrational levels of S4 now assigned the level at 44 28Ok 20 cm- ’ coincides with the vibra-
to the observed features in the ESSA spectrum of dimethyl POPOP solution (1 x 10e4 M/l) in toluene
Transition assignment Between vibrational levels Electronic Te, v (cm-‘) state Lower Upper* Upper
20 400 19600 18210 17000
Al A2 A4 A5
24810 25 590 26 970 28200
45200+20 45200+20 45200+20 45 200 t 20
S, S, S‘% S,
20 070 19300 16690
Bl 82 B5
24810 25 590 28200
4489Ok 44890+ 448902
10 10 10
S, S, S‘+
17300 16 100
c4 c5
26970 28 200
44 280 f 20 44 280 f 20
S4 S‘%
18410 17 180
Dl D3
24810 26 040
43 220 43 220
SC! S,
15260
E3
26 040
41300
S3
Ground state absorption maximum submaximum t or shoulder (cm ‘)
44 290
41280
*These are the mean positions obtained by adding the wave numbers of the submaxima or shoulders in the to the vibrational levels of the state S, from which the corresponding transitions take place. t Blank space in this column indicates that the upper vibrational level Te, v lies in the region of moderately strong S, -+ S, absorption but no specific maximum, submaximum or shoulder is located at the corresponding position. ESSA
138
V. RAO et
Y ERNENI
22
21
I
I
xl I
A1
81
id
A2 82
al.
19
,6 Y km-’ )
,7
I
I
I
LX A4
++
C403A5 Iii
16
15
I
I
85
c5
E3
1
+
t
145
loo0
I
Fig. 5. Excited singlet state absorption (ESSA) spectrum of dimethyl POPOP solution (1 x 10-t M/l) ir toluene in the region 4580-6880 A at room temperature. Different submaxima and shoulders in the spectrum, indicated by arrows, are designated by numbered letters, Al, A2, A3, Bl etc; the number in each designation represents the number of the vibrational level in the S1 state,‘from which the transition responsible for the corresponding submaximum/shoulder originates, while eatih letter marks out a spedific group of submaxima and/or shoulders resulting from transitions terminating in a common vibrational level in the upper singlet electronic state S, or S,.
tional level of S4 at 44 290 cm-’ identified from the ground state absorption whereas the remaining three levels (at 43 220,44 890 f 10 and 45 200 + 20 cm- ‘) lie in the region of relatively strong absorption due to S, -+ S4 transition. For this reason, these four upper levels might be assumed to be four different vibrational levels belonging to the electronic singlet state &. It can be seen from the energy level diagram (Fig. 4) that the transition from the third vibrational level of S, state at 26040 cm- ’ to the singlet electronic state S3 at 41280 cm-’ is expected to show up at 15 240 cm- ’ in the ESSA almost at the same position at which a submaximum (15 260 cm- ‘) is observed. It is worth noting here that in contrast to the situation in POPOP, the S4 state responsible for almost all the observed ESSA in this molecule is the same as the one involved in the transition giving rise to one of the weak ground state absorption bands.
out the region studied and consist of several submaxima and shoulders which represent the vibrational structure. In both the cases it has been possible to tentatively interpret all the observed features in terms of transitions between the different vibrational levels of the lowest excited singlet state S1 and two other upper singlet electronic states S, and S, or S,. According to this interpretation, the energy gap between the lowest and the highest vibrational levels of the S, state involved in these transitions is as much as 2390 cm - 1in each molecule, which is in sharp contrast to the assumption that the fluorescence transition originates from the lowest vibrational level of S,. However, in the present investigation the pump and the probe pulses interact simultaneously with the dye molecules during the period of their overlap (-67 ns). Although the molecules in the higher vibrational levels of S1 decay, by radiationless interactions, to the lowest vibrational level so fast as to render the steady-state populations of these levels negligibly
DISCUSSION
small, continuous production of molecules in these levels over the entire pump pulse duration ensures detectable non-equilibrium populations in them. And it should be possible for these molecules in the higher
The excited singlet state absorption spectra of the two dye molecules show extensive absorption through-
Transient excited singlet state in POPOP vibrational levels* of S, to undergo absorptive transitions along with those in the lowest vibrational level of the S, state during the period of overlap of the two pulses. The above assumption that the steady-state population of the higher vibrational levels of S, is negligibly small needs to be closely examined. It is valid only when the temperature is so low that the higher vibrational levels are not already thermally populated by the Boltzman distribution of molecules among the different vibrational-rotational levels of S, appropriate to that temperature. At the room temperature at which the experiments were carried out, however, the S, state has been found to be populated up to a height of 112Ocm _ 1 in POPOP and 960 cm ’ in dimethyl POPOP as evidenced by the short wavelength edges of their fluorescence spectra. Since the first excited vibrational level of the S, state in either molecule lies within the filled energy band width of that state, it is possible for that level to have a “non-negligible” steady-state population. If so the submaxima or shoulders in ESSA due to transitions from the first excited vibrational level should be somewhat stronger than the other submaxima or shoulders but much weaker than those due to transitions from the lowest vibrational 1evel.t But in the present studies, the following has been observed. In dimethyl POPOP (Fig. 5) the submaxima or shoulders due to transitions from the first excited vibrational level of S, are roughly of the same strength as those due to transitions from the lowest vibrational level, whereas they are stronger than all the submaxima or shoulders, except one, corresponding to transitions from the second and third excited vibrational levels. In POPOP the submaxima or shoulders due to transitions from the first vibrational level of S, are somewhat stronger than the shoulders due to transitions from the lowest vibrational level while they are weaker than one submaximum and about as strong as six submaxima or shoulders associated with transitions from the other higher excited vibrational levels of S, (Fig. 3). The explanation for this might be sought in the Frank-Condon principle, according to which the intensity of an absorption band is proportional to the square of the overlap integral formed for the two vibrational levels involved in the transition responsible for that
band.
The values
of the overlap
integrals,
in
*It is pertinent to note here that thermal populations of the different vibrational levels of the ground state undergo analogous transitions [19] to the first excited state S, reabsorbing part of the stimulated emission in a process known as self-absorption; this affects the optimum wavelength at which the dye laser operates. t It should, however, be emphasized that these expectations are strictly valid in a situation where there is no temporal overlap of the pump and probe pulses but in cases like the present one in which the two pulses overlap, the intensity distribution in ESSA spectrum will be decided, almost wholly, by the non-equilibrium populations in the different vibrational levels of S, with the “shaping” provided by the Frank-Condon factors.
139
turn, depend on the relative positions of the classical turning points of motion in the initial and the final levels of each one of the transitions. It is therefore possible for the overlap integrals of the transitions from the different vibrational levels of the S, state to have appreciably different values and hence for the corresponding submaxima or shoulders to have the observed intensities. Another interesting observation, which finds a plausible explanation based on the Franck-Condon principle, relates to the contrasting nature of the final states involved in the transitions responsible for the ESSA in the two molecules. In POPOP the final state of the transition associated with ESSA is the sarhe as the one involved in the strongest absorptive transition from the ground state while in dimethyl POPOP it is the one that is involved in the weakest absorptive transition from the ground state. If the potential minima of the two states So and S4 occur at about the same internuclear distance r,and that of S, at a greater r,then one expects, on the basis ofthe Franck-Condon principle, the pattern of intensities observed in POPOP for So + S1, S, + S4 and S, --) S, transitions. If, on the other hand, the minima lie at three different internuclear distances such that r,(S,) < r&S,) < r,(S,) one expects the intensity pattern observed in dimethyl POPOP. A plausible explanation for the relative variation in the strength of different transitions from S, as well as the observed difference (or otherwise) in the strength of these transitions compared with those from S, in each molecule can be found on the basis of parity/symmetry selection rules. The molecules POPOP and dimethyl POPOP do not have centers of symmetry and, therefore, no parity selection rules exist. Further, the highest symmetry point group expected for dimethyl POPOP is C, or C, and in either case, no symmetry forbidden selection rules operate. The highest symmetry point group possible for POPOP, on the other hand, is Cl0 with irreducible representations, A 1, A,, B, and B2. Here transitions between any two vibronic states whose product wavefunction forms a basis for the irreducible representation A, are forbidden while all others are allowed. However, as POPOP has 44 atoms, there will be 126 internal modes of vibrations and under low resolution symmetry selection rules do not give rise to any observable effects because of spectral congestion due to numerous overlaps in the vibronic spectrum. It would be interesting to carrry out high resolution work using polarization techniques with a view to study the several expected internal modes of vibration in molecules like POPOP. It is also informative to note that the ground state absorption band due to S, 4 S3 transition is stronger than that due to S, -+ S, transition suggesting that the electronic wave function of S1 is probably of A2 type as that of So is generally known to be of Al type. If so, pure electronic transition So -+ S, will be forbidden though transitions are allowed through vibronic coupling explaining the weak nature of the corresponding
Y ERNENIV. RAO et al.
140
absorption band. The transient absorption observed in the present experiment is fairly strong indicating that the product wave function of the two pure electronic states S1 and S3 is not of A2 symmetry. This points to the possibility that the wave function of S3 is of B, or B2 symmetry in which case the electronic transitions So --*S3 and S1 + S3 become allowed. As there are no such selection rules prohibiting transitions between any two electronic states in dimethyl POPOP the absorption band due to the So + S, transition in this molecule is strong unlike in POPOP. Acknowledgement-This work is supported by a grant from the US National Science Foundation (Grant No. Rll8409956). REFERENCES F. P. SCHAFER,in Topics in Applied Physics; 1 Dye Lasers. Springer, Berlin (1983). 1. WIEDER,Appl. Phys. Lett. 21, 318 (1972). E. SAHARand 1. WIEDER,Chem. Phys. ht. 23, 518 f:; (1973). c41 0. TESCHKEand A. DIENES, Opt. Commun. 9, 128 (1973). c51 E. SAHARand 1. WIEDER.IEEE J. Ouantum Electron. QE-10,612 (1974). C61 J. SHAH and R. F. LEHENY,Appl. Phys. Lett. 24, 562 (1974).
PI
R. F. LEHENYand J. SHAH,IEEE J. Quantum Electron. QE-I I, 70 (1975). 0. TESCHKEand A. DIENES,Appl. Phys. Lett. 26, 13 PI (1975). c91 0. TESCHKE, A. DIENES and G. HOLTOM, Opt. Commun. 13, 318 (1975). G. DOLAN~~~ C. R. GOLDSCHMIDT, Chem. Phys. Lett. WI 39, 320 (1976). Cl11 E. SAHAR,D. TREvEsand I. WIEDER,Opt. Commun. 16, 124 (1976). 0. TESCHKE,A. DIENEsand J. R. WHINNERY,ZEEE J. WI Quantum Electron. QE-12, 383 (1976). Cl31 E. SAHARand D. TREVES,IEEE J. Quantum Electron. QE-13, 962 (1977). and H. TASHIRO, Cl41 A. MULLER, J. SCHULTZ-HENNING Appl. Phys. 12, 333 (1977). A. PENZKOFER and W. KAISER,Opt. Cl51 W. FALKENSTEIN, Commun. 27, 151 (1978). Cl61 D. MAGDE,S. T. GAFFNEYand B. F. CAMPBELL,IEEE J. Quantum Electron. QE-17, 489 (1981). M. C. GEORGE, Y. V. RAO. H. C17al P. VENKATESWARLU. JAGANNATH and G. CHAKRAPANI,Pr&ana (J. Phys.) 28, 59 (1987). M. C. GEORGE,H. JAGANNATH, C17bl P. VENKATESWARLU, Y. V. RAO, G. CHAKRAPANIand A. MIAHNAHRI, Optical Studies in Laser Dyes, Tech. Rep. BMD DASG60-83-C-0060 (1985). Cl81 T. G. PAVLOPOULOS‘~~~‘P. R. HAMMOND, J. Am. them. Sot. 96, 6568 (1974). in Methqds ojExperimental Physics: Cl91 0. G. PETERSON, 15A Quantum Electronics. Academic Press, New York (1979. c71
Acto. Vol .44/Y
No. 2. pp. 131-140. 198R.
Printed m Great Bntain.
Transient excited singlet state absorption in POPOP POPOP
and dimethyl
YERNENI V. RAO,* PUTCHA VENKATESWARLU,t M. C. GEORGE, H. JAGANNATH and G. CHAKRAPANI$ Department of Physics, Alabama A & M University, Normal, (Recbpiued 3 March
Alabama
1987; in,final form Y June 1987; accepted
35762, U.S.A.
18 June 1987)
Abstract-Transient excited singlet state absorption (ESSA) has been measured in POPOP solution in ethanol and dimethyl POPOP solution in toluene at room temperature in the region 458%6880 A using a nitrogen laser and nitrogen laser pumped dye laser. Extensive absorption with several submaxima and shoulders, which represent the vibrational structure, has been observed in both molecules in the region covered in the present study. Energy level schemes of the two molecules have been obtained with the help of the ground state absorption and fluorescence spectra recorded for the purpose. The observed structure in the ESSA has been tentatively interpreted to be due to transitions between the different vibrational levels of the lowest excited state S, and two other upper singlet electronic states S3 and S4 or S,. The occurrence of transitions from the higher vibrational levels of S1 in addition to those from its lowest vibrational level could be understood on the basis of the fact that the pump and the probe beams are made to overlap for a period of - 6-7 ns during which they interact simultaneously with the dye molecules. A plausible explanation for the relative variation in the strength of different transitions from S, as well as the observed difference (or otherwise) in the strength of these transitions relative to those of S, in each molecule is given on the basis of parity/symmetry selection rules. The experimental results obtained and the transition assignments made are being reported for the first time.
INTRODUCTION Laser dyes such as scintillators, coumarines, xanthenes, oxazenes and polymethines constitute a class of organic molecules that can lase under appropriate conditions of stimulation. It is well known that the lasing transition occurs from the first excited singlet electronic state S, to the ground state S, of the molecule in most of the laser dyes [l]. However, in addition to the lasing transitions, the molecules in the S, state might either undergo absorptive transitions to the other upper singlet electronic states S,, S,, S, etc. absorbing either the pump radiation or the stimulated emission or both, or relax to the lowest triplet electronic state T, through perturbative processes thereby becoming unavailable for laser action. Both these processes thus cause loss of laser efficiency. Of the two processes, the first is of greater significance in the context of the pulsed mode and the second in the context of the cw mode of operation of the dye lasers. Because of this the study of these two processes, especially the former, has attracted the attention of several groups of workers in recent times [2-161. In spite of such wide ranging interest and activity the problem of scarcity of data on the excited singlet state absorption (ESSA) in laser dyes still remains, indicating the need for further work. Transient excited singlet state absorption in rhodamine 6G solution in ethanol has been studied in detail in this laboratory [17a]. Preliminary studies on
*On leave from A. N. R. College, Gudivada, 521 301, A.P., India. tAuthor to whom correspondence should be addressed. :On leave from Indian Institute of Technology, Kanpur 208 016, India.
POPOP, dimethyl POPOP and a-NPO have also been carried out earlier [17b]. In view of the interesting results obtained in rhodamine 6G, a detailed investigation of the transient ESSA in these three molecules has recently been undertaken and the results obtained in POPOP solution in ethanol and dimethyl POPOP solution in toluene are presented in this paper. SAHAR and
WIEDER [S] reported
the cross-sections
for excited singlet state absorption at the nitrogen laser wavelength of 3371 A in some dyes including POPOP. MAGDE et al. [16] subsequently studied the excited singlet state absorption (ESSA) of POPOP solution in toluene in the region 35W6500 A and observed two broad absorption maxima at 3500 A and 4200 A. But the transitions responsible for the excited state absorption have yet to be identified. No such study has ever been undertaken in the case of dimethyl POPOP as far as the authors’ knowledge goes. The transient excited singlet state absorption spectra of both POPOP in ethanol and dimethyl POPOP in toluene obtained in the present study reveal extensive absorption with a structure consisting of several submaxima and shoulders in the region 458@6880 A. In order to understand the origin of these spectral features as well as the maxima observed by MAGDE et al. [16] one needs the energy level diagrams of these two molecules. So ground state absorption and fluorescence spectra of these two molecules, which yield the necessary data for constructing the energy level diagrams, have also been recorded. EXPERIMENTAL Laser grade dyes obtained from Kodak and the solvents of reagent grade obtained from Baker Chemical Co. were used 131
Y ERNENI V. RAO et
132
without further purification. Experimental details for recording the ground state absorption and the fluorescence spectra and for measuring the transient ESSA of the two dyes in solution at room temperature are essentially the same as those given previously [17] and only a brief description will be presented here. The ground state absorption spectrum in the region 2@%7000 A and the fluorescence spectrum excited by a IMW Molectron model UV24 pulsed nitrogen laser were recorded using a Perkin-Elmer Model 323 u.v.-via near i.r. spectrophotometer. In the experiment for measuring absorption from the excited singlet state, the probe and the pump beams were obtained, as shown in Fig. 1, from a dye laser (DL) and the IMW nitrogen laser (NL) respectively. To maximize pumping efficiency, the pump beam was focussed into the dye solution close to the front wall of the absorption cell (AC) using a cylindrical quartz lens (L) and to ensure maximum interaction with the excited state population the probe beam was made to pass through the focussed region in the dye solution. Furthermore it was seen that the pump and probe pulses of duration 10 ns and 6-8 ns respectively overlapped for a period of _ 6-7 ns by introducing an optimum delay of -3 ns between the two by means of an optical delay line (ODL) so that they simultaneously interact with the dye molecules during the period of overlap. This was done with the twin objective of (i) seeing that the probe pulse starts interacting with the excited state population just when the latter attains its peak value and (ii) investigating transitions from higher vibrational levels of S, in addition to those from its lowest vibrational level since such transitions should occur as the probe pulse samples the molecules in these higher levels while they are continuously produced by the pump pulse but before they can thermally relax into the lowest level. The prohe beam after emerging from the absorption cell (AC) was detected by a photodetector (PD) and the output signal A after being amplified (AP) and averaged by a boxcar integrator (BXR) was normalized with respect to a similarly processed signal B from a second photo detector (PD) used for monitoring the input intensity of the probe beam. The normalized signal A/B which takes into account any fluctuations in the intensity of the probe beam was recorded on a strip chart recorder by scanning the dye laser through the region of interest. The normalized signals A/B, and Al/B and A”lB obtained when the probe beam passed through the cell containing the pure solvent, and when it passed through the dye solution with the pump beam turned off and the pump beam turned on respectively were recorded by scanning the dye laser through the tuning ranges of six different laser dyes (coumarin 440,460
al.
and 503 and rhodamine 6G and B and RhB plus cresyl violet) to cover the region of the ESSA spectrum from 4580 to 6880A. However, there are two gaps in the region covered, one from 4760 to 4820 A and the other from 6260 to 6520 A which could not be spanned for want of suitable laser dyes in the laboratory. The reproducibility of the recordings was checked after each scan. ANALYSISAND RESULTS Analysis of the data on normalized transmitted probe beam intensities at different wavelengths is based on the following model. The pump beam excites
the solute molecules in the ground state S, to one or more higher singlet electronic states S, and the molecules so excited decay back, by non-radiative interactions such as internal conversion and solvent environmental relaxation, to the lowest excited singlet state S, in a very short time of the order of l-10 ps. Intersystem crossing from S, into the lowest triplet state 7’i is assumed to be negligible in view of the fact that the intersystem crossing rate (- 10’ s-l) K, 6 l/T,, TP being the duration of the pump pulse (N 10 ns). Of the normalized signals recorded as described above (A/B) represents absorption due to the solvent, while (N/B) and (N/E) represent respectively the combined absorption due to the solvent and the solute molecules in the ground state S,, and that due to solvent and the solute molecules in the excited singlet state S, as well as the ground state S,, at the probe wavelength. It therefore follows that: A, = B, exp( - a)L, Ai = B, exp [ - (u + (a,) N)L]
and
A;’ = &w[
-(a + (a,) (N - 4 + WWI,
where L is the absorption path length, a is the linear absorption coefficient of the solvent, (a,) and (a,) are respectively the cross-sections for absorption out of the ground and the excited states S, and S, of the solute molecule at the probe wavelength and N and n are respectively the total number of solute molecules per
0s NL
=’
DL
/_ / BS
M
-
[]
ODL
!I AC l-r
I
A
l-l
81 BXR I I
Fig. 1. Schematic diagram of the experimental arrangement to measure the excited singlet state absorption (ESSA). AC Absorption cell, AP Amplifier, BS Beam splitter, BXR Boxcar integrator, DL Dye laser, M Mirror, NL Nitrogen laser, ODL Optical delay line, PD Photo-diode detector, R strip chart recorder.
Transient
volume and the number of those lifted to the S, state by the pump beam which are encountered by the probe beam. In the expression for A” the effect of stimulated emission on n at the probe wavelength is neglected since there is no evidence of such emission in the region covered by the present study in either molecule. The ground state absorption parameter is given by unit
N((T~)~L = In(A/B),
--ln(A’/B),
and the ground state absorption to its maximum value by
Similarly the differential absorption parameter between the first excited singlet state S, and the ground state S, is given by ~(a, -u,)~
L = In(A’/B), -In(A”/B),
and the relative differential (a, -us h/(0,
absorption
= In(A’/A”),, cross-section
relative
(as)/(a, h,,,, = In(,4/A’),/ln(A/A’),.,
-us ),,, = In(A’/A”),/ln(A’/A”),,,~X.
(a) Absorption and fluorescence. The ground state absorption spectrum of POPOP solution in ethanol shows two broad absorption bands, one with a maximum at 27850cm-’ and two subsidiary maxima or
l%?230
=, '49470. 49435 48980 48110' 4mj *469x)/: 462co/, :4"g/ l43m-
39230 36130
37080
POPOP 2,2’-p-Phenylenebis
Mel form
:
by
(i) POPOP
= ln(A/A’),,
cross-section
133
excited singlet state in POPOP
(5-phenyloxarole)
C,,H,,N,O,
2990 1830 700 0 Intensity
Fig. 2. Energy level diagram of POPOP. Levels marked with an asterisk are newly assigned from the ESSA spectrum while the others are assigned from the ground state absorption or fluorescence spectra. The pairs of energy levels at 46 170 and 46 200 cm-‘, and 49 430 and 49 470 cm-‘, though recognized as representing single levels. are shown separately in the figure to indicate the difference in their origins.
134
Y ERNENI V. RAO et al.
shoulders at 26480 and 29060 cm-‘, and the other with two maxima at 48 110 and 48 980 cm- ’ and three subsidiary maxima or shoulders at 46200, 47 230, 49 430 cm-‘, besides a group of three close lying maxima at 37080, 38 130 and 39230 cm-‘. The first band extending over the region 25000-32OOOcm-’ very probably represents the transition from the lowest vibrational level of the ground state S, to three different vibrational levels of the first excited singlet electronic state S, while the group of three maxima, and the second absorption band extending over the region 4100&50 000 cm- ’ represent transitions to different vibrational levels of two other upper singlet electronic states labeled S, and Ss respectively. All the energy levels together with the approximate intensity profiles of the absorption bands are shown in Fig. 2. The fluorescence spectrum of POPOP solution in ethanol extends over the region 26 450-28 000 cm- ’ and shows a peak at 23 840 cm-’ accompanied by two shoulders at 22680 and 24970 cm-‘. As expected, the fluorescence spectrum mirrors the absorption spectrum in the region 2500&32000 cm-‘. The normalized fluorescence Q(n)/Q,, and absorption ~@)/s,,,~,,, spectral wings in the overlapping region of these spectra have been found to rise steeply and intersect each other at 25 670 cm- ‘.* The energy corresponding to the point of intersection represents the energy separation between the lowest vibrational level of the first excited singlet state S, and that of the ground state S, [l] and is shown accordingly in Fig. 2. Assuming that transitions from the lowest vibrational level of S, to three different vibrational levels of the ground state S, are responsible for the fluorescence peak and the two accompanying shoulders, the levels at 700, 1830 and 299Ocm-’ in the ground state have been identified. (b) Excited singlet state absorption. The normalized transmitted probe beam intensities through POPOP solution in ethanol at a concentration of 1 x 10m4 M/l were recorded in the tuning ranges of six different laser dyes covering the region 4580-6880A as described earlier. The middle four tuning ranges span the region 482&6260 A without any break as their adjacent ends overlap while the first and the last tuning ranges are separated by gaps from this region. The ground state absorption parameter N(os)L and the differential absorption parameter n(u, -a&L at different wave-
*It may he noted here that the point of intersection of the absorption and fluorescence spectral wings of POPOP in solid solution of the tetrahydro-2-methyl furan at 77 K as determined from the spectra reported by PAVLOPOLJLOSand HAMMOND [18] is about 25 500 cm-’ which represents the position of the lowest vibrational level of S, above that of S, in the solid solution. tThis is primarily a consequence of the inevitable variations in the pulse to pulse intensity of the probing dye laser when it was operating at the extremes of its tuning range. But the reproducibility of the data over the rest of the dye laser tuning range was checked and found to be very good.
lengths were calculated using the relations derived above. As the calculated values of the parameters in the overlapping portions of the adjacent tuning ranges are found to vary?, a consistent set of values of these quantities over the mid region has been obtained by multiplying the calculated values in the different tuning ranges by appropriate matching factors; no such exercise was attempted in the extreme two tuning ranges. The values of the relative differential absorption cross-section (c~ -a,)J(o, -as),,, are plotted against wavelengths in Fig. 3. In order to assess the effect of the ground state absorption on the relative differential absorption at different probe wavelengths, the relative ground state absorption cross-sections (c,)J(es)maX after being multiplied by a factor N(c, )&n(c, - 0s L are also plotted against wavelength. In the extreme case when all the molecules are lifted to the excited singlet state S, by the pump beam, i.e. when n = N, the sum of the two ordinates corresponding to any wavelength gives (~,)~/(a, - ~7s)~~~ at that wavelength. But in any practical situation, n will be some small fraction of N and the ordinate of the second plot will have to be reduced by the same fraction before so adding to get (a,),/(~, - u,),,,~~ Thus it is obvious from the figure that the plot of (a, -u,)J(u, -as),,,,, versus 1 essentially represents the excited singlet state absorption spectrum. The spectrum shows extensive absorption throughout the region studied and consists of five submaxima and six shoulders listed in Table 1. All these submaxima and shoulders can be tentatively interpreted to be due to transitions between the different vibrational levels of the first excited singlet state S, and another upper singlet electronic state S,. Thus the two shoulders at 21270 and 20440cm-‘, the submaximum at 19080cm-’ and another shoulder at 17850cm-’ probably arise as a result of transitions respectively from four different vibrational levels (at 25 670,26 480, 27 850 and 29 060 cm- ‘) belonging to S,, to a common upper level at 46 920 + 20 cm-‘, while the shoulder at 19680cm-‘, the submaximum at 18310cm-’ and another shoulder at 17 120 cm-’ may be attributed to transitions respectively from the second, third and fourth vibrational levels of S, to another common upper level at 46 170+ 10 cm- ‘. Similarly the shoulder at 19910 cm-’ and the two submaximumat 19080and’ 17 720 cm- ’ probably represent transitions from the first, second and third vibrational levels of S, respectively to yet another common upper level at 45 570 + lOcm_‘, while the remaining two submaxima at 16610and1524Ocm-’ may be due to transitions from the second and the third vibrational levels of S, respectively to a fourth common upper level at 43090cm-‘. Of these four upper levels the one at 46 170+ 10 cm-’ almost coincides with the level at 46 200 cm- ’ and the other at 46 920 f 20 cm-’ is close to the level at 47 230 cm-’ obtained from the ground state absorption and shown in Fig. 2, while the remaining two lie in the region of strong ground state absorption although no specific maxima or shoulders
Transient
excited singlet state in POPOP
Y (in cm-‘) 72
21
20
19
I8
17
16
15
145)
I
I
I
I
I
I
I
I
I
Fig. 3. Excited singlet state absorption (ESSA) spectrum of POPOP solution (1 x lOmA M/l) in ethanol in the region 4500-6880 4 at room temperature. Different submaxima and shoulders in the spectrum, indicated
by arrows, are designated by numbered letters, Al, A2, A3, Bl etc; the number in each designation represents the number of the vibrational level in the S, state, from which the transition responsible for the corresponding submaximum submaxima and/or shoulders
or shoulder originates, while each letter marks out a specific group of resulting from transitions terminating in a common vibrational level in the upper singlet electronic state S, or S,.
are located at the corresponding positions. It is therefore reasonable to assume that these four levels are four different vibrational levels of the S, state. That the same state responsible for the strongest and the most extensive absorption from the ground state S, is involved in the excited singlet state absorption is what one would ordinarily expect for such a complex molecule in which the parity selection rules do not operate as it has no center of symmetry. As can be seen from Fig. 3 the excited singlet state absorption in the present study falls off to zero at 4580 A (21830 cm-‘) and no search was made for the maxima observed by MADGE et al. [16] at 42OO.A (23 800 cm- ‘) and 3500 A (28 560 cm- ‘). Of these the one at 23 800 cm- ’ may be attributed to a transition from the lowest vibrational level of S, to an upper level at 49 470 cm _ ’ which almost coincides with the level at 49 430 cm-’ belonging to the S, state while the other at 28 560 cm-’ is probably due to a transition from S, to a state higher than S, which falls outside the region of the present study of the ground state absorption. The transitions assigned to the submaxima and shoulders observed in the present study as well as the maxima
reported by MAGDE shown in Fig. 2.
et al.
are given in Table
1 and
(ii) DimethyI POPOP (a) Absorption and fluorescence. The ground state absorption spectrum of dimethyl POPOP solution in toluene consists of a prominent band in the region 23 60@-32 300 cm-’ and three weak bands with maxima at 34 950,41280 and 44 290 cm-‘. The strong band having a peak at 26 970 cm ’ and three shoulders at 25 590,26 040 and 28 200 cm- ’ probably arise as a result of transitions from the lowest vibrational level of the ground state S, to three different vibrational levels of the lowest excited singlet electronic state while the three weak bands represent transitions from the S, state to three other upper singlet electronic states labeled S,, S, and S,. The energy levels and the approximate intensity profiles of the absorption bands are shown in Fig. 4. The fluorescence spectrum of dimethyl POPOP solution extending from 25 770 to 17 670 cm-’ showsa peak at 23 170 cm- ’ and two shoulders at 22 000 and 24320cm-’ and appears as a mirror image of
136
YERNENI
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V. RAO et al.
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Dimethyl popop ?,2’-p-Phenylenebis (4-methyl-5-phenytoxozole) H3C
\
P3
Kc00
Mel form.
:
CP6H2,N,0,
2810 1640 490 0 Intensity
Fig. 4. Energy level diagram of dimethyl POPOP. Levels marked with an asterisk are newly assigned from the ESSA spectrum, while the others are assigned from the ground state absorption or fluorescence spectra. The pairs of energy levels at 41280 and 413OOcm-‘, and 44280 and 44290cm-‘, though recognized as representing single levels, are shown separately in the figure to indicate the difference in their origins.
region spectrum in the the absorption The normalized fluorescence 23600-323OOcm-t. and absorption s(&s_ spectral wings rise Q(MQL steeply in the overlapping region 25 770-23 600 cm- ’ and intersect each other at 24 810 cm-‘. The energy corresponding to the point of intersection represents the height of the lowest vibrational level of the first excited singlet state S, above that of the ground state and the position of the lowest vibrational level of St state is shown accordingly in Fig. 4. Using this value for the lowest vibrational level of St, it is then possible to use the fluorescence spectrum to assign three vibrational levels of S, at 490, 1640, 2810 cm-‘.
(b) Excited singlet state absorption. The excited singlet state absorption of dimethyl POPOP solution in toluene at a concentration of 1 x 10m4M/l was measured in the region 4580-6880A following the same procedure as that used in the case of POPOP. The values of the relative differential absorption crosssection (a, -a,)J(u, -or),_ as well as the quantity W,),/n(c, -QlaX are plotted against wavelength in Fig. 5. The plots clearly show that the effect of the ground state absorption on the differential absorption between the ground and the first excited singlet states is negligibly small. It is, therefore, reasonable to treat the (a, - a,),/(~, -crg)max versus I plot as essentially the
Transient
Table
1. Transition
ESSA maximum submaximum or shoulder (vcm-‘)
assignments
137
excited singlet state in POPOP
to the observed features in the ESSA spectrum of POPOP solution (1 x 10m4 M/l) in ethanol
Designation
Transition assignment Between vibrational levels Electronic Te, u (cm-‘) state Lower Upper* Upper
21270 20 440 19080 17 850
Al A2 A3 A4
25 670 26480 21850 29 060
46 46 46 46
20 20 20 20
S3 S, S, S,
47 47 47 47
19 680 18310 17 120
B2 83 84
26 480 27 850 29 060
46170+ 10 46 170 f 10 461705 10
S3 S, S,
46 200 46 200 46 200
19910 19080 17720
Cl c2 c,
25 670 26 480 27 850
45 570 f 10 45 570 + 10 45 570 f 10
S, S, S,
16610 15240
02 03
26 480 27 850
43 090 43 090
S, S,
23 800::
El F1
25 670
49 470
S,
25 670
54 230
S”
28 560::
920 920 920 920
+ k f +
Ground state absorption maximum submaximumt or shoulder (cm I) 230 230 230 230
49 430
*These are the mean positions obtained by adding the wave numbers of the submaxima or shoulders in the ESSA to the vibrational levels of the state S, from which the corresponding transitions take place. t Blank space in this column indicates that the upper vibrational level Te, t’ lies in the region of strong S,, + S, absorption but no specific maximum, submaximum or shoulder is located at the corresponding position. : Magde et al.
spectrum as is done excited singlet state absorption here. The spectrum shows extensive absorption in the whole region covered in the present study. There are as many as five submaxima and seven shoulders in the spectrum, all of which are listed in Table 2. As in the case of POPOP it is possible to tentatively attribute all
Table 2. Transition
ESSA maximum submaximum or shoulder (vcm- ‘)
assignments
Designation
but one submaxima and the seven shoulders to transitions from the different vibrational levels of the first excited singlet state S, to those of another upper singlet electronic state S,. The transition assignments made are presented in Table 2 and shown in Fig. 4. Of the four new vibrational levels of S4 now assigned the level at 44 28Ok 20 cm- ’ coincides with the vibra-
to the observed features in the ESSA spectrum of dimethyl POPOP solution (1 x 10e4 M/l) in toluene
Transition assignment Between vibrational levels Electronic Te, v (cm-‘) state Lower Upper* Upper
20 400 19600 18210 17000
Al A2 A4 A5
24810 25 590 26 970 28200
45200+20 45200+20 45200+20 45 200 t 20
S, S, S‘% S,
20 070 19300 16690
Bl 82 B5
24810 25 590 28200
4489Ok 44890+ 448902
10 10 10
S, S, S‘+
17300 16 100
c4 c5
26970 28 200
44 280 f 20 44 280 f 20
S4 S‘%
18410 17 180
Dl D3
24810 26 040
43 220 43 220
SC! S,
15260
E3
26 040
41300
S3
Ground state absorption maximum submaximum t or shoulder (cm ‘)
44 290
41280
*These are the mean positions obtained by adding the wave numbers of the submaxima or shoulders in the to the vibrational levels of the state S, from which the corresponding transitions take place. t Blank space in this column indicates that the upper vibrational level Te, v lies in the region of moderately strong S, -+ S, absorption but no specific maximum, submaximum or shoulder is located at the corresponding position. ESSA
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A1
81
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A2 82
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Fig. 5. Excited singlet state absorption (ESSA) spectrum of dimethyl POPOP solution (1 x 10-t M/l) ir toluene in the region 4580-6880 A at room temperature. Different submaxima and shoulders in the spectrum, indicated by arrows, are designated by numbered letters, Al, A2, A3, Bl etc; the number in each designation represents the number of the vibrational level in the S1 state,‘from which the transition responsible for the corresponding submaximum/shoulder originates, while eatih letter marks out a spedific group of submaxima and/or shoulders resulting from transitions terminating in a common vibrational level in the upper singlet electronic state S, or S,.
tional level of S4 at 44 290 cm-’ identified from the ground state absorption whereas the remaining three levels (at 43 220,44 890 f 10 and 45 200 + 20 cm- ‘) lie in the region of relatively strong absorption due to S, -+ S4 transition. For this reason, these four upper levels might be assumed to be four different vibrational levels belonging to the electronic singlet state &. It can be seen from the energy level diagram (Fig. 4) that the transition from the third vibrational level of S, state at 26040 cm- ’ to the singlet electronic state S3 at 41280 cm-’ is expected to show up at 15 240 cm- ’ in the ESSA almost at the same position at which a submaximum (15 260 cm- ‘) is observed. It is worth noting here that in contrast to the situation in POPOP, the S4 state responsible for almost all the observed ESSA in this molecule is the same as the one involved in the transition giving rise to one of the weak ground state absorption bands.
out the region studied and consist of several submaxima and shoulders which represent the vibrational structure. In both the cases it has been possible to tentatively interpret all the observed features in terms of transitions between the different vibrational levels of the lowest excited singlet state S1 and two other upper singlet electronic states S, and S, or S,. According to this interpretation, the energy gap between the lowest and the highest vibrational levels of the S, state involved in these transitions is as much as 2390 cm - 1in each molecule, which is in sharp contrast to the assumption that the fluorescence transition originates from the lowest vibrational level of S,. However, in the present investigation the pump and the probe pulses interact simultaneously with the dye molecules during the period of their overlap (-67 ns). Although the molecules in the higher vibrational levels of S1 decay, by radiationless interactions, to the lowest vibrational level so fast as to render the steady-state populations of these levels negligibly
DISCUSSION
small, continuous production of molecules in these levels over the entire pump pulse duration ensures detectable non-equilibrium populations in them. And it should be possible for these molecules in the higher
The excited singlet state absorption spectra of the two dye molecules show extensive absorption through-
Transient excited singlet state in POPOP vibrational levels* of S, to undergo absorptive transitions along with those in the lowest vibrational level of the S, state during the period of overlap of the two pulses. The above assumption that the steady-state population of the higher vibrational levels of S, is negligibly small needs to be closely examined. It is valid only when the temperature is so low that the higher vibrational levels are not already thermally populated by the Boltzman distribution of molecules among the different vibrational-rotational levels of S, appropriate to that temperature. At the room temperature at which the experiments were carried out, however, the S, state has been found to be populated up to a height of 112Ocm _ 1 in POPOP and 960 cm ’ in dimethyl POPOP as evidenced by the short wavelength edges of their fluorescence spectra. Since the first excited vibrational level of the S, state in either molecule lies within the filled energy band width of that state, it is possible for that level to have a “non-negligible” steady-state population. If so the submaxima or shoulders in ESSA due to transitions from the first excited vibrational level should be somewhat stronger than the other submaxima or shoulders but much weaker than those due to transitions from the lowest vibrational 1evel.t But in the present studies, the following has been observed. In dimethyl POPOP (Fig. 5) the submaxima or shoulders due to transitions from the first excited vibrational level of S, are roughly of the same strength as those due to transitions from the lowest vibrational level, whereas they are stronger than all the submaxima or shoulders, except one, corresponding to transitions from the second and third excited vibrational levels. In POPOP the submaxima or shoulders due to transitions from the first vibrational level of S, are somewhat stronger than the shoulders due to transitions from the lowest vibrational level while they are weaker than one submaximum and about as strong as six submaxima or shoulders associated with transitions from the other higher excited vibrational levels of S, (Fig. 3). The explanation for this might be sought in the Frank-Condon principle, according to which the intensity of an absorption band is proportional to the square of the overlap integral formed for the two vibrational levels involved in the transition responsible for that
band.
The values
of the overlap
integrals,
in
*It is pertinent to note here that thermal populations of the different vibrational levels of the ground state undergo analogous transitions [19] to the first excited state S, reabsorbing part of the stimulated emission in a process known as self-absorption; this affects the optimum wavelength at which the dye laser operates. t It should, however, be emphasized that these expectations are strictly valid in a situation where there is no temporal overlap of the pump and probe pulses but in cases like the present one in which the two pulses overlap, the intensity distribution in ESSA spectrum will be decided, almost wholly, by the non-equilibrium populations in the different vibrational levels of S, with the “shaping” provided by the Frank-Condon factors.
139
turn, depend on the relative positions of the classical turning points of motion in the initial and the final levels of each one of the transitions. It is therefore possible for the overlap integrals of the transitions from the different vibrational levels of the S, state to have appreciably different values and hence for the corresponding submaxima or shoulders to have the observed intensities. Another interesting observation, which finds a plausible explanation based on the Franck-Condon principle, relates to the contrasting nature of the final states involved in the transitions responsible for the ESSA in the two molecules. In POPOP the final state of the transition associated with ESSA is the sarhe as the one involved in the strongest absorptive transition from the ground state while in dimethyl POPOP it is the one that is involved in the weakest absorptive transition from the ground state. If the potential minima of the two states So and S4 occur at about the same internuclear distance r,and that of S, at a greater r,then one expects, on the basis ofthe Franck-Condon principle, the pattern of intensities observed in POPOP for So + S1, S, + S4 and S, --) S, transitions. If, on the other hand, the minima lie at three different internuclear distances such that r,(S,) < r&S,) < r,(S,) one expects the intensity pattern observed in dimethyl POPOP. A plausible explanation for the relative variation in the strength of different transitions from S, as well as the observed difference (or otherwise) in the strength of these transitions compared with those from S, in each molecule can be found on the basis of parity/symmetry selection rules. The molecules POPOP and dimethyl POPOP do not have centers of symmetry and, therefore, no parity selection rules exist. Further, the highest symmetry point group expected for dimethyl POPOP is C, or C, and in either case, no symmetry forbidden selection rules operate. The highest symmetry point group possible for POPOP, on the other hand, is Cl0 with irreducible representations, A 1, A,, B, and B2. Here transitions between any two vibronic states whose product wavefunction forms a basis for the irreducible representation A, are forbidden while all others are allowed. However, as POPOP has 44 atoms, there will be 126 internal modes of vibrations and under low resolution symmetry selection rules do not give rise to any observable effects because of spectral congestion due to numerous overlaps in the vibronic spectrum. It would be interesting to carrry out high resolution work using polarization techniques with a view to study the several expected internal modes of vibration in molecules like POPOP. It is also informative to note that the ground state absorption band due to S, 4 S3 transition is stronger than that due to S, -+ S, transition suggesting that the electronic wave function of S1 is probably of A2 type as that of So is generally known to be of Al type. If so, pure electronic transition So -+ S, will be forbidden though transitions are allowed through vibronic coupling explaining the weak nature of the corresponding
Y ERNENIV. RAO et al.
140
absorption band. The transient absorption observed in the present experiment is fairly strong indicating that the product wave function of the two pure electronic states S1 and S3 is not of A2 symmetry. This points to the possibility that the wave function of S3 is of B, or B2 symmetry in which case the electronic transitions So --*S3 and S1 + S3 become allowed. As there are no such selection rules prohibiting transitions between any two electronic states in dimethyl POPOP the absorption band due to the So + S, transition in this molecule is strong unlike in POPOP. Acknowledgement-This work is supported by a grant from the US National Science Foundation (Grant No. Rll8409956). REFERENCES F. P. SCHAFER,in Topics in Applied Physics; 1 Dye Lasers. Springer, Berlin (1983). 1. WIEDER,Appl. Phys. Lett. 21, 318 (1972). E. SAHARand 1. WIEDER,Chem. Phys. ht. 23, 518 f:; (1973). c41 0. TESCHKEand A. DIENES, Opt. Commun. 9, 128 (1973). c51 E. SAHARand 1. WIEDER.IEEE J. Ouantum Electron. QE-10,612 (1974). C61 J. SHAH and R. F. LEHENY,Appl. Phys. Lett. 24, 562 (1974).
PI
R. F. LEHENYand J. SHAH,IEEE J. Quantum Electron. QE-I I, 70 (1975). 0. TESCHKEand A. DIENES,Appl. Phys. Lett. 26, 13 PI (1975). c91 0. TESCHKE, A. DIENES and G. HOLTOM, Opt. Commun. 13, 318 (1975). G. DOLAN~~~ C. R. GOLDSCHMIDT, Chem. Phys. Lett. WI 39, 320 (1976). Cl11 E. SAHAR,D. TREvEsand I. WIEDER,Opt. Commun. 16, 124 (1976). 0. TESCHKE,A. DIENEsand J. R. WHINNERY,ZEEE J. WI Quantum Electron. QE-12, 383 (1976). Cl31 E. SAHARand D. TREVES,IEEE J. Quantum Electron. QE-13, 962 (1977). and H. TASHIRO, Cl41 A. MULLER, J. SCHULTZ-HENNING Appl. Phys. 12, 333 (1977). A. PENZKOFER and W. KAISER,Opt. Cl51 W. FALKENSTEIN, Commun. 27, 151 (1978). Cl61 D. MAGDE,S. T. GAFFNEYand B. F. CAMPBELL,IEEE J. Quantum Electron. QE-17, 489 (1981). M. C. GEORGE, Y. V. RAO. H. C17al P. VENKATESWARLU. JAGANNATH and G. CHAKRAPANI,Pr&ana (J. Phys.) 28, 59 (1987). M. C. GEORGE,H. JAGANNATH, C17bl P. VENKATESWARLU, Y. V. RAO, G. CHAKRAPANIand A. MIAHNAHRI, Optical Studies in Laser Dyes, Tech. Rep. BMD DASG60-83-C-0060 (1985). Cl81 T. G. PAVLOPOULOS‘~~~‘P. R. HAMMOND, J. Am. them. Sot. 96, 6568 (1974). in Methqds ojExperimental Physics: Cl91 0. G. PETERSON, 15A Quantum Electronics. Academic Press, New York (1979. c71