J . Phys. Chem. 1993,97, 8146-8151
8146
Spectroscopy and Dynamics of Jet-Cooled 4-Aminobenzonitrile (4-ABN) Haiping Yu, Evelyn Josh, Ben Crystall, Trevor Smith,' Wayne Siclair, and David Phillips' Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK Received: November 24, 1992; In Final Form: April 5, I993
Laser-induced fluorescence (LIF) excitation and emission spectra and fluorescence lifetimes of jet-cooled 4-aminobenzonitrile (4-ABN) are reported. The dispersed fluorescence following excitation of transitions in the medium and high excess energy region in the SImanifold are examined. Strong Fermi resonances between the 1; (0; 807 cm-*) and 1: (0," 815 cm-l) bands are observed. The inversion mode dominates in most dispersed fluorescence spectra, confirming the importance of the fluxional amino group in 4-ABN. Broad and unstructured emission in this region is observed for 4-ABN under jet conditions, which may be associated with intramolecular vibrational redistribution (IVR). The relatively low onset of IVR in 4-ABN is attributed mainly to the activity of the fluxional amino group.
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I. Introduction A great deal of effort has been applied to the understanding of the gas-phasespectroscopy and dynamicsof substitutedbenzene derivatives, including 1,Cdifluorobenzene,1,2 4-fl~orotoluene,3.~ aniline,- substituted a l k y l b e n ~ e n e sbenzonitrile,l6 ,~~~ and methylamino species.17-19 Investigations have focused on the flow of energy within these photoexcited molecules and particularly on the effects that substitution in the benzene ring have on the rate and nature of these processes. A comprehensive study of IVR in jet-cooled alkylbenzenesgJOand related molecules (phenalkynes," phenoxyalknes,l2 and p-alkylanilinesl3), carried out by Smalleyandceworkers using the techniqueof singlevibrational level fluorescence (SVLF), and the recent study by Gruner and Brume+ of normal modes and their energy distribution of alkylbenzenes support the findingsthat therateof IVRis enhanced by an increase in the vibrational degrees of freedom of the alkyl side group, and that the traditional simple mode assignments based on the Varslnyi nomenclatureZoare considerable oversimplifications of the actual normal mode motion. Additionally a direct measurement of mode-dependent, dissipative IVR rates has been reported,21 which provides evidence that the onset of IVR is dependent not only on excess vibrational energy but also upon which vibrational mode carries the initial excitation.22.23 As part of a continuing study of the amino-substituted benzonitrilechromophore,an account is given here of spectroscopy and dynamics of jet-cooled 4-ABN. The basic spectrosopy of 4-ABN and its complexes with various partners has been reported ear1ier,l7J8where it was shown that spectroscopically 4-ABN closely resembles aniline, rather than benzonitrile, and that the amino inversion mode is very active in both excitation and emission spectra. However information is scant on the relaxation processes following excitation of the medium and high excess energy region above the SIorigin. Here we present information on the normal modes of jet-cooled 4-ABN in the region up to 3000 cm-1 excess energy above the electronic origin, compared with vapor phase absorption mea~urements,2~ infrared st~dies,2~.2~ and earlier measurements in the jet.17.18 The dependence of the onset of unstructured emission upon excess energy and density of states as well as the influence of the nature and symmetry of coupled modes upon the rates of relaxation processes is also investigated. 11. Experimental Section
A. LIFSpectra Measurements. The continuousflow expansion supersonic jet apparatus used has been discussed in detail Preaent address: Schoolof Chemistry,University of Melbourne, Parkville 3052, Victoria, Australia. f
0022-3654/93/2097-8 146304.00/0
elsewhere.27 Briefly, the sample of 4-ABN was heated in a stainless steel oven and then mixed with helium carrier gas at a backing pressure of typically 4 bar to expand into the vacuum chamber through a nozzle of 100-rm orifice. An excimer-pumped dye laser system (Lambda Physik EMG103 MSC/FL2002) was used as the excitation source crossing the molecular beam at right angles 4 mm downstream from the nozzle. Fluorescence was detected mutually perpendicular to both laser and the molecular beams, and the dispersed fluorescence spectra were recorded by dispersing the total fluorescence into a monochromator (Rank Precision 1000). The signal from the PMT was fed into a boxcar averager (Stanford Research System)and interfacod to a personal computer. The sample of 4-ABN (98%) was obtained from Aldrich Ltd. and was used without further purification. B. Decay Measurements. Fluorescence decays were recorded using the well established technique of time-correlated single photon counting (TCSPC).2* The excitation source used was the frequency-doubled output of a synchronously pumped, cavitydumped dye laser (Coherent Antares Nd:YAG/Coherent CR 590). The signal from the PMT (XP202OQ) was fed via a X10 amplifier (R9009) to a constant fraction discriminator (Tennelec TC 453) to start thevoltage ramp of a time-to-amplitudeconverter (TAC). The TAC (Ortec 457) was operated in reverse mode and stopped by the laser pulses. The output pulse from the TAC was fed into a multichannel analyser (MCA, Canberra 30) and transferred onto a personal computer for analysis. Instrument response functions were recorded periodically to reflect any changes in the system over time, and thus realistic deconvolution was possible. The time resolution of the system was estimated to be approximately 400 ps, which was limited by the resolution of the PMT, and was sufficient for the experiments described here.
III. Results A. LIF Excitation Spectrum and Ro-vibronic Band Contours. Figure 1 shows the laser-induced fluorescenceexcitation spectrum of jet-cooled 4-ABN. It is in good agreement with that reported by Gibson et al.17J8in the first 1000 cm-', displaying an intense origin transition 0; at 33 493 cm-l and seven intense vibronic transitions in the first 1170 cm-l. In addition many less intense bands are reported here for the first time together with the higher energy excitation spectrum in the region up to 3000 cm-1 above the origin. The S1 SO electronic excitation in 4-ABN corresponds to the optically allowed lB2(mr*) 'AI transition in point group Ca. Thus al vibrational modes are expected to
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0 1993 American Chemical Society
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Jet-Cooled 4-ABN
-100
100
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8147
300
700
500
900
Relative enagy (an-1)
I
800
-
lo00
900
1100
1200
1300
1400
1500
amgy (-1)
I
x3
2100 UOO 2300 2400 2soO
2600
2700 2800
2900
Rdativc enagy(cm-I)
Figure 1. Laser-induced fluorescence (LIF)excitation spectrum of jet-cooled 4-aminobcnzonitrile(CABN). The electronic transition corresponds to the l B z ( r r * ) + 'A, transition showing the origin 0; as the most intense band at 33 493 cm-l.
TABLE I: Bands Observed in the Laser-Induced Fluorescence (LIF) Excitation Spectrum of Jet-Cooled 4-Aminobenzonitrile (QABN)' 6v (cm-1) re1 intens -33 7 5 -28 0 100 224 6 382 65 494 40 678 33 742 2 761 8 781
7
807 815
59 74
877
7
1012 1045 1059 1068 1165
16 10
1174 a
13 17
74 10
assignt
bv (cm-1)
1460 1486 1490 1493 1512 1537 1592 1605 1615 1625 1630 1640 1649 1658 1672 1694 1737
1761 1764
re1 intens 10 10
assignt
22 15 4
7 4 19 21 13 33
4 3
9 6 12 4 7
bv (cm-1)
1191 1199 1207
1224 1240 1246 1255 1276 1305 1312
1336 1347 1402 1423 1434 1438 1442 1450
re1 intens 28 44 8
assignt
bv (cm-1)
1773 1781 1783
34 13 8
1796 1809
10
1826 1830 1840
7
9 39 13 3 3 4 11 13
4
1711
1872 1878 1950
1963 1980 1989 2013 2037
re1 intens 4
assignt
8 9 6 9 7 11
10 7
20 10 7 2 24
8 10 10
13
8
bv is the shift from the electronic vibrationless origin 0; at 33 493 cm-l. Assignments are based on Varshyi's nomenclature.m
be active in absorption from the SOvibrationless level and in emission from SIvibronic levels. Many of the intense bands have been assigned as vibronic transitions in these totally symmetrical modes and their combinations in the excited state, although some overtones of non-totally symmetric vibrations are also observed. The assignments were carried out by analogy with the vapour phase absorption spectrum,24 the Raman spectrum,24 and the infrared spectra.25.26 Bands are assigned in accordance with Varsinyi's nomenclaturem to allow comparison with previous work. The energies, relative intensities and assignments of observed transitions are listed in Table I. Since the transition 'Bz (m*) 'A1 is short axis polarized, the 0; band exhibits a double-headed, B-type, rotational contour, similar to those observed in jet-cooled aniline' and jet-cooled benzonitrile.16 All of the intense vibronic bands exhibit similar double-headed
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structure and thus correspond to totally symmetric transitions. Some ro-vibronic band contours are presented in Figure 2. The origin transition of 4-ABN occurs at 33 493 cm-I, 1771 cm-1 higher in energy than for 3-ABN.19 Meta substitution with an amino group thus lowers the electronicorigin of the benzonitrile chromophore to a greater extent than para substitution. A similar effect has been observed for the related isomers.Cdimethylaminobenzonitrile (CDMABN) and 3-DMABN,29 where the corresponding frequency difference was 2179 cm-'. Spectroscopic similarities between 4- and 3-ABN are limited, as expected, due to the different symmetries of the two molecules. It is evident that pcyano substitution produces only a few changes in the vibrational structure relative to anilinez-8.13 and to 4-methylaniline.13 Only ring modes involving motion in the ring carbon at the point of substitution are expected to shift greatly in frequency
Yu et al.
8148 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
I
4
2
0
-2-4
nl.tivr nmwICm-1
-2.
Figure 3. Dispersed fluorescencespectra of jet-cooled CABN follouing excitation of the transitions occurring at exas8 energies of (a) 807, (b) 815, (c) 1191, and (d) 1199 cm-1.
as a result of substitution. Hence the ring breathing mode, 1, shows little dependence on the substituent for 4-ABN, aniline and 4-methylaniline with frequencies of 8 15,817, and 796 cm-1 in the excited state. Other ring modes show a substantial reshuffling produced by the para substitution. For example, the 6a mode in 4-ABN is lower in energy relative to that in Cmethylaniline, which is in turn lower in energy than that in aniline (382,491, and 433 cm-l), due to the increasing mass of thesubstituent. The 12mode is particularly effected by this with a substantial lowering in energy relative to aniline of 955 to 678 cm-l, similar to the case of the 13 mode from 1311 to 1165 cm-l. Little dependence on substitution is predicted for the inversion mode as this transition is reasonably well localized in the amino group. Variation in relative frequencies of this vibration for the above three molecules is small (807, 759, and 737 cm-l). B. Dispersed Fluorescence Spectra. Dispersed fluorescence spectra from the lower frequency fundamentals are in good agreement with those reported17J8 and illustrate again the similarities between 4-ABN, aniline, and 4-methylaniline (see Table 111). The fluorescence spectrum following 0; excitation is dominated by the 6a mode,in progression and in combination with modes 1,9a, 12,13, and 18a and with I:, which are in line with Gibson’s res~lts.1~ Transitions involving the 6a and 12 modes are prominent in dispersed fluorescence spectra following excitation of all transitions in 4-ABN. Dispersed fluorescence spectra following excitation of the intense vibronic modes 0 : 807 and : 0 + 8 15 (1;) cm-1 are presented in Figure 3. The fluorescence spectrum from the 0; 807 cm-l level is anomalous in that the most intense transition occurs at 1365 cm-l, attributed to I: with the other intense transitions at 1779, 2199, and 2617 cm-1 assigned to I: in combination with the 6a&6a;, and 6a; modes respectively. Theinversion frequency (v” = 43Ocm-l) observed here for 4-ABN is comparable in magnitude to that of aniline (v” = 417 cm-l), suggesting a similar amino group geometry in the two molecules
and indicating a similar degree of nonplanarity. The emission from the 0; 815 cm-1 band displays characteristic symmetric a1fundamentals, similar to the fluorescence from the origin. The inversion mode is particularly active in this spectrum, with the 1 and 1Ac transitions dominating the spectrum. This unpredicted behavior suggests the presence of coupling between this vibration and the close lying amino group inversion. Fermi resonances could play an important role in coupling the ro-vibronic 1: and 1: modes. Dispersed fluorescence spectra following excitation of the intense 0; 1191 and 0; 1199 cm-1 transitions were recorded and are also presented in Figure 3. The overall form of the spectra closely resemble those of fluorescence from I: and 1; respectively, which supports the assignment of these transitions to If6aA and lA64. Both spectra show a clear combination of discrete vibronic transitions superimposed on a rising congested background. Here the redistributed, broad background is relatively enhanced compared to that from excitation of the and 1; modes (Figure 3a, and b), with very little change in the spectral envelop and an appearance of extensively red-shifted @-level fluorescence, which can be associated with IVR. The assignments of these spectra in Figure 3 with relative intensities and shift to the excitation light positions are listed in Table 11. Figure 4 shows fluorescence features following excitation of transitions occurring with an e x w s energy of 1224, 1312, and 1336cm-I. The fluorescence spectrum resulting from excitation at 0; 1224 m-1, like that from exenergies of 493 and 807 cm-1 (lobi and modes), in anomalous in that few transitions occur at low relative energies. This transition is therefore not assigned as an a1 fundamental, but to excitation of a single quantum of the 14 mode (of b symmetry). The most intense discrete band, occuring at 1750cm-l below the excitation energy, is assigned to the 14;6ay combination, thus confirming the assignment of this excitation band as 14;. In contrast to the above spectrum, excitation at an excess energy of 0; + 1312 cm-1 results in prominently discrete fluorescence. The most active transition at a shift of 1307cm-1 is the first member of the strong progression in the 6a mode on 1. Other prominent modes include
ShM trom band center (cml)
O b e e r v e d r ~ v i b r o n i c b a n d c o n t o m o f t h‘cfeatures es~ occurring at excess energies above the electronic origin of (a) 0, (b) 1165, (c) 1224, (d) 1630, and (e) 2127 cm-1 in the LIF excitation spcctrum of jet-cooled CABN. All these exhibit a B-type contour, which indicates the electronic transition to be short axis polarized.
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+
(g)
+
:c
+
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The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8149
Jet-Cooled 4-ABN
I
"
tJ I
2wa
1 m
0
-lm
-2mo
0
-1m-2oO0Jooo4oQodooQ
nl.tivr mugy Ian-1
nl.tivr .Imgy/cml
F'igure 4. Dispersed fluorescencespectra of jet-cooled 4-ABN following excitation of the transitions occurring at excws energies of (a) 1224, (b) 1312, and (c) 1336 cm-l.
combinations in the 10bmode, which further confirms the excited state assignment of the 1;lObicombination mode. fluorescence from the 0; 1336 cm-1 band is largely redistributed, with the most dominant features being a transition at a relative energy of 1454 cm-1, which corresponds to Au = 0, the 12: transition, and the 6a progression from this. Dispersed fluorescence was also recorded upon excitation at higher excess energies (1434, 1438, 1456,1618,and 163Ocm-1). Thesespectraaredepictedin Figure 5 and are characterized by the absence (or low intensity) of disrete fluorescence, and by spectral envelopes which change very little as the excitation energy changes and have the appearance of extensively broadened, red-shifted @-level fluorescence. A number of the observed fundamentals vibrations of 4-ABN are listed in Table 111, compared to those of aniline,5 6*8 13,20 4-MA,13 and b e n z ~ n i t r i l e . ~ ~ ~ ~ ~ C. Decay Measurements. Table IV lists the fluorescence lifetimes at the selected excitation energies. Decays were fit to functions of the form
+
1
I ( t ) = A, exp(-t/T,)
I
1
2mo1m
3ooo
+ A, exp(-t/T,) + B + C
where A, B, C and 7 are adjustable parameters; A = the preexponential factor, B * the background, and C = the shift between the fluorescence decay and the instrument response function. Laboratory written software was used in the analysis of decays, which involved iterative dcconvolution using the Marquardt algorithm. All levels exhibited exponential decays with two components. A short lifetime appeared with relative yields of 55%and was attributed to background fluorescencefrom residual "hot" sample situated inside the chamber. Due to this all decays were analyzed globally. The short component was set to be equal over the data set, while the longer lifetimes were allowed to vary freely relative to the others. All preexponential factors were allowed to vary. This analysis method has the advantage of allowing the "true" trend in lifetimes to be examined, excluding any possible perturbing effects the sccond constant lifetime may have and reduces the effects of parameter correlation. The lifetimes at various low energy excitation bands compare well with those reported by Gibson et al.32 (Table IV). The possible
Figure 5. Dispersed fluorescencespectra of jet-cooled 4-ABN following excitation of the transitions occurring at excas energia of (a) 1434, (b) 1438, (c) 1618, (d) 1630, and (e) 1456 cm-l.
reason for the systematic small difference in results is due to the different convolutionproceduresused. 71for the origin transition is 13 ns and is approximately four times greater than that found in nonpolar solvent at room temperature where 71 is 3.3 ns in hexane (& = 0.14 and kf= 4.1 X lO'/~).3~This implies that & i n the jet for the zero point level 00 is 13/3.3 X 0.14 = 0.55. A large proportion of the initially excited molecules thus decay via some nonradiative process. The onset of IVR can be accompanied by anomalies in fluorescence decay curves;23 in the high energy region where a dense manifold of background levels are coupled, the so-called "dissipative" case, the IVR is associated with a fast subpicosecond component of fluorescence, plus longer, "normal" decay time corresponding to electronic relaxati0n.3~In the present case, that relaxation includes nonradiative decay, almost certainly to the triplet state, and it might be expected that there would be some mode-specificity for this process; hence, the overall relaxation rate would be expected to be influenced by IVR. In the dissipative case, strong mode-dependence would be absent (saturation) and there is some evidence for this at the highest excess energies around 1400 cm-1 and above, where the decay time is reasonably constant at 8.6-8.1 ns. In the intermediate excess energy region where IVR and intersystem crossing will have strong modedependence, the overall decay rate should be mode-dependent, as is seen to be the case in Figure 6.
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IV. Discussion Fluorescence s p t r a can be divided into three groups, corresponding to low, medium, and high energy excitation. Dispersed fluorescence following the excess energies below 0; 1150 cm-1 consists of discrete bands relatively free from congestion, in which all the major features are assigned in terms of the optically active ground-state vibrational modes of the initially prepared excited state. This indicates the absence of mode-mixing. Fluorescence lifetimes show a weak dependence on excitation energy and decrease progressivqly from 12.99 to 10.69 ns over the 1150-cm-l range. All these features suggest that in this energy regime the coupling of the optically prepared vibrational levels to other levels in the SImanifold is minimal.
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8150 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
Yu et al.
TABLE II: Bands Observed in the Dio rsed Fluorescence Spectra of Jet-Cooled QABN Following Excitation of the Tramitions Occur?ing at Excm Emeq#ea of (a) 80r(b) 815, (c) 1191 and (d) 1199 cm-'*
g excitation+: + 807 cm-1
g 6 4 excitation+:
re1 bu (an-1)
intens
assignt
+ 1191 cm-l
re1 intens
6v (cm-I)
assignt
0
0
6v (cm-1)
re1 intens
assignt
0 418 821 837 1105 1136 1221 1239 1253 1298 1306 1430 1514 1594
1779 1806 2050-2 120
1897 1964 2127 2190
1771 208 1 2140
2321 2376-243 1 2542-2594
2185 2510 2604
denotes the possible assignment for the 1l:l$
transition.
TABLE III: Com II of the Active Fundamentab Observed in the Spectra of Jet-Cooled QABN and Related Molecules 4-ABNb
Mode
J
6a 6b 10b 1 12 13 18a 9a 8a I(u = 2) I(u-4)
382 412 491,c492d 247 815 678 1165
J'
273 886 728 1223 1039 1275 1592 1593 807 430 1365
4-MA
aniline Jf
J
539
(177) 796.c798d 9552 1311,d1307'
759,'761d 1729h
J
Jf
433c 479c
benzonitrilc J Jf 459#
233,'217* 824c 817C 851' 946f lOOlr 10135 70Y 769$758# 1278' 119D 1040C 128Y 12810 417s 1089h
737C 4430
Vadnyi's notation for benzene derivatives.20 Present work. Reference 13. Reference 8. * Reference 5. f Reference 30. # Reference 31. b Calculated value from ref 6.
A marked increase in thedensity of intensetransitionsis evident in the LIF excitation spectrum above 1150 cm-l (see Figure 1). In addition some overtone and combination bands show large anharmonicities, such as the 12; transition, which occurs at 0; + 1336 cm-I, some 20 cm-l lower in energy than predicted. SVL fluorescence spectra following excitation of excess energies between 0; + 1150 to 0; 1400 cm-1 are characterized by congested, though resolvable, structure that occur both near to and to the red of the 0; transition energy. In addition some of the emission spectra exhibit significantly intense fluorescence bands, which are assignable in terms of transitions from the optically prepared level. The extent of resolvedversus unresolved fluorescenceis found to be strongly dependent upon the character
+
assignt
1632 1643 1669 2015 2025 2043 2060 2075 2193 2448 2490 2513
2617
aX
re1 intens
6u (cm-1)
0
418 1225 1316 1357 1482 1551 1612 17W1770
+ 1199 cm-l
$ 6 4 excitation+:
416 886 917 1039 1280 1299 1311 1360 1688 1717
408 425 1232 1280 1365 1640-1 7 10
2199 2460-2500
+ 815 cm-l
$ excitation+:
TABLE Iv: Fluorescence Decay Times of Jet-Cooled QABN aa a Function of Excm Energy excess energy (cm-9
0
7r (ns)
13.0 13.0/12.6 12.0 10.9 11.1 10.6 11.1
382 678 807 815 1012 1165
722
(ns)
14.4 13.4 12.8 11.4 10.2
excess energy (an-')
7r (ns)
1191/1199 1224 1312 1435/1438 1451/1460 1490/1493 1615/1630
8.9 9.3 10.4 8.6 8.2 8.6 8.1
7?2
(ns)
of the initiallyexcited vibrational motion. Thedispersed emission spectra from bands at 0; 1199 and 0; + 1312 cm-1 (assigned as lA6a; and @Ob; respectively) display a much greater proportion of resolved fluorescence than do the emission spectra from the bands at 0; + 1224 (14;) and 0; 1336 (1%) cm-1, the fluorescence from which is largely unresolved. Transitions involving combinationswith the 1mode thus exhibit more resolved fluorescence than other transitionsofsimilar energy. The emission from the 0; 1191 cm-1 (G6ai) band presents some combination bands, fundamental and overtone, with other optically active modes. Thus the bands at 1225, 1316, 1551, and 2127 cm-' correspond to the modes I;6a;13:, 1:641:, Ii6ai12: and Ii6ail: respectively. There is a rather intense band at 1482cm-1, which is not assignablein terms of the optically active fundamental and overtones, or combination bands, implying modecoupling and modemixing in this region. However, the appearance of a few discrete bands indicates that the number of coupled levels is small, i.e., on the order of 10 or less. Features of other SVL emission spectra in this energy region support the picture of optically prepared states coupled to a coarse manifold of levels.
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+
+
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8151
Jet-Cooled 4-ABN
presence of the fluxional amino group. It is unclear whether this effect is simply a result of the subsequent increase in state density caused by low frequency bath modes or whether the fluxional metion has a similar role as that of the methyl rotor in 4-fluorotoluene,3 coupling vibrations through van der Waals interactions with the benzene ring, or both. Normal mode calculations do however reveal a profound influence of the amino group in the vibrations of the benzene frame. A certain amount of mode selectivityis evident in the relaxation processes occurring in jet-cooled 4-ABN, with transitions involving combinations with the “1” mode in the intermediate energy region exhibiting proportionally more discrete fluorescencethan that of other modes in the comparable energy region, which shows that factors other than density of states may influence IVR in this region.
Acknowledgment. We would like to thank the SERC for the financial support necessary to carry out these experiments. We are also grateful to Coherent (U.K.) Ltd. for the provision of a CASE studentship for E.J.
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References and Notes
‘i
(1) Covelski, R. A.; Dolson, D. A.; Parmenter, C. S . J. Phys. Chem. 1985, 86, 645, 655.
s
0
1100 Ex-
lo00 1600 .Imw Icml)
zoo0
F’igure 6. Fluorescence decay rate (l/sf)as a function of SIvibrational energy. N.b. the lines connecting points have no significance.
All the spectra in the region 115&1400 cm-l, though congested, have resolvable structure in the spectral region near the 0; transition energy, even though this distribution of emission bands strongly differs from spectrum to spectrum. Figure 6 shows the plot of fluorescencedecayrate (1/ ~ fversus ) excess energy. There is a fluctuation in decay rate in the region between lo00 and 1500 cm-l, for example the bands at excess energies of 1224 and 1312 cm-1 have fluorescence lifetimes of 9.3 and 10.4 ns respectively; coincidentally,the correspondingdispersed fluorescencespectrum following excitation of 0; 1312 cm-1 band has a higher ratio of structured to unstructured emission than that following excitation of 0; 1224 cm-l band. Fluorescence decay curves observingresolved fluorescencefollowingexcitation in this region might be expected to exhibit quantum beats. Such experiments are currently under investigation. Dispersed fluorescence spectra corresponding to excitation in the SImanifold to vibrational energies greater than 0; 1400 cm-l are characterized by the absenceof (or very little) structured fluorescence and by spectral envelopes which change very little as the excitation energy changes and appear as extensively broadened, red shifted @-level fluorescence. The broadness of the spectral features is almost certainly due to the initially populated level coupling with the large number of zero-order levels in S1. Additionallymeasured fluorescence lifetimesat exctss energies greater than 1400 cm-l become relatively constant, at about 8.4 h 0.2 ns, which is in line with the lifetime saturation effect observed by Zewailet a1.34for anthraceneat energiesabove 1800 cm-1. This trend seems to be indicative of extensive IVR and is consistent with a large distribution of emitting levels, as is the case in dissipative IVR.
+
+
+
V. Conclusion The spectral and temporal measurements reported here give a reasonably coherent picture of the spectroscopy and dynamics of jet-cooled 4-ABN. A relatively low energy threshold observed for intramolecular vibrational relaxation processes in 4-ABN, similar to that reported for 3-ABN,19is explained in terms of the
(2) 1119. (3) (4) 86, 51. (5) (6)
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