J. Phys. Chem. 1989, 93, 101-107
101
in Table I. The most dramatic reductions upon electronic excitation occur for the out-of-plane bending modes, indicating that the molecule becomes far more floppy in the 'BZustate, in accord with the a,-a,* nature of the transition.
Moore for providing us with his fluorescence excitation data in advance of publication and for several helpful discussions. This work is supported by the U S . Department of Energy, Office of Basic Energy Sciences.
Acknowledgment. We thank Dan Ferko for his technical assistance in the experimental portion of this work and Prof. Robert
Registry No. p-DCB, 106-46-7;He, 7440-59-7; 'Tl, 13981-72-1; 37Cl, 13981-73-2.
Fluorescence Excitation Spectroscopy of Jet-Cooled p -Dichlorobenzene and p -Dichlorobenzene-d, W. D. Sands? and R. Moore* Department of Chemistry, Virginia Commonwealth University, VCU Box 2006, Richmond, Virginia 23284-2006 (Received: March 18, 1988; In Final Form: June 7, 1988)
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In this paper are reported the fluorescence excitation spectra ('A, 'B2J of jet-cooled p-dichlorobenzene and p-dichlorobenzene-d,. The spectra of both molecules exhibit much vibronic activity. In all, the vibrational frequencies of 11 normal modes are determined in S1p-C6H4C12. The frequencies of 10 normal modes are determined in SIp-C6D4C12.Chlorine isotope effects are observed in those vibronic transitions that include normal modes in which the Cl atoms undergo substantial displacements. The measured C1 and deuterium isotope shifts in S1 are in good agreement with those determined from experimental data and ab initio calculations of vibrational frequencies in So.
-
Introduction The near ultraviolet absorption spectra (So S,) of the dichlorinated benzenes were first interpreted in the early 1940s by Sponer.I Since that time other workers have offered vibrational analyses of the and low-temperature neat crystal, absorption spectra of p-dichlorobenzene. These analyses were hampered by spectral broadening. In the present paper we report the fluorescence excitation spectra of p-dichlorobenzene and p-dichlorobenzene-d, in a supersonic jet expansion. The spectra of these molecules are rich in vibronic structure, revealing a complexity that was obscured by spectral congestion in the prior studies.14 The results require a reinterpretation of the spectrum of p-dichlorobenzene and a reassignment of SI vibrational frequencies. We also assign the spectrum of p-dichlorobenzene-d4 and list the SI vibrational frequencies. Experimental Section Fluorescence excitation spectra of p-C6H4C12(Aldrich, 99+%) and p-C6D4C12(ICN Biomedicals 98% D / H ) were obtained by using a pulsed supersonic jet apparatus. A 4-in. cylindrical cross chamber was employed with a 6411. oil diffusion pump. During a run chamber pressures were maintained in the range 10-3-10-2 Torr. The pulsed nozzle was a General Valve Series 9 axial flow valve with a 0.76-mm nozzle diameter. The nozzle was pulsed a t the laser repetition rate of 20 H z by a home-built pulse generator. A backing pressure of 3 atm of helium was used for all experiments. The helium was passed over a pulverized room temperature sample of p-C6H4C12or p C & c l 2 that was contained in a replaceable element filter located immediately before the nozzle. The nozzle typically was placed 15-24 nozzle diameters from the excitation source, which consisted of a frequency-doubled Quantel TDL-50 dye laser. The dye laser was operated by using rhodamine 560, basic fluoroscein 548, or coumarin 500 (Exciton). When rhodamine 560 or fluorescein 548 was used, the dye laser was operated in a narrow-band mode, with the band d d t h of the doubled laser -0.15 cm-I. When Coumarin 500 was used, the laser was operated in a broad-band mode, with laser band widths 'Permanent address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA.
0022-3654/89/2093-0101$01.50/0
of about 1 cm-'. The laser beam entered and exited the chamber through 18-in. baffle arms equipped with Brewster windows. Fluorescence was detected at 90° to both the jet and the laser beam. Fluorescence was collected by an f/1 lens, which imaged the fluorescence through a 1 X 7 mm vertical slit and onto the photomultiplier tube (EM1 9558). A Schott WG-295 filter was placed immediately before the photomultiplier to discriminate against the excitation source. Signals from the photomultiplier tube were processed by a PAR 162/166 boxcar signal averager and displayed on a strip chart recorder. The spectra are not corrected for laser intensity. Excitation wavelengths were determined by measuring the laser wavelength with an arc lamp calibrated 0.85-m monochromator (Spex) and are estimated to be accurate to within f l cm-I. Results and Discussion The fluorescence excitation spectrum of p-C6H4C12is shown in Figure 1, and a listing of the vibronic transitions and assignments is given in Table I. The spectrum of p-C6D4C12is shown in Figure 2, with the corresponding vibronic transitions and assignments listed in Table 11. We observe no vibrational hot bands in either spectrum and estimate a rotational temperature of 1 5 K based on a preliminary rotational contour analysis. The vibrational frequencies of p-C6H4C12 and p-C6D4C12in soand SI are listed in Tables I11 and IV, respectively. The Mulliken notation is used to label the normal modes of the molecules. For correspondence with forthcoming papers5,6 on O-C&C12 and mC&C12 the Wilson notation is also provided in the tables. Higher resolution scans of the fluorescence excitation spectra of p-C6H4C12and p-C6D4C12are shown Figures 3-7. Vibronic assignments are also given in the figures. The axis system for p-C6H4C12is chosen as follows: the z axis is along the C-Cl bonds, they axis is in the plane of the molecule bisecting the C-C bonds, and the x axis is perpendicular to the plane of the molecule. The authors of ref 1 and 2 used different axis conventions. In our discussion of their work we have rotated (1) Sponer, H. Rev. Mod. Phys. 1942, 14, 224. (2) Anno, T.; Matubara, I. J. Chem. Phys. 1955, 23, 796. (3) Rai, J. N.; Upadhya, K.N. Indian J . Pure Appl. Phys. 1970,8, 352. (4) Castro, G.; Hochstrasser, R. M. J. Chem. Phys. 1966, 46, 3617. ( 5 ) Moore, R.; Jones, L. F.; Sands, W. D.; Shillady, D. D., manuscript in
preparation. (6) Sands, W. D.; Moore, R., manuscript in preparation.
0 1989 American Chemical Society
102 The Journal of Physical Chemistry, Vol. 93, No.
I, 1989
Sands and Moore
x
Y
. I
5
Y
Frequency (cm-')
Figure 1. Survey fluorescence excitation spectrum of p-C6H4CI2in a He expansion. This spectrum and all others shown in this paper are linear in wavelength but are plotted on a wavenumber scale. The break in the spectrum occurs where the laser dye was changed from rhodamine 560 to coumarin 500.
Figure 2. Survey fluorescence excitation spectrum of p-C6D4Cl2in a He expansion. The break in the spectrum occurs where the laser dye was changed from fluorescein 548 to coumarin 500.
297
302
G (cm-1) -40
0
100
200
300
400
Frequency (cm-')
Figure 3. Fluorescence excitation spectra of p-C6H4C12(a) and p C6D4C12(b) in the region 0-400 cm-' from the electronic ongins. Bands
marked with an asterisk in the p-C6D4CI2spectrum are due to impurity p-C&D$I* in the sample.
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their axis systems to coincide with ours. The So S1 absorption spectrum of gas-phase p-C6H4C12was first reported by Sponer.' She identified the transition as 'A, lBZuand assigned a few vibronic bands, including a vibronically induced b3, fundamental at 531 cm-' above the origin. A more complete vibrational analysis of the spectrum was provided by Anno and Matubara.z From their spectrum they assigned five a, fundamentals and a b3, fundamental. They also assigned other strong features to be. overtones of non-totally symmetric vibrations. Rai and Upadhya3 obtained a high-resolution absorption spectrum of the vapor and from their data assigned four a, fundamentals and a bjg fundamental. Castro and Hochstrasser reported the absorption spectrum of the neat crystal a t 4.2 The vibronic structure of the crystal spectrum is similar to the vapor. They
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Figure 4. High-resolutionscan of the transition 6; in p-C6H4C12.The three bands are due to the natural distribution of C1 isotopes in the
sample. assigned four a, fundamentals and three b3, fundamentals and reassigned one of Anno and Matubara's a, fundamentals to be an overtone of a bSufundamental. These previous assignments are summarized in Table V. The absorption spectrum of p C6D4ClZhas not been measured prior to this report. At the beginning of the preparation of this report, we became aware of the resonantly enhanced multiphoton ionization spectra of p-CsH4ClZby Rohlfing and R ~ h l f i n g . ~They utilized massselective detection, thereby simplifying the spectroscopy by removing inhomogeneous broadening due to the natural distribution of C1 isotopes. In addition to corroborating our assignments, they assigned some bands that we had observed but had been unable to identif). Bands that we observe and were assigned by Rohlfing and Rohlfing are listed Table I for the sake of completeness and include the following: 16;, 831 cm-'; 6;26;, 838 cm-I; 8;26& 870 ( 7 ) Rohlfing, E. A,; Rohlfing, C. M. J . Phys. Chem., preceding paper in this issue.
Jet-Cooled p-Dichlorobenzene and p-Dichlorobenzene-d4
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 103 TABLE I: Vibrational Band Assignments for So P -C~HICI~
vibr energy, cm-'
400
500
600
700
800
Frequency (cm-')
Figure 5. Fluorescence excitation spectra of p-c6H4CI2(a) and p -
C6D4C12(b) in the region 400-900 cm-i from the electronic origins. Bands marked with an asterisk in the p-C6D4C12spectrum are due to impurity p-C6HD3Ci2in the sample.
Frequency
(ern-')
Figure 6. Fluorescence excitation spectra of p-C6H4C12(a) and. p C6D4C12(b) in the region 900-1200 cm-' from the ekctronic origins. Breaks in the spectra occur where the laser dye was changed from rhodamine 560 or fluorescein 548 to coumarin 500.
0 151 296 299 302 335 341 539 588 633 637 641 672 694 729 831 835 837 841 872 877 1021 1025 1029 1054 1060 1064 1082 1087 1093 1139 1207 1219 1227 1261 1384 1393 1457 1472 1601 1753 1782 1794 1815 1821 1987 2105 2118 2134 2140 2196 228 1 2316 2523 2536
comb freq, cm-I
-
SISpectrum of
interpretation 00,
637, 643 676 5; 163 841
6b26P (?)
874 880 1025 1028 1031 4:, 1064 1070
1268 1389 1395 1458 1593 1760 1783 1799 1816 1822 1997 2108 21 18, 2124 2201
5;24;
2322 2526
4'5'26' 4x2;
Reference 7.
1200
1200
1300
1400
1500
1300
1400
1500
Frequency (crn-l) Figure 7. Fluorescence excitation spectra of p-C6H4C12(a) and p C6D4CIz(b) in the region 1200-1500 cm-' from the electronic origins.
cm-'; 5;30:, 877 cm-'; 14;, 1093 cm-l. They also detect a weak band at 452 cm-' that they assign to 22;. We do not observe this feature in our spectrum. The vibrational assignments given in Figures 3-7 and in Tables I and I1 are based on correspondence with known ground-state frequenciess-l0 and on C1 and D isotope shifts. Isotope shifts for a given normal mode are compared to those derived from experimental data9 and from ab initio calculations of So vibrational frequencies and normal modes. Observed and calculated isotopic or vshift ratios (defined as v(C6D4C12)/~(C6H4C12) (C6H437C1,)/v(C,H,35C12)) for normal modes relevant to the spectral assignments are given in Table VI. The results of the calculations are reported e l s e ~ h e r e . The ~ assignments of the (8) Stojiljkovic, A,; Whiffen, D. H. Spectrochim. Acta 1958, 22, 47. (9) Scherer, J. R.; Evans, J. C . Spectrochim. Acta 1963, 29, 1739. (10) Green, J. H. S.Spectrochim. Acta, Part A 1970, 26, 1503.
104 The Journal of Physical Chemistry, Vol, 93, No. 1 , 1989 TABLE II: Vibrational Band Assignments for So SI Spectrum of P -c6D4clZ vibr energy, cm-l comb freq, cm-l interpretation -38 0 148 21 1 258 284 298 30 1 316 492 502 26: 530 583 585 600 600 614 614 618 617 654 666 695 705 714 780 817 824 814 8a26; 835 60260, 5h30; (?) 831, 833 858 983 979 989 101 1 101 1 1021 1031 1036 1040 1054 1063 1204 1202 1225 1225 1294 1306 1333 1338 1369 1370 1398 1390 1441 1586 1584 1751 1749 1765 1915 1920 2104 2108 2119 2137 2136 2273 2279 2290 2498 2495 -+
spectra of p-C6H4CI2and p-C6D4CI2are discussed separately below. Only those bands that are observed for the first time or fqr which our assignment differs from that of prior workers are included in the discussion. p-C6H4C12. The fluorescence excitation spectrum and band assignments of p-C6H4C12in the region from t h e origin to 4 0 0 cm-l above the origin are shown in Figure 3a. The 0; transition is observed at 35 739 cm-'. The band at 151 cm-l in our spectrum is assigned 30;. Castro and Hochstrasser4 observed this band at 146 cm-' in their crystal spectrum but did not assign it. Instead, they assigned a band at 247 cm-l to 30;. A multiplet is observed in the vicinity of 300 cm-l with the most intense feature at 302 cm-'. Under higher resolution (Figure 4 ) three peaks with an equal spacing of 3.4 cm-' are observed in a 1:6:9 intensity ratio. The transitions are assigned to 6;, the C-CI symmetric stretch, with the least intense, lowest frequency band attributed to p-C6H437C12,the central band to p-C6H435C137C1, and the most intense, highest frequency band to pC6H2SC12 The measured v6(37c1-37cl)/v6(35c1ratio -3sc of 1) 0.977 is in excellent
Sands and Moore TABLE 111: Vibrational Frequencies of p-C&CIz in Soand SI sym Mulliken no. Wilson no. Sn frea" cm-l SI frea. cm-' aB 1 2 3072 2 8a 1574 9a 1169 3 1 4 1096 1054b 747 7 29 5 6a 6 328 302c 7a a" 7 95 1 17a 8 16a 405 167.5 1Oa bl, 9 815 13 3078 bl" 10 11 19a 1477 12 1Sa 1090 12 1015 13 14 547t-'d 20a 550 b28 15 5 934 4 16 687 415.5d 17 10b 298 b2u 18 20b 3087 19 19b 1394 14 1220 20 1107 21 18b 15 22 226 225.5d 7b 23 3065 b38 24 8b 1574 1472 25 3 1290 26 6b 626 539 27 9b 350 34 1 17b 819 b3" 28 29 16b 48 5 294 11 30 122 75.5 "Reference 10. bNot corrected for Fermi resonance. cBoth chlorines 3sC1. dReference 7.
TABLE I V Vibrational Frequencies of p-C,D4CIZ in So and SI sym Mulliken no. Wilson no. So freqf cm-' SI freq, cm-' aB 1 2 2299 2 1558 Sa 3b 9a 867 46 1 1074 1054 5 6a 716 695 328 3Olc 6 7a a, 7 780 17a 367 142 8 16a 632 bl, 9 1Oa 229 1 bIU 10 13 19a 11 1365 18a 1020 12 13 12 815 14 531 20a 788 b2g 15 5 599 16 4 351 17 289 10b 2300 b2u 18 20b 19 1312 19b 1208 14 20 840 18b 21 22 226 15 228 1 b3B 23 7b 1531 1441 8b 24
b3"
25
3
1000
26 27 28 29 30
6b 9b 17b 16b 11
609 330 692 417 125
530 316 251 74
"Reference 9. bThe mode numbering is the same as in p-C&C12 even though the frequency ordering is reversed upon deuteration. CBothchlorines 35Cl.
agreement with the ab initio valueS of 0.976. Previous workers2s3 assigned a transition in the neighborhood of 250 cm-I in absorption spectra of the vapor as 6;. Since we observe no band at 250 cm-' in our spectrum, it is evident that the 250-cm-I band in the vapor spectra represents a hot band transition. In these prior spectra
Jet-Cooled p-Dichlorobenzene and p-Dichlorobenzene-d4
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The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 105
The transitions at 1082, 1087, and 1093 cm-I are assigned by Rohlfing and Rohlfing7 to be the three isotopic components of 14;. v i 4 is the C-Cl asymmetric stretch and has an isotopically averaged frequency of 550 cm-' in the ground state.I0 There are 3 3fi 1096.8 1 4fi 1054/1065 two difficulties with the assignment of this set of bands to 14;. 61, 728' 1053.7 First, in our spectrum the bands are not in the correct 1:6:9 26; 53 1 727.2 intensity ratio as observed in the case of 6;. The intensity of the 251.0 2 21 1461/ 1466 transition attributed to p-C6H435C12is actually less than that 533.9 3' 1061 26; attributed top-C6H~5C137C1. Second, the bands are far too intense 1098 1051 for an overtone that undergoes a frequency change of about 1% 1060 725 738 61 244 between So and SI (in the harmonic approximation the overtone 318 23; 528/536 6; of a non-totally symmetric vibration that changes frequency by 1284 25; 1% would be much less than 0.1% as intense as the origin bandI2). 569 26' Nevertheless, the mass-selected photoionization spectra of Rohlfing 370 27' and Rohlfing7 clearly show these bands as being derived from the 247 308 three isotopic components of the sample. We suggest that these 452 220 findings can be reconciled by proposing 142is in Fermi resonance with a nearby totally symmetric vibrational level that is at a lower TABLE VI: Comparison between Observed and Calculated Isotope frequency than 1082 cm-I, probably 4' (these levels are proposed Shift Ratios to be in Fermi resonance in So8). In a perturbative treatment the normal obsd SI D/H obsd So D / H magnitudes of the second-order correction to the energy and the mode isotope shift ratio isotope shift ratio" first-order correction to the vibrational wave functions will vary 4 1.000 f 0.001 0.980 inversely with the separation of the zero-order energies of the 5 0.953 f 0.002 0.959 coupled levels. Since the zero-order energies of v4 and vI4 are 6 0.997 f 0.005 1.000 isotopically dependent, the degree of the mixing due to the Fermi 8 0.848 f 0.004 0.898 resonance is also isotopically dependent. Thus, the intensities of 16 0.859 f 0.002 0.872 the bands due to the 35C1-35C1,the 35C1-37C1,and the 37C1-37C1 24 0.979 f 0.001 0.973 species would not necessarily reflect their populations in the sample 26 0.983 f 0.003 0.973 27 0.927 f 0.004 0.943 but would also depend on the extent of mixing due to the Fermi 29 0.854 f 0.002 0.860 resonance. Likewise, the C1 isotope shift for 14; would be reduced 30 0.980 f 0.009 1.ooo and that for 4; would be increased as a result of the mode mixing. normal obsd SI 37CI/35Clb ab initio So 37C1/35Clb Indeed, the observed isotopic shift of 14; is only about 80% of that mode isotope shift ratio isotope shift ratioC calculated by ab initio methods by Rohlfing and Rohlfing7 and by US.^ Rohlfing and Rohlfing also observe isotope shifts for 4; 6 0.977 f 0.001 0.976 that are larger than those predicted from their ab initio frequency 14 0.990 f 0.001 0.987 calculations. These bands were not resolved in the prior spectra Reference 9. *p-C6H4C12 only, V ( ~ ~ C I - ~ ~ C ~ ) / V ( ~ ~ Cand ~ - ~were ~ C Iassigned ). as the fundamental band 3;.3,4 We see no Reference 5. evidence for 3; in this spectrum or in the spectrum of p-C6D4C12. The fluorescence excitation spectrum in the region 1200-1 500 6; was buried in hot band transitions built on 8; and 27; and could cm-I is shown in Figure 7a. The only band that is assigned to not have been identified. In the low-temperature crystal spectrum a fundamental in this region of the spectrum occurs at 1472 cm-'. of Castro and Hochstrasser4 6; is observed at 318 cm-I, but the This band could be assigned to the totally symmetric vibration band is sufficiently broad that the isotopic splittings are not v2 or to the b3gvibration ~ 2 4 . If the vibrational component of this resolved. transition is totally symmetric, then one might expect to observe In vapor spectral3an unidentified band is reported in the vicinity combination bands involving non-totally symmetric vibrations. of 335 cm-'. Our data show that this band is in fact double, with In fact, every totally symmetric fundamental is observed in comthe component at 335 cm-I assigned to 8; and the component at bination with every b3g fundamental observed in the spectrum. 7 of b3* symmetry and is present 341 cm-I assigned to 27;. ~ 2 is The absence of combinations involving b3* fundamentals v26 or in the spectrum due to vibronic coupling with S2,which has B,, ~ 2 with 7 the 1472 fundamental in the spectrum is consistent with symmetry." Castro and Hochstrasser4 observed a band at 370 the assignment of the transition to 24;. It should be noted, cm-I that they assigned to 27;. 8; has not been identified in any however, that the absence of predicted bands is negative evidence of the prior spectra,I4 most likely because of spectral congestion. and does not preclude the possibility that the transition is actually The fluorescence excitation spectrum and band assignments 2;. Anno and MatubaraZassign this band to 2; and Castro and of p-C6H4C12in the region from 400 to 900 cm-' above the origin Hochstrasser assign it to 5i.4 In both the vapor and crystal spectra 5a. The prominent bands in this region of are shown in Figure the bands are sufficiently broad that 5; and 24; are not resolved. the spectrum are assigned to 26; and 5;. Our assignments are All bands observed in the region 1500-2800 cm-l are expected in agreement with those of earlier although Sponer' to be combination bands and are listed in Table I. The bands and Rai and Upadhya3 believed that v5 was the C-C1 symmetric appear to be clustered together, particularly in the vicinity of the stretch. expected positions of 4;5: and 4:. The assignments of the comPreviously unidentified transitions in this spectral region are bination bands are based on the agreement of the observed band due to overtones of the non-totally symmetric modes V I 6 (assigned positions with those predicted from the fundamental frequencies. by Rohlfing and Rohlfing7) and vZ9and combination bands that Some of the observed band positions deviate from the predicted involve Vgr us, and ~ 2 7 . These features are far too weak to have positions by as much as 10 cm-I. This, together with the clustering been identified in absorption spectra of the vapor. 29; is assigned of the bands, is suggestive of anharmonic mixing of the pure on the basis of its deuterium isotope shift and is discussed below. normal modes. The spectrum of p-C6H4C12in the region 900-1200 cm-' is Due to experimental limitations we presently cannot take spectra shown in Figure 6a. Combination bands of v5 with vg, us, ~27,and beyond about 2800 cm-' above the origin. However, we observe ~ 3 are 0 observed in the spectrum. The feature at 1054 cm-' is no bands with vibrational energy greater than 2550 cm-I in the assigned to the transition 4;. ,.v is identified as the main progression forming mode in previously reported absorption spectra.'+
TABLE V Previous Assignments of Vibronic Bands in the So SI Absorption Spectrum of p-C&I4CI2 ref assgnt SIfreq, cm-I ref assgnt SI freq, cm-'
Scharping, H.; Zetzsch, C. Phorochemistry and Photobiology; Zewail, A. H., Ed.; Harwocd Academic: New York, 1983; Vol. 2, pp 1371-1380. ( 1 1)
(12) Herzberg, G.Molecular Spectra and Molecular Structure: Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966; Vol. 111, p 150.
106 The Journal of Physical Chemistry, Vol. 93, No. I , 1989
fluorescence excitation spectrum of C6H4C12or C6D4C12. We attribute this absence of bands to the combined effect of decreasing absorption cross sectionI2 and declining fluorescence quantum yie1dl3 with increasing vibrational energy. p-Cg4C12.We rely heavily on experimental deuterium isotope shifts in So as reported by Scherer and Evans9 to predict isotope shifts in SI p-C6D4C12. In most cases, we are able to predict the positions of the vibronic bands to within a few wavenumbers. The fluorescence excitation spectrum and band assignments of p-C6D4C12in the region 0-400 cm-I above the origin are shown in Figure 3b. The origin in p-C6D4C12is observed at 35 884 cm-I, a 145-cm-’ blue shift from thep-C6H4C12origin. A weak feature is observed at -38 cm-’ from the p-C6D4C12origin and is assigned as the origin band for C6HD3C12.Shifts of this magnitude ( E +36 cm-I per D/H substitution) are comparable to those observed in deuterated benzenes. l 4 In analogy to p-C6H4C12,a multiplet is observed in p-C6D4C12 near 300 cm-I and is assigned to 6;, with the p-C6D435C12band observed at 301 cm-I. The deuterium isotope shift of 1 cm-I is within the range of our experimental error. In So v6 is also insensitive to deuterium substitution, with a frequency of 328 cm-I reported for both the fully protonated and fully deuterated s p e ~ i e s . ~ 8; and 27; are observed in the p-C6D4C12 spectrum at 284 and 316 cm-l, respectively. The deuterium shifts of these bands are consistent with those observed in So (ref 9) and are given in Table VI. The band at 258 cm-’ (marked with an asterisk in Figure 3b) is assigned to 86 in C6HD3C12and is located 296 cm-’ above the C6HD3C12origin. Prominent in the fluorescence excitation spectrum in the region 400-900 cm-’ (Figure 5b) are bands assignable to 26; and 5;. The deuterium shifts are also consistent with those observed in So and are given in Table VI. Two weak bands (marked with asterisks) are observed at 492 and 666 cm-I and can be assigned to 26; and 5; in p-C6HD3C12. The deuterium shift observed for 16; is consistent with that observed in So and provides corroborating evidence for the assignment by Rohlfing and Rohlfing for p-C6H4C12.7The transition 29; is assigned in p-C6H4C12and p-C6D4CI2on the basis of the agreement of the isotope shift with that observed for So (Table VI). This transition is also observed in the spectra of other aromatic systems such as s-tetrazine.15 The fluorescence excitation spectrum in the region 900-1 200 cm-I is shown in Figure 6b. The spectrum is extraordinarily congested in comparison to the spectrum of p-C6H4C12shown in Figure 6a. The most prominent feature in the spectrum is observed at 1054 cm-I, which we assign to 4;. This band is much more intense than the analogous band in p-C6H4C12(compare Figures 1 and 2); it also exhibits no discernible deuterium isotope shift. We believe substitution of deuterium detunes the Fermi resonance that is proposed to occur between 4’ and 142 in p-C6H4C12,resulting in a more intense 4; band. We see no hard evidence of 14; in the spectrum of p-C6D4C12,although the transitions may occur in the extremely congested region of the spectrum between 1000 and 1040 cm-’. Combination bands 5;8: and $27; are the only bands in this region of the spectrum that can be assigned with any degree of certainty. Additional experiments are required to complete the assignment of this region of the spectrum. Mass-selective resonantly enhanced photoionization experiments would help by eliminating inhomogeneous broadening caused by incomplete deuteration and the natural distribution of C1 isotopes. The fluorescence excitation spectrum in the region 1200-1 500 cm-I is shown in Figure 7b. By analogy with pC6H4C12,we assign the feature at 1441 cm-I to 24; rather than to 2;. As in the case of p-C6H4C12,we observe no combinations of this band with v26 or ~27.Deuterium isotope shifts provide further evidence for this assignment; the SIisotopic shift ratio, v ~ ~ ( C ~ D ~ C ~ ~ ) / V ~ of 0.979 is in somewhat better agreement with the So value of (13) Shimoda, A.; Hikida, T.; Mori, Y . J . Phys. Chem. 1979, 83, 1309. (14) Sur, A.; Knee, J.; Johnson, P. J . Chem. Phys. 1982, 77, 654. (15) Innes, K. K.; Franks, L. A.; M e w , A. J.; Velmulapalli, G. K.; Cassen, T.; Lowry, J. J . Mol. Spectrosc. 1977, 66, 465.
Sands and Moore 0.973 for ~ 2 than 4 the v2 value of 0.990. All bands with vibrational energies above 1500 cm-’ are assumed to be combination bands and are given in Table 11. The clustering of bands observed in the p-C6H4Cl2spectrum is also observed in this spectrum (Figure 2). Differences between observed combination band positions and those predicted from fundamental frequencies are as large as 6 cm-I. We take the clustering and differences between observed and predicted band positions as evidence of anharmonic interactions between the pure normal modes. SI Vibrations. Eleven SI normal modes are identified in the fluorescence excitation spectrum of p-C6H4CI2. Rohlfing and Rohlfing7 identify one additional mode, v22. The vibrational frequencies are listed in Table 111. Ten SI normal modes are identified in p-C6D4C12;these are listed in Table IV. The totally symmetric modes that are observed in the spectra are v4, the ring breathing mode, v5, which is described as a C-C-C in-plane bend,I6 and v6, the c - c l symmetric stretch. v6 has markedly different vibrational frequencies for the three isotopic species (35C1-35C1,35C1-37C1,and 37C1-37CI). The calculated C1 isotope shifts for v4 and v5 are much smaller than v6,597and we do not observe any evidence of C1 isotope effects in transitions containing v4 and v5. Rohlfing and Rohlfing7 are able to measure these small shifts using mass-selected detection. We also identify three b3gvibrations in the spectrum: ~24,a C-C stretching motion;I6 v26, which in benzene is the degenerate partner of the C-C-C in-plane bend vs; ~27,which is an in-plane C-Cl bend.I6 Even though the amplitude of the C1 motion is quite large in ~27,the calculated C1 isotope shift is sma115s7and we do not observe any evidence of C1 isotope effects in our spectra. Again, Rohlfing and Rohlfing utilize a detection method that is sensitive to these small shifts and observe a shift of -0.7 cm-’ in 27; for p-C6H25C137Clrelative to p-c6H435c12.7The transitions involving these vibrations have similar intensies to those involving ag vibrations only and are present in the spectrum due to intensity borrowing from S2,which has Bl, symmetry.’ The rest of the vibrational assignments are obtained from overtones of non-totally symmetric modes. The only one of those observed in our spectra that involves an in-plane motion, 14;, is assigned by Rohlfing and Rohlfing7 to be the antisymmetric C-Cl stretch. We contend that 14; derives its intensity through a Fermi resonance with 4’. The Fermi resonance is weaker for pC6HZ5Cl2 than for p-C6H25C137C1because the zero-order levels are more separated in the case of p-C6H435C12and the energy denominator in the second-order correction term is correspondingly larger. Rohlfing and Rohlfing also observe one additional overtone of an in-plane bend, 22:, in their mass-selected photoionization spectra. Four transitions attributable to overtones of out-of-plane bends are observed in our spectra, 8:, 16;,7 29;, and 30;. The frequencies of the observed out-of-plane bends are significantly less in SI than in Soloin both p-C6H4C12in p-C6D4C12. This indicates that the 0 be molecules are significantly less rigid in SI than in So. ~ 3 can described as an “umbrella mode”, in which the H and C1 atoms move in opposition to the C atoms. The SI frequency in p-C6H4C12 is 60% of the sovalue. vg and ~ 2 are 9 degenerate partners in benzene. Their motion is described as a C-C-C out-of-plane bend.16 The SI frequency is only 41% of the sovalue for vg and 61% for vZ9 in p-C6H4Cl2. v I 6 is a c-c-c puckering motionI6 with the p-C6H4C12SI frequency 60% of the sovalue. These large frequency changes are in contrast to the in-plane modes, in which 9 all SI frequencies are >90% of the So value except for ~ 2 (86%). The frequency shifts observed in p-C@4C12 are similar, with the fractional frequencies of the out-of-plane modes somewhat smaller than in p-C6H4C12. ~Summary (C~H~C~~), The fluorescence excitation spectra of pC6H4Cl2and p-C6D4C12 have been measured and assigned. The assignments are brought into correspondence with prior assignments derived from ab(16) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic:
New York, 1969.
J . Phys. Chem. 1989, 93, 107-113 sorption spectra of room-temperature vapor and the low-temperature (4.2 K) ~ r y s t a l ,as~ well as the coincident mass-selected resonantly enhanced multiphoton ionization spectra of Rohlfing and R ~ h l f i n g . From ~ this work the SI vibrational frequencies of 11 normal modes of p-C6H&l2 are determined as are the frequencies of 10 s1normal modes of p-C6D4C12. Three of the normal modes are b38 fundamentals that derive spectral intensity via vibronic coupling with the lBlu S2 state." The S1 frequencies of out-of-plane bending motions obtained from overtone bands are much less than those reported for So,l0indicating a significant decrease in rigidity upon excitation. In the case of p-C6H4C12,we propose that the transition 14; derives intensity from a Fermi resonance between levels 4' and 142.
107
Acknowledgment. This work was supported by the National Science Foundation, Research Corp., and the Thomas F. Jeffress and Kate Miller Jeffress Memorial Trust. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Drs. Eric Rohlfing and Celeste Rohlfing for sharing with us in advance of publication their resonantly enhanced multiphoton ionization spectra and results of ab initio calculations of vibrational frequencies of isotopically substituted p-dichlorobenzene and for helpful discussions. Registry No. p-C6H4C12,25321-22-6; D1,7782-39-0; 3sC1, 1398172-1; "Cl, 13981-73-2; He, 7440-59-7.
Molecular Rotational Coherences at High Intensities Andre D. Bandrauk* and Lorraine Claveau DPpartement de Chimie, Facult&des Sciences, UniversitP de Sherbrooke, Sherbrooke, Quebec, Canada, J1 K 2 R l (Received: March 30, 1988)
Rotational excitations at high intensities are investigated in the nonperturbative regime by the dressed molecule method for pure rotations in linear molecules and also for the 001-100 IR vibrational transition of COzwhich is an example of an isolated molecular two-level system. It is shown that multiphoton excitation in such systems can be modeled by one-dimensional extended models of energy transfer. These models are used to establish the extent of delocalization of the dressed states over the rotational states of the molecule. Quasiperiodicityon a picosecond time scale of the energy of a molecule is maintained at intensities as high as 10l2 W/cm2 and is correlated to the extent of delocalization of the dressed states over the rotational states of the molecule.
I. Introduction Isolated two-level systems exhibit interesting coherence phenomena and have been studied extensively in atomic physics.' In particular, distortionless pulse propagation in such two-level systems is possible even for high intensities and is called self-induced t r a n ~ p a r e n c y . ~At ~ ~much higher field strengths, such coherences can be destroyed due to the lifting of the degeneracy of the magnetic levels of a particular atomic t r a n ~ i t i o n . ~In molecules, isolated two-level systems can be found in certain lasing systems, in particular the C02 laser, in which the upper 001 vibration level is resonantly 'coupled to the lower 100 and 020 levels by 10.4- and 9.6-pm IR radiation. Simultaneous rotational transitions occur as R and P branches so that, at high intensities, rotational coherences will arise as a result of virtual (nonresonant) transitions. Recently, amplification of a picosecond pulse in a COz laser has been achieved up to intensities 2f 10l2 W/cm2.s This corresponds to a Rabi frequency wR = jbE0 = 8 cm-' in a system where the rotational spacing becomes comparable: 2BJ 16 cm-l for a R(20) transition ( B = 0.39 cm-I, p(OO1 100) = 0.035 D = 0.014 au).6 We examine here the effect of multiple rotational transitions in this molecular two-level system using the dressed molecule representation of field-molecule interaction^.^,^ We reemphasize
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(1) Allen, J.; Eberly, J. H. Optical Resonance and Two Leuel Atoms; Wiley: New York, 1975. (2) Lamb, G. L., Jr. Rev. Mod. Phys. 1971,43, 99. (3) Lamb, G. L., Jr. Elements of Soliton Theory;.Wiley: New York, 1980. (4) Salamo, G.; Gibbs, H.; Churchill, G. Phys. Rev. Lett. 1974,33, 273. (5) Corkum, P. B. IEEE J . Quant. Electron. 1985,QE-21, 216. (6) Cousin, C.; Rossetti, C.; Meyer, C. C. R . Acad. Sci. (Paris) 1969, 268B,1640. (7) Bandrauk, A. D.; Turcotte, G. J . Phys. Chem. 1983,87,5099.
0022-3654/89/2093-0107.$01.50/0
that the C 0 2 system is well isolated from possible multiple transitions to other vibrational manifolds since, in the case of 10.4-pm radiations as an example, the IR photon is only resonant with the 100-001 transition. It can be estimated from the known transition energies and moments to othergJOlevels that multiphoton transitions become important around 1013 W cm-2 (see section IV). Thus the behavior of the present system at high intensities will enable us to examine rotational coherences on a single vibrational transition. As we show further below, the present system can be reduced to a one-dimensional hopping model of energy transfer. This will enable us to relate the high-intensity behavior of the above transition to previous simple models of quantum motion under the action of time-dependent perturbations, such as the periodically kicked rotator11J2 and the phenomenon of quantum nonlinear resonance^.^^-^^ It will be shown that, in the present system, localization in angular-momentum space of the dressed states of the field-molecule system prevail at intensities as high as 1Ol2 W/cm2. As a result, the quantum evolution of the system is recurrent. The relationship between quantum recurrence, discrete quasienergy (dressed) spectrum, the state localization has been examined previously in the atomic case1sJ6 (8) Bandrauk, A. D.; Atabek, 0. J. Phys. Chem. 1987,91, 6469. (9) Herzberg, G. Infrared and Raman Spectra; Van Nostrand: New York, 1945. (10) Cai, W. Q.; Gough, T. E.; Gu, X. J.; Isenor, N. R.; Scoles, G. Phys. Rev. A 1987,36, 4722. (1 1) Grempel, D. R.; Prange, R. E.; Fishman, S. Phys. Rev. A 1984,29, 1639. (12) Blumel, R.; Fishman, S.; Smilansky, U. J. Chem. Phys. 1986,84, 2604. (13) Zaslavskii, G. M. Stochasticity in Dynamical Systems; Nauka: Moscow, USSR, 1984. (14) Berman, G. P.; Zaslavskii, G. M.; Kolovskii, A. R. Sou. Phys. (JETE') 1981,54, 272.
0 1989 American Chemical Society
