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J. Phys. Chem. 1980, 84, 3340-3348
in a unique environment. A similar intensity variation is to be found when the spectra of two different crystals are compared (Table I). Polarization effects might also account for the negative Lorentzian lines. Possibly only a small fraction of the molecules is properly aligned to be excited, with a consequent lowering of the excited state population, thus leading to the negative lines. A very broad resonance is also evident in Figure 2 which shifts on change of crystal orientation. Without further experimentation,this phenomenon remains to be explained. Finally, it is also gratifying to note that more resonances are observed in the crystal spectra, which are hidden in the solution spectrum. We have already mentioned the appearance of the line at 1540 cm-’ but also remarkable are the weak lines at 1660 and 1674 cm-l. A tentative assignment of these is to the C(2)=0 stretching mode, which is usually found at 1670 cm-l.ll It would be expected, however, that this carbonyl stretching interaction would be coupled to the ~ - k transition * only if it is involved in the conjugated system. It can be concluded that the amount of information that can be retrieved from crystal spectra is quite high. Proper interpretation of the orientation effects on the spectra requires further systematic study. The resonance CARS spectrum of crystals of the flavodoxin semiquinone h also been obtained and it shows a relation to the solution spectrum analogous to that shown here for the oxidized flavodoxin.
(a ’A2, 4‘)
Rotational Relaxation of D2C0 Fluorescence Emission Spectra
Acknowledgment. This work was supported by grants from the National Science Foundation, PCM 79-11064, CHE 78-21162, National Institutes of Health, GM 28139, Environmental Protection Agency, R-806-4300, NATO Grant 1912, and by the Netherlands Foundation for Chemical Research (S. 0. N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).
References and Notes (1) W. A. Eaton, 2. Hofrlchter, M. W. Maklnen, R. D. Anderson, and M. L. Ludwia. 8bchem/sfrv. 14. 2146 (1975). (2) R. J. Plabnkamp, H. 6.van.Osnabiugge,’and A. J. W. 0. Vlsser, Chem. Phys. Lett., 72, 104 (1980). (3) R. M. Irwin, A. J. W. G. Visser, J. Lee, and L. A. Carreira, 8bchemlsfry, 10, 4639 (1980). (4) P. K. Dutta and T. G. Spiro, 8/ochem/sfry, 19, 1590 (1980). (5) R. M. Burnett, G. D. Darling, D. S. Kendall, M. F. LeQuesne, S.G. Mayhew, W. W. Smith, and M. L. Ludwlg, J. Siol. Chem., 249,4383 (1974). (6) K. D. Watenpaugh, L. C. Sikker, and L. H. Jensen, Roc. Natl. Acad. Sc/. U.S.A.. 70. 3857 (1973). (7) M. Dubourdieu and J. Le‘Gall, Biochem. Siophys. Res. Commun., 38, 965 (1970). (8) K. D. Watenpaugh, L. C. Sieker, L. H. Jensen, J. Le Gall, and M. Dubourdieu, Proc. Natl. Acad. Sci. U.S.A., 69, 3185 (1972). (9) L. A. Carreira, L. B. Rogers, L. P. Goss, 0. W. Martin, R. M. Irwin, R. Von Wandruszka, and D. A. Betkowttz. Chem. Bkmed., Envkon. Insfrum., 10, 249 (1980). (10) L. A. Carreira, L. P. Goss, and T. B. Maiioy, Jr., J. Chem. Phys., 89, 855 (1978). (1 1) Y. Nlshlmura and M. Tsubol, “Proceedings of the 7th International Conference on Ramen Spectroscopy”, W. F. Murphy, Ed., NorthHolland, Amsterdam, 1980, p 568.
Studied by Time-Resolved, Dispersed
Paul W. Fairchlld and Edward K. C. Lee” Department of Chemistry, University of Californk, Ifvine, Callfornk 9271 7 (Rece/vd: September 25, 1980)
The rotational structure in the fluorescence emission from a selected rotational level in the 4l vibronic level of the DzCO (S,) has been partially resolved. The time-resolved, dispersed fluorescence emission spectra of the 4; emission band shows that the overall rotational relaxation rate due to collisions is somewhat slower than the 4l 4O vibrational relaxation rate.
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Nonradiative behavior of a single rotational level (SRL) of SIin formaldehyde has received much recent attention through fluorescence quantum yield studies1i2as well as lifetime studies.3~~ While these measurements were carried out under experimental conditions close to the collisionfree pressure limit in a bulb study or in a supersonic free jet study, some degree of relaxation through long-range intermolecular interaction cannot entirely be ignored. Collision-induced rotational relaxation in an electronically excited molecule is of considerable interest, and particularly the collisional effects observed in radiationless processes of small molecules such as SOz, CHz, HzCO, and glyoxal have been studied in some detail.5 It has been shown that rotational relaxation of S1 glyoxal by the ground state glyoxal occurs with 240-A2cross section showing no “propensity” rule6for changes in rotational quantum numbers (AJ’, AK’),whereas rotational relaxation of SOz (A lAz) by the ground state SO2 (X lAl) has been shown to occur with 100-500-A2 cross section, 100 A2 of which is due to a dipole-type mechanism with a
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0022-3654/80/2084-3346$01.OO/O
specific quantum change of = 1.’ Since a dipole-type mechanism for collision-induced rotational transitions8 involves a long-range interaction and can predominate over other mechanisms in molecules with large dipole moments, it should play an important role in formaldehyde, e.g., HzCO with a dipole moment of 2.34 D foz So and 1.56 D for S1,9as compared to 1.615 D for SO2 (X lAJ. If collision-induced rotational relaxation of SI formaldehyde occurs with a rate 10 times gas kinetic as observed with the So formaldehyde in a microwave rotational line broadening experiment (collision-inducedAJ transitions),1° the S1rotational relaxation should occur with a relaxation time of 0.5 ps at 20 mtorr pressure. One direct way of observing collision-induced rotational transitions of electronically excited molecules is to study the rotationally resolved fluorescence emission spectrum from a laser-excited SRL as a function of time and also as a function of pressure. We were able to obtain a time-resolved and rotationally resolved fluorescence emissibn spectra from D2C0 using the fluorescence cell 0 1980 American Chemical Society
The Journal of Physical Chemistty, Vol. 84, No. 25, 1980 3347
Letters
5p SIN
2p Slec
IpSoc
op St!C - 1
Delay
I
I
I
3750 3760 3770 3780 16)
Flgure 1. Timcyesolved 4’ emission band of the D2C0 fluorescence excited by the A ‘A, +- 2 ?Al, 4; ‘R8 (13) transition at 28426.7 cm-’. The laser excitatbn pulse width was 51 ps and the gate width of the integrator was 0.5 p. The emission was analyzed wlth a spectrometer resolution of -42 A. Three r-form subbands are ‘R, ‘Q, and ‘P (very weak) from leit to right, and likewise three p-form subbands are PR (very weak), PCI and PP. Three peaks in the center are due to q-form subbands (see text).‘* 130th upper and lower states are near prolate tops and the K doublets are nearly degenerate for the levels involved here. Therefoire, we uise the transition notation for a symmetric top here.
equipped with multipath White and Welsh optics, gated integrators/e,ignalaverager, and a pulsed, etalon-tuned dye laser which have been described elsewhere.2J A set of fluorescence emission spectra of the 4; emission band from the J’ = 14, K’ = 9 level of D,CO (at 25 mtorr) obtained with gated delay times of 0, 1, 2, and 5 ps is shown in Figure 1. The laser pulse width of the dye laser was I1 ps, and the integrahr gate width for the fluorescence signal was 0.5 ps, much shorter than the typical fluorescence decay time of -4 jus at this pressure. The rotational structure shown for the zero delay time in Figure 1 was assigned froin a better quality spectrum obtained with a greater averaging time, using the spectroscopic constants given in the literature.’lJZ Although the rotational structure in the resonance fluorescence emission from the initially prepared SRL was resolved unambiguously, the satellite rovibronic emission feature from the collisionally relaxed levehi was not as well resolved. However, the most interesting feature im the time-resolved emission spectrum is the rotational relaxation, Le., the progressive spectral congestion resulting from rotational relaxation at longer delay times, and it is clearly demonstrated in Figure 1. According to the dipole-type “propensity” rule8 for an asymmetric top, AJ‘ = 0, f l with AK; = 0, f 2 and AK; = f l are allowed in the collision-induced transition of DzCO (or HZCO), a case of pA # 0. In the supersonic nozzle expansion beurn of HzCO,the AK, = f l collision-
induced transition was not observed! The AKd = f l collision-induced transition for a case of yc # 0 possible for the nonplainar excited state of D2C0 (or H&O) is, however, not allowed on account of the nuclear spins. This means that collisional relaxation routes in DzCO (or H2CO) through a long-range, dipole-type mechanism are rather constrained by the AJ’ = f l , AKL = 0 “propensity” rule, although an aidditional, slower route by a dipole-type mechanism could involve AK,I = f2. Hence, the K’ relaxation should be considerably slower than the J’relaxation, We feel that the rotational relaxation shown in Figure 1is predominantly in J’ occurring in the microsecond time scale (-5 times gas kinetic). This is also supported independently in our study of the pressure dependence of fluorescence quantum yields.14 The collision-induced vilbrational relaxation 4l- 4O in D2C0occws very efficiently, and the bimolecular rate constant has been measured to be 6.9 X 10-lo cm3 molecule-l s-l (31 ys-’ torr-’)16 and 9.6 X cm3 molecule-l s-l (3 times gas kinetic).16 Therefore, the 4l- 4O vibrational relaxation should occur with a lifetime of 1.3-0.9 ps a t 25 mtorr of DzCO, and we indeed observe the rotationally resolved DzCO 2y4: emission band from the collisionally populated 4O level. For the J’ = 14, K’ = 9 level of DzCO (4% the energy separation is 25-26 cm-l for the adjacent J’levels (AJ’ = f l ) , 61-68 cm-’ for the adjacent K’ levels (AK; = i l ) , which are collisionally disallowed by dipole-type rules, and 115-144 cni-l for the next adjacent K’levels (AK;= 1 2 ) which are collisionally allowed by dipole-type rules, whereas the energy gap between the 4l and 4O vibrational levels is 68 cm-l. Since the 4l-4O vibrational relaxation can also proceed by a long-range dipole mechanism and the energy gaps involved are similar, it is not surprising that it competes closely with the J’ rotational relaxation involving a smaller energy gap. It is likely that the overall rotational relaxation is quite slow mainly due to the slowness of the K’ relaxation involving a larger energy gap, while the extent of the contribution by nonspecific rotational relaxation due to a short-range interaction is probably small. A similar result is expected for H,CO, but we were unable to (obtaina well-resolved emission spectrum due to a low fluorescence yield from H2C0. The present observation of rapid J’relaxation combined with slow K’ relaxation points out the importance of the constrained collisional relaxation mechanism to be considered in analyzing the curved Stern-Volmer plots observed in the fluorescence lifetime studies of HzCO and DzCO. We are making attempts to improve the S/N ratio and hence the spectral resolution, so that we can obtain further details on the rotational effects and curved Stern-Volmer plots relevant to formaldehyde photophysics.
Acknowledgment. This report is based upon research supported by tlhe National Science Foundation under Grant CHE-79-25451. References and Notes (1) K. Y. Tang, P. W. Fairchild, and E. K. C. Lee, J . Chem. Phys., 66, 3303 (1977). (2) K. Shlbuya anld E. K. C. Lee, J. Chem. Phys.. 6B. 5558 (1978). (3) (a) J. C. Welscihaar and C. B. Moore, J. Chem. Phys., 70, 5135 (1979); (b) Ibrd., 72, 5415 (1980). (4) H. L. Selzle arid E. W. Schlag, Chem. Phys., 45, 111 (1979). (5) See for recent reviews (a) P. Avouris, W. M. Gelbart, and M. A. El-Sayed, Cheim. Rev., 77, 793 (1977); (b) E. K. C, Lee and G. L. Loper in “Radkitionless Transitions”, S.H. Un, Ed., Academic Press, New York, 1980, p 1; (c) K. F. Freed, Chem. Phys. Lett., 37, 47 (1976). (6) R. F. Rordorf, A. E. W. Knight, and C. S.Parmenter, Chem. phys., 27, 11 (1978). (7) D. L. Holtermann, E. K. C. Lee, and R. Nanes, Chem. Phys. Lett., to be published.
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J. Phys. Chem. 1980,84,3348-3351
(8) (a) See for a review on collisiiinduced rotatkml transitions, T. Oka, Adv. At. Mol. Phys., 9, 127 (1973); (b) T. Oka, J. Chem. Phys., 47, 13 (1967). (9) D. E. Freeman and W. Klemperer, J . Chem. Phys., 45, 52 (1968). (IO) D. V. Rogers and J. A. Roberts, J. Mol. Spsctrosc.,48,200 (1973). (1 I) (a) V. A. Job, V. Sethuraman, and K. K. Innes, J . Mol. Specrrosc., 30, 365 (1969); (b) T. Oka, IbM., 14, 27 (1964). (12) The A X 4; transition is a perpendicular, type 8, band and only r-form and p-form subbands (A& = +1 aryl -1, respectively) have been observed.” However, In the A X, 4: transition of D,CO, we observe what we tentatively assign as parallel q-form subband
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-+
(13)
(14) (15)
(16)
transitions (AK, = 0). Axis switching for the nonplanar-planar transition and/or the hybrid transition in the emission may explain this ~bservation.‘~ 0. Herrberg, “Electronic Spectra of Polyatomic Molecules”, van Nostrand, New York, 1966, pp 208 and 248. K. Shlbuya, P. W. Fairchild, and E. K. C. Lee, an unpublished work on the presswe dependence of fluorescence quantum yields in H&O. K. Shibuya, D. L. Holtermann, J. R. Peacock, and E. K. C. Lee, J. Phvs. Chem.. 63. 940 11979). J. C. Welsshaar, A. P. Bironavskl, A. Cabello, and C. 6. Moore, J. Chem. Phys., 6@,4720 (1978).
Chemllumlnescence of CN Radicals Formed from Reaction of Nitric Oxide with Multiphoton Electronic Excltation Photofragments of Toluene Jonathan B. Lurle and M. A. El-Sayed” Department of Chemlstty, University of Callfornla, 10s Angeles, Callfornla 90024 (Received: October 6, 1980; In Final Form: October H,1880)
We report the vibrational distribution and rate of formation of excited CN(B2Z)radicals produced by 266-nm irradiation of toluene/NO mixtures. At low NO pressures, perturbed rotational lines of CN(A%, u = 10) appear within the CN(B2Z)spectrum. Evidence is presented which indicates that the reaction which gives rise to this luminescence is Cz(a3d+ NO CN(B22,A%) + CO. -+
Introduction The creation of high concentrations of gas-phase free radicals has been greatly facilitated through the use of high-powered pulsed laser systems. Most of the research in this area has attempted to address three different general problems. First, for molecules requiring more than one photon to dissociate, the mechanisms of dissociation (the number of photons, the wavelength dependence) of the molecule giving rise to the photofragments have been investigated. Second, the rovibronic distributions of the photofragments have been determined. Third, quenching experiments have been performed on the photofragmenta, by adding foreign gases and monitoring the time evolution of the fragment concentration and internal energy distribution. In some of the quenching experiments, the quenching mechanism is believed to be chemical reaction between the photofragment and foreign gas. The best way to determine that a reaction is actually taking place is to observe a known spectroscopic property of a reaction product. Krausel has observed CN violet emission (B22 X2Z) in a crossed molecular beam experiment, using a beam from a graphite sublimation source and a nitric oxide beam. Upon infrared laser photolysis of vinyl cyanide/NO mixtures, Reisler, Mangir, and Wittig2 (RMW) observed CN luminescence from the B2Z and A211 states, with the population of the latter state higher than that of the former. In both experiments, the reaction following was determined to be the source of the observed luminescence: C2(a311)+ NO(X2rI) CN(B,A) + CO(XIZ+)
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The studies of Zandee and BernsteinS have demonstrated extensive fragmentation of benzene upon multiphoton electronic excitation with a focused dye laser in the visible region. However, none of the reactions between products of such a fragmentation process and added gases have been experimentally investigated. In this study, we report the high-resolution luminescence spectrum of CN0022-3654iaoi2oa4-334a~oi.ooio
(B22)radicals produced by the reaction of nitric oxide with carbon-containing compounds produced by multiphoton ultraviolet dissociation of toluene. In our experiment, the higher resolution has enabled us to observe rotational structure in the CN violet spectrum. In addition, we report time-resolved measurements of the CN(B22 X22) luminescence. We also discuss the different mechanisms for the formation of CN(B22) in terms of energy and symmetry considerations.
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Experimental Section The photolysis source was a Quanta-Ray Nd:YAG laser (DCR-l), frequency quadrupled to 266 nm. The laser had an output energy of 10-20 mJ over a 6-ns fwhm pulse. The laser was focused with a 10-cmfocal length lens and passed through a cell fitted with Suprasil windows along the excitation axis. The cell was roughly cylindrical in design, 1.5 in. in diameter and 4.75 in. long, with 1.75 in. viewing windows. In spite of the extremely high energy fluence, no indication of dielectric breakdown was noted in any of the experiments. For recording spectra, fluorescence was collected at right angles to the excitation beam by two lenses and focused through the entrance slit of a 0.5-m polychromator onto a Princeton Applied Research optical multichannel analyzer. A 50-pm slit width and an 1800 lines/mm grating yielded resolution of 1-2 cm-’. Lifetime measurements were carried out by focusing the fluorescence onto the entrance slit of a 0.5-m Jarrell-Ash monochromator and recording the signal with an RCA 8575 photomultiplier tube and a Princeton Applied Research boxcar integrator with a 10-ns gate. Analytical grade toluene (Mallinckrodt) and NO (Matheson) were purified by vacuum distillation, retaining only the middle fraction. Results We irradiated a mixture of 30 mtorr of toluene and 0.5 torr of NO with the focused 266-nm laser, and a search of the fluorescence spectrum from 2000 to 6500 A yielded N
0 1980 American Chemical Society
