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The study reported here opens an entirely new and interesting domain, since it offers the possibility to prepare an inverse microlatex of small size with rather narrow polydispersity. Furthermore, the microlatices can be used as models for neutron-scattering studies on the statistics
of high molecular weight, confined chains.
Acknowledgment. We thank S. Candau for much assistance in the quasi-elastic light-scattering experiments and R. Fitch for stimulating discussions.
Photolysis Wavelength Dependence of the Effect of Xenon on the Production of C,"(G3n,) by Ultraviolet Multiphoton Dissociation C. T. Lln,+ Ph. Avourls,' and Y. J. Thefalne IBM Thomas J. Watson Research Center, Yorktown Helghts, New York 10598 (Received:January 8, 1982; In F/m/ form: March 9, 1982)
We report on the effect of Xe buffer gas on the C2(a3II,+I3II,) emission which results from the excimer laser-induced multiphonon fragmentation of acetylene, triethylenediamine (Dabco), and benzene. We find that, upon addition of Xe, normal quenching of this emission occurs for 248-nm (KrF) photolysis of the above molecules, while an unusual enhecemept is found for-193-nm_(ArF) photolysis. Other radical emissions resulting from the same process, e.g., CH(A2A+X211)and CN(B2Z++X2Z+), show normal quenching at both wavelengths. The possible mechanisms of this unusual enhancement are discussed.
Introduction Laser (excimer or COP)photolysis provides an excellent way to produce ground- and excited-state free radicals, particularly diatomic species such as Cz, CH, CN, etc. Bimolecular reaction rates or excited-state collisional quenching cross sections of these radicals can easily be obtained by monitoring the laser-induced fluorescence or luminescence of the particular radical as a function of the increasing pressure of the reaction partner or quenching gas.1-6 In the above types of studies the nature of the radical precursor or the mode of the radical preparation is not considered to be important in the subsequent free-radical interactions. Here we discuss cases where an apparent deviation from this behavior is observed. Specifically, in certain multiphonon dissociation? the effect of Xe "buffer" gas on the intensity of the CZ*(d3ng-ii3n,) emission depends on the photolysis wavelength itself. In fact, while the normal quenching of this emission is observed for 248-nm (KrF laser) photolysis, an unusual enhancement is found for 193-nm (ArF laser) photolysis. In this Letter we demonstrate this effect and discuss possible interpretations. Experimental Section Triethylenediamine (Aldrich) was purified by vacuum sublimation while the other chemicals, Xe (Airco), acetylene (Matheson), and benzene (Aldrich), were used as received. The vapor pressure of the gases was measured by a MKS Baratron capacitance manometer. The photolysis laser was a Lambda Physik EMG 500 excimer laser which had an output of 100 mJ/pulse at 248 nm (KrF) and 40 mJ/pulse at 193 nm (ArF). The laser power was measured by a pyroelectric detector (Gentec ED 500) and the laser beam was focused in the reaction-fluorescence cell by a fused silica 8-in. focal length lens. The radical luminescence was viewed at right angles to the laser beam and the t On leave from Instituto de Quimica, Universidade Estadual de Campinas, Brazil.
0022-365418212086-2271$01.25/0
spectrum was obtained by the use of a 0.3-m polychromator and a PAR (OMA 11) optical multichannel analyzer equipped with a SIT detector.
Results In Figure 1 we show radical luminescence spectra obtained by the excimer laser-induced multiphoton fragmentatip of 20s mtorr of Qabco. -Luminescence bands of CN*(B2P-X22+), CH*(A2A-X211), and Cz*(d311g-.g3IIU)are observed. The three peaks of the Cz* spectrum at about 470,517, and 550 nm correspond to the u' = 1 u" = 0 (and overlapping Au = -1 sequence bands), u' = 0 u" = 0 (and Au = 0 sequence bands) and u' = 0 u" = 1 (and Au = +1 sequence bands) vibronic transitions, respectively.' Both ArF (193 nm) and KrF (248 nm) excitations result in the same radical emissions; however, the effect of added Xe buffer gas is different in the two cases. With 248-nm excitation we observe that all radical emissions (spectrum A) are quenched upon addition of Xe gas (10 torr, spectrum B). This is the normal collisional quenching behavior. For 193-nm excitations (spectra C and D), however, we observe that, while the CN* and CH* emissions are quenched upon Xe (10 torr) addition (spectrum D), the C2*emission is intensified. In particular, the intensity ratios for the different radicals are as follows: Ic,,(with Xe)/lc,.(without Xe) = 1.5 while IcN,(with Xe)/ICN*(withoutXe) = 0.7 and IcH.(with Xe)/IcH*(without Xe) = 0.6. He gas, on the other hand, quenches all radical emissions including C2*at both 248 and 193 nm. An enhancement of the 311g 311uCz* emission by Xe is also observed when acetylene is photolyzed by the ArF
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(1)L. Pasternack, W.M. Pitts, and J. R. McDonald, Chem. Phys., 57, 19 (1981). (2)H. Reiser, M. S. Mangir, and C. Wittig, J. Chem. Phys., 73,2280 (1980). (3)J. B. Lurie and M. A. El-Sayed, Chem. Phys. Lett., 70,251 (1980). (4)J. E. Butler, L. P. Goss, M. C. Lin, and J. W. Hudgens, Chem. Phys. Lett., 63,104 (1979). (5)V. M. Donnelly and L. Pasternack, Chem. Phys., 39,427 (1979). (6)A.P. Baronavski and J. R. McDonald, Chem. Phys. Lett., 56,369 (1978). (7)R. W.B. Pearse and A. G. Gaydon, "The Identification of Molecular Spectra", Chapman and Hall, London, 1976.
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
Letters 241
193nm EXCITATION
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Flgure 1. Radlcal luminescence spectra produced by the multiphoton photolysis of Dabco at 193 (top) and 248 nm (bottom). Spectra A and C were obtained with 200 mtorr of Dabco while spectra B and D were obtained with a mixture of 200 mtorr of Dabco and 10 torr of Xe. The broad band underlying the CN' and CH' emissions in spectra A and B is due to the tail of the Dabco' molecular emission. The short wavelength cutoff of this broad emisslon results from the use of a Corning 0-52 filter.
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Figure 3. Plot of relative luminescence intensity of CH' and C,' radicals from the ArF (193 nm) laser multiphoton photolysis of CpH, (100 mtorr) vs. Xe pressure. The triangles and squares represent luminescence data before and after correction for the quenching of C2* by Xe, respectively.
we found that both CH* and Cz* radicals are quenched by Xe with the same quenching cross section as the corresponding radicals from the 248-nm photolysis of Dabco. Photolysis of benzene at 193 nm did not give measurable radical luminescence.
Discussion From the presented results it is seen that the Cz*(a311,-i3311,) emission resulting from the multiphoton UV photolysis of polyatomics can be either quenched or enhanced by the addition of Xe gas. A t least two types of interpretations could be put forth for the observed behavior. In one case, the observed behavior is the result of the different interaction of Xe with two different precursors of Cz* produced by the photolysis at the two wavelengths (193 and 248 nm). Xe is a high atomic number atom and is known to enhance the spin-orbit coupling of other molecules in collisions or upon formation of weak charge-transfer complexes. The C2*(311) state is a triplet state and its formation, presumably from a singlet or doublet precursor, depending also on the multiplicities of the other fragments produced concurrently, can be a spin-forbidden process which will be enhanced by spinorbit coupling. In the case of Dabco: the different radical luminescences have a different laser power dependence, and this power dependence is in turn different at the two laser wavelengths. Similarly, for C2H2,the production of CH*(A2A) requi_restwo 193-nm photons, while the production of C2*(d3n,) requires at least three photons.1° These results suggest different precursors for the different radicals and that these precursors are different at the two wavelengths. It is possible, therefore, that both spin-allowed and spin-forbidden paths exist for the formation of Cz*(311,),and that the latter (occurring for 193-nm photolysis) are enhanced by Xe. This interpretation suggests that Cz*(d311,)formation at 193 nm proceeds via a common (8) N. J. Turro, 'Molecular Photochemistry", W. A. Benjamin, New
York, 1967.
(9) C. T. Lin and Ph. Avouris, to be submitted for publication. (10) J. R. McDonald, A. P. Baronavski, and V. M. Donnelly, Chem. Phys., 33, 161 (1978).
J. Phys. Chem. 1982, 86,2273-2276
precursor in both Dabco and acetylene. The nature of this precursor is not clear. In the case of the simplest molecule, C2H2,the precursor can be an excited singlet state of the parent, a state of the C2Hradical, or an excited singlet (e.g., Cln,) of C2 itself. We find that the excited radicals are formed fast, the radical fluorescence risetime is the same as that of the laser pulse, and the dependence of the radical luminescence on parent molecule pressure is linear, indicating a noncollisional radical production sequence. The enhancement by Xe, however, is observed even at low pressure (-1 torr). An enhancement of the a311u-+X1X,+ intersystem crossing in Cz by Xe at low pressures has been reported recently.lV2 However, unlike the case of the metastable_g3lIustate, the precursor resulting in the formation of d311ustate must be short lived. The observation of strong enhancement even at low pressures is therefore somewhat puzzling. A second type of interpretation involves the effect of the laser light on Xe itself. Xe does not exemplify linear absorption at 248 or 193 nm; however, under our focused beam conditions near two-photon resonant, three-photon ionization at 248 nm and two-photon ionization at 193 nm (IP(Xe) N 12 eV) take place. In an earlier study of the multiphoton photolysis of CO" at 193 nm, it was reported that the C(33Po-+23P) emission was enhanced while the C(31Po-21S) emission was quenched when Xe was introduced. It was assumed'l that the spin-flipping collisional process C(3lPo) + e- C(33P0)+ e- takes place, where the electrons are supplied by the two-photon nonresonant ionization of Xe at 193 nm. It is possible then that the behavior we have observed can be explained on the basis that the two-photon ionization of Xe at 193 nm is more efficient than three-photon ionization at 248 nm. The free
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(11) J. Bokor, J. Zavelovich, and C. K. Rhodes, J.Chem. Phys., 72,965 (1980).
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electrons have higher mobility than Xe and can induce efficient spin-flipping processes in the precursor or the Cz* radical itself. It should be noted that, if the E'&jtate (fnst singlet above d311u)of C2is the precursor of the d3H, state, the intersystem crossing will result in highly vibrationally excited ( u L 8) d311u. Such excitation has not been observed even at the lowest pressures used. Based on the cross-section data given in ref 12, about 10" electrons/cm3 are produced by the ionization of Xe (10 torr) under our experimental conditions. Using a photoionization cell analogous to the one described in ref 13 we find that the ionization at 193 nm is a factor of 3 higher than the ionization at 248 nm under the laser power conditions employed here. It is not clear if the factor of three higher electron yield at 193 nm can account for the observed enhancement of the C2*emission. However, in addition to the total ion yield the energy of the electrons produced in the photoionization process, i.e., 0.7 eV at 193 nm and 2.9 eV at 248 nm, may be of importance. It is quite possible that the spin-flipping process has a higher cross section with the slower electrons because of a resonance scattering behavior.14J5 In summary, we have demonjtrated an unusual collisional enhancement of the Cz*(d3JIg)formation in multiphoton fragmentation processed by Xe "buffer" gas. The effect of Xe on the C2*(311) formation is excitation wavelength dependent. The otserved effect is tentatively associated with a collisional enhancement, by Xe or free electrons, of a spin-forbidden process which results in d311u state formation. (12) D. Klingler, D. Pritchard, W. K. Bischel, and C. K. Rhodes, J . Appl. Phys., 49, 2219 (1978). (13) W. Zapka and F. P. Schiifer, Appl. Phys., 20, 287 (1979). (14) G. J. Schulz, Rev. Mod. Phys., 45, 423 (1973). (15) G. J. Schulz in "Principles of Laser Plasmas", G. Bekefi, Ed., Wiley, New York, 1976.
Excited-State Energetics and Dynamics of Zinc Tetrabenzoporphine in Supersonic Expansions Url Even, Joshua Jortner, Department of Chemistry, Tel-Aviv University, 69978 Tel Aviv, Israel
and Joel Friedman Bell Laboratories, Murray Hill, New Jersey 07974 (Received: February 9, 1982; I n Final Form: April 15, 1982)
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Laser-inducedfluorescence excitation spectra of the zinc tetrabenzoporphine molecule in seeded pulsed supersonic expansions of He provide information on the electronic-vibrational level structure of the So SI and the So S2 excitations. Electronic relaxation in the SI manifold was interrogated by time-resolved spectroscopy, while energy-resolved,line-broadening data provided semiquantitative information on interstate radiationless relaxation from the electronic origin of the S2 state.
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Introduction An extremely interesting application of the techniques of laser spectroscopy of ultracold, isolated, large molecules in supersonic expansions' will involve the investigation of (I)(a) M.P. Sinha, A. Schultz, and R. N. Zare, J. Chem. Phys., 58, 549 (1973); (b) R. E. Smalley, B. L. Ramakrishna, D. H.Levy, and L. Wharton, C h e p Phys.,61,4363 (!974); (c) D. H. Levy, L. Wharton, and R. E. Smalley 111 'Chemical and Biochemical Applications of Lasers", Vol. 2, Academic Press, New York, 1977, p 1; (d) D. H. Levy, L. Wharton, and R. E. Smalley, Acc. Chem. Res., 10, 134 (1977); (e) D. H. Levy, Annu. Rev. Phys. Chem., 31, 197 (1980).
electronically-vibrationally excited states of large molecules, which are relevant for the elucidation of basic photobiological processes. In this context, the interrogation of the excited-state level structure and radiationless transitions of "isolated" porphyrins are of considerable interest. The work of Fitch et aL2on the Q, and Qy bands of the phthalocyanine molecule in supersonic expansions (2) P. S. Fitch, L. Wharton, and D. H. Levy, J.Chem. Phys., 70,2019 (1979).
0022-3654/82/2086-2273$01.25/0 0 1982 American Chemical Society
