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J . Phys. Chem. 1993,97, 2228-2235
Continuously Scanned Resonant Raman Excitation Profiles for Iodobenzene Excited in the B Continuum Cbung-Yi Kung, Bor-Yu Chang, C. Kittrell, Bruce R. Johnson, and James L. Kinsey' Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77251 Received: September 30, 1992; In Final Form: November 30, 1992
A new technique for obtaining continuously scanned Raman excitation profiles (REP's) is described. The technique relies on simultaneous stepping of the laser wavelength (excitation frequency), the angle of a frequencydoubling crystal, and spectrometer grating angle under control of a single computer. The method is applied to the R E P S of four vibrational bands of iodobenzene (265, 530, 1065, and 1575 cm-I) excited in the B continuum (216-233 nm). The results are interpreted in terms of a dynamical model of the ground and excited B electronic states.
I. Introduction Within the past few years, it has become widely recognized that the interplay between spectroscopy and the dynamics of photodissociative processes provides a uniquely sharp tool for investigating the latter.I4 Measurement of both absorption spectra in dissociative continua and resonance Raman scattering for excitation within such continua have proven to be quite useful to this end. In dissociative resonance Raman spectroscopy (DRRS), a single excitation frequency is usually used, and the emission spectrum is scanned in wavelength. Such single excitation frequencyRaman spectra can provide valuable insight into the qualitative nature of the "reaction coordinate" for the photodi~sociation.~Recently, however, it has become evident that, in order toextract quantitative information about the reaction dynamics,it is extremely important to determine the dependence of these Raman scatteringcross sectionsas functionsof the exciting wavelength throughout the resonant dissociative band;5-7i.e.,one ' needs the Raman excitation profiles (REP's) for a clear-cut dynamical interpretation. In the limited number of cases in which REP's have been obtained in the past (including our own earlier work), the usual procedure was to obtain the full Raman spectrum at a number of excitation wavelengths and then to patch together the dependence of those features of interest as a function of this excitation wavelength. This is problematical in two respects: Only a llimited number of wavelengths at finite intervals from each other can be sampled, and it has been our experience that experimental differences between runs at different wavelengths are quite difficult to register with each other (thus leading to significant errors). The second difficulty has sometimes been addressed by using a nonresonant internal standard such as nitrogen gas. However, it is not always possible to find a standard whose Raman spectrdm is free of overlap with the features of the molecule under study. In this paper, we report implementation of a new technique in which a computer controls the simultaneous scanning of excitation wavelength and the spectrometer so that one stays on a given vibrational feature as the excitation wavelength is varied. During each scan, the measured Raman intensity is referenced to the total undispersed light scattering by the sample, which is then calibrated to a known standard. Although DRRS finds its greatest strength in applications to relatively small polyatomic molecules, there are still important applicationsto larger molecules. Our earlier work on iodobenzene is such an example.' In that work, a crude REP was determined by taking the entire Raman spectrum at a number of excitation wavelengths and then comparing the relative intensities of the various featuresas functionsof tMs wavelength. It was not possible todetermine the absolutecalibrationofone wavelength toanother. 0022-3654/93/2097-2228$04.00/0
In thispaper,wereporta reinvestigationoftheiodobenzeneDRRS spectrum, using our newly developed continuous scanning technique with calibration relative to the total scattering of N2 (via comparison at each wavelength to the total scattering by iodobenzene). The B band of iodobenzene (216-233 nm) is assigned* as 'A1 IAl, which correlates to the ring-based u* u 8 lBlu X 'Al, transition in benzene (183-217 nm). The excited state in C6H51is expected, however, to be significantly mixed with a charge-transfer configurations corresponding to C6Hs+I-. The C6H6transition is actuallydipole-forbidden (vibronically-allowed) in Dsh symmetry, although it becomes dipole-allowed in Cz, symmetry. The majority of the Raman activity in C6H5I is observed to be in the planar vibrational mode^,^^^ severalof which come in closely-spaced al-b2 pairs correlating to degenerate e modes in C&,. One of the strongest bands in the resonance Raman spectrum (- 1580 cm-I) is a member of such a pair of modes, although there is some uncertainty as to whether it is the a l or bz partner.y-'OThis point is addressed in our theoretical analysis. Our theoretical analysis of the observed REP's for three different vibrational features is based on a displaced harmonic oscillator model, with inclusion of nuclear coordinate dependence of the transition moment ("non-Condon" effects). In the timedependent formulationof Raman scattering, the differential cross section appears as the absolute square of an amplitude which is the transform of the product of a unit step function in time and a dynamical "correlation function" of the type C,(t) = (&l&(t)). the function 14,) is the j vibrational eigenfunction times appropriatecomponents of the electronic transition moment, and l~$,(t)) represents the same function after it has evolved for a time t on the repulsive potential surface of the excited state. In section IV we work with a model excited-state potential energy surface (PES) with adjustable parameters, which allows computation of the desired Cn(t),from which the REP's can be obtained. The PES parameters as well as parameters related to the non-condon effects are adjusted to produce satisfactory agreement between the theory and the observations. It has been widely acceptedthat more direct methods to obtain the correlation functions directly from the data by inversion are not possible because of the loss of phase information in taking the absolute square of the Raman scattering amplitude. There have, however, recently appeared some quite promising new results which suggest that significant progress in overcoming this limitation may be possible." Since these more direct approaches are still in an early stage of development, our analysis in this paper continues to rely on dynamical models with parameters that can be adjusted to maximize agreement with the experiments.
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0 1993 American Chemical Society
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Raman Excitation Profiles for Iodobenzene 11. Experimental Section The basic apparatus used in this work is similar to those previously described' for resonance Raman experiments. However, there have been some substantial changes in the apparatus to improve the quality of the data in general and more importantly to implement the real-time REP recording. Only the changes that are crucial to recording REP'S will be covered in detail here. The excimer laser is now free-running at 5-10 Hz instead of being triggered externally. A fast photodiode (EGCG FDNlOOQ,
