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J . Phys. Chem. 1989, 93, 2876-2878
Femtosecond Transient Absorption Spectroscopy of Cr(CO), in Methanol: Observation of Initial Excited States and CO Dissociation Alan G . Joly and Keith A. Nelson*!+ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 (Received: July 6, 1988; In Final Form: December 27, 1988)
Femtosecond time-resolved absorption spectroscopy has been used to study the photochemistry of Cr(CO)6 in methanol. Following photoexcitation at 308 nm, transient absorption measurements of Cr(C0)6 show a nonexponential feature at short times (less than 500 fs) which is interpreted in terms of dissociation of CO on a repulsive potential energy surface. Complexation of the resulting "naked" Cr(CO)5 with a solvent molecule is observed on a several picosecond time scale, consistent with earlier results. Slower (50 ps) spectral evolution is interpreted in terms of electronic and vibrational relaxation of the product, Cr(CO)5(MeOH). Similar results are observed in preliminary experiments in other solvents and on the molybdenum and tungsten analogues of Cr(C0)6.
Introduction Cr(CO)6 has long been considered a prototype for transitionmetal carbonyl photochemistry. In the gas phase, irradiation with ultraviolet light of virtually any wavelength produces Cr(CO), fragments with n varying between 0 and 5. In contrast, in solution or matrix environments, a single C O dissociates quickly and efficiently. Flash photolysis studies in matrices and solutions have shown that the photoproduct formed is a solvent-complexed This pentacarbonyl species, Cr(CO),S, where S is the is evidenced by a strong solvent dependence of the visible spectrum of Cr(CO)$. Initial solution-phase experiments performed on the nanosecond time scale were interpreted in terms of C O recombination kinetics of Cr(C0)5S.2 Picosecond studies by Simon and Peters3 indicated that the solvated species was present within 25 ps, but more recently Simon and Xie4 have shown that the solvent coordination time is 2.5 p in methanol and less than 1 ps in cyclohexane. Very little is known about the evolution of the system from the initially excited state of Cr(C0)6 to the C , pentacarbonyl species. Here we report solution-phase flash photolysis studies of Cr(C0)6 using < 100-fs pump and probe pulses which reveal the subpicosecond dynamics of C O dissociation. The subsequent solvent complexation kinetics are also observed. Femtosecond time-resolved spectroscopy experiments are unique in the sense that the pulse duration may be shorter than the time required for many types of elementary nuclear motions, including those involved in chemical change. As illustrated in Figure 1, photoexcitation with a sufficiently short pulse into a bound excited state (SI)produces excited molecules whose motions are in phase. Time-dependent molecular vibrational oscillations about the excited-state potential minimum influence absorption, birefringence, and other optical properties which can be measured with variably delayed probe pulses. Recently, these oscillations have been seen in organic dye molecules and information about the excited-state vibrational frequency and dephasing were ~ b t a i n e d . ~Absorption into a repulsive potential such as S3 in Figure 1 results not in oscillation but in a monotonic increase in vibrational displacement corresponding, for example, to photodissociation of the molecule along the vibrational coordinate. Depending upon the details of the reactive and other potential energy surfaces, this may lead to monotonic change in absorption, stimulated emission, or other optical properties.6* Rapid spectral shifts occurring during the photodissociation process can be used to obtain information about the shape of the reactive potential energy surface and about the influence of a condensed-phase environment on motion along the surface. Experimental Section The femtosecond laser system used has been described previ0us1y.~ Briefly, a CW mode-locked Nd:YAG laser synchronously Presidential Young Investigator Awardee and A. P. Sloan Fellow.
0022-3654/89/2093-2876$01.50/0
pumps a femtosecond dye laser with an antiresonant ring. The dye laser output consists of 65-fs pulses centered at 615 nm with a repetition rate of 82 MHz. These pulses are amplified in a three-stage dye amplifier pumped with the frequency-doubled output of a Nd:YAG regenerative amplifier. Seventy percent of the amplified output (500 Hz, 65 fs, 7 pJ) is separated and frequency-doubled by using a 1-mm KDP crystal to yield 308-nm, 0.5-pJ excitation pulses. These are focused to a 100-pm-diameter spot inside a 2-mm quartz flow cell containing the sample. The remaining 30% of the red light is focused into a 2-mm cell of D20 liquid to generate a white-light continuum. Ten-nanometer interference filters are used to select the wavelength of the probe pulses, which are variably delayed along a 1-pm stepping-motor delay line and overlapped with the pump pulses inside the sample. The polarizations of the beams are adjusted to be 54.7' apart to eliminate effects of molecular rotation on the signal. The intensities of probe light before and after the sample (Io and I , respectively) were measured and ( I - Io)/Iowas determined. The intensity of each excitation pulse was also measured, and only results of pulses within a 5% energy "window" were accepted. Two thousand such results were averaged at each point on the delay line. Cross-correlation involving the UV and probe light indicates an instrument response time of approximately 95 fs which is consistent with the fastest rise times observed in the transient absorption measurements. The samples were saturated (M) solutions in high purity methanol. To avoid damage from the high-intensity excitation pulses, flow cells were used and the cells were continuously translated horizontally and vertically on motorized translation stages. Measurements at each point on the delay line were made both with and without the excitation pulse present to subtract the background scatter of the probe pulse, which sometimes varied slightly during an experiment due to slow cell deterioration. Typical values of ( I - Io)/Io were between 2 and 15%. Results and Discussion Ultraviolet pulses at 308 nm excite predominantly the ]Alg.*TI,ligand field transitionlo in Cr(C0)6. This transition, which (1) Perutz, R. N.; Turner, J. J. J . Am. Chem. SOC.1975, 97, 4791. (2) Kelly, J. M.; Long, C.; Bonneau, R. J . Phys. Chem. 1983,87, 3344. (3) Simon, J. D.; Peters, K. S. Chem. Phys. Lett. 1983, 98, 53. (4) Simon, J. D.; Xie, X. J . Phys. Chem. 1986, 90, 6751. (5) Rosker, M. J.; Wise, F. W.; Tang, C. L. Phys. Rev. Lex 1987.57, 321. Nelson, K. A.; Williams, L. R. Phys. Rev. Lett. 1987, 58, 745. Chesnoy, J.; MokHtari, A. Phys. Rev. A 1988, 58, 3566. (6) Dantus, M.; Rosker, M. J.; Zewail, A. H. J . Chem. Phys. 1987, 87, 2395. Rose, T. S.; Rosker, M. J.; Zewail, A. H. J . Chem. Phys. 1988, 88, 6672. (7) Williams, L. R.; Nelson, K. A. J . Chem. Phys. 1987, 87, 7346. (8) Mathies, R. A,; Brito Cruz, C. H.; Pollard, W. T.; Shank, C. V. Science 1988, 240, 700. Dobler. J.; Zinth, W.; Oesterhelt, D. Chem. Phys. Lett. 1988, 144, 215. (9) Ruhman, S.;Kohler, B.; Joly, A. G.; Nelson, K. A. IEEE J . Quantum Electron 1988, QE-24, 470.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 2877
Letters
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QSchematic illustration of electronic potential energies as functions of vibrational coordinate Q. Optical absorption of an ultrashort pulse into SI leads to coherent oscillations about the excited-state potential minimum. Absorption into Szor S3leads to coherent wavepacket propagation from the excited state to the dissociated product.
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Time (ps) Figure 2. Transient absorption of Cr(C0)6 in methanol with a probe wavelength of 480 nm. The solid line is data which shows the spectral evolution as CO dissociates. The dotted line is a fit to the functional form for the potential, V(Q) = sechZ(oQ), shown in the left inset. The right inset shows an exponential increase in absorption at longer times associated with solvent complexation which yields Cr(CO)5(MeOH). is formally symmetry-forbidden but occurs through vibronic coupling, reduces r backbonding and places electron density in a Cr-CO u* orbital. This labilizes the Cr-CO bonds. If the excitation pulse is short in duration compared to the C O dissociation time then phase-coherent dissociation should be initiated. Figure 2 shows transient absorption measurements taken with 480-nm probe pulses. This wavelength corresponds to the absorption maximum of Cr(CO)5(MeOH). The data show three unique regions: a pulse duration-limited rise, a rapid (
