Optical Control of Excited-State Vibrational Coherences of a Molecule in Solution: The Influence of the Excitation Pulse Spectrum and Phase in LD690 † A. C. Florean, E. C. Carroll, K. G. Spears, ‡ R. J. Sension,* and P. H. Bucksbaum* FOCUS Center, Randall Laboratory, 450 Church Street, UniVersity of Michigan, Ann Arbor, Michigan 48109-1040 ReceiVed: May 5, 2006; In Final Form: July 17, 2006 Spectral and phase shaping of femtosecond laser pulses is used to selectively excite vibrational wave packets on the ground (S 0 ) and excited (S 1 ) electronic states in the laser dye LD690. The transient absorption signals observed following excitation near the peak of the ground-state absorption spectrum are characterized by a dominant 586 cm -1 vibrational mode. This vibration is assigned to a wave packet on the S 0 potential energy surface. When the excitation pulse is tuned to the blue wing of the absorption spectrum, a lower frequency 568 cm -1 vibration dominates the response. This lower frequency mode is assigned to a vibrational wave packet on the S 1 electronic state. The spectrum and phase of the excitation pulse also influence both the dephasing of the vibrational wave packet and the amplitude profiles of the oscillations as a function of probe wavelength. Excitation by blue-tuned, positively chirped pulses slows the apparent dephasing of the vibrational coherences compared with a transform-limited pulse having the same spectrum. Blue-tuned negatively chirped excitation pulses suppress the observation of coherent oscillations in the ground state. 1. Introduction The past decade has witnessed increasing interest in the field of coherent control of molecular dynamics. This interest is supported by technical advances in short pulse generation and, more importantly, by advances in methods to manipulate or shape the phase and amplitude of spectrally broad optical pulses. 1-4 A large number of systems in both the gas and condensed phases have been investigated. 5-19 Among these, dye molecules in solution have been studied extensively. 10-17 Dye molecules have important characteristic advantages for such study. Most dyes exhibit strong absorption in the visible part of the electromagnetic spectrum, where reliable ultrafast laser sources are readily available; they are photostable over a large number of excitation de-excitation cycles; the ground and excited-state dynamics and structure are in general well characterized; and their size and structures make them good prototypes for more complex biological chromophores. For the most part, coherent control studies on dyes have been concerned with manipulation of the ground and excited-state population. Linear chirp has emerged as the single most important control parameter. 10,11,13,14 Pioneering work in this field was done by Shank and co-workers, who studied the LD690 (oxazine 4 or 3-ethylamino-7-ethylimino-2, 8-dimeth- ylphenoxazin-5-ium perchlorate) dye molecule among other systems. 20-24 These experiments demonstrated that high-power positively chirped excitation enhances the total fluorescence signal (i.e., the excited-state population), while high-power negatively chirped excitation suppresses fluorescence. 22,25 An intrapulse pump-dump model was invoked to explain this result. In the case of negative chirp, excitation is initiated by the blue edge of the spectrum. The instantaneous frequency of the remainder of the pulse follows the dynamic red shift of the molecular absorption. As a consequence, population is reso- nantly dumped back to the ground state by stimulated emission. In contrast, positive chirp pumps population to the upper level, beginning in the red and continuing to pump higher into the excited state as the pulse tunes blue, rather than dumping population back to the ground state. Coherent ground and excited-state dynamics may play an important role in many chemical reactions. 25-30 Selective excitation of the vibrational modes of a molecule can lead to bond breaking and/or steer the system toward a desired target state. Optical pulse shaping can be extended from the control of population in electronic states to the control of vibrational coherences through Raman-type processes. 21,23,30,31 In many cases, it is not trivial to separate the ground and excited-state contributions to the coherence signal. A number of experimental and theoretical papers have dealt with this problem. 32-37 The amplitude, frequency, phase, and damping times of the coherent oscillations can be used to deduce the ground or excited-state nature of the vibrational wave packet(s) generated in the excitation process. Chirped pulse excitation was found to have a significant impact on the amplitude of the wave packet oscillations. 21,23 Negatively chirped pulses stimulate ground- state coherences, while transform-limited and positively chirped pulses enhance the excited-state component. 20,21,23,24 In this paper, we explore the influence of both spectral shaping and phase shaping of the excitation pulse on the observation of vibrational coherences for the LD690 dye molecule. These results are compared with the results of prior experimental investigations. LD690 is a convenient molecule as it has a rigid structure and exhibits a single dominant Franck- Condon active mode at about 586 cm -1 , assigned to a ring- breathing motion. 20 In the present study, we find that changing the pump spectrum allows selective excitation of ground or excited-state coherences. For a given pump spectrum, phase shaping can further improve control of the vibrational coher- † Part of the special issue “Charles B. Harris Festschrift”. * To whom correspondence should be addressed. E-mail: rsension@umich.edu; phb@slac.stanford.edu. ‡ Permanent Address: Department of Chemistry, Northwestern Univer- sity, Evanston, Illinois. 20023 J. Phys. Chem. B 2006, 110, 20023-20031 10.1021/jp0627628 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/29/2006