6983
J. Phys. Chem. 1993,97, 6983-6985
Ground-State and Excited-State Rotational Constants of pCyclohexylaniline Philip G. Smith, Thomas Troxler, and Michael R. Topp’ Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania I91 04-6323 Received: March 31, 1993; In Final Form: May 6, 1993
Picosecond time-correlated single-photon counting spectroscopy has been used in rotational coherence measurements to determine the excited-state moments of inertia ofp-cyclohexylaniline. This work complements earlier experiments involving single-colortime-resolved fluorescence depletion spectroscopy, allowing the separation and identification of the rotational constants in the SOand SIstates. Simulations have shown that the persistence of the K-type recurrences over several dozen cycles is consistent with a difference of -2% between the excitedstate B and Cconstants. The final results were as follows: (ground state) A“ = 2410 MHz, (B”+ C”) = 725 MHz; (excited state) A’ = 2355 f 15 MHz, B’ = 365 f 4 MHz, C’= 360 f 4 MHz.
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1. Introduction Rotationally resolved fluorescence excitation spectroscopy is currently providing an important means to interrogate the structures of large aromatic molecules and some of their van der Waals complexes in collimated molecular beams.’ Many other experiments involve supersonic free jets and/or large aggregates. Also, one commonly encounters short-lived excited states, such as in xanthione complexes,*or complicatedspectra resulting from vibrational mode mixing. Under these circumstances, spectral congestion tends to reduce the effectiveness of high-resolution spectroscopy, and the time-domain technique of rotational coherence spectroscopy (RCS) becomes i m p ~ r t a n t . ~Like the frequency-domain counterpart, RCS experiments provide a sensitive test of rotational level structure. Both techniques are therefore sensitive to the overall symmetry of the system, and to the orientation of the electronic transition moment with respect to the inertial axes of the molecule. In RCS, the actual coherence signals are Fourier transforms of the beat-frequency spectra associated with the coherent population of many levels. The method, depending on the differences in transition energies, is therefore not restricted by Doppler broadening. Although the time-domain approach tends to be less precise than the highresolution analogue where both are applicable, it provides a valuable means for estimating rotational constants of molecules in supersonic free jets. There are several different ways for implementing picosecond RCS experiments. The two main types of approach involve multiple-resonance pumpprobe techniques originally derived from time-resolved fluorescencedepletion (TRFD) spectroscopy,u and time-correlated single-photon counting (TCSPC) spectroscop^.^-^^ The TRFD technique, as originally applied by McDonald and c o - ~ o r k e r s ~ involves JJ double-resonance experiments in which the pump and probe pulses are derived by splitting a single picosecond pulse. RCS transients are detected in TRFD experiments, by arranging the polarizations of the two pulses to be either parallel or perpendicular. Time resolution is achieved by measuring the total fluorescenceintensity as a function of the temporal delay between the two pulses. Limited only by the laser pulse width, the resolution may be
