Torsional Wave-Packet Dynamics in 2-Fluorobiphenyl Investigated by State-Selective Ionization-Detected Impulsive Stimulated Raman Spectroscopy

We report the creation and observation of vibrational wave packets pertinent to torsional motion in a biphenyl derivative in its electronic ground-state manifold. Adiabatically cooled molecular samples of 2-fluorobiphenyl were irradiated by intense nonresonant ultrashort laser pulses to drive impulsive stimulated Raman excitation of torsional motion. Spectral change due to the nonadiabatic vibrational excitation is probed in a state-selective manner using resonance-enhanced two-photon ionization through the S₁ ← S₀ electronic transition. The coherent nature of the excitation was exemplified by adopting irradiation with a pair of pump pulses: observed signals for excited torsional levels exhibit oscillatory variations against the mutual delay between the pump pulses due to wave-packet interference. By taking the Fourier transform of the time course of the signals, energy intervals among torsional levels with v = 0-3 were determined and utilized to calibrate a density functional theory (DFT)-calculated torsional potential-energy function. Time variation of populations in the excited torsional levels was assessed experimentally by measuring integrated intensities of the corresponding transitions while scanning the delay. Early time enhancement of the population (up to ∼2 ps) and gradual degradation of coherence (within ∼20 ps) appears. To explain the observed distinctive features, we developed a four-dimensional (4D) dynamical calculation in which one-dimensional (1D) quantum-mechanical propagation of the torsional motion was followed by solving the time-dependent Schrödinger equation, whereas three-dimensional (3D) molecular rotation was tracked by classical trajectory calculations. This hybrid approach enabled us to reproduce experimental results at a reasonable computational cost and provided a deeper insight into rotational effects on vibrational wave-packet dynamics.

Jet-cooled vibronic spectroscopy and asymmetric torsional potentials of phenylcyclopentene

The ultraviolet spectroscopy of the S(1) <-- S(0) transition of 1-phenylcyclopentene (PCP) was studied by resonant-two-photon ionization (R2PI), laser-induced fluorescence (LIF) and single vibronic level fluorescence (SVLF). UV-UV hole-burning (UVHB) spectroscopy was used to determine that there is only one spectroscopically distinct conformer in the supersonic expansion. The excitation spectrum shows extensive vibronic structure extending to over 1000 cm(-1) above the electronic origin (34,646 cm(-1)). Much of the vibronic structure is similar to that of styrene and other singly substituted benzene derivatives, with Franck-Condon (FC) activity predominantly in substituent-sensitive benzene modes. Sizeable FC progressions were also found in the inter-ring torsion, reflecting a large displacement in the inter-ring angle upon electronic excitation. No evidence for FC activity in the ring-puckering coordinate is observed. The torsional potentials of the ground and excited states were determined from the experimental transition frequencies by fitting the calculated to the experimental torsional frequency spacings in an automated least-squares fitting procedure. The S(1) torsional potential is a symmetric single-well potential centered around a locally planar equilibrium geometry at a torsional angle of varphi = 0 degrees . The energy levels are reproduced by a cosine term potential function with torsional parameters V(2) = 3765 cm(-1) and V(4) = -183 cm(-1). The S(0) torsional potential possesses a twisted equilibrium geometry that is strongly asymmetric about varphi = 0 degrees due to the non-planarity of the cyclopentene ring. The best-fit potential parameters uses a sin/cos potential function (odd/even), with V = 948 cm(-1), V = -195 cm(-1), V = -162 cm(-1) and V = -268 cm(-1). The shape of the potentials are similar to those predicted by relaxed potential energy scans calculated at the DFT, CIS and TDDFT//CIS levels of theory. The change in the torsional angle varphi upon electronic excitation was determined to be approximately 15 degrees from fits of the displacement delta of the S(0) torsional potential with respect to the S(1) potential. The simulated shift of the S(0) potential with respect to the S(1) potential of approximately 15 degrees is in very good agreement with that obtained from B3LYP calculations.