Separation of vibrational and rotational coherences with polarized femtosecond time-resolved four-wave mixing spectroscopy

Abstract

One of the degrees of freedom available to femtosecond time-resolved four-wave mixing spectroscopy is the geometry with which the polarization of the three independent lasers employed in this technique are oriented relative to one another. In the work presented here, different polarization geometries were used to demonstrate the selectivity that can be achieved with respect to the rotational and vibrational dynamics observed in a femtosecond time-resolved four-wave mixing transient. Two of the polarization geometries presented here allow for the simultaneous observation of rotational and vibrational coherences and furthermore illustrate the mechanism with which the rotational dynamics enter into the signal. A third 'magic angle' polarization geometry allows for the complete elimination of the rotational coherences from the signal, which makes full characterization of the vibrational dynamics in the four-wave mixing signal possible. This is shown for the dynamics in both an electronically excited and the electronic ground state of gaseous iodine, which serves as a model system. The results are discussed in a wave-packet picture which applies under the present experimental conditions where any damping mechanism can safely be neglected.

Rotational and vibrational dynamics in the excited electronic state of deprotonated and protonated fluorescein studied by time-resolved photofragmentation in an ion trap

“…A vibrational wavepacket, comprised of several superimposed vibrational states with a favorable phase relation, is then launched, which oscillates with a period close to the relevant vibrational mode and finally decays by coupling to other vibrational modes. Since the vibrational timescale is usually much shorter than the rotational one, a superposition of vibrational and rotational wavepackets may be observed, as reported, e.g., for polarized four-wave mixing spectroscopy on gaseous iodine 42 . In the case of [FL − H] − , Horke et al .…”

Tracking molecular wave packets in cesium dimers by coherent Raman scattering

“…The study of the vibrational and rotational wave packets in molecules on femtosecond time scale has been of interest for several decades [3]. Recently, the vibrational and rotational wave-packet dynamics in different molecules have been observed by adopting the two-color, pump probe [4][5][6] and coherent femtosecond four-wave mixing (FWM) spectroscopic [7][8][9][10] techniques. Resonant coherent anti-Stokes Raman scattering (CARS) spectroscopy has been used to extract a time-dependent wave function of a reacting molecule [11].…”

Time- and frequency-resolved coherent anti-Stokes Raman scattering spectroscopy with sub-25fs laser pulses

“…Due to the inversion symmetry of molecules in bulk noncrystalline media, the lowest order NLO response of general utility is at third order, of which coherent anti-Stokes Raman scattering ͑CARS͒ is a well known example. When combined with ultrashort laser pulses, CARS can reveal detailed dynamics within excited molecules in both gas 8,9 and condensed phases. The CARS process involves the interaction of three laser fields, generating a third order polarization whose associated emitted field is measured in a phasematched direction.…”