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Measuring coherence dynamics of methanol using transient coherent spontaneous Raman scattering

Seth J. Meiselman, Offir Cohen, M. F. DeCamp, Virginia O. Lorenz

Published Jan 1, 2014• 0 citations
Publisher

Abstract

We demonstrate the measurement of vibrational state coherence dynamics in liquid methanol using transient coherent spontaneous Raman scattering. The resulting lifetimes and quantum beat frequency agree with frequency-domain and coherent anti-Stokes Raman scattering measurements.
DOI
10.1364/cleo_at.2014.jth2a.110

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We demonstrate coherence measurements of single-photon-level collective excitations of vibrational states using transient coherent spontaneous Raman scattering in liquid methanol. We observe the decay of the 1033 cm−1 mode and coherence oscillations due to simultaneous excitation of the 2834 and 2944 cm−1 modes. The coherence life-times and oscillation frequencies agree with frequency-domain line-shape measurements and femtosecond coherent anti-Stokes Raman scattering measurements. The demonstrated technique is complementary to and, in some cases, simpler than traditional stimulated spectroscopy techniques in that it does not require more than one laser and is free of nonresonant background.
Spontaneous vibrational Raman scattering is a ubiquitous form of light-matter interaction whose description necessitates quantization of the electromagnetic field. It is usually considered as an incoherent process because the scattered field lacks any predictable phase relationship with the incoming field. When probing an ensemble of molecules, the question therefore arises: What quantum state should be used to describe the molecular ensemble following spontaneous Stokes scattering? We experimentally address this question by measuring time-resolved Stokes-anti-Stokes two-photon coincidences on a molecular liquid consisting of several sub-ensembles with slightly different vibrational frequencies. When spontaneously scattered Stokes photons and subsequent anti-Stokes photons are detected into a single spatiotemporal mode, the observed dynamics is inconsistent with a statistical mixture of individually excited molecules. Instead, we show that the data are reproduced if Stokes-anti-Stokes correlations are mediated by a collective vibrational quantum, i.e. a coherent superposition of all molecules interacting with light. Our results demonstrate that the degree of coherence in the vibrational state of the liquid is not an intrinsic property of the material system, but rather depends on the optical excitation and detection geometry.
Stimulated Raman scattering is conventionally employed to probe vibrational dynamics by inducing a coherent oscillation within a molecular ensemble. In contrast, spontaneous Raman scattering is considered as an incoherent process because the scattered field lacks any predictable phase relationship with the incoming laser pulse. It raises questions about the nature of the quantum state of a molecular ensemble following spontaneous Stokes scattering: should it be described as a statistical mixture of vibrationally excited molecules, where collective coherence is absent? Here, we measure time-resolved Stokes--anti-Stokes two-photon coincidences on a molecular liquid and find that a coherent superposition of two collective vibrational modes from two isotopic sub-ensembles is generated under spontaneous Raman scattering. Our results confirm that a spatially extended vibrational coherence emerges when a single collective excitation is spontaneously scattered and detected in a single spatiotemporal mode.
A new method to measure picosecond or subpicosecond vibrational-dephasing dynamics with the use of incoherent or broadband light is presented theoretically and experimentally. It is based on a transient coherent Raman process with three beams. In our experimental scheme, two of them are incoherent light from a single source, and a delay time between the two is variable. A third beam has a higher frequency than the incoherent light by a vibrational energy in a molecule of interest, and is coherent in the delay-time range of the measurement. The delay-time dependence of coherent Stokes Raman scattering (CSRS) intensity offers the information about the coherence dynamics of the vibrational transition with a resolution time determined only by the correlation time of the incoherent light. For the theoretical calculation, a three-level model of the molecular system with homogeneous broadening is used, and delay-time dependences of the CSRS signal are calculated for off-resonance and resonance cases. Dephasing dynamics of the 2915-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode in dimethylsulfoxide was observed experimentally by the new method with the use of nanosecond laser pulses, and the result agrees well with that obtained with picosecond pulses.
Time-resolved, electronically resonant, coherent anti-Stokes Raman scattering is used to prepare and interrogate vibronic coherences of molecular iodine in matrix Ar. Coherences that involve evolution on the excited B(3Π0u) state, first- and third-order coherences, decay in less than one vibrational period (τ<300 fs). In contrast, as many as 200 vibrational periods of motion can be observed for Raman-prepared wave packets consisting of zero-phonon vibrational superpositions on the ground electronic state (second-order coherence). Packets consisting of v=4, 5 and v=3, 4, 5 on the X(1Σg) state decay with a half-life of 10±1 ps at 31 K, allowing a more accurate measure of vibrational level spacings and decoherence time than has been possible in frequency domain. The harmonic frequency of the molecule is reduced by 1.5 cm−1 (0.7%) in the matrix. The lack of recurrence in the excited electronic state ensures that the resonant anti-Stokes scattering arises only from the negative momentum component of the Raman packet. This momentum filter, which should be ubiquitous in condensed media, leads to a signal with deeper modulation than in the gas phase.