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The Journal of Chemical Physics

The determination of an inhomogeneous linewidth for a strongly coupled adsorbate system

Ian M. Lane, David Anthony King, Heike Arnolds

Published Jan 12, 2007• 18 citations

Abstract

We present a set of experiments that provide a complete mapping of coherent and incoherent vibrational relaxation times for a molecule on a metal surface, CO/Ir{111}. Included is the first detection of a midinfrared photon echo from a metallic surface, some 15 years after the analogous measurement on a semiconductor surface, which sets a precedent for the ability to manipulate and rephase polarization on a subpicosecond time scale on surfaces. For the C-O stretch in a strongly dipole-coupled CO layer we obtain a total linewidth of 5.6 cm-1, composed of a homogeneous width of 2.7 cm-1 and an inhomogeneous contribution of 3.0 cm-1. Pure dephasing is negligible at liquid nitrogen temperatures, making CO/Ir{111} an attractive model system for quantum computing.

DOI: 10.1063/1.2408413

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Vibrational photon echo experiments were performed on the asymmetric CO stretching mode of tungsten hexacarbonyl in glassy dibutylphthalate as a function of temperature using sub-picosecond infrared pulses (1976 cm−1) from a free electron laser. The echo decays display pronounced beats and are bi-exponential. The beats and bi-exponential decays arise because the bandwith of the pulses exceed the vibrational anharmonicity, leading to the excitation and dephasing of a multilevel coherence. From the beat frequency, the anharmonicity is determined to be 14.7 cm−1. From the bi-exponential decay components, the temperature-dependent vibrational dephasing of both the v = 0 → 1 and v = 1 → 2 transitions are determined.

Using the model of a weakly‐anharmonic, underdamped oscillator coupled to a bath, we demonstrate that the 2D time‐resolved Raman signal carries information about the population decay T1, the homogeneous dephasing T2, and the inhomogeneous dephasing T1 relaxation timescales. We distinguish between two projections of the 2D signal: first, the echo signal, which is stretched in the diagonal direction decays with T2, and the second which is stretched along one axis is related to the population relaxation and decays with T1. The width of both signals reflects T1. Equations of motion for vibrational coherence and population variables are employed in these calculations.

The first infrared vibrational photon echo experiments conducted in a liquid and a glass are reported. The experiments were performed on the CO stretching mode of tungsten hexacarbonyl at 5.1 micrometers (1960 cm-1) in 2-methyltetrahydrofuran over the temperature range 300 K to 16 K using picosecond pulses from the free electron laser at Stanford University. In addition, the first vibrational population relaxation measurements spanning a temperature range that takes a system from a liquid to a supercooled liquid to a glass are reported.

Time-resolved coherent anti-Stokes Raman scattering is applied to prepare and interrogate vibrational coherences on the ground electronic surface of molecular iodine isolated in Ar matrices. The coherence decay time shows a linear dependence on vibrational quantum numbers for v=3–15. The temperature dependence of decoherence rates is negligible for v<7 in the experimental range T=18–32 K. For a v=13, 14 superposition, the temperature dependence indicates dephasing by a 66 cm−1 pseudo-local phonon, just outside the Debye edge of the solid. The accuracy of the data is limited due to two-photon induced dissociation of the molecule, a process which is characterized using polarized fields. The T→0 limit of dephasing is discussed.