Onset of decoherence: Six-wave mixing measurements of vibrational decoherence on the excited electronic state of I2 in solid argon

Abstract

Pump–probe, four-wave, and six-wave mixing measurements of ${\mathrm{I}}_2$ isolated in solid argon are used to provide a clear experimental measure for the onset of vibrational quantum decoherence on the excited electronic state. The electronically resonant, six-wave mixing measurements bypass the rapid electronic dephasing, and measure the quantum cross-correlation between two packets launched on the B-state. The vibrational quantum coherence survives one period of motion, 400 fs, during which ∼2000 cm⁻¹ of energy is transferred to the lattice. The decoherence occurs during the second cycle of motion, while classically coherent motion measured via pump–probe spectroscopy using the same electronic resonances continues for ∼15 periods. This is contrasted with vibrational dephasing on the ground electronic surface, which lasts for ${10}^2$ periods, as measured through time-resolved coherent anti-Stokes Raman scattering. The measurements and observables are discussed through time-circuit diagrams, and a mechanistic description of decoherence is derived through semiclassical analysis and simulations that reproduce the experiments.