Instantaneous vibrational frequencies of diffusing and desorbing adsorbates: CO/Pt(111)

Area of Science:

• Surface Science
• Physical Chemistry
• Laser Physics

Background:

• Intense laser pulses induce adsorbate nuclear motion via nonadiabatic coupling.
• Precursor states, with highly excited vibrational modes, mediate adsorbate diffusion and desorption.
• Understanding these dynamics is crucial for controlling surface reactions.

Purpose of the Study:

• To investigate the dynamics of precursor states of carbon monoxide (CO) on a platinum(111) surface.
• To probe the vibrational response function and its evolution under laser irradiation.
• To elucidate the mechanisms of laser-induced diffusion and desorption.

Main Methods:

• Utilized infrared-visible sum frequency generation with phase-sensitive detection.
• Obtained the second-order nonlinear susceptibility and vibrational response function.
• Analyzed the free induction decay of vibrational polarization perturbed by a 400-nm pump pulse.

Main Results:

• Observed a redshift in C-O stretching frequency followed by a reverse shift upon high-fluence laser pumping.
• Demonstrated that laser pulses induce diffusion and desorption of CO adsorbates.
• Retrieved time evolution of instantaneous C-O stretching frequency from perturbed free induction decay.

Conclusions:

• Laser excitation significantly excites frustrated modes of CO precursors.
• Adsorbate collisions convert parallel momentum to normal momentum, exciting external stretching modes.
• This provides insights into laser-driven surface dynamics and adsorbate manipulation.