Nonadiabatic vibrational dynamics in the HCO2 (-)⋅H2O complex

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Hydrogen-bonded complexes exhibit complex infrared spectra, including red shifts and broadening of OH stretch bands.
• These spectral features reflect strong anharmonicity in the vibrational modes.

Purpose of the Study:

• To investigate the vibrational relaxation dynamics of the OH stretch in the HCO2(-)⋅H2O complex.
• To elucidate the spectral signatures arising from these dynamics.
• To understand the role of vibrational conical intersections in energy transfer.

Main Methods:

• Extensive ab initio calculations and time-propagation of the nuclear Schrödinger equation.
• Adiabatic separation of time scales for intramolecular and intermolecular modes.
• Calculation of potential energy surfaces (PESs) for ground and excited states.
• Identification of a vibrational conical intersection between OH stretch and HOH bend overtone states.

Main Results:

• A vibrational conical intersection was identified between the OH stretch fundamental and HOH bend overtone states.
• Coherent population transfer occurs via this conical intersection, with an ultrafast (60 fs) and irreversible initial step.
• Subsequent vibrational energy relaxation into the HOH bend and ground state is rapid (1 ps), despite PES separation.

Conclusions:

• The conical intersection significantly influences vibrational relaxation dynamics and spectral properties.
• The model achieves semiquantitative agreement with experimental Ar-tag action spectra.
• Adiabatic approximations are less significant for vibrations than for electronic processes due to mass differences.