Abstract
We present a methodology for computing vibrationally and time-resolved pump-probe spectra, which takes into account all vibrational degrees of freedom and is based on the combination of the thawed Gaussian approximation with on-the-fly ab initio evaluation of the electronic structure. The method is applied to the phenyl radical and compared with two more approximate approaches based on the global harmonic approximation-the global harmonic method expands both the ground- and excited-state potential energy surfaces to the second order about the corresponding minima, while the combined global harmonic/on-the-fly method retains the on-the-fly scheme for the excited-state wavepacket propagation. We also compare the spectra by considering their means and widths, and show analytically how these measures are related to the properties of the semiclassical wavepacket. We find that the combined approach is better than the global harmonic one in describing the vibrational structure, while the global harmonic approximation estimates better the overall means and widths of the spectra due to a partial cancellation of errors. Although the full-dimensional on-the-fly ab initio result seems to reflect the dynamics of only one mode, we show, by performing exact quantum calculations, that this simple structure cannot be recovered using a one-dimensional model. Yet, the agreement between the quantum and semiclassical spectra in this simple, but anharmonic model lends additional support for the full-dimensional ab initio thawed Gaussian calculation of the phenyl radical spectra. We conclude that the thawed Gaussian approximation provides a viable alternative to the expensive or unfeasible exact quantum calculations in cases, where low-dimensional models are not sufficiently accurate to represent the full system.
Highlights
The method is applied to the phenyl radical and compared with two more approximate approaches based on the global harmonic approximation—the global harmonic method expands both the ground- and excited-state potential energy surfaces to the second order about the corresponding minima, while the combined global harmonic/on-the-fly method retains the on-the-fly scheme for the excited-state wavepacket propagation
The full-dimensional on-the-fly ab initio result seems to reflect the dynamics of only one mode, we show, by performing exact quantum calculations, that this simple structure cannot be recovered using a one-dimensional model
The evaluation of vibrationally resolved electronic spectra is often limited to approximate potential energy surfaces, e.g., global harmonic models,12 or exact quantum calculations taking into account only a few degrees of freedom
Summary
Pulses, led to the direct observation of nuclear wavepacket dynamics in various systems. These spectra are commonly analyzed with fitting procedures to recover the kinetics, with Fourier transformation to identify the main frequency components, or with simple low-dimensional models to simulate the vibronic features. computationally feasible full-dimensional methods for simulating vibrationally and time-resolved spectra of large polyatomic molecules are missing. In the hybrid semiclassical approach of Grossmann, the original thawed Gaussian approximation is applied to model the “harmonic” degrees of freedom, while treating the rest by more sophisticated semiclassical methods In this context, the success of the on-the-fly ab initio thawed Gaussian approximation in reproducing steady-state absorption, emission, and photoelectron spectra of rather large systems is somewhat surprising. We adapt the on-the-fly ab initio thawed Gaussian approximation in order to evaluate vibrationally and time-resolved electronic spectra, and compare it with the commonly used adiabatic global harmonic approach, in which the two potential energy surfaces are expanded to the second order about the corresponding minima. To justify our prediction of time-resolved electronic spectra of the phenyl radical, we construct a Morse potential corresponding to the single vibrational mode and evaluate the pump-probe spectra using both the thawed Gaussian approximation and exact quantum dynamics. The results imply that the thawed Gaussian approximation is rather accurate at this level of anharmonicity and that more-dimensional calculations are required to reproduce the on-the-fly ab initio spectrum
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