Femtosecond Time-Resolved Observation of Relaxation and Wave Packet Dynamics of the S1 State in Electronically Excited o-Fluoroaniline

Abstract

Quantum beat frequency is the basis for understanding interference effects and vibrational wave packet dynamics and has important applications. Using femtosecond time-resolved mass spectrometry and femtosecond time-resolved photoelectron image combined with theoretical calculations, we study the electronic excited-state relaxation of o-fluoraniline molecule and the time-dependent evolution of vibrational wave packets between different eigenstates. After the molecule absorbs a photon of 288.3 nm and is excited to the S₁ state, intramolecular vibrational redistribution first occurs on the time scale τ₁ = 349 fs, and then the transition to the triplet state occurs through the intersystem crossing on the time scale τ₂ = 583 ps, and finally, the triplet state occurs decays slowly through the time scale τ₃ = 2074 ps. We find the intramolecular vibrational redistribution is caused by the 0⁰, 10b¹ and 16a¹ vibrational modes of the S_l state origin. That is, the 288.3 nm femtosecond laser excites the molecule to the S₁ state, and the continuous flow of the vibrational wave packet prepares a coherent superposition state of three vibrational modes. Through extracting the oscillation of different peak intensities in the photoelectron spectrum, we observe reversible changes caused by mutual interference of the S₁ 0⁰, S₁ 10b¹ and S₁ 16a¹ states when the wave packets flow. When the pump pulse is 280 nm, the beat frequency disappears completely. This is explained in terms of increases in the vibrational field density and characteristic period of oscillation, and statistical averaging makes the quantum effect smooth and indistinguishable. In addition, the Rydberg component of the S₁ state is more clearly resolved by combining experiment and theory.

Keywords: o-fluoraniline; photoelectron image; quantum beat frequency; relaxation dynamic; wave packet dynamic.

Figure A1

The infrared spectrum of the ground state, S₁ state, and T₁ state of the molecule shows no imaginary frequency. For the S₀ state, calculation method and basis set: DFT and CAM-B3LYP/6-311+G(d,p). For S₁ and T₁ states, calculation method and basis set: TD-DFT and CAM-B3LYP/6-311+G(d,p).

Figure A2

Molecular structure and Cartesian coordinate system of the ground state, S₁ state and T₁ state, and the Z axis is perpendicular to the paper surface. For the S₀ state, calculation method and basis set: DFT and CAM-B3LYP/6-311+G(d,p). For S₁ and T₁ states, calculation method and basis set: TD-DFT and CAM-B3LYP/6-311+G(d,p).

Figure A3

The molecular orbitals of the first to the fifth and eighth peaks.

Figure 1

o-Fluoroaniline energy varies with angle, and the five structures of the molecule are given. Calculation method and basis set: DFT and CAM-B3LYP/6-311+G(d,p).

Figure 2

MO29, MO30, MO32 orbitals of o-fluoroaniline S₁ state. Calculation method and basis set: TD-DFT and CAM-B3LYP/6-311+G(d,p).

Figure 3

Time decay curves of the parent ion measured with (a) a 288.3 nm pump pulse and 800 nm probe pulse, (b) a 280 nm pump pulse and 800 nm probe pulse. The circles are experimental data, and the solid lines are fitted data. Both curves are fitted using a Gaussian cross-correlation function and a convolution of three exponential decay functions, resulting in different decay times.

Figure 4

Photoelectron spectra at delay times of (a) Δt = 0.027 ps and (b) Δt = 1.429 ps. The ordinates of the two plots are normalized, and (b) is normalized with the maximum value of (a) as a reference. The insets are the corresponding optoelectronic images (the original image on the left and BASEX transformed image on the right).

Figure 5

Time decay curves of the parent ion measured with (a) 266 nm pump pulse and 800 nm probe pulse, (b) 260 nm pump pulse and 800 nm probe pulse, (c) 250 nm pump pulse and 800 nm probe pulse, and (d) 240 nm pump pulse and 800 nm probe pulse. The circles are experimental data, and the solid lines are fitted data. All curves are fitted using a Gaussian cross-correlation function and a convolution of three exponential decay functions to obtain different decay times.

Figure 6

(a) Time-resolved photoelectron spectrum obtained with a 288.3 nm pump pulse and 800 nm probe pulse. (b) The spectrum obtained by Fourier transform of the residual data at the fourth peak. (c) Signal intensities of the six peaks as functions of delay time. Experimental data are given as open circles, while solid lines are fitted results.

Figure 7

(a) Time-resolved photoelectron spectrum obtained with 280 nm pump pulse and 800 nm probe pulse. (b) Signal intensities of the six peaks as functions of delay time. Experimental data are given as open circles, while solid lines are fitted results.