Qin Chaochao

Coherent Control of Molecular Alignment and Orientation by a Femtosecond Two-Color Laser Pulse

The coherent control of molecular alignment and orientation by a femtosecond two-color laser pulse is studied theoretically. The effect of the carrier-envelope phase of the femtosecond two-color laser pulse on molecular alignment and orientation is discussed, and it is shown that the enhancement or suppression of the molecular orientation can be coherently manipulated by precisely controlling the carrier-envelope phase of the femtosecond two-color laser pulse. In addition, the time-dependent angular distributions of the molecular axis are presented.

Ultrafast dynamics of o-fluorophenol studied with femtosecond time-resolved photoelectron and photoion spectroscopy

The ultrafast dynamics of o-fluorophenol via the excited states has been studied by femtosecond time-resolved photoelectron imaging. The photoion and photoelectron spectra taken with a time delay between 267 nm pump laser and 800 nm probe laser provide a longer-lived S1 electronic state of about ns timescale. In comparison, the spectra obtained by exciting the S2 state with femtosecond laser pulses at 400 nm and ionizing with pulses at 800 nm suggest that the S2 state has an ultrashort lifetime about 102 fs and reflects the internal conversion dynamics of the S2 state to the S1 state.

Ultrafast Predissociation Dynamics of Excited State of Acrylic Acid

The ultrafast predissociation dynamics of acrylic acid after excitation to the second electronically excited state (S-2) with a 200 nm pump pulse were studied using a femtosecond pump-probe technique combined with time-of-flight mass spectroscopy (TOF-MS). The time-resolved mass spectra signals of the parent ion and fragment ions were collected. By using the kinetic equations to fit and analyze the time-resolved mass spectra ion signals, the existence of the predissociation channel was revealed. The excited molecule populated in the S-2 state decayed to the first electronically excited state (S-1) through a fast internal conversion process over a period of 210 fs. The excited molecule populated on the S-1 state then decayed to the vibrationally hot ground state (S-0) through another internal conversion process over a period of 1.49 ps. Finally, on the vibrationally hot ground state surface, the molecule dissociated to the neutral fragments, H2C=CH and HOCO, H2C=CHCO and OH via C - C bond fission and C - O bond fission, respectively. The corresponding predissociation time constants were determined to be approximately 4 and 3 ps, respectively. The generation of fragment ions can occur in two ways, both from the dissociation of the parent ion and the ionization of the neutral fragments on the vibrationally hot ground state surface.

Field-free molecular orientation induced by combined femtosecond single- and dual-color laser pulses: The role of delay time and quantum interference

The coherent control of field-free molecular orientation of CO with combined femtosecond single- and dual-color laser pulses has been theoretically studied. The effect of the delay time between the femtosecond single- and dual-color laser pulses is discussed, and the physical mechanism of the enhancement of molecular orientation with pre-alignment of the molecule is investigated. It is found that the basic mechanism is based on the creation of a rotational wave packet by the femtosecond single-color laser pulse. Furthermore, we investigate the interference between multiple rotational excitation pathways following pre-alignment with femtosecond single-color laser pulse. It is shown that such interference can lead to an enhancement of the orientation of CO molecule by a factor of 1.6.

Ultrafast Internal Conversion Dynamics of 2-Chloropyridine by Femtosecond Time-Resolved Photoelectron Imaging

Ultrafast internal conversion dynamics of 2-chloropyridine were studied by femtosecond time-resolved photoelectron imaging spectroscopy coupled with time-resolved mass spectroscopy. The ultrafast internal conversion from the second excited state (S-2) to the first excited state (S-1) via an adjacent conical intersection within (162 +/- 5) fs was clearly observed from the time-dependence of the photoelectron spectra. The subsequent deactivations involved the coupling of S-2/S-0 (the ground state) and S-1/S-0 conical intersections, which occurred on a timescale of about (5.5 +/- 0.3) ps, and led to the internal conversion to the ground state from the S-2 and S-1 states.