Chengyin Wu

Laser-induced dissociation and explosion of methane and methanol

Femtosecond laser-field-induced dissociation and explosion of polyatomic molecules methane and methanol were experimentally investigated by using a time-of-flight mass spectrometer and a chirped pulse amplifier laser. Both linearly polarized and circularly polarized femtosecond laser pulses were used. The intensity varied from 6×1013 to 3×1015 W cm-2. Strong molecular ions, singly charged fragmental ions and highly charged atomic ions were observed for both methane and methanol. The singly charged fragmental ions had a similar dependence on laser intensity compared with the molecular ions. This indicated that the ionization of the inner-valence electron led to the formation of highly excited molecular ions that rapidly dissociated into the singly charged fragmental ions. The highly charged atomic ions were evidence of Coulomb explosion. The kinetic energies of the atomic carbon ions were almost zero for methane and relatively large for methanol. This indicated that the explosion of polyatomic molecules methane and methanol was a concerted process and all the bonds in the molecules were broken simultaneously.

Mass and photoelectron spectrometer for studying field-induced ionization of molecules

Abstract We present a simple experimental apparatus, a time-of-flight mass spectrometer (TOFMS) and a photoelectron spectrometer (PES), for studying the ionization of molecules induced by intense femtosecond laser fields. Both the angular and the kinetic energy distributions of the ions and the photoelectrons can be precisely measured. As an example, the dynamics data for CS 2 were measured using the spectrometer, and the results revealed that field-induced ionization played the main contribution for CS 2 in an intense femtosecond laser field.

Resonantly enhanced two photon ionization and zero kinetic energy spectroscopy of jet-cooled 4-aminopyridine

We report studies of supersonically cooled 4-aminopyridine (4-AP) using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 62291+/-6 cm(-1). The REMPI spectrum of the S1 state is dominated by ring deformation modes and the inversion mode of the amino group, while the ZEKE spectra demonstrate a strong propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S1. In addition, the ZEKE spectra obtained via different vibrational levels of the S1 state contain four common features, corresponding to the activation of four different vibrational modes of the cation. These observations are explained in terms of the structural changes from the ground state to S1 and further to the cation. The vibrational mode distributions in both the REMPI and the ZEKE spectra, the excitation energy of the S1 state, and the ionization potential of 4-AP, are remarkably similar to those of aniline, suggesting that the electronic activity is centered on the ring.

Fragmentation dynamics of methane by few-cycle femtosecond laser pulses

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.

Zero kinetic energy photoelectron spectroscopy of p-amino benzoic acid

We report studies of supersonically cooled p-amino benzoic acid using one-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state S(1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 64 540+/-5 cm(-1). A common pattern involving the activation of five vibrational modes of the cation is recognizable among all the ZEKE spectra. A propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S(1), is confirmed, and the origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. A puzzling observation is the doublet splitting of 37 cm(-1) in the ZEKE spectrum obtained via the inversion mode of the S(1) state. This splitting cannot be explained from our density functional calculations.

Field-free alignment of molecules at room temperature.

Field-free alignment of N(2), O(2), CO, CO(2), CS(2), and C(2)H(4) molecules was experimentally achieved at room temperature by using 800 nm, 110 fs laser pulses at an intensity of 6x10(13) W/cm(2). An enhanced degree of alignment was also demonstrated by using two pulses with appropriate separation times. These results indicate that multi-pulse alignment is a feasible approach to provide macroscopic ensembles of highly aligned molecules for practical applications.

Observation of rotamers of m-aminobenzoic acid: Zero kinetic energy photoelectron and hole-burning resonantly enhanced multiphoton ionization spectroscopy

We report studies of supersonically cooled m-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. Two conformers have been identified and characterized using the hole-burning method in the REMPI experiment. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S(1)) of the neutral species and those of the ground state cation (D(0)) have been assigned, and the adiabatic ionization potentials have been determined for both conformers. The REMPI spectra are dominated by in-plane motions of the substituents and ring deformation modes. A propensity of Deltav=0, where Deltav is the change in vibrational quantum number from the S(1) to the D(0) state, is observed in the ZEKE spectra. The origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. Comparisons of these results with those of p-aminobenzoic acid will be analyzed.

Controlling molecular rotational population by wave-packet interference

We propose a control scheme for selecting populations of molecular rotational states by wave-packet interference. A series of coherent rotational wave packets is created by nonadiabatic rotational excitation of molecules using two strong femtosecond laser pulses. By adjusting the time delay between the two laser pulses, constructive or destructive interference among these wave packets enables the population to be enhanced or suppressed for a specific rotational state. The evolution of the rotational wave packet with selected populations produces interference patterns with controlled spatial symmetries. This method provides an approach to prepare a molecular ensemble with selected quantum-state distributions and controlled spatial distributions under field-free condition.

Measurement of the Field-Free Alignment of Diatomic Molecules

An apparatus was constructed to experimentally quantify the field-free alignment of diatomic molecules irradiated by strong femtosecond laser pulses. In this apparatus, both homodyne and pure heterodyne detections were realized. The alignment signal is proportional to [ - 1/3](2) for homodyne detection and ( - 1/3) for pure heterodyne detection, where theta is the polar angle between the molecular axis and the laser polarization direction. Fourier transform spectra of the homodyne signal and the pure heterodyne signal were also studied. By comparing the alignment signal and its Fourier transform spectrum with the numerical calculation of the time-dependent Schrödinger equation, we demonstrated that the pure heterodyne signal directly reproduced the alignment parameter , and its Fourier transform spectrum provided information regarding the populations of different J states in the rotational wavepacket.

Molecular Rotational Excitation by Strong Femtosecond Laser Pulses

We study the rotational wave packet created by nonadiabatic rotational excitation of molecules with strong femtosecond laser pulses. The applicable condition of the Delta-Kick method is obtained by comparing the laser intensity and pulse duration dependences of the wave packet calculated with different methods. The wave packet evolution is traced analytically with the Delta-Kick method. The calculations demonstrate that the rotational populations can be controlled for the rotational wave packet created by two femtosecond laser pulses. The evolution of the rotational wave packet with controlled populations produces interference patterns with exotic spatial symmetries. These calculations are validated by comparing the theoretical calculations with our experimental measurements for the rotational wave packet created by thermal ensemble CO(2) and two strong femtosecond laser pulses. Potential applications in molecular science are also discussed for the rotational wave packet with controlled populations and spatial symmetries.