S. D. Chao

Experimental and theoretical investigations of ionization/dissociation of cyclopentanone molecule in a femtosecond laser field

The ionization/dissociation mechanism of cyclopentanone has been experimentally investigated in molecular beam by irradiating with intense 394 and 788 nm laser fields with pulse duration of 90 fs. The range of laser intensities varied from 3 x 10(13) to 4 x 10(14) W/cm(2). For both wavelengths, the singly charged parent ion is observable while the doubly charged one cannot be found easily, although the fragmentation pattern supports its presence. Meanwhile, the extent of fragmentation at 788 nm is less than that in the 394 nm case. We quantitatively analyze the ionization processes of cyclopentanone in intense femtosecond laser by comparing the calculation results of ionization rate constants obtained from Ammosov-Delone-Krainov, Keldysh, and Keldysh-Faisal-Reiss (KFR) theories based on hydrogenlike atom model. We also compare the experimental and theoretical results; the generalized KFR theory is found to be useful in predicting the ionization yields of singly and doubly charged cyclopentanone ion. To interpret the dissociation patterns of the cyclopentanone ions, we have used the Rice-Ramsperger-Kassel-Marcus theory with the potential surfaces obtained from the ab initio quantum chemical calculations.

l-mixing dynamics of Rydberg states of hydrogen atom in a static electric field

Both exact analytical formula and numerical calculations are performed to study the l-mixing population decay of the n = 10 and n = 100 high Rydberg states of hydrogen atoms in an electric field. The two time constants of and observed in zero-electron-kinetic-energy (ZEKE) experiments in the common stray field condition, i.e. - can be obtained by 2-exponential fitting of the exact decay curve. The principle of high-resolution ZEKE spctroscopy based on the two time constants is then elucidated. The inverse Born-Oppenheimer approximation will be introduced to treat the ZEKE states of complex systems like many-electron atoms, molecules, clusters etc.

Comment on the spontaneous emission rates of ZEKE states

Abstract The numerical results of the transition rate for spontaneous emission from highly excited states ( n > 50) have been calculated with the corresponding lifetimes. These results can provide a helpful reference to understand the extraordinary long lifetimes found in ZEKE states, which are responsible for the high resolution ZEKE spectroscopy.

Highly multiphoton molecular excitation by an intense laser pulse

Multiphoton excitation of molecules by an intense laser field is treated. When the first excitation energy is much larger than both the photon energy and the transition energy between excited states, excited states are strongly coupled with each other by the photon, even though coupling with the ground state is weak. In such a case, a kind of collective excitation takes places, in which the excitation probability of every excited state is almost linear as functions of intensity on a log–log plot, and the slope is independent of the final state. By taking methane and formaldehyde as examples we numerically study this process.

A master equation approach to the dynamics of zero electron kinetic energy (ZEKE) states and ZEKE spectroscopy

We have theoretically studied important dynamic processes involved in zero electron kinetic energy (ZEKE) spectroscopy using the density matrix method with the inverse Born-Oppenheimer approximation basis sets. In ZEKE spectroscopy, the ZEKE Rydberg states are populated by laser excitation (either a one- or two-photon process), which is followed by autoionizations and l-mixing due to a stray field. The discrimination field is then applied to ionize loosely bound electrons in the ZEKE states. This is followed by using the extraction field to extract electrons from the ZEKE levels which have a strength comparable to that of the extraction field. These extracted electrons are measured for the relative intensities of the ion states under investigation. The spectral positions are determined by the applied laser wavelength and modified by the extraction electric field. In this paper, all of these processes are conducted within the context of the density matrix method. The density matrix method can provide not only the dynamics of systems population and coherence (or phase) but also the rate constants of the processes involved in the ZEKE spectroscopy. Numerical examples are given to demonstrate the theoretical treatments.

Molecular ionization of cyclohexanone in femtosecond laser fields: An application of ADK theory

The mechanisms of ionization and dissociation of cyclohexanone (C6H10O) in a 90 fs, 788 nm linearly polarized laser field ranging from 1013 to 1014 W/cm2 by a time-of-flight mass spectrometer (TOF-MS) have been investigated. The ion yields as a function of laser intensity have been measured experimentally. By comparison with the Ammosov-Delone-Krainov (ADK) theory based on a hydrogen-like model, the ionization mechanism of cyclohexanone in this intense femtosecond laser field has been understood. Considering the importance of molecular nuclear motions, we propose that the Franck-Condon (F-C) factor can provide the excess vibrational energy in the molecular ion. This energy is required for the decomposition of the molecular ion which finally results in the observed mass spectrum.

Theoretical and experimental investigations of ionization-dissociation of polyatomic molecules

The Keldysh theory originally developed for the ionization of 1s state of hydrogen atom has been generalized for molecules by introducing the molecular orbital theory and the Born-Oppenheimer approximation. Ionization and dissociation mechanisms of cyclopentanone in the intense 800 nm laser field (1013 to 1014 W/cm2) have been experimentally investigated by using a method of molecular beam and time-of-flight mass spectrometer (TOF-MS). Singly-charged parent ion and numerous fragment ions are observed in the mass spectra, which are analyzed by using the molecular Keldysh theory.