Photoionization/Photodissociation Studies Of Cycloketone Molecules And Their Methyl Substitutes

(Ⅰ) As the development of femtosecond laser technique, many studies have been interested in the ionization and dissociation of polyatomic molecules in intense laser fields. These studies can reveal various information on the dynamics of molecular processes. The intensity of electronic field observed by a focused femtosecond beam can reach or overwhelm the Coulomb field interaction between nucleus and electrons inside a molecule. When the laser intensity is in the range of 1013～1014 W/cm2, many factors, such as wavelength, intensity, character of the molecule, can have strong influence on the interaction of laser-field and molecules.In this work, the mass spectra of numerours cycloketone molecules and their methyl substitutes have been measured in a 90 fs laser field with 788 nm or 394 nm and within the intensity range from 3×1013 W/cm2 to 5×1014 W/cm2. It is verified by a calculation of the structure-based Keldysh parameter for these molecules that under the conditions of these experiments the dominant ionization mechanism is the field ionization mechanism, including tunneling ionization and barrier suppression ionization. Because the time of electrons tunneling barrier is shorter than the duration of laser pulse, it is expected that the mechanism of photoionization/photodissociation in the fs laser field is ionization dissociation. Therefore the character of the cation absorption is a key factor in the process of photodissociation. The yields of parent and fragment ions depending on the intensity and wavelength of the laser have been observed.It is found that, for a 788 nm fs laser field with an intensity of 6×1013 W/cm2, the mass spectra of cyclopentanone, cyclohexanone, 2-methly cyclopentanone, 3-methly cyclopentanone and 2-methly cyclohexanone have dominant contribution from their parent ions, and the yields of their fragment ions increase as the laser intensity increasing, but the parent ions are still the strongest signal in the mass spectra. As to the mass spectra of other molecules, including cycloheptanone, cyclooctanone, 3-methly cyclohexanone and 4-methly cyclohexanone, the yields of their parent ions are low and their fragment ions are dominant in the mass spectra. In the case of the 394 nm fs laser field, all the molecules have been dissociated extensively. For analyzing these phenomena, using DFT method, we calculate the transition energies and the oscillator strengths of these molecular cations between their ground and excited states based on the optimized molecular geometric structures obtained. The optical absorption spectra of these molecular cations are obtained from these calculated results. The absorption spectra show different features for different cycloketone molecules at the excitation wavelengths used in the experiment. Comparing the experimental and theoretical results, it indicates that the parent ions are dominant contribution in mass spectra if the cations have no allowed electronic transition at the wavelength of the laser field. In contrast, if the electronic levels of molecular cation are resonance with the wavelength, heavy fragmentation will occur. We suggest the molecules are ionized and produce the parent ions of vibronic excitation and during the laser pulse duration the energetic ions may relax to the ground state of the parent ion. Then if the electronic level of the parent cation is nonresonant with the laser wavelength, the parent ion will be hardly to obtain enough energy by absorbing the laser photons for dissociation. On the contrast, if the laser wavelength is resonant with their electronic absorption, the parent ions will be easily to absorb more photons from the laser field to accumulating enough energy through the intramolecular vibronic redistribution for dissociation, and finally the parent ion decompose to fragmental ions.It is noticed that the multicharged parent ions are not observed in the mass spectra for these molecules. It is interpreted by a high ionization threshold for further ionizing (i.e., for cyclohexanone it is about 17eV) and a low-lying dissociation states for these cycloketone cations. In this case, the vibronic excited ions may dissociate immediately. This explaination has been verified by the following experiments of VUV photoionization/dissociation of cycloketones. Furthermore, in the mass spectra under a higher laser intensity, multicharged ion of carbon (C2+) has been observed and its signal peak exhibits the double-peak structure which indicates that a Coulomb explosion happened. It is suggested that even they may be generated during the process of ionization in such intense laser field, these multicharged parent ions may hardly survive because of the nonsteady structure of the cycloketone cations.(Ⅱ) VUV photoionization/dissociation of cycloketones is performed by using a time-of-flight mass spectrometer (TOF-MS) in Hefei synchrotron radiation source. The photoionization TOF mass spectra and photoionization efficiency (PIE) curves of their fragment ions are measured. The appearance potentials (AP) of these ions are obtained from their PIE curves.For analyze the dissociation channels of the cyclopentanone, the energies of neutrals and cations of parent and its fragments have been calculated using DFT/G3 method. From these data, the dissociation energies of cyclopentanone along the dissociation channels have been evaluated. Comparing the theoretical and experimental results, the most possible channel of dissociation has been proposed.By studying the cycloketone molecules and their methyl substitutes, the ionization energy and the dissociation energies of the corresponding channels are also observed, the ionization energies of some molecules are consisted with the results obtained by PE method in other previous studies reported in references. The ionization energy of 2-methly cyclopentanone, 4-methly cyclohexanone and most appearance potentials of the fragment ions are measured first time in experiment. The data indicate that cycloketone and their methyl substitutes have similar dissociation channels and dissociation energies. And most dissociation energies of them are lower than 11 eV, which indicate the structure of these cycloketone and their substitutes are nonsteady. In addition, at the lower intensity of the femtosecond experiments, the slope of the ion yield depending on the laser intensity indicate the energy of multiphoton absorptioin are agree with the appearance energy detected by VUV photoionization.