Ionization And Dissociation Mechanism Of Cyclopentanone Within Femtosecond Laser Field

The study of molecules in intense (I>10¹⁴ W/cm²) laser field with ultra-short pulse duration (Ï„<10^-13s) has been an area of considerable activity over ten years. The early observation of new phenomena, e.g. above-threshold ionization and high order harmonic generation, stimulated further experimental and theoretical works. There are so many studies about photoionization and fragmentation processes induced by ultra-intense lasers in organic molecules. For the lower intensity limit the ionization/dissociation processes can be understood by a multiphoton ionization mechanism (MPI), while for intensities increasing, molecular ionization is attributed to so-called"field ionization"i.e., ionization induced by the electric field of the laser beam (tunneling, barrier suppression ionization, electron recollision, etc). However, on the other hand, the ionization/dissociation mechanism involved in experiments with laser intensities in the intermediate range (10¹³ï½ž10¹⁵ W/cm²) is less clear, although it has been the subject of many recent publications. The motivation for the present work is to gain more insight about the interaction of molecules with lasers at high intensities.The interaction of cyclopentanone with(10¹³W/cm²ï½ž10¹⁴ W/cm²) femtosecond laser fields is studied in the present thesis by means of a time-of-flight mass spectrometer. The spectra were obtained with 394 nm and 788 nm, 90 fs laser pulses respectively. These compounds are particularly interesting because they are often found as structural units in large biological substances such as steroids and polyether antibiotics. Furthermore, there are so many studies about these two compounds.When the wavelength is 394 nm, the mass spectrum of each compound was recorded at typically five different laser intensities covering the range between 1×10¹³ W/cm² and 1×10¹⁴ W/cm². From the mass spectra we found that there are some common characteristics among cyclopentanone: the parent ion peak is far more less than the fragmental ion peaks. This implies that MPI is the main mechanism in ionization/dissociation processes, and the ionization-dissociation mechanism is dominated because of the pulse width (90 fs) is less than the time of parent dissociation for cyclopentanone. However, when the wavelength is 788 nm, this implies that FI is the main mechanism in ionization dissociation processes. In the mass spectra the parent ion is obvious, and the quantities of fragmental ions which focus on large ones are relative small.The different absorption of cyclopentanone molecule ion to various wavelength laser have a primary effect on the degree of fragmentation. In femtosecond laser field, the cyclopentanone molecule experience a ladder-climbing chiefly, then the molecule ion have a definite absorptive probability to 394nm laser and continue to absorb the photon easily, so the ion dissociate to fragmental ion. There is little absorption for cyclopentanone molecule ion in 788nm, scarcely any, so it's very difficult to absorb the photon for dissociation. Comparing to the 394nm, we can obtain more parent ion and little fragmental ion.Therefore, the intensity of laser plays an important role in the absorption of molecule ion to photon durning the ionization and dissociation process of cyclopentanone molecule. In 788nm, although the cyclopentanone molecule ion can hardly absorb the photon, but the increase of laser intensity will enlarge the absorptive probability for photon, so it can open the corresponding absorption channel and dissociate into fragmental ion. In experimental region, the ionization of cyclopentanone molecule ion being saturated gradually . The fragmental ions such as C₃H₃O⁺,C₃H₄O⁺,C₃H₅⁺ can be get from the direct dissociation of parent ion, and other less m/z fragmental ions can be get from the consequent dissociation of more m/z fragmental ions.