Rotational State Selection Of Chloroform Molecules By The Electrostatic Hexapole

In the study of molecular ultrafast dynamics, the preparation of a molecular system with specific rotational quantum state is of great significance to learn the ultrafast process of molecules, in many ultrafast dynamics experiments, choosing a number of quantum states as few as possible or even choosing a single one can make us understand the internal micro-reactions of the molecular more intuitively, moreover, it can reduce the difficulty of experimental analysis and the tedious data handling process. Usually the inhomogeneous electrostatic field is used to choose the molecular rotational state and in this article the non-uniform electrostatic field is generated by electrostatic hexapole. The application of electrostatic hexapole in China is still relatively rare. The principle of electrostatic hexapole is to arrange six electrodes in a circle equally and connect every electrode alternately to high positive and negative voltages, due to the electromagnetic effects this process can generate an inhomogeneous electrostatic field with an approximately cylindrical symmetry in the axial direction. The molecular energy level in this field can split due to the so-called Stark effect, and as a result, the molecules that have positive Stark effect will endure a linear force which converges to the axis in the molecular beam axis direction, while the molecules that have negative Stark effect will endure a emanative force which departures from the axis. The combination of this effect and the application of supersonic molecular beam realizes the selection of molecules in different quantum rotational states.In this thesis the electrostatic hexapole device and femtosecond laser are used to study the rotational state selection of gaseous chloroform molecules. The length of our hexapole is 50 cm, the rod radius is 3 mm, and the device radius is 6 mm. We can change the selection of the molecule rotational states through changing the voltage applied to the electrostatic hexapole device gradually. The laser for ionizing the molecules is with a wavelength of 800 nm and a repetition rate of 10 Hz. After the molecular ionization with the femtosecond laser, we can probe the ion signal intensity through the time of flight mass spectrometry,then we can get the relationship between the ion signal intensity and the applied voltage of the hexapole device. According to the theoretical simulation developed in this group, we simulate the focus situation of the molecule's different rotational state in the same condition, by fitting with the experimental results, we can infer the rotational state chosen in the experiment. In the case of the hexapole parameter determination, the selection of molecule rotational state is not only related to the voltage which is applied on the hexapole, but also related to the velocity of the molecular beam. As a result, we choose two different pressures of carrier gas to change the speed of molecular beam, then detect and simulate the selections of molecule rotational state in different velocities. By comparing the fitting results we find that the rotational state selected by the molecules at different velocities is different. Besides that, the higher the velocity of the molecules, the greater the focus voltage required.