Time-of-fight mass spectrometer and high vacuum system

The main components of our time-of-fight mass spectrometer are in a high vacuum system. The high vacuum system can be roughly divided into four regions: 1. molecular beam source region (chamber); 2. molecular beam-laser interaction region; 3. ion flight tube; 4. ion detection region.

      The molecular beam source chamber is where the molecular beam is generated. It is equipped with a pulsed valve for generating supersonic molecular beam, a conical skimmer and a gas conduit. The volume of the molecular beam source chamber is about 20 L (molecular beam chamber shell and all components are made of stainless steel). The bottom portion of the beam source chamber is connected in sequence with an electric valve (electropneumatic gate valve), a diffusion pump (or turbomolecular pump) and a mechanical pump at the front end. Electropneumatic gate valves are designed to automatically close in the event of a power failure or an emergency, isolating the beam source chamber from the pump to prevent the oil and gas in the pump from flowing back and causing pollution. Therefore, electropneumatic gate valves are installed between the beam source chambers and pumps. The pumping rate of the diffusion pump (or turbomolecular pump) is about 1250 L/s. Because the diffusion pump (or turbomolecular pump) can only work in the moderate vacuum range (~ 1 x 10^-2 Torr or less), a mechanical pump is connected to the front end. A molecular sieve adsorber is installed between the diffusion pump (or turbomolecular pump) and the mechanical pump. The purpose of which is to absorb the oil and gas flowing back to avoid polluting the entire vacuum system. The pressure of the beam source gas chamber is measured by an ionization gauge connected to the upper part of the gas chamber. The background pressure of the gas chamber (background pressure) is maintained below 5 x 10^-8 Torr; while the pressure between the diffusion pump (or turbomolecular pump)and the mechanical pump is maintained at 10^-3 Torr, measured by a Convectron gauge.
      The important component of the pulsed molecular beam source is a pulsed gas valve. When the valve is closed, the high-pressure chamber (inner part of the pulse valve) and the vacuum chamber are isolated from each other. When the valve is opened, the gas is rushed into the vacuum chamber from the high-pressure chamber (inner part of the pulse valve) through the nozzle orifice. If the pressure of the high-pressure chamber is high enough and the hole (nozzle orifice) at the exit is small enough, the condition of free expansion can be achieved and a supersonic molecular beam can be generated. There are many types of pulse valves that generate pulsed molecular beams. The pulse valve (General Valve Corp, Series 9) used in this instrument (our time-of-fight mass spectrometer) is made of the mechanical principle of solenoid conductance. This pulse valve is composed of a pulse valve The controller controls the frequency and trigger time of the pulse valve. In our experiments, the trigger opening time is set to be 100 microseconds, and the frequency is 10 Hz. Because the unit molecular density of the pulse type is higher than that of the continuous type, and the molecular velocity distribution is narrower, it is easy to assemble and has low background noise. The required pumping pump does not have to have very high pumping speed, which is more in line with economic benefits. The nozzle orifice of the pulse valve we use has diameter of 0.15 mm, the tip angle is 30 degrees, and 10.5 mm in front of the skimmer. The skimmer is a conical shape stainless steel with a height of 20 mm, a top diameter of 1 mm, and a bottom diameter of 20 mm. The supersonic molecules in the middle part enter the molecular beam-laser interaction area through this skimmer. Under general experimental conditions, the stagnation pressure P₀ of the high-pressure chamber of the pulsed gas valve is about 2000 Torr, while the pressure P₁ of the molecular beam source area increases from the background pressure of 5 x 10^-8 Torr to 1 x 10^-5 Torr.

      The molecular beam-laser interaction region is the place where molecules are excited or ionized by laser. It contains time-of-flight lens (TOF lens) and focusing lens (einzel lens) assembly. The volume of this free gas chamber is about 31 L, and the electropneumatic gate, 8" molecular turbo pump (turbomolecular pump, Balzers TPU-520, TP) with a pumping speed of 520 L/s and the supporting machinery of the turbo pump are connected underneath it. The pump (mechanical pump, Balzer DUO-016B) pumping rate is 5.38 L/s. There are two supersil UV grade windows at the left and right ends of this gas chamber. The  molecular beam-laser interaction center is 50mm away from the skimmer. The laser beam enters and exits the air chamber through this window and passes through the interaction center. The pressure of the molecule and laser interaction area is measured by ionization gauge.  The background pressure is usually maintained below 5 x 10^-9 Torr. The electrode plates of the time-of-flight mirror group from near to far of the conical skimmer are U1, U2, and U3 respectively; the distance between each of these three electrode plates is 1 cm, the central aperture is 13 mm, and they are all covered with a layer of nickel mesh (Ni mesh) make the central hole a uniform electric field. The voltages of U1 and U2 are respectively provided by two power supplies (Standford Research System Inc. Model PS350). Usually, the voltage of U1 is higher than that of U2 and U3 is at zero potential. Therefore, an electric field and an acceleration zone is formed between U1 and U2. A second acceleration zone is formed between U2 and U3, so these three sets of lenses are called two-stage time-of-flight lenses. the U4, U5, and U6 after U1, U2, and U3 are called einzel lens, which is used to focus ions; usually U4 and U6 are grounded, and U5 is applied with some voltage. The connection between the time-of-flight electron optics and the focusing electron optics with hollow supporting ceramic rods. The entire electronic optics lenses are directly locked by 0-80 stainless steel rods. These electron optics lenses are the core of the time-of-flight mass spectrometer.

       An ion detector is placed in this area. It is composed of two stacked microchannel (MCP) ion/particle detection plates. Each MCP has a diameter of 33 mm and a thickness of 0.4 mm. There are about 2 x 10⁵ channels on it, the channel diameter is only 10 micrometers and has a bent angle of 12°. In front of the two MCPs is a grounded electrode plate, which ensures that the detected ions are in a flat zero-field (field free) region before entering the MCP. Each channel acts as an electron multiplier. When the ion enters the channel and hits the tube wall, several electrons will escape, and the escaped secondary electrons will repeatedly collide in the channel, and finally the electrons generated by the second MCP are considerable. As a result, a large number of electrons are generated (i.e. forming an electric current). After the current is converted, it is sent to the signal collection device, and then the mass spectrum is generated
       There are two types of signal collection instruments used in this experiment. One is the multichannel signal collector MCS (multichannel scaler, Standford Research System SR430) for collecting ion signals (electric currents) for mass spectrometer. Another one is a digital oscilloscope (LeCroy9450, 350Hz), which is mainly used to measure the intensity of laser light. The MCS signal collector uses the time window as its collection unit, and a basic unit is called a bin (bin widths are 5ns, 40ns, ... 10ms, etc. to choose from). Taking 40 ns as an example, if the length of the recorded signal is determined to be 2K (1K = 1024 bins), the range of the time window is 82 microseconds (40ns x 2 x 1024), and all ions within this flight time can be detected and recorded. The range of the time window can be determined according to the needs of the experiment. The larger ion mass range must be collected, the larger the range of time window must be included. In order to reduce the interference of noise, we usually set the signal discrimination voltage collected by MCS at -15 mV, (15= N x [4 x 10⁷] x 1.6 x 10^-19 x 50) MCP gain value N = 4 x 10⁷, that is, when 5 x 10⁷ ions are collected by MCP to amplify its current, the count value of ions will be accumulated beyond -15 mV record it. Digital oscilloscopes can also be used to record flight mass spectra. The recording method is the average of the signals, not the nature of accumulation. This advantage is that it can present the original appearance of the signals, but it cannot accumulate weak ion signals. To improve the reliability of the spectrum. In this experiment, the oscilloscope is used to record the average intensity of the pulsed laser as a light source intensity correction for the spectrum.