# Spatial and Momentum Mapping Modes for Velocity Map Imaging Spectrometer
## 4 Experiment
### Spatial Mapping Mode  
It is necessary to find the overlap between two laser focuses in a pump-probe experiment. This procedure could be pretty tedious if we do it in a vacuum chamber. Previously, the preliminary spatial overlap was found by either the optical mapping method [28] or the spatial mapping mode of VMI [19]. Here, we try to add magnification in the spatial mapping mode.  
We capture spatial mode ion images, as shown in Figure 6. The absolute size is determined with reference to the 80 mm diameter of the phosphor screen. As indicated by the inserted diagram in Figure 6a, the supersonic molecular beam propagates up while the laser propagates to the right side. Through the adiabatic expansion, the supersonic molecular beam is cooled down to about 10 K with a mean speed higher than the room-temperature thermo-speed. In comparison, the residual gas is at room temperature. The laser is focused with a Rayleigh length of 4 mm and a beam waist of 68 \(\upmu\)m. The diameter of the Ar jet is 1.2 mm. Upon interaction between the laser and Ar jet, the produced Ar\({}^{+}\) ions are characterized by narrow velocity distribution and compact shape in the acquired image, shifting from a more dispersed image from the residual gas. So, the spot or narrower stripe with higher intensity in each subplot is attributed to Ar\({}^{+}\), while the weaker and broader stripe is attributed to the ions from residual gas. It should be noted that since the ions from residual gas move isotropically, their image represents the reaction center better than the ion image from the jet does.  
Figure 6a-e show the ion images with the simulated magnification factors: 1.0, 2.2, 2.8, 5.9 and 7.6, respectively. The actual magnification factors in the measurement are 1.0, 1.5, 1.8, 5.2 and 6.4, respectively. The jet image shifts more upward as the magnification factor increases. In the voltage setting of the higher magnification factor, the ions are retarded more, which leads to a longer ion time of flight and a larger jet image offset.  
Spatial mapping mode is helpful for preliminarily overlapping two laser pulses in the Z-X plane in a pump-probe experiment. We compare the performance of spatial imaging of different spectrometers in Table 2. The best performance was achieved by [PERSON] [21] using an ion microscope. Although we did not provide a high magnification factor,the magnified images help us to find good alignment. By monitoring the shift of ion image when switching the VMI lens between the spatial mapping mode and focusing mode, the alignment of the laser focus can be improved further to below 0.2 mm.  
In Figure 6, the observed grids are from the metal mesh on the ninth electrode, where the microlens effect from the mesh cell can distort the ion images. This distortion is more severe at a larger magnification factor, which leads to a clearer grid pattern. Although Figure 6e is acquired at the simulated magnification factor of 7.6, the experimental magnification factor is 6.4. We speculate this is also due to the microlens effect beyond our simulation.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Author** & **Magnification Factor** & **Type** \\ \hline Tsatrafyllis [21] & 80.0 & Ion microscope \\ [PERSON] [22] & \(-\)14.6 & Ion microscope \\ [PERSON] [20] & 6.8 & Ion microscope \\ [PERSON] [19] & \(-\)6.0 & Two-mode VMI \\ This work & 1.0, 1.5, 1.8, 5.2, 6.4 & Two-mode VMI \\ \hline \hline \end{tabular}
\end{table}
Table 2: List of the performance of spatial imaging measured in spectrometers.  
Figure 6: Spatial mode ion images in spatial mapping mode with different magnification factors. The intensities are given in log scale to make the contribution from the residual gas visible. The simulated magnification factors are **(a)** 1.0, **(b)** 2.2, **(c)** 2.8, **(d)** 6.0 and **(e)** 7.6, respectively, while the measured magnification factors are **(a)** 1.0, **(b)** 1.5, **(c)** 1.8, **(d)** 5.2 and **(e)** 6.4, respectively.