Microwave Ionization of Sodium at High Scaled Frequency

Microwave ionization at high-scaled frequency (n$^{3}\omega )$ is very interesting because it is the connection between field ionization via MPI (Multi-photon Ionization) to photoionization. We measure the ionization rates for sodium with 12.54GHz and 25.15GHz linearly polarized microwave. With a stray field presented we can only go scaled frequency up to 20 with different field amplitudes and microwave pulse lengths. In general the 50{\%} ionization fields increase with the scaled frequency. There are some persistent structures at scaled-frequency around 7, and the thresholds no longer decrease monotonically with binding energy. These measurements will help us to understand the basic mechanism of microwave ionization at high scaled-frequency.