X-Rays
The spectrum below the UV has very short wavelength, and consequently each photon has a very high frequency and carries far more energy than the visible or IR spectrum. This is the x-ray spectrum, and soft x-rays refer to the region just beyond the UV which can readily destroy some molecular bonds. Hard x-rays are at very short wavelengths where photon energies are high enough to penetrate most materials.
Related Terms: Far Infrared, Infrared (IR), Near Infrared, Ultra-Violet (UV), Visible Spectrum (VIS)