Answer
It takes more energy to ionize an atom (completely remove an electron from it) than to excite the electron to a higher energy state.
Work Step by Step
For photons, E = hf, where E is the energy, h is Planck's constant, and f is the photon's vibrational frequency.
A higher-frequency uv photon has more energy than a visible-light photon. This is discussed on page 564.
The visible-light photon isn't energetic enough to ionize the atoms, only to excite them.
An ultraviolet photon has sufficient energy to eject the electron altogether.
