1923 H. L. Brose tr. A. J. W. Sommerfeld vi. 325 In our diagram the absorption lines would have to be represented by arrows that start out from the natural or ground level and are directed upwards.
1953 L. H. Aller 25 The excitation potential in ev is the potential through which a bombarding electron must drop in order to acquire sufficient energy to excite an atom from the ground level to the level in question.