(a) <sup>22</sup>Ne<sup>+</sup> yield from uncondensed atoms (closed symbols) and from clusters (open symbols) as a function of the GMD peak height

<p><strong>Figure 3.</strong> (a) <sup>22</sup>Ne<sup>+</sup> yield from uncondensed atoms (closed symbols) and from clusters (open symbols) as a function of the GMD peak height. The intensities of these two components are re-normalized at the highest point of the GMD scale. The horizontal error bars denote the standard deviation, and the vertical error bars denote the statistical errors, which are defined by the square root of the integrated number of <sup>22</sup>Ne<sup>+</sup>. The line guides a quadratic dependence on the laser power for atoms and a linear dependence for clusters. (b) Log–log plot of the <sup>22</sup>Ne<sup>+</sup> yields. The lines are guides for the eyes the same as (a).</p> <p><strong>Abstract</strong></p> <p>We carried out time-of-flight mass spectrometry for neon clusters that were exposed to intense free electron laser pulses with the wavelength of 62 nm, which induce optical transition from the ground state (2s<sup>2</sup> 2p<sup>6</sup>) to an excited state (2s<sup>2</sup> 2p<sup>5</sup><em>nl</em> ) in the Ne atoms. In contrast to Ne<sup>+</sup> ions produced by two-photon absorption from isolated Ne atoms, the Ne<sup>+</sup> ion yield from Ne clusters shows a linear dependence on the laser intensity (<em>I</em>). We discuss the ionization mechanisms which give the linear behaviour with respect to <em>I</em> and expected features in the electron emission spectrum.</p>