Eur. Phys. J. D 9, 123–126 (1999) T HE E UROPEAN P HYSICAL J OURNAL D EDP Sciences  c Societ`a Italiana di Fisica Springer-Verlag 1999 Near-threshold photoionization of germanium clusters in the 248–144 nm region: ionization potentials for Ge n K. Fuke a and S. Yoshida Department of Chemistry, Kobe University, Nada-ku, Kobe 657-8501, Japan Received: 2 September 1998 Abstract. We examine the photoionization thresholds of Gen (n =2 - 34) with a wide photon energy (5.0–8.6 eV) using a laser photoionization time-of-flight mass spectrometry. A high-output vacuum ultra- violet light generated with stimulated Raman scattering is used as the ionization light source in the energy above 6.0 eV. A characteristic size dependence of ionization potential (IP) with a maximum at n = 10 is found for clusters smaller than 22 atoms. The rather high IP of Ge 10 in comparison with its neighbors is con- sistent with the results on the photodissociation study of Ge + n . We also find that IPs decrease rapidly from n = 16 to 22, and then decrease at a much slower rate for larger clusters. These features in IPs are similar to those of Sin reported in our previous paper, except for the smaller IP gap of Gen at n ≈ 20. We discuss these results on IPs in relation to their electronic structure and stability. PACS. 36.40.Mr Spectroscopy and geometrical structure of clusters – 71.24.+q Electronic structure of clus- ters and nanoparticles 1 Introduction The structures and properties of small semiconductor clus- ters have been the subject of intensive study because of their importance in both fundamental and applied sci- ences. These studies include the reactivities of Si n toward small molecules such as O 2 , NH 3 ,C 2 H 4 , etc. [1–3]. Pho- todissociation [4–6] and collision-induced dissociation [7] experiments have also been conducted, so that informa- tion on the stabilities and binding energies of Si n and Ge n may be gained. However, little is known about the struc- tures of silicon clusters. Recently, Jarrold and others have measured the mobilities of Si + n and Ge + n , using injected-ion drift-tube techniques to obtain information on the struc- tures of these clusters [8, 9]. The results of Si + n indicate the existence of isomers having different mobilities and the oc- currence of a structural transition between these isomers. Although they have made substantial progress in obtain- ing the gross structures of cluster ions, it has not yet been possible to obtain detailed experimental information on the structures of Si n and Ge n , and thus, most of what we know about the structures comes from theoretical calcula- tions [10–14]. In order to gain information on the growth of the electronic-level structure of semiconductor clusters, photo- electron spectroscopy [15–19] of negatively charged silicon and germanium clusters, and electronic absorption spec- troscopy [20, 21] of size-selected clusters have been con- ducted. Other physical properties, such as ionization po- a e-mail: fuke@kobe-u.ac.jp tentials (IPs) are also important for understanding the electronic structure, chemical reactivities, and dissociation processes. In our previous papers [22, 23], we have reported the photoionization thresholds of Si n , n =2 - 200. The IPs have been found to exhibit a large gap in between n = 20 and 22. This gap has been tentatively ascribed to the struc- tural transition of neutral silicon clusters in analogy with that of the cluster ions observed recently in the mobility measurements [8]. In the present work, we examine the photoionization thresholds of Ge n , n =2 - 34 in the energy region of 5.0–8.6 eV to obtain further information on the size de- pendence of IPs for semiconductor clusters. In the energy region above 6.42 eV (ArF laser), high-output vacuum ultraviolet (VUV) laser light, generated by anti-Stokes (AS) conversion, is used as the photoionization light source to bracket the IPs. 2 Experimental methods Clusters of germanium atoms are produced using a laser vaporization source [23]. A pulsed, frequency-doubled Nd:YAG laser (ca. 10 mJ/pulse) is focused onto the surface of a 0.6-cm-diameter germanium rod, which is translated and rotated within an aluminum source block. Germa- nium atoms evaporated from the rod surface are mixed with helium and flowed into a cylindrical flow tube (5 cm long by 0.35 cm i.d.), where cooling and cluster growth occur. To produce cold clusters, the tube is maintained