High-resolution atomic-beam ultraviolet (UV) laser spectroscopy in Dy I and Er I has been performed. Isotope shifts have been measured for two transitions in Dy I and one transition in Er I. Specific mass shifts and field shifts have been derived for the studied transitions, and large differences between the two – transitions in Dy I have been found. From the derived specific mass shifts and field shifts, configuration mixing at the upper levels of transitions has been discussed. 1. Introduction High-resolution laser spectroscopy continues to play an important role in obtaining atomic spectroscopic data such as isotope shift (IS). The IS contains fundamental atomic properties such as electronic configuration and wave function. Recently, theoretical calculations of IS have been reported by different groups using various methods for light elements of Li, Na, and Mg [1–3]. Dysprosium and erbium, two typical rare-earth elements, have the , , , open shells, which yield complicated atomic structures. Such a kind of complex heavy atoms provides a challenge to theoretical atomic calculations of many-electron atoms [4]. Moreover, Dy and Er are prime candidates for the study of ultracold dipolar physics [5, 6]. On the other hand, measurements of IS yield the mass dependence of nuclear charge radii, a key information for the study of unstable nuclei [7, 8]. Study of IS is, therefore, of much interest not only from the point of view of atomic physics but also from the point of view of nuclear physics. Many studies of IS have been reported for Dy I and Er I in the visible and near-infrared regions by laser spectroscopy [5, 9–16]. For the ultraviolet (UV) region, corresponding to high lying levels at energy about 25000？cm？1, only several measurements have been reported. ISs in high lying levels of Dy I have been measured by using a Fabry-Perot spectrometer [17] and those of Er I by Doppler-reduced saturation absorption spectroscopy [18]. Strong configuration mixing is considered in such high lying levels and may yield different ISs. In our previous papers [19–21], we reported high-resolution atomic-beam UV laser spectroscopy in Gd I and Er I around 395？nm by frequency doubling of a diode laser beam. The present work is to extend previous measurements to the wavelength region of about 402？nm in Dy I and Er I. In this paper, ISs are measured for two UV transitions in Dy I and one transition in Er I. From ISs, field shifts and specific mass shifts are obtained, and results are discussed. 2. Experiment The present experiment was performed using an atomic beam and a UV
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