A cold pulsed source of hydrogen and hydrogen clusters: development and extreme-ultraviolet studies

Abstract

We describe the development and application of a differentially pumped pulsed cold H2 and (H2)2 beam source. The nozzle was cooled with liquid nitrogen and, actuated by a piezoelectric valve with pressures up to 5 bar, produced gas pulses 250 μs long. Strong limitations by collisions with the fast hydrogen molecules returning from the walls of the source chamber could be overcome by surrounding the volume around nozzle and skimmer with cold surfaces. As a consequence, the beam intensities for both the monomer and the clusters could be greatly enhanced. The valve was operated without any realignment for more than one year. The beam source was characterized by a number of photo-ionization studies. As a part of these efforts the presumably first laser-based single-photon ionization spectra of H2 and (H2)2 were obtained. We estimate that more than ten $\mathrm{H}_{3}^{+}$ ions could be detected per laser shot at a wavelength of 74.2 nm and a pressure of 5 bar. The auto-ionization lines of H2 were recorded with a resolution of about 0.54 cm−1. A line width of about 629 MHz was obtained in the B←X band system of hydrogen observed by resonance-enhanced two-photon ionization. Further potential improvements of the experiment are discussed.