An experimental study of laser-produced plasmas is performed by irradiating a planar tin target by laser pulses, of 4.8 ns duration, produced from a KTP-based 2-µm-wavelength master oscillator power amplifier. Comparative spectroscopic investigations are performed for plasmas driven by 1-µm- and 2-µm-wavelength pulsed lasers, over a wide range of laser intensities spanning 0.5 − 5 × 1011 W/cm 2. Similar extreme ultraviolet (EUV) spectra in the 5.5–25.5 nm wavelength range and underlying plasma ionicities are obtained when the intensity ratio is kept fixed at I1µm/I2µm = 2.4(7). Crucially, the conversion efficiency (CE) of 2-µm-laser energy into radiation within a 2% bandwidth centered at 13.5 nm relevant for industrial applications is found to be a factor of two larger, at a 60 degree observation angle, than in the case of the denser 1-µm-laser-driven plasma. Our findings regarding the scaling of the optimum laser intensity for efficient EUV generation and CE with drive laser wavelength are extended to other laser wavelengths using available literature data.

Experimental spectroscopic studies are presented, in a 5.5-25.5 nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2µm-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1-µmwavelength Nd:YAG laser light over a range of laser intensities spanning approximately 0.3 − 5 × 10 11 W cm −2 , under otherwise identical conditions. Over this range of drive laser intensities, we find that similar spectra and underlying plasma charge state distributions are obtained when keeping the ratio of 1-µm to 2-µm laser intensities fixed at a value of 2.1(6), which is in good agreement with RALEF-2D radiation-hydrodynamic simulations. Our experimental findings, supported by the simulations, indicate an approximately inversely proportional scaling ∼ λ −1 of the relevant plasma electron density, and of the aforementioned required drive laser intensities, with drive laser wavelength λ . This scaling also extends to the optical depth that is captured in the observed changes in spectra over a range of droplet diameters spanning 16-51 µm at a constant laser intensity that maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for EUV lithography. The significant improvement of the spectral performance of the 2-µm-vs 1-µm driven plasma provides strong motivation for the development of high-power, high-energy nearinfrared lasers to enable the development of more efficient and powerful sources of EUV light.

The universal response of a sudden created core hole, predicted to occur on an attosecond (10(-18)  s) time scale, lacks an experimental demonstration. With a two-dimensional coincidence spectrometer, we demonstrate an extensive energy sharing between the Ag 4p photoelectron and the N2,3VV Auger electron exceeding 10 eV. This energy width provides access to the time scale of the emission process. This is the fingerprint of the dynamic fluctuation process 4p(-1)⇌4d(-2)4f. The shakeup induced interband transitions from the Ag(100) surface are also identified by comparing the coincidence spectrum with the M4,5VV Auger transitions.

A simulation model for the direct contact condensation of steam in subcooled water is presented that allows determination of major parameters of the process, such as the jet penetration length. Entrainment of water by the steam jet is modeled based on the Kelvin–Helmholtz and Rayleigh–Taylor instability theories. Primary atomization due to acceleration of interfacial waves and secondary atomization due to aerodynamic forces account for the initial size of entrained droplets. The resulting steam-water two-phase flow is simulated based on a one-dimensional two-fluid model. An interfacial area transport equation is used to track changes of the interfacial area density due to droplet entrainment and steam condensation. Interfacial heat and mass transfer rates during condensation are calculated using the two-resistance model. The resulting two-phase flow equations constitute a system of ordinary differential equations, which is solved by means of the explicit Runge–Kutta–Fehlberg algorithm. The simulation results are in good qualitative agreement with published experimental data over a wide range of pool temperatures and mass flow rates.