Interaction of Argon Clusters with Intense VUV-Laser Radiation: The Role of Electronic Structure in the Energy-Deposition Process

The response of Ar clusters to intense vacuum-ultraviolet pulses is investigated with photoion spectroscopy. By varying the laser wavelength, the initial excitation was either tuned to absorption bands of surface or bulk atoms of clusters. Multiple ionization is observed, which leads to Coulomb explosion. The efficiency of resonant 2-photon ionization for initial bulk and surface excitation is compared with that of the nonresonant process at different laser intensities. The specific electronic structure of clusters plays almost no role in the explosion dynamics at a peak intensity larger than $1.8\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. The inner ionization of atoms for resonant and nonresonant excitation is then saturated and the energy deposition is mainly controlled by the plasma heating rate. Molecular dynamics simulations indicate that standard collisional heating cannot fully account for the strong energy absorption.

Figure 1

Photoabsorption spectrum of ${\mathrm{Ar}}_{800}$ clusters [8]. As indicated, the FEL radiation wavelength was tuned to surface and bulk excitons as well as to nonresonant excitation.

Figure 2

TOF mass spectra recorded after irradiation of ${\mathrm{Ar}}_{900}$ clusters at three different wavelengths (surface, bulk, and nonresonant excitation) and peak intensity [(a) $1.5\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ and (b) $1.9\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$, 3 mm outside the focus].

Figure 3

Relative abundance of ${\mathrm{Ar}}_2^{+}$, ${\mathrm{Ar}}_3^{+}$ resulting from irradiation of ${\mathrm{Ar}}_{900}$ clusters depending on the laser peak intensity. Initial resonant bulk excitation is compared with nonresonant excitation.

Figure 4

Comparison between simulated (a) and measured (b) TOF mass spectra of irradiated ${\mathrm{Ar}}_{170}$ clusters (100.8 nm, $3.2\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$). ${\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}^{2+}$ ejected into the half-sphere facing away from the detector are indicated.

Figure 5

Calculated energy absorption of the ${\mathrm{Ar}}_{147}$ cluster plasma containing two quasifree electrons per ion in the laser field ($3.2\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ for both 100.8 and 800 nm).