Excited-State Relaxation of ${\mathrm{Ag}}_8$ Clusters Embedded in Helium Droplets

Neutral silver clusters ${\mathrm{Ag}}_N$ are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for ${\mathrm{Ag}}_8$, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of $(6.89\pm 0.09)\mathrm{eV}$ for the vertical ionization potential of ${\mathrm{Ag}}_8$. The influence of the helium matrix on photoemission is discussed.

Figure 1

Mass spectrum of silver clusters embedded in helium droplets ionized by electron impact.

Figure 2

Silver clusters embedded in helium droplets are exposed to laser light with a wavelength of 312.8 nm (3.96 eV). The resulting R2PI signals of ${\mathrm{Ag}}_2$ and ${\mathrm{Ag}}_8$, the dominant species in the mass spectrum [6], are recorded as a function of the nozzle temperature (and thus the mean droplet size) to identify the photoelectron signals.

Figure 3

Photoelectron spectra of neutral silver clusters in He droplets taken at various nozzle temperatures between 9.0 and 12.5 K, corresponding to mean droplet diameters between 60 and 5 nm, respectively. Laser pulses of 312.8 nm ($2h\nu =7.93\mathrm{eV}$) are used for resonant two-photon ionization. According to the R2PI result shown in Fig. 2, the peak in the shaded region can uniquely be assigned to ${\mathrm{Ag}}_8$. From the threshold kinetic energy of 1.03 eV, an IP of 6.90 eV is deduced. For comparison, the results of quantum chemical calculations (CI: 6.8 eV) [19] and electron impact ionization measurements (EI: 7.1 eV) [20] for the vertical ionization potential are marked by arrows.

Figure 4

Measured electron kinetic energies at the ionization threshold plotted against the photon energy shows an almost exact one-to-one correspondence, indicating only a single photon process. From the slope $E_L$ can be determined to be 2.95 eV below the IP. Inset: At the low energy side of $E^{*}$, evidence for the transition to a direct two-photon ionization (2P, dashed line) is observed.

Figure 5

Schematic diagram of the ionization dynamics of ${\mathrm{Ag}}_8$. After excitation to the unoccupied band $E^{*}$ roughly 4.0 eV above the ground state, the cluster quickly relaxes to the lower edge $E_L$. In a R2PI experiment, ionization occurs from this long-living level. Thus, via excitation into $E^{*}$, an occupation inversion between $E_L$ and the ground state could be reached, a requirement for laser operation. Indeed, ${\mathrm{Ag}}_8$ fluorescence measurements in argon matrix [24] show a radiative decay in this spectral region.