Structural relaxation dynamics of electronically excited XeArN clusters
Authors: Goldberg Alexander , Jortner Joshua 
Published: July 2002

In this article we explore the structural, dynamic, and spectroscopic implications of large local configurational changes in electronically excited Xe*ArN (N=12,54,146,199) heteroclusters, where the Xe* [≡Xe(3P1)] atom is excited to the lowest dipole-allowed extravalence Rydberg excitation. The ultrafast femtosecond and picosecond dynamics driven by the short-range repulsive interaction between the vertically excited Xe* Rydberg and the cluster Ar atoms was studied by molecular dynamics simulations. From the analysis of the time dependence of the structural parameters for site-specific Xe excitations in medium-sized (N=54) and large (N=146,199) clusters, two general configurational relaxation phenomena were established: a “bubble” formation (i.e., a large configurational dilation around Xe*) for Xe interior sites and a “spring” formation (i.e., the stretching of Xe* outside the cluster) for Xe surface sites. General Xe site-specific features of both bubble and spring formation involve ultrashort (Gaussian) energy transfer to the cluster (∼50–100 fs characteristic times τET) inducing configurational relaxation, which manifests a multimodal time solution. The initial (Gaussian) temporal mode (∼150–300 fs characteristic times τ0&gt;τET) is followed by an exponential mode (ps lifetime τ1), with subsequent impact induced, damped vibrational coherence effects with frequencies (ω2,ω3), and exponential decay (ps lifetimes τ2,τ3). The bubble formation for the central site of Xe*Ar146 or Xe*Ar54 is induced by energy transfer of τET≅60 fs followed by subsequent multimodal dilation with τ0≅170 fs and τ1≅2 ps, and a subsequent expansion with coherent motion of vibrational wave packets with ω2,ω3≅20, 40 cm−1 and τ2,τ3≅2, 6 ps. The bubble reaches an equilibrium configuration after ∼10 ps with asymptotic spatial expansion of ΔRb*=0.7–0.8 Å. The spring formation for an exterior surface site of Xe*Ar146 is τET≅80 fs and τ0≅210 fs, which is followed by a substantial (≅1.2 Å) Xe* stretching and a subsequent contraction accompanied by vibrational coherence effects with ω2≅10 cm−1 and τ2≅20 ps, with the asymptotic spring spatial extension ΔRs*≅0.6 Å, being accomplished after ∼30 ps. Regarding dynamic cluster size effects we established that following vertical excitation at initial temperatures Ti=10–30 K, the following phenomena are manifested: (i) Large Xe*Ar146 and Xe*Ar199 clusters exhibit short-time (10–20 ps) configurational relaxation in rigid clusters. (ii) The central site in a medium-sized Xe*Ar54 cluster undergoes a rigid–nonrigid (“melting”) transition induced by the electronic excitation, with the Xe* manifesting long-time (100–200 ps) mass transport from the interior bubble to the surface spring. (iii) Small Xe*Ar12 clusters exhibit stepwise reactive dissociation on the ps time scale. The spectroscopic implications of large configurational relaxation in Xe*ArN (N=54,146) clusters were interrogated by the simulations of the Xe site-specific time-dependent spectral shifts in emission, which decrease from the initial large values [e.g., δνe(t=0)=0.92 eV at Ti=10 K for the central site] to low values. The time evolution of the emission spectral shifts is qualitatively similar to the structural dynamics, which involves initial ultrafast (∼50–100 fs) decay, a (ps) exponential contribution, and a damped oscillatory behavior. The time-resolved Xe site-specific emission spectral shifts obey an exponential structure-spectral relationship which is isomorphous with time-independent relations for the absorption spectral shifts and for the emission asymptotic spectral shifts. Finally, predictions are provided for the spectroscopic interrogation (by energy-resolved fluorescence) of the longer time (∼150 ps) Xe* bubble mass transport in nonrigid Xe*Ar54 clusters. The long-time fluorescence spectra, which were simulated by the spectral density method, exhibit: (i) A Gaussian line shape, corresponding to the slow modulation limit. (ii) Spectral shifts (〈δνe〉=0.01–0.1 eV) exhibiting a site-specific hierarchy, i.e., 〈δνe〉(central)&gt;〈δνe〉(interior)&gt;〈δνe〉(surface)&gt;〈δνe〉(top). (iii) Linewidths (full width at half-maximum) which follow the order of the site-specific hierarchy of the spectral shifts. The calculated site-specific emission spectral shifts and linewidths and the calculated Stokes shifts for central and interior bubble sites and for surface spring sites in Xe*Ar146 are in reasonable agreement with the experimental results for Xe*Ar1400 clusters. Our overall picture regarding the dynamic and spectroscopic implications of large excited-state configurational relaxation provides guidance, predictions, and insight for the fate of Rydberg states in clusters and in the condensed phase.