## Abstract

In this article we explore the structural, dynamic, and spectroscopic implications of large local configurational changes in electronically excited Xe*Ar_{N} (N = 12,54,146,199) heteroclusters, where the Xe* [ ≡ Xe(^{3}P_{1})] 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 τ_{ET}) inducing configurational relaxation, which manifests a multimodal time solution. The initial (Gaussian) temporal mode (∼150-300 fs characteristic times τ_{0}>τ_{ET}) is followed by an exponential mode (ps lifetime τ_{1}), with subsequent impact induced, damped vibrational coherence effects with frequencies (ω_{2},ω_{3}), and exponential decay (ps lifetimes τ_{2},τ_{3}). The bubble formation for the central site of Xe*Ar_{146} or Xe*Ar_{54} is induced by energy transfer of τ_{ET}≅60 fs followed by subsequent multimodal dilation with τ_{0}≅170 fs and τ_{1} ≅2 ps, and a subsequent expansion with coherent motion of vibrational wave packets with ω_{2},ω_{3}≅20, 40 cm^{-1} and τ_{2},τ_{3}≅2, 6 ps. The bubble reaches an equilibrium configuration after ∼10 ps with asymptotic spatial expansion of ΔR_{b}* = 0.7-0.8 Å. The spring formation for an exterior surface site of Xe*Ar_{146} is τ_{ET}≅80 fs and τ_{0}≅210 fs, which is followed by a substantial (≅ 1.2 Å) Xe* stretching and a subsequent contraction accompanied by vibrational coherence effects with ω_{2}≅ 10 cm^{-1} and τ_{2}=20 ps, with the asymptotic spring spatial extension ΔR_{s}* ≅ 0.6 Å, being accomplished after ∼ 30 ps. Regarding dynamic cluster size effects we established that following vertical excitation at initial temperatures T_{i} = 10-30 K, the following phenomena are manifested: (i) Large Xe*Ar_{146} and Xe*Ar_{199} clusters exhibit short-time (10-20 ps) configurational relaxation in rigid clusters. (ii) The central site in a medium-sized Xe*Ar_{54} 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*Ar_{12} clusters exhibit stepwise reactive dissociation on the ps time scale. The spectroscopic implications of large configurational relaxation in Xe*Ar_{N} (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., δν_{e}(t = 0) = 0.92 eV at T_{i} = 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*Ar_{54} 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 (〈δν_{e}〉 = 0.01-0.1 eV) exhibiting a site-specific hierarchy, i.e., 〈δν_{e}〉(central)>〈δν _{e}〉(interior)>〈δν_{e}〉 X(surface)>〈δν_{e}〉(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*Ar_{146} are in reasonable agreement with the experimental results for Xe*Ar_{1400} 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.

Original language | English |
---|---|

Pages (from-to) | 8994-9017 |

Number of pages | 24 |

Journal | Journal of Chemical Physics |

Volume | 107 |

Issue number | 21 |

DOIs | |

State | Published - 1 Dec 1997 |

## Fingerprint

Dive into the research topics of 'Structural relaxation dynamics of electronically excited XeAr_{N}clusters'. Together they form a unique fingerprint.